There are two types of losses in MOSFET. One is conduction loss and the other one is switching loss. Conduction loss is explained in detail here. Switching loss is not as easy as computing the conduction loss. There are many parameters to consider. You need to understand these parameters to select the correct values. However, you don’t need any more to be an expert in power electronics as I will guide you on how to compute MOSFET switching losses.
MOSFET switching loss is a complicated thing. Switching losses are contributors to overall power dissipation in a continuously switching circuit like SMPS, DCDC converters, chargers, and inverters to name a few. Switching losses are not happening at static condition like when the MOSFET is conducting. It is the losses associated during turn on and turn off the MOSFET. Design engineers’ techniques will vary from each other. In this article, let me share with you the technique I used on how to compute MOSFET switching losses.
What are the things to Consider on How to Compute MOSFET Switching Losses
There are three main contributors of MOSFET switching losses. The first is gate loss (due to total gate charge), second is output capacitor loss (due to COSS) and the third one is loss due to rise time and fall time. These are not standard nor the absolute correct considerations. These are just the things I am using in my designs. These are great references during design stage to size the MOSFET and select components. Optimization will take place when there is an actual sample for testing.
How to Compute MOSFET Switching Loss due to Gate Charge
There is energy needed to turn on the MOSFET from off state. For non-switching applications, this is not a concern because it will only happen during the first time on, of the MOSFET. However, for continuous switching applications, this amount of energy will be repetitive thus will result in power dissipation.
Gate charge loss is also related to gate capacitance. They are interchangeable, at least for me, and my derivations are based on this.
The energy needed to charge the gate capacitor is given below
Energy = ½ C x V2
Energy is also can be expressed as
Energy = Power / Frequency
Equating both equations will yield
Power / Frequency = ½ C x V2
Charge of a capacitor could be expressed as
Charge, Q = C x V
Expressing C
C = Charge, Q / V
Then substitute C to the equated equations above to give the gate charge power loss
Power loss gate charge = ½ x Qtotal x V x Fsw
Where:
Qtotal is the total gate charge specified in the MOSFET datasheet
V is the applied VGS (gate to source voltage)
Fsw is the switching frequency
Below table is how IRFR320TRPBF-BE3 (MOSFET from Vishay) datasheet specified the total gate charge. For worst case power loss, consider using the maximum value. For typical value, use the typical value instead. Sometimes datasheets will only indicate the maximum values though.
Gate charge loss is a function of voltage (VGS) and switching frequency (Fsw). This simply means that the applied VGS could not be exaggerated as it will result in higher gate loss. To guide you on what must be the proper VGS to apply, read How to Determine the Correct MOSFET VGS Threshold.
In the same manner, the switching frequency must be selected such that the gate loss is not too much that it will jeopardize the system efficiency and the MOSFET health.
How to Compute MOSFET Switching Loss due to Output Capacitance
The switching loss due to MOSFET output capacitance COSS is another complex parameter. It is difficult to derive the exact or even near exact value. However, there is a way to determine the possible maximum value which is good enough for worst case analysis.
The COSS will charge the moment the MOSFET is turned off. The energy needed during this time will cause power loss.
The energy of a capacitor is given in below equation
Energy = ½ C x V2
Energy is also can be expressed as
Energy = Power / Fsw
Both equations when equated will yield
Power / Fsw = ½ C x V2
Re-arranging the equation will give
Power loss output capacitance = ½ x COSS x V2 x Fsw
Where:
COSS is the output capacitance specified in the MOSFET datasheet
V is the drain to source voltage
Fsw is the switching frequency
The maximum power loss due to the output capacitance is maximum at the maximum value of the drain to source voltage. The choice of switching frequency is also a concern to minimize this switching loss.
The IRFR320TRPBF-BE3 MOSFET datasheet specified the COSS as below table.
Switching Loss due to Rise and Fall Times
The last thing to consider on how to compute MOSFET switching loss that I want to share is the one due to rise time and fall time. For some, this is analogous to the COSS loss thus no need to consider separately. In my designs, I still consider this as separate source of switching loss though.
Below is the equation of this switching loss.
Power loss rise and fall times = ½ x (trise + tfall) x Irms x V x Fsw
Where:
trise is the rise time specified in the MOSFET datasheet
tfall is the fall time specified in the MOSFET datasheet
Irms is the rms drain current of the MOSFET. This is a circuit value.
V is the drain to source voltage
Fsw is the switching frequency
Below is the rise and fall times specification of the sample MOSFET.
The total switching loss is the sum of all the individual losses. If the datasheet provides the typical values of each parameter needed, use it if your goal is to derive the typical switching loss value. On the other hand, If the objective is to get the maximum switching loss, the maximum parameters in the datasheet would be selected.
Summarizing How to Compute MOSFET Switching Losses
I hope this article can give you insights and guidance when you start your own design. Summarizing here the key points to consider on how to compute MOSFET switching losses.
1. Switching loss is happening during the time the MOSFET is either switching to on or to off state.
2. Switching losses are dependent on the switching frequency.
3. Gate loss is due to the total gate charge. It is dependent on the VGS aside from the switching frequency.
4. Output capacitance loss is due to the COSS. The maximum COSS loss will happen at the maximum drain to source voltage.
5. The total switching loss of a MOSFET is the sum of the total gate charge loss, COSS loss and the loss due to the rise and fall times.
6. The parameters needed on how to compute MOSFET switching losses; the total gate charge, COSS, rise and fall times are specified in the datasheet under dynamic characteristics.
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