Selecting a thin film or electrolytic capacitor for a power conversion circuit (II)

Choose the right capacitor:

Analysis of some common power conversion circuits can show how choosing different capacitor technologies can profoundly impact the size, weight, and cost of the system, depending on whether the capacitors need to be used to store energy or handle ripple noise. For example, for a bulk capacitor used as a 1 kW off-line converter, the difference in characteristics between the two types of capacitors can be clearly illustrated by comparing the electrolytic capacitor and the film capacitor. The converter has a power factor corrected front end and has a nominal DC bus voltage (Vn) of 400V.

Assuming an efficiency of 90% and a voltage drop (Vd) of 300V, below this value, the output regulation function will be lost. In the event of a power outage, the bulk capacitor C1 provides energy to maintain a constant output power when the bus voltage drops from 400V to 300V. We can calculate the C1 value required to make a 20ms pass before the voltage drops below 300V.

In contrast, the use of film capacitor solutions can lead to unrealistically large volumes: up to 15 film capacitors may be required in parallel, resulting in a total volume of 1500 cm3 (91 cubic inches).

If a capacitor is only needed to control the ripple voltage on a DC line such as an electric vehicle power system, it will be significantly different when selecting a capacitor. The bus voltage may be 400V as before, but it is powered by a battery, so there is no traversal requirement. When the downstream converter extracts 80 Arms pulse current at a switching frequency of 20 kHz, it is very realistic to limit the ripple to within 4 Vrms.

There are other reasons why film capacitors are the best choice. The parallel connection of multiple electrolytic capacitors results in an excessive capacitance, which may cause problems such as controlling the energy in the surge current. In addition, DC-connected transient overvoltages are common in light-duty traction applications such as electric vehicles, and film capacitors are more robust.

Similar analysis is also suitable for applications such as UPS systems, power conditioning for wind or solar generators, general grid-tied inverters and welders.

As the preferred film capacitor:

The relative cost of a thin film or electrolytic capacitor can be analyzed from the perspective of mass storage or handling of corrugation capabilities. The data published in 2013 compares the typical cost of a DC bus powered by a rectified 440 VAC power supply.

Considering the above analysis, film capacitors are an excellent choice for filtering applications such as decoupling, switching buffering and EMI suppression or inverter output.

Decoupling capacitors placed on the inverter or converter DC bus provide a low inductance path for high frequency current cycling. The rule of thumb is to use approximately 1μF of capacitance per 100A switch. It is worth noting that the connection to the capacitor should be as short as possible to avoid transient voltages. The magnitude of the change of 1000 A/μs is possible at high currents and high frequencies. Considering that the PCB traces may have an inductance of about 1 nH / mm, a transient voltage of 1 V can be generated per millimeter.

In the switch snubber circuit, the capacitor is connected in series with the resistor/diode and connected via a power switch (usually an IGBT or MOSFET) to control dV/dt.

The snubber capacitor reduces ringing, controls EMI, and prevents spurious on/off. The size of the snubber capacitor is usually chosen to be approximately twice the sum of the switch output capacitor and the mounted capacitor. The resistor value is chosen to prevent all ringing as standard.

EMI suppression:

Thin film capacitors are also ideally used as X-type and Y-type capacitors to reduce differential mode and common mode noise, respectively, due to their self-restoring and transient overvoltage capabilities. Usually of the polypropylene type, the capacitance is usually a few μF and is required to comply with the applicable EMC standards.

A Y-type capacitor with a low connection inductance is at the position where the input line is connected to ground. Here, the nominal transient voltages of the Y1 and Y2 capacitors are 8kV and 5kV, respectively, as shown in the input-to-ground connection. Considerations for leakage current limit the amount of capacitance that can be applied. Although the low connection inductance of the film capacitor helps maintain high self-resonance, the external grounding system should be kept short.

Inverter output filtering:

Non-polarized film capacitors and series inductors can typically be integrated into a single module, which forms a low-pass filter that attenuates high-frequency harmonics in the AC output of the driver and inverter. These are increasingly used to meet system EMC requirements and reduce the stress associated with dV/dt on cables and motors, especially when the load is remote from the drive unit.

in conclusion:

For power conversion applications, understanding the relative advantages of electrolytic and film capacitors can help designers make the right choice for the best overall size, weight and BOM cost. Can be summarized as follows:

Electrolytic capacitor:

Has a higher energy storage density (Joules / cm 3)

Low cost for "straight through" bulk capacitors for DC bus voltage

Maintain ripple current rating at higher temperatures

Film capacitors:

Lower ESR for excellent ripple handling

Higher surge voltage rating

Self-healing improves system reliability and service life