Switch Mode Power Supplies

1. Switchmode Power Supplies
Power Supplies
– An Overview

2. This presentation will discuss about Power
converters
• Power conversion from high voltage AC (input)
supplies to low voltage DC(output)
• One DC voltage(input) to another DC voltage(ouput)
• Output > Input or Output < Input
but, Input Power > Output Power
(.... + losses in converter)

3. Types of Regulators
 Linear Regulators
 Switching Regulators

4.  Linear Regulators
 a simple, inexpensive circuit that produces a
quiet output voltage less than the input
voltage
 Switching Regulators
 more complex circuit that’s efficient,
compact, and versatile in its input/output
voltages

5. Linear Regulators
- Simple, inexpensive
+ Electrically “quiet” pure DC output
+ Vout < Vin
+ Poor Efficiency
- Can be physically large
Step Down
Or
Step Up
Transformer
Rectifier Linear
Regulator
DC o/p
AC i/p Vi
n
Vo
Classical Linear Regulator

6. Switchmode Regulators
+ Wide range of input voltages
+ Multiple output voltages possible
+ High Efficiency
+ Compact
- Complex, more expensive
- Electrically “noisy” (not pure DC)
Rectifier Switching
Regulator
DC o/p
AC i/p Vi
n
Vo
Typical SMPS

7. Switchmode Topologies
A few types…Non isolated
 Buck :output DC< input DC
Application:
• To Produce a lower O/P voltage than DC I/P voltage
– Step down converter.
• Regulating DC power supplies.
• As a replacement for the Linear regulator – avoiding
high power dissipating series pass transistor

8. Topologies
Basic buck converter:
Basic buck converter:
Vo = D * Vin
D = Ton / T
where T=Ton+Toff

9. Configuration depending on state of
switch (Buck)

10. Topologies
 Boost : output DC > input DC
Application:
 In battery operated devices, where the required
operational voltage is more than battery voltage
 To achieve holdup time in critical embedded systems
 Where high voltages are required e.g., TV picture
tubes, Cathode ray tube.

11. Topologies
Basic boost converter:
Basic boost converter:
D = Ton / T
where T=Ton+Toff

12. Configuration depending on state of
switch (Boost)

13. Topologies
 Buck-boost (Inverting)
- an output voltage is generated opposite in polarity to the
input.
D = Ton / T
where T=Ton+Toff

14. Configuration depending on state of
switch (Buck-Boost)

15. Isolated Converters - Requires
Transformer
 The transformer functions as:
 An isolation between the input and output
circuit.
 Energy storage element.
 Stepping up or down.
 Providing Multiple outputs.

16. Switchmode Topologies
(Isolated)
 Flyback:
- an output voltage that is less than or
greater than the input can be generated,
as well as multiple outputs.

17. Topologies
 Forward:
an output voltage that is less than the
input can be generated.

18. Topologies
Push-Pull:
A two-transistor converter that is
especially efficient at low input voltages.

19. Topologies
Half-Bridge:
A two-transistor converter used in many
off-line applications.

20. Topologies
Full-Bridge:
A four-transistor converter (usually used in off-
line designs) that can generate the highest
output power of all the types listed.

21. Modes of operation in converters:
Converters may be operated in two modes, according
to the current in its main magnetic component
[inductor or transformer]

Discontinuous mode:
- the current fluctuates during the cycle , goes down to
zero at the end of each cycle.
 Continuous mode :
- the current fluctuates but never goes down to zero.

22. Modes of operation in converters:
 Discontinuous mode of operation:
 Secondary peak currents are higher – I2
R Losses, Skin effect
losses
 Bigger I/P filter to reduce EMI problems
 O/P capacitors to be large enough to handle larger ripple
current rating
 Responds faster for the load variation and I/P voltage
variations

23. Modes of operation in converters:
 Continuous mode of operation:
 Low conduction losses due to lower currents.
 Requires fast recovery O/P rectifier diodes – Higher cost

24. Control techniques in converters:
 Voltage Mode Control:
The difference between the desired and actual
output voltages (error) controls the voltage
applied across the inductor.
 Current Mode Control:
The difference between the desired and actual
output voltages (error) controls the peak
inductor current.

25. Control techniques in converters:
 Voltage Mode Control:
Advantages:
- A single feedback loop is easier to design and analyze.
- Less noise, Less power loss, More resolution.
- A low-impedance power output provides better cross-
regulation for multiple output supplies.
- Provides Stable Operation.

26. Control techniques in converters:
Disadvantages:
- Any change in line or load must first be sensed as an
output change and then corrected by the feedback
loop. This usually means slow response.
- Compensation is further complicated by the fact that
the loop gain varies with input voltage

27. Control techniques in converters:
 Current Mode Control:
Advantages:
- Responds immediately to input voltage changes.
- Since the Error Amplifier is used to command an output
current rather than voltage, the effect of the output
inductor is minimized .This allows both simpler
compensation and a higher gain bandwidth over a
comparable voltage-mode circuit.

28. Control techniques in converters:
- Additional benefits with current-mode circuits include
inherent pulse-by-pulse current limiting.
- Disadvantages:
- There are two feedback loops, making circuit analysis more
difficult.
- The control loop becomes unstable at duty cycles above
50%.

30. Thanks
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