High value current source design for thermal stability

jcx

Member
2003-02-17 7:38 pm
..
headroom/dropout V, output compliance V range, output impednace vs frequency requirements are also useful for design in addition to actual Tempco/power stability requirement number

LM317 cascoded with IXYS depletion mode 6 A power MOSFET may meet some sets of requirements
 
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Fusion916

Member
2012-02-27 5:02 am
headroom/dropout V, output compliance V range, output impednace vs frequency requirements are also useful for design in addition to actual Tempco/power stability requirement number

LM317 cascoded with IXYS depletion mode 6 A power MOSFET may meet some sets of requirements

I'm looking for a solution that only uses discrete devices.

Here is the scenario I'm trying to avoid for a circuit such what is attached (sorry for the crude drawing):

Constant bias being applied to the BJT, say 1V, and vbe at t=0 is 0.7, so the current is 0.3/R. And lets just say R is 0.3 for a 1A bias. I'm looking to avoid this:

BJT heats up > reduction of vbe > vbe/r current goes up > device heats up more > further reduction in vbe > etc

Then thermal runaway. Basically I'm looking for a circuit that has good tracking of the bias being applied to the BJT. I suppose I could mirror the current and thermal match, but that would add another power device. Looking to avoid that too.
 

jcx

Member
2003-02-17 7:38 pm
..
or 2 Q in "loop" feedback?


"ring of two" bjt feedback ccs uses the Vbe of the feedback/control Q - the heating of the pass Q only has an effect through h_fe tempco (or MOSFET Vth TC divided by loop gain)

pic below was just something I had on hand - hope you can parse pnp source version of the topologies

left most uses a higer Vref - 10 V Zener, and "Baxandal" or "complementary Darlington" - the Vbe nominally cancel - but you would have to bolt both to the same heatsink if large power were involved - and the tracking would have long thermal time constant

the other 2 are the "ring of two" - middle one cascoded
 

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