The first article in this three-part FAQ series reviewed safety capacitors (sometimes called high-frequency bypass capacitors), primarily for filtering electromagnetic interference (EMI) on the input of mains-connected power converters such as power supplies, battery chargers, and motor drives. This FAQ moves deeper inside the various types of power converters and will consider DC link capacitors, the holdup capacitors for energy storage in AC/DC power supplies, and pulse power capacitors.
A DC link is typically used to connect a rectifier (or other DC source such as a battery) and an inverter. A DC link capacitor is used as a load-balancing energy storage device. This capacitor is connected in parallel between the positive and the negative rails and helps prevent the transients on the load side from going back to the input side. It also serves to smooth the pulses in the rectified DC input.
The selection of the correct DC link capacitor is important to achieve the proper performance of the system. For example, an under-designed DC link capacitor can cause premature failure or will cause EMI resulting in problems with electronic circuitry. An over-designed DC link capacitor is not as cost-effective or size-efficient. Two important characteristics of DC link capacitors are low ESR and high ripple current rating. Aluminum electrolytic and metalized film capacitors are the most commonly used technologies in DC link applications.
Film capacitors are available with higher voltage ratings than aluminum electrolytics. In some applications, lower-cost aluminum electrolytic capacitors are used in series to increase the effective voltage rating. An active or passive balancing circuit is often needed to ensure a uniform distribution of the DC link voltage across the individual capacitors and ensure reliability and enhance lifetimes.
Cost is an important differentiator between aluminum electrolytic and film DC link capacitors. Aluminum electrolytics can store a given amount of energy for a lower cost compared with film capacitors. However, film capacitors have superior current-carrying capabilities and are better than aluminum electrolytics in terms of cost per amp.
Whichever capacitor technology is used, designers have a choice of using discrete capacitors or capacitor modules. The use of individual capacitors can reduce component costs, but at the increase of system integration costs. Modules can enable the elimination of bus bar assemblies and lower integration costs. Modules also support integrating functional components such as laminate bus bars, multiple capacitor bandwidths, bleed resistors, and externally mounted devices.
Holdup capacitors
A holdup capacitor is a specialized DC link capacitor found in AC/DC power supplies. In addition to acting as a load balancing device between the rectifier and inverter sections, holdup capacitors provide extra energy storage to support the output voltage for a specified “holdup time” after removing the AC input power. Holdup time for a power supply is defined as the time during which the output voltage stays in regulation after removal of the input voltage. For AC/DC power supplies, it is typically specified at full load and both high ac line and low ac line conditions. For example, for a 3.3Vdc output with a ±10% (±0.33V) regulated output, the hold-up time is measured from the time the input is removed to the time that the output voltage drops to 2.97V. Typical holdup times for today’s systems range from 15ms to 50ms.
Important characteristics for holdup capacitors include capacitance, ESR, and ripple current rating. Typical capacitor types used as holdup capacitors include various aluminum and tantalum electrolytic devices. They typically have several times the power density of film capacitors. That makes them well suited for applications requiring high capacitance to handle peak load requirements and voltage ride-though (when there is a momentary dip in voltage level).
Due to the availability of high capacitance (100µF, or more) multilayer ceramic capacitors (MLCCs), it is sometimes possible to replace electrolytic capacitors with smaller and lower profile MLCCs. However, at low frequencies often employed for holdup capacitors, higher dielectric losses in MLCCs typically results in higher ESRs compared with electrolytic capacitors, reducing the effectiveness of MLCCs as holdup capacitors.
Pulse power capacitors
Specialized capacitors have been developed for pulse power systems. For example, high-voltage resin encapsulated disc capacitors from CeramTec feature a new strontium based, low-loss, high permittivity dielectric. It has been specifically designed to function in circuits with high peak current and high repetition rates, such as those found in gas lasers, DC power supplies, X-ray power supplies, and medical systems.
To meet the needs of military and avionics applications, Vishay Intertechnology has extended its EP1 wet tantalum capacitor with new ratings in the B and C case codes. Offering the industry’s highest capacitance per voltage rating and case size for this device type, the capacitor is available with radial through-hole or surface-mount terminations, each with a stud-mount option, for increased design flexibility. The enhanced EP1 now features ultra-high capacitance from 3,600µF to 40,00µF in the B case code and 5,300 µF to 58,000µF in the C case code. Voltage ratings for the device range from 25Vdc to 125Vdc. Optimized for pulse power and energy hold-up applications in laser guidance, radar, and avionics systems, the EP1 is housed in an all-tantalum, hermetically sealed case for increased reliability.
High-power pulse capacitors
More and more, assemblies of capacitors are used as energy storage banks to deliver high energy bursts during several 100ms. Contrary to batteries and supercapacitors, power capacitors have no limitation in terms of discharge time. These high energy systems require large numbers of big capacitors mechanically mounted in low inductance and low resistance assemblies. These energy storage banks can deliver several kV and energy levels from 100kJ to several MJ per installation.
The first article in this three-part FAQ series considered safety capacitors for filtering electromagnetic interference on the input of AC mains connected power converters. The third and final article in this series will look at capacitors for power converter output filtering.
References:
Energy Bank Capacitor Applications, AVX
How to Select DC Link Capacitor, electrocube
Power Factor Correction (PFC) Handbook, ON Semiconductor
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