Paralleled transistors driver

Hello,

I was using 3 paralleled 2N3055 to drive a transformer, of an inverter.
I think that those transistors were haunted or something, because, always the leftmost was getting very hot, the middle one was getting hot, and the rightmost was getting warm.

Everytime I took the leftmost transistor off, the transistor, which was the middle one, was getting very hot and the rightmost was getting hot.

Well, the leftmost was the one that always gets hotter then the rest.

Well, I tried some configurations, but none of them worked:


Do you have any clue?

I know paralleling bipolar transistors is not good idea, so I'll change to MOSTs.

There is a power supply project that used three 2N3055 transistors with 0.1 ohm emitter resistors and one guy had the same problem as you. Changing the value of the emitter resistors from the original 0.1 ohm to 0.33 ohm fixed it so he didn't have to replace the transistors with matched ones.

The transistor with the highest current gain hogs the load since they have a very wide range of current gain. Then the hogging transistor gets hot which makes it worse. Mosfets turn off a little when they get hot which helps to make paralleled ones share the current.

increase the emitter resistor as stated, or manually match the transistor betas. Otherwise, you're asking for thermal runaway.

!

audioguru said:

There is a power supply project that used three 2N3055 transistors with 0.1 ohm emitter resistors and one guy had the same problem as you. Changing the value of the emitter resistors from the original 0.1 ohm to 0.33 ohm fixed it so he didn't have to replace the transistors with matched ones.

The transistor with the highest current gain hogs the load since they have a very wide range of current gain. Then the hogging transistor gets hot which makes it worse. Mosfets turn off a little when they get hot which helps to make paralleled ones share the current.

Click to expand...

Thanks, Guru.

Yes, I forgot to mention I was using 0.33 ohms resistors on the emitter.

But the funniest thing is that always the more left is located the transistor, hotter it is. What a "beta" coincidence, huh?


Another funny thing is that this only happens when I use the 2N3055, when I use others like 2SC4106 or MJE13007, they heat almost the same.