MOSFET NPN fails as Switch device while PNP doesn't! - Page 1
Forum section: electronics / projects

author: ruiseixas
author_rank: Contributor
timestamp: May 09, 2017, 01:38:47 am
content: 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?

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: May 09, 2017, 01:47:19 am
content: 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.

author: ruiseixas
author_rank: Contributor
timestamp: May 09, 2017, 01:58:43 am
content: Quote from: HackedFridgeMagnet on May 09, 2017, 01:47:19 amLooking at the datasheet, it speaks about V_GS (Gate Threshold Voltage), meaning, voltage betweenGate and Source... In the N case is around 2 and 4 voltshigher than the Sourceto start conduction from Drain to Source, supposedly!

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: May 09, 2017, 02:07:00 am
content: 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

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 09, 2017, 02:11:15 am
content: You didn't complete the N-to-P transformation.Vertically swap R2 and M2.Tim

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 09, 2017, 02:12:16 am
content: Quote from: ruiseixas on May 09, 2017, 01:58:43 amthats 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.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: May 09, 2017, 02:16:57 am
content: Quote from: T3sl4co1l on May 09, 2017, 02:11:15 amPretty sure he means R1 and M1.

author: ruiseixas
author_rank: Contributor
timestamp: May 09, 2017, 02:18:17 am
content: Quote from: HackedFridgeMagnet on May 09, 2017, 02:07:00 amWell, I switch the resistor position, and now it works as is supposed to andthere is no need of a Gate voltage superior to a Drain voltageas 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...

author: ruiseixas
author_rank: Contributor
timestamp: May 09, 2017, 02:33:00 am
content: 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!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 09, 2017, 04:04:07 am
content: 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

author: ruiseixas
author_rank: Contributor
timestamp: May 09, 2017, 04:51:57 am
content: Quote from: T3sl4co1l on May 09, 2017, 04:04:07 amBottom line, chose N or P MOSFET/Transistor so that the Source/Emitter voltage remains constant.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 09, 2017, 05:02:26 am
content: For a switch application, yes!Tim

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 09, 2017, 05:07:18 am
content: 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.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: May 09, 2017, 05:17:58 am
content: Quote from: ruiseixas on May 09, 2017, 04:51:57 amNot even sure why you say that.I would say to switch on a mosfet (eh), make sure Vgs > Vth

author: VEGETA
author_rank: Super Contributor
timestamp: May 09, 2017, 05:55:19 am
content: 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.