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Posted on 29 June 2019

The major difference between a conventional IGBT and RB-IGBT is the reverse blocking capability

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Power conversion efficiency and reduction of power loss is the main target of power electronics.

The development of 3-Level power converters realized by neutral-pointclamped technology (NPC) was one big step in higher efficiency converters. The presented power module realizes an advanced NPC (A- NPC) topology to minimize power loss with Fuji’s 6th generation IGBT and Fuji’s unique reverse blocking IGBT (RB-IGBT) dies. 

By Daniel Hofmann, Thomas Heinzel and Fred Eschrich, Fuji Electric Europe

 

Due to increase in power consumption the power conversion efficiency and reduction of power losses in power conversion systems become more and more important. A multilevel topology is one of the most effective configurations in power conversion for DClink power. A very common solution for improved efficiency regarding to multi-level power converters is the Neutral-Point-Clamped (NPC) 3-level power converter. A 3-level topology allows decreasing switching losses as well as a reduction in size of filter by enhanced spectral performance of output voltage and increase of switching frequency.

Fuji investigated another type of NPC 3-level power module named advanced NPC (ANPC). This generation of power module utilizes a reverse blocking IGBT (RB-IGBT) for the clamping of output to the neutral point. Besides, the reduction of switching loss, a minimization of filters and low conduction loss could be achieved.

This article shows the basic function of the RB-IGBT chip and module as well as possible applications for Wind Power, Photovoltaic (PV) and Drives circuit.

3-Level Topologies

Fuji Electric's RB-IGBT is not the only way to create a 3-Level topology. There are several other topologies to realize a 3-Level configuration. Two well-known topologies which are called NPC1 and NPC2 reach comparable advantages like Fuji's 3-Level solution.

NPC1:

The advantage of the topology is that the chips only have to block half of applied voltage by reason of two IGBT chips in series. Additionally the breakdown voltages Vces of all switches (T1 to T4) are only half of a 2-Level inverter which leads to less loss. The disadvantage of this topology is the usage of many switches and therefore many components. The on-state voltage Von increases as a result of all IGBTs in series. Every single IGBT on state loss contributes to the onstate loss and leads to higher loss than in a 2-Level inverter.

NPC2:

This topology has less components than NPC1 and looks quite similar to Fuji’s A-NPC solution. Instead of RB-IGBT this topology uses two IGBT chips and two diodes. One IGBT is connected in series with a diode in blocking direction and connected antiparallel with the other IGBT and diode. The merit of this topology is the reduction of Von loss because of using only a single IGBT in series and hence less switches. Therefore one IGBT chips has to block the full voltage. Consequentially the chip needs to be higher rated and implies higher switching loss. T1 and T2 have the same breakdown voltage as the 2-Level half bridge without any advantage.

A-NPC:

The A-NPC solution by Fuji has the same advantages as NPC2 according to number of components and low Von loss due to single switches in series. The additional merit of this topology is even lower loss by reason of the usage of RB-IGBT. A single RB-IGBT can substitute an IGBT and diode in series which leads to a miniaturization and less loss. The disadvantage as seen in NPC2 is the needed full blocking voltage capability of main switches and the high breakdown voltage of T1, T2 as in 2-Level topologies.

Three widely used 3-Level topologies

Regarding to the device loss of the topologies in dependency of switching frequency it is quite obvious that up to 30 kHz the A-NPC has an advantage compared to all other NPC topologies (Figure 2). Above 30 kHz the switching loss of A-NPC becomes higher than the NPC1 configuration.

Devices loss against switching frequency for 2-level, NPC 3-level and A-NPC configurations.

The 3-level module is a single-phase module in M403 package with footprint of 110mm x 80mm with optimized arrangement of the terminals for the construction of A-NPC power converters. According to Figure 3 the external view and the circuit diagram of the module with main terminals U, N, M, and P on the top and auxiliary terminals arranged at the edge is shown. The equivalent circuit in Figure 3a shows two main switches T1 and T2 made of V-Series IGBT chips (6th generation) by Fuji and the clamping switches T3 and T4 (RBIGBT) which are connected in anti-parallel. Main switch IGBT’s are rated with 1200V/300A for instance for the A-NPC module 4MBI300VG-120R-50. Due to only half DC-link voltage applied to clamping of output U to the neutral point M the needed RB-IGBT’s capability is 600V/300A. The usage of RB-IGBT dies with its bi-directional switching capability is leading to an improvement of loss. Conventional IGBTs don’t have enough reverse blocking capability and therefore a FWD in series connection is needed. Otherwise a leakage current flows along the surface of the IGBT die when a reverse voltage is applied between collector and emitter.

3-Level module 4MBi300VG-120R-50 in M403 package and Equivalent circuit of ANPC module with T1, T2 as main switches and T3, T4 as RB-IGBT switches.

The major difference between a conventional IGBT and RB-IGBT is the reverse blocking capability of RB-IGBT. The IGBT structure which belongs to NPT technology was improved by adding a junction isolation region. As shown in figure 4 (b) the RB-IGBT structure has a junction isolation region which is missing in the conventional IGBT figure 4 (a). The dicing surface or also lateral face is generated during the chip manufacturing process. The chips are cut out from the wafer with crystal deformations and high–density crystal defects. At this lateral face a continuous generation of carriers from the crystal defects is transported by an electric field and lead to a large leakage current at negative voltage. Due to the applied reverse voltage from Emitter to Collector holes are generated. An additional hole will be created at the other edge of the cell between Collector and p+ area. According to the whole generation a current flows through the Base into the Emitter and a large leakage current occurs.

