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Table of Contents
4.0 SMPS Design Examples
4.1 Generic SMPS ICs
4.2 LM78S40 Universal Switching Regulator
4.3 MAX641 Step-Up Switching Regulator
Assignment Questions
For Further Research
An Introduction to Power Supplies
i
_____ Notes _____
4.0 SMPS Design Examples
Objectives
• Identify various types of switch mode converters.
• Determine the approximate current waveforms in SMPS circuits.
•
4.1
Generic SMPS ICs
Of these components, only the switch and diode can be integrated; the inductor
and capacitor are external components. In high power applications, even the
switch and diode are discrete. It may therefore seem pointless to integrate a
switching regulator. However, this is not the case. The real trick is to control the
switch.
Switching converters can achieve a power conversion efficiency of 70 – 90%.
However, they also create electro-magnetic interference (EMI), which can have
adverse effects on other nearby electronics. Low-loss ferrite materials, high
permeability magnetic shielding, and smaller semi-conductors can reduce EMI.
Bipolar switching transistors, with a gain-bandwidth product in excess of 4 MHz
are often used as the principle switching element. To minimize adverse effects
from inductive kickback, Schottky or fast-recovery diodes are also used.
Buck Switching Regulator
An Introduction to Power Supplies
4-1
Switch Mode Power Supplies
_____ Notes _____
Boost Switching Regulator
Most switching regulators consist of:
Switching transistor
Diode clamp
LC filter
Control logic
It is possible to design switching power supplies to operate directly from the
hydro input. This eliminates the need for a transformer, rectification, and prefiltering.
A switching regulator IC contains the four basic components found in a linear
regulator, but adds an oscillator and some control logic in order to control the
transistor switch (control element).
Generic Switching Regulator IC
There are several different control methods that can be used to control the
conduction in the series control element:
PWM – pulse width modulation: the frequency is held constant and the ON
time is varied. This is the most common technique used.
PFM – pulse frequency modulation: the on or the off time is held constant
and the frequency is varied.
4-2
An Introduction to Power Supplies
Switch Mode Power Supplies
_____ Notes _____
PBM – pulse burst modulation: the oscillator frequency and duty cycle is
held constant and oscillator cycles are gated on or off. This technique is
used in simple converters below 10 watts.
As the switching frequency increases, the size of the magnetic components
decreases and the switching losses increase. If the switching frequency is in the
audio range, it is possible for the coil windings to vibrate, thus creating an
annoying singing tone. This is a common phenomenon in TV sets, where the
flyback transformer operates at 15.75 KHz.
The sampling circuit generally consists of a simple voltage divider. Under
normal operating conditions, it produces an output equal to the built-in reference
voltage. An op amp is used to compare the sampled and reference voltages. This
creates a difference signal that used to control the series pass device (switch). A
similar technique is used in linear regulators, the principle difference is that the
series device is operated at the extremes of its load line instead of the linear
region. This subtle difference is what gives switching converters their high
power conversion efficiency.
4.2
LM78S40 Universal Switching Regulator
LM78S40 Datasheet by National Semiconductor
uA78S40 Datasheet by OnSemi
AN-711 LM78S40 Applications by National Semiconductor
Many of the functional blocks in this circuit are disconnected. This gives the
designer a great deal of design flexibility. If required, the transistor switch can be
used directly in low power applications, or it can be used to drive a high power
series pass element.
An Introduction to Power Supplies
4-3
Switch Mode Power Supplies
_____ Notes _____
Each SMPS IC has its own design peculiarities. In some cases, the restriction
placed on the designer may preclude the use of any SMPS IC and a completely
discrete circuit design must be considered. This situation however, is beyond the
scope of this presentation.
Transistor Driver and Switch
The 78S40 uses a Darlington pair in the switching arrangement. The collectors
of both transistors are brought out to external pins. This allows them to be
connected together as is the standard configuration, or an external resistor can be
placed in the driver collector to control the switch saturation current.
Oscillator
The 78S40 chip is designed to operate within a switching frequency range of 100
Hz to 100 KHz. Increasing the switching frequency increases the
electromagnetic radiation and PCB layout problems but decreases the size of the
inductor. Most designs based on this IC have an operating frequency of 20 - 30
KHz.
The oscillator, the charge/discharge ratio is preset to approximately 6:1. The
overall switching duty cycle can be varied from approximately 17% to 50% by
means of two feedback loops.
The switching frequency and duty cycle are controlled by current and voltage
feedback. This will at times make triggering an oscilloscope to the switching
waveforms somewhat problematic. As the load increases, the switching
frequency tends to increases.
Reference Voltage
The 78S40 has an internal 1.245 volt temperature compensated, band-gap
voltage reference which is available at an external pin. This reference voltage
should be bypassed by a 0.1fd capacitor to ground to help insure stability.
Current Feedback
The current feedback circuit modifies the switch ON time. A current sensing
resistor RSC generates a voltage proportional to the switching current. When this
potential exceeds approximately 0.3 volts, the oscillator (and hence switch) is
turned OFF. This control mechanism takes priority over voltage feedback.
Voltage Feedback
The voltage feedback loop, consisting of the voltage divider and comparator
controls the switch ON time. If the output voltage is too low, the ON time is
increased.
4-4
An Introduction to Power Supplies
Switch Mode Power Supplies
_____ Notes _____
Current and Voltage Limitations
The internal Darlington transistor switch can handle a maximum peak current of
1.5 amps during the t on period, and a maximum of 40 volts during the t off
period.
An external transistor switch is needed if the design requires either more current
or a higher input voltage.
Voltage Sensing Resistors
The voltage divider at the output represents a minimum load. The voltage at the
junction of the two resistors must equal the 1.245 reference voltage when one
resistor is attached to ground and the other to the output.
Efficiency
The efficiency of a well-designed power converter can be in excess of 90%.
The output voltage divide constitutes a minimum load on the switching converter
and therefore reduces the efficiency. The current drawn by the divider can be as
low as 100 a, but is more typically in the region of 1 ma. This may not be
significant with high load currents, but it becomes more dominant as load current
decreases.
The saturation voltage of the Darlington transistor can be as high as 1.3 volts.
This decreases efficiency as load current and t on increase.
The internal Darlington transistor has a switching speed of 300 – 500 nSec.
During this time, the transistor is neither ON nor OFF, and therefore dissipates
power.
Any current sensing resistor in series will also dissipate power.
In the following formulas:
Vin = Nominal input voltage
Vout = Desired output voltage (determined by the ratio R2/R1)
Vsat = ON voltage drop across the switching element
VD = Forward voltage drop across the flyback diode.
I out = Desired output current.
I s = Voltage divider sampling current (~ 1 ma)
Vripple = Desired peak-to-peak ripple voltage.
RSC = Short circuit current sensing resistor.
An Introduction to Power Supplies
4-5
Switch Mode Power Supplies
_____ Notes _____
NOTE:
When breadboarding these circuits:
It may be necessary to reduce RSC to 0 .
Always keep the circuit leads as short as possible.
Always use a large decoupling capacitor at the circuit input.
Spread Sheet Design
Buck (Step down) Circuit
Simplified Circuit
Buck Design Formulas
Formula
 R 
Vout  Vref 1  2 
 R1 
I pk  2 I out(max)
Comment
The internal reference voltage is
Vref = 1.245 volts.
I pk is the peak inductor current.
I out (max) is the maximum output
RSC 
0.33
I pk
t on
Vout  VD

