Hi all. We are testing a new design based on the ucc28070 controller, which is very similar to the PR779 1,2 kw PFC and having difficulties. On the first two tests the mosfets on one phase have blown up (very high input currents, large output voltage fluctuations with peaks of over 1000v and also sounds coming from the inductor). Each time the ucc28070 was also affected, as between pins 15 and 17 we measured a small resistance (22ohms aproximately). We were using MCP1407 to drive the mosfets, but now we switched to a 2-transistor buffer because the drivers also blew up.
On the third test we disconnected both mosfets and supplied power only to the ucc. we also supplied Vsense with 1,3V to keep the circuit running.
Without applying any voltage on the CSA and CSB pins, the duty factor was the programmed maximum (90%). When we applied voltage to CSA (0-3V) , the duty factor started to decrease with increased voltage.The decrease started at about 1,4V.
The VINAC pin seemed very sensitive as even a very small voltage (milivolts) caused the duty factor to go maximum and stay there regardless of the voltage being disconnected afterwards. We mounted 4,7nF capacitors on the VSENSE and VINAC pins. The threshold at which the duty factor started to decrease was not 1,4V anymore but 0,8V.
We observed that if we apply voltage to both CSA and CSB the threshold moved back to 1,4V.
The circuit seemed insensitive to VSENSE voltage variations up to 3V , value at which it went into OVP.
Another yet more important problem is that any AC voltage applied at VINAC caused the duty factor to go to maximum (shouldn't it vary with applied voltage, going up close to the zero crossings and down on the peaks?).
The values were calculated using ucc28070 excel design spreadsheet. We took a close look at the pcbs and there is no error there. The ucc was soldered with hot air in the minimum amount of time. We have experience with switchmode power supplies and designed the circuit with regard to the PR779 evaluation board layout. The inductors used were wound on 2 T200-2 cores each with a inductance of 130uH. Current transformers were wound on 43 mix ferrite cores at a 1:100 ratio.
Thanks in advance for any suggestions.
Regards
