MOSFET could be used as a switch or amplifier. Regardless of the application, there is a conduction loss. Let’s explore what is the meaning of conduction loss and how to compute MOSFET conduction loss. Conduction loss is the main contributor to MOSFET power dissipation when operating in a non-switching application. Even in switching applications, conduction loss is still very significant and must be considered carefully for a robust design.
What is a Conduction Loss
In MOSFET circuits, a conduction loss is a term given to the power loss or power dissipation that is due to the drain current and the on-state resistance. It is the power loss when the MOSFET is in on-state. Basically, when the VGS threshold requirement is achieved, the MOSFET will conduct, and the conduction loss will take effect. If you missed the article How to Determine the Correct MOSFET VGS Threshold, read here.
How to Compute MOSFET Conduction Loss Using Mathematical Approach
The parameters involved to compute the MOSFET conduction loss are drain current (ID) and the drain to source on-state resistance (RDSon). The equation for conduction loss is
Conduction loss = ID2 x RDSon
Where;
ID – drain current
RDSon – drain to source on-state resistance
The drain current is the current that flows to the MOSFET drain. There are two ways to compute the drain current. First is to consider the worst case. Second is just to aim at the typical value. The worst case drain current is important to consider so that the MOSFET will not over stressed. On the other hand, the typical value is more realistic to use in computing the efficiency of the circuit. The worst drain current will give the worst conduction loss while the typical drain current will give the typical conduction loss.
Worst Case Drain Current
To illustrate how to derive the worst case drain current or the maximum drain current, refer to simple MOSFET circuit below. To get the maximum drain current, neglect the voltage drop of the MOSFET M1 when at on-state. Consider the maximum value of the VDD. VDD is a power supply, and all power supplies have voltage variation. For instance, the voltage variation is 5%, this means that the VCC could increase to a level 5% higher than its typical value.
Typical value is the nominal or usual value or the designed value.
Then, consider the minimum value of the Rdrain. Rdrain is a resistor, and all resistors has tolerance. For instance, the tolerance is 5%, which means the resistance value could reach as high as 5% higher than the typical value.
Mathematically, the worst drain current is given by below equation
Drain Current Maximum = VDD maximum / Rdrain minimum
or
Drain Current Maximum = VDD x ( 1 + VDD tolerance) / [ Rdrain x ( 1 – Rdrain tolerance) ]
Considering the given values of VDD = 10V (+/-5%) and Rdrain = 100 ohms (+/5%), the maximum drain current is then
Drain Current Maximum = 10V x ( 1 + 0.05) / [ 100 ohms x ( 1 – 0.05) ] = 10.5V / 95 ohms = 0.111 A
Typical Value Drain Current
Unlike the worst-case method, typical drain current will only consider the typical value. It will not care about the VDD and the Rdrain tolerances. However, it will consider getting the typical MOSFET drain to source on-state resistance (RDSon).
The typical drain current is given by the equation below
Drain Current Typical = VDD typical / [ Rdrain typical + RDSon typical ]
To learn in detail how to get the typical MOSFET RDSon, read How to Get MOSFET Correct RDSon Value.
Supposing the typical value of RDSon is 0.5 ohm, then the typical drain current is
Drain Current Typical = VDD typical / [ Rdrain typical + RDSon typical ] = 10V / [ 100 ohms + 0.5 ohm ] = 0.0995 A
How to Compute MOSFET Conduction Loss – Worst Case Value
The worst-case conduction loss will be used to size the MOSFET such that it will not get damaged and to ensure a high reliability design. Below is the general equation of the conduction loss. Re-writing this to show the worst-case conduction loss in the next equation below.
Conduction loss = ID2 x RDSon
Worst Case Conduction Loss = ID maximum2 x RDSon maximum
Using above derived values for the drain current and the RDSon will result to
Worst Case Conduction Loss = ID maximum2 x RDSon maximum
Worst Case Conduction Loss = (0.111 A)2 x (0.5 ohm) = 0.0062 W
The value of the RDSon used here is just the typical value as assumed above. In your design, you can use the maximum RDSon specified in the datasheet to exaggerate the design margin. Based on the result here, the maximum conduction loss is 0.0062W. Thus, select a MOSFET that could handle more than this value.
How to Compute MOSFET Conduction Loss – Typical Value
The typical value of the conduction loss is
Conduction loss = ID typical2 x RDSon typical = (0.0995 A)2 x 0.5 ohm = 0.00495 W
If you are going after the realistic system efficiency, you should use this computed conduction loss. Typical conduction loss could also be used to size the MOSFET rating but should add more design margin.
For MOSFET working in non-switching applications, the conduction loss is already the total power dissipation of the MOSFET.
MOSFET Conduction Loss Summary
These are the key points on how to compute MOSFET Conduction Loss.
1. Conduction loss is MOSFET’s power dissipation during on-state.
2. Conduction loss is dependent mainly on the drain current and the drain to source on-state resistance.
3. The drain current could be worst-case (maximum value) or typical value.
4. The Conduction loss is could also be a maximum or typical value.
5. Maximum value conduction loss is used to size the MOSFET so that it will not damage when operating in the entire range.
6. The typical value conduction loss is more realistic in computing the system efficiency.
7. Typical value conduction loss is not considering the tolerances but needs the typical value of the RDSon.
8. The typical value conduction loss could also be used to size the MOSFET rating but with the added big design margin.
9. For a MOSFET in a non-switching circuit, the conduction loss is the total power dissipation.
10. All specifications and parameters needed on how to compute MOSFET conduction loss are written in the datasheet.
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