This technical report describes the design, simulation, construction and testing of a positive DC switch mode power supply with short circuit current limiting. The design was based on the SG3524 regulating pulse width modulator integrated circuit. Key measurements showed the circuit maintained a constant 8.5V output voltage and was able to supply 250mA of load current or go into current limiting mode during a short circuit. The efficiency was 74.5% and line and load regulation were both under 1%. While the design met its goals, the output ripple voltage was greater than intended due to a larger than expected filter capacitor being required.

1.
Submitted By: Alex Kremnitzer
Date: 05-04-2011
Date Performed: 04-26-2011
Lab Partners: None
Switch Mode Power Supply
Technical Report 4
Milwaukee School of Engineering
ET-3100 Electronic Circuit Design

2. 1
Abstract
In this design, a positive DC Switch Mode Power Supply with short
circuit hard current limiting was designed, evaluated and tested. The design was
based on the SG3524 Regulating Pulse Width Modulator integrated circuit.
The tested circuit verified the validity of the design. The load voltage
remained constant at 8.5VDC and was able to supply the required load current
of 250mA. The circuit was able to supply the load current until the short circuit
current was reached upon which it then went into hard current limiting mode.
Efficiency was an improvement based on prior linear power supply
circuits, being 74.5%. Both line and load regulation was less than 1%. The
design does have the limitation of being noisy and the ripple voltage on the load
was greater than with previous linear designs.
Introduction
The design constraints of the power supply were to provide 8.5VDC for
load current of 250mA. The output current is to be hard limited at 125% of the
normal load current and the load ripple voltage is to be less than 50mVpp. The
circuit was designed, simulated and constructed.
The circuit shown in the Appendix was constructed. Component
measurements were taken using a Meterman Model LCR55 LCR Meter. In the
lab, key measurements were taken using a Fluke Model 87III Multimeter and a
Hewlett Packard 54602A Oscilloscope. The circuit was supplied from a Tenma
Model UTC 72-5085 Power Supply. The following formulas were used in the
design of the power supply circuit:

3. 2

4. 3

5. 4
Calculations for Line Regulation:

6. 5
Calculations for Load Regulation:
Calculations for Power Supply Impedance:
Calculations for Power Supply Efficiency:
PIN includes the power dissipated by all the components of the circuit while POUT is the power
dissipated just in the load.

7. 6
Calculations for Ripple Rejection Ratio:
Error Analysis Calculation Formulas:
Simulation Validation
Table 1: Calculated versus Simulated Results
Table 2: Simulated Line Regulation (mV/V)

8. 7
Table 3: Simulated Load Regulation (mV/A)
Table 4: Simulated Power Supply Impedance (Ω)
Table 5: Simulated Ripple Rejection
Table 6: Simulated Power Supply Efficiency (%)

9. 8
Analysis of Simulation Results
Overall the simulation verified the calculated values as shown in Table 1.
The Load Voltage and Current were close to the calculated values. However the
oscillator frequency was much lower than calculated. The values for the timing
resistor (RT) and capacitor (CT) were verified to be correct. The duty cycle was
also lower than calculated. Since the frequency timing component values and
calculations were verified to be correct, I assumed the variances were from the
circuit model of the SG3524 Regulating Pulse Width Modulator integrated
circuit.
With the exclusion of the ripple rejection, the simulated circuit’s
regulation and efficiency was good. See Tables 2 through 6. The simulated
values for the Line Regulation were 4mV/V. Load Regulation was good at
0.16mV/V. Power Supply impedance simulations yielded a value of 0.16Ω. The
simulated efficiency for rated load current of 250mA was 84% and at 50% of
rated current was 78%. Ripple Rejection was simulated with an input ripple of
50mVpp and was poor yielding -32dB for a filter capacitor of 6.8uF and -12dB
for 68uF.

10.
Design Validation and Tes
Figur
Figu
Figur
sting
Figure 1: V
e 2: Voltage
ure 3: ΔVo w
re 3: ΔVo wi
Voltage befo
e at Base of
with 6.8uF F
ith 68uF Fil
re Inductor
f Pass Trans
Filter Capac
lter Capacit
r
sistor
citor
tor
9

11. 10
RL=2Ω
Table 7: Component Value Error Analysis
Table 8: Calculated versus Measured Values

12. 11
Table 9: Line Regulation (mV/V)
Table 10: Load Regulation (mV/A)
Table 11: Power Supply Impedance (Ω)
Table 12: Ripple Rejection
Table 13: Power Supply Efficiency (%)

