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					                   Section 4 – Power Transformer Design

                                                                      the volt-seconds per turn applied to the windings
Power Transformer Design
                                                                      and is independent of load current.
     This Section covers the design of power trans-
formers used in buck-derived topologies: forward                  Undesirable Effects of Energy Storage
converter, bridge, half-bridge, and full-wave center-                  Leakage inductance delays the transfer of current
tap. Flyback transformers (actually coupled induc-                between switches and rectifiers during switching
tors) are covered in a later Section. For more spe-               transitions. These delays, proportional to load cur-
cialized applications, the principles discussed herein            rent, are the main cause of regulation and cross regu-
will generally apply.                                             lation problems. Reference (R4) included in this
                                                                  manual explains this in detail.
Functions of a Transformer
                                                                       Mutual inductance and leakage inductance energy
    The purpose of a power transformer in Switch-
                                                                  causes voltage spikes during switching transitions
Mode Power Supplies is to transfer power efficiently
                                                                  resulting in EMI and damage or destruction of
and instantaneously from an external electrical source
                                                                  switches and rectifiers. Protective snubbers and
to an external load. In doing so, the transformer also
                                                                  clamps are required. The stored energy then ends up
provides important additional capabilities:
                                                                  as loss in the snubbers or clamps. If the loss is exces-
• The primary to secondary turns ratio can be es-
                                                                  sive, non-dissipative snubber circuits (more complex)
    tablished to efficiently accommodate widely dif-
                                                                  must be used in order to reclaim most of this energy.
    ferent input/output voltage levels.
                                                                       Leakage and mutual inductance energy is some-
• Multiple secondaries with different numbers of
                                                                  times put to good use in zero voltage transition (ZVT)
    turns can be used to achieve multiple outputs at
                                                                  circuits. This requires caution–leakage inductance
    different voltage levels.
                                                                  energy disappears at light load, and mutual induc-
• Separate primary and secondary windings facili-
                                                                  tance energy is often unpredictable, depending on
    tate high voltage input/output isolation, especially
                                                                  factors like how well the core halves are mated to-
    important for safety in off-line applications.
                                                                  gether.
Energy Storage in a Transformer
     Ideally, a transformer stores no energy–all energy           Losses and Temperature Rise
is transferred instantaneously from input to output. In                Transformer loss is sometimes limited directly by
practice, all transformers do store some undesired                the need to achieve a required overall power supply
energy:                                                           efficiency. More often, transformer losses are limited
• Leakage inductance represents energy stored in                  by a maximum “hot spot” temperature rise at the core
     the non-magnetic regions between windings,                   surface inside the center of the windings. Tempera-
     caused by imperfect flux coupling. In the                    ture rise (°C) equals thermal resistance (°C/Watt)
     equivalent electrical circuit, leakage inductance is         times power loss (Watts).
     in series with the windings, and the stored energy
                                                                                   ∆T = RT × PL
     is proportional to load current squared.
• Mutual inductance (magnetizing inductance) rep-                      Ultimately, the appropriate core size for the ap-
     resents energy stored in the finite permeability of          plication is the smallest core that will handle the re-
     the magnetic core and in small gaps where the                quired power with losses that are acceptable in terms
     core halves come together. In the equivalent cir-            of transformer temperature rise or power supply effi-
     cuit, mutual inductance appears in parallel with             ciency.
     the windings. The energy stored is a function of




