I design a 4kW PFC with the control IC UCC28070.
I refer TI's application note,then design the circuit.
The problem is that the input current have zero crossing, that makes THD is higher than 5.5%.So I have try to let the input current's THD be decrease.
I have tried to improve the compensation components and the capacitor after the full-bridge. But it is no used.
The first problem is when voltage is small, duty cycle can not full open .So I measure the output of current amplifier output, finding that at zero crossing the voltage of current amplifier output can not rise.I think it is the main problem cause duty cycle can not open close to 100%.(I measure the demo board , but the result is same.Whether UCC28070 application have this problem?)
(The yellow line is the Iin , red line is the voltage of current amplifier output, blue line is the Vgs1 )
I also doubt another reason!
So I measure the current of two inductances and one of switch's Vgs,Vds.
I find at the zero crossing, both Vds and Vgs are zero .At that time, inductance's current would be reverse.
(The yellow line is the IL2 , red line is the Vds2 , blue line is the Vgs2 , green line is the IL1)
So I add a diode before the inductance ,hoping the diode can obstruct the reverse current.
When I add a diode,the input current's THD is decrease, it is good.But , when I operating at the full load 4kW , the circuit just can work only about 10 seconds.Then,the switch will damage.I have try to add the heatsink and the fan, it can let the switch live more long(But also about 20 seconds).
The next picture is the 4kW measure with diode.(The yellow line is the Iin , red line is the Vds1 , blue line is the Vo , green line is the Isw1)
All data looks good.But the switch still damage.I think whether is the switching capacitor hard switching cause switch overtemperature?
So,my main two problems are :(1). What the reason cause the duty cycle can not open close 100%?Is it can be improve?
(2).Whether have another good method to avoid switch damaging?
