I am trying to understand the Power MOSFETs. I am wondering about the usefulness of the Ciss, Coss, Crss capacitances which can be found in the datasheets – aren’t they small signal capacitances? If so, how to deal with them in the case of MOSFET switching, which is connected with large signal behavior?
Thanks in advance
Electronic – Power MOSFET capacitances
capacitancemosfetpower
Related Solutions
You're being confounded by Cgd (same as Crss). Two main points:
- Cgd is very dependent on Vds.
- Cgd gets swept through Vds-peak + Vgs-peak. Not just Vgs.
First, look at Figure 11 and Cdg. Note that at Vds = 25V, Cgd is the tabulated 220pF. But what is it at 5V ... 700pF. And what is it at 1V ... ~8000pF. 18nF not looking so unreasonable now.
So, to get the 185nC number in the datasheet, as shown in Figure 10, you would have to integrate Cdg from 125V to 0V + Vgs-peak, and then add the Cgs contribution.
As Tom hints this is reverse voltage protection on the +12V line, however you may find it confusing for two reasons:
- The MOSFET is used 'backwards' when it is 'on'. Not a problem for MOSFETs, they conduct equally well in either direction. This is because the body diode must be reverse-biased when it is blocking negative input on the +12 input pin.
The MOSFET symbol is wrong - the part is a p-channel MOSFET, but an n-channel symbol is used. Try redrawing it with a p-channel symbol that includes the body diode.
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Best Answer
Yes the capacitances are small signal which is only a guide to large signal behaviour.Yes the capacitances are significant in powermosfets because after all they are comprised of millions of small signal devives called cells in paralell.The number of cells has gone up as chips became more densely packed.This means that modern low resistance powermosfets have much more capacitance than the early devices that were around in the late 1970s.In fact most powermosfets are optimised for low on resistance at the expense of capacitance these days.All the capacitances change with voltage which makes things much worse at large signals.Also the millar effect which is documented elsewhere can make gate source capacitance many times worse than the spec.Semiconductor manufacturers have realised this and state a gate charge spec which is more realistic when designing gate drive for a hard switched supply,and if you are going resonant the capacitance figure isnt too bad.