RC-Monster Forums - View Single Post - Linear Regulator using FETs for pass device

BrianG · 06.06.2009, 01:05 AM

I didn't look at the specs of your linked FET, but it looks like a typical FET and those are meant to be used in switching applications. They typically don't have good performance when used in linear applications due to the hot spots on the die (despite their high rated power dissipation). And the proper FETs are expensive, so the FET idea is dead. Although, I might be able to wring a little more efficiency out of the supply at lower currents by switching the recified output of the transformer by using FETs at the switches, but not sure if I want to go there. I got these transformers for free so I don't want to spend a lot of cash.

As far as bipolar transistors go, I aloso have to have a decent hfe (current gain) value. At high currents, hfe goes down, so I need to make sure the gate current will be fairly low. Otherwise, I would need another transistor in a darlington configuration to get more gain. But each stage introduces more drop out voltage. so I'd rather not do that. I plan on using an LM317T at the actual regulator, so I should only need one stage.

Anyway, the BGMicro store is not my store (I wish it was!), but they have components at FAR better prices than RadioShack.

So, I'm looking at using 10 PNP transistors, each with the following specs:
- 40V Collector-Emitter voltage min
- 10A min collector current (like to have a little headroom)
- Current gain of at least 20 @ 10A (will be higher at lower currents, but I like to spec for worst case)
- 50w power dissipation or better.

	(#15)
BrianG RC-Monster Admin Offline Posts: 14,609 Join Date: Nov 2005 Location: Des Moines, IA	06.06.2009, 01:05 AM I didn't look at the specs of your linked FET, but it looks like a typical FET and those are meant to be used in switching applications. They typically don't have good performance when used in linear applications due to the hot spots on the die (despite their high rated power dissipation). And the proper FETs are expensive, so the FET idea is dead. Although, I might be able to wring a little more efficiency out of the supply at lower currents by switching the recified output of the transformer by using FETs at the switches, but not sure if I want to go there. I got these transformers for free so I don't want to spend a lot of cash. As far as bipolar transistors go, I aloso have to have a decent hfe (current gain) value. At high currents, hfe goes down, so I need to make sure the gate current will be fairly low. Otherwise, I would need another transistor in a darlington configuration to get more gain. But each stage introduces more drop out voltage. so I'd rather not do that. I plan on using an LM317T at the actual regulator, so I should only need one stage. Anyway, the BGMicro store is not my store (I wish it was!), but they have components at FAR better prices than RadioShack. So, I'm looking at using 10 PNP transistors, each with the following specs: - 40V Collector-Emitter voltage min - 10A min collector current (like to have a little headroom) - Current gain of at least 20 @ 10A (will be higher at lower currents, but I like to spec for worst case) - 50w power dissipation or better.