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I'm designing a programmable bench power supply (PS) that has multiple isolated channels (each powered by a separate transformer winding). I like the idea of isolated channels so that multiple channels can be connected in series to generate dual-polarity output (like -15/0/+15 V). The idea was to use a microcontroller (MCU) to set the output voltage and current limits of each channel using analog voltages from the MCU's DAC (or an external DAC IC controlled by the MCU). Given that channels are able to float with respect to one another, what would be the best way to ensure that the control voltages sent from the MCU to each PS output channel are properly referenced to the channel's voltage level?

The options that I've come across so far are to either:

  1. (analog approach) use an isolation amplifier between the DAC and each PS channel
  2. (digital approach) use a separate DAC IC for each PS channel and digital isolators to interface each DAC with the MCU.

I'm curious to know if there is a standard approach to this type of problem as I imagine that this comes up in many bench supply designs. Thanks!

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  • \$\begingroup\$ You probably need to both set voltage/current and sense voltage/current. It is easier to use two digital couplers for isolated UART and place secondary MCU behind the isolation. (Not sure about supplies, but this is how multimeters work) \$\endgroup\$ – venny Sep 2 '14 at 20:43
  • \$\begingroup\$ Is 15V the maximum voltage for any channel? And how much current? (One problem for any bench supply is getting rid of the excess heat...power in the regulating element.) \$\endgroup\$ – George Herold Sep 2 '14 at 21:33
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Most of the power supplies that I have looked at have used some combination of analog and digital isolation, depending on the supply. Most of this is drawn from repairing a couple of HP/Agilent/Keysight E3600 series power supplies. Generally, there are at least two processors: one is an I/O processor for the GPIB/serial interface, and a different one manages the user interface and direct control over at least one power supply channel. These two generally communicate via a pair of optoisolators. The I/O controller shares a ground with the I/O connectors on the back. The main controller is grounded to the common of one of the power supplies. In supplies with more than one output, the other output is connected with analog optoisolators to the main supply. I think there was something like 6 optoisolators connecting the two channels together in my E3649A dual outut supply, some for analog signals and some for digital signals. I have a triple output Tektronix supply as well, but I can't remember how it's wired offhand. With optoisolators, I think they do a good job of passing an analog signal through if they are driven in current mode, so you will need op amps on both sides to condition the signal.

MCUs are very cheap these days; it may be advisable to put an entire microcontroller on each channel and then use some sort of serial protocol for communication through digital isolators. The Atmel XMEGA series has built-in DACs and ADCs and lots of serial ports (4+), for example. One of these per channel and one more for a main controller could be a very nice design, and it would only require two digital isolators per channel.

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I think the MCU-per-channel approach is a good one, assuming you're simply setting static voltages and current limits. You could use opto-isolators or magnetic isolators and send data serially bidirectionally (such as fault data in the reverse direction). I would not necessarily limit thinking to UART-to-UART communication- it might make more sense to send the data via SPI, which could avoid the necessity of closely matched clock frequencies. In fact, with the right SPI DACs you might be able to pass the digital data directly without an MCU (at some loss in flexibility).

Another possibility, which could be cheaper, would be to use PWM to send data, which could allow you to use a single MCU, if it had enough independent PWM channels. Then the isolated control circuitry could consist of not much more than a reference, a gate and a low pass filter.

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