DC-DC converters 12V to 120VDC 20W by MJE2955

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The design is a simple saturation-limited push-pull converter. There is no special reason to use PNP transistors; I used them simply because I had a box full of them around. You may well turn over the design to use NPN transistors.
The 2SC945 is a bias switch for startup. When applying 12V power, this transistor applies enough bias to the power transistors to get the oscillation started. Soon later, the 100uF capacitor charges up, the transistor goes off, and the power transistors self-bias into cut-off, such that cross-conduction is eliminated. After removing power, the 6k8 resistor discharges the bias timing capacitor, as otherwise the circuit would be unable to restart!
The secondary rectifiers are ultrafast diodes. These are NOT 1N4007! And the 220nF capacitors for the secondary filter are no typos; the diodes deliver almost pure DC, since the oscillation waveform is square, so only some noise filtering is needed. No electrolytics are necessary here.

Read More Source:
http://ludens.cl/Electron/dcdc/dcdc.html
Thank you.

This entry was posted on Saturday, January 26th, 2008 at 10:24 am and is filed under Converter Circuit. You can follow any responses to this entry through the RSS 2.0 feed.