How to calculate the Switching losses for MOSFET and Diode?

M

mady

Member

• Aug 15, 2015

• #1

Hello,

For MOSFET Switching losses:

How to calculate the MOSFET Switching Losses, If they are not given the Ton and Toff time in the datasheet? Is they any another formula?

Pswi = Vds x Id x Fsw x ((Ton x Toff)/2)


For Diode Switching losses:

How to calculate the Diode Switching Losses, If they are not given the Qrrd in the datasheet? Is they any another formula?

Pswi = Qrrd x Vrrm x Fswi

 

Attachments

• MOSFET Datasheet.pdf

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• Diode Datasheet.pdf

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Last edited: Aug 15, 2015

crutschow

Well-Known Member

Most Helpful Member

• Aug 15, 2015

• #2

If you don't know the switching speed that you can't calculate the switching losses.
If you use Schottky diode, the switching losses are generally negligible, only the forward loss needs to be calculated.

 

Last edited: Aug 16, 2015

alec_t

Well-Known Member

Most Helpful Member

• Aug 16, 2015

• #3

Ton and Toff will depend on how you drive the FET and what external impedances are associated with the FET. For example, a resistor is often used in series with the FET gate to suppress spurious oscillation, but introduces an RC time constant because of the inherent gate capacitance of the FET.

 

M

mady

Member

• Aug 16, 2015

• #4

Using MOSFET....

I have attached the datasheet too....

 

M

mady

Member

• Aug 16, 2015

• #5

Hello alec_t,

How to calculate the diode losses?

 

alec_t

Well-Known Member

Most Helpful Member

• Aug 16, 2015

• #6

The junction capacitance is specified, so knowing the reverse voltage you can calculate the stored energy which is lost per cycle. Probably negligible compared to If x Vf in the forwards direction, as Crutschow says.

 

crutschow

Well-Known Member

Most Helpful Member

• Aug 16, 2015

• #7

The Qrrd for the diode is basically C*V where V is the switching voltage across the diode and C is the diode junction capacitance.

 

S

simonbramble

Active Member

• Aug 19, 2015

• #8

You could try looking at his datasheet (page 14), but it gets ugly

http://cds.linear.com/docs/en/datasheet/3864fa.pdf