CDE Screw-Terminal Aluminum Electrolytic Capacitors for Power Conversion Applications
For high current power conversion applications several suppliers offer screw-terminal aluminum electrolytic capacitors for the bus capacitor or dc-link capacitor application. CDE Cornell Dubilier is preeminent and offers types with more than 50 amps ripple capability with expected lifetimes approaching 50 years. To avoid over design and assure best value for the application CDE provides Java applet life/temperature calculators.
CDE Cornell Dubilier is a broad-line capacitor maker with power film, RF mica and aluminum electrolytic capacitors as its focus products. The steady demand is for screw-terminal aluminum electrolytic capacitors for power conversion applications. CDE with manufacturing in Liberty South Carolina and Mexicali Mexico has found success in serving the large equipment, power conversion market that remains in North America after the exodus of consumer electronics to Southeast Asia and China. Its international sales are expanding and its largest customer is in China.
The principal end markets are power supplies, instruments, UPS systems, motor drives, welders, avionic systems, military and medical systems. The screw terminal aluminum electrolytic capacitors are typically at the power input acting as bus capacitors filtering the dc bus voltage after rectification. The ac voltage from the power line is converted to dc voltage-rectified-by input diodes or thyristors and applied to the aluminum electrolytic capacitors for smoothing into a flat dc voltage.
Before the advent of readily available switching semiconductors power conversion was linear and the topologies included heavy 60 Hz transformers to step the voltage up or down and low voltage aluminum electrolytic capacitors. But today the evolution to switching regulators is nearly complete, and the input transformers have shrunk to multi-kilohertz switching transformers further upstream. The topologies are off-line topologies with the load to which the capacitors connect being typically a dc-to-dc converter or a dc-to-ac inverter. And the bus capacitors-now separated from the input ac power by only rectifying diodes and an input filter-have a voltage rating of 250 to 600 Vdc to withstand the rectified power line voltage.
For power conversion circuits on printed circuit boards it is routine to use plug-in style aluminum electrolytic capacitors, radial leaded or snap-in terminals. But for the higher power conversion circuits the screw-terminal aluminum electrolytic capacitor is king. It readily fits the laminated bus and power cable input circuitry needed to handle high current. And high-current is more and more the selection key for choosing the aluminum electrolytic bus capacitor or capacitors. The ripple voltage riding on the aluminum electrolytic's bias voltage creates ripple current in the capacitor that heats the capacitor. Capacitors vary greatly in how much ripple current they can withstand and still deliver acceptable service life.
CDE Cornell Dubilier is the largest maker of screw terminal aluminum electrolytic capacitors and has the widest variety of performance features. The challenge is to find the best value capacitor design for each application and to do it in a way that is easy for the project design engineer. The strategy is to have a wide variety of types to meet the market price points and performance needs and to have electronic selection tools on the CDE website to quickly find the right one for each application.
CDE Cornell Dubilier catalogs seventeen types of screw-terminal aluminum electrolytic capacitors spanning types for military applications (101C, 125), inverters (520C, 550C), long life (500C, 3188) and high-capacitance (DCMC, 3186). Some of the types are established types originally developed by Mallory and NA Phillips and BC Components. All are on the CDE website at www.cde.com/catalog/alum/#screw, and many are in distributor catalogs.
Differences in ripple current handling and expected life are from differences in rated temperature, base life capability and thermal characteristics. Many types are available with ThermalPak(TM) or rilled construction that conducts the heat from the capacitor element to the case for easy cooling. So that designers can know the life and operating temperature CDE provides Java applet life-temperature calculators on its website with temperature extrapolations accurate to within 2°C. The calculators are at www.cde.com/calc/. Designers can see the effect of ripple current at different frequencies, the effect of different ambient temperatures, different cooling air speeds and different heatsinks to the capacitor.
The incremental value of the CDE approach shows in the table below. Expected life and ripple current capability steps up as you move into better types. The Java applet calculators allow designers to do "what if" comparisons between types and verify expected life for different types at expected operating conditions. In the table here four types are compared, all rated 4700 µF 450 V with 400 V applied and operating at 60 °C with 12 A of 120 Hz ripple current. The table starts with a low cost type, Type DCMC with prices starting a less than $10 in OEM quantities and steps up to ever higher ripple current and expected life types.
Type Case Size in mm (in)
4,700 µF, 450 V Life (hours)
60 °C, 400 V,
12 A @ 120 Hz
DCMC 76x130 (3x5⅛) 40,000
500C 76x143 (3x5⅝) 83,000
520C 76x143 (3x5⅝) 208,000
550C 89x143 (3½x5⅝) 428,000
It shows that paying roughly 10% more for a step up in grade doubles the life and permits the designer to get the right capacitor for the application. Stepping through the four types here shows a more than ten times expected life improvement for the same operating conditions.
