benmack
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I've been evaluating a few off-the-shelf flyback converters, and they all appear to run the output capacitor at well over its max rated ripple current - am I missing something, or will that give poor lifetime?
For example a mains to 5V 3A DC supply I'm looking at uses a 330uF 25V capacitor which is rated 840mA RMS max at 100kHz. However the actual RMS ripple it sees must be well in excess of 3A! Running thermal tests into full load, in an ambient of 20 deg C the capacitor can reaches 88 deg C, so in a 40 ambient it will exceed its 105 deg C absolute max. And this PSU is from a reputable North American manufacturer, with a 3 year warranty
They all avoid the voltage ripple problem by using an LC filter on the output, thus allowing use of a smaller output capacitor.
We've had a batch of PSUs fail in the field in a matter of months due to the output capacitors cooking, hence my concern
Any thoughts?
For example a mains to 5V 3A DC supply I'm looking at uses a 330uF 25V capacitor which is rated 840mA RMS max at 100kHz. However the actual RMS ripple it sees must be well in excess of 3A! Running thermal tests into full load, in an ambient of 20 deg C the capacitor can reaches 88 deg C, so in a 40 ambient it will exceed its 105 deg C absolute max. And this PSU is from a reputable North American manufacturer, with a 3 year warranty
They all avoid the voltage ripple problem by using an LC filter on the output, thus allowing use of a smaller output capacitor.
We've had a batch of PSUs fail in the field in a matter of months due to the output capacitors cooking, hence my concern
Any thoughts?