t off Vin  Vsat  Vout
load current.
The value for the current sensing
resistor is a given, not derived,
formula.
ton and toff > 10 s
ton + toff < 50 s
 V  VD 
t off
L   out
 I

pk


I pk L
t off 
Vout  VD
4-6
An Introduction to Power Supplies
Switch Mode Power Supplies
_____ Notes _____
CT  45 105 toff
CO 
Timing capacitor
I pk t on  t off 
Minimum output filter capacitor
8 Vripple
 Vin  Vsat  VD  Vout


V
in

 Vout  VD
I pk  Vout  VD

I in( ave)(max load) 
2  Vin  Vsat  VD
Converter efficiency.



Input current.
Buck Design Example (AN711)
Vin = 25 volts
Vout = 10 volts
Iout (max) = 500 ma
Vripple < 1%
Step 1 - Calculate the peak current
I pk  2 I out(max) = 1 amp
Step 2 - Calculate the current sense resistance
RSC 
0.33
= 0.33 
I pk
Step 3 - Calculate the ton/toff ratio
t on
Vout  VD
10  1.25


 0.8
t off Vin  Vsat  Vout 25  1.1  10
Step 4 - Calculate the values for ton
and toff
Since t on  0.8 t off
and
t on  t off  50  sec
Let t off  22 sec
then
t on  18 sec
Step 5 - Calculate the timing capacitor CT
CT  45 105 toff  45 105  22 106  0.01fd
An Introduction to Power Supplies
4-7
Switch Mode Power Supplies
_____ Notes _____
Step 6 – Determine the inductor size
 V  VD
L   out
 I
pk


t off   10  1.25 22  10 6  250H

1



Step 7 – Determine the minimum storage capacitor size
CO 
I pk t on  t off 
8 Vripple



