Synchronous Cuk converter with totem pole driver

Jan 02, 2024

Because of the continuous conduction at the input and output, I can't seem to leave Cuk converters alone.

The simulated circuit(scroll down to see just the Cuk part)

And a mini-obsession over the last few years has been trying to find ways of driving a synchronous output rectifier mosfet – a challenge because the output circuit is at a negative potential with respect to the input circuit (click here for an earlier, p-fet, idea).

In an idle moment (while avoiding digging up a sewer over Easter) I spotted that the two mosfets in an imagined synchronous Cuk converter would form a totem pole, and so might be amenable to a standard low-side high-side driver.

And if LTspice has done its work well, it looks like this technique would indeed work as the output (lower line on graph left) settles to -2x the input voltage in this converter, whose input fet (top fet in diagram above) has a 2:1 on-off ratio .

LTspice note: I had to delay the start of the input ramp to avoid giving the simulation tool a headache.

Huge drawback

There is a huge drawback with this scheme, as neither the input+output 0V (triangle symbol in circuit diag), nor the negative output, have a dc connection to the IC's local 0V.

The graph right shows the horrendous waveform across which feedback signals would have to jump – the local IC 0V is a -34V 333kHz square wave relative to the output's 0V.

However

However, the traditional TL341+optocoupler feedback used in isolated ac-dc power supplied could easily cross this, I claim….

Powering the switching IC is not such a problem, as the top of C4 – the Cuk capacitor, is at a relatively stable dc voltage relative to local-IC-0V, and so a three-terminal regulator (LDO or dc-dc) could easily do that, maybe with a diode to prevent the regulators input capacitor from messing with the Cuk capacitor, or Cuk capacitors triangle wave from pushing through the regulator.

Getting any control signals in and out of this circuit would also be an EMC nightmare unless more isolation was applied.

For reference: here are the essential components of a synchronous Cuk converter – The 30Ω load is only an example and in real life you would have to add an input capacitor.

As always, hats off to to Analog Devices for keeping LTspice free to use.

Ps, Fixed the sewer eventually…