driving transformer primary

[johnfm]

Hi,

I need a circuit to provide a few mA at 3-5V across an isolated barrier. The hard part is that my total available power on my primary side is 5V at 3mA.

I am trying to debug a low power oscillator/xfmr/rectifier circuit. I seem to have problems driving my primary side xfmr with the output of a schmitt trigger inverter IC. The input waveforms look nothing like my oscillator output. Should I try driving a FET, rather than the primary directly (any idea where I can get low power FETs for driving inductive loads)? Any suggestions for improving the driver circuit? Thanks for your time...

 

[Heydave]

I think I first saw the idea used for data transfer in some data aquisition modules I beleive from Burr-Brown many years ago. I have not seen the technique used anywhere else nor for power but that does not mean it does not already exist. Any mismatch in the driver signals, coupling caps, or imperfect phase will end up being a common mode noise. This is not much different from the problems encountered driving a transformer but my guess is that energy losses are less than transformer approaches. How much noise can be estimated and appropriate common mode supression can be implemented. Use the impedance of the caps at the switching frequency and the power needed to pick the minimum size to do the job. Since the noise will only couple through the bridge diodes, the amount of coupling is diectly related to the power transfer. A very rough estimate could be along the lines of:
Power = 3ma @ 5volts = 15mw or < 12dBm
Assuming a mismatch of 1% or 20dB therefor mismatch noise is -8dBm. A good common mode filter should deal with this fairly easily since the switching frequency is rather high.