Analysis and practice of replacing aluminum electrolytic capacitors with film capacitors

1. Foreword

Aluminum electrolytic capacitors are the most critical component that restricts the service life of the inverter. The main reason is the service life of aluminum electrolytic capacitors, especially in high-harmonic current and high-temperature applications such as inverters. Compared with other components, the life of aluminum capacitor is the shortest.

2. The role of "DC support" and "DC-Link" capacitors

When dc power is used as the power supply of the inverter, since the dc power supply needs to be connected to the inverter through a dc bus, this power supply method is also called "DC-Link". Since the inverter needs to obtain the pulsating current with high effective value and high amplitude from the "DC-Link", a high pulsating voltage will be generated on the "DC-Link", which makes the inverter unbearable. For this reason, "DC-Link" needs to be "supported" to ensure the power supply quality of "DC-Link".

In most cases, the components supporting the "DC-Link" are capacitors. The function of the "DC-Link" capacitor is mainly to absorb the high-amplitude pulsating current from the inverter from the "DC-Link", preventing it from generating a high-amplitude pulsating voltage on the impedance of the The power supply voltage fluctuation at the converter end is kept within the allowable range.

The second function of the "DC-Link" capacitor is to prevent the voltage overshoot and instantaneous overvoltage from the "DC-Link" from affecting the inverter.

3.The role of power frequency multiphase rectified DC bus capacitors

A three-phase bridge rectifier circuit or a 12-phase rectifier circuit is used in applications where there is no sudden change in the load current. It is not necessary to connect a DC bus capacitor across the rectifier output end. Because there is no sudden change in current, the induced potential produced by the parasitic inductance of the rectifier and AC power supply Will not be very high to affect the output voltage.

However, when the load is a switching power converter, the switching power converter will take the ripple power at the switching frequency from the dc bus. If this current flows into the dc bus and the parasitic inductance on the ac side, it will produce ripple voltage at the switching frequency that cannot be tolerated. From this point of view, the dc power supply is no longer only providing dc current, but needs to provide pulse current with rich ac components. At this time, the dc power supply not only needs low dc internal resistance, but also needs to have good low impedance in a very wide frequency band width. However, this wide-band low-impedance will not be provided as a dc power supply for the rectifier. In order to obtain a good wide-band low-impedance, a capacitor with good performance must be used. Utilize the characteristic that the capacitor voltage cannot jump and the capacitance of the capacitor decreases with the increase of frequency, the capacitor is used to reduce the ac impedance of the dc bus.

From this perspective, the capacitor connected in parallel to the three-phase bridge rectifier or 12-phase rectifier output DC bus is no longer a smoothing voltage filter capacitor, but a power bypass capacitor, or known as "DC support", "DC-link" capacitor.

"DC support" and "DC-Link" capacitors can choose aluminum electrolytic capacitors or film capacitors.Since aluminum electrolytic capacitors can withstand a relatively low ripple current value, they need to be able to withstand high-amplitude ripple currents in "DC support" and "DC-Link" applications. This requires that the aluminum electrolytic capacitor should be selected according to the ripple current when selecting the aluminum electrolytic capacitor. If the load generates a ripple current of 20A, a capacitance of 1000µF should be selected.

It can be concluded from the above description that the function of the DC support capacitor is to provide a “non-inductive” DC “power” to the load when the load current fluctuates, eliminating the inestimable and large parasitic inductance between the switch and the power supply. The resulting undesirable induced TV voltage spikes.

Although this solution may be optimal, the price may be very high, and general applications will not accept it. So, is there a better solution? The conclusion is that there are several low-cost and high-performance solutions.

4.Price issues

The key to replacing aluminum electrolytic capacitors with film capacitors is price. If a film capacitor with a rated voltage of 700V can achieve a capacitance of 1µF~2µF per RMB, it can replace aluminum electrolytic capacitors.

Some capacitor manufacturers believe that the price of 2µF per RMB is absolutely impossible to manufacture, and some capacitor manufacturers believe that it is achievable after careful calculation. If there is enough capacitance and the price is close to or lower than aluminum electrolytic capacitors, such a solution will be better. When film capacitors are needed to replace aluminum electrolytic capacitors, they should be replaced according to the replacement ratio of small capacity.

5. Film capacitor replaces aluminum electrolytic capacitor scheme

The biggest obstacle for film capacitors to replace aluminum electrolytic capacitors is the price of film capacitors. If the capacitor is replaced by a one-to-one ratio, it is definitely unrealistic in terms of price.From the above analysis, it can be known that the multi-phase rectifier output filter capacitor is no longer a smoothing function, but a power bypass of the DC bus. Therefore, as long as the capacitance can meet the requirements of the power bypass. In fact, as the power supply bypass of the DC bus, the requirement on the capacitance is not as large as that of the aluminum electrolytic capacitor, which makes the current carrying capacity of the aluminum electrolytic capacitor poor and the reason for the relatively high ESR.

Because the capacitor filtering of the three-phase bridge rectifier is actually a DC support, DC-Link or DC bus bypass capacitor. The key to this problem is whether the current carrying capacity of the "filter" capacitor meets the requirements. Film capacitor manufacturers have produced corresponding metallized polypropylene film capacitors. The data of EACO's SHD series DC-Link single-sided metallized polypropylene film capacitor is shown in Table 1.

Table 1 Partial data of EACO's SHD and SHE series DC-Link single-sided metallized polypropylene film capacitors

EACO's SHD and SHE series of DC-Link single-sided metallized polypropylene film capacitors are similar in appearance to bolt-type aluminum electrolytic capacitors. The replacement of aluminum electrolytic capacitors with this film capacitor requires only minor changes.

As can be seen from Table 1, EACO's SHD series DC-Link single-sided metallized polypropylene film capacitors can withstand very high ripple currents and are basically not limited by operating temperature.For example: SHD-700-300 polypropylene film capacitor with 300µF/700V has a current withstand capacity of 50A, and it can reach 100A when two are connected in parallel.Far higher than 3900µF/400V aluminum electrolytic capacitor can withstand about 15A ripple current value, higher than 15kW inverter "filter" capacitor needs to "filter" about 60A rectifier filter and inverter generated switch ripple current.From this data, aluminum electrolytic capacitor heating is inevitable, and the life is short.