The Ultimate Advantage of Power MOSFET over Power Bipolar Transistor

The Ultimate advantage of Power MOSFET over Power Bipolar Transistor

There are various discussions found on the Internet about the advantages of Power MOSFET over Power BJT.

MOSFETs are portable, use low power, draw no current and are compatible with silicon processing technology.

Science.com link

BJT will consume more power because it’s wasting current when it’s switched on. Also, the BJT generally has a 0.3v voltage drop in the input pin, and it takes a lot of base current to do that. MOSFET’s have high input impedance( goes infinity at lower frequencies). MOSFET’s are power efficient as the outputs are controlled by input voltage instead of input current. MOSFET’s can be directly connected in parallel but BJT can’t.

Quora.com link

Parametric differences between MOSFETs and bipolar transistors

EEtimes.com link

The Ultimate Advantage:

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

With the proliferation of battery-related applications such as electric vehicles, handheld equipment, mobile phones, portable devices, energy storage, energy loss becomes a key concern. Power MOSFET has been the ultimate 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.

Conclusion

The ultimate 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.

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