# 100 watt CC LED Driver for $12



## MikeAusC (Dec 9, 2011)

There still aren't many 30 watt LED drivers around for the SST-90, so I wasn't expecting to find a quick or cheap solution when I wanted to drive my 100watt LED array from a 12 volt battery. The array is ten strings in parallel, with 10 LEDs in each string, so I need 30 volts at 3.3. amps to achieve full drive.

Sure, there are plenty of 100 watt DC-DC converters around, but they're all Constant Voltage. For stable LED operation as temperature varies, we want *Constant Current*.


Theoretically you can convert any CV converter to CC by sensing feedback voltage across a current sensing resistor instead of voltage across the load. To be efficient, you want as small a voltage as possible across the current sensing resistor, but then you have to amplify that voltage, and that's where the problem arises - any phase shift in the amplifier can make the feedback control unstable.


Long before op-amps (yes, I've been around that long) a simple way of current threshold sensing was to use the non-linear conduction knee of a transistor by simply connecting the Base and Emitter of a PNP transistor across the current sensing resistor. As soon as the voltage drop approaches 0.5 volt, the transistor starts to conduct heavily. If you feed this into the voltage-sensing input of a Constant Voltage DC-DC Converter you have a CC driver with high-side current sensing. The single transistor causes minimal phase shift compared with an OpAmp.


If you're driving a single white LED this way, the efficiency is poor, 3.5 volt across the load and 0.5 volt across the shunt - 12% loss. But with a 30 volt load the numbers work out much better - less than 2% loss.


But how to build the whole lot for $12 ???? I was amazed to see small 150 watt DC-DC Step Up Converters available for less than $10US that really work. With only needing 100 watts here, there's a good margin - although for long running at full power, you need to add good heatsinking or airflow. By adding a few standard components for a couple of dollars, you can convert it from CV to CC for LED driving. You can power it from 12 to 28 volts if youre driving a 30 volt LED>


This is the one that I have - http://www.satistronics.com/dc-1032...step-up-converter-for-car-notebook_p2897.html 


This one seems to be an improved version with better heatsinks. http://www.satistronics.com/150w-dcdc-1032v-to-1235v-converter-boost-charger_p2985.html


On the lower one in this photo, I've had a go at mounting the additional components on the original PCB. It worked ok - except that the Out+ terminal is double-sided - that's why the LED+ connection is raised off the board. The upper assembly is the 120 watt version - the same PCB with some components changed and different windings on the toroid.
https://lh6.googleusercontent.com/-k2Q5gGtFsvY/TuB4ctcjiMI/AAAAAAAAAC4/UsdWX1e-Wdk/s800/IMG_4251 100watt Driver Cu.JPG

https://lh6.googleusercontent.com/-raLm5hr6Al8/TuB4dfr4vvI/AAAAAAAAAC8/bcR7A4XqXyk/s800/IMG_4254 100watt Driver top.JPG


This shows the few components I've added to create a CC Driver from a CV supply. You can use any small-signal PNP transistor. To protect the LED from current surges if the LED is connected when the power is on, set the open-circuit output voltage to be 1 volt greater than the maximum voltage across the LED in normal operation.
https://lh3.googleusercontent.com/-PNKtbQsnFFU/TuB4fAICAZI/AAAAAAAAADE/y4ElruSSqwM/s800/IMG_4268 100watt Driver Circuit.JPG



Here is the 100 watt LED that I'm using.
https://lh4.googleusercontent.com/-tiDY89GMdQU/TuB4eeBuhII/AAAAAAAAADQ/3nSd42dIdAc/s800/IMG_4256 100watt LED.JPG



I'm working on selecting three brightness settings - 10/30/100 watts - by switching three different sensing resistors'. But my tip is - don't try and switch the resistors using mechanical switches !


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## MikeAusC (Dec 9, 2011)

Reserved for updates


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## VegasF6 (Dec 9, 2011)

Bravo, thank you for sharing. I was just looking at this boost circuit literally 2 days ago for a project I am doing. I need 24V, I wasn't looking at making a CC circuit as you have done, but good to get some feedback on the original circuit as well.
Thanks!


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## MikeAusC (Dec 10, 2011)

VegasF6 said:


> Bravo, thank you for sharing. I was just looking at this boost circuit literally 2 days ago for a project I am doing. I need 24V, I wasn't looking at making a CC circuit as you have done, but good to get some feedback on the original circuit as well.
> Thanks!



If 3 amps is enough, you could use this one - http://www.satistronics.com/mini-dc...nsformer-converter-power-regulator_p2210.html


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## mpteach (Dec 20, 2011)

I found that 100w cv driver for 8$ plus free shipping on ebay. I also found a 100w cc/cv driver for $16 which would need no mods and is in a blue case.

How efficient is your driver at 30v?

Wouldn't a germanium transistor be more efficient than a silicon with a lower drop?
How much does the current output drop as the transistor warms up?

Would an AD8217 work better or would that have problems with phase shift? It looks likes its designed for current sensing.


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## MikeAusC (Dec 20, 2011)

mpteach said:


> I found that 100w cv driver for 8$ plus free shipping on ebay. I also found a 100w cc/cv driver for $16 which would need no mods and is in a blue case. . . . .



Yes, this is a great option for 100 watt LEDS.




mpteach said:


> . . . . . . How efficient is your driver at 30v?
> 
> Wouldn't a germanium transistor be more efficient than a silicon with a lower drop?
> How much does the current output drop as the transistor warms up?
> ...



A silicon transistor drops 0.5 volt so that's 1.5 watt in a 100 watt load - I wouldn't worry about it.


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## mpteach (Dec 20, 2011)

What is the efficiency of the $8 driver? They only list the efficiency at 16v in 19v out lol not 12in 30vout.


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## MikeAusC (Jul 27, 2012)

Here is some more info for less technical people who want to do this mod.

I used a 2N4356 transistor, but any PNP transistor capable of driving 100milliamp and 40 volt Vce rating will do.

All the resistors are 0.5 watt. 1k=1000 ohms. 4k7=4700 ohms. The current sensing resistor needs to be 0.22 ohm for 3 amps.

The capacitor is a 1.0 microfarad, but it must be the monolithic type to cope with the high AC current through it.

I do NOT recommend building it onto the existing board. Just add the components on a separate board and connect to the output from the Driver using short wires. Wire back to the sensing point on the Driver as shown by the brown wire.


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## MikeAusC (Oct 29, 2012)

If you want to avoid soldering, QSKJ have now made a Constant Voltage / Constant Current 100watt Driver that's ideal for this purpose - QS-1235CCBA-100W. It can output up to 35 volts at full power from 10 to 32 volts input. You set maximum Output Current from 0.5 to 5 amps with a screwdriver adjustment.

The Input must always be lower than the output - including under short-circuit conditions. So don't try to drive 3 LEDs in series from a 12 volt input - the Switching Diode will start to conduct, feeding the input directly to the output

The only supplier I've found charges $18 - Google "dx sku 157473".


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