For a "100v 25A Transistor," how can I tell how much current/voltage will be needed?

Question

I'm finding it hard to determine what the threshold is for a transistor to allow current through the semiconductor. Does it depend on the transistor? The description reads "100v 25A Transistor," but I assume those are the maximums for the transistor. How much current must be applied to allow that to pass through the transistor? I hope this is clear, I'm having a hard time articulating the question.

Answer 1

first you need to read a little about the transistor and understand how it works with gate, drain and source. it's like water channel.

you got a gate that controls opens the flow through the transistor :)



look at the transistor charateristic, V-I graphs etc

Answer 2

The figures given are what is called Absolute Maximum Ratings and if you exceed them may lead to failure of the transistor. You also need to consider the power rating for the transistor and the allowable temperature range. The temperature must allow for the effects of ambient temperature and the thermal resistance of the heat path which should include all variables such as extreme conditions (like heatsink obstruction when the user forgets to clean the dust channels).



As well, the Beta of a transistor falls with increasing current. At low current levels the transistor you are considering may have a Beta of 50-100 at low currents but this may fall to less than 10 at high currents. This is not usually a problem as it can be compensated with additional transistors or you may be able to use Darlington transistors (a transistor that already has an additional transistor built in).



Transistors also have a failure mechanism at certain currents and voltages known as secondary breakdown failure mode. In this mode, certain parts of the die (the actual piece of silicon inside the device) get hotter than the others. As a result, the hottest part of the die conducts the most current causing it to get hotter still until the device short-circuits internally. This occurs at lower voltages and currents than the absolute maximum values and prudent design makes allowances for this by derating. For more information, consult the design sheets for the device.

Answer 3

Assuming this is a bipolar transistor, they are most common.



You are correct, 100V/25 amps are the max ratings, and should not be approached too closely.



Assume you want to operate it at 20 amps. Look up on the spec sheet for HFE. This is the current gain. Let's say it is 50 minimum. Also check what current it is tested at, if not at 20 amps, we may have to look at some curves to guess what it would be at 20 amps.



So if HFE is 50, then base current is 20/50 or 400mA, which is the maximum current you need to get 50a in the collector.



For this discussion to move any further, I'd need to know a lot of detail, with the first item, is this a saturated application, like a switch or a linear application, like an amplifier.



Watch out for heat. In the example above, 20 amps and, say, 50 volts is 1000 watts, a huge amount of power that would require a very large heat sink.

Answer 4

Bipolar transistors conduct when you put current into them! There is a band gap voltage to overcome and for silicon the voltage needs to be over 0.7V. The amount that the transistor can conduct is given by the "gain" and this is the hfe value in the data sheet

Answer 5

Good idea. Either you're at the beginning of a blooming business, or your husband will never ask you to get a 'real' job again!