                                                            4-1
Temperature Rise Limit                                           with high velocity forced air cooling), and while RI
    In consumer or industrial applications, a trans-             shouldn’t be ignored, it usually is not critically im-
former temperature rise of 40-50°C may be accept-                portant compared with RE.
able, resulting in a maximum internal temperature of                 External RE is mainly a function of air convection
100°C. However, it may be wiser to use the next size             across the surface of the transformer–either natural
larger core to obtain reduced temperature rise and               convection or forced air. RE with natural convection
reduced losses for better power supply efficiency.               cooling depends greatly upon how the transformer is
                                                                 mounted and impediments to air flow in its vicinity.
Losses                                                           A transformer mounted on a horizontal surface and
    Losses are difficult to predict with accuracy.
                                                                 surrounded by tall components, or mounted in a rela-
Core loss data from core manufacturers is not always
                                                                 tively small enclosure will have considerably greater
dependable, partly because measurements are made
                                                                 RE than if it were mounted on a vertical surface,
under sinusoidal drive conditions. Low frequency
                                                                 benefiting from the “chimney effect”. With forced air
winding losses are easy to calculate, but high fre-
                                                                 cooling, RE can be driven down to a very small value,
quency eddy current losses are difficult to determine
                                                                 depending on air velocity, in which case internal RI
accurately, because of the high frequency harmonic
                                                                 becomes the primary concern. With forced air cool-
content of the switched rectangular current wave-
                                                                 ing, thermal resistance and temperature rise often be-
shape. Section 3 discusses this problem extensively.
                                                                 come irrelevant, because an absolute loss limit to
Computer software can greatly ease the difficulty of
                                                                 achieve power supply efficiency goals becomes
calculating the winding losses, including high order
                                                                 dominant.
harmonics(1).
                                                                     For the average situation with natural convection
Thermal Resistance                                               cooling, a crude “rule of thumb” can be used:
     Temperature rise depends not only upon trans-
former losses, but also upon the thermal resistance,                      800° C - cm 2 / Watt
RT (°C/Watt), from the external ambient to the central               RE =                            ° C / Watt
                                                                              AS in cm 2
hot spot. Thermal resistance is a key parameter, un-
fortunately very difficult to define with a reasonable                Where AS is the total surface area of the trans-
degree of accuracy. It has two main components: in-              former, excluding the mounting surface. Calculating
ternal thermal resistance RI between the heat sources            AS is time-consuming, but another rule of thumb sim-
(core and windings) and the transformer surface, and             plifies this, as well. For a given class of cores, such as
the external thermal resistance RE from the surface to           E-E cores in the ETD or EC series, the relative pro-
the external ambient.                                            portions are quite similar for all core sizes. Thus for
     Internal thermal resistance depends greatly upon            all cores in the ETD or EC series, the usable surface
the physical construction. It is difficult to quantify           area, AS, is approximately 22 times the winding win-
because the heat sources are distributed throughout              dow area, AW. Combining this with the equation
the transformer. RI from surface to internal hot spot is         above enables the window area, AW, from the core
not relevant because very little heat is actually gener-         data sheet, to be used to directly calculate the exter-
ated at that point. Most of the heat generated in the            nal thermal resistance:
core (other than in toroids) is near the transformer
surface. Heat generated within the winding is distrib-                         36
                                                                     RE =                   ° C / Watt
uted from the surface to the internal core. Although                        AW in cm 2
copper has very low thermal resistance, electrical in-
sulation and voids raises the RT within the winding.                 For pot cores or PQ cores, window areas are pro-
This is a design area where expertise and experience             portionately smaller, and not as consistent. AS/AW
is very helpful. Fortunately, internal thermal resis-            may range from 25 to 50, so that RE may range from
tance is considerably smaller than external RE (except           16/AW to 32/AW °C/W.



                                                           4-2
    Experience is a great help in minimizing and                   Ferrite cores: In most ferrite materials used in
crudely quantifying thermal resistance. In the final           SMPS applications, hysteresis losses dominate up to
analysis, an operational check should be conducted             200-300kHz. At higher frequencies, eddy current
with a thermocouple cemented at the hot spot near the          losses take over, because they tend to vary with fre-
middle of the centerpost, with the transformer                 quency squared (for the same flux swing and wave-
mounted in a power supply prototype or mockup.                 shape).
Worst Case Losses                                                  Thus, at frequencies up to 200-300kHz, worst
     Transformer losses should be examined under               case is at low VIN and full load because of high
worst-case conditions that the power supply is ex-             winding losses. Once core eddy current losses be-
pected to operate over long periods of time, not under         come significant, they rise rapidly with frequency,
transient conditions.                                          especially at high VIN. (The increase in eddy current
     Transformer losses can be put into three major            loss with high VIN, small D, is not shown in core
categories: core hysteresis losses, core eddy current          manufacturer’s loss curves because they assume sinu-
losses, and winding losses.                                    soidal waveforms.) Winding losses also rise with fre-
     Core hysteresis losses are a function of flux             quency, especially at low VIN. To maintain a reason-
swing and frequency. In all buck-derived applications          able RAC/RDC, Litz wire with more strands of finer
under steady-state conditions, VIN•D = n•VO. Under             wire must be used, raising RDC because increased in-
fixed frequency operation, volt-seconds and therefore          sulation and voids reduce the copper area. Thus, at
flux swing are constant. Hysteresis loss is therefore          frequencies where core eddy current losses dominate,
constant, regardless of changes in VIN or load cur-            core loss worst case is at high VIN, full load. Winding
rent.                                                          loss worst case is always at low VIN, full load..
     Core eddy current loss, on the other hand, is                 Laminated metal alloy and powdered metal
really I2R loss in the core material. If VIN doubles,          cores: Core eddy current losses dominate, hence
Peak I2R loss quadruples, but since D is halved, aver-         worst case is at high VIN, full load. Winding losses
age I2R loss doubles. Thus core eddy current loss is           are worst case at low VIN, full load.
proportional to VIN. Worst case is at high VIN.                Balancing Core and Winding Losses
     Winding losses: In buck-derived regulators, peak              At SMPS operating frequencies, when the core is
secondary current equals load current and peak pri-            usually loss-limited, not saturation limited, total
mary current equals load current divided by the turns          losses are at a broad minimum when core losses are
ratio:                                                         approximately equal to or a little less than winding
                                                               losses. Likewise, winding losses are at a minimum
        I Spk = I L ; I Ppk = I L / n                          and well distributed by making the rms current den-
                                                               sity approximately equal in all windings. With a
    Peak currents are independent of VIN. But at con-
                                                               bridge or half-bridge primary, which has good wind-
stant peak currents (constant load), rms current
                                                               ing utilization, and center-tapped secondaries which
squared (and I2R loss) is proportional to duty cycle D
                                                               have poor utilization, rms current densities will be
and inversely proportional to VIN.. (With constant
                                                               approximately equalized when the primary conductor
peak current, high order harmonics depend mostly on
                                                               cross-section area is 40% and the secondaries 60% of
switching transitions and do not vary significantly
                                                               the available area. In most other cases, primary and
with D.)
                                                               secondary conductor areas should be 50%/50%, in-
    In buck-derived regulators, winding loss is al-
                                                               cluding: Forward converter (single-ended pri-
ways greatest at low VIN.
                                                               mary/secondary SE/SE), C.T. primary/C.T. secon-
                                                               dary, bridge-half bridge primary/bridge secondary.




