Stealth Diode And Smps Mosfet Help In Controlling Emi In Power Supplies by ps94506

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       Stealth Diode And SMPS MOSFET
            Help In Controlling EMI
               In Power Supplies
                 Technological Advances Can Reduce EMI And
           Provide Increased Power Density With Fewer Components


      witch-mode power supplies               produce harmonics that can cause severe

S     (SMPS) and white goods are
      increasingly being designed with
an active power factor correction (PFC)
                                              interference with other equipment and
                                              reduce the maximum power that can be
                                              drawn. Distorted line voltage causes
                                                                                                       Sampat S. Shekhawat,
                                                                                                      Praveen M. Shenoy, and
                                                                                                            Ronald H. Randall
input stage to meet international             overheating of capacitors dielectric
regulatory standards for harmonic             stress and over voltage in insulation.                 Fairchild Semiconductor
content. Historically, power factor           Distorted line current increases
correction circuits have used a boost         distribution losses and reduces available    process by which the input is made to
converter topology that combined a            power. Utilizing power factor correction     look more like a resistor. This is
power switch and boost diode. Due to          not only ensures compliance with             desirable since the resistor has a unity
the recovery losses of the boost diode,       regulatory specifications and eliminates      power factor, compared to the typical
snubber circuitry was necessary to            the above failures, but also improves        SMPS power factor values of 0.6 to 0.7,
reduce voltage ringing and hence              device efficiency by increasing the          and allows the power distribution
electro-magnetic interference (EMI).          maximum power that can be drawn              system to operate at its maximum
                                              from the source.                             efficiency. Power factor is defined as the
However, with the introduction of the                                                      ratio of real power to apparent power.
soft recovery hyperfast “StealthTM”           Instituting laws and restrictions that       Real power is the power measured by a
diode, snubber circuitry can be               require active power factor control has      wattmeter
eliminated or reduced, and the boost          been an on-going effort since 1992, with
converter can be implemented in the           the most recent approved standard being
hard-switched mode. When the new              EN 61000-3-2 with Amendment A14.
diode is combined with new SMPS               The objective of this standard is to limit   and apparent power is the product of
MOSFET technology, designers can              the magnitude of the individual              rms current times rms voltage. The rms
obtain lower conduction losses,               harmonic currents up to the 39th             current is
simplified gate drive, lower switching         harmonic. While Amendment A14 is
losses and reduced EMI. In addition,          targeted at relaxing the requirements for
previous limitations to PFC design may        some equipment currently in the “Class       where n is the order of the respective
be overcome and optimal PFC                   D” grouping, equipment such as               harmonic currents (i.e., 2nd, 3rd, etc.).
performance achieved, making it easier        personal computers, monitors and             Making the power supply input current
to meet harmonic standards such as EN         television sets are still maintained under   approach a sinusoid removes the
61000-3-2.                                    the original EN 61000-3-2 “Class D”          harmonic currents, decreasing the rms
                                              requirements. IEC 61000-3-4 EMC-             input current, and increasing the circuit
Traditionally, the AC source is rectified      Part3: Limits, Section 4: Limits for         power factor. The result is maximum
into a large capacitor filter and current is   harmonic current emissions (equipment        utilization of the source power and
drawn from the source in narrow high          with input current >16A per phase).          reduction of harmonic content.
amplitude pulses. This rectifier filter
stage forms the front end of the SMPS.        Power Factor Corrections Basics              Most active PFC designs incorporate the
The high amplitude current pulses             Power factor correction refers to the        continuous current mode (CCM) boost


12   CONFORMITY ®: SEPTEMBER 2002
                   FEATURE                                                                                       www.conformity.com




converter topology because of its             figure 2, 92 µJ or 15% of the total            the turn-on loss during ta is 297 µJ.
simplicity and broad AC input voltage         switch turn-on loss occurs during tr.
range. Another PFC operating mode,            This time period and the losses               During the third period, tb, the diode
known as discontinuous current mode           associated with it are a function of the      recovery period, the voltage across the
(DCM), is used at very low power              switch turn-on di/dt rate. Unfortunately,     switch falls and reduces to its saturated
levels. The increased input pulse current     slowing down the switch turn-on di/dt to      value and the diode blocks the output
associated with DCM increases EMI,            minimize EMI and reduce the diode             voltage. In Figure 2 tb is short (35ns)
limiting this operating mode to low           reverse voltage spike increases the tr        with a loss of 230µJ in the boost switch.
power applications. The CCM boost             time and its associated losses.               The total turn-on loss in this case was
converter (as shown in Figure 1) places                                                     629µJ. Typical designs contain
the boost diode D5 and switching device       The second period, ta (32nsec), when the      MOSFET as the switching device (S1 in
S1 in the hard-switched mode.                 diode current reverses to its IRRM value is   Figure 1) and the MOSFET input
                                              both diode and power circuit dependent.       capacitance, CISS, and reverse transfer
The drawback to hard switching is that        The diode IRRM is a function of the           capacitance, CRSS, increase the switch
the diode reverse recovery                    forward current, recovery current di/dt       voltage fall time resulting in additional
characteristics increase the switching        and the diode junction temperature.           switch losses. The total circuit loss
device’s turn-on losses and the               Increasing any of these will increase the     during tb is a function of the diode
generated EMI. The diode’s reverse            IRRM. The major portion of the switch         current since this current exists at the
recovery characteristics describe how it      turn-on loss occurs during ta. In Figure 2    full 400V output potential. The rate of
transitions from the forward conducting
state to the reverse voltage blocking
state. If the return of the reverse
recovery current from IRRM to zero is too
abrupt or “snappy,” voltage spikes and
severe EMI are generated.

