OFTEN when making portable devices there is a need for a regulated 5V supply, typically to power either logic or a microcontroller. Usually a linear regulator, such as a 7805, is used to derive this supply. Such regulators are quite wasteful because the current into such a regulator is always greater than the current out of it and there is a voltage drop across the regulator as well.

Switch Mode Regulators – Saves Batteries

OFTEN when making portable devices there is a need for a regulated 5V supply, typically to power either logic or a microcontroller. Usually a linear regulator, such as a 7805, is used to derive this supply. Such regulators are quite wasteful because the current into such a regulator is always greater than the current out of it and there is a voltage drop across the regulator as well.

When running 5V logic from a 9V supply the regulator is wasting almost as much power as the whole device is using. If running the system from a 12V battery, such as a key fob battery, then more power is wasted than is used. For example, with an input of 9V, an output 5V @ 500mA, using P=IV the input power = 4·5Watts, output power = 2·5W. Wasted power = 2W, efficiency = 2·5 / 4 ·5 = 55%.

If the input is 12V then the input power = 6W, the wasted power is 3·5W and the efficiency falls to 42%. The wasted power is  dissipated as heat, and so the regulator must be mounted on a heatsink.

Switch mode power supplies (SMPSUs) convert one voltage to another voltage much more efficiently. Under the right conditions they can draw less current (at a higher voltage) than they supply to the load (at lower voltage).

In the past, switch mode supplies have been extremely tricky devices to use, requiring specialist knowledge, but now several manufacturers produce switch mode chips which have everything needed to make switch mode converters. The datasheets for these devices give very good application data to assist designers and some manufacturers even give out software to design the whole circuit according to user input parameters.

As an example, the circuit diagram of Fig.1 is based on National Semiconductor’s LM3578 switch mode converter. The entire circuit costs about half as much as a 9V PP3 alkaline battery. The circuit’s efficiency is summarised below:

Output 5V@180mA Input 9V@141mA
efficiency: 71%
Output 5V@360mA Input 9V@290mA
efficiency: 69%
Output 5V@540mA Input 9V@450mA
efficiency: 67%
Output 5V@720mA Input 9V@620mA
efficiency: 65%
Output 5V@180mA Input 12V@108mA
efficiency: 69%
Output 5V@360mA Input 12V@220mA
efficiency: 68%
Output 5V@540mA Input 12V@380mA
efficiency: 59%
Output 5V@720mA Input 12V@600mA
efficiency: 50%

In addition the circuit can output 500mA for long periods without any need for a heatsink. The minimum input voltage that the circuit needs to produce regulated 5V output is about 7·3V. The circuit is generally not recommended for sensitive analogue circuitry as it does have a small amount of ripple on the output.

The circuit is straightforward to build. The inductor current rating should be about the same as the load current for best operation, and inductors can be connected in series or in parallel to achieve the desired value. The diode D1 should be a Schottky type as the circuit runs at high frequency. It is possible to add current limiting by inserting a resistor into the circuit between pin 8 and pins 7 and 6.

Imaginative

The circuit’s operation may be understood intuitively. Imagine a square wave with a 50% duty cycle: the average voltage out of such a circuit is half the peak voltage of the square wave. All that is needed to convert such a square wave to d.c. is a circuit thattakes the average of its input; a low-pass filter is such a circuit.

Many low-pass filters use resistors and capacitors, but resistors are not desirable in power circuitry and therefore in the SMPSU the low-pass filter uses an inductor and a capacitor. The Schottky diode provides a path for the inductor current when the “switch”, which produces the square wave, is off.

Regulation is achieved by feeding back the output voltage via a resistor divider back into the device. Inside the i.c. the voltage on pin 1 is compared with a 1V reference. If the voltage on pin 1 is too high then the output voltage must be too high and so the i.c. reduces the duty cycle of the square wave, which drops the output voltage – a classical application of negative feedback.

We can now see why the circuit is efficient. If the input voltage is double that of the output voltage then the square wave’s duty cycle will be about 0·5 (50%). This means that the circuit is only drawing current half the time. In addition the only major losses in the circuit are in the switch which produces the square wave, and in the diode.

These losses are small owing to the low voltage drops across the devices when in their on state. So next time an efficient power supply is needed, consider using these simple switch mode devices in your application.

Article reproduced by permission of Wimborne Publishing. www.epemag.com