Re: Linear Power Supply Design Question



giroup01@xxxxxxxxxxxx wrote:

> Just a few questions on upgrading my old linear power supply. I haven't
> touched electronics in years, I forgot quite a bit...
>
> Schematic here:
> http://img230.imageshack.us/img230/6576/alimpage47fg.jpg
>
> Originally rated 0-30V/0-3A, I want it to go to 0-50V/0-5A.
>
> The 'new, improved' power section consists of a 400VA toroid
> transformer, 35A bridge, 7400 uF 250 VDC filter capacitor, a TIP31C
> driving a bank of 5 parallel 2n3773 (with 0R22 emitter resistors)
> mounted on heatsinks with 110v fans at both ends. Following my first
> unsuccessful upgrade I fused the output to 5 amps to minimize
> fireworks.
>
> The transformer supplies 53 VAC when both its secondary windings are
> connected in series. With this configuration the DC voltage at the
> collectors is 73.5 V *but* I'm currently using a single secondary
> winding giving me approx. 35 VDC output.
>
> Will the circuit survive with 53 VAC on input? I'm asking this because
> I don't want the thing to blow up again like it did the first time when
> I was setting the max amp limit with 53 VAC. (Should have used a
> fuse!!) My first attempt at this upgrade involved adding 2 2n3055, they
> shorted out at about 4 amp.
>
> It works great now with only half the supply voltage and I think it
> should work with full voltage because of the 2n3773s, though I'm a
> little unclear on the concept of 'secondary breakdown'. And I know the
> heatsinks may run a bit hotter but they're huge and well ventilated.

Secondary breakdown was in fact the first thing I thought of when you
mentioned what you wanted to do.

In short it reduces the allowable dissipation of power transistors when
operating at higher collector-emitter voltages. Regardless of temperature.

I assume you have the data sheets. See where the 'kink' is on the I vs V
allowable dissipation graph. 2N3773s are definitely better than 2N 30055s
in this regard though. They don't need derating for secondary breakdown
below 100V so no problem there.

Graham



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