I'm trying to debug a pair of cascaded DC/DC converters and have ran into a brick wall. The local FAE said that it is probably something to do with "Negative Input Inductance" on the second converter messing up the stability of the first converter (but the FAE isn't around to "help" more). The problem is that I can't find any app notes, papers, books, etc. on this issue.
My question is: Do you know of any literature on such issues? Or better yet, some ideas on things to try or look at?
Here's my setup...
Converter 1: +4v to +12v@1 amp out boost converter. Switching frequency is about 350 KHz. Converter 2: This is actually a 10-ish watt Class-D audio amp (which is basically a switching buck converter). Switching frequency is about 310 KHz.
And the problem...
Converter 1 works fine with a resistive load instead of converter 2. It even works if the resistor is switched in/out at audio frequencies.
Converter 2 works fine when powered from a benchtop power supply.
When Converter 1 is feeding Converter 2, C1 will shut down due to over-current through the MOSFET. It shuts down easier if the audio frequency is lower. Above a 1 KHz sine wave it seems to work fine. When it shuts down, the power output is only about 50% of what the converters are able to do separately.
Update: I found the problem.
There were two bugs...
Basically, Olin was correct. I did a miscalculation. The first converter should have been able to supply twice the current that it was providing. Instead of +12v at 1A, we needed 2 amps.
Converter 1 is a current mode converter-- meaning that it has a current sense resistor between the MOSFET and GND. It appears that the PCB traces and vias for this signal path was not up to the task. I tried several resistors in the 4 to 24 milli-ohm range, but suspect that the traces/vias were adding another 5 or 10 mOhm. The end result is that we were overcurrenting sooner than we wanted to.
In the debug process, I isolated Converter 1 from the rest of the circuit and tweaked it to provide a solid 2 amps into a resistor load. Once it was solid, I connected it back up to the audio amp and it worked fine under all expected loads and audio frequencies.
So, apparently, it had nothing to do with negative inductance or whatever.
For being a mostly-digital guy, I sure am getting a lot better at analog stuff! :)