PMP21455: PMP20708

Hi,

PMP20708 uses Q1 and R7 to form a current source to supply UCC28881 to keep it operating. From the UCC28881 datasheet, we will need to supply up to 120uA current to VDD. The current from Q1 and R7 is determined by |Vgs(th),Q1| / R7. Assume Vgs(th),Q1 is -1.6V, we have a supply current of 176uA.

And the maximum input voltage of VDD is clamped to 7.5V. The minimum input voltage to allow Q1 & R7 current source works as expected can be calculated from the equation: VDD clamping voltage + Vgs(th) + Q1 drain-to-source voltage drop when conducting current. Theoretically, With 12VDC input, the controller should already operate. So yes, it should work with 19VDC input. But the on-time will be closed to UCC28881 limitation with current transformer design in PMP20708 with 19VDC input and 12V, 0.1A output.

Regards,

Sheng-Yang Yu

I expect the IC temperature should be at similar level with 12V, 0.1A output and 19Vinput. You can allow more copper on the trace around the IC or even apply heatsink to the IC if you have concern.

Answer to question 1: Yes, you can use optocoupler feedback instead of feedback from the auxiliary winding.

Answer to question 2: Yes, UCC28881 allows both CCM and DCM operations. So you can increase the transformer inductance. However, it will likely increase the transformer size.

Answer to question 3: If you want to use non-isolated topology to deliver power to 12V, 0.1A output, it is also possible. However, you have to make sure the converter is operates under DCM as the maximum duty cycle of UCC28881 is ~50%. 19Vin to 12Vout will require >50% duty cycle in CCM operation. Also, need to make sure UCC28881 peak current is below its spec so you can deliver full output power. 

Yes, PMP20708 is operating under DCM for the entire range. While PMP10098 will operate under CCM when input voltage is low.