By Tom Colella, Engineering Manager
What is a DC Link Capacitor?
The DC Link Capacitor is a part of power electronics found in inverters, converters, and motor drives. Although its primary function is to smooth out and steady direct current (DC) voltage, it also identifies any sudden jumps in voltage in the DC link circuit. As a result, this capacitor is similar to a steady bridge between the input (beginning) and output (end) of power electronic converters. In electric vehicles, the DC link capacitor protects the inverter circuit by absorbing sudden voltage increases.
The DC link capacitor is made to be strong and capable of holding a signification amount of energy. Given the ability to handle large changes in voltage and temperature, picking the right DC link capacitor is very important for the best performance and life of a system.
Why is a DC Link Capacitor Needed?
In power conversion, when AC is converted to low voltage DC, or AC from one frequency to another, the AC is usually rectified and smoothed. Once this is accomplished, the power is then routed to an inverter to obtain the final output. The DC that is fed into the inverter is called the DC link. As the name implies, the two sources are linked together with a filter capacitor [see Figure 1: DC Link Circuit].
The Role of a DC Link Capacitor in Electric Vehicles
In electric vehicle applications, the DC link capacitor is used as a load-balancing energy storage device. The DC link capacitor is placed between the DC (in this case, the battery) and the AC (which is the load side) of the voltage inverter. The capacitor is placed parallel to the battery, which maintains a solid voltage across the inverter. The device helps protect the inverter network from momentary voltage spikes, surges and EMI. The noise is the result of the pulsed inverter current and stray inductance on the DC bus [see Figure 2: Typical Electric Vehicle Circuit].
Why Is the Selection of a Proper DC Link Capacitor Important?
The selection of the proper DC link capacitor is important to achieve the proper performance of the system. For example, an under-designed DC link capacitor can cause a premature failure, or will cause EMI interference resulting in problems with other electronic circuitry. An over-designed DC link capacitor is not as cost- or size-efficient.
What are the Different Types of Capacitors?
Electrolytic Capacitors: Characteristics and Considerations
An electrolytic capacitor is polarized – with a positive and negative terminal similar to a battery – and is filled either with a chemical or solid polymer electrolyte, which gives the characteristic of achieving extremely large capacitance values. The major drawbacks to an electrolytic capacitor are that it has a limited life, the Equivalent Series Resistance (ESR) is extremely large, the leakage current is high, the insulation resistance is low, and the tolerances are broad. The primary advantage of an electrolytic capacitor is large capacity in a small package size at a relatively low cost. A major consideration in selecting an electrolytic capacitor is the temperature range of the device. Remember, even though the capacitor operates at room temperature, the device will increase in temperature rapidly with the voltages and high frequencies present. Electrolytic capacitors should have high ripple and pulse current ratings.