I read somewhere that connecting transistors in parallel will increase the current handling capability of the circuit when used as a switch. Can anyone explain why? Also, what happens when you connect them in series?

 

If you connect two battery in parallel the current will increase and voltage will be same ( when both are eg. 12V 12V each)
when they are in series the voltage increase ( eg. 12V +6V = 18V) but current will remain same..!!

 

With transistors in parallel each one can conduct up to its allowable current so total current is the sum of capability of the transistors connected in parallel.

Rocket science.

 

In principle, transistors can be used in parallel to increase current handling, or in series to increase voltage handling.

In practice, both these arrangements have their pitfalls, the series connection being really quite hard to realise. It was used for instance in the line deflection circuits of some TV sets at a time when high voltage transistors were hard to manufacture. Unfortunately, the base drive to the series connected transistors required critical adjustment to make them switch at the same instant, sharing the voltage evenly. If this was not achieved, one or other transistor would be overstressed, and in time both would fail.

The connection of transistors in parallel for higher current handling also requires the transistors to be driven on and off at the same time, but transistors may be more tolerant of a momentary over-current. Generally, the difficulty is more a question of current sharing, which may be easier to achieve with MOSFETs than it would be with bipolar transistors.

A MOSFET tends to get less conductive as it heats up, but the reverse is true for a BJT. There is a danger therefore that any one of a set of parallel BJTs may tend to run away with the lion's share of the current: more current = more heat = more current = more heat......BOOM!

 

BJTs may tend to run away with the lion's share of the current: more current = more heat = more current = more heat......BOOM!

Having worked with switch mode power supplies in the 80's with numerous parallel transistors I can tell you from first hand experience the do NOT go Boom. It sounds more like a sub machine gun:

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In order for a BJT transistor to work, you must exceed the Vbe of that transistor. Vbe is an indeterminate value somewhere around 0.65v.
When any number of transistors are connected in parallel, the one with the lowest Vbe will begin to conduct first. It will take most of the current causing it to heat more which lowers its Vbe which causes it to conduct more which causes it to heat more blah blah blah. Vbe drops around 1.8mv/deg C. BJT's can be used as a thermometer.

MOSFETs are the opposite, they self regulate because their on resistance increases with temperature, lowering dissipation forcing the rest of the MOSFETs in parallel with it to share a bit more of the load.

 

Parallel connection of BJTs is possible, but a number of precautions have to be taken if this is going to be successful.

One of the most basic things to do is to de-rate the transistors, i.e. to assume a lower current rating, and so use more in parallel. It can also be helpful to use transistors which have been selected for consistent performance or "matched", but this may be costly and inconvenient. Mounting the transistors close together on a common heat-sink helps to keep their case temperatures as close as possible, but unfortunately internal temperature differences can still occur.

One effective way of improving the current share is to put a low-value resistance in series with each emitter. This is quite commonly done in audio amplifiers. This wastes power, but may be better than seeing the system go up in smoke.