Film capacitors are widely used in high power invertible welding machine because of its heavy withstanding current, high withstanding voltage, little capacitive reactance and good frequency property, which use in invertible circuit for high frequency filter, IGBT snubber and resonance. This paper analyzes the reliability of film capacitor, the relationship with voltage, current and frequency, and the alternative proceeding in the using process, for that it can help the engineers of invertible welding machine manufacturers to choose the film capacitor better.
1. The role of film capacitors in inverter welding machine rectification and filtering
Electrolytic capacitors are generally used as rectifier filter capacitors for inverter power supplies and inverters. Electrolytic capacitors are widely used because of their small size, large capacity and low price. For filtering and surge absorption, film capacitors need to be selected. Metallized film capacitors are wound by metallized film. The obvious advantage of metallized film capacitors over other capacitors is that they have self-healing properties, that is, when metallized capacitors break down due to defects in the dielectric, the breakdown An arc current is generated immediately, and this current density is concentrated in the center of the breakdown. Since the metallization film is very thin (less than 0.1 um), the heat generated by the current is enough to melt and evaporate the metal near the breakdown, forming a five-metal area around the breakdown area, restoring the insulation between the two poles of the capacitor, so that the capacitor returns to normal Work. The self-healing effect of the metallized film can increase the voltage of the film. When the smoothing filter voltage is above 450VDC and the ripple current is large, film capacitors are generally used. The overcurrent capability of film capacitors with the same capacity and voltage level is more than 10 times that of point solution capacitors. Therefore, the main inverter welding machine manufacturers at home and abroad mostly use film capacitors instead of electrolytic capacitors when choosing high-frequency filter capacitors.
The effective current borne by the filter capacitor is mainly the superposition of the ripple current at the output frequency generated by the inverter and the harmonic current at the switching frequency.
2. Relation of Film Capacitor Reliability to Effective Current and Pulse Current
The inverter welding machine must determine the effective current and pulse current flowing through the capacitor when selecting the filter capacitor, because the failure of the capacitor is usually caused by the excessive effective current and pulse current flowing through the capacitor.
(1) Relation between effective current and temperature rise
Each time the current passes through the coil, the core and coil losses of the inductor and transformer produce a temperature rise. These losses are limited by the total losses allowed by the application (power budget) or the maximum temperature rise allowed.
(2) Relation with pulse current
Pulse current is a current whose direction is constant and its intensity changes periodically with time, also known as pulsating current. Some users' load needs intermittent power, that is, according to a certain time rule, power to the load for a certain time, and then power off for a certain time, on and off a cycle. This is repeated to form a pulse current
3. The relationship between reliability and voltage of film capacitors
(1) Rated Voltage (Un ) - The approximate sine wave (positive and negative peak-to-peak) peak operating peak cycle voltage at which the capacitor is designed.
(2) Rated DC Voltage (Un ) - The highest operating peak voltage at which the capacitor is designed with a DC or single periodic waveform and can operate continuously.
(3) Actual working voltage (Un)——When the capacitor is working, the voltage value actually applied between the two poles of the capacitor should be Un>=Uw when the voltage is selected. It is recommended that the actual working voltage of the capacitor should be 70%~80% of the rated voltage.
(4) Ripple Voltage (Un ) - The peak-to-peak AC component of a single-phase voltage.
(5) Non-periodic impulse voltage (Un ) - A peak voltage caused by switching or any other disturbance in the system, which is only allowed to last for a time shorter than the fundamental period and to occur a limited number of times.
(6) Insulation Voltage (Ui)—The voltage rating (rms) of the capacitor element and terminals to case or to ground.
The voltage level of the high-frequency filter capacitor selected by the inverter welding machine is: the rated voltage of the single-phase input filter capacitor is 400VDC; the rated voltage of the three-phase input filter capacitor is 800VDC; the voltage level of the IGBT absorption capacitor is generally selected according to the rated voltage of the IGBT module.
4. Reliability and Frequency of Film Capacitors
When the working frequency of the capacitor is lower than the rated power, the capacitor can work normally; but when the working frequency of the capacitor is higher than the rated power, the voltage and current of the capacitor will be limited by the loss on the ESR and the heating caused by the dielectric loss, and the applied AC The voltage must decrease with increasing frequency to maintain the power dissipation PE of the capacitor itself .
In order to prevent permanent damage to the capacitor, the steady state overtemperature is obtained at the hottest part of the capacitor surface when the ambient environment does not exceed a certain value, so that the power generated is equal to the power dissipated in the form of thermal energy.
5. Precautions for Using Film Capacitors
When confirming the use and installation environment, as a capacitor used within the rated performance range specified in the product catalog specification, it should be avoided to use in the following situations:
(1) The surface temperature rise shall not exceed the specified value (the temperature rise of the capacitor is less than 25 degrees Celsius during normal operation).
(2) Overcurrent. The working current should not exceed the rated current for a long time.
(3) Overvoltage. The working voltage should not exceed the rated voltage for a long time. When the inverter welding machine uses a high-frequency filter capacitor, it is best to connect a discharge resistor in parallel at both ends of the filter capacitor. The resistance value can be calculated according to the standard, and will not be introduced in detail here.
For the installation of capacitors, copper nuts are generally used to lead out high-frequency filter capacitors. Note that the torque should not be too large during installation. If the torque is too large, the terminal will be broken, or the contact will be poor, which will increase the contact resistance in the circuit.
