Power transistors in parallel

[humanus]

I must connect 5 pcs. 2N 3055 in parallel in order to increase pass current in a variable power supply. I want to learn about the calculation of series emitter resistances. In practice I have seen 5 pcs. of 0.1 Ohm/5W resistors. But in another design there were 0.47Ohm/5W resistors. What is the ideal value to use?

 

[Mosaic]

The purpose of the emitter resistors is to equally spread the current load across the transistors. Based on your required current load u can clac the resistors using V=IR. the resistor power rating should be 2VI, where V is the voltage drop across the resistor. Use a small value to reduce power loss due to heat generation .

 

[humanus]

Thank you Mosaic. I of course know the purpose of these emitter resistances and Ohm's Law. But I wonder how many volts must be on the emitter resistance? For example , if we will pass 5 amperes from 5 parallel connected 2N3055, i.e. 1 ampere for each what must be the value of voltage on each emitter resistance? What is the ideal voltage falling on each emitter resistance?

 

[crutschow]

The required emitter resistor value is rather empirical, so there's no way to calculate the ideal value. It depends upon the variation in base-emitter voltage between units and how well the transistors are thermally coupled on the heat sink (all transistors should be on the same heat sink). A value of 0.1 ohm should likely be sufficient if the transistors are well thermally coupled.

 

[#12]

I would bet money that 1 ohm would be overkill, but I can't find any data sheets that show dVbe/dT or dVbe matching in a "lot" or "batch" of transistors. I think you will have to build it and try it out. I'd start with .47 ohms. Can you afford 1/2 volt of loss?

 

[Mr RB]

I've repaired a lot of old '70's linear supplies using stacks of 3055's with 5W emitter resistors. 0.1 ohm was pretty typical on commercial units, for larger current use, sometimes 0.47 ohm but that was less common.

Part of the secret is choosing sets of transistors with similar Vbe, you can measure that in a second with your multimeter on diode test and that one reading gives you a good enough idea for matching in sets.

If the resistor drops 0.15 to 0.2v at full curtrent that is high enough to give safe current sharing between transistors.