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NPC2 Topology of RB-IGBT

Issuing time:2018-02-01 14:01
Keywords: RB-IGBT NPC2 topology Danfoss

Source: Danfoss


For many years, three-level topologies have been widely used in various applications. Such applications are usually based on the classical neutral point clamp (NPC1) topology, with four power switches (IGBTs) per half bridge and two clamp diodes. A variant of this topology is called the NPC2 topology, where two IGBTs are used for each half-bridge, and two IGBTs are connected in a clamping circuit in a common-collector connection. This topology can also use two reverse-resistive (RB) IGBTs instead of two common-collector-connected IGBTs to reduce the number of conduction elements. For NPC1/NPC2 and NPC2 topologies with RB-IGBTs, the gate driver requirements (especially protection functions such as desaturation monitoring and active clamping) are different. This article will discuss these differences and provide a proven solution to modify standard gate drivers for use with NPC2 topologies with RB-IGBTs.

1 Introduction

The traditional two-level converter topology (Figure 1a) is characterized by two switching states: the positive and negative voltage states of the DC bus (DC+, DC-). In order to reduce the total harmonic distortion in the output waveform, more switching states are needed. The well-known three-level NPC1 topology (Figure 1b) provides this additional switching state: the N-point is a neutral state of 0V. Filtering requirements can be reduced due to lower voltage waveform distortion. This makes the attraction of these topologies higher and higher because the cost of the filter has become an important factor in the design of the converter system. The disadvantage of using a three-level topology is that the number of switching devices (IGBTs and diodes) increases, increasing the complexity and partially increasing the overall system cost [1]. By applying the NPC2 topology (Figure 1c), the number of power semiconductors can be further reduced compared to the classical NPC1 device.
Two- and Three-Level NPC1/NPC2 Half-Bridge Topologies Overview
Figure 1 Overview of two-level and three-level NPC1/NPC2 half-bridge topology

Instead of the two IGBTs and diodes connected in a common-collector mode in the NPC2 topology, two reverse-resistive (RB) IGBTs can be used. The internal structure of the RB-IGBT is improved so that the IGBT can withstand the same level of forward and reverse blocking voltages. In contrast, the standard IGBT can withstand a reverse blocking voltage that is only a fraction of the forward blocking voltage. Therefore, using RB-IGBTs means that two diodes can be reduced in the NPC2 topology (Figure 1d). This can bring many advantages, such as reducing conduction losses, improving the utilization of the package area, and simplifying the layout of the auxiliary terminals of the power module [2].

2 IGBT Driver Considerations

The various topologies shown in Figure 1 require different IGBT drivers. For example, two-level topologies usually require short-circuit protection and overvoltage protection. The more commonly used short-circuit protection scheme is called VCEsat or desaturation monitoring, as shown in Figure 6. Overvoltage protection is typically achieved by actively clamping the IGBT's collector-emitter voltage. Figure 4 shows an example of this application.
If a three-level NPC1 topology is used, the turn-off sequence of the IGBT in the event of a short circuit will be very important. In this case, the external IGBT of the half-bridge must first be switched off and then the internal IGBT must be switched off. If not shut down in this order, the internal IGBT will withstand the entire DC bus voltage and will be damaged because the rated voltage of the IGBT in the three-level NPC1 topology is “only” half of the DC bus voltage [1]. Therefore, in the event of a short circuit, the drive should not automatically shut down the IGBT. Instead, the fault condition should be reported to the control unit and the control unit ensures the correct shutdown sequence. The shutdown sequence can only be ignored if the internal IGBT uses advanced active clamping and the driver is allowed to perform an automatic shutdown [3].
The common characteristic of the NPC topology shown in Figure 1 is that, during normal operation, the voltage at the phase output U alternates between +1/2DC and -1/2DC with respect to the neutral point N, ie, the polarity changes. . This is particularly significant for the bidirectional switching of the IGBT between the N and U points in the NPC2 topology. Figure 2 shows the voltages obtained by these IGBTs when external switches (not shown here) are turned on and off, respectively.
Ideal voltage distribution for bidirectional switches
Figure 2 Idealized Voltage Distribution for Bidirectional Switches
The collector-emitter voltage of the IGBT in Figure 2a is always a positive voltage or (idealized) zero, depending on the actual phase output voltage at U. Therefore, there are no special requirements for short circuit and overvoltage protection. However, if RB-IGBTs are used for bi-directional switching, the situation is different. The alternating voltage present at U-points requires modification of the classic short circuit protection and overvoltage protection circuits. Otherwise, the driver will be damaged and eventually damage the IGBT.
Figure 3 (left) illustrates the tests performed using Fuji Electric's NPC2 Power Module 4MBI650VB-120R1-50. The load in this example is connected between U and DC- with the top switch T1 turned on and off. The waveform of channel 2 ("CE RB-IGBT T3") shows the alternating voltage between N-U at the turn-on and turn-off of IGBT T1.

