Re: SMPS with two control loops

On Sat, 26 Nov 2005 15:13:08 +1100, Adam S <no@xxxxxxxxxxxxx> wrote:


>The current loop is stable, 0 to 25A at short circuit to open circuit,
>and I measured crossover point at 10kHz with about 50~80 degree margin
>(using transformer injection method), giving 1/10th the switching
>frequency as recommended by the experts. The current error amplifier has
>a flat gain after a zero located at 1kHz. This gives good current
>tracking at lower frequencies. All is well...
>
>The voltage loop is causing problems, I cannot get it stable at
>light/zero loads. Am I in a situation where I'm best to plug in various
>caps/resistor values around the voltage error amplifier until the thing
>just works ?
>

At light and zero loads, a voltage regulator runs into a brick wall,
unless it can absorb reverse energy, or (at least) cycle skip, without
the drive train going completely haywire. Magnetic and capacitive
couplers in thecontol loop may object strongly to this treatment,
producing unexpected bifurcations that aren't as pretty as
cycle-skipping.

Synchronous rectification is useful in this case, as current can be
drawn from the load, as well as being delivered to it, on a per-cycle
basis. Simpler methods involve applying a switched minimum load.

RL
.

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