To soften this response, designers have
either slowed down the switch turn on
di/dt and/or added snubber circuits.
With prior diode technology, the
designer was limited to having either a
soft or fast diode. The large IRRM value
of previous soft diode technology
generates large switching device turn-on
loss during the diode trr recovery period.   Figure 1: Boost PFC Circuit

However, slowing down the switch turn-
on rate increases the switch turn-on loss.
And, adding snubber circuitry adds to
circuit cost and complexity and reduces
circuit reliability. The snubber circuits
often involve complex energy recovery
schemes since the basic RC approach
results in high power dissipation in the
snubber resistor.

Hard-switched turn-on tradeoffs are
better understood by breaking the
switch turn-on period into discrete time
segments. Figure 2 illustrates the
recovery of a snappy diode operating in
the Figure 1 circuit. The diode is
conducting 15A and commutated by a
switch turning on with a di/dt of 500
A/µs. During the first period, tr (48ns),
the switching device current increases to
the 15A diode forward-conduction
current value and the diode current
reduces to zero. The potential across the
switch is nearly 400Vdc during tr. In the    Figure 2: Hard Switched Turn-On Recovery of a “Snappy” Diode DSEP1506.


14   CONFORMITY ®: SEPTEMBER 2002
                                                                                                    FEATURE

switch voltage fall during tb determines     Figure 3 are exactly identical to those of     SMPS MOSFET
the distribution of this loss between the    Figure 2, except that the snappy diode is      At the same time, recent developments
diode and the switch.                        replaced with a soft-recovery diode.           in SMPS MOSFET and diode
                                             Stealth technology provides a diode that       technology provide a greatly improved
New Components Improve                       recovers in a soft monotonic mode              component pair for the hard-switched
PFC Design                                   resulting in lower EMI. In addition, the       boost PFC topology. Advances in SMPS
So-called “Stealth” diodes combine fast      ratio of IRRM/Ifwd is decreased resulting in   MOSFET switching performance
recovery characteristics of hyperfast        lower ta losses. The lower IRRM allows         provide a device capable of hard-
technology with soft recovery                the designer to turn on the switch at a        switched operation in excess of 100
characteristics to achieve lower             higher di/dt rate (in some cases over 500      kHz. These new MOSFETs have a
switching losses and low EMI. Such           A/µs) reducing the tr losses in the switch     much lower Rdson when compared to
diodes are designed for use in high          by as much as 30%. The Figure 3                earlier MOSFETs.
power, high frequency circuits and are       switch tr, ta and tb turn-on losses are 86,
ideally suited for PFC circuits as a boost   185 and 219µJ, respectively. The total         Key features of the latest generation of
diode. SMPS, uninterruptible power           turn-on loss in this case was 492µJ. It        MOSFETs include:
supplies (UPS), inverters and motor          may be noted that the losses during ta
drives are typical applications. A diode’s   phase is much lower with the Stealth           ● Lower specific on resistance in regular
softness rating, S is defined as tb/ta.       diode (297µJ vs 185µJ) due to lower              DMOS technology
                                             IRRM (12A vs 17.6A). Also, the recovery
Typical values for commonly available                                                       ● Reduced total input gate charge (Qg)
diodes range from 0.3 to 0.6. Stealth        waveform of Stealth diodes is
                                             exceptionally smooth with no ringing           ● Lower Miller capacitance (Cgd)
diodes provide softness values greater
than 1.2 and have a trr reverse recovery     and hence produces much lower EMI.             ● Lower Cgd/Cgs, and Qgd/Qgs ratio
time of less than 25ns.                      The soft recovery diode not only results       ● Input gate capacitance is less and its
                                             in a reduction in the switch losses but it       variation is much less with the
Figure 3 illustrates the advantages of a     also permits the removal of the parallel         variation in Vds
soft recovery diode in the PFC circuit of    RC snubber across diode.                       ● Lower turn-off energy
Figure 1 circuit. The conditions of                                                         ● Higher avalanche energy rating




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● Lower thermal resistance due to
  reduced die thickness

Power quality requirements and
improved efficiency of SMPS are
driving the demand of improved power
devices. Front end PFCs for laptop
battery chargers, PC and server power
supplies, telecom power, medical
equipment and household appliances
have become a reality. The requirement
of improved reliability and efficiency
have forced device designers to improve
the device performance at increased
voltage and temperature.

