PWM & High Speed Optocouplers

by Ivan Hamilton 8/25/2011 7:53:00 PM

One of the signals needed in my servo controller is a PWM signal to control the motor driver. From the Papilio FPGA board I plan to use, comes a 3.3V signal and there's two things that it will need.

  1. Isolate it from the "nasty" high voltage/current sections
  2. Translate it from 3.3V to 5V+

I had an 8 channel PC817 based Futurlec Opto-Isolator Mini Board ($5) lying around (it uses 560Ω series input & 1kΩ output pull ups), and wondered how it would perform. I rigged it up and was initially pleased.

PC817 based opto @ 1Khz

The bottom trace is the 3.3V input, and the top trace is the 5V output  (both 2V/div).

Then I moved the frequency from 1Khz up to 25Khz... 

PC817 based opto @ 25Khz 

At 20Khz the output is barely exceeding 3V and certainly couldn't be called a square wave. At 20% duty cycle, it's basically off, and at 80% - completely on. If we produce a 25kHz signal, and want ~1% accuracy on the duty cycle, we'll need an opto capable of switching in around 1% x 25Khz Cycle = 1% x 40μS = 0.4μS. The PC817 has a quoted 4µS rise & fall time.

It was time to find a higher performance opto, and "common" DIP optos include the 6N13n family (manufactured by Fairchild, Vishay, Avago, etc):

  • 6N138 & 6N139 - 0.1Mbs  10/35μS (High-Low/Low-High) switching
  • 6N135 & 6N136 - 1.0Mbs 1.5 & 0.8μS switching
  • 6N137 - 10Mbs - 0.1μS switching

It looks like the 6N136 should perform as required. A dual channel version, the HCPL2531 is available as well (and so I ordered a couple).

The HCPL2531 has a stated Input Forward Voltage of 1.45V and maximum average mitter current of 25mA. Passing 3.3V thru 220Ω will give IF=(3.3-1.45)/220=8.4mA. Maxium Average Output Current is a paltry 8mA (vs slower 6N138/9's 60mA), so the output side required a 2.2KΩ pull up to 5V (2.2mA). HCPL2531's propagation delay charts show the best "standard" results for IF=16mA & RL=1.9KΩ (close enough to my 8.4Ma/2.2KΩ setup).

HCPL2531 based opto @ 25Khz

The HCPL2531 was much better, and much more suitable to the PWM signals. Top is 3.3V input and the bottom is 5V output (the upper & lower traces are swapped from previous photos before).

The PC817 would still be suitable for slower signals (errors,home switches, etc), and will probably get used for exactly that.

Currently rated 3.3 by 12 people

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CNC | Electronics

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