Any comments to yet another SMPS - but this time a resonant half bridge? - Page 1
Forum section: electronics / projects

author: Kremmen
author_rank: Super Contributor
timestamp: December 16, 2013, 04:24:40 pm
content: Hi all,Since there are several forum members known to have experience in SMPS design, i thought to ventilate my recent ad hoc design with you in hope of comments.My purpose is to build no more than 5 pieces of this supply, all of them strictly for hobby purposes (small CNC servo power). Thus the price of individual components is not a primary criterion.The schematic and layout are attached. Some notes:Firstly, the design is a modification of IR app note 1160 documenting a resonant half bridge PSU. The original circuit is spec'ed for 24 V secondary but mine will be 75V@6A+24V@1A. The currents are maximums with some margin.The main operation (excluding startup) will occur between 50 and 100 kHz depending on the load. The lower frequency corresponds to max load.The main transformer is a EC70/3C80 core with solid copper windings, primary 3xAWG19 and 75V secondaries 4xAWG19. The 24V secondary is a double AWG19 winding. The xformer will be underutilized at the max power of this PSU but i happen to have a few of these lying around.The app note documents using the stray inductance of the transformer as a distributed component of the resonant LC circuit. In order to "maximize" the stray inductance, a 2 slot bobbin is used to wind the transformer. I have chosen to use a more traditional single slot bobbin and to include a separate resonance inductor. This way in needs be, the inductor can be modified without tearing dowh the whole transformer. Also my intention is to optimize the distribution of MMF inside the transformer, although detailed design of the winding layout is still in progress.One point is that now the 24V secondary is one-sided. I may add a winding to create a symmetric coupling to the xformer core.At this time I have copied the detailed circuit around the TL431 and opto straight from the app note. It remains to be seen if that is the way it stays. The considerably higher output voltage may necessitate some modifications due to the voltge limitation of the 431 (or it may not). In any case the loop transfer function still needs to be properly worked out. In that sense the feedback part should be considered more a placeholder than the finished thing.One point of concern is the long trace from the opto transistor back to the controller chip. I have tried to route it as far as reasonably possible from the switching point that is bound to radiate the most EMI. Any suggestions to improve the circuit there would be welcome. The trace is not very visible in the attached images but you can see the opto next to the main xformer. The trace follows the border between the PE skirt and primary side COM copper fill, and turns in towards the controller passing under the EMI filter inductors. This may not be the all time best route but at least it avoids the switch resonant inductor which is on the other side of the board. Any views to this are welcome.I have considered synchronous rectification but in the end decided to rely on fast Schottky diodes to avoid the hassle of arranging the synchronous control circuitry.There is as yet no inrush limiting but i am thinking of adding at least a simple NTC based limiter.

author: nickm
author_rank: Regular Contributor
timestamp: December 17, 2013, 05:23:04 am
content: I don't see any snubbers on your FETs or Schottkeys. You might consider having the placeholder for them. I'm not sure if this topology needs them. Also your 431 is only set up for a type II. Since this is voltage mode you might want the option for a type III even though you're regulating before the LC.

author: Kremmen
author_rank: Super Contributor
timestamp: December 17, 2013, 07:00:47 am
content: Quote from: nickm on December 17, 2013, 05:23:04 amThanks for your comments.I'll do some checking but i think that since this is supposed to be a resonant design with ZVS, snubbers are not needed or indeed desirable (at least the lower FET snubber could negatively impact the resonant circuit, i think). Certainly the appnote circuit didn't have them. The example circuit there was one of IR's example boards so it is a practical design. But i will look into that.Basically the same thing with the feedback. In the app note circuit the feedback is type II and it is not obvious whether there is need for the D-term. Then again, it won't cost anything to put in the placeholders for the caps and whatnot. So thanks for the note, i'll include the places in both lanes to have a type III response. The next step is to model the transfer functions anyway so then we know.By the way, the IR chip does have current mode control, it just measures the current across the lower switch Rdson (no separate sense resistor). That functions as the inner loop and around that we have the normal output voltage feedback.