Filter
Top Products
23 June 2005 HE104MAN-V8 Manual
Tri-M Engineering Tel: 800.665.5600, 604.945.9565
1407 Kebet Way, Unit 100 Fax: 604.945.9566
Port Coquitlam, BC V3C 6L3 E-mail: info@tri-m.com
Canada Web site: www.tri-m.com
18
3.5 Filter Capacitors
At 10kHz and above, the impedance of filter capacitors is essentially their effective series resistance
(ESR) and this parasitic resistance limits the filtering effectiveness of the capacitors. The filter
capacitors absorb the “switching ripple” current with their 100m0hm ESR that absorbs a 5A ripple
current and will dissipate 2.5W of heat.
The capacitors used for filtering in the HE104 are organic semiconductor (OS-CON) capacitors. The
OS-CON is an aluminium solid capacitor with organic semi-conductive electrolyte used as cathode
conductive materials. The OS-CON has many advantages over the conventional electrolytic:
- Very low ESR values, less than 8 times lower for same package.
- High ripple current rating, over 4 times higher for same package.
- No degrade in operation at extended low temperatures. (ESR value of conventional
electrolytics can increase 25 fold at –40C).
The life expectation for a filter capacitor is typically 2,000 to 6,000 hours @ 105C. For a conventional
electrolytic capacitor the temperature acceleration coefficient = 2 for a 10C increase, while the OS-
CON has a temperature acceleration coefficient = 10 for a 20C increase. For example, a capacitor
rated for 2,000 hours @ 105C would have an expected life of:
For conventional electrolytic capacitor
32,000 hours (3.6 years) @ 65C
128,000 hours (14.6 years) @ 45C
For OS-CON capacitor
200,000 hours (22 years) @ 65C
2,000,000 hours (220 years) @ 45C
This means that the OS-CON has extremely longer life in practical use, even under the same
warranty of 2,000 hours @ 105C.
In a buck convertor, output ripple voltage is determined by both the inductor value and the output
filter capacitor (for continuous mode).
ESR
•
Vout
•
(1- (Vout))
Vin
Vp –p
L1
•
frequency