                                                         4-3
    The above allocations can be impossible to                       There is a great deal of overlap in topology us-
achieve because the number of turns in each winding             age. Flyback circuits (flyback transformers are cov-
must be an integral number. In a low voltage secon-             ered in Section 5) are used primarily at power levels
dary, 1.5 turns may be required for optimum balance             in the range of 0 to 150 Watts, Forward converters in
between core and winding losses. With one turn, the             the range of 50 to 500 Watts, half-bridge from 100 to
flux swing and core loss may be much too large; with            1000 Watts, and full bridge usually over 500 Watts.
two turns the winding loss becomes too great. At ei-                 Full bridge and half-bridge topologies with full
ther extreme, it may be impossible to meet tempera-             bridge secondaries have the best transformer effi-
ture rise or absolute loss limits. A larger core may be         ciency because the core and the windings are fully
required to resolve this problem.                               utilized. With center-tapped secondaries, winding
Window Utilization                                              utilization and efficiency are reduced. With center-
    This subject is discussed extensively in Section 3.         tapped primary and secondaries, winding utilization
As a reminder:                                                  and efficiency are further reduced. All of the push-
• Safety isolation requirements impose minimum                  pull topologies have the further advantage that for a
    dimensional limits for creepage and insulation              given switching frequency, giving the same output
    thickness which can waste a high percentage of              ripple filtering and closed loop capability, the fre-
    window area, especially in a small transformer. A           quency at which the transformer core and windings
    bobbin also reduces the area available for wind-            operate is halved, reducing core and ac winding
    ings.                                                       losses.
         Triple insulated wire satisfies the insulation              Forward converter transformers have the poorest
    thickness requirement and eliminates the cree-              utilization and efficiency because neither the core nor
    page requirement. It is worth considering, espe-            the windings are used during the lengthy core reset
    cially for small transformers where creepage dis-           interval.
    tances take up a large percentage of window area.           Frequency
• In the reduced window area that is available for                    There are several meanings to the term “fre-
    the windings, much of the actual winding area is            quency” in switching power supply applications, and
    taken up by voids between round wires and by                it is easy for confusion to arise.
    wire insulation. In a winding consisting of many                  In this paper, “switching frequency”, fS, is de-
    turns of single, round, insulated wires, only 70 -          fined as the frequency at which switch drive pulses
    75% of the area available for that winding is               are generated. It is the frequency seen by the output
    likely to be conductor metal -- “copper”. With              filter, the frequency of the output ripple and input
    Litz wire, the copper area is reduced further. For          ripple current, and is an important concept in control
    every level of twisting , an additional 0.75 factor         loop design. In a single-ended power circuit such as
    (approximate) applies. For example, with Litz               the forward converter, the power switch, the trans-
    wire 7 strands of 7 strands (49 total wires), the           former, and the output rectifier all operate at the
    copper area would be .75•.75•.75 = 42% of the               switching frequency and there is no confusion. The
    area available for that winding. On the other               transformer frequency and the switching frequency
    hand, a winding consisting of layers (turns) of             are the same.
    copper foil or strap, there are no voids, only the                “Clock frequency” is the frequency of clock
    insulation between turns. Winding area utiliza-             pulses generated in the control IC. Usually, the
    tion could be as much as 80 - 90% copper area.              switching frequency is the same as the clock fre-
Topology                                                        quency, but not always. Occasionally, the control IC
    The choice of circuit topology obviously has                may divide the clock frequency to obtain a lower
great impact on the transformer design, but a detailed          switching frequency. It is not unusual for a push-pull
discussion is beyond the scope of this topic.                   control IC to be used in a single-ended forward con-
                                                                verter application, where only one of the two switch


