MikeAusC
Enlightened
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 !
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 !
