What is a DC link?
A DC link is a connection which connects a rectifier and an inverter. These links are found in converter circuits and in VFD circuits. The AC supply of a specific frequency is converted into DC. This DC, in turn, is converted into AC voltage.
The DC link is the connection between these two circuits. The DC link usually has a capacitor known as the DC link capacitor. This capacitor is connected in parallel between the positive and negative conductors.
The DC capacitor helps prevent the transients from the load side from going back to the distributor side. It also serves to smoothen the pulses in the rectified DC.
Why is a DC Link Capacitor Needed?
DC-link capacitors are used in order to provide a more stable DC voltage, limiting its fluctuations even under heavy current absorption by the inverter: in practice they act as filters. In inverters the supply current is highly distorted due to commutation.
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].
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]. 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.
Figure 1: DC Link Circuit
Figure 2: Typical Electric Vehicle Circuit
Figure 3: Capacitor Construction
Film or Electrolytic?
Because the ripple current ends up being the driving requirement, most modern inverters use film capacitors. Compared to electrolytics, film caps have high ripple current rating due to their low ESR and ESL. Electrolytics have a higher capacitance/volume ratio than film caps, but the ESR and ESL is much higher so you need to put many of them in parallel in order to satisfy the ripple current requirement. The volumetric efficiency typically ends up being much higher if film capacitors are used.
Additionally, the working lifetime rating of electrolytics is around 10k hours, whereas for film it’s 100k hours. This is because the electrolyte dries out and leads to increased ESR which increases power loss and ultimately results in failure. There is a lot to be said about the advantages of film caps over electrolytics. Our GDHY DC link film capacitor C51 below can be an option.
Summary
Film capacitor selection is extremely important to achieve the best voltage and current-carrying capability for DC link capacitors. GDHY can utilize various combinations of dielectric materials and terminations in the construction of each type of capacitor. That is why it is important for the designer to consider the following:
- What is the nominal capacitance? Is the capacitance fixed or dependent on the ripple current?
- What is the temperature range? What is the temperature profile?
- What are the DC and AC ripple voltages?
- Is there cooling and, if so, what type?
- What is the ripple current and frequency? Is the ripple current steady-state or intermittent?
- What type of mounting and lead configuration is needed?
To achieve the desired final design, GDHY has a variety of film capacitors – including C51, designed specifically for DC link applications which require long life under extreme operating conditions. Perfect candidates for C51 include EV power conversion, aircraft power systems, battery chargers and other high pulse circuit applications. Custom designs are also available.