                                                          4-4
drivers is used, to guarantee 50% max. duty cycle. In
this case the switching frequency is half the clock                                             V IN D nVO '
                                                                                  V IN t on =         =
frequency.                                                                                         fS   fS
    Confusion often arises with push-pull topologies.
                                                                      The maximum duty cycle, Dmax, associated with
Think of the push-pull power circuit as a 2:1 fre-
                                                                 minimum VIN in normal steady-state operation, is
quency divider, with the transformer and the individ-
                                                                 limited by a variety of considerations:
ual switches and individual rectifiers operating at a
                                                                      In a forward converter, a substantial portion of
“transformer frequency”, fT, which is one-half of the
                                                                 each switching period must be allowed for core reset.
switching frequency. Collectively, the switches and
                                                                 If the voltage backswing during reset is clamped to
rectifiers operate at the switching frequency, but the
                                                                 VIN, the duty cycle must be limited to less than 50%
transformer operates at the transformer frequency.
                                                                 because the time required for reset equals the switch
Some designers define “switching frequency” as the
                                                                 ON time.
frequency that the individual switch and the trans-
                                                                      In a push-pull converter (bridge, half-bridge,
former operate at, but this requires redefining the
                                                                 PPCT) duty cycle can approach 100% at the switch-
term “switching frequency” when dealing with output
                                                                 ing frequency (always think of D at the switching
ripple and in control loop design.
                                                                 frequency, not the transformer frequency). However,
Duty Cycle                                                       it may be necessary to limit D to less than 90% to
    Duty cycle, D, is defined as the amount of time              allow a current transformer to self-reset.
the power switch is on in relation to the switching                   Often the control IC limits the duty cycle for sev-
period: D = tON/TS.                                              eral reasons including allowing time for delays in
    In a single-ended forward converter, this is                 turning off the switch.
clearly understood, but in a push-pull circuit, ambi-                 At low VIN, if normal Dmax is right at the duty
guity often arises. For example, in a half-bridge cir-           cycle limit, the regulator has no reserve volt-second
cuit operating at minimum VIN, the duty cycle is                 capability and cannot respond rapidly to a sudden
likely to be in the vicinity of 90% (D = 0.9). The               load increase occuring when VIN is low. It may be
transformer is delivering power to the output 90% of             desirable to make the “normal” Dmax less than the
the time, there is a voltage pulse applied to the filter         absolute limit, Dlim, to provide a little headroom in
input 90% of the time, etc. But individual power                 this situation.
switches and individual rectifiers, which conduct                     A potentially serious problem needs to be consid-
only during alternate switching periods, can be said to          ered: During initial start-up of the power supply, or
operate at a duty cycle of 45%. That is true, but it is          following a sudden large increase in load current
better tothink of them as operating at D/2, retaining a          which temporarily pulls down Vout, the control loop
consistent definition of D throughout the power sup-             calls for full current, pushing the duty cycle to its ab-
ply design.                                                      solute maximum limit, Dlim. The output filter in-
Maximum Duty Cycle                                               ductor limits the current rate of rise, so that for sev-
     In normal steady-state operation of a buck-                 eral switching frequency periods, the duty cycle is at
derived regulator, VIN•D is constant. The control                the limit, Dlim. During the transient event described
loop changes duty cycle D inversely proportional to              above, Dlim could occur when VIN is maximum.
VIN to maintain a constant output voltage, VO. (VIND             Thus, the volt-seconds applied to the transformer
= n•VO'), where n is the turns ratio NP/NS, and VO’              windings could be several times larger than normal:
equals output voltage VO plus diode forward voltage
drop at full load.                                                   Limit VIND = VINmaxDlim
     At a fixed switching frequency and with normal                  Normal VIND = VINminDmax
steady-state operation, the volt-seconds applied to
the transformer windings are constant, independent
of line voltage or load current.

                                                           4-5
     The flux swing, also several times greater than                  tentatively selected, the turns ratios will translate into
normal, could saturate the core. (The increased core                  specific turns, but these are not likely to be the inte-
loss is not a problem–it is only temporary.)                          gral numbers required in practice. It then becomes a
     This may not be a problem if the ratio                           juggling act, testing several approaches, before
limit/normal VIND is small and/or if the normal flux                  reaching the best compromise with integral turns. The
density swing, limited by core loss, is a small fraction              lowest voltage secondary usually dominates this pro-
of Bsat (Bsat - Br for a forward converter). For ex-                  cess, because with small numbers the jumps between
ample, if limit/normal VIND is 3:1, and if normal ∆B                  integral turns are a larger percentage. Especially if
is 0.08T, then with Bsat greater than 0.24T, there is                 the lowest voltage output has the greatest load power,
no problem.                                                           which is often the case, the lowest voltage secondary
     If this problem exists, soft-start circuitry can                 is rounded up or down to the nearest integral.
eliminate it during start-up, but soft-start has no effect            Rounding down will increase core loss, rounding up
when the load increases rapidly. A few IC control                     will increase winding loss. If the increased loss is
circuits have volt-second limiting capability, but the                unacceptable, a different core must be used that will
vast majority do not. The soft saturation characteris-                require less adjustment to reach an integral number of
tic of power ferrite material may be forgiving enough                 turns. The low voltage output is usually regulated by
to allow the core to saturate, with the absolute current              the main control loop.
limit providing protection, but with sharp-saturation                     Higher voltage secondaries can be rounded up to
core materials, this is a likely disaster. If all else fails,         the next integral with less difficulty because they
the normal flux swing must be reduced to the point                    have more turns. However, it is unlikely that accuracy
where the abnormal flux swing does not reach satu-                    or load regulation will be acceptable, requiring linear
ration.                                                               or switched post-regulation.
Restrictions on Number of Turns                                           Since the primary is usually higher voltage, the
     Choices regarding the number of turns and turns                  primary turns can usually be set to achieve the de-
ratios are often severely limited by low voltage sec-                 sired turns ratio without difficulty.
ondaries. For a 5 Volt output the alternatives might                      Once the turns have been established, the initial
be a 1-turn or a 2-turn secondary–a 2 to 1 step in the                calculations must be redefined.
number of turns in every winding. For the same size                   Flux Walking
core and window, this doubles the current density in                       Faraday’s Law states that the flux through a
the windings and accordingly increases the loss.                      winding is equal to the integral volt-seconds per turn.
     Choices may be further restricted when there are                 This requires that the voltage across any winding of
multiple low voltage secondaries. For example, a 2.5                  any magnetic device must average zero over a period
to 1 turns ratio may be desirable between a 12 Volt                   of time. The smallest dc voltage component in an ap-
and a 5 Volt output. This is easily accomplished with                 plied ac waveform will slowly but inevitably “walk”
a 2-turn 5V secondary and a 5-turn 12V winding. But                   the flux into saturation.
if the 5V secondary has only 1 turn, the only choice                       In a low frequency mains transformer, the resis-
for the 12V secondary is 3 turns, which may result in                 tance of the primary winding is usually sufficient to
excessive linear post-regulator loss. This problem                    control this problem. As a small dc voltage compo-
could be handled by the use of fractional turns -- see                nent pushes the flux slowly toward saturation, the
reference (R6).                                                       magnetizing current becomes asymmetrical. The in-
     There are no hard and fast rules to follow in es-                creasing dc component of the magnetizing current
tablishing the optimum turns for each winding, but                    causes an IR drop in the winding which eventually
there is some general guidance. First, define the ideal               cancels the dc voltage component of the drive wave-
turns ratios between windings that will achieve the                   form, hopefully well short of saturation.
desired output voltages with the normal VIND estab-
lished earlier. Later, when a specific core has been


