Discharge resistors play a crucial role in the safe discharge of DC link capacitors. These components discharge the current after an electric vehicle is switched off and convert the energy into heat. This ensures that the DC link capacitor is discharged safely.
There are legal regulations and various technical implementation options for safe discharge. The measures required are explained below.
According to applicable regulations, intermediate circuit capacitors and other capacitors must be discharged to below 60 V within a maximum of 5 seconds after the ignition is switched off. Once the vehicle battery is disconnected from the intermediate circuit after the vehicle is turned off, residual energy remains in the capacitors. This must be dissipated quickly to ensure safety.
Discharge resistors from Miba, also known as DIScharge Resistors, ensure that this discharge process in the intermediate circuit capacitor is carried out quickly and reliably. This regulation applies to all electrically powered vehicles that drive on public roads, such as HEVs, PEVs, BEVs, and FEVs. As the trend toward electrification is also increasing in areas such as construction machinery, agricultural vehicles, and special-purpose machines, the issue of safe discharge is also gaining importance in these areas.
In the single-pulse discharge method, all of the energy stored in the capacitor is fed into the discharge resistor at once. This leads to a high energy flow, especially at the beginning of the discharge process, which causes significant heating. However, the energy conversion decreases significantly in the final phase.
In order to withstand these thermal loads, the resistor must have a high heat capacity. Accordingly, they must be dimensioned large enough to safely dissipate the heat. The advantage of this method is its ease of use, as no additional control electronics are required.
The disadvantage, however, is the uneven discharge process, which requires a large housing and cooling surface due to the high energy conversion in the initial phase. This increases both material and manufacturing costs.
When discharging the DC link capacitor using constant power, intelligent control is required to supply the resistor with several constant power pulses at a high frequency. This ensures an even distribution of discharge energy throughout the entire process.
Since the initial energy is lower in this process, the discharge resistors can be made more compact and lighter. In contrast to single-pulse discharge, the energy is released evenly over the entire discharge period, which enables controlled heat generation.
The only disadvantage of this method is the need for constant-power control of the resistor, which can, however, be implemented at low cost. In many cases, it can be implemented with existing ICs, keeping the effort to a minimum.
The advantages of the constant power method clearly outweigh the disadvantages:
More compact discharge resistors for capacitors are cheaper to manufacture than large ones, while the required control electronics only represent a minor additional expense. In the overall calculation, the combination of a small resistor and intelligent control is around 40% more cost-efficient than a large resistor in the single-pulse process.
In addition, the smaller design offers new possibilities in terms of housing shape and mounting options, such as direct integration on the circuit board.
The special circuit design of the constant-power pulse discharge of the DC link capacitor reduces the size of the discharge resistor, thereby saving costs, space, and weight. In addition, temperature monitoring can be integrated and multiple resistors can be integrated into one component (for example, for active and passive discharge).
Constant-power discharge therefore offers clear advantages over the single-pulse method thanks to its compact design, lower weight, and uniform energy release.
