Looking for a synchronous mosfet rectifier circuit that works - Page 1
Forum section: electronics / projects

author: e100
author_rank: Frequent Contributor
timestamp: October 09, 2012, 10:25:05 am
content: This is for a 50Hz 9Vac 20A transformer with no centre tap.The output is a purely resistive load with no smoothing cap so it doesn't have to block in the reverse direction.The goal is to rectify without generating 20W of heat so I can use a small plastic enclosure for the entire project (sans transformer) instead of a large metal enclosure.The driving part of the rectifier can be self powered, or powered externally.I've found several theoretical self powered circuit ideas which people that have critisized for not fully driving the mosfets properly etc.There are also specialist recifier chips designed for switch mode power supplies but these seem overly complex to use as they use sense windings.So, does anyone have a circuit which has been used in a real project that isn't too complicated?If it helps, I have various single and dual rail op amps, some logic level N channel mosfets and comparators etc. I also have some N channel high side mosfet drivers and non-logic level N channel mosfets, and isolated dc-dc converters on order for another project, but currently don't have any P channel mosfets.ThanksMike

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: October 09, 2012, 11:26:11 am
content: I think you will have trouble satisfying your requirements with a linear power supply.Any idea how many watts loss you are going to get in the transformer alone?As far as your requirements, say you want 18 watts loss and you have output of roughly 20a at say 10v dcthis should be about 200/218 = 91.7% efficiency. I don't think you will get it even if you manage to lose the diode drops.Then if you want it regulated voltage then your efficiency will go way down at these low voltages.Even switched modes will have trouble meeting your specs.

author: e100
author_rank: Frequent Contributor
timestamp: October 09, 2012, 12:12:41 pm
content: The transformer is in its own metal box for safety and thermal reasons.Overall efficiency isn't the prime goal. I don't have any particular targets. I would just like to get everything else in one plastic box, hence the desire to minimise power dissipation inside the box.The load is outside somewhere else and uses rectified AC with no smoothing or linear regulation.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: October 09, 2012, 12:53:27 pm
content: I guess the reason I was talking about efficiency was that any losses will be your heat.Ok I think your idea is feasible If you take the transformer out of the equation but I dont have a circuit for you.Can you put a bridge rectifier in the metal box?For the circuit.You just need zero crossing detection, fet drivers, and some sort of failsafe dead time mechanism between your fets. which I guess you already know.I don't know whether you should go micro controlled or analogue. I would go micro but I think many others would go analogue.Good luck.

author: BravoV
author_rank: Super Contributor
timestamp: October 09, 2012, 01:57:46 pm
content: Need 2 pairs of N and P-mos which you don't have.Detail here ->http://www.extremecircuits.net/2010/05/power-mosfet-bridge-rectifier.html

author: e100
author_rank: Frequent Contributor
timestamp: October 10, 2012, 11:06:05 am
content: My concern with that circuit (and the other circuits I've seen) is the lack of evidence that it actually works as intended - it might just be another theoretical circuit idea that never actually got built and tested. That's why I'm keen to find something that is proven.Mike

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: October 10, 2012, 11:44:42 am
content: You could try it, it seems simple enough.You have the zero crossing detection, dead time( I think), and a power stage.Put it in Ltspice or you can test it on low power and see if it works.Then beef up the driver stage and Fets.

author: BravoV
author_rank: Super Contributor
timestamp: October 10, 2012, 12:29:49 pm
content: That circuit works, I built it many-many years ago when I was totally clueless how it works.Fyi, this circuit originated from Elektor magazine, I think its pre 2000 edition, quite old and if my memory serves me well, it was for 5 Volt linear power supply with LDO regulator for about 2 - 3 Amp if I'm not mistaken, sorry, its been quite a while.It was built exactly like the schematic and was using different mosfets, and I remember in that era mosfets with RDS ON at sub < 100 mOhm was still quite rare and expensive, I used easy to find & cheap > 100 mOhm TO-220 style mosfets that were available at that time.Agree with HackedFridgeMagnet, if I'm going to build it again now, I'd beefy up the protection by clamping the voltage using zeners at the Vgs, Vds and the op-amp's supply lines.Or.. if you going to build a better but pricierusing the IRF's IR1167 smart rectifier control IC, this one definitely "proven" circuit since its designed by IRF themself, here ->http://www.irf.com/technical-info/whitepaper/TP-080527.pdf