HI,

i have designed a Boost Converter, that generates 250V from 5V input. It operates in DCM, because of high impedance output (5MOhm) and low output current (50uA). The Mosfet input signal has 40kHz frequency with 0.9 Duty Cycle.

The circuit works well, but the inductor current looks strange for the third interval. The current falls in the negative current region - usually DCM says it should stay zero. The peak inductor current is around 740mA and the minimum inductor current is -40mA.

For the simulation in Cadence i am using a real external diode model from Vishay (byg20j).

What can be the reason for the negative inductor current?
Its bad for the efficiency, but is there anything else i have to worry about?
How can i avoid this?

Thanks for your help!

Hi bki，

I guess it is caused by the real diode. Obviously its threshold is not zero. And I have similar problem in a buck converter, http://www.designers-guide.org/Forum/YaBB.pl?num=1403496724. It has oscillation in DCM mode because the inductor current is not exactly zero because the delay from cross-zero point to the time when the switch is turned off.

Best Regards,
Yawei