What is a DC Link Capacitor?

DC Link is a connection between a rectifier and an inverter. It acts as energy storage device between two stages. DC Link circuits are found in converter circuits or variable frequency drives (VFD). DC link capacitors, energy storage components in these circuits, are crucial in electric vehicles, serving as buffer between different power sources and loads, protecting systems from spikes and EMI. They play a key role in inverters and converters, keep voltage stable and absorb current ripples for efficient operations of systems. Today, advanced power electronics technology has spread the use of DC to AC converters, and DC link circuits are found everywhere.

In these circuits, AC or DC source voltage is first converted to DC voltage of suitable level and rectified voltage is fed to a capacitor (DC Link capacitor). The capacitor terminals are in turn connected to an inverter, which converts the DC back to AC with different frequency. In VFDs, output frequency can be varied to control a motor speed. DC Link capacitor prevents transients from load side going over to the other side. It also serves to smoothen rectified DC input, and works as energy storage for inverter. The capacitor gets rectified input voltage, comprising of a base DC voltage, superimposed with high ripple. Capacitors placed after power line rectifier face frequency twice that of supply frequency, plus the ripple content.

High frequency switching circuits with DC link enable to get rid of transformers, and allow large flexibility in output voltage and frequency levels. DC Link capacitor is subject to large high frequency ripple currents with wide spectrum of harmonics. It carries all the ripple current, and its terminals have to bear the circuit DC current without compromising its performance and life. DC Link capacitors are extensively used in automotive industry, three phase pulse width modulation (PWM), photovoltaic and power inverters, motor drives, on-board chargers and power supplies. Often, DC link capacitors need to have high capacitance and voltage ratings,

Main functions of DC link capacitors are:

1. Provide low impedance path for high frequency AC ripple currents, and decouple DC source from AC load.
2. Stabilize bus voltage and minimize voltage ripple, important for proper operation of inverter.

Selection of DC link capacitor has to be done with following key considerations.

• Capacitor technology- Plastic Film. Ceramic or Electrolytic. Film capacitors generally have better AC characteristics and life.
• Capacitance value determined by permissible voltage ripple level.
• Ripple current rating- It should be able to handle the high frequency currents.
• Voltage rating – Should be higher than maximum DC bus voltage.
• Packaging and size- Design as compact as possible
• Reliability and expected lifetime

Electrolytic capacitors may be used as DC link capacitors, but they have limitations of voltage (not over 550 V DC), and their capability to withstand high frequency ripples is rather limited due to high loss factor. Aluminium electrolytic capacitors also dry out over time, thereby get degraded over time. Hence their specifications have to be quite liberal in terms of values and voltages.

Ceramic capacitors are selected for DC link applications in low power levels. Values of these capacitors are in nF range, going to few hundred nF, peak currents up to 100 Amp, and voltages up to 500 VDC. They have advantage of high working temperatures up to 150 ℃, and their small size. They are characterized by long working life, and can easily be mounted on PCB.

Plastic film capacitors are the most common for DC Link capacitors. Plastic films are characterized by high dielectric strength, low losses, stability over large frequency range and smaller size. Another advantage over electrolytic capacitors is their higher temperature endurance. Advantages of plastic film capacitors are:

• Smaller size
• Low ESR
• Low self-inductance
• Higher temperature range
• Non-polarized nature
• Long operational life
• Virtually unlimited shelf-storage life

High frequency components in voltage means heavy currents, and metallized capacitors are designed to have low ESR, low losses and low inductance. Working voltage ratings can go up to 1100 V DC, dV/dt is quite high (typically 40 V/μSec), which may give peak ripple currents (C dV /Ddt) of 240 Amp for 6 μf. Life expectancy may be up to 100,000 hours.