MOSFET Usage

I am in the process of designing a device that uses 30A MOSFET for the output. I need it to apply a 12 Vdc source to a switchable load on the ground side. Are there any major issues using a MOSFET in this way?

Reply to
Ron M.
Loading thread data ...

What is the load type, inductive (motor etc) capacitive, resistive?

What is the peak current on switch on?

Have you done thermal calcuations of the MOSFET and heat sink?

What is the operating temprature range? Do the thermal calcualtions meet the MOSFET thermal requirements?

What is the operating voltage? Is the max_Vds of the MOSEFT much greater than the operating voltage.

What is the Vgs_turn_voltage? Is it much greater than the MOSFET Vgs_turn_on_threshold?

If you answer these questions then you are well on your way to operating the MOSFET correctly.

PS... good clues on MOSFET operation may be obtained from here.

formatting link

Joe

Reply to
Joe G (Home)

Make sure you examine the data sheet for "silicon limiting values" and "package limiting values" - some manufactures headline the current the die can handle but aren't so keen to draw attention to the fact the package can't match that value.

There was a thread somewhere about that recently (may still be going), IR were taking a bashing as being one of the worst offenders.

Reply to
ian field

You forgot to mention eye-squared-tee considerations.

Reply to
Robert Baer

You'll probably need to divide the load over 2 or more MOSFETs due to PCB and package limits.

--
Failure does not prove something is impossible, failure simply
indicates you are not using the right tools...
 Click to see the full signature
Reply to
Nico Coesel

Oh bugger,,, I forgot that one Joe

Reply to
Joe G (Home)

another MOSFET web site that might be useful.

formatting link

Reply to
Joe G (Home)

et

er

the

I knew this would go crazy if didn't spell things out. OK here we go. The MOSFET will be switching a 2 ohm (resistive) load for approx 100 -

200 ms. It will be controlled via 555 output. The MOSFET needs to have plus attached all the time and the load will be between the MOSFET and ground and will be several loads switched in one at a time. Basically this is the output for a sequencer that will be used to provide power to several different strings of devices (9 outputs in sequence by 20 sequences). The different sequences will be switched in manually using diode matrixing and siwtched grounds (relay). I know the MOSFET can handle the amount of current and power factors being used. Just wasn't sure if the plus could be connected to the MOSFET all the time and the load between the MOSFET and ground. Haven't used them much and wanted to save myself some grief during the design stages. They aren't that expensive $1.80 each but why blow em up if you don't gotta. THANKS for all the responses. Hopefully this will be enough for an informed opinion. THANKS folks.
Reply to
Ron M.

.
e

e?

meet

ater

g the

Do you want to use N-channel or P-channel mosfets. Using the latter would be easier for switching grounded loads.

Reply to
neddie

...

he

the

ive?

