Power supply recovery time after a heavy load much longer than expected

Hi,
I've been testing a class AB amp on a simple 150W toroidal transformer + bridge + 2x 10,000uF filter caps power supply. However, it seems to be taking a long time to recover after a short period of heavy load (approx 70-80% load for 5-10 seconds). It's in the region of 8-11 seconds before the voltage rises back up to the normal value for a minimal load condition (approx 100mA) and the ripple goes back down. Does anybody else have the same experience? If so, how did you improve the recovery time?

I suspect it's due to the transformer itself as the secondary VAC also drops and it seems to take the same amount of time to recover.

Here are some pictures. Blue trace is the B+ supply in question. First is under heavy load with 1kHz signal. Second is few seconds after 1kHz signal is turned off. Third is about 10s after the 1kHz signal has been turned off when it gets to the normal minimal load condition.

1.PNG


2.PNG


3.PNG
 
If the transformer had internal thermal protection it would be indicated on the label and/or data sheet, and thermal protection does not trip and recover in a matter of seconds anyway, so that's not the reason.
What you're seeing is the normal performance of a simple unregulated power supply. The recovery time is due to the capacitors being discharged at 1 KHz but only being charged at 120 Hz or 2x the line frequency.
If you need the output voltage to be more stable then a bigger transformer is needed, as you initially suspected. Larger capacitors will help some too, but the main cause is the transformer.
As long as the voltage drop is not causing the amp to clip with the loads you intend to drive, it's not really a problem. It's just the way these circuits work.
 
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Could the bias current of the amp be increasing drastically during the heavy loading (getting close to thermal runaway) and it is this loading the supply and causing ripple. After a few seconds the bias current manages to fall back down as the junctions cool.

Disable the bias generator in the amp and try the test again.
 
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Could the bias current of the amp be increasing drastically during the heavy loading (getting close to thermal runaway) and it is this loading the supply and causing ripple. After a few seconds the bias current manages to fall back down as the junctions cool.
I'd suspect this as well. The thermal compensation may not be well matched, although its never going to be perfect. There might be too much distance between the relevant sensing device and the relevant junctions.
 
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I doubt it's the bridge rectifier, if it were too small for the design it would just overheat and short out.

Does the power supply have some "soft-start" circuitry? I'm wondering if that gets activated during high output and then takes several seconds to recover.
 
Could the bias current of the amp be increasing drastically during the heavy loading (getting close to thermal runaway) and it is this loading the supply and causing ripple. After a few seconds the bias current manages to fall back down as the junctions cool.

Disable the bias generator in the amp and try the test again.
I think you're onto something here, I need to make some measurements to see what's happening to the bias after the signal is removed.