                                                                4-6
     In a high frequency switchmode power supply, a              volt-second asymmetry is thereby corrected, peak
push-pull driver will theoretically apply equal and              magnetizing currents are approximately equal in both
opposite volt-seconds to the windings during alter-              directions, and flux walking is minimized.
nate switching periods, thus “resetting” the core                    However, with the half-bridge topology this cre-
(bringing the flux and the magnetizing current back to           ates a new problem. When current mode control cor-
its starting point). But there are usually small volt-           rects the volt-second inequality by shortening and
second asymmetries in the driving waveform due to                lengthening alternate pulse widths, an ampere-second
inequalities in MOSFET RDSon or switching speeds.                (charge) inequality is created in alternate switching
The resulting small dc component will cause the flux             periods. This is of no consequence in full bridge or
to “walk”. The high frequency transformer, with                  push-pull center-tap circuits, but in the half-bridge,
relatively few primary turns, has extremely low dc               the charge inequality causes the capacitor divider
resistance, and the IR drop from the dc magnetizing              voltage to walk toward the positive or negative rail.
current component is usually not sufficient to cancel            As the capacitor divider voltage moves away from the
the volt-second asymmetry until the core reaches                 mid-point, the volt-second unbalance is made worse,
saturation.                                                      resulting in further pulse width correction by the cur-
     Flux walking is not a problem with the forward              rent mode control. A runaway situation exists, and
converter. When the switch turns off, the transformer            the voltage will walk (or run) to one of the rails. This
magnetizing current causes the voltage to backswing,             problem is corrected by adding a pair of diodes and a
usually into a clamp. The reverse voltage causes the             low-power winding to the transformer, as detailed in
magnetizing current to decrease back to zero, from               the Unitrode Applications Handbook.
whence it started. The reverse volt-seconds will ex-             Core Selection: Material
actly equal the volt-seconds when the switch was ON.                  Select a core material appropriate for the desired
Thus the forward converter automatically resets itself           transformer frequency.
(assuming sufficient reset time is allowed, by limiting               With power ferrites, higher frequency materials
the maximum duty cycle).                                         have higher resistivity, hence lower eddy current
     The flux walking problem is a serious concern               losses. However, the permeability is generally lower,
with any push-pull topology (bridge, half-bridge or              resulting in greater magnetizing current, which must
push-pull CT), when using voltage mode control..                 be dealt with in snubbers and clamps.
     One solution is to put a small gap in series with                With metal alloy cores, the higher frequency
the core. This will raise the magnetizing current so             materials have higher resistivity and require very thin
that the IR drop in the circuit resistances will be able         laminations. Although saturation flux density is usu-
to offset the dc asymmetry in the drive waveform.                ally very much greater than with ferrite materials, this
But the increased magnetizing current represents in-             is usually irrelevant because flux swing is severely
creased energy in the mutual inductance which usu-               limited by eddy current losses.
ally ends up in a snubber or clamp, increasing circuit                Ferrite is the best choice in transformer applica-
losses.                                                          tions except for mechanical ruggedness.
     A more elegant solution to the asymmetry prob-
lem is an automatic benefit of using current mode                Core Selection: Shape
                                                                      The window configuration is extremely impor-
control (peak or average CMC). As the dc flux starts
                                                                 tant. The window should be as wide as possible to
to walk in one direction due to volt-second drive
                                                                 maximize winding breadth and minimize the number
asymmetry, the peak magnetizing current becomes
                                                                 of layers. This results in minimized Rac and leakage
progressively asymmetrical in alternate switching
                                                                 inductance. Also, with a wide window, the fixed
periods. However, current mode control senses cur-
                                                                 creepage allowance dimension has less impact. With
rent and turns off the switches at the same peak cur-
                                                                 a wider window, less winding height is required, and
rent level in each switching period, so that ON times
                                                                 the window area can be better utilized.
are alternately lengthened and shortened. The initial


