Author Topic: Transistor question  (Read 1861 times)

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Online neo

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Transistor question
« on: October 09, 2016, 12:37:51 pm »
How is their rated current measured, i think if a transistor can do 5 amps at 60 volts it should be able to do 25 amps at 12v, am i wrong?
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Offline IanB

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Re: Transistor question
« Reply #1 on: October 09, 2016, 01:03:06 pm »
How is their rated current measured, i think if a transistor can do 5 amps at 60 volts it should be able to do 25 amps at 12v, am i wrong?

Yes.

A transistor may be able to handle 5 amps, and it may be able to handle 60 volts, but not at the same time.

If a transistor were working in linear mode and passing 5 A with a voltage drop of 60 V it would be dissipating 300 W. That would be a very, very beefy transistor (and would need a huge heat sink with a fan). Is that what you mean?

Transistors are mostly used in switching applications these days. When they are switched on they can pass a large current with a low voltage drop (= little power dissipated), and when they are switched off they can block a significant voltage with no current flowing (= no power dissipated). If they spend too long in the middle they will generally explode.

When you consider transistor ratings, you cannot just say "rated at". You need to look at and quote the specific entry in the data sheet for the rating of interest.

« Last Edit: October 09, 2016, 01:04:44 pm by IanB »
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Online neo

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Re: Transistor question
« Reply #2 on: October 09, 2016, 01:06:23 pm »
what im asking is from a theoretical view point, i know im reality it could never do max but its simpler to just use max for theory. My question was is the amperage rating just what it can do at max regardless of volts or does it vary with voltage.
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Offline smoothVTer

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Re: Transistor question
« Reply #3 on: October 09, 2016, 01:08:22 pm »
You're not wrong, but your statement is not true for every mosfet.

5A * 60V == 300W    and    25A * 12V == 300W    so if you're considering only power dissipation, yes it is equivalent.

However, the figure of merit you need to check on the datasheet is typically called "Continuous Drain Current"  and you must not exceed this figure in steady-state. The mosfet may be able to handle short quick pulses of a few ms higher than this current ( typically called "Pulsed Drain Current" on the data sheet specs )  depending on the duty cycle you are running.   As always, ensure you do not violate the maximum drain to source voltage for the mosfet, too.

 
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Online neo

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Re: Transistor question
« Reply #4 on: October 09, 2016, 01:32:53 pm »
You're not wrong, but your statement is not true for every mosfet.

5A * 60V == 300W    and    25A * 12V == 300W    so if you're considering only power dissipation, yes it is equivalent.

However, the figure of merit you need to check on the datasheet is typically called "Continuous Drain Current"  and you must not exceed this figure in steady-state. The mosfet may be able to handle short quick pulses of a few ms higher than this current ( typically called "Pulsed Drain Current" on the data sheet specs )  depending on the duty cycle you are running.   As always, ensure you do not violate the maximum drain to source voltage for the mosfet, too.

i actually just meant a big gigantic transistor
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Online Ian.M

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Re: Transistor question
« Reply #5 on: October 09, 2016, 01:50:50 pm »
Here's a concept you need: https://en.wikipedia.org/wiki/Safe_operating_area
IC_max and VCEO_max (bipolar) or ID_max and VDS_max (MOSFET) are just two of the limits on the S.O.A graph.  Often it isn't explicitly given in the datasheet and you have to calculate where your operating point falls on it from the datasheet parameters.  Power devices frequently give multiple lines on their  S.O.A graph for short term pulsed operation outside their static S.O.A.   

There's a further complication with bipolar transistors,  the difference between VCEO_max and VCBO_max.  VCBO_max is the fundamental one and is the limiting voltage at which the reverse biassed C-B junction will not break down.  However the leakage current may be quite significant and will be amplified by the transistor if the emitter is connected, hence the lower VCEO_max rating.  Unless you have a low enough resistance pulldown on the base to sink *ALL* of the leakage current, don't expect to operate a transistor reliably between the two!
 

Online rstofer

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Re: Transistor question
« Reply #6 on: October 09, 2016, 04:53:01 pm »
Look at VCE.  Take a power transistor like the 2N3055 http://www.onsemi.com/pub_link/Collateral/2N3055-D.PDF

It is rated for a maximum 15A collector current and it really can't do anything like that.  When you look at VCE you will find a more realistic operating point of 10A with 3V between the collector and emitter.  So, the transistor is dissipating 30W just from the load.  But look at the matching base current: 3A!  Or 2A depending on which figure you take.  In any event, that is passing through the base-emitter junction so it has a voltage drop of 1.5Vl so there's another 4.5w.  Note that the gain at high current is as low as 3 or 5 depending on which number you take.  That's why welders have a bazillion 2N3055s driven by ranks of other 2N3055s.

Figure 2 shows the dissipation limits.
 

Offline ebclr

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Re: Transistor question
« Reply #7 on: October 09, 2016, 04:59:39 pm »
You are  wrong

In transistor world you need to consider Safe operating area https://en.wikipedia.org/wiki/Safe_operating_area

One critical spec is the power dissipation
 

Offline danadak

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Re: Transistor question
« Reply #8 on: October 09, 2016, 09:24:29 pm »
How is their rated current measured, i think if a transistor can do 5 amps at 60 volts it should be able to do 25 amps at 12v, am i wrong?


Not necessarily, depends on the silicon  design. Current density a big factor in
transistor, whether or not the junction can handle that much current. Same is
true for bond wires and pads, what they were designed for.

And as prior discussion safe area considerations, both thermal and electrical.


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