How to Select MOSFET Switch for Buck Converter

MOSFET is one of the commonly used switches in Buck Converter. It may be easy to select it but there are some parameters to consider on how to select MOSFET switch for buck converter. This article will focus on what are the parameters to consider. After reading this full article, you will be ready to select the right MOSFET switch.

Below circuit is a simple power section of a buck converter. The MOSFET switch is highlighted in yellow. To know how buck converter works, read How Buck Converter Works and Operates.

Parameters to Consider on How to Select a MOSFET Switch

1. RMS Current
2. DC Current
3. Peak Current
4. Drain Peak Voltage
5. Gate to Source Voltage
6. Power Dissipation or Power Losses
7. Thermal Managment
8. Thermal Resistance

Consider the RMS and DC Currents in Selecting a MOSFET Switch for Buck Converter

First to consider on how to select MOSFET switch for buck converter is the current. In a Buck converter, the current shape is trapezoidal. It has RMS and DC values. These current rating must not be exceeded. Otherwise, the device will fail. A good design is to ensure an allowance of at least 20%. For instance, the device rating is 100A, your actual application should not exceed 80A. This will result to 80% current stress (80A/100A = 80%). Thus, you still have 20% allowance from 100% stress which is the limit of the device. If you can select a MOSFET with even higher current rating and no cost impact at all, then go for it. The lesser the device stress, the longer its life. In a buck converter, the RMS and DC currents are not easy to derive since it involves trapezoidal waveforms. But don’t worry, read the article Buck Converter Switch Current Derivation (RMS and DC).

MOSFET datasheets specifies continuous drain current. Compare the computed RMS value to this specification and apply the margin recommended above.

In my personal choice, I always use the RMS value since it is higher than the DC value. In this way, I ensure I am considering the worst case.

Continuous Drain Current Specification

MOSFET Peak Current an Important Parameter to Select a Right MOSFET Switch for Buck Converter

MOSFET datasheet specifies pulsed drain current. In a buck converter, the current waveform is trapezoidal, and it is occurring in a repetitive manner. The peak of the trapezoid waveform must not exceed the pulsed drain current rating of a MOSFET. In most cases, there are current peaks on top of the trapezoidal peak due to parasitic elements. This must not exceed the pulsed drain current. A good margin is to set at least 20% allowance as explain above.

Pulsed Drain Current Specification

MOSFET Peak Voltage is a Key Parameter to Consider in Selecting a MOSFET Switch to Avoid Overvoltage Failure

Aside from current, another parameter to consider on how to select MOSFET switch for buck converter is voltage. MOSFET datasheet specifies drain voltage or drain to source voltage. The actual measured or computed peak drain voltage must not exceed this rating. Otherwise, the device will fail. A 20% allowance is good enough.

Drain to Source Voltage Specification

MOSFET Gate to Source Voltage

There are two parameters to consider. First is the voltage limit while the second is the threshold voltage. The voltage limit is simply the maximum gate to source voltage the device can withstand. The threshold voltage is the level of voltage needed for the device to guarantee turn on. The actual voltage applied to the MOSFET must be below the limit while high enough to guarantee MOSFET turn on. At least 20% margin from the voltage limit is a good practice. On the other hand, an allowance of at least 30% is good enough for the threshold voltage to ensure turn on.

Gate to Source Voltage Specification

Gate to Source Threshold Voltage Specification

Power Dissipation or Power Loss is a Must to Consider during MOSFET Selection to Avoid Immediate Failure

Power dissipation is equally important parameter as voltage and current and this also dictate on how to select MOSFET switch for buck converter. MOSFET power dissipation are due to conduction of the channel and the switching action. The former is also referred to conduction losses. The latter on the other hand is called switching losses. The total power loss is the sum of the conduction and the switching losses.

MOSFET datasheet specifies power dissipation. The computed power losses must not exceed this rating. At least 20% margin is a good number.

Power Dissipation Specification

Static Characteristics Relevant for Conduction Losses

Dynamic Characteristics Relevant for Conduction Losses

Thermal Management is the Key to Extend MOSFET life

In power electronics, the power dissipation or losses result to heat. This heat must be managed so that the power devices will not burn. Most common technique is to use a heat sink. In conjunction with the heatsink, cooling devices like fan or blower are also commonly used.

Both thermals and power dissipations are directly related. To know more details, read MOSFET Power Dissipation and Junction Temperature Calculation and Heat Sink Thermal Resistance Calculation Easy Explanation.

Thermal Resistance

The smaller the thermal resistance, the higher the MOSFET power capability. This is because the heat generated on the device is lesser. A smaller thermal resistance will also require smaller heatsink size and minimal airflow if using air cooling.

Thermal Resistance Specifications