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I'm building a parallel isolated DC/DC converter setup, using the Murata DBE0125V2NBSC (datasheet here). The datasheet recommends the following for parallel current sharing:

Turn all units off before configuring the output voltage via PMBus commands; all units must have the same output voltage configuration.

It is recommended to turn on one unit first and then turn other unit (s) on after the output for the first one has settled. Turn on the next unit (s) after the previous unit reaches its regulated output voltage for at l

I have control logic on the isolated side, but what is the most common way to control the primary on/off on these converters? Would I use relays? If so, how would I best choose one (try to match the isolation ratings, etc.)?

Thanks for your input.

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If you're going to be using the PMBUS version/feature of this supply, you should be able to control the power supply enable state with a PMBUS I2C transaction. This will require some sort of PMBUS master device powered by the 48V input supply. This could be as simple as a low-pincount micro-controller with an I2C interface. You can even bit-bang the thing with two GPIO pins, but I digress...

IF you want to enable the supply using a simple on/off Remote Control signal, you will need some sort of logic circuit on the input side. It wouldn't make sense to have control circuits on the output (isolated) side, because when the supply is off, the circuits are unpowered.

This particular module has a pin called RC (Remote Control). After, reading the data sheet several times to makes sense of the "P" and "N" suffix, it looks like they want you to either ground the RC pin or leave it open to control the output state. You could use a simple BSS138 N-channel MOSFET with the gate tied to your 3.3/5V logic circuit, source tied to GND (-Vin), and drain tied to the RC pin. Just make sure that your control circuit GND is connected to -Vin, which it already should be.

In my experience, we like to set things up so that the lack of drive to the power supply remote input (for example, the control cable fell out) will disable the output. If the MOSFET described above were OFF (logic low at the gate) then you want the supply to be off (I'm guessing). Either way works but one way is slightly safer than the other. I cannot definitively determine which suffix (N or P)would do that from the data sheet. The data sheet is written poorly to describe the feature IMO. Your best bet is to call the Murata tech support and ask this question: "If I use the P suffix module and I ground the RC pin, will the output be enabled or disabled?".

Good Luck!

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  • \$\begingroup\$ I have separated isolated converters that power a lower voltage 3v3 and 5V control circuits. This part of the circuit is high power and is supposed to be fully under control of the microcontrollers. I know I could just slap on a MOSFET and tie the grounds to -Vin, but this defeats the purpose of the isolation. Which is why I was thinking that I'd use a miniature relay. As for the P suffix modules, in most cases, they are ON when left open, and turn off when pulled low. I already have a relay controlled by the MCU that controls the overall input, but it seems I would need - \$\endgroup\$
    – Shreyas
    Commented Dec 11, 2017 at 9:46
  • \$\begingroup\$ - individual relays too. I do have the PMBUS version, but the datasheet clearly says that I should not use the PMBUS enable/disable features ever in a parallel current sharing setup! \$\endgroup\$
    – Shreyas
    Commented Dec 11, 2017 at 9:48
  • \$\begingroup\$ If you don't want to tie the control circuit GND to -Vin, a PS2501 opto-isolator will isolate your RC pin GND (-Vin) from the control circuit GND. In general, we try not to use mechanical relays due to the lower reliability of moving parts. \$\endgroup\$
    – Randy Nuss
    Commented Dec 11, 2017 at 14:41
  • \$\begingroup\$ Actually, that's a very nice idea, thank you! This is a one-time board and will be used only by us, and replacing the relays is not a big deal (especially since we need at least one anyway, to cut off all input to the converters). Also, the reed relays I was looking at have way too large of a lifetime to even matter. Nevertheless, I should try to use more things and expand my horizons; thanks for the useful input! \$\endgroup\$
    – Shreyas
    Commented Dec 11, 2017 at 16:38

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