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Aluminium Electrolytic Capacitors' performance in Very High Ripple Current and Temperature Applications
Leif Eliasson Evox Rifa AB, a Kemet company Box 98, SE- 563 22 Gränna, Sweden Phone +46 390 124 82 Introduction The main limiting factor, for electrolytic capacitors, automotive power applications, is very often the ripple current capability. This article is focusing on the possibility to dramatically increa the capacitor performance, by using capacitors designed with low internal thermal resistance, together with optimization of the application thermal management. Ripple current capability A low ESR value is normally considered to be the most important parameter to achieve a high ripple current capability. The ESR and ripple current, results in power loss and in internal temperature ri. The operational life is reduced if the capacitor operates at high temperature. The internal capacitor temperature (hot-spot temperature, Th), is not only dependent of the power loss. The thermal parameters of the capacitor also have a significant impact on the hot-spot temperature. A low thermal resistance path will reduce hot-spot temperature and/or increa the ripple current capability. Thermal model The capacitor thermal model described below is built up by thermal resistances (Rthhc and Rthca) and thermal capacitances (Ch and Cc). With known thermal parameters, it is possible to simulate and predict the capacitor temperature and calculate operational life. thermal capacitor model:
视野英语Heat- sinking of the capacitor body By reducing the external thermal resistance, it is possible to dramatically increa the ripple current capability. Even with “standard” electrolytic capacitors heat- sinking of the capacitor body will result in a significant improvement. Achieving 60- 70% incread current capability, is possible, as a result of reducing the external heat resistance (Rthca).
New capacitor generation - significant performance gain Evox Rifa has introduced a new generation of axial capacitors. The PEG 220-226 ries of capacitors, are designed with 50- 70% reduced internal thermal resistance, ( Rthhc, this compared with all other comparable axial capacitors available on the market ). The low thermal resistance is achieved by metallic contact between the catholic foil and bottom of the capacitor casing. The winding is mounted with axial pressure ⇒ Metallic contact between the cathode foil and the bottom of ca, resulting in low thermal resistance Winding tabs, welded to bottom of capacitor Bottom of capacitor ca (cut) Winding with extended cathodic foil
英文版Capacitor, extended cathode design
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Axial force on the Al-deck, gives pressure to the winding The extended cathode technology has durin
g decades been in u for large Alelectrolytic screw terminal capacitors. High power applications in cars, now brings similar requirements also for smaller capacitors. Winding with extended cathode, together with other improvements, results in a significant increa in ripple current capability. Up to 28A ripple current, continuous load, is possible to specify, at a capacitor ca temperature of 125ºC (∅20x 43mmca). To achieve this performance the capacitor body needs to be heat- sinked. As a comparison, the ripple current for some of the best capacitors on the market today is 14-15 A (heat- sinked, 125ºC, same ca-size). Example - Continuous load: Capacitor ripple current: 26.7A, ≥5 kHz. (Cont. load). Applied DC voltage: ≤ 18VDC Application chassis temp.: 116.7ºC. External thermal resistance (cap. body to metallic chassis): Rthca= 1.6 ºC/W This value is achievable by heat sinking approximately one third of the capacitor body, using thermal conductive paste or glue.
Capacitor article, fulfilling above requirements: PEG 225 HJ4480Q ( 4800µF, 25V, ∅20x 35mm), ESR(≥5kHz, ≥125ºC)= 7.3mΩ (max), Rthhc= 2.4ºC/W (internal thermal resistance) Calculated temperatures and operational life (Lop): Power loss: 26,72x 0.0073= 5.2 W, Steady state, thermal conditions (cont. load): Cap. ca temp. (Tc): 116.7ºC+ 5,2W* 1.6 ºC/W= 125ºC. Capacitor, hot-spot temperature (Th =winding temp.): 116.7ºC+ 5.2W* (2.4+1.6) ºC/W= 137.5ºC. Operational life for an electrolytic capacitor is direct related to the capacitor hot-spot temperature (max winding temperatur
建筑学大学排名e). The above described capacitor type is capable of minimum 4 kh operational life at described conditions ( ⇒ Th= 137.5 ºC). Test results and experience verify that the operational life (Lop, minimum) can be described by following formula:
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typer( 12ºC decread temperature results in an factor of 2 longer operational life. Specified Lop for PEG 225 is 85kh at Th= 85ºC and 2 kh at 150ºC) Test results, verifying capacitor specification [ accelerated test, 12ºC incread temperature compared with specified temp., at rated ripple current ]: Tested capacitor article: PEG 225 HJ4480Q, Test conditions, as described in above example but tested at 12 ºC higher temperature⇒ Ca temperature 137 ºC, hot-spot temperature 149,5 ºC. Ripple current load: 27 A. Test duration: 2000 h Parameter change after test: ∆Cap (µF) -6,4% -5,3% -10,8% ∆tan d (%) +8,1% +45% -4,5% ∆ESR(100kHz) +5,2% +23% ±0,0%
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porinComments: The capacitor specification are fulfilled, also after 2000h at accelerated conditions. [at +12 ºC incread temp] Example, Intermittent load (simulation and measurements): 50% Duty Cycle, 40A, 5kHz, 30s “on”, 30s “off”, applied DC voltage: 16V, application chassis temperature: 117ºC Capacitor mounted with low thermal resistance path, to metallic chassis, Rthca= 1.6 ºC/W.
Capacitor article: PEG 226 KL 4270Q ( 2700µF, 40V, ∅20x 43mm), ESR(>5kHz, >125ºC)= 6.7mΩ (max), Rthhc= 2.4ºC/W, Ch=18.4 ºC/ J, Cc=4.6 ºC/ J Thermal simulation:
Calculated hot-spot temperature (Th) and capacitor ca temperature (Tc) Verifying measurement, intermittent load 50% Duty Cycle, 40A, 5kHz, 30s “on”, 30s “off”, applied DC voltage: 16V, PEG 226 KL 4270Q ( 2700µF, 40V, ∅20x 43mm)
Measurements, intermittent load, (50%, 40A, 5kHz) (Ca temperatures, two capacitors):
xtrapParametric changes after 1000h of intermittent operation: ESR (100kHz, 20º): + 3.5%, Capacitance (100Hz, 20º): -2.5%. Remark: The capacitors still fulfil the specification for new capacitors
Test t-up, capacitors mounted with low thermal resistance path, to metallic chassis: Summary In high ripple current applications like automotive power, it’s possible to gain a significant advantage by using capacitors designed with optimized thermal parameters. Further significant improvement is possible to achieve by heat-sinking the capacitors.
