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Switched-Mode Power Supplies Report
D. N. Borysiuk, student at Heritage College studying Electronics Engineering Technology, Computers
and Satellites/Telecommunications.
B. Temperature-Compensated Current-Limiting Circuit
I. INTRODUCTION
The current limiting circuitry sets the oscillation on-time by
A power supply is a device that transfers and converts
energy from the source to the output. Switchers are the
most common PSUs. They can be found almost anywhere, in
sensing the transistor switching current across the external
resistor RSC. Therefore the peak current is limited and in
result the power diode and transistor are protected.
computers, cars and portable electronics. They are cheaper to
build, lighter, smaller, produce less heat, more efficient and
more powerful, but are more complex than linear power C. Temperature-Compensated Voltage Reference
supplies. The drawback of SMPS is that they create EMI A 1.3V voltage reference is used to compare to the sampled
which has unfavorable effects on the nearby electronics. Buck, fraction of the output. It is capable of providing a maximum of
Boost and Inverter SMPS use a switching transistor, a diode 10mA current without implementation of external pass
clamp, control logic and at least one storage element (a transistor.
capacitor and/or an inductor). This report will analyze the
functions and applications of these DC/DC regulators based D. High Gain Differential Comparator
on the LM78S40 IC. When the output voltage is too high, the comparator inhibits
the gating signal produced by the oscillator which turns on the
transistor switch.
E. Power Switching Circuit
The power switching transistors in a Darlington
configuration can provide up to 1.5A peak current and 40V
collector-emitter voltage. The collector and emitter pins of the
Darlington pair are externally accessible for design flexibility.
Switching times are between 300-500ns.
III. INTRO TO TOPOLOGY
A voltage regulator provides a constant voltage output
regardless of the line voltage, load, or ambient temperature.
Fig. 1 PC Power Supply
II. LM78S40
T HE LM78S40 was designed to maximize flexibility,
reduce the amount of parts but still include all of the
fundamental components while keeping simplicity and cost
effective design. The fundamental blocks of this IC are:
A. Current Controlled Oscillator
The current controlled oscillator sets the on/off transistor
switching frequency (20-30 kHz) and is controlled by a single
external capacitor. Even though the oscillation duty cycle is
fixed at 6:1, it can be adjusted by the current-limiting circuit to
frequency range of 100 Hz to 100 kHz.
Fig.2 LM78S40 IC Pin Layout
The LM78S40 IC offers an optional OP amp to increase
flexibility. It can be configured to many purposes such as to
satisfy certain functions for complex applications, provide
www.morpheustechnology.com series pass regulation or a second output voltage.
www.wikipedia.com
www.national.com
www.smps.us
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IV. BUCK TOPOLOGY
A. Description
The Buck SMPS or Step Down PSU is used when the The current during the ON period is:
required DC output must be lower that the source DC voltage.
The average DC inductor current of the buck is equal to the
output current. It is commonly used in places where the source
voltage is low power but the output is required to have high
current. The output voltage is determined by the switching The maximum current to be reached is:
frequency. The following formula is used to determine the
output voltage:
3. Step2 (opened switch)
A buck SMPS can be found powering a mobile processor.
B. Circuit Analysis
In this section you will be guided step by step through the
simplified buck regulator operation.
1) Initial State
Fig. 5 Buck (switch set to OFF)
As the switch opens, the diode starts to conduct, the voltage
across the inductor reverses polarity, and the stored energy
from the inductor releases into the capacitor and the load. The
capacitor will discharge into the load.
Fig. 3 Simplified Buck Circuit The current during the OFF period is:
Starting off with the assumption that the inductor and the
capacitor are not yet charged, no current is flowing due to the
open switch.
The falling slope of the inductor current in the OFF period is:
2) Step 1 (closed switch)
4) Step3 (closed switch)
Fig. 4 Buck (Switch set to ON)
Fig. 6 Buck (switch set to ON)
As the switch flips to the ON position, the inductor starts
energy storing. The diode does not conduct. The current flows As the switch closes, the voltage polarity of the inductor
in the load and the capacitor. reverses and the process repeats.
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C. Layout E. Lab comparison
The figure below demonstrates how to build a buck circuit During lab experiments, the results (shown in Fig. 9) were
using LM78S40. as shown in the ideal waveforms (Fig. 8) except for the
voltage output. The output looked nothing like a sine wave.
The expected sketch was very bad. The ideal waveforms were
also drawn for a continuous mode circuit while the actual
circuit built was in discontinuous mode.
