Isolated Low-Power Capacitively Coupled Supply

[Dave Kearney]

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

[Andrew Lohmann]

You could add soft inductors in series with the power transfer capacitor s. They are low Q inductors (a resistor in parallel with each inductor). Also use a multiple buffer gates in parallel as the driver driven from a microcontroller timer ports.

You could also replace the capacitor coupling with a gate drive transformer.​

[BPetr.1]

Hello Dave,

For real DC isolation you need a transformer. There are many transformer drivers like MAX845. Exactly this has 0.2A output current.

Because they work with fixed frequency the circuit can be tuned for ZVS or ZCS. A small LC filter on the input side and caps in parallel to diodes can do the job.

As Andrew stated, the currents through the caps can be significant, therefore I do not expect a charge pump to be less noisy than tuned transformer.

Of course you need linear regulator after the rectifier and some dummy load to keep the tuning in range. May be only resistor and zenner will be sufficient.

Cheers.