Switching Waveform of NPC2 Topology Using RB-IGBT
Figure 3 Switching waveforms for NPC2 topology using RB-IGBT (VDC = 800V, Iload = 650A)

2.1 Overvoltage protection

In general, in order to prevent the IGBT from being broken by overvoltage, an active clamp circuit is usually used. (For low-power applications, alternatives such as "two-level shutdown" or "soft shutdown" can also be used [1]). Overvoltage is caused by the stray inductance in the commutation circuit and the rate of change of the current (di/dt). Active clamping reliably suppresses overvoltages and has proven to drive IGBTs into active areas to reduce di/dt in a wide range of applications.

a) Standard IGBTs and b), c) RB-IGBT active clamp circuits
Figure 4 a) Standard IGBT and b), c) RB-IGBT active clamp circuit

Figure 4a shows the active clamp setting of a standard IGBT T1. The TVS (D2...x) is selected according to the actual application conditions (for example, the DC bus voltage, the VCES level of the IGBT), and is connected from the collector to the gate through a low voltage Schottky diode or a PIN diode (D1). This low-voltage diode is an essential component that prevents current from flowing from the gate of the IGBT to the collector, requiring only a blocking capability of 40V. However, if you choose an NPC2 topology with RB-IGBTs, you cannot use typical active clamp circuits with unidirectional TVS and low-voltage diodes. This is because the voltage across the RB-IGBT will change polarity depending on the switching state (Figure 4b). As long as the polarity of the corresponding IGBT collector is positive, the TVS of the corresponding driver can block the voltage from the driver. However, after the voltage polarity of the collector is reversed, the TVS diode starts to conduct and the entire collector potential is applied to the anode of the low-voltage diode D1. This voltage is approximately equal to half of the DC bus voltage and will cause damage to the IGBT driver and related IGBTs.

There are two optional precautions. In the first solution, bidirectional rather than unidirectional TVS must be used, as shown in Figure 4c. However, it can be seen from FIG. 3 that the negative voltage “ax(C2)” is likely to reach the level corresponding to the bidirectional TVS breakdown voltage. This will still subject the diode D1 to excessive reverse voltage. Therefore, this method is not recommended. A second solution is recommended to replace the low-voltage diode D1 with a high-voltage diode. The blocking voltage of the high voltage diode must be at least half of the DC bus voltage. Please note that in addition to blocking the voltage, the creepage distance and clearance of the diode must also be considered. In some cases it may be necessary to use multiple diodes in series.

2.1.1 Advanced Active Clamp Function

In order to increase the efficiency of the active clamp circuit, CONCEPT is equipped with a so-called advanced active clamp in its multiple drivers.

(AAC) features. AAC uses an additional feedback circuit in the driver's internal output stage. According to the actual clamp current/overvoltage condition, the internal push stage MOSFET will be turned off linearly [4].

a) Standard IGBT and b) Advanced active clamp circuit for RB-IGBT
Figure 5 a) Advanced Active Clamp Circuits for Standard IGBTs and b) RB-IGBTs

If you use an NPC2 topology with RB-IGBTs, you need to modify the conventional AAC design. Figure 5a shows the commonly used AAC for standard IGBTs containing D1 and D3 (low voltage diodes). Due to the use of only one bi-directional TVS (Dx), point A will generate a dangerously high voltage when –C/2 is applied to terminal U. This will cause the diodes D1 and D3 and 20R to be overloaded, eventually causing the entire driver to be overloaded. To prevent this high voltage, it is recommended to replace all unidirectional TVSs with bidirectional TVSs. In addition, these TVSs need to be connected in series with a unidirectional TVS D4 (Figure 5b). The asymmetric breakdown voltage of the TVS network ensures that the voltage generated at point A is within a safe range (assuming the TVS is selected according to the actual application conditions) when U is negative (-DC/2), and when U is Active clamping works normally at positive voltage (+DC/2).

