# DC/DC converter with 12 outputs, low noise?

I need to 5V to 5V. It might seem surprising but let me expain. I need to convert a voltage of 5V with 60mV ripple (coming from a PSU doing 120V AC to 5V DC) to say 5V with low ripple.

I'm however struggling to know how I would achieve that at a decent cost. I thought about doing a flyback boost converter to have 5V->6V and then having a LDO on each input subcircuit. I would need to reproduce this x12 times, so the cost would be steep for 12 flybacks ICs + 12 transformers + 12 LDOs.

Given the 12 supplies I must make must all be isolated from one another, the current consumption would be about 20mA per supply, the ripple must be very low, would you have suggestions on how to do this without breaking the bank too much?

Is it possible to have a single flyback IC with 12 outputs? Something like that?

More details for those who would ask questions on why I need this: I have to make 6 input subcircuits for current sensing and 6 for voltage sensing.

The total of 12 inputs must be isolated.

All this is done using a 12 TI AMC1301 isolation amplifiers to either sense the current accross a shunt or the voltage accross a step-down transformer.

Since the AMC1301 is an isolation transformer it's fully differential and necessitates two supplies. It takes about 20 mA from each supply voltage.

I need 12 so it's about 240mA of current consumption. I'm mainly worried about the isolated side (the non isolated side can share the same supply.)

EDIT: I need about 2800VDC isolation for 1 minute.

• Why not one 5 V to 6 V boost converter and 12 LDOs? Commented Jan 31, 2020 at 4:38
• @ThePhoton Each subcircuit must be isolated from one another, so that means 12 isolated supplies derived from one 5V. 12 LDOs would not provide the required isolation. Ultimately I need 12 transformers somewhat, or whatever can give approx 2800Vdc isolation (for 1 minute). Commented Jan 31, 2020 at 7:45
• Why must they be isolated? What will be the AC impedance between each channel as a function of frequency? Given your level of experience, is it possible your assumptions are wrong for isolation impedance? Commented Jan 31, 2020 at 7:48
• It's going to be used for protection relays inside big boy power line transformers. It's frequent to have conditions that cause high voltages across inputs for a ton of different reasons. For example phase faults, arcs, etc. Moreover it's a requirement from the client. Commented Jan 31, 2020 at 7:51
• Given the isolation requirements, and the conditions it's going to be used in, you might be better off finding an isolated 5V to 5V module that fits your requirements. This isn't something you want to pinch pennies on. If you're going to build them by the thousands, then, yeah, it might be good to design your own powersupplies. Otherwise, get premade and tested modules.
– JRE
Commented Jan 31, 2020 at 12:38

You could do a flyback with multiple outputs.

You can wind multiple output windings and a single primary to get a bunch of 6V outputs followed by an LDO on each. The regulation of the non-regulated outputs won't be great, but you don't care because you have LDOs on the output.

Also, a flyback doesn't have great ripple performance so you need an LDO with good PSRR at your switching frequency. You'll need very good LDOs.

Finally, I've never tried a 12 output flyback and it might be problematic from a transformer design viewpoint. (Proximity effects, leakage inductance due to poor coupling, poor efficiency due to fringing and AC losses, etc.)

You probably want to limit the secondary windings to 4, and use 3 flyback controllers. You could experiment with more because your current is low.

• I would need one winding at the primary and 12 at the secondary? (or 6 if I were to use two ICs or whatever, or 4, etc). My question being... do those even exist? If so can we get them readily on digikey for example? Would you happen to have one as an example? Commented Jan 31, 2020 at 7:56
• Not all linear regulators are created equal. You'll want to pair your switching regulator with a linear regulator with good ripple rejection at your switching frequency. Or, use a capacitor multiplier to knock down the ripple before the linear regulator as Dave suggests in the video in the linked question.
– JRE
Commented Jan 31, 2020 at 12:12
• @Yannick With the exception of one dual output flyback, every flyback converter I've done has a custom transformer. You can wind your own or get them from Payton or Coilcraft for example if you're going into production. Here's a design example for a 3 output flyback: ti.com/lit/an/slva666/slva666.pdf -As I said in my answer I probably would not attempt more than 4 outputs per flyback as the transformer construction and coupling would become an issue. But with your low currents you could certainly experiment. Commented Jan 31, 2020 at 15:33

While anything is possible with enough filtering, a flyback switcher is not a good start for a low noise power source. If you want to go this route, Linear Tech has chips and app notes targeted at low noise outputs. However ...

240 mA is low enough that you could do this with a sine wave oscillator driving audio transformers, each one followed by a traditional bridge rectifier, filter, regulator, and output filter. If you have a higher source voltage available, such as 12 V, then one power block could be one LM386 as a power oscillator driving four transformers. 3 identical blocks equals 12 outputs. If I recall correctly, there is a sine oscillator app circuit in the datasheet.

The oscillator frequency doesn't have to be 50/60 Hz; it can be anything that doesn't interfere with the signals being measured. For anything above 100 Hz, use a signal diode such as 1N914 / 1N4148 as the rectifiers.

What are the isolation requirements? Do the transformers need safety certifications?

• 2800VDC for 1 minute is the requirement is the isolation requirement. A lot of SMD transformers permit this kind of isolation. Sadly I'm stuck with 5V PSU, those are special ones used for relay transformers that convert line power to 5V DC (i.e. the power is taken directly from the power lines). Commented Jan 31, 2020 at 7:55