Thursday, December 1, 2011

Connecting Two or More Transistors in Parallel


How to Connect two or more transistors in parallel

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 transistors or MOSFETs.

Using power transistors are more common due to the greater ease of  configuring them. However when higher outputs are desired, it becomes necessary to add more number of these devices together.



And as per the rules, it becomes necessary to connect them in parallel. Though using transistors in electronic circuits is pretty easy, connecting them in parallel needs some attention due to the one significant drawback with transistor characteristics.

As per transistor specs, the devices needs to be operated under reasonably cooler conditions and that's why we install heatsinks on them to maintain the above criterion. Moreover, transistors have the "bad habit" of conducting proportionately rising currents through them as they get heated up.

Therefore if its case temperature tends to increase, the current through it also increases, which in turn heats it up further. The process may get aggravated until the devices become too hot to sustain and gets permanently damaged. This situation is called thermal runaway, in transistors.

When connected in parallel, due to non-consistent characteristics, the transistors in the group may dissipate varying amounts of current through them. Consequently, the transistor which passes more current through it starts getting heated up faster and pretty soon we find the device entering into the above thermal runaway situation  damaging itself and promoting the phenomenon to the remaining devices in due course of time.

The situation can be effectively tackled by adding a small value resistor at 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.






15 comments:

  1. nice circuit! I was looking at your 50 watt inverter circuit and if I used the bc547 and the tip127, what value could the resistors be?

    thanks,
    david

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  2. Thanks David!
    The entire parts list is included in the article, the resistors values are also given, so can refer it :-)

    Regards.

    ReplyDelete
    Replies
    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 .

      Delete
    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.

      Delete
  3. hi mr swagatam how can i calculate the emmiter resistor is it similar to the resistor base regards

    ReplyDelete
    Replies
    1. Hi Roger,

      They can be simply calculated using Ohms law:

      R = U/I, where U is the load/collector voltage and I is an approximate current level that's may be a little below the maximum tolerable collector/emitter current rating of of the transistor.
      Regards.

      Delete
    2. thank alot mr swagatam it helped!

      Delete
  4. hi again mr swagatam i have another question on paralleling two or more transistors lets say collector current is 30A and total power is 200 watts! my question is does both current and power dissipation raises or just one of them does? thanxs for your time! regards

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    Replies
    1. Hi Roger,

      Suppose we connect two transistors in parallel, each having a max collector tolerance of say 15 amps. We apply 25 amps, to them thinking that the current will be equally divided to safe 12 amps between each of them.
      However if the emitter resistor is not connected, due to a slight variation with the spec of the transistors, one of them will start passing more current, say 13 amps.
      Due to this it will start heating up more than the other transistor, this will lead to passing more current through it say about 14 amps at some instant, this will again heat it up even more so that it starts passing even more current until eventually the transistor fuses.
      When this happens the second transistor is without any support and therefore it also gets damaged within seconds.
      So its power dissipation which starts the problem and that needs to be controlled first.
      Regards.

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  5. Hi swagatam, i appreciate all your effort in making electronics easier for electronic enthusiasts. We stil can do more, i believe that. Pls sir, what is the minimum and maximum input current that can be applied to two identical parrallelly connected transistors rated say 30A each with equal current limitter resistors, and how are they going to share the current or will the same current going to flow through them following Kirchorf's junction law? And what formular could be used to calculate the minimum and maximum input current. Thanks man.

    ReplyDelete
    Replies
    1. Thanks!

      The current through the collector to emitter of the transistors will be unequal due to the differences in their characteristics and gain.

      The resistors connected at the emitters will not allow current more than a specified safe limit, in the process, the resistor which receives more current, becomes hotter than the other, and thus a current balance is maintained across transistors....

      Delete
  6. you show 4 resistors in the picture, are they all the same low value protection resistors?

    ReplyDelete
    Replies
    1. It will depend upon the watt output taken from the transistors, the rating will increase proportionately with increase in the required output power

      Delete
  7. I'm currently working on a radio transmitter and the schematic calls for a MRF947T1 Low Noise, High-Frequency Transistor. I have a bunch of old electronics I'm cannibalizing so parts aren't really that much of a problem. Is there a way I can wire up a bunch of the abundant generic Transistors to do the same job? Also, wouldn't a light bulb instead of a resistor help maintain stability more efficiently due to its ability to heat up? Just like an old fashioned oscillator circuit?

    ReplyDelete
    Replies
    1. I don't think that would be possible because RF transmitters involve highly sensitive configurations which require critical calculations and wiring. Using parallel transistors could easily affect the parameters making the circuit ineffective, so it won't work.

      bulbs might work but the modern circuits are not designed to work with them, so it won't suit them.

      Delete

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