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Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
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Posts: 1567
How do I determine the minimum values? I have a NPN 175W transistor. It can take a voltage of 1000v C-E and 10A collector current. I can connect the emitter to any value/wattage resistor that goes to ground.
If I am going to run 500v at the collector, how do I calculate how low a resistance value I can use without blowing the transistor? Is the answer 50 ohm as this will limit the current to 10A, not including any resistance offered by the transistor?
Just so I understand the wattage rating of the transistor I want to clear up a point of confusion for myself. If it is rated at 175W, does this mean that Vce * Ie < 175W (assuming we have everything heatsinked properly and cooled)?
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
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The 175W rating is maximum heat dissipation with perfect heatsink. However there is a phenomenon called second breakdown, this limits the transistor dissipation with relatively higher collector voltages, this is because "hot spots" can form on the silicon die and the maximum operating temperature is exceeded in these spots.
Check your transistor's datasheet, there will be "Safe operating area" graph which will show you the curve of maximum allowed dissipation vs. collector voltage.
Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
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Jmartis, thanks. There is still the main question about sizing.
Can I presume, due to the conservation of energy, that if I have 125W input (125v, 1A) and the drop on the resistor is 100ohm*1A = 100v, I will be safe as there must be no more than 25w lost in the transistor?
Is there a way to figure out for a given Rload on the emitter what the voltage drop across the CE junction should be?
Registered Member #15
Joined: Thu Feb 02 2006, 01:11PM
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IamSmooth wrote ...
How do I determine the minimum values? I have a NPN 175W transistor. It can take a voltage of 1000v C-E and 10A collector current. I can connect the emitter to any value/wattage resistor that goes to ground.
If I am going to run 500v at the collector, how do I calculate how low a resistance value I can use without blowing the transistor? Is the answer 50 ohm as this will limit the current to 10A, not including any resistance offered by the transistor?
Just so I understand the wattage rating of the transistor I want to clear up a point of confusion for myself. If it is rated at 175W, does this mean that Vce * Ie < 175W (assuming we have everything heatsinked properly and cooled)?
As JMartis stated, it all depends on how cool you can maintain the device.
For all practical purposes, those 175W max dissipation ratings are usually bogus unless you have a perfect, water-cooled heatsink. If the transistor was free-air (no-heatsink) you'd probably be limited to less than 1W (it should state this in the datasheet), and even with a very good heatsink / fan, you probably still be limited to less than 10-15Watts.
The important value to look at is junction temperature. Thats the number you don't want to exceed.
The MIT courseware has a good example on this here:
Registered Member #27
Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058
Can I presume, due to the conservation of energy, that if I have 125W input (125v, 1A) and the drop on the resistor is 100ohm*1A = 100v, I will be safe as there must be no more than 25w lost in the transistor?
Yes, you can assume that they are purely resistive devices that follows Ohms law.
Is there a way to figure out for a given Rload on the emitter what the voltage drop across the CE junction should be?
If you want to make a regulator you need to use feedback of some sort.
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Sizing resistor/transistor for power dissipation
