How to calculate Mosfet Switching loss and conduction loss?

MOSFET Switching Losses

Can anybody tell me how i can calculate Mosfet Switching losses and conduction losses ? any reference ? and how it can be associate with smps. also if available diode losses. thank you



regards,
mengghee

MOSFET Switching Losses

It is hard to predict by simulation, since it is mainly depends on the layout. If you can do the RC extraction of the layout. You can do a better calculation for switching loss.

Re: MOSFET Switching Losses

conduction losses are given by
\[P_{cond}= I^2_{on} R_{ds,on}\]
where \[I_{on}\] is the drain current when the MOSFET is on and \[R_{ds,on}\] is the drain-source resistance of the MOSFET when it is on. To get the average value of the conduction loss, simply multiply the above expression by the duty ratio of the MOSFET.

Switching losses are due to the nonzero product of the drain current and drain to source voltage. If the MOSFET were an ideal switch the rise and fall times of the current and voltage would be zero and you would not have any switching loss.
\[P_{sw1} = \frac{t_{sw,on} V_{off} I_{on} f_{sw}}{2} + \frac{t_{sw,off} V_{off} I_{on} f_{sw}}{2} \]
where \[V_{off}\] is the drain source voltage when the MOSFET is off, \[t_{sw,on}\] and \[t_{sw,off}\] are the time to turn the MOSFET on and off and \[f_{sw}\] is the switching frequency. This could be simplified by assuming that switching on time and off time are equal, but this is not necessary.

There is another form of switching loss which has to do with the charging/discharging of the drain-source parasitic capacitor of the MOSFET. This is given by
\[P_{sw2} = C_{oss} V^2_{off} f_{sw}\]
where \[C_{oss}\] is the output capacitance of the MOSFET
(you should be able to find this in the datasheet).

Best regards,
v_c

MOSFET Switching Losses

Hi Mengghee;
I think in switching device (mosfet) we have 3 region to calculate power loss.
1- ON state loss
2- OFF state loss
3- switching loss

power loss in itmes 1 & 2 are simple to calculate.
as v_c said
P1= DF * I^2 * Rds on
P2= (1-DF) * V(supply) * I(leakage)
usualy P2<<P1 and we ignore it.

but power loss of P3 is difficult to calculate.
because apart of rise and fall times (tr, tf), type of output load and circuit behavior must be considered.
If we assume load is resistor it is simplest way.
in this case:
P3= (1/6) V * Ion (tr + tf)/T
switching power loss achived with average value of instantinus power V(t)*I(t)

Hello, can I ask something related to this? I also have this problem of calculating the switching loss of my MOSFET. From what is answered in this post, it may be represented by two formulas. One involving the rise times and fall times and one involving the Coss of the mosfet. Should calculations using these two formulas match? because when I do I find that the results are significantly greater. Which is the more accurate one? And should I expect the result from these calculations to match what I would get in an actual circuit?

You can use a simulation tool like PSpice to simulate and switching circuit and measure switching losses. However you need to have accurate MOS models, gate drive circuit for accurate switching loss. One need to be careful while drawing the simulation circuit and defining component to get close to actual switching loss. This is very much possible.

Would changing the gate drive affect the switching loss significantly? I think the formulas given in the previous posts focused more on the drain-source capacitance of the mosfet but not the gate capacitance. If for example I have a 4.7 ohm resistor in series with the gate of my mosfet (I've read that it can reduce noise), would my switching loss reduce if I remove the resistor (to allow larger charging currents) or replace it with a lower one? I can't seem to find a spice model for my mosfet so I am trying to focus on theoretical stuff so I can understand what's going on with my hardware

A resistor is series with the gate has no effect on the gate losses due to gate capacitance (it's always CfV^2 where C is the gate capacitance, f is the switching frequency, and V is the gate voltage), but it will have an effect on the drain-source switching losses if the resistor is large enough to slow the transistor rise and fall times.

Edit: Note that the gate power loss occurs in the gate driver circuit, not the MOSFET itself.

Re: MOSFET Switching Losses

v_c said:

There is another form of switching loss which has to do with the charging/discharging of the drain-source parasitic capacitor of the MOSFET. This is given by
\[P_{sw2} = C_{oss} V^2_{off} f_{sw}\]
where \[C_{oss}\] is the output capacitance of the MOSFET
(you should be able to find this in the datasheet).

Best regards,
v_c

Click to expand...

Here, Voff is consider as a peak voltage or Vrms?