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Connecting Two or More Transistors in Parallel

In this post we will learn how to safely connect multiple transistors in parallel, these can be BJTs or mosfets, we will discuss both.

Why Parallel Transistor become Necessary

While making power electronic circuits, configuring the power output stage correctly becomes very crucial.

These stages primarily may consist of power devices like the power BJTs or MOSFETs. Normally, single BJTs become sufficient for getting moderate output current, however when higher output current is required, it becomes necessary to add more number of these devices together. Therefore it becomes necessary to connect theses devices in parallel. Though using single BJTs is relatively easier, connecting them in parallel needs some attention due to the one significant drawback with transistor characteristics.

What is "Thermal Runaway" in BJTs


As per their specs, transistors (BJTs) need to be operated under reasonably cooler conditions, so that their power dissipation does not exceed the maximum specified value. And that's why we install heatsinks on them to maintain the above criterion.

Moreover, BJTs have a negative temperature coefficient characteristic which force them to increase their rate of conduction proportionately as their case temperature increases.

As its case temperature tends to increase, the current through the transistor also increases, which forces the device to heat up further.

The process gets into a kind of chain reaction heating the device rapidly until the device becomes too hot to sustain and gets permanently damaged. This situation is called thermal runaway, in transistors.

When two or more transistors are connected in parallel, due to their slightly differing individual characteristics (hFE), the transistors in the group may dissipate at different rates, some a little faster and others a little slower.

Consequently, the transistor which may be conducting slightly more current through it might start getting heated up faster than the neighboring devices, and soon we may find the device entering into a thermal runaway situation  damaging itself and subsequently transferring the phenomenon to the remaining devices as well, in the process.

The situation can be effectively tackled by adding a small value resistor in series with the emitter of each transistor connected in parallel. The resistor inhibits and controls the amount of current passing through the transistors and never allows it to go to dangerous levels.

The value should be appropriately calculated, as per the magnitude of the current passing through them.

How it's connected? See the figure below.


how to connect two or more transistors in parallel



How to Calculate the Emitter Current Limiting Resistor in Parallel BJTs


It is actually very simple, and could be calculated using Ohm's Law:

R = V/I,

Where V is the supply voltage used in the circuit, and "I" could be 70% of the transistor's maximum current handling capacity.

For example let's say if you used 2N3055 for the BJT, since the max current handling capacity of the device is around 15 amps, 70% of this would be around 10.5 A.

Therefore, assuming the V= 12V, then

R = 12/10.5 = 1.14 Ohms

How to Avoid Emitter Resistors in Parallel BJTs

Although the use of emitter current limiter resistors looks good and technically correct, a simpler and a smarter approach could be to mount the BJTs over a common heatsink with a lot of heatsink paste applied to their contact surfaces.

This idea will allow you to get rid of the messy wire-wound emitter resistors.

Mounting over a common heatsink will ensure quick and uniform sharing of heat and eliminating the dreaded thermal runaway situation.

Moreover since the collectors of the transistors are supposed to be in parallel and joined with each other, the use of mica isolators no longer become essential and makes things much convenient as the body of the transistors get connected in parallel through their heatsink metal itself.

It's like a win-win situation...transistors easily combining in parallel through the heatsink metal, getting rid of the bulky emitter resistors, a well as eliminating the thermal runaway situation.

easy connections of transistor in parallel by using common heatsink



Connecting Mosfets in Parallel


In the above section we learned how to safely connect BJTs in parallel, when it comes to mosfets the conditions become entirely the opposite, and much in favor of these devices.

Unlike the BJTs, mosfets do not have the negative temperature coefficient problems, and therefore are free from the thermal runaway situations due to overheating.

On the contrary, these devices exhibit a positive temperature coefficient characteristics, meaning the devices begin conducting less efficiently and begin blocking current as it begins getting warmer.

Therefore while connecting mosfets in parallel we do not have to worry much about anything, and you may simply go ahead hooking them up in parallel, without depending on any current limiting resistors, as shown below. However using separate gate resistors for each of the mosfets should be considered....although this is not too critical..


how to connect mosfets in parallel



Need Help? Please leave a comment, I'll get back soon with a reply!




Comments

  1. swagatam sir i am free lance electronic hobbyist too. I want to know from scratch of basic electronics to build a strong foundation for my knowledge purpose.please suggest some books or upload some tutorial so that i can learn from you .

    ReplyDelete
  2. Hi Kathiravan,

    I am myself planning to write a related book and sell the publication at cheap rates for the hobbyists here, presently there's no such info online which would enable a new hobbyist to grasp the facts quickly within 6 months or so, if i find any will let you know for sure.

    ReplyDelete
  3. yes, but why would you require parallel transistors for a relay??

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  4. Hi!
    The piece of information I cannot find anywhere is what the specs should be for the emitter resistor (that is resistance and max amperage).
    In my project I have two screwdrivers with controller, transistor and motor. I want to let one controller switch both transistors, put them in parallell and so regulate both motors from one potentiometer.
    I don't have the specs for the transistors, but know of course that they can handle full load from one motor each. The motors operates on 18 V and drops at max load to 15,6 V/18Amps.
    What specs should my emitter resistors have, and how does one dimension such in the general case. I cannot find this answere anywhere, and I highly suspect that i e electric car situations would require something rather different from low voltage/power circuit boards?

    Thanks!

    ReplyDelete
  5. Hi, In general the resistors must be rated such that even with a short circuit condition the resistors are able to restrict the current from exceeding the BJTs breakdown limit.

    for example if the breakdown limit of a tramsistor is 15Amp, we can consider 14 amp as the safe limit and then use Ohms law for dimesnsionng the resistor as given below, ....assuming 12V as the supply voltage:

    R = 12/14 = 0.85 ohms

    ReplyDelete
  6. Hello Hitman,
    Even by your standards this is an awesome post, particularly the idea of preventing thermal runaway by sharing heat sinks. I'm actually revisiting your solar tracker circuit and needed more power, so here i am. Keep up the good work,
    Brian

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  7. Thanks very much Brian, I am glad you liked the post!!

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  8. I always thought that BJT'S had a POSITIVE Temp Coefficient and MOSFETS had a NEGATIVE Temp Coefficient.

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  9. Please how can I combine two pnp and two npn transistors together in parallel

    ReplyDelete
  10. that's not possible, you cannot connect NPN and PNP in parallel

    ReplyDelete

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