Fig. 7 Buck Configuration for the LM78S40 (Vin=30V, Vout=5V)
D. Ideal Waveforms
Fig. 9 Oscilloscope screen prints
Unlike the capacitor, the inductor and load currents are DC.
Discharge to the load happens during the positive side of the
waveform while the negative represents it charging by the
inductor (Fig. 9). Typically the general losses happen across
the switch and a small loss happens across the inductor.
The minimum inductor size can be obtained by:
Fig. 8 Ideal Waveforms of a Buck Regulator (Va=Collector Node of TIP32)
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V. BOOST TOPOLOGY
A. Description
The maximum current to be reached is:
Boost or step-up regulator is used when the voltage source
is lower than the required DC output. The resulting output
current is smaller than the source. The average DC inductor
current of the boost is equal to the input current. The
following formula is used to determine the output voltage: 3) Step2 (opened switch)
In order for batteries to achieve high voltage, they must
have a very large amount of cells. Boost SMPS is used in
hybrid electric vehicles. Toyota Prius engine requires 500
volts. In order for the battery to actually achieve this voltage,
it must consist of 417 cells. The actual battery only contains
168 cells. The Boost SMPS takes the source voltage of 202
volts and brings it to the required 500 V. Other application of
boost is in some flashlights that for example require 3.3 volts
to power an LED with only one 1.5 V battery. Boost SMPS
are also used in cathode fluorescent lights such as in LCD
Fig. 13 Boost (Switch set to OFF)
backlights.
B. Circuit Analysis As the switch opens, the diode starts to conduct, the voltage
In this section you will be guided step by step through the across the inductor reverses polarity, and the stored energy
simplified boost regulator operation. from the inductor releases into the capacitor and the load. The
capacitor will discharge into the load.
1) Initial State (simplified circuit)
The inductor current during the OFF period is:
The falling slope of the inductor current in the OFF period is:
Fig. 11 Simplified Boost Circuit
Starting off with the assumption that the inductor and the The voltage output is:
capacitor are not yet charged, no current is flowing due to the
open switch.
2) Step1 (closed switch) 4) Step3 (open the switch)
Fig. 12 Boost (Switch set to ON)
As the switch flips to the ON position, the inductor starts
energy storing. The diode does not conduct. The current flows
Fig. 14 Boost (Switch set to ON)
in the load and the capacitor.
As the switch closes, the voltage polarity of the inductor
The current in the inductor during the ON period is:
reverses and it begins to charge and then discharge into the
load. The process repeats.
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C. Layout E. Lab Comparison
The figure below demonstrates how to build a boost circuit During the lab experiments (shown in Fig. 17), the
using LM78S40. oscilloscope results looked just like expected (shown in Fig.
16).
Fig. 11 Boost Configuration for the LM78S40 (Vin=12V, Vout=15V)
D. Ideal Waveforms
Fig. 17 Oscilloscope screens prints of the IL, Va, Vout and Id (boost lab)
An output several times the input voltage can be obtained
by increasing the ON time.
When the switch is off, the energy stored in the inductor is
transferred to the load. The result is a high surge current
through the diode and very high voltages across the switch.
The minimum inductor size can be obtained by:
Fig. 8 Ideal Waveforms of a Boost Regulator
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VI. INVERTER TOPOLOGY
A. Description
The maximum current the inductor reaches is calculated
The inverter, also known as the buck-boost SMPS, inverts using the rule:
the input polarity. It can provide negative voltage. The
average DC inductor current of the inverter is equal to the sum
of the input and output currents. The following formula is
used to determine the output voltage:
3) Step2 (open the switch)
Negative voltage is useful in OP Amp rails.
B. The Inverter
In this section you will be guided step by step through the
simplified inverter regulator operation.
1) Initial State (simplified circuit)
Fig. 19 Inverter (open the switch)
As the switch opens, the diode starts to conduct, the voltage
across the inductor reverses polarity, and the stored energy
from the inductor releases into the capacitor and the load. The
inductor current during the OFF period is:
Fig. 11 Simplified Boost Circuit
Starting off with the assumption that the inductor and the
capacitor are not yet charged, no current is flowing due to the
open switch. The slope of the inductor current in the OFF period is:
2) Step1 (close the switch)
4) Step3 (close the switch)
Fig. 18 Inverter (close the switch)
As the switch flips to the ON position, the inductor starts
energy storing. The diode does not conduct. Fig. 20 Inverter (close the switch)
The current in the inductor during the ON time is: As the switch closes, the voltage polarity of the inductor
reverses and it begins to charge while the capacitor discharges
into the load. The process repeats.
