Stability Test for a Full bridge converter.

[abhinand rd]

Hello,
i came across this tech. document " https://www.ti.com/lit/an/snva364a/snva364a.pdf " which says we can conduct a stability test for power supply using oscilloscope and signal generator.
i wanted to know if this procedure can be applied at high power levels ~2kW.
How to choose the measurement resistor and how to find the low impedance node to insert this resistor?

 

[mtwieg]

You can perform a stability test like this, but you have to be careful where you inject the test signal. Basically you want it to be at at a node which sees a very high impedance in one direction. That document shows doing the injection between Vout and the feedback attenuator, which can work if the attenuator impedance is high. Another common place to do it is between the error amp output and the modulator (usually the signal is a lot "cleaner" there than the output), or even in the feedback network of the error amp. But there's no reason it doesn't work at high power levels.

As for the resistor, it's not really critical, it just has to be much less than the normal impedance of the insertion path, so that it doesn't affect the transfer function by being there. So if your attenuator impedance was 10K, then you would probably want much less than 100 ohms. But if Rs is too low your injected perturbation will be very small.

 

[abhinand rd]

Thank you

 

[abhinand rd]

i have certain follow up questions:
1) Isolation transfomer mentioned in this document is a standard 110V/12V one. my question is can we use this transformer for sweeping through frequency range of about 50Hz to 100kHz with signal amplitude between 30mV-100mV?
2) As an alternative, can i use ferrite core transformer made for kHz operation for sweeping the same signal through 50Hz-100kHz(30mV-100mV).
does the core get saturated at low frequencies?

 

[mtwieg]

The transformer does not have to be very broadband. Remember the method does not require an exact injection amplitude across all frequencies, you're just interested in the ratio of two nodes, so you always measure both simultaneously. Saturation should be avoided though, otherwise analysis can be tricky or misleading, always make sure your injected waveforms look sinusoidal or something is wrong.

A line power transformer might work okay, but I bet an audio step down transformer would have better bandwidth. I've also used small ferrite transformers in the past, but usually they cut off at low frequencies.

 

[abhinand rd]

Hii

thanks
I tried with a ferrite core transformer at low frequencies.. it didn't work.
I guess that they cut off.
let me use a line frequency transformer and check how things go.