2.2 Short Circuit Protection Function

In order to protect any topological IGBT in the event of a short circuit, a reliable desaturation monitoring function is required. Figure 6 shows a classic desaturation monitoring scheme using high voltage diodes. This setting is usually used to detect short circuits. A more advanced solution is to replace the high voltage diode with a resistor network (Rvce in Figure 7a) that measures the VCE voltage while the IGBT is on. This solution can avoid short-circuit protection malfunction [1]. Both schemes can be used for two-level and three-level NPC1/NPC2 topologies.

However, if an NPC2 topology with RB-IGBTs is selected, desaturation monitoring with high voltage diodes will no longer work. The same reason as explained in the overvoltage protection function, as long as the corresponding collector voltage is positive (relative to the emitter), and the corresponding driver's high voltage diode can block the collector and the low voltage side of the driver between the detection input Voltage, this method works. But as long as the polarity goes to a negative potential, the diode starts to conduct, and excessive current will flow through the diode, which will damage the driver and/or IGBT.

In a short-circuit protection function using a resistor network, the resistor Rvce lowers the collector voltage and limits the current flowing from the collector to the drive's sense input. The next part of this article will briefly explain the principle of this circuit. [5]

Deuterium saturation monitoring using high voltage diodes
Figure 6 Desaturation monitoring using a high voltage diode

2.2.1 Short Circuit Protection Using Resistor Networks

The following description refers to Figure 7. In the IGBT off state, the MOSFET's internal MOSFET connects the detection pin to

COM (the negative potential of the gate driver). Then, the capacitor Cax is precharged/discharged to the negative supply voltage. If there is no diode D1, K point will produce voltage VK, which can be calculated according to formula 1.

Formula 1:

The function of D1 is to clamp the voltage VK at the positive supply voltage VCC to prevent the gate driver's detection input from being damaged by the high voltage. The maximum current flowing through point K can be calculated by the following formula:

Formula 2:

In order to limit losses in the resistor network and diode D1, it is recommended that the current be adjusted to the maximum DC bus voltage

0.6...mA. The current flowing through point F can be calculated according to Equation 3. This current will charge Cax while the IGBT is on. The time required for Cax charging determines the response time of the short-circuit protection function.

Formula 3:

The MOSFET turns off when the IGBT is turned on and in the on state. As VCE decreases, Cax charges from COM potential to IGBT saturation voltage. The voltage on Cax is always compared with the reference voltage determined by Rref. In the event of a short circuit, the voltage of the capacitor Cax increases as the IGBT de-saturates. When the voltage of Cax is higher than the reference voltage, the driver regards this as a fault condition. Figure 7b depicts the short circuit protection process.

Saturation monitoring function schematic using a resistor network
Figure 7 Schematic diagram of desaturation monitoring using a resistor network

If a negative voltage appears at the collector of the IGBT in the OFF state, the voltage at the K point will also be a negative voltage. In order to prevent the detection pin output current of the driver, it is necessary to add a diode D2 in the circuit (Figure 8). Otherwise, substrate current will be generated in the driver circuit and an unexpected latch-up effect occurs (Note: Active rectification can also be implemented in the ASIC to solve this problem). Diode D2 clamps the K-point voltage at the emitter potential, preventing/limiting any current flowing out of the driver's sense pin.

Improved desaturation monitoring function using a resistor network
Figure 8 Improved desaturation monitoring using a resistor network

Figure 9 demonstrates the use of Fuji Electric's RB-IGBT NPC2 4MBI300VG-120R-50 power module with CONCEPT standard version of the 2SC0106T driver core (other drivers such as the 2SC0108T and 2SC0435T are also applicable), and the recommended circuit for short-circuit and active clamp Bits are modified to enable short circuit protection. Using a standard device without a snubber capacitor, the applied DC bus voltage is 800V.
Short circuit test after modifying the driver according to the recommended circuit
Figure 9 Short circuit test after modifying the driver according to the recommended circuit

3 Conclusion

In summary, for NPC2 topologies using RB-IGBTs, classical protection functions such as desaturation monitoring and active clamping need to be modified. These modifications can be easily implemented using the standard driver core provided by CONCEPT. Without these modifications, the negative voltage at the phase output will overload the driver, damaging the driver and eventually damaging the entire power cell. The solution proposed in this paper opens a new path for the application of new RB-IGBTs technology in the fields of solar power generation and UPS.
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