A
Adam S
- Jan 1, 1970
- 0
I designed a 12V 25A step down converter switching at 300kHz. Input
capacitor ripple current is around 11A RMS. In an attempt to reduce
board space and cost, for the input capacitors I chose 6 x 10uF 25V
ceramics over low ESR electrolytics.
Unfortunately I discovered the hard way that these large ceramics
experience horrifying drop in capacitance at their rated voltage.
Here are some measurements I made of various capacitors I bought from
Farnell.
capacitor Capacitance at DC bias
type 0 VDC 25 VDC %
-------------------------------------------------------
Kemet 10uF 25V X7R 1210 9.3u 4.0u %43
Taiyo 10uF 25V X5R 1210 11.2u 3.1u %27
Taiyo 10uF 25V X5R 1206 9.9u 1.7u %17
Kemet 10uF 10V X7R 1206 9.2u 2.2u %24
? 1uF 25V X7R 1206 0.97u 0.83u %85
Should this be expected ? It appears one needs to triple the capacitor
count to achieve the rated CV value.
Adam
capacitor ripple current is around 11A RMS. In an attempt to reduce
board space and cost, for the input capacitors I chose 6 x 10uF 25V
ceramics over low ESR electrolytics.
Unfortunately I discovered the hard way that these large ceramics
experience horrifying drop in capacitance at their rated voltage.
Here are some measurements I made of various capacitors I bought from
Farnell.
capacitor Capacitance at DC bias
type 0 VDC 25 VDC %
-------------------------------------------------------
Kemet 10uF 25V X7R 1210 9.3u 4.0u %43
Taiyo 10uF 25V X5R 1210 11.2u 3.1u %27
Taiyo 10uF 25V X5R 1206 9.9u 1.7u %17
Kemet 10uF 10V X7R 1206 9.2u 2.2u %24
? 1uF 25V X7R 1206 0.97u 0.83u %85
Should this be expected ? It appears one needs to triple the capacitor
count to achieve the rated CV value.
Adam