KR20240026550A - Regenerative clamp circuits for series connection of power switching devices - Google Patents

Abstract

Voltage clamp circuits for series connection of power switching elements are presented. Voltage clamp circuits include a regenerative discharge circuit to reduce power loss. By using high-frequency alternating current in the regenerative discharge circuit, the size of the isolation transformers is reduced. By ladder connecting the transformers, the required insulation voltage between the primary and secondary windings is lowered. Circuits that further include regenerative snubbers are also presented.

Description

{Regenerative clamp circuits for series connection of power switching devices}

The present invention relates to circuits for series connection of switching elements for power, and more specifically to regenerative clamp circuits.

switching power converter

In Figure 1, a direct current to three-phase alternating current voltage source converter is shown as an example of a switching power converter. An ideal switch is a lossless device, so switching power converters composed of ideal switches have 100% energy efficiency.

Figure 2 shows a converter composed of actual semiconductor switching elements. Actual semiconductor switching devices are not lossless but have low-loss characteristics. Actual switching power converters can have efficiencies as high as about 99%, depending on the application.

Energy efficiency of 99% may be satisfactory for some applications, but it is not particularly satisfactory for high-power applications. For example, in a converter handling 500 MW of power, a 1% loss is 5 MW of power. Power loss is a loss in itself and a burden on the heat dissipation device.

Series connection of switching elements

Actual semiconductor switching devices have limitations in their voltage blocking capabilities. The blocking voltage of currently used semiconductor switching devices is up to several thousand V.

To implement higher voltage power converters, a series connection of switching elements is used. As an example, Figure 3 shows a converter in which four switching elements are connected in series. In the case of HVDC systems of hundreds of kV, hundreds of switching elements are connected in series.

Circuits for voltage distribution

There is a problem with the distribution of blocking voltage in the series connection of switching elements. If the off-state characteristics of the switching elements are the same, the blocking voltages will be equally divided, but it is known that there is a limit to producing switching elements with the same off-state characteristics. Therefore, a method of connecting additional resistors for voltage distribution is used. This is shown in Figure 4. The values of the voltage distribution resistors should be manufactured to the same values as precisely as possible.

Even if voltage distribution resistors are connected, another problem arises with the switching transient time. Even if the switching elements are turned off and driven at the same time, since the characteristics of the switching elements and driving circuits are not completely the same, the time when the switching elements are actually turned off is not completely the same. Therefore, a high blocking voltage is applied to the switching elements that are turned off first. Additionally, even when turning on, a high blocking voltage is applied to switching elements that are turned on later.

To alleviate the above problem, a method of additionally connecting snubber circuits for voltage distribution is used. This is shown in Figure 5. These snubber circuits help distribute the blocking voltages over the switching transient time by allowing the blocking voltages to rise linearly rather slowly.

The above method has been widely used, but additional losses occur in additional voltage distribution resistors and voltage distribution snubbers. In addition, since the blocking voltage distribution function is not strict and there is no overvoltage prevention function, the number of switching elements connected in series must be increased to ensure reliability, which leads to more on-state loss in the switching elements.

voltage clamp circuit

A method of additionally connecting Zener diodes to prevent overvoltage is proposed. This is shown in Figure 6. The voltage applied to each switching element does not become greater than the Zener voltage, and this function is called voltage clamp.

By connecting Zener diodes, voltage division resistors or voltage division snubber circuits can be omitted. This is shown in Figure 7. In this case, losses similar to those occurring in voltage distribution snubbers occur in Zener diodes. When turning off, load current flows through the Zener diodes next to the switching elements that are turned off first, causing loss. When turning on, load current flows through the Zener diodes next to the switching elements that are turned on later, causing loss. Even in the off state, the voltage is clamped and leakage current can flow to the Zener diodes, but in general, the off state leakage current is very small.

Regenerative voltage clamp circuit

Clamping the voltage to prevent overvoltage can be done with diodes and voltage sources rather than Zener diodes. This is shown in Figure 8. In this circuit, losses occurring in Zener diodes are charged to voltage sources, so ideally the losses can be eliminated. However, the problem lies in how to implement the voltage sources.

