Switched Mode Power Supplies

Introduction

Switched Mode Power Supplies, (often abbreviated to SMPS) are considerably more complex than the linear regulated power supplies described in Power Supplies Module 2. The main advantage of this added complexity is that switched mode operation gives regulated DC supplies that can deliver more power for a given size, cost and weight of power unit.

Switched Mode Designs

A number of different design types are used. Where the input is the AC mains (line) supply the AC is rectified and smoothed by a reservoir capacitor before being processed by what is in effect a DC to DC converter, to produce a regulated DC output at the required level. Hence a SMPS can be used as an AC to DC converter, for use in many mains powered circuits, or DC to DC, either stepping the DC voltage up or down as required, in battery powered systems.

Switched Mode Block Diagram

Fig. 3.0.1 shows a block diagram example of a typical SMPS with an AC Mains (line) input and a regulated DC output. The output rectification and filter are isolated from the High Frequency switching section by a high frequency transformer, and voltage control feedback is via an opto isolator. The control circuit block is typical of specialist ICs containing the high frequency oscillator, pulse width modulation, voltage and current control and output shut down sections.

Whatever the purpose of a SMPS, a common feature (after conversion of AC to DC if required) is the use of a high frequency square wave to drive an electronic power switching circuit. This circuit is used to convert the DC supply into high frequency, high current AC, which by various means, depending on the design of the circuit, is reconverted into a regulated DC output. The reason for this double conversion process is that, by changing the DC or mains frequency AC to a high frequency AC, the components, such as transformers, inductors and capacitors, needed for conversion back to a regulated DC supply, can be much smaller and cheaper than those needed to do the same job at mains (line) frequency.

The high frequency AC produced during the conversion process is a square wave, which provides a means of controlling the output voltage by means of pulse width modulation. This allows the regulation of the output to be much more efficient than is possible in linear regulated supplies.

The combination of a square wave oscillator and switch used in switched mode supplies can also be used to convert DC to AC. In this way the switched mode technique also be used as an ‘inverter’ to create an AC supply at mains potential from DC supplies such as batteries, solar panels etc.

Voltage Regulation

In most switched mode supplies, regulation of both line (input voltage) and load (output voltage) is normally provided. This is achieved by altering the mark to space ratio of the oscillator waveform before applying it to the switches. Control of the mark to space ratio is achieved by comparing voltage feedback from the output of the supply with a stable reference voltage. By using this feedback to control the mark to space ratio of the oscillator, the duty cycle and therefore the average DC output of the circuit can be controlled. In this way, protection from both over voltage and over current may be provided.

Where it is important to maintain electrical isolation from the mains supply, this is provided by using a transformer, either at the AC input where it may also be used to alter the AC voltage prior to rectification, or between the control section of the power supply and the output section where, as well as providing isolation, a transformer with multiple secondary windings can produce several different voltage outputs.

To provide a well regulated output, a sample of the DC output voltage is normally fed back to the control circuitry and compared with a stable reference voltage. Any error produced is used to control the output voltage. To maintain electrical isolation between input and output, feedback will usually be via a device such as an opto-isolator.

HF Switching

Using high frequency for the switching drive gives several advantages:

• The transformer will be of a HF type, which is much smaller than a standard mains transformer.

• The ripple frequency will be much higher (e.g. 100kHz) than in a linear supply, and so it needs a smaller value of smoothing capacitor.

• Also using a square wave to drive the switching transistors (switched mode operation) ensures that they dissipate much less power than a conventional series regulator transistor. Again this means that, for a given amount of power output, smaller and cheaper transistors can be used, than in similarly rated linear power supplies.

• The use of smaller transformers and smoothing capacitors makes switched mode power supplies lighter and less bulky. The added cost of the complex control circuitry is also offset by the smaller, and therefore cheaper transformers and smoothing capacitors, making some switched mode designs less expensive than equivalent linear supplies.

Although linear supplies can provide better regulation and better ripple rejection at low power levels than switched mode supplies, the above advantages make the SMPS the most common choice for power supply units in any equipment where a stabilised supply is needed to deliver medium to large amounts of power.

A disadvantage of using such a high frequency square wave in a powerful circuit such as a SMPS is that many powerful high frequency harmonics are created, so that without very effective RF screening and filtering, there is a danger of the SMPS creating RF interference.