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Understanding Vce & Vbe (newbie)


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Hi Andres,
Transistors need a minimum amount of base current to turn on, depending on the amount of collector current and amount of saturation voltage that you desire.  A transistor also has a maximum amount of base-emitter voltage that must be provided by the base current source. Its max Vbe is 1.5V with a fairly low base current of less than 400mA and is higher (but not rated) for higher base currents.
As you can see by the decreasing ratio of collector current to base current, as the collector current is higher in order for it to saturate as well as it can then it needs a massive base current. For a collector current of 10A, a base current of 3.3A is required for the weakest one to saturate to only 3V. The collector will dissipate 30W of heat and the base-emitter will dissipate  probably about 6.6W of heat. That is inefficient!

The hFE is rated with a 4V collector voltage and much more base current is needed in order for it to saturate well with a lower saturation voltage.

A modern power Mosfet is much more efficient if you have 10V to drive its gate. The Mosfet dissipates far less power ( less heat wasted) than a 2N3055 and its gate doesn't dissipate anything since it is a very high resistance. ;D

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Ok. So if I would use a power MOSFET like IRF640 (most common over here, rated at 200V) I can apply 1 or 10V to the gate and make it conduct... that simple...

And if I want to use and inductive load..., is the same?

What's the difference if I use a SCR or a TRIAC...?

sorry for so many basic questions..., I'm still learning...  :-\

Basicly I want to use them for driving relays and maybe automotive coils...

Thanks for your help.

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Hi Andres,
If you want to use an IRF640 Mosfet then look at its datasheet:

At room temperature if you apply 1V to the gate then it will do nothing. Its gate threshold voltage where it will conduct a tiny amount of current (1mA) is from 2V to 4V. The voltage is a range from 2V to 4V because each one is a little different. If it is very cold then the gate voltage must be at least 6V to turn it on a tiny amount.

The Rds drain-source resistance is spec'd with a gate voltage of 10V. Curves in the datasheet show how typical devices perform poorly with gate voltages less than 10V.

Some Mosfets are designed to be driven by 5V logic. They perform even better with more gate voltage.

The Mosfet's gate drive isn't simple if you want it to switch on and off quickly. It has a high gate to source capacitance that needs to be charged with a high current for fast switching.

For SCRs and TRIACs, look them up in our Articles section above, or in www.google.com .

A relay and a coil are inductors. When current in an inductor is stopped, it develops a very high voltage that could destroy the driver if the voltage isn't arrested. For DC circuits a rectifier diode is wired across the coil in reverse to arrest the high voltage. For AC circuits other protection circuits are used. ;D


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If I use a MOSFET and I want it to switch fast a need to make sure that the gate has a high current and a voltage from 2v to 4v?{/quote]
Yes, a high gate drive current to switch fast, but with only 2V to 4V for the gate then the Mosfet will conduct typically only 1mA. Use 10V for the gate for the Mosfet to conduct nealy as hard as it can.

I read that a reverse diode across the inductor protects the driver, but it make it switch off later.

Yes, the switch-off is a little slower but you might not notice.

I also read that I could use a zener diode for fast switching..., but how do you know the rate of the zener?

Selcet a zener voltage about the same or less than the max voltage rating of the transistor driving it.

I also would like to use SCR, but how do I look for the article in this page?

Click on the Articles button at the top of this page and find and click on SCR in the index. ;D

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Hi Prateek,
A modern (expensive) Mosfet has a very low on resistance so it ts much less when used for switching, than a BJT.
I think a Mosfet is more linear.
A Mosfet doesn't have thermal runaway like a BJT.
A power Mosfet is capable of operating at higher frequencies than a BJT.
I can't think of anything except low surplus cost that makes a power BJT better than a power Mosfet. ;D

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