I would like to ask what is causing the following phenomenon. I've spotted this on my real buck converter at a low duty cycle first, then tried in my SPICE simulation as well which confirmed this.
I've set V2 to 50kHz and 1% duty cycle. The FET on time should be 200ns at this point. But the transient analysis shows this:
Voltage pulse at V2 really takes 200 ns but voltage Q1-D roughly 600ns! Interesting is that the voltage Q1-D rises almost instantly at turn-on but takes a long delay at turn-off.
Datasheet for given FET says:
- tdon 22ns
- tr 99ns
- tdoff 110ns
- tf 92ns
These are for slightly different test conditions but that should not be the matter here. At least not the reason for such a long turn-off time. For these times I would expect the Q1-D pulse to last ca 300ns instead of 600ns.
Is there some additional restriction about switching MOSFETs on for a very short time?
Note: This simulation has been done in Simplis Elements 8.1. In real application I use a different MOSFET even with shorter datasheet times but the off-delay is even longer.