piclist 2002\09\30\195032a >
Thread: 2-tran SMPS 5v reg now 90% efficicient!
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face BY : Russell McMahon email (remove spam text)



> Hey Roman hows the high voltage version coming?
> I am in need of a smps that can take 200 - 300 DC volts input and deliver
> ~13.6Vdc out at 10 amps

I'd think really hard about using something else for a design of that
voltage and power level. I suspect Roman may agree. While Roman's design
appears superb it is still relatively untried and may yet have subtle
"gotchas"  (although it's hard to see what they would be)(because, if they
exist,  they are subtle :-) ).  Also at those power levels well proven
current limit with fold back and bullet proof operation under all
conceivable transient conditions is required. My 3 transistor design
operates at up to 200v in and 600 mA out in practice but I wouldn't
recommend it for the above application without considerable investigation.

Amongst other things, at high voltages the energy levels become non trivial
in the components linking the upper and lower sections of the circuit. As
the current requirement rises the drive requirement of the pass transistor
also rises again increasing dissipation. - especially the power required in
the resistor from lower transistor collector to upper transistor base. This
resistor's average (as opposed to peak) dissipation tends to stay constant
with increasing voltage because of the changing mark space ratio as voltage
is increased BUT beware what happens under eg short circuit conditions.
Depending on its design a current limit may protect the output but expose
this resistor to full dissipation with "interesting" results.

Even the zener resistor can become non trivial with increasing voltage as it
has to be "stiff enough" to provide reference voltage at lowest input
voltage and not die at highest input voltage. Addition of a current source
to drive the zener may be indicated. A useful alternative is to drive the
zener from both the input (for startup) and from the output (most of its
current once started). In this design this would require "tapping down" the
output voltage resistively to feed the emitter of the control (lower)
transistor so that the zener has some headroom. Nasty complexity for such a
beautifully simple circuit.

Use of a P FET* as the pass element can be very worthwhile here as it
greatly reduce drive power requirements - BUT may then need extra bipolar
drivers depending on switching speed and desired efficiency. I ended up
adding several bipolars to the basic design to improve performance over the
widest possible range of input voltages. This may not be needed if you can
optimise for a limited input range. A few extra small transistors cost very
little but you may wish to avoid them if the "purity" (and fun value!) of
the 2 transistor design is valued.

* 300 volt PFETS are about as rare as hen's teeth. The advantage of a buck
design is that you can operate the pass element almost at its rated voltage.
Especial care with switching transients is then indicated although the buck
regulator is very forgiving in this respect. .


       Russell McMahon

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