                                                           4-7
     Pot cores and PQ cores have small window area                     There are many variables involved in estimating
in relation to core size, and the window shape is al-             the appropriate core size. Core power handling capa-
most square. The creepage allowance wastes a large                bility does not scale linearly with area product or
fraction of the window area, and the winding breadth              with core volume. A larger transformer must operate
is far from optimum. These cores are not as well                  at a lower power density because heat dissipating sur-
suited for high frequency SMPS applications. One                  face area increases less than heat-producing volume.
advantage of pot cores and PQ cores is that they pro-             The thermal environment is difficult to evaluate accu-
vide better magnetic shielding than E-E cores, re-                rately, whether by forced air or natural convection.
ducing EMI propagation.                                                Some core manufacturers no longer provide area
     EC, ETD, LP cores are all E-E core shapes. They              product information on their data sheets, often sub-
have large window area in relation to core size, and              stituting their own methodology to make an initial
the window has the desirable wide configuration.                  core size choice for various applications.
     Toroidal cores, properly wound, must have all                     The following formula provides a crude indica-
windings distributed uniformly around the entire                  tion of the area product required:
core. Thus the winding breadth is essentially the cir-
cumference of the core, resulting in the lowest possi-                                              4
                                                                                  æ PO          ö       3
ble leakage inductance and minimizing the number of                   AP = AWAE = ç
                                                                                  ç K ∆B f      ÷
                                                                                                ÷           cm4
winding layers. There is no creepage allowance be-                                è        T    ø
cause there is no end to the windings. (But there is a
                                                                      where:
problem bringing the leads out.) Stray magnetic flux
                                                                         PO       = Power Output
and EMI propagation are also very low.
                                                                         ∆B       = Flux density swing, Tesla
     The big problem with toroidal cores is the wind-
                                                                         fT       = Transformer operating frequency
ing difficulty, especially with the shapes and gauge of
                                                                         K        = .014 (Forward converter,PPCT)
conductors used in SMPS transformers. How can a 1-
                                                                                  = .017 (Bridge, half bridge)
turn secondary be spread around the entire toroid?
Automatic winding is virtually impossible. For this
                                                                       This formula is based on current density of
reason, toroidal shapes are seldom used in SMPS
                                                                  420A/cm2 in the windings, and assumes a window
transformers.
                                                                  utilization of 40% copper. At low frequencies, the
     Planar cores with their low profile are becoming
                                                                  flux swing is limited by saturation, but above 50kHz
more popular as SMPS frequencies progressively in-
                                                                  (ferrite), ∆B is usually limited by core losses. Use the
crease. Planar cores introduce a new set of unique
                                                                  ∆B value that results in a core loss of 100mW/cm3 (2
problems which are beyond the scope of this discus-
                                                                  times the “flux density” given in the core loss
sion. Be assured that Faraday’s and Ampere’s Laws
                                                                  curves).
still apply, but in a planar core, flux density and field
                                                                       These initial estimates of core size are not very
intensity change considerably throughout the impor-
                                                                  accurate, but they do reduce the number of trial solu-
tant regions, making calculation much more difficult.
                                                                  tions that might otherwise be required. In the final
Core Selection: Size                                              analysis, the validity of the design should be checked
    A novice in the art of transformer design usually             with a prototype transformer operated in the circuit
needs some guidance in making an initial estimate of              and the environment of the application, with the hot
the core size appropriate for the application require-            spot temperature rise measured by means of a ther-
ments. One widely used method, with many varia-                   mocouple cemented to the center of the centerpost.
tions, is based on the core Area Product, obtained by
multiplying the core magnetic cross-section area by
the window area available for the winding.




