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High Voltage DC power supplies.

Warning .... High voltages can kill. Ask the chap laying on the workshop floor !.

Introduction

Sometimes we need relatively high voltages for some electronic applications. Typical examples are Geiger tubes (800v), backlighting (80v), TENS units (80v), florescent tubes (240v) and Nixie tubes (170v). In many cases we need a stabilised voltage. In the past much use was made of fly-back oscillator circuits and push pull multi-vibrators but they did not easily lend them selves to accurate stabilisation. The slightly more modern switched mode supply not only is smaller, lighter, cheaper and simplifies stabilisation to better than 20mV, at several hundred volts, with an efficiency around 90%.

Switched mode power supplies.

As you may have noticed I am a PIC fan. So let us have a look at the theory of switched mode and how it can be done with PIC's. First download and read TB053 from ....

http://ww1.microchip.com/downloads/en/AppNotes/91053b.pdf

This article gives a good theoretical understanding of the method and a practical design for a 170v power supply. It should be noted that any PIC chip with a PWM module (16F877, 18F1320 etc) can be used in this application providing that you write your own code. The source code provided will only work with the PIC16C781/782. Please note that SMPS circuits usually work at several hundred kHz and diodes such as the 1N4000 series cannot work at this frequency. You must use fast switching diodes such as the UF4000 series (Ultra Fast). The working voltage of the output capacitor must exceed the working voltage and preferably be a low ESR type.

SMPS integrated circuits.

A wide range of special purpose SMPS integrated circuits exist on the market. One very common one is the MC34063 which can be used in step up / step down / voltage invert modes. Step up is also commonly known as 'buck boost and an appropriate circuit can boost a supply voltage of 5 volts up to several hundred volts using an external switching MOSFET transistor. A typical 'step down' application would be to step a vehicles 12 volts down to 3.3 volts to recharge a mobile telephone. One problem is that amateurs may find it difficult to source chips such as these. However, with the knowledge that most mobile telephone charger adaptors contain an MC34063 you have a possible source at your local car scrap yard. Mine gave me a sack of mobile telephone accessories, that get left in vehicles when they are scrapped !.

One of my favorite SMPS chips is the MAX1771 housed in an 8 pin DIL package. This uses an external switching MOSFET and is capable of driving 24 watt loads with an efficiency of 90%. You can download the data sheet for this chip direct from the Maxim website at ......

http://datasheets.maxim-ic.com/en/ds/MAX1771.pdf

I think it wise to take heed of any layout suggestions given in the application notes. To change the actual operating output voltage only requires changing the two feedback voltage divider resistors. With the right values the voltage fed back to pin 3 (FB) needs to be about 1.5 volts. This basically tells the chip that the output voltage has charged up the output capacitor to the correct voltage and can switch off until the output voltage starts to fall under load. I think this is an important lesson to learn .... the chip only produces an output when required .... so if you scope the output, do not expect to see a continuous output.

A MAX1771, 0 - 300 volts DC PSU for bench development work. DC power in on the left terminal block. The inductor is just above the MOSFET transistor. The two preset resistors are for setting the output voltage. The bottom one sets the maximum output and the top on adjusts the output voltage between O and Max. The right hand electrolytic capacitor is rated at 400volts DC and is the output capacitor. The high voltage output is available on the right hand side terminal block. No attempt has been made to minaturise the layout. I have since moved the 'shut down' signal terminal block to the bottom of the board and introduced a third terminal block at bottom left. The reason for this is that I wanted the PIC to measure and monitor the High voltage output. To do this the top resistor and top left preset resistor form a voltage divider to step the high voltage down to a 0 - 5 volts range that can be directly fed into a PIC analogue input. A 5.1 volt Zenner diode across the preset protects the PIC against any accidental over-volt conditions.

In circuits where there is a light load, the output capacitor can retain a charge after the supply has been removed from the circuit. Do not work on the circuit until after you have discharged the output capacitor with a resistor.