This reminds me of an interesting problem I recently had.

I had a circuit which contained a 48V to 12V switcher, from which
several other supplies were derived. Since it was a new design, I
decided to bring it up slowly with a bench power supply set to a
current limit of something like 200mA. Everything was working fine
until I tried to turn on one additional section of the circuit (I
think it was an FPGA if I remember). Every time I would do that, the
whole thing would suddenly turn off. My initial suspicion was that the
FPGA was doing this (since it had active control of the the power
on/off circuit). However, it turned out ultimately that what was
happening was that the FPGA put the input current JUST a hair over
200mA, causing the power supply to drop the output voltage a bit.
However, this had the effect of increasing the input current drain -
which became a runaway positive feedback which dropped the voltage to
0V long enough to shut down the circuit, but short enough that it
wasn't even noticed on the power supply display.

Sean


On Dec 5, 2007 1:33 AM, Richard Prosser <rhprosser@gmail.com> wrote:
> I'm not sure what you mean about simulating a constant power load in real
> life. In real life you're not simulating.
> It is pretty easy to simulate a constant power load in pSpice. In real life
> - how about a wide  input range DC-DC feeding a constant load?
>
> RP
>
>
>
> On 05/12/2007, Xiaofan Chen <xiaofanc@gmail.com> wrote:
> >
> > Long ago, I was simulating some DC/DC converter
> > circuits and it is easy to simulate a constant
> > power load in PSpice.
> >
> > In the real life, it is relative easy to simulate a constant
> > current load using current source circuit. It is also relative
> > easy to simulate a constant voltage load (battery).
> > But how to simulate a constant power load (low power,
> > say around 1W-10W)? So when you inject a change
> > in the DC/DC converter, the output voltage will change
> > in one direction but the load current will change in the other
> > direction.
> >
> >
> > Xiaofan
> > --
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