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I would like to obtain or build a converter that takes DC in, and has multiple (i.e. 6 or 8) isolated outputs. Step up or step down is not important, but might be useful, however is less important than figuring out the overall way of achieving this.

I can design to accommodate with respect to the DC input voltage. Output I require 6 or 8 isolated 5-25V (ideally around 7V) outputs. Each output needs a capability for 15W.

My initial thought is some kind of TL494 circuit as an inverter and a transformer / inductor with several separate windings.

Has anyone got any ideas, or even schematics, of how I might (cheaply) achieve multiple isolated outputs from a single DC source?

Each of the isolated outputs are intended to drive a buck converter based CC / CV circuit. I understand this is a complicated way of going about things but it is part of a wider idea that I want to test.

I have scoured the internet and its not clear how one might easily achieve this.

Thank you in advance.

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My first knee-jerk reaction is to do this with flyback converters. Normally for a few Watts, you can leverage the high volumes of power over ethernet transformers. 15 W might be a bit high for that. Look around.

If you can tolerate some ripple, all you need is a free-running oscillator producing pulses to drive the gates of the low side FET switches. Each isolated side drives a opto-isolator when the desired voltage is reached. The output of the opto back on the non-isolated side forces that FET gate low despite the oscillator signal.

I've done supplies like that using the PWM output of a microcontroller, with the opto signal feeding the fault input of the PWM generator. That takes no firmware overhead once the hardware is set up.

If you don't need any other logic, then just a free running oscillator at maybe 200 kHz is sufficient instead of a microcontroller. Each FET needs its own mechanism to force the gate low independently when its opto signal is asserted.

If you keep the input voltage to below 30 V, then you can use "logic level" FETs that can be driven directly by digital logic. For example, the IRLML6344 would do nicely. It has only 29 mΩ Rdson with 4.5 V gate drive.

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  • \$\begingroup\$ Thank you Olin. I have considered flybacks, I need 0V / ground isolation too, would I still manage multiple outputs from one source? Ripple is not an issue in this application. I am presuming I should only need one 'clock' for all the drives, or would it be better for all the flybacks to be seperate? I will look at the PoE adapters, thank you. \$\endgroup\$ – Rendeverance Oct 8 '17 at 14:41
  • \$\begingroup\$ +1 for this answer. I would go for a Flyback converter too even if 15W x 8 = 120W is a bit high for Flyback. But it can be used anyways. Besides, Since the outputs will be isolated, care must be taken on regulating these outputs. Note that even if you keep the number of secondaries same for all outputs, if you regulate one (by using, for example, TL431-optocoupler approach) the others may not regulate under load. \$\endgroup\$ – Rohat Kılıç Oct 8 '17 at 15:59
  • \$\begingroup\$ @Rohat: I was thinking a separate transformer for each output. At 15 W, that's within reasonable flyback range. \$\endgroup\$ – Olin Lathrop Oct 9 '17 at 13:19
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The easy way is to use forward converters with multiple windings on the secondary.

But locating a source may be hard or DIY core and winding may be best option.

I suggest INTUSOFT free transformer design software.

http://www.intusoft.com/mag.htm

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