1. Converter principles and modelling
Dr John Fletcher

2. Power Supply Principles
Topics
Voltage control modes
Load and line regulation
Multiple outputs
Post-regulation
Paralleling
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Power Supply Principles

3. Power Supply Principles
Voltage-mode control
The buck converter steps down input voltage.
Output voltage is a function of the duty ratio, D.
One goal is to regulate the output voltage during line disturbances (input voltage change) or load changes (output current changes).
Fig 2.3 and 2.4
In the loss-less converter the relationship between output and input voltage is not load current dependent. However, losses in practical converters mean that the relationship is load current dependent.
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Power Supply Principles

4. Power Supply Principles
Voltage-mode control
The output voltage of a converter is a function of input voltage, the duty ratio and the load current.
As the load current rises the output voltage will drop.
(load regulation)
If the input voltage rises the output voltage will tend to rise.
(line regulation)
As disturbances occur, the duty ratio must be adjusted to compensate.
E.g. if load current increases, D should be increased to compensate. D
P 324, fig 9.1 Erikson
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Power Supply Principles

5. Load and line regulation
Output voltage with minimum load
Output voltage at full load
Change in output voltage caused by change in input
voltage from nominal to min (or max) value
See p2, hnatek for definitions
Nominal output voltage, often no-load voltage
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Power Supply Principles

6. Power Supply Principles
Voltage-mode control
The output voltage is measured by a sensor.
An error amplifier produces an output proportional to the difference between the output voltage and the reference.
The magnitude of the error is fed to the compensator.
The compensator uses this error to determine the duty ratio, D, required to force the actual voltage towards the desired reference value.
This constitutes a closed-loop control system using negative feedback. A block diagram is shown. D
Circuit
P 325, fig 9.2 Erikson
Model
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Power Supply Principles

7. Power Supply Principles
Voltage-mode control
The pulse-width modulator takes the compensator output and produces a PWM signal whose duty-ratio, D, is proportional the voltage, vc.
This drives the transistor via the gate driver.
If the output voltage falls due to an increase in the load current, the compensator increases the duty-ratio, D.
If the input voltage falls, the output voltage falls, and the control circuit compensates by increasing D. D
Circuit
P 325, fig 9.2 Erikson
Model
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Power Supply Principles

8. Voltage-mode control (flyback)
The flyback converter with voltage-mode control.
PWM controller senses output voltage and adjusts D to regulate output.
Fig 2.37, p 88 Hnatek
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Power Supply Principles

9. Power Supply Principles
Voltage-mode control
Advantages
Voltage mode control is (at first glance) a simple control technique.
The voltage sensor is low-cost.
The relationship between D and the output voltage is simple to understand.
Disadvantages
No intrinsic control of current, especially switch current.
The dynamics of the system are complicated – this makes compensator design difficult.
P 325, fig 9.2 Erikson
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Power Supply Principles

10. Primary v’s secondary feedback
Primary feedback
Output voltage inferred from bias winding voltage
No isolation required – low cost
Poor regulation (± 5%)
Fig 3 and 4, AN-19
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Power Supply Principles

11. Primary v’s secondary feedback
Output voltage sensed and fedback to controller
Signal crosses isolation barrier – cost of barrier
Good or excellent regulation (± 1% ~ ± 0.1%)
Fig 3 and 4, AN-19
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Power Supply Principles

12. Voltage control - forward
The forward converter with voltage-mode control.
PWM controller senses output voltage and adjusts D to regulate output 1.
Fig 2.52, p 107 Hnatek
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Power Supply Principles

13. Forward – multiple outputs
PWM controller senses output voltage and adjusts D to regulate output 1.
Auxiliary outputs may require post-regulation, using a linear regulator or a mag-amp.
2 diodes, 1 inductor and 1 capacitor required for each additional output.
Fig 2.52, p 107 Hnatek
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Power Supply Principles

14. Power Supply Principles
Forward – paralleling
The converter opposite is two forward converters paralleled.
Switches Q1 and Q2 are arranged to switch in anti-phase.
This is commonly referred to as the push-pull converter.
Transformer saturation is a real issue in this converter requiring the use of balancing circuits to avoid a dc flux (and a drift towards saturation).
The half-bridge converter is a modification to the push-pull where balance is easier to obtain.
Fig 3.10, p 26 Chryssis also Mohan p314, 10.20
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Power Supply Principles

15. Power Supply Principles
Summary
Forward converter reviewed
Basic input-output relationships established
Basic model constructed in LTSpice
Voltage-mode control techniques
Primary and secondary feedback techniques
Multiple outputs
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Power Supply Principles

16. Power Supply Principles
Bibliography
Erickson, “Fundamentals of Power Electronics”, Kluwer Academic Publishers, ISBN
Mohan, Undeland, Robbins, “Power Electronics: Converters, Applications, and Design”, Wiley International, ISBN
Trzynadlowski, “Introduction to Modern Power Electronics”, Wiley, ISBN
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Power Supply Principles

17. Power Supply Principles
AC input converters
SMPS takes AC input and produces regulated DC outputs.
The rectifier stage generates a DC link voltage.
The DC link is used by the DC-DC converter to generate the desired outputs (in this case, more than one output voltage).
The DC-DC converter stage may include isolation.
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Power Supply Principles

18. Power Supply Principles
Additional Slides
Non-isolated switched-mode topologies
Boost
Isolated switched-mode topologies
Flyback, half-bridge, full-bridge
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Power Supply Principles

19. AC input with power factor correction
A boost converter with power factor correction (PFC).
Link current ig(t) is controlled to follow a full-wave rectified shape synchronised with input voltage.
The input current, iac(t), is then forced to be sinusoidal and in-phase with input voltage (unity-power factor input).
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Power Supply Principles

20. The boost converter circuit
Steps up input voltage.
Output voltage is a function of the duty ratio, D
Input current is continuous – EMI advantages
Figs 2.13b and 2.17
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Power Supply Principles

21. The boost converter modes
When switch is on
When switch is off
Figs 2.14 a and b
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Power Supply Principles

22. The boost converter equations
Inductor voltage waveform. Applying volt-second balance:
Voltage conversion ratio
Figs 2.15 a and Fig 2.16
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Power Supply Principles

23. The boost converter waveforms
Inductor current waveform.
Output voltage waveform
Figs and Fig 2.19
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Power Supply Principles

24. Flyback converter circuit
Derived from the buck-boost topology.
Transformer is really a ‘two-winding inductor’.
Energy stored in primary while switch ‘on’.
Energy transferred to secondary when switch ‘off’.
Fig 6.31 d 6.32 a
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Power Supply Principles

25. Flyback converter - modes
When switch is on:
When switch is off:
Fig 6.32 b 6.32 c
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Power Supply Principles

26. Flyback converter - waveforms
Applying volt-second balance:
Applying charge balance:
Fig 6.33
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Power Supply Principles

27. Two-switch forward converter
Q1 and Q2 driven by same signal.
No need for 3rd winding on transformer.
Reduced voltage stress on transistors.
Restriction
Requires two switches.
Fig 6.33
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Power Supply Principles

28. Half-bridge converter
Fig 6.33
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Power Supply Principles

29. Full-bridge converter
Fig 6.33
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Power Supply Principles