Friday, January 9, 2015

12 to 60 Volt 300 mA DC-DC Converter Design

I recently did a teardown of an LED light panel, and I wasn't thrilled with the quality of the power supply so I thought I look at designing my own.  Although part of my motivation to do this was the average quality of the LED driver that came with the panel, I also wanted to add some features like being able to dim the panels.  So I've spent the last week doing a bit of research about DC-DC converters and I thought I'd document what I've come up with so far.

Internal construction of the LED panel
I'm not kidding myself, I know this is going to be difficult.  I've never built a switch mode converter before, and I know there are a lot of things to take care of.  Component selection is important, the ESR of the capacitors, resistance of the inductor, on resistance of the switching element, switching times and diode voltage rating just to name a few.  Then you have to consider PCB layout, minimising loop areas to limit EMI, making sure traces are wide enough to handle currents, and keeping signal and power grounds separate.  As there are so many things to look at, I've decided to do what I think is the responsible thing and use an IC designed for the job.  It takes less board space and will generally have a more stable feedback loop than anything I could design.

Now what voltages am I trying to design for?  Well the output is at 60 Volts and requires 300 mA, and for the input I've decided to use a 12 Volt supply.  Why?  Well, there are a lot of products on the market for 12 Volts.  It gives me the option of running the device off a car battery or using a power supply designed for down lights.  It also allows me to offload safety requirements to the manufacturer of the power supply.  I actually intend to use an ATX power supply from a PC, they're safe and have become cheap commodity items providing a ridiculous amount of current at 12 volts for little money.

This is where I ran into a small problem.  I had originally planned to use a boost converter, but to convert 12 to 60 volts would require an ideal duty cycle for the switching regulator of around 0.8.  This number shouldn't really go above 0.6, high duty cycles cause component stress due to high currents.  So I'm left looking at other topologies like, SEPIC, flyback and forward converters.

To complicate things further, I have couple extra requirements,  I'd like to be able to shut down the light and to also dim it by reducing the current through the LED panel, not by using PWM switching.  I know this can change the colour of the emitted light slightly, but as I'm doing photography I don't want issues like rolling shutter to be a problem.  As I'm driving LEDs I also want to use feedback of load current to control the converter.  None of this is too hard, listed below is a short-list of several IC's that should do this perfectly.

Now I just need to choose a topology, and part of that will depend on what transformers and inductors are available. If you have any suggestions, ideas, or similar projects, I'd love to hear from you.

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