CMOS open drain PWM output to larger load?

Hello, I am working on a project that uses an off the shelf remote unit driving some RGB leds. The specs are:

Supply voltage : DC12V

Output: Three CMOS drain-open output

Connecting mode: Common anode

Max load current: 2A each color

I need to take this output and expand it to tens of amps per color to drive a much larger display. This install will be permanent so I want to make sure I take into account long term reliability.

I was thinking of just having this output drive the gate of another mosfet (including a pull up resistor) but I can’t find a reference diagram for that configuration. Any one got any pointers?

You probably want a pull-down resistor, not a pull-up.

Other than that, I think your idea will work, although it'll probably be slightly overkill :-) That may not matter for this case.

How many amps? 10A? 100A? :-)

You need something like this, with pullups, not pulldowns. The transistors are doing the pulling down.

You may need to lose the logic inversion of that circuit… If so a p-channel MOSFET can be used, driven from the existing open-drain output (and pull-up resistor). This then drives the main n-channel MOSFET (which has a pull-down). Also this configuration will be normally off when the controller is powered down - and doesn’t invert the sense of “on” from the original controller. Both MOSFETs can be non-logic level (which are easier to find).

Since you might want to switch quite fast and have a good driving capability of 2A, those pull-up and pull-down resistors can be quite low, say 120 ohm 3watt. That gives 100mA to the gates which will switch them off nice and quick.

So I’d suggest using the 12V supply for the p-channel source, 120 ohm from gate to source and gate driven directly by existing unit.

This then drives the main n-channel MOSFET (Rds(on) < 5mOhm, proper heatsink) which also has a 120ohm 3W resistor from gate to source.

If you get power resistors that mount onto the same heatsink this will help keep things cool.

Say load is 30A and 5mOhm n-chan MOSFET is used, then it dissipates a max of 4.5W, each resistor 1.2W, so heatsink needs to cope with 7W per channel (20W for R+G+B if 30A is the current).

Power dissipation depends on the square of current so you must do the calculations for your worse case current. Don’t be confused by the MOSFET current spec, this is not the relevant spec, Rds(on) is all-important.

If the PWM rate is high you will have to work out the switching losses too. Remember that during switching transients the MOSFET’s power dissipation spikes much higher than when off or on. Its important that such transients are much shorter in duration than the PWM period (a hundred times shorter in duration is a good place to aim for - this is why we need low value pull-up and pull-down resistors (MOSFET gates are VERY capacitive).

Thank you for the pointers, I will test every thing with low power first. If I can I will test the circuit both ways as described/drawn. I do not know the final drive current yet. This is for a 7ft by 7ft art install that is RGB back lit. LED density is TBD yet.

Thank you MarkT for the sample heat dissipation calculations. They help immensely. I am planning a large fined heat-sink, I have a few old audio amplifiers that may donate the heat sink, I also have a few of the older CPU passive units from when the CPU had to dissipate >75 watts. I also plan on design for 3 or 4x, and testing at 2x, the final power draw needs.

Next step is to hook the drive unit up to an O scope and see what the PWM rate is and then pick out some parts to order :)

It may take a while but I will keep updates on the progress. Thank you.

With "pull-down" I meant pulling down the gate. I call the resistors in front of the LEDs "series resistors." Also, those will probably heat up a bit -- you'd rather want more LEDs in series to get the voltage/current of each strand about right.

The installation sounds fun, though -- will you post videos when you're done?

jwatte:
With “pull-down” I meant pulling down the gate. I call the resistors in front of the LEDs “series resistors.” Also, those will probably heat up a bit – you’d rather want more LEDs in series to get the voltage/current of each strand about right.

The installation sounds fun, though – will you post videos when you’re done?

The configuration of the output of my device is open-drain. That means “off” it just floats, “ON” it pulls the out to ground. In its normal mode it has the LEDs pull up to VCC when off, when off the cmos shorts the low side of the LEDs to ground. If I want to use that as a feed for another mosfet I need to add a spring(resistor) to pull it fairly hard to VCC (pull up) and let the CMOS out pull hard to common to turn on the next device in the chain. What I was asking for (and got, thank you all) was the proper configuration for the resistors and the related math in picking them. Unfortunately I can remember how most of these devices work but not all the required biasing and current limiting resistors needed. I am using a premade control unit and LOTS of 12V RGB LED strips internally wired 3 leds in series with a resistor.

As for over kill? well at 7ft x 7ft I think it works out to just under 140A@12v. Heat management is going to be a huge problem for the whole project. It is 3 panels so I am going to end up with 3 units @50A each. The more I look at this the more scary it gets :wink: Sorry, I may not get any pics I can share of the finished unit, this is going into a private residence. But I will share every thing else I can!

N-channel MOSFETs are available in more varieties, have lower Rds, are are less expensive, from what I've seen.

Thanks CrossRoads, I have been looking at places like mouser and I see they have packages as small as SMD "rated" to 50A. I am thinking really? those small of leads are going to be reliable at that current? I know things like Rds count but I still think I want a package with some meat to the leads.

Try this one then http://www.fairchildsemi.com/ds/FD%2FFDL100N50F.pdf

Supposedly good for 2500W! Pretty sure proper heatsinking is a must.