1 18  10 6  22  10 6
 50fd
8  0.1
Step 8 – Determine the feedback resistor sizes
R2 
R1 
Vref
Is

1.245
 1.25 K
110 3
Vout  Vref
Is

(Select 1.3 K)
10  1.245
 8.76 K
110 3
(use a 10 K potentiometer)
Final Circuit
Note: the above schematic is missing the reference and input bypass capacitors.
4-8
An Introduction to Power Supplies
Switch Mode Power Supplies
_____ Notes _____
Basic Waveforms
Boost (Step Up) Circuit
Simplified Circuit
An Introduction to Power Supplies
4-9
Switch Mode Power Supplies
_____ Notes _____
Boost Design Formulas
I pk
Formula
 R 
Vout  1.25 1  2 
 R1 
 V  VD  Vsat 

 2 I out(max)  out
 Vin  Vsat 
RSC 
0.33
I pk
t on Vout  VD  Vin

t off
Vin  Vsat
Comment
The internal reference voltage is 1.245
volts.
I pk is the peak inductor current.
I out (max) is the maximum output load
current.
The value for the current sensing
resistor is a given, not derived,
formula.
ton and toff > 10 s
ton + toff < 50 s
 V  V D  Vin 
t off
L   out


I
pk


I pk L
t off 
Vout  VD  Vin
CT  45 105 toff
CO 
I
 I out  t off
2
pk
Timing capacitor
Minimum output filter capacitor
2 I pkVripple
 Vin  Vsat 
Vout


 Vin  Vout  VD  Vsat
I pk
I in( ave)(max load) 
2
Vin  Vref
R3 
IB
Converter efficiency.
Input current.
Driver collector resistor
IB 
I pk

, assume
  20
Boost Design Example (AN711)
Vin = 5 volts
Vout = 15 volts
Iout (max) = 150 ma
Vripple < 1%
Step 1 - Calculate the peak current
 V  VD  Vsat 
15  1.25  0.45 
  2  0.15
I pk  2I out(max)  out
  1 amp
5  0.45


 Vin  Vsat 
4 - 10
An Introduction to Power Supplies
Switch Mode Power Supplies
_____ Notes _____
Step 2 - Calculate the current sense resistance
RSC 
0.33
= 0.33 
I pk
Step 3 - Calculate the ton/toff ratio
t on Vout  VD  Vin 15  1.25  5


 2.5
t off
Vin  Vsat
5  0.45
Step 4 - Calculate the values for ton
and toff
Since t on  2.5 t off
and
t on  t off  50  sec
Let t off  10  sec
then
t on  25 sec
Step 5 - Calculate the timing capacitor CT
CT  45 105 toff  45 105 10 106  4.5 nfd
Step 6 – Determine the minimum inductor size
 V  VD  Vin 
t off   15  1.25  5  10 10 6  100 H
L   out


I pk
1




Step 7 – Determine the minimum storage capacitor size
CO 
I
 I out  t off
2
pk
2 I pkVripple

1  0.152 10 10 6
2  1 0.15
 24 fd
Step 8 – Determine the feedback resistor sizes
Let
I s  1 ma
R2 
R1 
Vref
Is

1.245
 1.25 K
110 3
Vout  Vref
Is

(Select 1.3 K)
15  1.245
 13.8 K
110 3
An Introduction to Power Supplies
(use a 25 K potentiometer)
4 - 11
Switch Mode Power Supplies
_____ Notes _____
Step 9 – Determine the base drive resistor size (if desired)
R3 
Vin  Vref
and
IB
R3 
IB 
I pk

, assume
  20
5  1.245
 72
1 20
Final Circuit
Note: the above schematic is missing the reference and input bypass capacitors.
4 - 12
An Introduction to Power Supplies
Switch Mode Power Supplies
_____ Notes _____
Basic Waveforms
Inverter Circuit
In some designs (such as the one below) the switch current is extremely large. As
a result, an external switching transistor may be required.
Simplified Circuit
An Introduction to Power Supplies
4 - 13
Switch Mode Power Supplies
_____ Notes _____
Inverter Design Formulas
I pk
Formula
 R 
Vout  1.25 1  2 
R1 