                                                            4-8
Transformer Design Cookbook                                              VINmaxDlim:                   89.3 V
     The steps for designing a power transformer for
SwitchMode Power Supplies is outlined below. A                       Step 3.    Calculate output voltages plus diode
typical example is carried through to illustrate the             and secondary IR drops at full load:
process. There are many approaches to transformer
design. The approach presented here appears the most                     VO1':      5.0 + 0.4 = 5.4 Volts
logical and straightforward to the author.                               VO2':      n/a
     It may be worthwhile to use software such as
“Magnetic Designer” from Intusoft(2) for the initial                 Step 4.     Calculate desired turns ratios: P-S1;
design, using the approach defined herein for verifi-            S1-S2, etc. Remember that choices with low voltage
cation and tune-up. The author has not evaluated                 secondaries will probably be limited.
“Magnetic Designer” sufficiently to make an unquali-
fied endorsement, but it should certainly make a good                    n = NP/NS1 = VIND/VO1': 42/5.4 = 7.8
starting point and take a great deal of drudgery out of                  Possible choices: 8:1 ; 7:1 ; 15:2
the process. It has the advantage of including an ex-            Core Selection
tensive core database.                                               Step 5:     Tentatively select core material,
Initial Preparation                                              shape and tentative size, using guidance from the
     The first few steps in this process define applica-         manufacturer’s data sheet or using the area product
tion parameters that should not change, regardless of            formula given previously in this paper. Will a bobbin
subsequent iterations in the selection of a specific             be used?:
core type and size.                                                      Core Material: Ferrite, Magnetics Type P
     If the results are not acceptable, start over from                  Core type, Family: ETD
the very beginning, if that seems appropriate. Great                     Core Size: 34mm -- ETD34
difficulty in achieving an acceptable forward con-
verter transformer design may be a subtle message                   Step 6: For the specific core selected, note:
that a half-bridge topology is perhaps a better choice.             Effective core Area, Volume, Path Length. (cm)
     Step 1.      Define the power supply parameters                      Ae:       0.97 cm2
pertaining to the transformer design:                                     Ve:       7.64 cm3
                                                                          le:       7.9 cm
        VIN Range:               100 - 190 V                        Window Area, Breadth, Height, Mean Length per
        Output 1:                5 V, 50 A                       Turn ( ' indicates net with bobbin, creepage).
        Output 2:                none                                     Aw / Aw': 1.89 / 1.23 cm2
        Circuit Topology:        Forward Converter                        bw / bw': 2.36 / 1.5 cm
        Switching Freq, fS:      200 kHz                                  hw / hw': 0.775 / 0.6 cm
        Transformer Freq, fT:    200 kHz                                 MLT:       5.8 / 6.1 cm
        Max Loss (absolute):     2.5 W
        Max °C Rise:             40°C                            Define RT and Loss Limit
        Cooling Method:          Natural Convection                  Step 7:    Obtain thermal resistance from data
                                                                 sheet or calculate from window area (not bobbin
   Step 2.      Define absolute duty cycle limit                 area) from formula for EC and ETD series:
Dlim, tentative normal Dmax at low VIN (to provide
headroom for dynamic response), and normal VIND:                                     800           36
                                                                     R E =                      =       = 19 °C/Watt
                                                                             2 2 • A W i n c m 2 1 .8 9
        Absolute Limit, Dlim:    0.47
        Normal Dmax:             0.42
        Normal VIN•D :           VINmin•Dmax = 42 V

                                                           4-9
     Calculate loss limit based on max. temperature                   Step 11: Finalize the choice of primary turns. A
rise:                                                            larger turns ratio results in lower peak current, larger
         Plim = °Crise/RT:      = 40/19 = 2.1 Watts              D (less reserve), and more copper loss. From the pos-
                                                                 sibilities defined in Step 4, trial solutions show the
    The 2.1W limit applies, since it is less than the            best choice to be NP = 15 turns (7.5:1 turns ratio).
absolute limit from Step 1. Tentatively apportion half                Recalculate normal VIND and flux swing under
to core loss, half to winding loss.                              worst case VINmaxDlim conditions:
         PClim: 1 Watt
         PWlim: 1.1 Watt                                             VIND = nVO’ = 7.5•5.4 = 40.5 V

    Step 8: Loss Limited Flux Swing                                  ∆Blim = 0.14T•89.3/40.5 = 0.31T -- OK
    Calculate max. core loss per cm3
        PClim/Ve = 1/7.64 = 131 mw/cm3 ( = kW/m3)                    Step 12: Define the winding structure.
    Using this core loss value, enter the core loss                  An interleaved structure will be used, as shown in
curve for the P material selected. At the transformer            Figure 4-1, to minimize leakage inductance and
frequency, find “flux density” (actually peak flux               winding losses.
density). Double it to obtain the loss-limited peak-
peak flux density swing, ∆B:

    At 131 mw/cm3 and 200kHz:
    ∆B = 2•800 Gauss = 1600G = 0.16 Tesla
    Normal ∆ φ = ∆B•Ae

    Step 9: Using Faraday’s Law, calculate the num-
ber of secondary turns:

    òES1 dt = VpkS1tON = VO1'•TS
    NS1 = òES1dt/∆ φ = VO1'•TS /∆ φ

         VO1′TS   5.4 × 5 × 10 −6
N S1 =          =                   = 1.74 Turns
         ∆B × AE .16 × .97 × 10 − 4
     Rounding down to 1 turn will greatly increase the
volts/turn, flux swing and core losses. Rounding up to
2 turns reduces core losses but increases winding                Figure 4-1
loss. Since the result above is much closer to 2 turns,
this will be adopted.                                                The interleaved structure results in two winding
     Step 10: Recalculate flux swing and core loss at            sections. Primary windings of 15 turns in each section
2 turns:                                                         are connected in parallel. Primary current divides
                          1.74turns                              equally in the two paralleled windings because this
    ∆B(2 turns) = 0.16T             = 0.14Tesla
                           2 turns                               results in the lowest energy transfer. Secondary
                                                                 windings of 1 turn copper foil in each section are
   From the core loss curves, loss at 0.14T/2 (700
                                                                 connected in series, resulting in a 2-turn secondary.
Gauss) is 110mw/cm3 x 7.64cm 3
                                                                 With only one turn in each section, the secondary
                                                                 windings can be much thicker than DPEN to minimize
         Core loss = 0.84 W
                                                                 dc resistance without increasing the ac resistance.



