High-speed Hybrid Modules for Fast Switching Applications

In recent years, the number of applications that require power conversion systems in the high frequency region in order to reduce size, weight and increase efficiency, has grown significantly. Leading this trend are applications such as Electric Vehicles (EV) and Hybrid Electric Vehicles (HEV), charging systems as well as distributed photovoltaic power generation systems. But also UPS, welding or medical systems are following the idea into fast switching devices. To fulfil the requirement for high-speed and low loss switching, Fuji Electric has developed high-speed hybrid modules which combine silicon insulated gate bipolar transistors (IGBT) with silicon carbide Schottky barrier diodes (SiC-SBD). As a result, power dissipation during highfrequency inverter operation can be reduced by approximately 50% compared to existing products.

 

Introduction

To reduce the emission of greenhouse gas CO2 in order to suppress global warming, the efficiency of power conversion systems, especially renewable energy generation needs to be increased. Also the switch from combustion vehicles to electric vehicles targets a more sustainable environment. To realize this change, a close-knit network of charging infrastructure is required. These EV charging systems needs higher switching frequencies to achieve a high level of compactness of the overall system and an increased efficiency. Besides the field of EV chargers also UPS systems tend to operate at higher switching frequencies. Figure 1 shows the target applications in relation to the power capacity and the switching frequency. Some of the main applications of high-speed hybrid modules are power conversion equipment, used for renewable energies, automotive applications and uninterruptible power systems (UPS). All these need to convert power at high frequencies.

 

Figure 1: Main applications of high-speed hybrid modules

 

Features of the high-speed hybrid modules

To achieve a further improvement, switching frequencies of 20kHz or higher are needed for semiconductor devices. Therefore, Fuji Electric developed a high-speed, low loss IGBT which can operate in a switching frequency region beyond 20kHz. These Si-IGBT’s are combined with SiC-SBD’s to reduce the switching losses and are offered in a half bridge configuration. To maintain the compatibility, the high-speed hybrid modules make use of the same packages like conventional Si-modules, for example the 62mm standard package and the EconoDual™3. The following chapters describe the characteristics of the high-speed IGBT and the SiC-SBD chip.

 

High-speed IGBT turn-off loss improvement

The IGBT utilizes an optimized chip for high speed switching which is based on the conventional IGBT technology. In order to improve the IGBT for high-speed switching the generated switching loss needs to be reduced. This could be achieved by changing the trade-off between the turn-off loss Eoff and the collector emitter saturation voltage Vce(sat). Figure 2 shows a comparison of these trade-off characteristics of the actual “X-Series IGBT” (Fuji Electric 7th IGBT Generation) and the high-speed optimized IGBT for a 200A / 1200V module.

 

Figure 2: 1200V high-speed IGBT Vce(sat) -Eoff characteristics

 

Turn-off losses of the high-speed IGBT are 33% less than conventional IGBT of the 7th generation while the Vce,sat is still suitable for high speed applications(1). This reduction is due to the improved tail current during turn-off. This improvement comes from the drastically reduced parasitic capacitance of the active chip structure as well as on the reduced concentration of impurities in the collector layer which is responsible for suppressing the hole injection(2).

 

SiC-SBD improvement in reverse recovery and turn-on loss

By using a SiC- SBD, the high-speed hybrid module can reduce the reverse recovery peak current by about 60%. The fact that a SiC-SBD is a unipolar device with no minority carrier injection, is the reason for this improvement. The reverse recovery losses ERR are reduced by 92% compared to a Si Free Wheeling Diode of the actual 7th Chip generation.

Besides the improvement during reverse recovery, the enhanced characteristics of a SiC-SBD results in an enhanced behaviour at turn-on of the IGBT in the opposing arm. The reduced current peak during reverse recovery is reflected in the peak of the turn-on current, which is approximately 60% lower. The turn-on loss reduction is therefore about 84% and the overall switching losses of the device are 66% lower(3). Table 1 displays an overview about the explained switching loss benefits of the high-speed hybrid module.

 

	Eon (mJ)	Eoff (mJ)	Err (mJ)	Total Loss (mJ)
X Series Si module	14.5	19.2	14.4	48.1
High-speed hybrid module	2.3	12.8	1.1	16.2
Reduction rate	84%	33%	92%	66%

 

Table 1: Switching loss comparison

 

Contribution for power conversion systems

Figure 3 shows the dependence between the reactor volume and the switching frequency of the power semiconductor device. If the switching frequency of a power conversion system (PCS) is increased from 10kHz to 30kHz the total inductor volume can be decreased by around 50%. By applying high-speed hybrid modules, the size of the entire unit can be decreased and the costs can be significantly reduced because of miniaturizing the passive components, such as capacitors, inductors and transformers which are used for filtering circuits at low frequency operation.

 

Figure 3: Dependence of reactor volume on switching frequency

 

To get an impression what the implementation of the high-speed hybrid IGBT module means for an inverter operation, a simulation result is displayed in figure 4. The simulation is done for a 200A / 1200V 62mm standard package in a small capacity PCS. For a switching frequency of 20kHz an overall loss reduction of about 50% can be achieved. Because of the lower switching losses, these benefit will even increase for higher switching frequencies and will contribute to a high-efficiency operation and miniaturization at the high frequency operation of the inverter system. Even with the slightly increased on-state losses because of the higher Vce,sat the improvement is remarkable.

The chip junction temperature, when mounted to the inverter, is displayed in Fig. 5. Comparing the junction temperature of the chip at a switching frequency of 20kHZ, there is already an advantage of around 18°C for the IGBT and 19°C for the SBD compared to the X-Series Si module. This enables a PCS to further increase the output current during high frequency.

 

 

Figure 4: Inverter loss comparison

 

Postscript

Large volume and mass of filter circuits like capacitors, inductors and transformers can be reduced by higher switching frequency enabled by the use of the high-speed hybrid module. The reduced turn-off losses by the high-speed IGBT and the low losses for turn-on and reverse recovery losses caused by the SiC-SBD allow high switching frequency operation. These benefits make the high-speed hybrid module to an optimal solution for fast switching applications such as EV quick charger’s, PV systems, UPS systems, plasma cutters and welding machines. Fuji Electric will continue pursuing ways to reduce losses to contribute to energy savings and a sustainable society.

 

Figure 5: Chip junction temperature when mounted to the inverter

 

References

1. Usui, R. et al. High Speed Hybrid Modules Combining High Speed IGBTs with SiC-SBDs. FUJI ELECTRIC REVIEW. 2018, vol.64,no.4, p.176-180.
2. Hara, Y. et al. High-Speed Discrete IGBT “High-Speed W-Series”. FUJI ELECTRIC REVIEW. 2015, vol.61,no.4, p.280-284.
3. Usui, R. et al. High Speed Hybrid Modules Combining High Speed IGBTs with SiC-SBDs. FUJI ELECTRIC REVIEW. 2018, vol.64,no.4, p.176-180.

 

This article originally appeared in the Bodo’s Power Systems magazine.