s meet

reater

ing the

This is a simple way to do it. Used 555's to generate signals , but just to show the principle. If your load is purely resistive , ok. If inductive , use a diode across it . Use components (Fet) that came with LTSpice. There are probably better ones to use, that's up to you :0) Version 4 SHEET 1 1132 680 WIRE 848 -208 752 -208 WIRE 864 -208 848 -208 WIRE 1008 -208 864 -208 WIRE 864 -160 864 -208 WIRE 752 -96 752 -128 WIRE -80 -80 -544 -80 WIRE 32 -80 32 -96 WIRE -224 -32 -624 -32 WIRE 1008 -16 1008 -208 WIRE -80 0 -80 -80 WIRE 32 0 -16 0 WIRE 864 0 864 -80 WIRE 960 0 864 0 WIRE 400 80 400 64 WIRE 496 80 400 80 WIRE -192 96 -192 80 WIRE -96 96 -192 96 WIRE 112 96 64 96 WIRE 400 96 400 80 WIRE 400 96 336 96 WIRE -480 112 -528 112 WIRE -192 112 -192 96 WIRE -192 112 -256 112 WIRE 32 160 32 0 WIRE 112 160 32 160 WIRE 496 160 336 160 WIRE -624 176 -624 -32 WIRE -480 176 -624 176 WIRE -96 176 -256 176 WIRE 864 192 864 0 WIRE -96 208 -96 176 WIRE 112 224 48 224 WIRE 448 224 336 224 WIRE -544 240 -544 -80 WIRE -480 240 -544 240 WIRE -224 240 -224 -32 WIRE -224 240 -256 240 WIRE -144 240 -224 240 WIRE 688 240 656 240 WIRE 800 240 768 240 WIRE 448 272 448 224 WIRE 496 272 496 160 WIRE 496 272 448 272 WIRE -144 288 -144 240 WIRE -96 288 -144 288 WIRE 112 288 96 288 WIRE -480 304 -496 304 WIRE 1008 304 1008 80 WIRE 496 320 496 272 WIRE -624 352 -624 176 WIRE -624 512 -624 416 WIRE -368 512 -624 512 WIRE 496 512 496 384 WIRE 496 512 -368 512 WIRE 864 512 864 288 WIRE 864 512 496 512 WIRE 1008 512 1008 384 WIRE 1008 512 864 512 WIRE -368 544 -368 512 FLAG -528 112 0 FLAG -368 544 0 FLAG 752 -96 0 FLAG 848 -208 Vcc FLAG -496 304 Vcc FLAG -192 80 Vcc FLAG 656 240 Out FLAG 64 96 0 FLAG 96 288 Vcc FLAG 400 64 Vcc FLAG 48 224 Out FLAG 32 -96 Vcc SYMBOL Misc\\NE555 -368 208 R0 SYMATTR InstName U1 SYMBOL cap -640 352 R0 SYMATTR InstName C1 SYMATTR Value 4.7=B5 SYMBOL res -112 80 R0 SYMATTR InstName R1 SYMATTR Value 1k SYMBOL res -112 192 R0 SYMATTR InstName R2 SYMATTR Value 200k SYMBOL voltage 752 -224 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value 12 SYMBOL pmos 960 80 M180 SYMATTR InstName M1 SYMATTR Value FDS6375 SYMBOL res 848 -176 R0 SYMATTR InstName R3 SYMATTR Value 1k SYMBOL res 784 224 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R4 SYMATTR Value 1k SYMBOL res 992 288 R0 SYMATTR InstName LOAD SYMATTR Value 2 SYMBOL npn 800 192 R0 SYMATTR InstName Q1 SYMATTR Value 2N3904 SYMBOL Misc\\NE555 224 192 R0 SYMATTR InstName U2 SYMBOL cap 480 320 R0 SYMATTR InstName C2 SYMATTR Value 1.8=B5 SYMBOL res 480 64 R0 SYMATTR InstName R6 SYMATTR Value 100k SYMBOL res 16 -96 R0 SYMATTR InstName R7 SYMATTR Value 10k SYMBOL cap -16 -16 R90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName C3 SYMATTR Value 100p TEXT -98 568 Left 0 !.tran 10

Reply to
neddie

Do you want to use N-channel or P-channel mosfets. Using the latter would be easier for switching grounded loads.

***************

If an old style iron cored transformer and rectifier is used as the power source its not too difficult to add a voltage doubling rectifier to provide an additional rail for n-channel gate drive.

It is usually possible to do something similar with an SMPSU, but usually this involves opening and modifying an approved assembly - something not everyone's willing to do.

Reply to
ian field

...

he

the

ive?

s meet

reater

ing the

The actual device is an NTE2376 30A MOSFET crossed over to an IRFP250NPbf. The device is an N-channel Enhancement type. Just want to know if I can place the load on the Source side instead of the drain side without undue stress on the component. It will be pulsed for

100-200 ms into a 2 ohm resistive load @ 12 vdc. provided by battery. This is a portable application for firing e-matches in fireworks in a sequential manner.
Reply to
Ron M.

