mosfet driver

[ghostman11]

can you have a look and let me know if the following circuit will work.View attachment 69046

ive tried to simplify the diagram a bit, i am driving 360 multiplexed leds, and just wanted to check the way i have the mosfets set up. and if the transistor arrangement i have used will drive the mosfets on. thanks in advance

 

[Jon Wilder]

Why are you using a bipolar? The PIC can drive the MOSFET gate directly and your logic won't be reversed (1 will be 'on' while 0 will be 'off').

 

[ghostman11]

i thought the gate needed 10v?? have i misread the datasheet??? i was sure theese blighters wernt logic level

 

[Jon Wilder]

Ah yes very true. Didn't think about that. In that case, logic 0 will turn on the MOSFET while logic 1 turns it off.

 

[be80be]

He could do a emitter follower but what he has will work. But why are you doing these like this
going to take a lot of 74hc595

I would do them in sets of 8

 

[ghostman11]

i am only using 2 shift registers i just drew in one set of top and bottom so it was clearer for people to see how i wanted to connect the transistors to the mosfets. i will have 2 unused outputs on both registers. also i only drew 1 set of 36 leds there are infact 10 sets of 36 leds, this is for some outside tree lights on 200' of cable , the leds fit into pingpong balls as defusers.

hmm i didnt make that very clear!!! what i meant to say was all 360 leds are driven from 2 registers so at any one time there will be a min 10 leds lit

 

[MikeMl]

Why bother with the emitter follower between U2 and Q3? The emitter follower can only pull Q3's gate to U2's Vdd - 0.65V. Since Q3 is a logic-level NFET, you could just connect the U2 pin to the gate of Q3.

If the Vdd of U1 is 12V, then you don't need the inverter between U1 and the gate of Q1. If it is not 12V, then you need the inverter as a level shifter.

 

[ghostman11]

hi mike, the VDD of U1 is 5v. I left loads out of the drawing for simplicity but in doing so you dont get the full picture! i will post a full drawing later

regards Jason

 

[alec_t]

Q4 will only put ~ 4.3V on the gate of Q3, so may not turn it fully on. Suggest you connect it as you have Q2.

 

[ghostman11]

hi alex can you please explain why its only 4.3v. it isnt working in sim but i dont understand why?? i thought if the collector of Q4 was 12v then the gate of Q3 would be 11.3v.

 

[alec_t]

Q4 base is driven at 0-5V by the '595; hence Q4 emitter and Q3 gate are only 0-4.3V. Q2 is ok.

 

[ghostman11]

looks like i have been thinking incorrectly!! i had assumed as it was a bipolar transistor then the base voltage wouldnt have an effect on the emitter voltage i have changed it as you have sugested but still dosnt work in sim so i will have to wait a couple of days untill the parts get here so i can bread board it thanks for the help much appreciated

 

[alec_t]

still dosnt work in sim

If you post your sim file perhaps someone will spot why.

 

[alec_t]

I've just run this sim ok. Note that Q2 and Q4 must be driven in anti-phase.
View attachment 69053

 

[ghostman11]

thanks alec i am using proteus and i think the p chanel mosfets are not working correctly in the sim. once the kids are in bed i will try again. only using the sim untill the parts arive! could you explain what antiphase means please

many thanks!
BTW i was in Cardiff not so long back!!! man there are some great pubs!! i have a mate that lives on the huge council estate there! talk about rabitt warren

 

[alec_t]

antiphase

When Q2 base is 'high' (5V), Q4 base is 'low' (0V) and vice versa.

BTW i was in Cardiff not so long back!!! man there are some great pubs!!

I'll take your word for that. My drinking days are long gone

 

[ghostman11]

thanks for that alec i understand now! i will try and get it to work on proteus later, day off tommorow so hopefuly get it tested out and board etched

my drinking days should be over as well but when in Rome..........

 

[ghostman11]

this is what i get.

View attachment 69055

surely in this one with Q1 held high the led should be off?? i will be anoyed if its a sim error lol

 

[alec_t]

I think Q1 is upside-down.

 

[ghostman11]

LMFAO!!! great catch alec i am a total muppet!! no wonder my sim wasnt working many thanks mate i doubt i will make that mistake again