MOSFET Rectification

Completely by accident last month when looking at options for single-cell boost converters, I came across a new Linear Technology chip called the LT4320.  This one-chip wonder simplifies a problem which has been dealt with in switched mode regulators for some time but hadn't made it across to the linear domain - the forward voltage drop of a rectifier diode bridge.

When you have a system that is pulling a large amount of current, no diode available will give a properly low voltage drop, Schottky or not.  What you essentially want is to use a solid state switch with very low on-resistance... for which the bog-standard N channel MOSFET is perfect.  The catch has always been to drive the gates, which is do-able but requires a lot of circuitry... Linear Technology has encapsulated it all onto one chip, dramatically simplifying things... four Nch MOSFETs, a couple of caps and you're away!

So I decided to do a little PCB for it... here's the schematic...

It's very simple, as you can see.  This is what the PCB ended up like... the usual 2oz copper, ENIG finish for power stuff... the 4oz used on power amp boards didn't feel necessary.

The LT4320 is rated for an absolute maximum of 80V potential so it can't be used in any application but is best suited for high current applications where the inefficiency of a rectifier diode is very obvious.

The board was designed so either SO8-like or D2PAK packages could be supported.  In any reasonable application the power dissipation should be very low from either of them, but it gives more options as to what packages to buy.

This particular board is to replace the rectifier diodes in my router UPS/PSU so went for NXP 30V MOSFETs of "79A" rating.

Here's the front side of the board... the double connections for the DC outputs mean either spades or block terminals can be installed.  There is space for a snubber on the transformer secondary but for this application it was deemed unnecessary.

Here's the old rectifier board to be pulled out...

 

And here's the new one slotted in place...

As the current draw of the PSU isn't that high, there isn't a big improvement in efficiency (less than 5% improvement, I think), but it does mean that the raw DC voltage is a bit higher which helps the battery charge properly under load.

Overall, worth it but there are projects which will benefit more, like valve filaments!