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I need to control a very high current in my project. I'm planning to use darlington formation with three transistors.

I'm planning to use BQ24450 to charge a group of 12V acid-lead battery.

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BQ24450 supplies only 25mA current to control the switching BJT transistor. Charge current will be variable up to 80A. I haven't decided for transistor model yet, but it looks like a darlington pair will not create enough current gain since power BJTs usually have low current gain. Therefore, I decided to make a darlington group with three BJTs.

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Is making darlington formation with three transistors practically okay? I have never seen it before in a professional design. Could it cause any problems (like leakage currents always keeping the BJTs on causing them to burn eventually)? If yes, are there any measures I can take?

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  • \$\begingroup\$ Can you find a BJT that will deal with 80A of collector current and 8A of base current? \$\endgroup\$
    – Scott Seidman
    Commented Sep 25, 2013 at 22:02
  • \$\begingroup\$ Keep in mind the voltage drop of the combined equivalent transistor. A darlington arrangement is not for for high current due to the voltage drop and therefore the large heat dissipation. \$\endgroup\$
    – Olin Lathrop
    Commented Sep 25, 2013 at 22:07
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    \$\begingroup\$ The real solution here is a DC-DC converter with a constant-current output. Trying to run 80A through a linear regulator is a generally bad idea, unless there are very, very specific reasons a switch-mode converter wouldn't work. \$\endgroup\$
    – Connor Wolf
    Commented Sep 26, 2013 at 6:13

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A darlington triple will drop about 2.1V across it at normal currents of up to 10A - I can see it being a bit higher at 80A, maybe around 3V - this means it will dissipate a power of 240W and this doesn't consider that the regulator is a linear type and there maybe as much as 5V constantly across the transistor (darlington or FET or single BJT).

This means a whopping great heatsink and a potential power dissipation of up to (and possibly exceeding) 400W.

May I recommend that you consider a switching battery charger using a big P channel mosfet - it might dissipate 50W in comparison.

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