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Registered Member #150
Joined: Sun Feb 12 2006, 09:43AM
Location:
Posts: 11
So im in the process of building a full wave bridge rectifier DC PSU and I'm stumped on what size my filter cap needs to be. I am looking at an output of 75vdc @ 20A. Is there a formula somewhere for this?
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
C ≈ ( Iload/ Vripple) * k * 1000 uF in which
k = 7 for 100Hz or 6 for 120Hz ripple frequency
I have given a general formula because you asked for one, but in practice I use a well-known rule of thumb which says that a load current of one amp will develop a 100Hz 700mV pk-pk ripple across a 10,000uF filter capacitor in full-wave rectification from 50Hz mains. The difference made by the 60Hz mains supply available in some other countries isn't worth worrying about.
Many applications will run swimmingly with a 2V or still larger ripple on their supply As with everything in electronic design, it all depends on the needs of the application.
So start out by defining the maximum ripple acceptable to you, your application and Jesus, and then scale up the rule of thumb from there
Watch out for your filter cap's ripple current rating. Ripple current is not at all easy to calculate because it is mostly dependent on the series resistance of the transformer and the mains supply itself, so be generous in your estimation of the ripple current rating required. If you exceed the recommended rating, then the capacitor's life expectancy (usually 2000hrs @ 85 Celsius unless otherwise stated) will grow shorter, sometimes very quickly so . This accelerated aging will cause the ESR to rise, creating still more heat for the same amount of ripple current.
Lastly, audiofools will sometimes exaggerate the amount of filter capacitance required for effective smoothing to absurdist proportions, but this should be politely ignored together with their other extra-rational claims!
These are just my views, those of a complete beginner, but we have a few members here professionally involved in electric power supply who will be able to fill in a lot more of the detail in my simple outline.
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DC PSU Filter Caps