The actual device is an NTE2376 30A MOSFET crossed over to an IRFP250NPbf. The device is an N-channel Enhancement type. Just want to know if I can place the load on the Source side instead of the drain side without undue stress on the component. It will be pulsed for

100-200 ms into a 2 ohm resistive load @ 12 vdc. provided by battery. This is a portable application for firing e-matches in fireworks in a sequential manner. *************

Shouldn't be a problem, but if you want to ground one side of the load and still use an N-channel MOSFET, you have to devise an extra supply rail above Vdd.

Look at the data sheet - there is a minimum Vgs to achieve the headline RDSon.

Reply to
ian field

om...

n

y?

-
e
d
g
t
e
r
d

ove

I checked the data sheet and am having difficulty decyphering what you mean. The Vgs is +/- 20 volts MAX but the Gate Threshold Voltage is between 2 and 4 volts. Like I said I'm kind of new to designing with MOSFET's. I'm more of a bi-polar kind of tech. Boy!! That didn't sound right but I guess you get the gist of what I mean.

Reply to
Ron M.

.
:

.com...

or

on

way?

e?

ns

o.

0 -

ave

and

ly

er

0

ing

n't

the

ed

t

for

and

above

Your gate needs to driven higher than the source , by at least the threshold voltage. Look at it like this. If you turn on the gate with 5V , the fet turns on. The source voltage (voltage across the load)now climbs and the fet will start to turn off. This will continue until you can drive the gate higher than the source by the threshold voltage. Your supply to the fet is 12V , so you want to drive the gate to at least 16V(assuming 4V threshold). As mentioned in a previous post , you can use a voltage doubler to achieve the higher voltage needed to drive the gate very easely. If no "ac" supply is around , maybe you can use another

555 to do the job. I'll modify the previous cct I posted to show you how. Just jive me a few minutes. Rob
Reply to
neddie

...

te:

ps.com...

for

ad on

s way?

ive?

ions

h
T

go.

100 -

have

T and

ally

ower

20

using

an

asn't

d the

nted

hat

S for

o

d and

l above

ne

I've modified the earlier cct to use N-channel mosfets. It's just a

555 astable driving a voltage doubler , to give you about 24V. The extra transistor is just to level shift and give you 24V drive to the gate of the FET. Note that the Vgs of the FET is not being exceeded , its still only seeing 12 across gate-source. It's not the best cct in the world , but it will work and uses a 555 which you are already using. If you have another AC source , replace 555 with that. If its is only 50 or 60 hz , you mat have to increace the cap values in the doubler cct a bit. P.S I've bees assuming you have LTSpice , if not get it from Linear.com. It's a great program and is free!!

