Discharging the DC link

What are the requirements for proper discharging?

According to the regulations, the DC link and other capacitors must be discharged to below 60 Volts within 5 seconds of the ignition being switched off.

After switching off, the vehicle battery is disconnected from the DC link. When the disconnection process has been completed, the capacitors in the DC link are still charged. This energy must be dissipated to below 60 Volts within 5 seconds. Discharge resistors from Miba ensure that the DC link is discharged reliably and quickly.

This 5-second rule applies to all electrically powered vehicles that are allowed to operate on public roads such as HEV, PEV, BEV and FEV. As the electrification of construction machinery, agricultural tractors and special machines continues to advance, discharging the DC link is an ongoing issue.

What are the options for discharging a DC link?

Single Pulse: Simple, but large resistances needed

When the DC link is discharged using a single pulse, all the energy is applied disproportionately to the resistor. As a result, the resistor must process a high amount of energy, especially in the initial phase, which then causes extremely high temperatures. On the other hand, only a low level of energy is converted at the end of the discharge.

For the resistor to withstand the enormous heat in the beginning, it must have a high thermal capacity. This means that it must be correspondingly large. Only in this way can it absorb the heat and, if necessary, dissipate it to a cooling medium.

The advantage of this method is that no complex control electronics are required to activate the discharge of the DC link. At first glance, the effort required by the customer is therefore low and theoretically cost effective.

The disadvantage, however, is that the unregulated discharge method described above, requires a larger housing and a large cooling surface area due to the high energy conversion during the initial phase. The larger the housing and the cooling surface of the resistor, the more expensive the costs are.

Constant power: Intelligent control required, but small, cheaper resistors are sufficient

When discharging the DC link using constant power, intelligent control electronics apply a sequence of constant power pulses to the resistor at a high frequency, typically referred to as PWM.

As a result, the discharge energy is distributed evenly over the entire discharge process of the DC link. Because the discharge energy is lower at the beginning, the resistor can be built correspondingly smaller and lighter. 

Compared to single pulse discharge, the constant power discharge method works with less energy at the beginning, but with higher energy at the end of the discharge process. This results in even energy and heat conversion over the entire DC link discharge process.

The disadvantage of this method is that the resistor needs a control system to regulate the constant power. However, the control unit can be integrated cost-effectively with just a few components. In most cases, integration using existing ICs only involves a very small amount of effort.

The numerous advantages, and above all the cost savings, outweigh the effort required:

• The resistor can be built much smaller and lighter.
• Service life is extended by discharging the DC link evenly.
• Consistent heat dissipation to the heat sink is easier to handle in the worst-case scenario.
• Temperature monitoring can be integrated into the component.
• The price of the components for a constant power discharge of the DC link, including control unit, are more cost effective than the large resistor required for single-pulse discharge.

Single pulse vs. constant power: what is the best way to discharge the DC link?

Smaller resistors can generally be produced more cost effectively than larger ones. Overall, the costs for the smaller resistor plus control electronics are about 40% lower than the large resistor required for single pulse discharge.  A smaller resistor design enables new housing shapes and assembly options, such as direct integration on the circuit board.