2
\$\begingroup\$

I want to drive the IRF9530 P-Channel FET with 5V PIC microcontroller. My load is 3.2A. schemtic shows one row in my LED matrix. Scanning @ 1/16th rate, so peak current per LED will be 64mA and the average will be 4mA. When one or two LEDs light up in a row, they are bright. However when more LEDs light up inthe same row, they are dimmer.

  • Can the FET can supply require current with the given gate voltage?
  • Is there any simple modification to be done within the given hardware, to supply the required current?

I cannot use another driving transistor to drive the FET (because I have a PCB). Also I cannot find logic level MOSFETs in my area.

Minor changes will be preferred.

schematic

\$\endgroup\$
4
  • \$\begingroup\$ Perhaps you should control the ULN2803's instead. Or if this is a global enable even try a relay. It sounds like a one-off, so doing a little 3d sculpture on the existing board in the vicinity of the FET footprint would not be outside of reason. \$\endgroup\$
    – Chris Stratton
    Commented Mar 5, 2019 at 16:29
  • \$\begingroup\$ Hi I cannot understand what tou mean...!!! The schemtic shows one row in my LED matrix.Scanning @ 1/16th rate.So peak current per LED will be 64mA and the average will be 4mA. \$\endgroup\$
    – Joseph143
    Commented Mar 5, 2019 at 16:45
  • \$\begingroup\$ Seems like there's a lot you left out of your question - that comment should be an edit. But it also seems like you have boxed yourself into a corner and may simply have to suspend the project for whatever amount of time it will take suitable parts to reach your location. \$\endgroup\$
    – Chris Stratton
    Commented Mar 5, 2019 at 16:46
  • \$\begingroup\$ ”Also I cannot find logic level MOSFETs in my area.” Mouser and Digi-Key ships all over the world, so that’s not an issue. Pick a transistor suited for your application. \$\endgroup\$
    – winny
    Commented Mar 5, 2019 at 17:55

1 Answer 1

Reset to default
2
\$\begingroup\$

Can the FET can supply require current with the given gate voltage?

Not really, you do not have a lot of voltage margin left.

From the left plot I can see that at \$I_D\$ = 3 A and \$V_{gs}\$ = -5 V the FET's \$V_{DS}\$ will be about -1 V.

That leaves 5 V - 1 V = 4 V for the LEDs + series resistor. If the LEDs actually have a 3 V forward voltage then this might just work but often LEDs do have a higher forward voltage especially White LEDs.

You could try to decrease the 27 ohms resistors such that you do get the required current. That will still not eliminate the 1 V drop across the MOSFET though.

This MOSFET is really better suited to a much higher \$V_{GS}\$ like 10 V as then the drop would be about 0.5 V, that's still a lot!

Also note that if you manage to get the 3 A and the MOSFET drops 1 V it will dissipate 3 W so it will get hot! It will need a heatsink!

You really need to consider using a "better" MOSFET with a much lower \$R_{DSon}\$, for example the AO3401. Since the 3.2 A is on the edge for this device I'd use 2 of them and let each power half of the LEDs.

\$\endgroup\$
3
  • 1
    \$\begingroup\$ More importantly, get a "better" MOSFET with a lower Vgs(th), commonly known as a logic-level MOSFET. For that matter, the AO341 is not a good choice, since it has a max rating of 4 amps at Vgs of 10 volts. \$\endgroup\$
    – WhatRoughBeast
    Commented Mar 5, 2019 at 15:35
  • 1
    \$\begingroup\$ @WhatRoughBeast You're a bit quick with your conclusions, I suggested to use two of those AO3401 each at half of that 3.4 A. At Vgs = -4.5 V Rdson is 60 mohm or less which is good enough for 3.4 /2 = 1.7 A. I never suggested to use the AO3401 at 4 A. Of course there are MOSFETs which need an even lower Vgs for even more current. The suggested AO3401 is very common and cheap and that's why I suggested it. If you have a better suggestion then feel free to add a comment below. \$\endgroup\$
    – Bimpelrekkie
    Commented Mar 5, 2019 at 15:44
  • \$\begingroup\$ Ok thanks.I plan to use Two of AO3401 SMD Parts in parallel.I hope I dont need to change the resistors.The One in the schematic 10K pullup & the 100R gate resistor will share both of the AO3401. \$\endgroup\$
    – Joseph143
    Commented Mar 5, 2019 at 23:49

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.