Hi all,
I have a small PCB and a small case which mitigates using small smd capacitors to smooth a low frequency AC input. Testing has shown the buck chip ideally needs 470uF @ 25v to lower the ripple enough for stability.
In addition, high reliability and long life is paramount and the capacitors will get a hard work out (low frequency SIN, 0-21V AC) thus I investigated using newer high capacitance ceramics. After prototyping with some 1812 100uF/25V ceramics which worked (but measured 75-80uF) I went ahead and purchased 6,000 100uF 25V 1210 ceramics from TDK in Japan at $0.12 each (I was wary how they could fit such capacitance in such a small volume but the product code checked out and it's TDK...).
Then came the catch, when I measure these they are 47-60uF, which mitigates using double the amount. This is not a problem room wise but it does add additional cost to each product. This checked out on the PCB, if I use 6 it's not stable, but 12 is ok. The buck prefers low ESR and these are X7R.
I went back to the supplier and she says my multimeter is wrong. Perhaps. If I measure any electrolytic it is 100% accurate. If I measure a .001uF ceramic it gets the value to within 20%. It doesn't explain why the PCB requires double and the multimeter states the capacitance as half.
She said to use a multimeter that allows switching the charge voltage to a 1v level, which apparently (can't test with mine) gives a reading closer to the 100uF. I explained that the reason I bought 25v rated ceramics was I expected the capacitance not to change that significantly (half!) as the voltage rises. The typical PCB voltage is ~7V and they've lost half at that point.
Could you advise if there's anything else I have overlooked?
I have a small PCB and a small case which mitigates using small smd capacitors to smooth a low frequency AC input. Testing has shown the buck chip ideally needs 470uF @ 25v to lower the ripple enough for stability.
In addition, high reliability and long life is paramount and the capacitors will get a hard work out (low frequency SIN, 0-21V AC) thus I investigated using newer high capacitance ceramics. After prototyping with some 1812 100uF/25V ceramics which worked (but measured 75-80uF) I went ahead and purchased 6,000 100uF 25V 1210 ceramics from TDK in Japan at $0.12 each (I was wary how they could fit such capacitance in such a small volume but the product code checked out and it's TDK...).
Then came the catch, when I measure these they are 47-60uF, which mitigates using double the amount. This is not a problem room wise but it does add additional cost to each product. This checked out on the PCB, if I use 6 it's not stable, but 12 is ok. The buck prefers low ESR and these are X7R.
I went back to the supplier and she says my multimeter is wrong. Perhaps. If I measure any electrolytic it is 100% accurate. If I measure a .001uF ceramic it gets the value to within 20%. It doesn't explain why the PCB requires double and the multimeter states the capacitance as half.
She said to use a multimeter that allows switching the charge voltage to a 1v level, which apparently (can't test with mine) gives a reading closer to the 100uF. I explained that the reason I bought 25v rated ceramics was I expected the capacitance not to change that significantly (half!) as the voltage rises. The typical PCB voltage is ~7V and they've lost half at that point.
Could you advise if there's anything else I have overlooked?
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