Other Parts Discussed in Thread: UCC28070
Hello,
I am currently working on a PFC with following specs:
- 230VAC input
- 425V output
- P_total=8kW equally shared between two PFC-controllers (4 interleaved phases with 2kW each).
- f_sw= 65kHz
In my first setup, I ran 4kW on a single controller smoothly. Load jumps went smoothly and the overall performance was really good without making any adjustments from the excel sheet provided.
I now want to go up to 8kW with two UCC28070 in multiphase operation from pg 18 of the datasheet and I am running into some issues.
My setup is the following:
- 130kHz clock from a signal generator with 200ns pulse width, 180° phase shifted as recommended
- adjusted value of R_RT with factor 1,1 due to external synchronization
- adjusted value R_DMAX with factor D_SYNC due to external synchronization
- CDR to V_REF1/2, RDM to clock
- schematics follow pg. 18
- for shared signals I always equally divided resistors and capacitor values and placed them close to the ICs
(so eg. for V_SENSE, the lower resistor of the divider is now placed with (2xR and 1/2C at IC1 and IC2)
I was able to run 3 phases simulatenously, 4 give some instability issues. Theses occur in all phases simultanously, so the current sharing appears to be working fine. I have two theories and would like to know your thoughts or any other issues that you might be aware of.
1. What is the impact of external synchronization on current loop gain exactly? It is written on pg. 16 of datasheet:
It must be noted that the PWM modulator gain is reduced by a factor equivalent to the scaled RRT due to a direct correlation between the PWM ramp current and RRT. Adjustments to the current loop gains should be made accordingly.
So far I didnt make any changes to the current loop. Could you explain how exactly these changes are supposed to be made?
2. Does R_IMO need to be adjusted if working with two controllers? I accidently changed the value, and received very different behavior.
Would be very grateful for any ideas or advice
Thanks
Julia