# How to calculate swithching losses for a mosfet using in power supply

#### mohan.n2293

Joined Jan 23, 2017
60
Hi All,

Please provide a datasheet to know how to calculate the switching losses for the mosfet.

#### Sensacell

Joined Jun 19, 2012
2,933
The switching losses are the product of current and voltage applied to the device - integrated over the entire operating cycle.
Very hard to calculate theoretically, unless the waveforms are very simple.

What do you have as a starting point?

#### mohan.n2293

Joined Jan 23, 2017
60
The switching losses are the product of current and voltage applied to the device - integrated over the entire operating cycle.
Very hard to calculate theoretically, unless the waveforms are very simple.

What do you have as a starting point?
My Application is flyback converter please provide any direct formulas to get switching loss calculations for mosfet

#### ian field

Joined Oct 27, 2012
6,539
My Application is flyback converter please provide any direct formulas to get switching loss calculations for mosfet
Its possible that some semiconductor manufacturers supply application notes covering this topic, but I'm fairly sure there's no standard formula.

The main items are power due to RDSon and drain current which is fairly easy, the hard work bit is the switching transition where the chanel is changing from carrying current to blocking voltage. Flyback is even more fun because the flyback can be a few multiples of Vdd depending on the behaviour of the magnetics.

#### crutschow

Joined Mar 14, 2008
28,558
As Ian noted, the conducting MOSFET power can be calculated by the I²R of the MOSFET when ON times the duty-cycle of it's ON vs. OFF time.

The switching losses can be estimated by assuming a linear (trapezoidal) rise time.
The rise (turn-off) dissipation in watts is then approximately equal to the on-current times the peak turn-off drain voltage times 1/2 the rise-time times the switching frequency.
The fall (turn-on) dissipation is normally low for a flyback, because transformer inductance typically allows very little current to build up during the short fall-time.

For a better estimation, you can do a Spice simulation of the flyback circuit.

#### ian field

Joined Oct 27, 2012
6,539
As Ian noted, the conducting MOSFET power can be calculated by the I²R of the MOSFET when ON times the duty-cycle of it's ON vs. OFF time.

The switching losses can be estimated by assuming a linear (trapezoidal) rise time.
The rise (turn-off) dissipation in watts is then approximately equal to the on-current times the peak turn-off drain voltage times 1/2 the rise-time times the switching frequency.
The fall (turn-on) dissipation is normally low for a flyback, because transformer inductance typically allows very little current to build up during the short fall-time.

For a better estimation, you can do a Spice simulation of the flyback circuit.
Something I forgot to mention - using a fast MOSFET may not count for much if the gate driver struggles to drive the gate capacitance. Its a not exactly rare cause of MOSFETs running hotter than expected.

#### mohan.n2293

Joined Jan 23, 2017
60