Topics: How to select input capacitor for Phase shift controlled full bridge converter
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How to select input capacitor for Phase shift controlled full bridge converter
I am designing phase shift full bridge converter with input voltage 90-210VDC and output 135V-216VDC. the out put current is 30A.The transformer ratio is 1:3.Please help me in selecting input capacitor.which is better among electrolytic and film capacitors?
09-10-2013 03:41 AM
I may be out of date with capacitor technology, but when i was designing high power bridges, you used both types of capacitor. The electrolytic handled the the low frequency from the ac rectifier bridge, and the film cap took care of the switching ripple to the bridge.
However, you have dc input, so it would depend a lot on whether you have holdup requirements, which keep the need for the bulk storage, or maybe you could just use the film.
09-10-2013 03:42 AM
To add to what Ray said, I also think it depends to some degree on where your DC input is coming from and how clean it is by the time it reaches your board. Is it nearby or remotely located? If it's physically far away, like a meter or more, the connecting cable will have more inductance. The switching frequency of your bridge will be important, too. These factors will affect how "good" the input DC looks to your switcher. The worse it looks, the more important the capacitors on your board.
I always like to use a local "bulk" capacitance with electrolytics. There are some very good electrolytics out there these days, available from "the usual suspects", like Nichicon, United Chemi-con, Panasonic, etc. No matter what you do, try to choose caps with the lowest ESR possible. You don't have to spend a lot, as it's pretty "main stream" technology these days.
Personally, I save the films for local bypassing of the switching transistors themselves. Once again, it depends on how far away from the input electrolytic they are. But something like 0.1 to 1.0uf might do. Watch for excess ringing under load; if this is a problem, you might also have to add to some snubbers near the transistors. And don't just randomly stick films in parallel with the electrolytics without thoroughly analyzing the effect, as multiple resonances can mess things up.
As in most switcher design, layout is everything!!
09-10-2013 03:44 AM
thank you for reply.The input is from fuel cell.Since fuel cells cannot give peak currents we want to use bulk capacitor at input side
09-10-2013 03:44 AM
Well, I'm afraid I'm out of my element here. I do know that there are many pieces of test equipment that will help you assess the output impedance of your fuel cell. Once you know how the fuel cell responds to loading of your DC/DC stage, it should be pretty straight-forward calculating the necessary capacitance. From what you're saying and from what little I know, it does sound like you'll need a lot of C, which should make a bank of electrolytics attractive.
Sorry I can't be of any more help.
09-10-2013 03:45 AM
It is likely that your fuel cell will need to have current with very low ripple. Ask your electro-chemist how much the ripple current is allowed to be. Often for fuel cell interfacing converters the allowed ripple is so small that converters with no ripple current needs to be used. Try using coupled magnetic components and multiple converters with phase shifted switching to lower the current ripple rather than just a capacitor. It is likely you fuel cell electro-chemist will thank you for it.
09-10-2013 03:45 AM
The phase-shift mentioned is interleaving, indeed it will reduce the input ripples current, 2 phase may not be enough, probably safe is 3 or 4 phases.
As mentioned by Ray, if there is hold-up time requirement, likely may not be, if yes, there is another consideration is using interleave Boost, lower down the input ripples at the same time take care of the hold-up time, and at the same time optimize the output Diode rating.
09-10-2013 03:46 AM
Its better to use wet tantalum CLR capacitor ..it has high surge current capability and it will great as a filter purpose .you have to fix voltage rating of capacitor like 250V. with this there is also a les input current ripple should be there.