 Vin  Vout  VD  Vsat
 2 I out(max) 
Vin  Vsat

RSC 
Comment
The internal reference
voltage is 1.245 volts




I pk is the peak inductor
current. I out (max) is the
maximum output load
current.
The value for the current
sensing resistor is a given,
not derived, formula.
ton and toff > 10 s
ton + toff < 50 s
0.33
I pk
t on Vout  VD

t off
Vin  Vsat
 Vout  V D 
t off
L


I
pk


I pk L
t off 
Vout  VD
CT  45 105 toff
CO 
I
Timing capacitor
 I out  t off
Minimum output filter
capacitor
2
pk
2 I pkVripple
Converter efficiency.
Vout
Vout  VD
I in( ave)(max load )
I pk 
Vout  VD



2  Vin  Vout  Vsat  VD




Input current.
Inverter Design Example (AN711)
Vin = 12 volts
Vout = -15 volts
Iout (max) = 500 ma
Vripple < 1%
Step 1 - Calculate the peak current
 Vin  Vout  VD  Vsat
I pk  2 I out(max) 
Vin  Vsat

4 - 14




An Introduction to Power Supplies
Switch Mode Power Supplies
_____ Notes _____
 12   15  1.25  2 
  2.6
I pk  2  0.5

12  2


Step 2 - Calculate the current sense resistance
RSC 
0.33
= 0.13 
I pk
Step 3 - Calculate the ton/toff ratio
t on Vout  VD  15  1.25


 1.625
t off Vin  Vsat
12  2
Step 4 - Calculate the values for ton
and toff
Since t on  1.6 t off
and
t on  t off  50  sec
Let t off  10  sec
then
t on  16 sec
Step 5 - Calculate the timing capacitor CT
CT  45 105 toff  45 105 10 106  4.5 nfd
Step 6 – Determine the inductor size
 V  VD
L   out
 I
pk


  15  1.25 
t off  
  10  10 6  70 H



2.6



Step 7 – Determine the minimum storage capacitor size
CO 
I
 I out  t off
2
pk
2 I pkVripple

2.6  0.52 10 10 6
2  2.6  0.15
 60 fd
Step 8 – Determine the feedback resistor sizes
R2 
Vref
Is

1.245
 1.25 K
110 3
An Introduction to Power Supplies
(Select 1.3 K)
4 - 15
Switch Mode Power Supplies
_____ Notes _____
Vout  Vref
R1 
Is

16  1.245
 17.5 K
1 10 3
(use a 25 K potentiometer)
Step 9 – Determine the switching transistor bias resistors
Select
100  R3  300
R4 
where

Vin  Vsat  VT  VBE
I pk 
VT  0.3 v (current sense threshold voltage)
hFE
(of the external transistor)
4
R4 
12  1.3  0.3  0.7
 720
2.6 190
Final Circuit
Note: the above schematic is missing the reference and input bypass capacitors.
4 - 16
An Introduction to Power Supplies
Switch Mode Power Supplies
_____ Notes _____
Basic Waveforms
An Introduction to Power Supplies
4 - 17
Switch Mode Power Supplies
_____ Notes _____
4.3
MAX641 Step-Up Switching Regulator
MAX641 Datasheet
MAX641 Block Diagram
This particular IC requires the designer to only select the value of the inductor.
Boost Design Example
Vin  5v  10%
Vout  15v
VD  0.4v
0.25v  Vsw  0.75v
f osc  50 KHz  20%
 8Sec  t on  12Sec
15ma  I load  450ma
To determine the value of the inductor, it is necessary to make two calculations,
one for a maximum size and the other for minimum.
Case 1 Maximum L
I pk 
I pk 
Vout  VD  Vin
 4  I load
Vin  Vsw
15  0.4  4.5
 4  15ma  174ma
4.5  0.75
L
4 - 18
Vin  Vsw
 t on
I pk
An Introduction to Power Supplies
Switch Mode Power Supplies
_____ Notes _____
L
4.5  0.75
 8Sec  172 H
174ma
Case 2 Minimum L
From the datasheet Ipk(max) = 450ma.
L
5.5  0.25
 12Sec  140H
450ma
A value of 160 H would be a reasonable compromise.
An Introduction to Power Supplies
4 - 19
Switch Mode Power Supplies
_____ Notes _____
Assignment Questions
On-Line Test
Quick Quiz
1.
Composition Questions
To answer these questions, it will be necessary to do some research.
1.
Analytical Questions
1.
Create a spreadsheet to implement the design of a Buck Regulator based on
the LM78S40.
2.
Create a spreadsheet to implement the design of an Inverter based on the
LM78S40.
3.
Create a spreadsheet to implement the design of a Boost Regulator based on
the LM78S40.
4.
4 - 20
An Introduction to Power Supplies
Switch Mode Power Supplies
_____ Notes _____
For Further Research
An Introduction to Power Supplies
4 - 21