Abstract: A power supply with a 360-900V DC input, which can be directly powered by the inverter's intermediate DC bus, sounds very appealing. However, such a power supply has special design requirements.
Power supplies for control systems and other applications have traditionally been supplied directly from single-phase or three-phase power grids. However, with the increasing use of frequency converters and servo motor amplifiers, a new power supply method has emerged: power from the intermediate DC bus of the frequency converter. The advantage of this is that it utilizes the "free" kinetic energy stored in the running motor to power the control system. If this becomes a reality, it will significantly improve the robustness of the power supply relative to grid fluctuations, eliminating the need for a battery buffer system that requires frequent maintenance.
To understand this application, consider a crane as an example: what happens if the power grid is suddenly interrupted just as the crane is lifting a load ? Typically, the control system must have a battery backup to ensure the load is safely lowered. However, if the control system is powered by an intermediate DC bus, the crane's motor acts as a generator as the load is lowered, maintaining voltage on the intermediate DC bus and the control system. This forced synchronization of the actuators and control system simplifies and enhances system design.
So, what requirements must a power supply meet to be suitable for applications powered by an intermediate DC bus ? Some might say there are no requirements, because based on fundamental principles, switching power supplies always internally rectify AC power, so they can be powered by both AC and DC. Furthermore, many switching power supply product specifications clearly indicate a certain DC input range, such as 450…750Vdc. So why do problems arise in practice when a regular switching power supply is directly connected to an intermediate DC bus ?
One reason is that the voltage on the intermediate DC bus is often a high-frequency AC voltage with a ground amplitude of several hundred volts , also known as "common-mode noise." The label "DC voltage" actually refers only to the voltage between the positive and negative terminals, not to ground, because the entire intermediate DC bus fluctuates to ground at the same rhythm. This effect is due to the high-frequency periodic connection of the positive and negative terminals to ground via the motor and other capacitors in the inverter using fast switches (IGBTs).
Although frequency inverters have built-in filters, these filters only operate externally, facing the input power grid; they have no effect internally, facing the intermediate DC bus. Furthermore, because this is within the inverter's internal system, there are no explicit electromagnetic compatibility (EMC) standards. EMC standards for power supplies do not cover this application because ordinary power supply inputs do not experience long-term interference in the 600V, kiloHz frequency range—interference that exceeds the input reliability requirements of ordinary power supplies by tens of times. Therefore, the requirements for power supplies supplying the intermediate DC bus are completely different from those for ordinary power supplies.
