In a previous post I mentioned a small DC-DC converter using 12 VDC and an IRS24531 driving two dual MOSFETs FDS6930B and two custom built ferrite transformers with an E187 core. I tested one set of transformers that I built, which have about 348 uH inductance and 25 H leakage inductance. The ones in this board have about 500 uH and only 2 H leakage.
I wanted to see the current draw of the circuit, so I applied 12 VDC, and the current shot up to 200-400 mA with no load. I was able to get it to run at about 10 VDC with only 20 mA draw. The other board with my transformers ran fine at 12 VDC. Here are some waveforms:
10 VDC, good waveform:
About 10.2 VDC, peaks on transitions:
At 11 VDC, unstable transitions and high current draw:
Output at 10 VDC:
Output at about 10.5 VDC, getting ugly:
Output at about 11 VDC:
This didn't make much sense, but then I observed that the frequency was about 22 kHz, and it is supposed to be about 65 kHz. The resistor is 10k and the capacitor should be 33 nF. According to the spec sheet, the period of oscillation is 1.453 *RC so that would be 479 uSec or 21 kHz. Apparently I neglected to change the resistor to about 3k.
I changed to 3.01k and frequency is 70.7 kHz. Works fine at 12.2V and 13.3V.
You can see the 1 uSec dead time:
So, found the problem, solved it, and all seems OK. Total current draw of the entire PCB at 13.3 VDC is about 10 mA. It draws about 750 mA when initiated, which is about as expected.
Paul