Cross sectional view of conventional chip and RB-IGBT [1].

By applying a positive Gate-Emitter voltage this leakage current can be reduced as demonstrated in fig. 5. As long as Vge = + 15V is applied the RB-IGBT is in on-state and the generated flow of electron due to reverse voltage decreases. Only a small leakage current remains. The blocking voltage characteristic is shown in figure 5. The blue curve represents the RB-IGBT with shorted Gate-Emitter. The forward and reverse blocking capability is related to the 600V chip with same blocking voltage value. Cyan colored wave form shows an improvement and extended reverse blocking behavior of RB-IGBT when positive voltage of 15V is applied between Gate and Emitter. Furthermore a conventional NPT-IGBT also with Vge of +15V is indicated in red. The insufficient capability of reverse blocking is visible.

Blocking voltage characteristics of conventional IGBT and RB-IGBT as well as the trade-off between saturation voltage and turn-off energy.

Anyway, in spite of the strong distinctions both IGBT types have almost the same structure of active area. Therefore switching speed and trade-off curve of Von are similar. In case of applied reverse voltage after forward conduction the RB-IGBT operates like a conventional FWD and reverse recovery behavior resembles each other. Regarding to trade-off relationship between saturation voltage Vce(sat) and turn-off loss Eoff for RB-IGBT and normal IGBT with diode in series a clear benefit is obvious in figure 5 for a 600V and 100A device at 125°C. Turn-off characteristic for RB-IGBT is similar to IGBT with additional diode. However, RB-IGBT has much lower Vce(sat) compared to IGBT and diode as a result of no additional diode. This improvement contributes to a minimization of power loss by using a bi-directional switch.

The advantage of a 3-Level inverter topology is quite evident in the comparison of loss. The series connection of two IGBTs like in NPC1 leads to high switching loss. In case of NPC2 and A-NPC Von loss are smaller. Four switches with FWD is the cause of higher Von loss in NPC2. In contrast, A-NPC uses two IGBT and two RB-IGBT which leads to less Von loss. Remarkable is the efficiency of A-NPC with 97,73% and NPC2 with 97,59%. This improvement of 0,14% is significant due to the save of energy over time when an inverter operates several years.

Operation modes of RB-IGBT module

In total there are three different switching modes available for the 3-Level module. Table 1 demonstrates an overview of operating switches in dependency of chosen switching mode. The on-state of a switch means always that a bias gate voltage level of +15V is applied. On the contrary, the off-state represents a reverse bias gate voltage level of -15V. When a switch is declared in sw-state then the IGBT is connected to the drive circuit and gets an input gate signal.

Switching mode A, B and C for RB-IGBT module

A-Mode:

This mode describes the usage of the main switches T1 and T2 as a half-bridge. The load is clamped between U and N. Hence T2 is in OFF state and T1 switches. RB-IGBTs are not in use and OFF. For the opposite way round the load is connected between U and P.

B-Mode:

In mode B one of the RB-IGBTs is switching during the other is in onstate. Consider T3 switches then T4 is used as FWD. In order to keep the leakage current inside of T4 as low as possible T4 has to be in on-state. In the first case the load is clamped between P and U. T3 switches while T4 is in on-state and acts like a FWD.

C-Mode:

The C-Mode is used in the case that the load is clamped between M and U. Here T1 switches while T3 is OFF and T4 is ON which again means that T4 behaves like a diode. T2 is OFF in this switching mode due to clamping of load. In the inverse case T3 is ON while T4 is OFF and T2 is now switching. T1 is for the whole time period in offstate. An idea of how to switch the IGBTs in a single-phase test setup is given by Figure 6. The load is clamped between U and N. T1 and T3 operate and the flowing current is marked in red. T4 is in a steady on-state meanwhile T2 is in off-state for the whole time.

Pulse wave form for the load clamped between U and N. Current flow is marked in red.

The resulting output wave form of a 3-level configuration contains three levels of energy (0 energy, 1/2 energy, 1 = full energy) and the shape is more sinusoidal than a 2-Level topology.

Switching Waveforms of RB-IGBT

An experimental setup was created to record turn-off, turn-on, reverse recovery and short circuit behavior of the RB-IGBT. In case of turn-on, turn-off and reverse recovery the conditions have been used with:
Tj = 25°C, Vcc = 400V, Vge = +/-15V, Rg = +8,2/-39Ω,
Snubber = 1,84uF, Ls = 34nH.

According to short circuit conditions the experimental setup used:
Tj = 125°C, Vcc = 400V, Vge = +15/-0V, Rg = +8,2/-100Ω,
Snubber = 0,67uF.

Turn-on of RB-IGBT is faster than the turn-off if we take care of the time scale. The current peak due to diode is approximately 190A which is appropriate. The reverse recovery of RB-IGBTs are demonstrated in figure 7 where the main switch IGBT T1 with Rg,on of 10Ω is switching while T4 RB-IGBT is in on-state.

Switching behavior of RB-IGBT (turn-on, turn-off, reverse recovery, short circuit).

The short circuit behavior for RB-IGBTs is obvious in figure 7 as well. The short circuit capability of the RB-IGBT is 10μs. The gate resistances are determined at +8,2Ω and -100Ω hence it is clearly that RB-IGBT is able to withstand such a short circuit condition for 10μs.

The new technology by Fuji Electric is unique and very promising. Applications like photovoltaic inverters or UPS already use this topology and emphasize the high efficiency of this configuration.

Line-Up of 3-Level modules

 

References:

1) Fuji Electric Journal Vol. 75 No. 8, 2002.

 

 

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