mosfet p channel led power supply

Thread Starter

hjhjr1

Joined Aug 8, 2021
7
This is a schematic of a 4 channel flasher. I would like to change output to a a positive voltage out, the LED Cathode directly to ground. Very little drain,to source resistance.
thanks
Hilton


1628884156294.png
 

Ian0

Joined Aug 7, 2020
3,754
Put MCP1401 MOSFET drivers between the PIC outputs and the MOSFET gates, and, as you say, use a P-channel MOSFET.
If you want a single IC to drive all four gates, us the TC4467/8/9
 

dl324

Joined Mar 30, 2015
13,506
Welcome to AAC!

The circuit would be simpler if the LED and PIC supply voltages were the same. Is that an option?
 

Thread Starter

hjhjr1

Joined Aug 8, 2021
7
I would like to replace the irz44 N channel with a P channel mosfet. I want the LED Cathode grounded (-) . i would like no extra IC to drive the mosfet,could add resistors or diodes.. This is a commercialy available circuit board.
 

Juhahoo

Joined Jun 3, 2019
249
I would like to replace the irz44 N channel with a P channel mosfet. I want the LED Cathode grounded (-) . i would like no extra IC to drive the mosfet,could add resistors or diodes.. This is a commercialy available circuit board.
Still, makes no sense to change perfectly arranged circuit with P FETs, but okay, Pfet to top side, gate to VCC via resistor and transistor pulling gate down via MCU. No special drivers needed . You can use IRLML6401 as PFET as long as the voltage remains below 12V

1628892801342.png
 

Thread Starter

hjhjr1

Joined Aug 8, 2021
7
Still, makes no sense to change perfectly arranged circuit with P FETs, but okay, Pfet to top side, gate to VCC via resistor and transistor pulling gate down via MCU. No special drivers needed . You can use IRLML6401 as PFET as long as the voltage remains below 12V

View attachment 245595
The voltage on a automobile can go over 14volts. The led is going on taillights are the same bulbs are used as brake lights. The mosfet has to be + voltage out. The cathode is has to be grounded to the ground of the automobile. I would just like to replace the mosfet not add any other components except resistors or diodes. Trying to find a p channel mosfet to replace the n channel mosfet.
 

Juhahoo

Joined Jun 3, 2019
249
The voltage on a automobile can go over 14volts. The led is going on taillights are the same bulbs are used as brake lights. The mosfet has to be + voltage out. The cathode is has to be grounded to the ground of the automobile. I would just like to replace the mosfet not add any other components except resistors or diodes. Trying to find a p channel mosfet to replace the n channel mosfet.
Changing mosfets just N to P will cause a lot of smoke, but good luck :D
 

dl324

Joined Mar 30, 2015
13,506
chech schematic
The circuit you drew won't work. When the GPIO is 5V, Vgs on the MOSFET will be -7V to -9V and that will turn on many MOSFETs. To insure that they turn off, you need a pull up resistor on the gate. That will cause a problem with forcing current in to the I/O. If you force enough current, the IC will latch-up. If latch-up occurs and you don't remove power quickly enough, it will be destroyed. I've used some devices that limited current forced in to the output to 100uA.
 

Ian0

Joined Aug 7, 2020
3,754
Wouldn't the whole point of using a P-channel device be to switch the POSITIVE supply to the LEDs so that the negative can be returned to the chassis?
But your circuit still has the P-channel device in the negative supply to the LEDs?

To drive a P-channel FET from a PIC use a MOSFET driver like this.
61A91135-226F-470E-8BD9-4975D5D71A6F.jpeg
Or use the circuit that @Juhahoo drew in post #6.
Why is this still going on after 13 posts?
 

dl324

Joined Mar 30, 2015
13,506
chech schematic
Here's a circuit that will work. It requires you to change the polarity of the output in the PIC but I guess you already knew that.

