I am currently using LT1370+ coilcraft NA5920, trying to step from +15V to 300V DC output. NA5920 is a 1:24 transformer.
However, I measured my output voltage ( I switched the ossiliscope to AC coupling), and I see very strong ripple. This ripple I can also see on the LT1370 VC pin. WHen I am driving the DCDC circuitry to output ~ 3W(300V 10mA), the VC voltage is ~ 1.6V, not 1.9V. I guess the current within LT1370 does not reach the saturation point.
I guess this is a kind of control loop problem. It seems to link closely with the output voltage. The higher voltage, the harder to control. I read LT1370's data sheet on the VC pins' resistor and capacitor section. And I chagned the resistor from 2K to 1K, even to zeros. Not useful at all.
I would like to ask the expert any clues on this output ripple. Please look at my schematic and PCBA.
1. Could that be of the 500Khz switching frequency?
2. Could that be part of the VC control loop?
3. Sometime, when the load is heavier, I can even hear the NA5920's sound(lower frequency sound). I guess the control loop also had some problems.
Application Note 39 may contain info that be of help. The datasheets of the LT3757 (page 30), LT3748 and LT8304-1 have reference circuits for similar application as well. I'd also recommend that you contact ADI local FAE to consult with.