Why Parallel Transistor become Necessary
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 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 10.5V.
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 combine in parallel through 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 its begins to get warmer.
Therefore while connecting mosfets in parallel you do not have to worry about anything, and you may simply go ahead hooking them up in parallel, without depending on any current limiting resistors as shown below, except the gate resistor which must be included with each mosfet separately....although this is not too critical..