I normally wouldn't recommend this hack, but those were the constraints you imposed. I've used this method as a quick and dirty method to level shift between 3.3V and 5V circuits and it usually becomes permanent because it works and takes less space than using proper level shifting.
1628948625705.png
The zener diode needs to be a voltage large enough to force the gate voltage high enough to turn the MOSFET off when the GPIO is HIGH, while allowing sufficient Vgs to turn it on hard enough when the GPIO is LOW.

When the GPIO is HIGH, R1 and the zener limit the current forced into the output. I didn't find anything in the datasheet that specified the maximum that this current can be, but it did say to not allow the voltage to forward bias the parasitic diode on the output to the rail. That current is what can cause latch-up.

From post #5, it would appear that this is for a motor vehicle.

You should be aware that external lighting is regulated in some countries (i.e. USA). If you're using DOT approved LEDs, they'll be designed to operate from 12V so R2,4,6,8 aren't needed. R3,5,7,9 also aren't needed. The PIC outputs should be able to drive the gate cap.
 
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Thread Starter

hjhjr1

Joined Aug 8, 2021
7
Here's a circuit that will work. It requires you to change the polarity of the output in the PIC but I guess you already knew that.

I normally wouldn't recommend this hack, but those were the constraints you imposed. I've used this method as a quick and dirty method to level shift between 3.3V and 5V circuits and it usually becomes permanent because it works and takes less space than using proper level shifting.
View attachment 245651
The zener diode needs to be a voltage large enough to force the gate voltage high enough to turn the MOSFET off when the GPIO is HIGH, while allowing sufficient Vgs to turn it on hard enough when the GPIO is LOW.

When the GPIO is HIGH, R1 and the zener limit the current forced into the output. I didn't find anything in the datasheet that specified the maximum that this current can be, but it did say to not allow the voltage to forward bias the parasitic diode on the output to the rail. That current is what can cause latch-up.

From post #5, it would appear that this is for a motor vehicle.

You should be aware that external lighting is regulated in some countries (i.e. USA). If you're using DOT approved LEDs, they'll be designed to operate from 12V so R2,4,6,8 aren't needed. R3,5,7,9 also aren't needed. The PIC outputs should be able to drive the gate cap.
This is what I am looking for but I need a to-220 case. why this should not be used?
 

dl324

Joined Mar 30, 2015
13,506
This is what I am looking for but I need a to-220 case.
When I looked for P MOSFETs, I used the case style from the N device you mentioned.

This is the package the MOSFET I referenced uses:
1628951290264.png

Note that I chose a device with a low threshold voltage to insure low on resistance. You didn't provide any information regarding drain current, so I chose something that was comparable to the original device. If you're switching a significantly lower current, other less expensive devices can be used, but they'll likely be surface mount.
why this should not be used?
If you're referring to using the diode for level shifting; purists might frown upon it. Diodes have long been used in integrated circuits for level shifting...
 

Ian0

Joined Aug 7, 2020
3,754
why this should not be used?
If you have two nice stable, accurate power supplies, and an accurate zener diode, and don't require any great speed from it, it will work fine.
With an automotive power supply, nominally 12V but 14.7V on full charge, and 9V on cranking, it's not going to cover the entire range. It will just about manage 12V to 14.7V, but it's not going to work at cranking voltage (and it probably doesn't need to).
You might have to select your zeners, because the one specified is 9.1V @ 20mA, considerably less at the 300uA that you are running it al.
 

Audioguru again

Joined Oct 21, 2019
3,829
The datasheet for your IRFZ44 says that it is fully turned on when its Vgs is 10V. The output of your PIC is only 5V when some IRFZ44 Mosfets barely turn on.
You should use a logic-level Mosfet like an IRLZ44 (see the L?) that turns on well with a Vgs of 4V or 5V.
 

eetech00

Joined Jun 8, 2013
2,644
I would like to replace the irz44 N channel with a P channel mosfet. I want the LED Cathode grounded (-) . i would like no extra IC to drive the mosfet,could add resistors or diodes.. This is a commercialy available circuit board.

Connect the driver this way.


1628963659777.png
 
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