As you may have seen I've been designing a DC DC constant current power supply to drive some LEDs from a 12 Volt supply. The main reason for doing it myself was because most of the drivers I've seen don't have a feature to dim the LEDs by reducing the drive current, they typically drive the LEDs at full power and pulse them on and off to vary the brightness. This can be an issue when used for photography.
I'm getting there slowly, and am close to laying out a PCB, but during my research on LED drivers I found a nice little unit from Mean Well power supplies that allows you to dim LEDs by PWM and Analogue dimming. Granted, they have one of the least reassuring names in the electronics industry, but I like their stuff and have never had an issue with them. (I think I've used their products 4 times before) So I thought I'd get one and see how a professionally designed unit performs. They're cheap enough, you can get one like mine for $20 excluding postage.
|MeanWell LDH-45A-350W LED Driver|
As the unit is potted I can't do much of a teardown, but I did pop the back off and you can see some of the signs of a switch-mode converter. In two locations you can see arrays of vias probably used to dissipate heat into the copper pour from switching elements like diodes or transistors, and small traces coming off large areas most likely indicate parts of the feedback circuit.
|Bottom side of LED driver PCB|
The banner specs are also worth a look. The datasheet is for a family of converters, I have the 350 mA model. I'm happy that they comply to an EMI standard, most cheap stuff from China doesn't worry too much about things like that. Short circuit protection is expected, there's most likely a transistor on the low side of the LED string that's used for PWM control, when an over current is detected this transistor simply shuts off disconnecting the load. Encapsulation is good as well, the unit feels rugged, like it would take some abuse.
|LDH-45 LED Driver Family Specs|
My unit is capable of outputting up to 86 Volts and my panels run at 60 Volts, so that's fine. The voltage input range is also fine, although there is a derating curve in the datasheet. Any Vin lower than 12 Volts won't be able to drive the LEDs at full current.
Usage is fairly straight forward, the input voltage is connected to the Vin+ and Vin- wires and the LEDs are connected to the Vout+ and Vout- ones. That's all you need to do if you dan't want to use the dimming features at all, but it will drive the load at full capacity, 350 mA, and my panel is rated for 300mA, so I need to reduce the drive current. For this post I'm not using PWM dimming, I'm just focussing on analogue dimming.
Reducing the drive current is as simple as placing a control voltage across the Analogue DIM and DIM- wires. I assembled a test rig as per the diagram below using an adjustable power supply. The LED panel I used to test the driver was the one I recently did a review of. Voltages and current were measured at various places around the circuit. You can get my measurements and the datasheet from here.
First up I wanted to check the relationship between the analog dimming voltage and the output current. The datasheet gives a graph for this. It's basically a linear relationship between 0.25 to 1.2 Volts.
|Analogue dimming voltage control from the datasheet|
Remarkably, my results were pretty much spot on. I couldn't go to 100% drive current, but the curve matches well for the tests I performed.
|Measured analogue dimming voltage control|
I then wanted to test the efficiency of the unit. The datasheet has a graph, presumably at full current, for the maximum efficiency of different number of LEDs with a Vf of 3.15 Volts. In my case that would be 19 LEDs for an efficiency of 0.932
|Efficiency for different LED configurations|
Once again my measurements compared well. Once a decent load was applied I was able to get a consistent efficiency of around 92 percent. Not too shabby at all. This is great, due to its efficiency, it doesn't get noticeably hot, which means no active cooling needed. (YMMV depending on how enclosed it is)
|Measured Efficiency for different load currents|
For my own reference I wanted to do some electrical tests. These are all performed with an output current of 300 mA
I wanted to see how noisy the converter was electrically and its operating frequency. The first test I did was to clip the ground lead to the probe tip and hold it over the device, a poor man's EMI probe if you will. You can notice that the device appears to be running at 95 kHz. This is surprising as my design runs at close to 600 kHz, mine could be smaller, but may have more loss. You can also see a few spikes created by switching elements. I could guess what they are, but as I don't know the exact topology used I'd only be guessing.
|Scope probe loop held over the converter|
I then check the input voltage ripple of the power supply, it's in the 50mV range.
|Input voltage ripple|
I then checked the input current. As I didn't have anything specifically set up to do this I clipped the probe leads across the input to a multimeter and used the internal shunt to measure the current. Quick and dirty.
You can see that the input current resembles the charge discharge cycle you'd expect from a boost converter.
As the input current is about 1.67 Amps, the ripple is around 30 mA.
|Input current ripple|
There is also a bit of ringing in the input current, that could be the EMI filter or just inductance from all the test leads I was using.
|Input current ringing|
I measured the output current in a similar way. It was measure to be 300 mA by multimeter.
The spikes in the output current have a magnitude of about 180 mA. Once again, this type of thing may be due to the way I'm measuring things.
|Output current ripple|
You can see ringing in this case as well. L and C as far as the eye can see.
|Output current ringing|
Nothing too exciting here, the output voltage is 60 Volts.
Ripple in the output voltage that looks like this could be down to the ESR of the capacitors in the output filter. As current flows into them, V=IR causes the voltage to rasise slightly, when they discharge the voltage drops. Just speculation, but at 60 Volts, what's a couple hundred milliVolts between friends.
|Output Voltage ripple|
I was very interested in the startup profile as I had a hard time getting mine to behave. From the the point the voltage starts to rise it takes about 25 ms to get to the a stable output voltage. There doesn't appear to be any noticable overshoot. All in all quite nice.
|Startup voltage waveform|
I really didn't want to build multiple units like this, don't get me wrong, I want to learn how to do it, but it's hard to beat a self contained module that sells for $20.
Update 03 August 2016
Andry posted a question in the comments and I thought it would be helpful to discuss the problem here.
Please give an advice with PWM and analog dimming on LDH-45B-350.
Input data: 24V DC as power supply. 8pcs in series 1W LED Red.
When I turned on power supply LEDs on the output start to shine on full brightness. Seems everything is fine. The problem is in that when I connect DIM- and PWM DIM pins together with a wire directly as short connection to switch of LEDs on the output my LDH- driver still supply voltage and current to the output. Also I've tried to connect DIM- and Analogue DIM pins together but unfortunately with the same result. Applying PWM signal (0-5V) to DIM- and PWM DIM pins gives no influence to the output independently of duty cycle from 0% to 100% on the output there is full brightness of the LEDs. I've trying everything above with two LDH-45B-350 devices with same negative result.
It took me a while to figure out what the problem was because I was focusing on how the the unit was connected. I started by testing mine to make sure I understood how it operated. By replicating the commenter's procedure I was able to turn my light on and off. It turns out the problem lies elsewhere.
Let's start by looking at the data sheet.
The thing to pay attention to here is that the output voltage range of the LDH-45B-350 is 21-86 Volts. Note 3 is also important. The output voltage will always be at least 3 Volts higher than the input. Andry is using a 24 Volt power supply. This means that the output will always be 27 Volts or higher, and as there are 8 x 1W red LEDs connected to the output, each one has about 3.375 Volts across it. The forward voltage of a 1W red LED is somewhere about 2.4 Volts. So I think that's the problem, The unit is trying to drive a load that is outside of its regulation range. As it stands, the LEDs are being over driven and are at risk of being damaged. There are a few ways to fix things though.
- Add more LEDs. Something like 12-14 LEDs should work properly.
- Add some power resistors in series. Not ideal, as there will always be a small current flowing.
- Add some diodes in series to drop the voltage.
The solution that fits best will depend on the situation. Hopefully this helps.