Other Parts Discussed in Thread: UC3842, OPA192, OPA237
Hello
I'm facing difficulties to check the stability margin with Bode 100 on a flyback built around UCC2801.
The flyback is in inverting step up mode with the following param :
Vin=19V...29V
Vout=-50V, Iout=30mA
DCM mode chosen with Duty cycle < 0.46 over the full operating range
The SIMPLIS schematics is the following:
The UCC2801 model is derivated from UC3842 and is not 100% correct. I modify some internal parameter to have a plant transfer function matching from my prototype to the simulation.
In the prototype, the op amp inverting the feedback is an OPA192 (final choice will be OPA237).
Lp=40µH, NS/NP=5
Fsw=115kHz
I performed the compensation to get a fc=2kHz and a phase margin of 70°.
Plant transfer function comparison
When I compare the measurement and simulation of the plant transfer function I get close results:
Compensator transfer function comparison
When I compare the compensator transfer function I get strange results with the BODE 100:
The phase is not in line with theory and simulation below 200Hz :
Loop gain comparison
And the loop gain is also different from simulation and measurements :
It's the first time I perform stability margin checking on a flyback in an inverting step up configuration.
I don't understand the phase on the loop gain below 200Hz. Is it a measurement issue ?
I think so because the transient is correct :
(yellow trace is the image of the 30mA load step).
Can you give me advice to check carefully the stability with this configuration ?
If needed, I can share my prototype schematics and my simulation file but it should be done via MP or email as it's confidential.
Thanks for your support