Replacement of voltage sources with capacitors is shown in Figure 9. Let us first charge the capacitors to a certain voltage using some means. If the voltage across the capacitors is maintained at a constant level, the capacitors are equivalent to voltage sources. However, when turning off, the capacitor is further charged as the load current flows through the diodes next to the switching elements that are turned off first. When turning on, the capacitor is further charged as the load current flows through the diodes next to the switching elements that are turned on later. If the capacity of the capacitors is slightly increased, the increase in the voltages of the capacitors at each switching may not be large, but discharging is necessary to maintain the voltages of the capacitors at a constant level in the long term. The problem now is how to discharge the more charged capacitors as needed and recover the energy.

A regenerative discharge circuit as shown in Figure 10 has been proposed. Figure 10 shows only one module among the series switching elements. A switching element for discharge (14) is further connected next to the clamp diode (13). By detecting when the load current flows through the main diode (11), turning off the main switching element (12) and turning on the discharging switching element (14), the main diode (11) is turned off and the load current is discharged. As it flows through the dedicated switching element 14, the capacitor is discharged. And the energy of the capacitor is recovered by the DC side voltage source of FIG. 3. After discharging as necessary, when the discharging switching element 14 is turned off, the load current flows through the main diode 11 again.

The regenerative clamp circuit of FIG. 10 is valuable as a technology that enables low-loss series connection of switching elements. However, it is not good for the regenerative discharge circuit to be included in the main circuit.

First, discharging operation is only possible when the load current flows through the main diode. The time available for regenerative discharge operation is limited.

Second, the size of the discharge current is the same as the load current, and the load current changes depending on the load. Discharge is fast at medium load, slow at light load, and no discharge at no load.

Third, because the size of the discharge current is the same as the size of the load current, the ratings of the switching elements for discharge must be the same as those of the main switching elements.

Fourth, the capacity of the capacitor must be quite large. This is because the time for the load current to flow through the main diode to discharge the capacitor is determined by the period of the alternating current. If the frequency on the alternating current side is 60Hz, 1/120 second is the time during which discharge cannot occur. In other words, the voltage of the capacitor must be maintained to a certain extent without discharge for a period of 1/120 of a second, so the capacity of the capacitor is required to be quite large.

Fifth, the operation of the discharge circuit becomes a disturbance in the pulse width modulation control of the main circuit.

B. Gemmell, J. Dorn, D. Retzmann and D. Soerangr, “Prospects of multilevel VSC technologies for power transmission,” 2008 IEEE/PES Transmission and Distribution Conference and Exposition, 2008. T. J. Hammons et al., “State of the Art in Ultrahigh-Voltage Transmission,” in Proceedings of the IEEE, vol. 100, no. 2, pp. 360-390, Feb. 2012. R. Withanage and N. Shammas, “Series Connection of Insulated Gate Bipolar Transistors (IGBTs),” in IEEE Transactions on Power Electronics, vol. 27, no. 4, pp. 2204-2212, April 2012. V. U. Pawaskar, G. Gohil and P. T. Balsara, "Study of Voltage Balancing Techniques for Series-Connected Insulated Gate Power Devices," in IEEE Journal of Emerging and Selected Topics in Power Electronics, vol. 10, no. 2, pp. 2380-2394, April 2022. F. V. Robinson and V. Hamidi, "Series connecting devices for high-voltage power conversion," 2007 42nd International Universities Power Engineering Conference, 2007. L. Yang, L. Zhu, P. Fu, L. Yue and X. Yao, "A Module-Based Self-Balancing Series Connection for IGBTs," in IEEE Transactions on Industrial Electronics, vol. 68, no. 10, pp. 9410-9419, Oct. 2021. J. Zhang, S. Shao, Y. Li, J. Zhang and K. Sheng, "A Voltage Balancing Method for Series-Connected Power Devices in an LLC Resonant Converter," in IEEE Transactions on Power Electronics, vol. 36, no. 4, pp. 3628-3632, April 2021. Z. Gao, S. Shao, W. Cui, J. Zhang, X. Chen and K. Sheng, "A Voltage Balancing Method for Series- Connected Power Devices Based on Active Clamping in Voltage Source Converters," in IEEE Transactions on Power Electronics, vol. 37, no. 9, pp. 10620-10632, Sept. 2022. J. Rodriguez, Jih-Sheng Lai and Fang Zheng Peng, “Multilevel inverters: a survey of topologies, controls, and applications,” in IEEE Transactions on Industrial Electronics, vol. 49, no. 4, pp. 724-738, Aug. 2002. S. Debnath, J. Qin, B. Bahrani, M. Saeedifard and P. Barbosa, “Operation, Control, and Applications of the Modular Multilevel Converter: A Review,” in IEEE Transactions on Power Electronics, vol. 30, no. 1, pp. 37-53, Jan. 2015.

