Converter principles and modelling Dr John Fletcher

Power Supply Principles Topics Voltage control modes Load and line regulation Multiple outputs Post-regulation Paralleling 13/09/2018 Power Supply Principles

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. 13/09/2018 Power Supply Principles

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 13/09/2018 Power Supply Principles

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 13/09/2018 Power Supply Principles

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 13/09/2018 Power Supply Principles

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 13/09/2018 Power Supply Principles

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 13/09/2018 Power Supply Principles

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 13/09/2018 Power Supply Principles

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 13/09/2018 Power Supply Principles

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 13/09/2018 Power Supply Principles

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 13/09/2018 Power Supply Principles

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 13/09/2018 Power Supply Principles

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 13/09/2018 Power Supply Principles

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 13/09/2018 Power Supply Principles

Power Supply Principles Bibliography Erickson, “Fundamentals of Power Electronics”, Kluwer Academic Publishers, ISBN 0412085410 Mohan, Undeland, Robbins, “Power Electronics: Converters, Applications, and Design”, Wiley International, ISBN 0471429082 Trzynadlowski, “Introduction to Modern Power Electronics”, Wiley, ISBN 0471153036 13/09/2018 Power Supply Principles

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. 13/09/2018 Power Supply Principles

Power Supply Principles Additional Slides Non-isolated switched-mode topologies Boost Isolated switched-mode topologies Flyback, half-bridge, full-bridge 13/09/2018 Power Supply Principles

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). 13/09/2018 Power Supply Principles

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 13/09/2018 Power Supply Principles

The boost converter modes When switch is on When switch is off Figs 2.14 a and b 13/09/2018 Power Supply Principles

The boost converter equations Inductor voltage waveform. Applying volt-second balance: Voltage conversion ratio Figs 2.15 a and Fig 2.16 13/09/2018 Power Supply Principles

The boost converter waveforms Inductor current waveform. Output voltage waveform Figs 2.18 and Fig 2.19 13/09/2018 Power Supply Principles

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 13/09/2018 Power Supply Principles

Flyback converter - modes When switch is on: When switch is off: Fig 6.32 b 6.32 c 13/09/2018 Power Supply Principles

Flyback converter - waveforms Applying volt-second balance: Applying charge balance: Fig 6.33 13/09/2018 Power Supply Principles

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 13/09/2018 Power Supply Principles

Half-bridge converter Fig 6.33 13/09/2018 Power Supply Principles

Full-bridge converter Fig 6.33 13/09/2018 Power Supply Principles