Power MOSFET Ciss, Coss, Crss test conditon understanding.

[parachutes30]

test condition such as: VGS = 0 V, f = 1 MHz, VDS = 25V

what does Vgs=0V mean? is it short - circuit or open - circuit of G and S?

 

[KlausST]

Hi,

"Floating" means the voltage is not defined.

V_gs = 0V definitely means G and S are short.

Klaus

 

[FvM]

The specification tells the measurement DC bias conditions.

To measure Ciss or Crss, you'll hardly short gate and source.

 

[KlausST]

Hi

True. I was too fast with my answer. I just read "open" and "short" .... and "open" was not true.
I didn't consider a third mode.

Klaus

 

[parachutes30]

The specification tells the measurement DC bias conditions.

To measure Ciss or Crss, you'll hardly short gate and source.

Ciss is the capacitance Cgs + Cgd, and I want to test it by my LCR meter.
So how can I do it? should I short-circuit D and S ? if i do this, obviusly I am short-circuiting the 15V power supply dc bias.

 

[KlausST]

Hi,

V_gs = 0V.....and as FvM correctly stated this is the DC operation point.
--> use a high ohmic resistor across G and S to ensure 0V DC.

But you want to test input capacitance:
--> connect a high quality foil capacitor from your ESR measurement device to G of the mosfet.
The capacitance need to be way higher than the exoected DUT capacitance to avoid influence in the measurement result.
If you take a 100 times higher capacitor...you should be safe.

Note: the AC voltage should be small enough to be below V_gs_th.

Klaus

 

[parachutes30]

Hi,

V_gs = 0V.....and as FvM correctly stated this is the DC operation point.
--> use a high ohmic resistor across G and S to ensure 0V DC.

But you want to test input capacitance:
--> connect a high quality foil capacitor from your ESR measurement device to G of the mosfet.
The capacitance need to be way higher than the exoected DUT capacitance to avoid influence in the measurement result.
If you take a 100 times higher capacitor...you should be safe.

Note: the AC voltage should be small enough to be below V_gs_th.

Klaus

so when I test Ciss , i short the D and S and the real test condition is : VGS = 0 V, f = 1 MHz,
and when I test Coss, I short the G and S and the real test condition is : VDS = 25V, f = 1 MHz,

 

[parachutes30]

After I contact with the support of a manufacturer. what i know is:
When test Ciss, LCR meter is across the G and S, Vds = 15V, and there is a big cap between the D and S to AC-short the D and S.
When test Coss, LCR meter is across the D and S, Vds = 15V, and G and S is really shorted.