How to choose a Power MOSFET?

How to choose a Power MOSFET?

Why do you want to use MOSFET (over BJT) in the first place?

You are probably using MOSFET in the portable battery-operated applications which require a very low power loss.

First of all, need to understand the application for Power MOSFET.

With the proliferation, battery related applications such as automotive ABS, handheld equipment, mobile phones, portable devices, and energy loss becomes the key concern.

Power MOSFET has been the popular choice

Energy (Power) Loss

In the battery-related applications, transistors are used as a switch or a driver.

The power loss of a Bipolar Transistor is mainly from Vce (sat.), the saturated voltage driven by high base current, typically 0.2 volts.

Power loss is equal to the current flow x Vce (sat). However, Vce cannot be reduced.

On the other hand, the power loss of a MOSFET is mainly from the Rds(on), the on-resistance of the MOSFET.

Power loss is equal to the square of current flow x Rds(on). If we can reduce the Rds(on), we can save the power loss.

If resistors are connected in parallel, the resistance will be reduced. MOSFET can be connected in parallel to reduce the Rds(on) in a similar fashion. However, in a portable application, there may not be enough space to fit many Power MOSFETs.

Manufacturing Process of Power MOSFET

The manufacturing process of Power MOSFET is very similar to DRAM. You can pack millions of DRAM cells in a square inch to increase the memory capacity. In a similar fashion, you can pack millions of MOSFET’s in a square inch to reduce the Rds(on), on-resistance.

In 1994, National Semiconductor was able to put 5 million MOSFETs in a square inch to produce MOSFET with very low on-resistance.

There are mainly 4 parameters in the selection of Power MOSFET:

1/ Vds (the breakdown voltage)

if Vds is higher than a few hundred volts, you have to use IGBT instead. MOSFETs are mostly below 100v Vds.

2/ Rds(on)

the lower, the less power loss, critical for the battery-operated applications.

3/ Space

if space is allowed, several MOSFETs can be used in parallel to reduce the Rds(on).

4/ Price

if space is limited, you may not have the luxury to parallel several MOSFETs to get a low Rds(on). 1 single MOSFET of low Rds(on) will be much more expensive than several MOSFETs in parallel to achieve the same Rds(on) value.

Conclusion

The key advantage of Power MOSFET over Bipolar Transistor is the ability to reduce power loss in the circuit with limited space available.

The author was the global business manager of the Discrete division of National Semiconductor in Silicon Valley in the late ’80s.

You can now buy some MOSFETs on the 1N4148.com site.