13. 12
Table 14: Individual Component Power Dissipation

14. 13
Analysis of Testing Results
Component Values
The values of all components measured were within their tolerance
ratings as shown in Table 7. The Current Sense Resistor, RCS was 0.64Ω to give
a short circuit current limit of 125% of rated current (250mA). However due to
component availability, a lower value was used. Resistors were placed in
parallel to obtain a total resistance of 0.56Ω. The Fluke 87III multimeter
measured 0.3Ω with the probe tips shorted, so the displayed measured value of
RCS was reduced by this amount.
Circuit Analysis
See Table 8, and Figures 1 thru 4. The measured Load voltage and
current were as expected from the circuit calculations. The measured reference
voltage was lower (4.93) than nominal (5.0VDC) and I was able to adjust for
this variance with the trimpot between R’1 and R’2.
The oscillator frequency was calculated to be 43.7kHz, but measured to
be 40.9kHz. This is a deviation of -6.61%.
The duty cycle was calculated to be slightly smaller than what was
measured. The calculated value was 48.6% and the measured value was 54.8%.
This is an increase of 12.76%. Even with the variance, the circuit was able to
maintain the load voltage at the set voltage (8.5VDC).
The short circuit current ISC was slightly lower (-6.7%) than calculated.
However as there was a 12.5% variation between the calculated value of RCS
and the available value. The deviation is to be expected.
Due to the reduced on time of the pass Transistor QP, a heat sink was not
required. The collector tab remained cool to the touch under all load conditions,
including short circuit.
The ripple on the load was much worse than designed for. The calculated
value for C was to obtain the design criteria of 50mVpp. However, with the
calculated capacitance of 6.8uF, the ripple voltage was 912.5mVpp as shown in
Figure 3. The value for C was increased to 68uF as shown in Figure 4 and there
was a dramatic improvement, with the ripple reduced to 71mVpp. After
reviewing the component values and design calculations, it was determined they
were all be correct and a cause for the variation was not determined.

15. 14
Line Regulation
Line regulation is the power supplies measure of its ability to maintain a
constant load voltage with a change in the supply voltage while at a constant
load resistance. The measured power supply’s line regulation was good. ∆Vout
/ ∆Vin was 5.4*10-2
mV/V of the output voltage as shown in Table 9. This value
is good, since the ideal percentage of line regulation is less than 1%.
Load Regulation
Load regulation is the power supplies measure of its ability to maintain a
constant load current over changes in the load resistance with a constant supply
voltage. In this design, the measured load regulation was good. ∆Vout / ∆Iout
was 0.01mV/A of the output current as shown in Table 10. This design is an
improvement over previous class designs. An ideal percentage of load
regulation should be less than 1% of the output current.
Power Supply Impedance
The power supply impedance calculated from the measured values was
0.10Ω as shown in Table 11. This value is good as the load impedance is much
greater than this value. An ideal power supply impedance is 0.0Ω where all
power would be transferred to the load.
Ripple Rejection
The Ripple Rejection of the circuit is the degree to which the circuit
reduces the output ripple in comparison to the input ripple. In this circuit, the
Ripple Rejection Ratio was not good in the circuit. Table 12 shows that for a
filter capacitor value of 6.8uF it was -7.2dB and for a value of 68uF was 15dB.
The calculated value of C was 6.8uF, but when measured, the output ripple did
not meet the design criteria of 50mVpp max. As the capacitance was increased
there was dramatic improvement. The design calculations and component
values were evaluated and found to be correct and a reason for the error could
not be found.

16. 15
Efficiency
Efficiency is the ratio of Power Out to Power In of a circuit. A higher
value means less power is lost due to heat and the circuit is more efficient. This
design has a more efficient power transfer from input to output than previous
linear power supply class designs. Table 13 shows that the measured power
efficiency for 100% rated current (250mA) was 75.4% and for 50% (125mA)
was 69.0%. Due to the design being more efficient, heat sinking of the pass
transistor was not required.
Power Out is referring to the power dissipated by the load. The Power In
is the total power consumed by the complete circuit. The individual component
power dissipations are shown in Table 14. Based on the individual calculation,
the total power dissipated in the circuit is 3.72 Watts. Using 3.72 Watts as the
Power In, the efficiency becomes only 45.7% which does not appear to be a
realistic number for a Switched Mode Power Supply.
Conclusion
This design met the design constraints of supplying 250mA to an 8.5VDC
load. The load voltage remained constant at 8.5VDC until the short circuit
current (ISC) was reached upon which it then went into hard current limiting
mode. The output ripple was greater than previous class designs, but the trade
off is improved circuit efficiency (74.5%). The increased efficiency also means
a cooler running circuit and reduced component power ratings. Provided that
the additional noise a Switch Mode Power Supply introduces to the load, it will
produce less heat than a linear power supply.

17. 0
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A A
B B
C C
D D
E E
F F
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ET-3100 Electronic Circuit Design
Date: Sheet of
Rev:Size:
Design 4 Switch Mode Power Supply Schm
2011-05-01 1 1
1.0A
Milwaukee School of Engineering
Alex Kremnitzer
Notes:
Unless specified:
1. All resistors are carbon composition, 10%, 1/4W. .
2. Capacitor C is electrolytic, 16WVDC.
3. Capacitor C2 is ceramic, 20WVDC>
R5
620Ω
1/2W
R_1
7.5kΩ
R_2
2.7kΩ
Rpot
2kΩ
Key=A
50%
Rcs
0.64Ω
C
6.8µF
Cc
1nF
(TIP32 SUBSTITUTE)
Rload
100Ω
Key=B
10W
34%
Vunreg
17.5 V
Hewlett
Packard
E3631A
Qp
MJE15033
D1
1N5818
L
.001µH
R6
420Ω
Rc
51kΩ
Ct
2.2nF
Rt
13kΩR4
5.1kΩ
R3
5.1kΩ
SG3524
C2C1VCCVREF
IN+
IN-
E1
E2
CL-
CL+
RT CT GND COMP
12 13
11
14
5
4
9876
2
1
16 15