bjt in parallel operation

[nanotech1]

can someone help me

i need info on how to operate bipolar transistor in parallel
for higher current output

is there any rule of thumbs
on the designing sutch circuit without blowing the bgt's all together

i want to build my own high power rf generator for induction heating

any help appreciated

 

[Larry Brasfield]

can someone help me

i need info on how to operate bipolar transistor in parallel
for higher current output

is there any rule of thumbs
on the designing sutch circuit without blowing the bgt's all together

I'm not sure if this is the rule-of-thumb you are looking for,
but you need to either degenerate with emitter resistors
(simple) or control relative base currents (harder) so as
to avoid having one transistor conduct most of the current
due to its Vbe getting lower because it is hotter because
it is conducting more current. (You want to get the gain
of that partially thermal feedback loop enough below 1
that the parts share current about equally.)

i want to build my own high power rf generator for induction heating

any help appreciated

The current hogging problem is almost nonexistent with
MOSFETs because of their positive TC of channel R.
For similar reasons, they are far less prone to second
breakdown effects too. I would look into using them.

 

[Active8]

" || "

I'm not sure if this is the rule-of-thumb you are looking for,
but you need to either degenerate with emitter resistors

Something I've *always seen* in || bjt apps.

(simple) or control relative base currents (harder) so as

Show me an example of one working circuit using this approach.

to avoid having one transistor conduct most of the current
due to its Vbe getting lower because it is hotter because
it is conducting more current. (You want to get the gain
of that partially thermal feedback loop enough below 1
that the parts share current about equally.)

At least you hinted that *Vbe required to maintain constant I_C*
drops with temperature instead of talking about DC beta increasing,
but that thermal feedback loop thing may be just a bit much for the
OP. We're in seb now.

But the OP can google on Ebers-Moll and Gummel-Poon for a
not-too-sadistic explaination of V_BE/V_T and all that.

The current hogging problem is almost nonexistent with
MOSFETs because of their positive TC of channel R.

Except in the sub-threshold region.

For similar reasons, they are far less prone to second
breakdown effects too. I would look into using them.

I would google on induction heater mosfet and induction heater IGBT.

IGBT applications induction heater

MOSFET applications induction heater

IGBTs seem to be cropping up in those things lately.

http://www.fairchildsemi.com/products/discrete/ind_heating.html

http://www.dansworkshop.com/Induction Heating.shtml