Other Parts Discussed in Thread: UCC28517,
I’m currently working on a battery charger implementing a two-transistor forward converter with PFC front end using TI’s UCC28517 dual controller and UCC27714 high side low side driver. I have experienced some catastrophic failures of the forward stage which on further investigation appears to be a result of latch up of the UCC27714 outputs. I have seen many instances of the UCC27714 High side driver latching in the high state with the low state driver latched in the off state, however, I have yet to capture a high and low side dual high state latch event due to the catastrophic result and difficulty of capture. On increasing the value of the gate drive resistor from 3R3 to 10R I have managed to reduce the instances of failure, which points to a dv/dt problem, but it doesn’t cure the problem and increasing the value further will push the converters efficiency out of spec. I have followed all of the UCC27714 application note guidelines (use of Schottky diode clamps, VDD filter, layout etc) but the problem remains.
The two-transistor forward converter takes the HS pin below COM ground but the datasheet claims -8V minimum compliance so this shouldn’t be a problem?
My design configuration is as follows:
Operating frequency: PWM 260kHz PFC 130kHz
Bulk Voltage: 400V ±20V (failures have occurred below 120Vdc)
HS/HB supplied from separate isolated 16V AUX supply (Bootstrap not used)
Switching FET gate pull down: 10k
Power FET gate resistor: 10R (3R3 increases failure rate)
VDD filter: 4R7 with 1uF and 100nF ceramic
PWM Output of UCC28517 driving both HI and LO inputs through separate 51R resistors and 100pF capacitors
VREF of UCC28517 driving the enable pin of UCC27714
Switching FET’s IPW60R080P7
TX clamp diodes: DESI8-060AS
TX magnetising current: 180mA
Layout points of note: UCC28517 located approx. 50mm away on daughter board, UCC27714 located across the two-switching power FET’s (as close as possible) gate source drive direct across FET pins
Are TI able to provide any further guidance on preventing the latch-up of UCC27714 outputs?
Are TI aware of the successful implementation of the UCC27714 in two transistor forward converters?
Thank you