OT - LED Dimming Output Stage

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oddbudman
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Posted by oddbudman: Tue. Sep 9, 2003 - 01:03 AM
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Hey there,

Currently I am developing a led dimmer and am sorta at cross roads on which route to take for the output staging. The dimmer works using PWM (dc).

Basically I want to build a 6 channel LED dimmer that can dim something like 2A per channel @ 12/24v.

I say 12/24v because many of the commercial modules that are hitting the market these days tend to want to work off one these voltages, however others do exist. So I was hoping I could design the dimmer so that it could switch which output voltage it was opperating at.

Some other design overheads include:
- Overload protection
- Small board space

At the moment it looks like I have 2 options for handling the output stage:

1. Use discrete transistors
2. Use opamps

I had the opamp idea the other day after reading an article on using a opamp as a variable voltage powersupply. It seemed ideal in that they often come with overload protection, and changing the output voltage is just a matter of changing the feedback. Since board space is at a premium I also think perhaps using opamps could save some space.

Since I really am not all that familiar with opamps I thought i may as well post here and hope for some feedback.

Is my idea of using an opamp ok? or should I be looking at just using transistors?

If you could suggest any good links too that would be appreciated. (I have googled already but i'm probably looking in the wrong places)

Look foward to some replys.

Thanks in advance,

odd.

Tags:

AVR Microcontrollers, megaAVR and tinyAVR
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admin
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Posted by admin: Tue. Sep 9, 2003 - 01:20 AM
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Hi!

If you are PWM:ing use transistors, it's cheaper. Othervise you need to use expensive OP's for 2A or extra boost transistors for the OP.

Björn

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admin
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Posted by admin: Tue. Sep 9, 2003 - 01:52 AM
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At 2A, using an op-amp is quite expensive, and the outputs do NOT get very close to the rails. Further, they tend to act like a voltage-souce either high or low and you would have to power them from the raw supply.

If your source is automotive (truck, tractor, bus, etc), you really need to watch out for "load dump". This is the situation where the battery is disconnected when the motor is running. A very high (65V for a 12V system) will result and it will last for milliseconds. You may think that you don't have to watch out for this, but mechanics are notorious for forgetting about this; they are not electricians, after all. In a 24V system (trucks and buses). the load dump transient is even higher.

You are much better off using transistors or FETs. In my book, a FET is much easier to use from the port-pin of a micro in this kind of application. Here is one example: To saturate a transistor, you need to supply a base current of about 1/10 the collector current. Yes, I know beta is a lot higher than this, but beta drops very much as it gets close to saturation. So, 2A/10 = 200ma that has to be provided to the base. You can't get this kind of current from a port pin so you need to use something like an emitter follower. With one emitter follower (per power transistor), you can probably get by with 20-25ma of base current, but you will be sucking a LOT of current from your 5V supply OR you will be dissipating a lot of power just in the driver. And, forget a Darlington because the saturation voltage is quite high and the power dissipated will be very high at 2A (2-3W) per transistor.

A FET may cost more, but you can drive it directly from a port pin and you can get lower saturation voltage (than a Darlington). This means less power dissipated and less heat to get rid of. With 6 LEDs, this can make a LOT of heat! With an on-resistance of 0.1 ohm, 2A gives you 0.2V of drop and the power is 0.4W. FETs are readily available with an on-resistance of 0.05 ohm but you really need to watch out for the gate-drain voltage rating (look for 65V in a 12V automotive system).

Hope this provides you some useful ideas.

Jim Wagner

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dwb240
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Posted by dwb240: Tue. Sep 9, 2003 - 01:56 AM
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Hi,

Go with transistors, MOSFETs maybe even.

The op-amp will function like a linear voltage regulator I assume, which means that if you want a 6 volt output, it will have to drop 6 volts, with 2 amps flowing through it. Thats 12 watts of power, quite a bit! The heatsinking will take up much much more space than anything else...

But if you use MOSFETs, they are either ON or OFF. In both those states there is basically no voltage drop (with low RDSon ones), so no power dissipated. Of course though the state between ON and OFF is a state of changing resistance, and during this time the MOSFET will have a large voltage drop and have to dissipate a lot of heat.

So if you use MOSFETs (or transistors as well), keep the switching frequency as low as possible for the dimmer. Too low and you'll get flicker of course, so maybe 100 Hz. This low frequency makes these transistion periods occur less frequently, for higher efficency (sorry for the bad spelling throughout my post ;-)

Look for 'logic level mosfets' for simplicity.

Regards,

-Colin

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dwb240
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Posted by dwb240: Tue. Sep 9, 2003 - 01:57 AM
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Looks like you got in there first Jim ;-)

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oddbudman
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Posted by oddbudman: Tue. Sep 9, 2003 - 04:31 AM
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Thanks for the fast replies! much appreciated.

Any ideas on the overload / short circuit protection?

After doing some reading MOSFETs seem to be the ideal solution thanks :)

odd

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Tobiase
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Posted by Tobiase: Tue. Sep 9, 2003 - 08:32 AM
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A FET and an op-amp will give you current control as well.
Just a thought.

Tobias

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Posted by admin: Tue. Sep 9, 2003 - 11:33 AM
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Just a thought - Look at high side switches from ON semiconductor.
They are complete output voltage switches often with thermal and short circuit protection built in.
Rob

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Posted by admin: Tue. Sep 9, 2003 - 11:44 AM
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There are Fets around such as the ST Omni FET range that have built in overload protection. If you drag too much current for too long they over heat and cut out untill they cool down again.
I have, to date, found them to be very robust.

Mike

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Posted by admin: Tue. Sep 9, 2003 - 12:16 PM
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A current sense resistor is a cheep solution. Maybe 0.1 ohm at 2 Amps you have 0.2 V. Opamp (comparator is very fast) with a ref. voltage of 0.25V will protect your FET's. With this comparator output you can cut off the gate of the FET and set an interrupt to switch off your PWM.

Michael

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Posted by admin: Tue. Sep 9, 2003 - 02:53 PM
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Once again, thanks for the replies.

Mike, I stumbled across the omni fets while looking in the farnell catalog for the 'logic level mosfets' Colin mentioned.

These look pretty good! What a sensible looking IC.

I think i will order a few tomorrow so I can do some testing, I'll try my best to get back to this thread for the results. Perhaps this will help some other avrfreaks :)

odd

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oddbudman
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Posted by oddbudman: Fri. Sep 12, 2003 - 05:24 AM
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The omnifets arrived and are now in operation.

They are teriffic! I have them hooked straight to the microcontroller and am using them to pull 24vdc down to switch on a series of LED modules.

Currently I am only switching 300mA or so per channel- total of 3 channels RGB mixing. I only wish I had more leds!

Once again thanks for all the help and assistance. I would definitely recomend omnifets for this application to any others out there.

happy tinkering.

odd

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