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C. Layout E. Lab Comparison
During the lab experiments shown in Fig. 23, it is clear
The figure below demonstrates how to build an inverter circuit
that the results were very similar to the expected except for the
using LM78S40.
current in the switch Fig. 22. The current in the switch should
have been sketched inverted. The circuit built in the lab had a
transistor with a different gain value. A few modifications had
to be done from the Fig. 21. The switching frequency of the
transistor was too low and the output value was slowly
climbing to the expected. Troubleshooting revealed two
problems. The transistor gain being too low and the timing
capacitor value being too high. A 200 ohm decrease of the R4
resistor drastically increased the transistor gain. The timing
capacitor being 0.005µF was decreased to 0.003 µF. Making
these changes, halved the inductor current and instantaneously
clipped the output voltage to the expected. The results were
text book waveforms.
Fig 21Inverter (Vin=12V, Vout= -15V)
D. Expected waveforms
Fig. 23 Oscilloscope screen prints of the Vsw and Id (inverter lab)
The minimum inductor size can be obtained by:
Fig. 22 Ideal waveforms of an inverter
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VII. PCB LAYOUT THEORY
A. Introduction
Noise reduction, minimization of voltage error and close to each other and make sure that they are going to the
stabilization rules are very important to keep in mind during a same ground location. A PCB board can act as a heat sink by
good PSU layout design. Problems usually occur in high placing extra copper which will then serve as a heat
current and large input to output ratio power supplies. The conductor.
high power components should be laid out first. The high
frequency nodes should be kept as small as possible in order G. Vias
to reduce the EMI generation area. The low impedance nodes Vias connect layers together and also parallel layers
such as the ground should be kept as big as possible. together to allow more current to be flowing between
components. Microvias are typically designed to carry 1A of
B. Guideline current. Vias of 14mils should pass no more than 2A of
Make most ground connections through vias. For each current while large vias of 40 mills or larger should see no
power stage, keep power ground and control ground more than 5A of current.
separately. Within multilayered boards, control ground plane
on an inner layer can act as a shield between power and
control circuits. The areas and lengths of the loops carrying
high frequency switching currents must be minimized. You
have to make sure that PCB traces won’t fuse at any abnormal
current (such as short circuit) that could develop in the circuit.
C. Inductor
The closed core EMI inductors such as toroid or encased E
core are the best because they produce less noise as of posed
to open and stick code inductors.
E. Feedback
To maximize efficiency feedback trace should be placed far
from the inductor. The inductor creates too much noise
therefore the traces are usually located on the opposite side of
the board
C. Filter Capacitor
If a small value ceramic capacitor is used, it should be
placed as close as possible to the VIN pin of the IC in order to
reduce trace inductance which causes the IC to pickup noise.
Surface mount capacitors are better to use in some cases Fig. 24. A Well-Designed Four Layer Board for the Buck
because they pick up less noise than the through hole.
The example layout incorporates the layout practices
D. Transistor recommended. The thick copper pours and all the power parts
When choosing the transistor, things to keep in mind are: its for high currents are contained within layer 1. The second
maximum voltage and current rating, and the frequency cutoff. layer is the ground plane. It connects to the rest of the circuit
The transistor must drop the entire input voltage plus flyback near the input at only one point. The result is no current being
effect. It must be able to handle at least twice the load current. passed. The bottom and third layer are dedicated to the power
The frequency cutoff must be a lot higher than the operating and signal traces. The bottom layer contains small
frequency. Bipolar switching transistors with 4MHz gain
components. Copper floods all the unused board area.
bandwidth are typically used.
F. Traces and Ground Planes
Denys N. Borysiuk was born in
Traces must be short and thick for high current. The Dnepropetrovsk, Ukraine, on May 16,
minimum thickness is 15 mills per amp. If space permits, 30 1989. He lived in Moscow until he was
mills per amp for 1 oz copper should be striven for, especially 11 and then moved to Ottawa. He
studied at Heritage College.
where high frequency switching currents flow. To decrease
He was a Corporal in the infantry
the EMI emitted from the inductor, several ground planes are with experience in Ceremonial Guards.
used and the components are arranged so that the switching His special fields of interest included
current loops curl together in the same direction, which also musical beat production and weight
lifting.
reduces magnetic field reversal. Another way to reduce the
EMI is to place the inductor, diode and the output capacitor