High Current High Side Switching

What would an ideal Q2 be? Something like an FQP7P06 ? This particular datasheet does not indicate that the tab is connected to any of the pins (I know that's a thing to be careful of on BJT's).

Since I'll be moving 2.5 amps (30 watts) through it, I assume a heatsink of some sort would be a good idea? If the tab is in fact electrically isolated, I could just run an aluminum bar across the tabs.

magnethead494:
What would an ideal Q2 be? Something like an FQP7P06 ?

It'll do the job. You could look for one with a lower on resistance and Id. It would be uncommon for the source or drain to not be connected to the tab.

Since I'll be moving 2.5 amps (30 watts) through it, I assume a heatsink of some sort would be a good idea? If the tab is in fact electrically isolated, I could just run an aluminum bar across the tabs.

You're using the MOSFET as a switch. A typical FQP7P06 would have an Rds(on) of about 350m ohms at 2.5A. Power dissipation at 2.5A would be (2.5A)(2.5A)(350m ohms) = 2.2W. A heatsink is still needed.

dl324:
It'll do the job. You could look for one with a lower on resistance and Id. It would be uncommon for the source or drain to not be connected to the tab.You're using the MOSFET as a switch. A typical FQP7P06 would have an Rds(on) of about 350m ohms at 2.5A. Power dissipation at 2.5A would be (2.5A)(2.5A)(350m ohms) = 2.2W. A heatsink is still needed.

Thanks. So i'll need to heatsink them individually somehow without allowing them to touch eachother, I take it. (I still find it odd the datasheet doesn't specify). Or use nylon nut and bolt and thermal pads.

You can buy insulators.

https://www.jameco.com/z/06-202-TEN--Heat-Sink-Mount-Kit-For-TO-220-Components-10-packs-_34121.html

magnethead494:
I still find it odd the datasheet doesn't specify

You are right! The datasheet is missing that critical detail. The tab is almost always the same as the middle pin on any TO-220 package; they are presumably punched in one piece.

Mind you, that datasheet is actually from 2001 (down near the bottom). That really is a rather poor device for your application, you would be much better off using a modern part requiring no heatsink. If it is a SMD part, just use these adapters:

The chip plus the adapter will be no bigger than the TO-220 itself. You will have a vastly more compact assembly. :sunglasses:

Paul__B:
You are right! The datasheet is missing that critical detail. The tab is almost always the same as the middle pin on any TO-220 package; they are presumably punched in one piece.

Mind you, that datasheet is actually from 2001 (down near the bottom). That really is a rather poor device for your application, you would be much better off using a modern part requiring no heatsink. If it is a SMD part, just use these adapters:

The chip plus the adapter will be no bigger than the TO-220 itself. You will have a vastly more compact assembly. :sunglasses:

What would be a better P-channel Mosfet? I've only done a few pieces in SMD and none of them were exactly pretty; granted 3 pins on an 0.1 spacing is easier than 10 pins on a .0254 spacing.

Searching "P-channel logic level FET" ...

Well, this one would be an excellent choice but is in a "DPAK" format - small but somewhat easier than a SOT-23.

This old one would appear to be adequate, not logic level but since you are going to drive it with at least 10 V, it should switch your 2.5 Amps easily with no heatsink.

And it is in a TO-220 package.

(By all means, someone can check my reading of the datasheets. :grinning: )

Paul__B:
Searching "P-channel logic level FET" ...

Well, this one would be an excellent choice but is in a "DPAK" format - small but somewhat easier than a SOT-23.

This old one would appear to be adequate, not logic level but since you are going to drive it with at least 10 V, it should switch your 2.5 Amps easily with no heatsink.

And it is in a TO-220 package.

(By all means, someone can check my reading of the datasheets. :grinning: )

I work 1 block from Mouser Headquarters, so I've been going through their website. I just don't know how to tell crap from, well, not crap...on top of limiting options by minimum voltage and current load.

So i'm about to start building this project.

Question: So using a ULN2803A as the low side driver to drive the FQP27P06 P-Channel QFET high side driver, what resistor do I need as a pull-up on the gate of the QFET? Just a 10K, or is there a specific value?

Also, is a series gate resistor necessary in this application? If so, what value would be best?

