EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: ruiseixas on May 09, 2017, 01:38:47 am
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Hi,
Probably i'm making some silly mistake here, however using a NPN MOSFET IRF540 in a breadboard or a IRF530 in LTSpice the results are the same, a linear response and not and ON / OFF type one as expected!
Here are the results in contrast with the PNP MOSFET 9640 for a DC Sweep (0 to 12v):
(SEE Picture)
Why doesn't the MOSFET NPN behaves like the PNP with a clear ON / OFF transition?
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yeah two silly mistakes.
NPN Mosfet?? u mean 'n channel'.
The other is to use n channel as a high side switch you need to have a gate voltage higher than the drain voltage.
Or you can go low side, ie move the resistor above the FET.
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The other is to use n channel as a high side switch you need to have a gate voltage higher than the drain voltage.
Looking at the datasheet, it speaks about V_GS (Gate Threshold Voltage), meaning, voltage between Gate and Source... In the N case is around 2 and 4 volts higher than the Source to start conduction from Drain to Source, supposedly!
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so when the FET is switched on the drain and source will be at almost the same voltage.
this is ok for a p channel but for an N channel unless you have a gate voltage higher than the Drain voltage you wont be able to turn on properly.
FET gate drivers often overcome this using a Bootstrap circuit.
https://en.wikipedia.org/wiki/Bootstrapping_%28electronics%29#Driving_MOS_transistors (https://en.wikipedia.org/wiki/Bootstrapping_%28electronics%29#Driving_MOS_transistors)
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You didn't complete the N-to-P transformation.
Vertically swap R2 and M2.
Tim
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The other is to use n channel as a high side switch you need to have a gate voltage higher than the drain voltage.
Looking at the datasheet, it speaks about V_GS (Gate Threshold Voltage), meaning, voltage between Gate and Source... In the N case is around 2 and 4 volts higher than the Source to start conduction from Drain to Source, supposedly!
thats right. thats why the drain will follow the gate voltage 2 to 4 volts lower, it cant go higher since it will shutdown, so it jiggle around there.. this is called linear region (emitter follower) in bjt term. otoh PMOSFET's drain will not be able to follow gate's voltage thats why when Vgs PMOSFET goes lower than some threshold it will go boom fully opened, this is "transistor as switch" mode, not "transistor as regulator nor amplifier" mode. to get fully opened NMOSFET as expected, swap R1 and M1.
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You didn't complete the N-to-P transformation.
Vertically swap R2 and M2.
Tim
Pretty sure he means R1 and M1.
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this is ok for a p channel but for an N channel unless you have a gate voltage higher than the Drain voltage you wont be able to turn on properly.
Well, I switch the resistor position, and now it works as is supposed to and there is no need of a Gate voltage superior to a Drain voltage as you can see. I think the issue here is that N channel MOSFET doesn't work as switch in a "common collector" configuration like a common BJT Transistor...
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Finally I got it, when the MOSFET starts to open, the voltage in the Source also starts to rise, and thus the Gate to Source Voltage never goes beyond the threshold level. You need to make sure the Source Voltage remains constant while switching ON and OFF so that the Gate to Source Voltage goes beyond the threshold level! :phew:
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MOSFETs depend on gate to source voltage, precisely the same as BJTs depend on base to emitter voltage.
The BJT common collector circuit has the same behavior as the MOSFET common drain circuit.
Tim
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MOSFETs depend on gate to source voltage, precisely the same as BJTs depend on base to emitter voltage.
The BJT common collector circuit has the same behavior as the MOSFET common drain circuit.
Tim
Bottom line, chose N or P MOSFET/Transistor so that the Source/Emitter voltage remains constant.
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For a switch application, yes!
Tim
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that will also depends on where you want to put the mosfet, low side or the high side. if its acceptable, most people will prefer low side nmosfet since nmosfet is a little bit more efficient than its complementary pmosfet. the same rule applies to bjt npn or pnp switch.
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Bottom line, chose N or P MOSFET/Transistor so that the Source/Emitter voltage remains constant.
Not even sure why you say that.
I would say to switch on a mosfet (eh), make sure Vgs > Vth
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You are using it as a linear device here like linear power supplies. If you want a switch, you should put the MOSFET in the last position before ground.