In this post we will learn how to safely connect two or more transistors in parallel, these can be BJTs or mosfets, we will discuss both.
Contents
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 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 benefiting from both ends...transistors easily combine in parallel through the heatsink which in turn eliminates the current hogging situation, and gets rid of the bulky emitter resistors.
Connecting Mosfets in Parellel
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..
Anthony Brown says
I always thought that BJT’S had a POSITIVE Temp Coefficient and MOSFETS had a NEGATIVE Temp Coefficient.
Brian O Callaghan says
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
Swagatam says
Thanks very much Brian, I am glad you liked the post!!
alfreds bloggerprofil says
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!
Swagatam says
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
Swagatam says
yes, but why would you require parallel transistors for a relay??
Swagatam says
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.
kathiravan says
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 .
Anonymous says
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?
Swagatam says
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.
Anonymous says
you show 4 resistors in the picture, are they all the same low value protection resistors?
Swagatam says
It will depend upon the watt output taken from the transistors, the rating will increase proportionately with increase in the required output power
Anonymous says
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.
Swagatam says
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….
techs up says
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