Generally, high speed switches, such as
conventional DMOS, are the natural
choice for these applications. The turn-
off switching loss of the MOSFET is
very low because this switch is a
majority carrier device with negligible
current tail. At the same time the turn-
off switching loss does not very widely    Figure 3: Hard-Switched Turn-On Recovery of A Soft Stealth Diode
with the variation in temperature. In
comparison, high speed IGBT turn-off
loss almost doubles from Tj=25°C to
125°C. The parasitic capacitances for
the SMPS MOSFET have been
optimized and the Miller capacitance
(Cgd) has been reduced in order to
improve the switching performance. The
output capacitance has also been
reduced, which in turn has reduced the
turn-off energy loss. At the same time,
the ringing in turn-off current has been
reduced compared to other MOSFET
technologies in order to reduce EMI.

Figure 4 compares the turn-off of an
SMPS MOSFET (FDH44N50) and a               Figure 4: Turn-off waveforms for a standard MOSFET (STW45NM50) and an
conventional MOSFET (STW45NM50)            SMPS MOSFET (FDH44N50) measured under identical conditions clearly shows
under identical conditions (Id=20A,        that FDH44N50 has much lower ringing and EMI.
Tj=125C, Vdd=400V, Rg=5.6Ohms). It

     Low line voltage=90Vac                                     Low line voltage=180Vac
     OUTPUT POWER         Boost Diode       Boost Switch        OUTPUT POWER        Boost Diode     Boost Switch
     <100W                ISL9R460P2        FDP15N50            <150W               ISL9R460P2      FDP15N50
     <250W                ISL9R460P2        FDH27N50            <400W               ISL9R460P2      FDH27N50
     <400W                ISL9R460P2        FDH44N50            <600W               ISL9R860P2      FDH27N50
                                                                                    ISL9R860P2
     <600W                ISL9R860P2        FDH27N50 * 2-3      <1000W              ISL9R1560P2     FDH45N50
     <900W                ISL9R1560P2       FDH44N50 * 2        <1500W              ISL9R1560P2     FDH44N50 * 2
     <1100W               ISL9R1560P2       FDH44N50 * 3-4      <2200W              ISL9R3060G2     FDH44N50 * 3-4

 Table 1: Boost switch and diode for various PFC power levels and input voltage ranges



16   CONFORMITY ®: SEPTEMBER 2002
                                                                                                 FEATURE

is quite clear that the SMPS MOSFIT         Table 1 gives the suggested Stealth          MOSFET output and Miller
has lower ringing and hence lower EMI       diode & SMPS MOSFET combinations             capacitance, turn-off loss and ringing at
than the standard MOSFET. The turn-         (Fairchild components are identified) as      turn-off have been reduced, reducing
off energy loss (Eoff) is also lower for    a function of PFC circuit power level        both losses and EMI. The new SMPS
SMPS MOSFIT (142 vs 173 µJ).                and AC input voltage. These are only         MOSFET turn-off switching loss is
                                            recommendations since the MOSFET             reduced to less than 50% of that of the
The combination of an SMPS MOSFET           and Stealth diode selection will also        fastest IGBTs without compromising
with Stealth diodes is a practical          depend on heat sink size, its thermal        EMI. The conduction loss is also
approach to low cost, reliable PFC          performance and other variables.             drastically reduced compared to that of
circuits. Designers are constantly                                                       the former benchmark MOSFETs.
looking for the following key features to   Conclusion                                   Through parametric improvements, a
reduce cost, improve reliability and        The electrical and thermal performances      500V SMPS MOSFET can compete
increase power density.                     of hard-switched topologies such as the      with some of the super-junction
                                            CCM boost PCF topology are heavily           MOSFETs.
1. Reduce the size of passive               dependent on the characteristics of the
   components such as inductors,            power switching devices. New                 About The Authors
   transformers, capacitors and current     generation SMPS MOSFETs and                  Sampat S. Shekhawat and Praveen M.
   sense components etc.                    Stealth diodes provide performance that      Shenoy are staff engineers at Fairchild
2. Reduce the size of active components     affords decreased component count,           Semiconductor. They can be
   such as power semiconductor              increased power density, and reduced         reached respectively at
   switches                                                                              sampat.shekhawat@fairchildsemi.com
                                            EMI.                                         and
3. Better thermal performance of power                                                   Praveen.shenoy@fairchildsemi.com.
   switches to reduce heat sink size        Because of their superior switching          Ronald H. Randall is a consultant to
4. Reduced input capacitance and gate       characteristics, power MOSFETs               Fairchild Semiconductor, and can be
   charge so that power switches can be     dominate the power supply market. The        reached at r.h.randall@ieee.org.
   driven directly from controller.         MOSFET is a majority carrier device so
                                            its turn-off loss is very low. By reducing
5. Reduce EMI




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