Version 4 SHEET 1 1220 680 WIRE -176 -592 -176 -608 WIRE -80 -592 -176 -592 WIRE -464 -576 -512 -576 WIRE -176 -576 -176 -592 WIRE -176 -576 -240 -576 WIRE -640 -544 -640 -592 WIRE -352 -512 -464 -512 WIRE -80 -512 -240 -512 WIRE -80 -480 -80 -512 WIRE -720 -448 -736 -448 WIRE -640 -448 -640 -480 WIRE -640 -448 -656 -448 WIRE -464 -448 -576 -448 WIRE -352 -448 -352 -512 WIRE -240 -448 -352 -448 WIRE -128 -448 -240 -448 WIRE -128 -400 -128 -448 WIRE -80 -400 -128 -400 WIRE -736 -368 -736 -448 WIRE -464 -384 -480 -384 WIRE -80 -352 -80 -400 WIRE 784 -208 768 -208 WIRE 848 -208 784 -208 WIRE 960 -208 848 -208 WIRE -736 -176 -736 -304 WIRE -352 -176 -736 -176 WIRE -80 -176 -80 -288 WIRE -80 -176 -352 -176 WIRE 240 -176 240 -208 WIRE -352 -144 -352 -176 WIRE 848 -144 848 -208 WIRE -80 -80 -544 -80 WIRE 32 -80 32 -96 WIRE 240 -80 240 -96 WIRE 784 -64 784 -208 WIRE -224 -32 -624 -32 WIRE 960 -32 960 -208 WIRE 1104 -16 1104 -208 WIRE -80 0 -80 -80 WIRE 32 0 -16 0 WIRE 848 16 848 -64 WIRE 896 16 848 16 WIRE 848 32 848 16 WIRE 1024 64 960 64 WIRE 1056 64 1024 64 WIRE 400 80 400 64 WIRE 496 80 400 80 WIRE -192 96 -192 80 WIRE -96 96 -192 96 WIRE 112 96 64 96 WIRE 400 96 400 80 WIRE 400 96 336 96 WIRE -480 112 -528 112 WIRE -192 112 -192 96 WIRE -192 112 -256 112 WIRE 784 112 784 16 WIRE 848 112 784 112 WIRE 32 160 32 0 WIRE 112 160 32 160 WIRE 496 160 336 160 WIRE -624 176 -624 -32 WIRE -480 176 -624 176 WIRE -96 176 -256 176 WIRE 784 192 784 112 WIRE -96 208 -96 176 WIRE 112 224 48 224 WIRE 448 224 336 224 WIRE 1024 224 1024 64 WIRE -544 240 -544 -80 WIRE -480 240 -544 240 WIRE -224 240 -224 -32 WIRE -224 240 -256 240 WIRE -144 240 -224 240 WIRE 608 240 576 240 WIRE 720 240 688 240 WIRE 448 272 448 224 WIRE 496 272 496 160 WIRE 496 272 448 272 WIRE -144 288 -144 240 WIRE -96 288 -144 288 WIRE 112 288 96 288 WIRE -480 304 -496 304 WIRE 1104 304 1104 80 WIRE 496 320 496 272 WIRE -624 352 -624 176 WIRE -624 512 -624 416 WIRE -368 512 -624 512 WIRE 496 512 496 384 WIRE 496 512 -368 512 WIRE 784 512 784 288 WIRE 784 512 496 512 WIRE 1024 512 1024 304 WIRE 1024 512 784 512 WIRE 1104 512 1104 384 WIRE 1104 512 1024 512 WIRE -368 544 -368 512 FLAG -528 112 0 FLAG -368 544 0 FLAG 768 -208 Vgate FLAG -496 304 Vcc FLAG -192 80 Vcc FLAG 576 240 Out FLAG 64 96 0 FLAG 96 288 Vcc FLAG 400 64 Vcc FLAG 48 224 Out FLAG 32 -96 Vcc FLAG -512 -576 0 FLAG -352 -144 0 FLAG -480 -384 Vcc FLAG -176 -608 Vcc FLAG -640 -592 Vcc FLAG -736 -448 Vgate FLAG 240 -80 0 FLAG 240 -208 Vcc FLAG 1104 -208 Vcc SYMBOL Misc\\NE555 -368 208 R0 SYMATTR InstName U1 SYMBOL cap -640 352 R0 SYMATTR InstName C1 SYMATTR Value 4.7=B5 SYMBOL res -112 80 R0 SYMATTR InstName R1 SYMATTR Value 1k SYMBOL res -112 192 R0 SYMATTR InstName R2 SYMATTR Value 200k SYMBOL res 768 -80 R0 SYMATTR InstName R3 SYMATTR Value 10k SYMBOL res 704 224 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R4 SYMATTR Value 1k SYMBOL res 1088 288 R0 SYMATTR InstName LOAD SYMATTR Value 2 SYMBOL npn 720 192 R0 SYMATTR InstName Q1 SYMATTR Value 2N3904 SYMBOL Misc\\NE555 224 192 R0 SYMATTR InstName U2 SYMBOL cap 480 320 R0 SYMATTR InstName C2 SYMATTR Value 1.8=B5 SYMBOL res 480 64 R0 SYMATTR InstName R6 SYMATTR Value 100k SYMBOL res 16 -96 R0 SYMATTR InstName R7 SYMATTR Value 10k SYMBOL cap -16 -16 R90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName C3 SYMATTR Value 100p SYMBOL Misc\\NE555 -352 -480 R0 SYMATTR InstName U3 SYMBOL cap -96 -352 R0 SYMATTR InstName C4 SYMATTR Value 22n SYMBOL res -96 -608 R0 SYMATTR InstName R5 SYMATTR Value 1k SYMBOL res -96 -496 R0 SYMATTR InstName R8 SYMATTR Value 22k SYMBOL cap -752 -368 R0 SYMATTR InstName C5 SYMATTR Value 100=B5 SYMBOL cap -576 -464 R90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName C6 SYMATTR Value 10=B5 SYMBOL schottky -656 -544 R0 WINDOW 0 -23 -27 Left 0 WINDOW 3 30 -35 Left 0 SYMATTR InstName D1 SYMATTR Value 1N5819 SYMATTR Description Diode SYMATTR Type diode SYMBOL schottky -656 -464 R90 WINDOW 0 68 27 VBottom 0 WINDOW 3 -103 -72 VTop 0 SYMATTR InstName D2 SYMATTR Value 1N5819 SYMATTR Description Diode SYMATTR Type diode SYMBOL nmos 1056 -16 R0 SYMATTR InstName M1 SYMATTR Value IRF7832 SYMBOL pnp 896 64 M180 SYMATTR InstName Q2 SYMATTR Value 2N2907 SYMBOL res 832 16 R0 SYMATTR InstName R9 SYMATTR Value 1k SYMBOL voltage 240 -192 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value 12 SYMBOL res 832 -160 R0 SYMATTR InstName R10 SYMATTR Value 10k SYMBOL res 1008 208 R0 SYMATTR InstName R11 SYMATTR Value 10k TEXT -96 568 Left 0 !.tran 10 startup

