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.