If it were me I would just go with a simple solution:

With the right heatsink

this P-channel Mosfet could handle the 7.5A Bank1 an Bank2 easily

Datasheet

Bank1 => Mosfet-1
Bank2 => Mosfet-2
Bank3 => Mosfet-3

Parts:
qty: 3 FQP27P06 mosfet
qty: 3 TO-220 heatsink

The mosfets are logic level so you can drive them directly with an arduino output.
You might want to add 1k series resistor on each gate and 10k resistor from gate to GND.

10k should work nicely.

A series gate resistor is needed if the Arduino was driving a MOSFET, this limits the Arduino output current to a safe level.

The 2803 is much more resilient.

BTW
It is always best to provide a proposed schematic as they are worth a billion words.

Paul__B:
You are right! The datasheet is missing that critical detail. The tab is almost always the same as the middle pin on any TO-220 package; they are presumably punched in one piece.

Mind you, that datasheet is actually from 2001 (down near the bottom). That really is a rather poor device for your application, you would be much better off using a modern part requiring no heatsink. If it is a SMD part, just use these adapters:

The chip plus the adapter will be no bigger than the TO-220 itself. You will have a vastly more compact assembly. :sunglasses:

I like that.

Can add to the list for one of a kind wiring.

Here is my current shopping cart.

I work 1 block from Mouser, so I'm going to call it into Will Call in the morning and pick up during lunch

https://www.mouser.com/ProductDetail/512-FQP27P06

https://www.mouser.com/ProductDetail/511-ULN2803A

https://www.mouser.com/ProductDetail/571-1-2199298-6

https://www.mouser.com/ProductDetail/603-CFR-25JT-52-10K

https://www.mouser.com/ProductDetail/279-CFR50J100R

https://www.mouser.com/ProductDetail/474-DEV-11113

https://www.mouser.com/ProductDetail/374-T5994ST

https://www.mouser.com/ProductDetail/855-M20-9992046

https://www.mouser.com/ProductDetail/755-BA07CC0T

Use the Link toolbutton

magnethead494:
Here is my current shopping cart.

I work 1 block from Mouser, so I'm going to call it into Will Call in the morning and pick up during lunch

FQP27P06 onsemi / Fairchild | Mouser Europe

ULN2803A STMicroelectronics | Mouser Europe

1-2199298-6 TE Connectivity | Mouser Europe

CFR-25JT-52-10K YAGEO | Mouser Europe

CFR50J100R TE Connectivity / Holsworthy | Mouser Europe

DEV-11113 SparkFun | Mouser Europe

T5994ST TT Electronics - IoT Solutions | Mouser Europe

M20-9992046 Harwin | Mouser Europe

BA07CC0T ROHM Semiconductor | Mouser Europe

All this can be shown in a schematic :wink: .

  1. FQP27P06 P-channel Mosfet

  2. ULN2803A Darlington Array

  3. 20-pin ic socket

  4. Carbon Film Resistor Through-hole 10k/1%/1/4W

  5. carbon film resistor 100 OHM/1/2W/5%

  6. Pro-Mini 5V/16Mhz

  7. 12V 5A ac/dc adaptor

  8. SIL Vertical Pin Header 20-pin

  9. 7V 1A LDO Voltage Regulator

Doesn't that look better ?

(done using the LINK toolbutton (chain link symbol).
Click the LINK toolbutton
Paste the URL, click OK,
Type in text label, click OK

I drew this up for the 2803A and FQP27P06.

This is to build a display for automotive lighting, as a demo sequencer for Switchback bulbs, trailer electronics, and third/fourth brake light assemblies.

Re:links I am used to the auto-parse of Xenforo and vBulletin

Hint

You can copy and paste images from the internet, draw lines in a program such as Paint.

That's a pretty drawing.

I have photoshop on my home computer and could have drawn that there, but not on my phone here at the workplace.

But yes, that's an accurate drawing for this project.

Should I include a 100 ohm current limiting resistor between the darlington collector and the MOSFET gate? For 3 cents each i figure it can't hurt.

Also, on the 7 volt LDO (to feed the VIN of the pro-mini), I seem to recall that there is supposed to be a supporting circuit for them to prevent oscilation. I'm not seeing one in the datasheet, so I assume everything is integrated?