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A SWITCHER FOR MANY REASONS

by Lawrence Foltzer

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I recently responded to a call for help on the Microchip web site, Knowledge Base, PICmicro Conference. The gentleman was looking for a device that could provide a programmable/variable (30ý150 V) DC supply voltage to actuate a piezoelectric mechanism. His idea was straightforward enoughýhave a PIC drive a DAC followed by an amplifier.

Previously, I developed a simpler approach to his problem, but for another variable high-voltage application. My technique operates off the same 3ý6-V supply the processor uses, doesn't require the DAC, and can all be done with a simple PIC12C508.

My approach (shown in Figure 1) was basically a variable burst-length, fixed duty-cycle switching power supply driven by an RC-clocked PIC microcontroller.

Figure 1ýHereýs a look at the basic circuit topology for my solution.

As Figure 2 shows, you won't need much power to drive a piezo transducer, a photomultiplier or avalanche photodiode, or a high peak current avalanche pulse generator, so you might try this approach if your system is short on power.

Figure 2ýThe design details of the switcher are all right here.

The transistor is a 2N6515 and youýll need a fast high-voltage diode like the MURS160 for the rectifier. Drive impedance needs to be low even though only low drive power is required, given the low switch current. The low-impedance drive suppresses feedback from output to input which otherwise would cause erratic operation of the switcher. You might try driving the switching transistor with a common emitter buffer amplifier and save I/O ports on the processor.

Listing 1 contains the code I used for my project.

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