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MOSFET switching speed

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Rahul Soni

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What makes the speed of MOSFET faster, is it Rds(on), or is it the insulated gate? Please explain.
What i know is the switching speed of MOS is greater than BJT but some people say opposite of it. What is the right answer (please give proper explanation).
Thanks in advance.
 

KlausST

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Hi,

Did you read some datasheets and application notes on that?. Please do so.

There is no general answer on your question, because it depends on many other parameters.
(most: gate drive current, 2 x gate capacitance, switched voltage, internal body diode...)

Klaus
 

Rahul Soni

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Hi,

Did you read some datasheets and application notes on that?. Please do so.

There is no general answer on your question, because it depends on many other parameters.
(most: gate drive current, 2 x gate capacitance, switched voltage, internal body diode...)

Klaus

I found that in general, MOSFET is faster than the BJT but I am still unable to find the reason for that.
Some people say that it is the insulated gate that reduce the current requirement and makes it faster while some says that Rds(on) reduce the power dissipation and makes it more faster. Can you please explain ?
 

KlausST

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Hi,

I found that in general, MOSFET is faster than the BJT but I am still unable to find the reason for that.

I found that in general, PEARS are sweeter than the APPLES but I am still unable to find the reason for that.

Different fruits, different electronic parts.

Klaus
 

Rahul Soni

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Hi,



I found that in general, PEARS are sweeter than the APPLES but I am still unable to find the reason for that.

Different fruits, different electronic parts.

Klaus

Why MOS are faster? Due to Rsd(on) or insulated gate ?
 

Vbase

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Rahul,
Welcome to the 'fruits and vegetables forum'.
 

dick_freebird

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Saturation (or the lack) - to get tolerable Vce on a bipolar
you have to load up the base with charge (current) and this
then has to be turned around, but has a long storage time.

Bipolars are real fast to turn on. It's turning off that's slow.
 

crutschow

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The main reason BJTs are considered slower is as Dick stated. The saturation delay for the BJT to turn off can be very long. But if you prevent the BJT from saturating (such as connecting a Schottky diode from base to collector as is used in Schottky TTL logic circuits) then you can basically eliminate the saturation delay and BJT speeds can be comparable to MOSFETs, at least for small devices.
This tends to be less true for power devices, however, as the recombination time of the minority carriers in a BJT junction (which MOSFETs, being a majority carrier device, don't have) increases the switching time.
 

KlausST

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Hi,

just out of curiosity i made a LTspice simulation.

four devices under test
1) N-Ch MOSFET Si1555
2) N-Ch MOSFET Si4482
3) NPN BJT FZT849
4) NPN BJT BC337

Connection:
* Source to GND, emitter to GND
* Drain pullup with 100R to 12V, collector pullup with 100R to 12V
* gate with 1k to squarewave, base with 1k to squarewave

squarewave: 50kHz, 0V, 4V, duty cycle 50%

This is what the simulation shows:
DUT: Turn ON / turn OFF (both @ 50% = 6V)
* Si1555: 30ns / 278ns
* BC337: 30ns / 1077ns
* FZT849: 440ns / 2019ns
* Si4482: 2236ns / 3560ns

SpeedTest.png

I know: nobody would drive FETs and BJTs with the same source (voltage, impedance). But here it´s done just to compare different devices.


Klaus
 

Vbase

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Klaus, I hope in real life you don't drive your mosfets with 1K gate resistor.
Can you repeat this test for apples and pears sweetness?
 

KlausST

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Hi,

Can you repeat this test for apples and pears sweetness?

It´s common knowledge, that you can´t repat this test with sweetness, that´s nonsense. But for sure you can repeat this test with softness.

While increased drive strength with FETs and BJTs decrease softness of output voltage it is the opposite with apples and pears.
This is demonstrated on youtube: look for "cooking an apple with electricity:) "

sorry for going a bit OT :)

Klaus
 
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