IRF5305S Power MOSFET switch down to 3V via optocoupler?

Thread Starter

powersupplyfix

Joined Jan 19, 2017
5
Hi all,
found this interesting (yyd-3) module which is some sord of four channel "silicon" relay.

It has four (817) optocouplers which each drive (VCC:10M->G->10K:GND) the gate of an IRF5305S (used as a high side switch).

I looked at the datasheet of that P-FET but am unsure if it will switch reliably at voltages down to 3V.
We'll need to switch minimum 100mA, at max 1.2A.
These will operate at 70C temperatures.

Has anybody worked with these FETs or does anybody know if there is a minimum (VCC) voltage or current required for these types of hexfets to switch on reliably?

Thanks!
 
Last edited:

Alec_t

Joined Sep 17, 2013
11,306
Vgs(thr) is quoted as min -2V and max -4V. So no, you can't be sure the one you buy would switch 100mA reliably at -3V. You need a 'logic level' FET with as small a Vgs(thr) as you can find (say~ -2V max).
 

crutschow

Joined Mar 14, 2008
24,983
To find the minimum voltage that it will reliably switch at, look in the data sheet for the Vgs voltage they use to measure the ON resistance [Rds(on)].
You need one where that Vgs voltage is no more than 3V.
 

Thread Starter

powersupplyfix

Joined Jan 19, 2017
5
Hi all, thank you for your replies.
Yes, those VGS (threshold) are specced from 2-4V, so don't the 3.3V fall right inbetween?
If I got that wrong, do you know a FET which you could kindly recommend which I could use (preferrably drop in, i.e. no change in the existing 1M/10K SMD resistors)) which could switch 3.3v @2A?
It needs to work between 12V VDD (max) and VDD (min) 2.5-3.3V and switch 2A maximum at 70C.

The stuff (LEDs lights, motors etc) which needs to be switched on/off is being powered by 3.3V only (but controlled via optocouplers by 12V).
It all works well with a relay version, but those 3.3V relays need 120mA each!
Thanks!
 

ronv

Joined Nov 12, 2008
3,770
Hi all, thank you for your replies.
Yes, those VGS (threshold) are specced from 2-4V, so don't the 3.3V fall right inbetween?
The 2 to 4 volt number is where the FET just begins to turn on - Idrain = 250 ua.
If I got that wrong, do you know a FET which you could kindly recommend which I could use (preferrably drop in, i.e. no change in the existing 1M/10K SMD resistors)) which could switch 3.3v @2A?
It needs to work between 12V VDD (max) and VDD (min) 2.5-3.3V and switch 2A maximum at 70C.
http://www.irf.com/product-info/datasheets/data/irf5305s.pdf
 

crutschow

Joined Mar 14, 2008
24,983
Yes, those VGS (threshold) are specced from 2-4V, so don't the 3.3V fall right inbetween?
Yes it does.
But that spec is the manufacturing variation between various units of the same type.
So some will have a value of 4V which means it will be off with an applied Vgs of 3.3V.
 

Thread Starter

powersupplyfix

Joined Jan 19, 2017
5
Ok, got it now, so it needs to be above 4V to work reliably, thank you for that!
Bummer!
Is there any other HEXFET (or FET for that matter) I could use (except said irf5305)?
I really looked at the tree and tried to find out, but am totally swamped by the many versions and what not.

If you could think of a part which could work with the existing pull up of 1M and 10K Gate resistor, and can switch 3.3V@2A@70C reliably with a gate going from 0V to 3.3V, and it's source (being high side switch) connected to same 3.3V it would be really kind of you if you could tell me which.

Thank you so much!!
 

ronv

Joined Nov 12, 2008
3,770
Ok, got it now, so it needs to be above 4V to work reliably, thank you for that!
Bummer!
Is there any other HEXFET (or FET for that matter) I could use (except said irf5305)?
I really looked at the tree and tried to find out, but am totally swamped by the many versions and what not.

If you could think of a part which could work with the existing pull up of 1M and 10K Gate resistor, and can switch 3.3V@2A@70C reliably with a gate going from 0V to 3.3V, and it's source (being high side switch) connected to same 3.3V it would be really kind of you if you could tell me which.

Thank you so much!!
Click the link in post #5.
 

crutschow

Joined Mar 14, 2008
24,983
Take a look at this one.
This from its data sheet shows it's ON resistance for a Vgs of -2.5V and -2.7V, so it will be fully turned on at -3V.

upload_2017-1-19_19-54-53.png
 
Last edited:

Thread Starter

powersupplyfix

Joined Jan 19, 2017
5
Crutschow, thank you very much for educating patiently with your knowledge and helping me out, even the pins match!!
Really appreciated!
And to all, thank you also for all your help!
 
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