Power Design Requirements and Issues Powered by an Intermediate DC Bus

With a 360-900V DC input that can be powered directly from the inverter intermediate DC bus, such a power supply sounds very exciting. But such power supplies have special design requirements.

With a 360-900V DC input that can be powered directly from the inverter intermediate DC bus, such a power supply sounds very exciting. But such power supplies have special design requirements.

Power supplies for control systems and other applications have traditionally been powered directly from a single-phase or three-phase grid. However, due to the increasing use of frequency converters and servo motor amplifiers, a new power supply may emerge: from the intermediate DC bus of the frequency converter (Figure 1). The advantage of this power supply is that the “free” kinetic energy stored in the running motor can be used to power the control system. If this possibility becomes a reality, it would greatly improve the robustness of the power supply against grid fluctuations without having to use a battery buffer system that requires frequent maintenance.

 Power Design Requirements and Issues Powered by an Intermediate DC Bus

To understand this application, consider a crane as an example: What happens if the power grid is suddenly interrupted while the crane is just lifting the load? Often, the control system must be equipped with battery buffers so that the hoisted cargo can be lowered safely. Whereas, if the control system is powered by the intermediate DC bus, the crane’s motor acts as a generator when the load is lowered, maintaining the voltage on the intermediate DC bus and the control system. This forced synchronization of actuators and control systems enables simpler and safer system design.

So, what requirements must a power supply meet to be suitable for an application powered by an intermediate DC bus? One might say that there is no requirement, since a switching power supply always rectifies the AC internally, inferred from the basic principle, so a switching power supply can be powered by both AC and DC. Moreover, the product description of many switching power supplies will also clearly point out a certain DC input range, such as 450…750Vdc. So why is there a problem in practice with an ordinary switching power supply directly connected to the intermediate DC bus?

 Power Design Requirements and Issues Powered by an Intermediate DC Bus

One reason is that the voltage on the intermediate DC bus is often a high frequency AC voltage with an amplitude of several hundred volts to ground (Figure 2), known as “common mode noise”. The designation “DC voltage” actually refers only to the voltage between the positive and negative poles, not to the ground, because the entire intermediate DC bus moves to the ground at the same rhythm. This effect is due to the fact that the fast switches (IGBTs) in the frequency converter periodically connect the positive and negative poles to ground at high frequency through the motor and other capacitors.

Although the frequency converter has its own filter, it only works externally, that is, facing the input power grid; and internally, that is, facing the intermediate DC bus, it does not work. And because this is the inside of the inverter, there is no clear electromagnetic compatibility standard requirement. The electromagnetic compatibility standard for power supply does not cover this application, because there will be no long-term interference in the frequency range of 600V and several thousand Hz on the input end of ordinary power supply, which exceeds a few percent of the input reliability required by ordinary power supply. ten times. Therefore, the requirements for the power supply powered by the intermediate DC bus are completely different from ordinary power supplies.

Therefore, PULS has developed a 24V/20A output power supply specially powered by the intermediate DC bus of the inverter, QTD20.241. An important feature of it is that it reverses the orientation of the filter. The filter is usually designed so that the interference generated by the switching power supply does not affect the power grid at the input end. In the application of intermediate DC bus power supply, the filter needs to shield the interference from the intermediate DC bus; the interference from the power supply does not need to be filtered out, because the intermediate DC bus is already very “clean”, and the filter that comes with the inverter The filter also filters out possible interference from the QTD20 power supply. In addition, according to the requirements of the UL508 standard, there is a special DC fuse on the input end of the QTD20. If the input end of the QTD20 is accidentally connected in reverse, the fuse will not break, and the power supply will not continue to work.

In fact, there are many applications in which the QTD series products launched by PULS can avoid personal injury and economic losses that may be caused by power grid fluctuations, such as textile, papermaking, grinding machinery, and the aforementioned hoisting machinery. QTD series products can provide enough energy to the control system to ensure the safe and controlled shutdown of the entire equipment in the event of grid fluctuations, thus becoming the best choice for drive system design.

The Links:   NL10276AC30-04 NL6448BC20-30F

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