Reply to
neddie

I checked the data sheet and am having difficulty decyphering what you mean. The Vgs is +/- 20 volts MAX but the Gate Threshold Voltage is between 2 and 4 volts. Like I said I'm kind of new to designing with MOSFET's. I'm more of a bi-polar kind of tech. Boy!! That didn't sound right but I guess you get the gist of what I mean.

Right - before you do anything else, protect the gate by connecting a 20V zener from gate to source.

If you're using logic level MOSFETs, Vgs of about 8V should absolutely guarantee the headline RDSon, so lets assume you're using a 12V lead acid battery to fire the igniters, you could either build a blocking oscillator voltage booster to get the higher voltage you need for the gate pull-up supply - or you could simply use a 9V alkaline battery which will last almost its shelf life because of the very low currents involved.

Once you have your 9V battery on top of your 12V rail (21V gate pull-up supply) you connect the rail via a pull-up resistor, 100k would do, now all you need is a switching transistor sitting on the ground rail to switch the MOSFET's gate - I would suggest including a small "gate stopper" resistor in series with the switching transistor's collector.

Reply to
ian field

Geez! Us>

Reply to
oparr

Geez! Using a P-Channel MOSFET like the IRF5305 would be so much simpler. Why has he latched onto the use of a N-Channel MOSFET?

***************

Sounds like he already has the N-channel devices, and is using a significant number, so changing route halfway and buying up a batch of P-channel devices might be too much over budget.

P-channel devices are more expensive and less efficient than N-channel and the OP might not have been able to find a reasonably priced P device that he liked for the current rating he wanted.

Reply to
ian field

OP, can you confirm? If so, then you put the cart before the horse.

d

he

You **may** be giv>

Reply to
oparr

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.