Low-Power 3.3V 50mA Isolated Capacitively-Coupled Supply Without Transformer?

Hi Guys,

I'm designing a low-power isolated supply (3.3V, Max ~50mA) and am keen to steer clear of transformers as EMI is critical to the design and I've had some noise issues with my previous transformer-based design.

I have been looking at using a capacitively coupled circuit plus bridge rectifier as per Maxim's AN4553 app note below (or a similar charge-pump type circuit which will allow me to get ~150mW of power across the isolation caps) but their MAX256 chip is far overspec'd from a power (and size/cost) point of view (3W) for my requirements.

Do you have any suggestions for a driver to replace the max256 here, which would differentially drive a square wave into a circuit like this at <2 MHz? Or is there an alternative way to achieve a capacitively coupled low power supply? I've had a look at some cmos drivers, even 555 timers, but nothing seems to provide the combination of (a) required source/sink output power and (b) dual, complimentary square wave output I require to drive into the capacitor/bridge circuit.

Since it is such a low power application, am hoping there is a cheap, small, quiet way to do this easily, but so far I haven't found it.

Thanks in advance,

Dave

1. How many volts of isolation are you hoping for?
2. What are you hoping to drive (I ask because there are some isolated D/A, A/Ds and Communications drivers with built in power supply across the barrier)

Have you considered a 50/60 Hz transformer running off the power line or inverter? These have been used to power EMI-sensitive AM-broadcast radio receivers for over ninety years.

Hi Aubrey,

Thanks for the reply. I only need 100V+ of isolation really, it's mainly to remove any potential for ground loops in my circuit. I need to separate isolated supplies of a few 10s of mA each, one to drive a GPS Antenna LNA and one to drive a bank of Digital IOs. Both need to be isolated from the rest of the system to avoid setting up a ground loop and to minimise risk of an overvoltage on one of them making it across to the rest of the system.

Any ideas you have would be great. Even any thoughts of ultra-low EMI solutions for this type of application in general would be appreciated. Is the capacitively-coupled charge-pump type approach not really used?
Many thanks,
Dave

mixtwitch, what frequency did your previous noisy transformer-based design run at?

A small linear power supply (fuse, 50/60Hz transformer, surge-limiting resistor or lamp, rectifier, filter capacitor, three-terminal linear regulator) will easily provide 3.3V @ 50mA, isolated to 1000V, with no noise. You are asking for just 165 mW output, so the 30% efficiency won't generate much heat.

Dave
I feel your pain!
One technique used is to use a capacitor as a series impedance element. There are 2 issues. Firsly there is a volt drop across the capacitor, and you need to increase the capacitance and/or frequency to reduce t his. Secondly, the isolation is determined by the breakdown voltage of the capacitor and that translates to physically bigger capacitors. Where safety is at stake it becomes more complicated, but safety is not at issue here. There are several companies that transfer data in this way but none who use the technique for power. Take a look at TI's AMC3306M25 (https://www.ti.com/lit/ds/symlink/amc3306m25.pdf?ts=1607702208014&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FAMC3306M25 ). In it you will see the data transfer via capacitors, but the power is transferred via magnetics.

There is a technique called "flying capacitor" isolation. In the good old days this technique was used to isolate analog inputs to an ADC. In principle you have a floating capacitor with each end connected to a SPDT (changeover) switch. The capacitor is connected to one side of the isolation to charge. The two switches are then connected to the other side of the isolation to discharge. A solid state switch would not work for that because you couldn't achieve isolation. But relays do work- reed relays were normally used.
Of course with power transferred this way, you would have a great deal of ripple as the capacitor discharges and recharges. I don't think this approach will work for you.

I cannot think of any other way a charge pump can work across the isolation.

I have worked with the MAX256 far less than the 3W power draw without any issues, but I must admit I never really tested for EMI- the products never really got to release.
Of course you could use almost any oscillator with a transformer, but I am not sure you would be able to create anything of greater superiority that the MAX256, or similar.

There are also dc to dc modules- they tend to be more expensive, but you need to check their noise and ripple.

Good luck!

While clearing my papers I came across this TI device- very similar to the MAX256, but with specs closer to your requirements. See SN6501 (https://www.ti.com/lit/ds/symlink/sn6501.pdf?HQS=dis-dk-null-digikeymode-dsf-pf-null-wwe&ts=1608494798398 ). Still transformer coupled, I think the package is smaller, but the rest is about the same size.