                                                          4-10
    Step 12: Calculate DPEN at 200 kHz:                            wires deep. Q is approximately 1/10 the value for
                                                                   solid wire, or 0.3, resulting in Rac/Rdc of 1.2. Thus,
    Dpen = 7.6/√f = 7.6/√200,000 = .017 cm                         Rac = Rdc•1.2, or .06Ω.
                                                                       Multiplying by 1.65A squared, the ac loss is
   Step 13: Calculate dc and rms ac currents in each               0.16W in each section, for a total primary ac loss of
winding at VINmin and Dmax. (Ref. Section 3):                      0.32W. Adding the 0.18W dc loss,

    ISdc = 50A•Dmax = 50•0.405 = 20.25A                                Total primary power loss = 0.5 Watts.

    ISac = ISdc((1-D)/D)1/2 = 24.5A                                     Step 15: Define the secondary winding.
                                                                        The secondary consists of two turns (two layers)
    IPdc = ISdc/n = 20.25/7.5 = 2.7A                               of copper strip or foil, 1.3cm wide (full available
                                                                   winding breadth), and 0.13cm thick. There is one
    IPac = ISac/n = 24.5/7.5 = 3.27A                               secondary layer in each of the two sections of the in-
                                                                   terleaved winding structure. This permits the thick-
    Primary current in each of the two paralleled sec-             ness of the copper strip to be much greater than DPEN
tions is one-half the total primary current: 1.35Adc               to minimize dc losses, without increasing ac losses.
and 1.65Aac.                                                       This is because ac current flows only on the outer
                                                                   side of each turn. As the conductor is made thicker,
    Step 14: Define the primary winding:                           Rac/Rdc becomes larger, but Rdc decreases and Rac
    One layer of 15 turns spread across the available              remains the same.
winding breadth of 1.3cm allows a maximum insu-                         With a solid copper secondary, the layer thick-
lated wire diameter of 0.87mm. AWG 21 – 0.72mm                     ness is the same as the conductor thickness, 0.1cm.
copper will be used.                                                    Q = Layer thickness/DPEN = 0.13/.017 = 7.6
    From Ref R2, pg 9, the effective layer thickness                    Rac/Rdc = 7.5
equals 0.83•dia(dia/spacing)1/2.
                                                                       This will be acceptable because the dc resistance
    Q = (layer thickness)/DPEN                                     is very low.
    Q = 0.83•.072(.072/.087)1/2/.017 = 3.19
                                                                       Rdc = ρ•MLT•Ns/(bw’h)
    From Dowell’s curves, Rac/Rdc for 1 layer is 3.1.                  Rdc = 2.3•10-6•6.1•2/(1.3•0.13) = 166µΩ
This will result in unacceptable ac losses.                            Pdc = 166µΩ•20.252 = .068 W
    A Litz wire consisting of 100 strands #42 wire                         Pac = Rdc•Rac/Rdc•Iac2 = 166µΩ•7.5•24.52
has a diameter of 0.81mm and a resistance of                           Pac = 0.75 W
0.545mΩ/cm.                                                            Total secondary loss:
    The dc resistance of the single layer is:                              .068W + 0.75W = 0.82 W
                                                                       Total copper loss:
                                                                           0.82W + 0.5W = 1.32 W
    Rdc = Ω/cm•MLT•Ns = .00055•6.1•15 = .05Ω
                                                                       Total core plus copper loss:
                                 2
     Multiplying by (1.35Adc) , dc power loss is                           0.84W + 1.32W = 2.16 Watts
.091W in each section, for a total primary dc loss of
0.18W.                                                                 Thus, the total power loss is under the absolute
     The diameter of each #42 wire is .064mm, but                  limit of 2.5Watts, but slightly over the 2.1 Watt limit
there are effectively ten layers of fine wire in the sin-          based on the desired max. temperature rise of 40°C.
gle layer of Litz wire. This is because the 100 strands
are roughly equivalent to a 10 x 10 array, thus ten


                                                            4-11
References:

(R2)    “Eddy Current Losses in Transformer Windings
and Circuit Wiring,” Unitrode Seminar Manual SEM600,
1988 (reprinted in the Reference Section at the back of this
Manual)

(R4)     “The Effects of Leakage Inductance on Switching
Power Supply Performance,” Unitrode Seminar Manual
SEM100, 1982 (reprinted in the Reference Section at the
back of this Manual)

(R6)     “How to Design a Transformer with Fractional
Turns,” Unitrode Seminar Manual SEM500, 1987 (re-
printed in the Reference Section at the back of this Manual)

(1)    PROXY -- Proximity effect analysis, KO Systems,
Chatsworth, CA, 818-341-3864

(2)      “Magnetics Designer,” Magnetics design software,
IntuSoft, San Pedro, CA 310-833-0710




                                                               4-12
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