The object of the present invention is to provide clamp circuits for serial connection of power switching devices as independent circuits without including regenerative discharge circuits in the main circuit.

1. In circuits for serial connection of switching elements for power;

Diodes and clamp capacitors connected in parallel to the switching elements;

High-frequency inverters connected in parallel to the clamp capacitors;

High-frequency isolation transformers connected to the high-frequency inverters; and

a high-frequency alternating current voltage source to which the high-frequency isolation transformers are connected;

Regenerative clamp circuits comprising a.

2. The regenerative clamp circuits according to item 1 above, wherein the high-frequency isolation transformers and the high-frequency alternating current voltage source are connected in parallel.

3. The regenerative clamp circuits according to item 1 above, wherein the connection between the high-frequency isolation transformers and the high-frequency alternating current voltage source is a ladder connection.

4. The regenerative clamp circuits according to item 1 above, wherein the connection between the high-frequency isolation transformers and the high-frequency alternating current voltage source is a combination of a ladder connection and a parallel connection.

5. In paragraph 1, 2, 3, or 4 above,

Diodes and snubber capacitors connected in parallel to the switching elements;

Diodes connected in parallel to the snubber capacitors; and

DC-DC boost converters connected between the snubber capacitors and the clamp capacitors;

Regenerative clamp circuits further comprising:

6. In paragraph 5 above,

Snubber inductors connected in series to the switching elements;

Regenerative clamp circuits further comprising:

The present invention is a regenerative clamp circuit that enables low-loss series connection of switching elements. Additionally, by providing the regenerative discharge circuits as independent circuits rather than including them in the main circuit, the following effects are achieved.

First, the discharging operation is independent, so discharging operation is always possible regardless of the operation of the main circuit.

Second, since the discharge operation is always possible, the size of the discharge current may be small.

Third, because the size of the discharge current can be small, the current rating of the switching elements of the high-frequency inverters can be small.

Fourth, since discharging operation is always possible, the capacity of the clamp capacitors may be small.

Fifth, because the operation of the discharge circuit is independent, it does not affect the pulse width modulation control of the main circuit.

Figure 1 shows an example of a switching power converter.
Figure 2 shows a converter composed of actual semiconductor switching elements.
Figure 3 shows an example of a converter in which switching elements are connected in series.
Figure 4 shows additional voltage dividing resistors.
Figure 5 shows additional voltage distribution snubbers.
Figure 6 shows additional voltage clamps.
Figure 7 shows only the voltage clamps connected.
Figure 8 shows ideal voltage clamps.
Figure 9 shows unfinished voltage clamps.
Figure 10 shows regenerative voltage clamp circuits.
Figure 11 shows the first embodiment of the present invention.
Figure 12 shows a second embodiment of the present invention.
Figure 13 shows further inclusion of unfinished turn-off snubbers.
Figure 14 shows a third embodiment of the present invention.
Figure 15 shows the fourth embodiment of the present invention.

.

Figure 11 shows the first embodiment of the present invention. High-frequency inverters 21 are connected to the clamp capacitors, and the high-frequency inverters 21 are connected to high-frequency isolation transformers 22 via inductors. The high-frequency isolation transformers 22 are connected in parallel to the high-frequency alternating current voltage source 23. The energy charged in the clamp capacitors by the voltage clamp operation for the main switching elements is converted into high-frequency alternating current through the high-frequency inverters 21 and recovered to the high-frequency alternating current voltage source 23 through the high-frequency isolation transformers 22. . The high-frequency inverters 21 regulate the voltages of the clamp capacitors by controlling the discharge currents, respectively.

If the frequency of the high-frequency alternating current voltage source 23 in which the recovered energy is stored is increased, the size and weight of the high-frequency isolation transformers 22 become very small.

Discharging of the clamp capacitors is achieved through control of independent high-frequency inverters 21 and is always possible. Therefore, the capacity of the capacitors does not have to be large, and the current rating of the high-frequency inverters can be small.

The problem with the Figure 11 circuit is that high-voltage isolation is required between the primary and secondary windings in the high-frequency isolation transformers 22. If hundreds of switching elements are connected in series in an HVDC system of several hundred kV, several hundred kV of insulation is required between the primary and secondary windings of the high-frequency isolation transformers 22.

