# Bipolar pass transistor to Mosfet conversion

#### yourownfree

Joined Jul 16, 2008
99
I have a power supply a Kenwood PS-30. It usesd two 2n5885 transistors. I have seen other power supplies use 2n3772 transistors. In any case I would like to use power mosfets instead of bipolar transistors if it would be better. I know the mosfet wont be turned on all the way so cant take advantage of the low ohms there, so I guess my question is: Will a mosfet be better than a bipolar, running cooler and dissipating less heat? If so what kind of modification would I need to make? I am thinking of adding a resistor from the gate to ground and adjusting the gate voltage accordingly to obtain the proper output. But I could be way off here. What is your take on this? How can I use a mosfet as a pass transistor in a power supply or is it a bad idea?

#### Jony130

Joined Feb 17, 2009
5,211
In linear regulator MOSFET will dissipate the same amount of heat as a bipolar transistor. Because power dissipation is equal to P = (Vin - Vout) * Iload. And it does not depend on the type of transistor we use.

#### AnalogKid

Joined Aug 1, 2013
8,804
Without a schematic it's hard to say, but three things:.

1. This is a linear power supply, so the power dissipation in the pass elements will be *exactly* the same no matter what the pass elements are.

2. The drive requirements for a power MOSFET are significantly different from those of bipolar transistors.

3. There are significant differences between transistors designed to be used as high power switches (such as low cost power MOSFETs) and transistors designed to be used as linear amplifiers - gain flatness, safe operating area, etc. These are more factors complicating the substitution you are proposing.

ak

#### yourownfree

Joined Jul 16, 2008
99