Offline isolated SMPS with discrete synchronous rectifier drives is best? - Page 1
Forum section: electronics / general

author: ocset
author_rank: Super Contributor
timestamp: December 15, 2019, 07:22:44 pm
content: Hi,There are virtually no IC’s on the market that manage synchronous rectifier drive for isolated offline Two Transistor Forward Converters, or even for any other type of offline isolated converter. (eg Flyback, LLC, Full Bridge).This is a very bad state of affairs. As such, we have designed our own discrete synchronous rectifier drive circuitry. This is as attached in the LTspice simulation and schematic.All that’s basically needed is to invert and delay and trim the waveforms sufficiently to achieve the correct dead times between the respective gate drives, and of course, reverse overcurrent protection.However , a microcontroller is needed, one on the primary and one on the secondary. The secondary one simply manages the reverse overcurrent latch.The primary side microcontroller provides the 280ns signal delay that’s needed for the main gate drive signal from the PWM controller. This delay is essential and is part of getting the dead time for the synchronous rectifier gate drives.I actually believe that a third small micro would be adviseable, on the secondary side, to replace the shown digital logic gates. Also, the 50ns delay block could in reality, only be done with a microcontroller.There are a number of offtheshelf synchronous rectifier controllers available which have no connection to the primary side PWM controller…eg UCC24630…these are not useful, as they rely on detection of noisy switching nodes which may mean noise tripping and malfunction….after all, switching a synchronous rectifier on at the wrong time can be pretty catastrophic.UCC24630http://www.ti.com/lit/ds/symlink/ucc24630.pdfSo would you agree that these kind of “Home-brew” synchronous rectifier controllers are the way forward?Amongst other things, they use microcontrollers as “delay buffers”.Attached is the LTspcie sim and schem of a Two transistor forward with sync rects, and our home-brew sync rect driver circuitry.

author: David Hess
author_rank: Super Contributor
timestamp: December 15, 2019, 07:52:35 pm
content: There are several solutions including from On Semiconductor and Linear Technology which use a pulse transformer from the primary to secondary to control the the synchronous rectifier.

author: ocset
author_rank: Super Contributor
timestamp: December 16, 2019, 03:39:37 pm
content: Thanks, yes, but they are terribly expensive (the ICs, not the pulse transformer)...the below solution is so cheap, i wonder why no-one uses it?The attached schematic ,(and LTspice simulation) would you agree, is the de facto way to drive synchronous FETs for a Full Bridge, Two Transistor forward, or Flyback. Its shown here for a Two transistor forward, but its easy to see how this transfers to Flybacks, Full and Half Bridges too.The cost is very low, simply using a few MC74VHC1G126 logic buffer ICs, and some RC delays, and a digital isolator IC.MC74VHC1G126 logic buffer datasheethttps://www.onsemi.com/pub/Collateral/MC74VHC1G126-D.PDFI was thinking I may need Schmitt Trigger buffers, but the above ones have the same effect as Schmitt outputs anyway, would you agree?All the offTheShelf solutions for this (driving synchronous rectifiers) , involve ICs that are ridiculously expensive. Eg, look at the LT1681/LTC1698 combo, for driving Two transistor forwards with synchronous rectifiers…they are $6 each !!!Why are people not using this technique?All it involves is delaying the drive output of a PWM controller…..then also inverting the output of the controller…..then delaying the inverted output a bit less, and trimming it in time so it fits between the PWM output pulses, giving the required dead time.So simple & cheap and yet nobody doing this? Why?(obviously reverse overcurrent limiting needs adding…but add a comparator and sense resistor or CST, and Bob’s your uncle!)Would you agree, from the datasheet, that the output impedance of the MC74VHC1G126 is 7V/0.025A = 280R?

author: XFDDesign
author_rank: Frequent Contributor
timestamp: December 17, 2019, 07:26:41 am
content: For my home-rolled controllers, I use a classic 555 timer, a few 3906s, a comparator, and a 74AC74.The only thing required to make it work in isolated applications is an optocoupler feedback block.