Figure 12 shows a second embodiment of the present invention. The configuration and operating principle are similar to the first embodiment, but the difference is that the high-frequency isolation transformers 32 are ladder-connected to the high-frequency alternating current voltage source 33.

The high frequency isolation transformers 32 of the Figure 12 circuit do not require high voltage isolation between the primary and secondary windings. The voltages between the primary and secondary windings of the high-frequency isolation transformers 32 are within twice the clamp capacitor voltages. The amount of current flowing through the high-frequency isolation transformers 32 is not the same. As currents continue to merge into the ladder, the lower high-frequency isolation transformers 32 carry more current.

In the circuit of Figure 12, if too many high-frequency isolation transformers are ladder-connected, leakage inductances of the high-frequency isolation transformers may accumulate, causing problems with voltage stability. In this case, voltage stability can be improved by appropriately adding damping resistors to the ladder circuit. Alternatively, the number of ladder connections may be limited and ladder connections and parallel connections may be appropriately combined.

A turn-off snubber circuit can be added to reduce the turn-off losses of the main switching element. This is shown in Figure 13. Figure 13 shows only one module among the series switching elements. By adding a snubber capacitor 42 in parallel so that the blocking voltage increases linearly and slightly slowly from 0V, the turn-off loss occurring in the main switching element 41 is reduced. This is the so-called zero voltage turn-off. However, while the main switching element 41 is turned on, the energy charged in the snubber capacitor 42 must be discharged to 0V to play the role of a snubber at the next turn-off.

Figure 14 shows a voltage clamp circuit further including a regenerative turn-off snubber circuit. The discharge circuit of the snubber capacitor is a type of DC-DC boost converter. Discharging of the snubber capacitor 42 occurs while the main switching element 41 is turned on. When the discharge switching element 43 is turned on, the energy of the snubber capacitor 42 is transferred to the inductor 44, and the snubber capacitor 42 is discharged to 0V. When the discharge switching element 43 is turned off, the energy stored in the inductor 44 is charged to the clamp capacitor 46 through the discharge diode 45.

A turn-on snubber circuit can also be added to reduce the turn-on losses of the main switching element. Adding a snubber inductor in series to allow the conduction current to increase linearly and slowly from 0A reduces the turn-on losses occurring in the main switching element. This is the so-called zero current turn-on. And while the switching element is turned off and in the off state, the energy charged in the snubber inductor must be discharged to 0A to play the role of a snubber at the next turn on.

Figure 15 shows a voltage clamp circuit further including a turn-on snubber circuit. When the main switching element 41 is turned off, the energy stored in the snubber inductor 47 is transferred to the snubber capacitor 42, and the snubber inductor 47 is discharged to 0A.

In each module of series switching elements, the voltage of the clamp capacitor undergoes appropriate power conversion and can be used as a driving power source for the main switching element and other switching elements and as a power source for other electronic circuits.

When starting, a high-frequency alternating current voltage source is first applied to the high-frequency isolation transformer circuit, and the high-frequency inverters operate as diode rectifiers, charging the clamp capacitors and securing the clamp voltages. Then, in each module of the series switching elements, the driving power of the switching elements and the power of other electronic circuits are secured and ready for operation.

Claims (6)

In circuits for serial connection of switching elements for power;
Diodes and clamp capacitors connected in parallel to the switching elements;
High-frequency inverters connected in parallel to the clamp capacitors;
High-frequency isolation transformers connected to the high-frequency inverters; and
A high-frequency alternating current voltage source to which the high-frequency isolation transformers are connected;
Regenerative clamp circuits comprising a. The regenerative clamp circuits according to claim 1, wherein the high-frequency isolation transformers and the high-frequency alternating current voltage source are connected in parallel. The regenerative clamp circuits according to claim 1, wherein the connection between the high-frequency isolation transformers and the high-frequency alternating current voltage source is a ladder connection. The regenerative clamp circuits according to claim 1, wherein the connection between the high-frequency isolation transformers and the high-frequency alternating current voltage source is a combination of a ladder connection and a parallel connection. In claim 1, claim 2, claim 3, or claim 4,
Diodes and snubber capacitors connected in parallel to the switching elements;
Diodes connected in parallel to the snubber capacitors; and
DC-DC boost converters connected between the snubber capacitors and the clamp capacitors;
Regenerative clamp circuits further comprising: In claim 5,
Snubber inductors connected in series to the switching elements;
Regenerative clamp circuits further comprising: