High voltage and current LED driver needed

[snarfer]

I need a driver that can drive a string of fairly high voltage LEDs off of a battery pack, which could have voltage between 12 and 36 volts. I need to be able to drive two parallel LED strings, approximately 55v at 700mA, and the circuit also needs to be PWM dimmable at frequency of at least a few kilohertz.

I've been working on making my own driver based on Linear LTC3476. They have a design note that describes how it can be done, but even in simulation their circuit generates transient voltages in the kilovolts and can hardly manage the necessary current. I've been experimenting with some microprocessor based designs as well, but again the transients tend to kill the MCU.

I wonder if there is a commercially available driver and/or driver IC that can provide my required specifications. I have spent a lot of time searching and so far no luck.

Thanks for any suggestions.

 

[SemiMan]

Are you planning to make one driver circuit per LED string?

You are looking at about 40watts per string. With a 12V input, you are looking at some pretty high peak currents. You are going to need a boost regulator with at least a 4 amp peak current not to mention one that has a 60V output. I think you may look a long time for that, I do not think a single chip exists that will do that. That said, I would not be surprised if you can not find an application note for something suitable. Those are pretty simple specs for discrete MOSFETS.

Look up the National Semiconductor LM5022. There are some application notes that may be of use.

 

[SemiMan]

Are you planning to make one driver circuit per LED string?

You are looking at about 40watts per string. With a 12V input, you are looking at some pretty high peak currents. You are going to need a boost regulator with at least a 4 amp peak current not to mention one that has a 60V output. I think you may look a long time for that, I do not think a single chip exists that will do that. That said, I would not be surprised if you can not find an application note for something suitable. Those are pretty simple specs for discrete MOSFETS.

Look up the National Semiconductor LM5022. There are some application notes that may be of use as starting points and looks like there is a design note too.

 

[snarfer]

In fact, yes, I was hoping to use two 35 watt strings. The other alternatives would be to use 10 parallel buck drivers at 7 watts each, or 5 parallel buck-boost drivers at 14 watts each. And those seem pretty awful too!

Thanks for the very useful application note. I had not thought of using a voltage mode boost regulator along with a current sense amplifier, as explained in the application note. That's something I will try simulating next.

You are indeed correct that it would take a very long time to find a boost LED driver with characteristics I describe. In fact I have spent a very long time already! The only one I could find was the previously mentioned IC from Linear. It seems their design note is untested and probably doesn't work though. I sent an email to customer support. Perhaps they can help.

 

[Peufeu]

If you cannot find what you need, perhaps you need to rethink your requirements. Higher voltages like 60V are not very easy to design for. Most of the really good MOSFETs and switching regulators are below 40V if not below 30V, due to the supremacy of the computer industry. Same thing for the good low-ESR capacitors...

Now, do you really need 36V (actually, with lead acid batteries, 36V=something like 45V maximum...) ? If you could drop the input voltage to 24V (30V max), and the output voltage to the same 24V (using more strings of LEDs) you could use a lot more chips, most of the LED drivers available are lower-voltage.

I am currently designing a driver for my bike around the LTC3780. I have not built it yet, but I chose this one because this is the only one that does buck-boost and has an excellent efficiency. However it is limited to 36V.

http://www.candlepowerforums.com/vb/showthread.php?t=195146

So basically you want 80W of LEDs. This means you need an efficiency of 90-95% or you'll have thermal management problems.
Now, at 12V this will make close to 6.5 amps average current, peak current being a lot more (count 10 amps). This shouldn't be out of reach of your average MOSFET so it's OK.

The LTC3780 has an application example for a 12V-5A regulator, with input from 5V to 32V. This is close to what you need. It works straight out of the box in LTSpice and they provide the schematics for the evaluation board.

It doesn't provide PWM dimming, though. You could hack the feedback loop for analog dimming instead, though.

 

[snarfer]

yes i'm thinking right now about just using parallel buck-boost drivers. seems like a lot of pcb real estate though. and also they might need to be synchronized.

i had wanted to be able to use the same LED module in both mains-powered and battery-powered versions so I wanted to keep the voltage high for max efficiency when mains powered. experimenting now with LT1871 external mosfet based boost.