I tried to design a step up SMPS to create ~200-300v from a 24V input with an expected power of ~12W max. Since the project is for a hobby, I chose to attempt a hybrid boost-flyback converter to get the needed voltage gain with a reasonable duty cycle. I think i have a non-stable system at some output voltages, there is a low frequency ~7hz triangle wave of ~10V on my output of 250V. Other then that it seems to work fine with ~85% efficiency. When I tune it to a stable voltage (~50% of random voltages are stable), there is no low frequency ripple, only the expected ~1V output ripple at the switching frequency.
For compensation I made some assumptions (transformed the load / output voltage to primary side) and followed a typical boost compensation procedure. I attempted to follow suggested design steps in the PMIC (LM3478) datasheet. I added a low frequency pole but, apparently that did not work so well.
Basically I built a compensation network for ~48v output, with a ~1.5mH inductor, ignoring the fact it is a flyback. Any easy ideas of how to stabilize the output? I'm at the point to blindly play with the compensation network and see if it changes anything.
- \$ f_s=120kHz \$ ; \$ V_i = 24V \$ ; \$ V_o = 200V-300V \$
- Load is steady, resistive.
- Transformer is 2 mil air gaped, 52uH Primary, 1.8mH Secondary, 1:5.6 ratio.
Any ideas to better stabilize the system would be helpful... Unfortunately Controls was not my forte in collage.
Steps I have Tried:
- The controller uses current mode feedback as well as voltage, I overcompensated the current feedback to almost disable it per the data sheet, it had no effect. This is not sub harmonic anyways...
- I have tested with multiple loads ranging from 3W to 15W, low loads exhibit this effect slightly more.
- I tested this approximate design and transformer using a TL495 on a breadboard and it was successful and stable with no load, and no compensation network.