Tweet

Posted on 17 July 2019

Efficiency Improvement with Silicon Carbide-Based Power Modules

Free Bodo's Power Magazines!

 

 

 

SiC to become the next generation of power semiconductors

In recent years, discrete Silicon Carbide-based (SiC) devices have been introduced to the market. This article describes the utilization of SiC-based diodes and switches in power modules, and compares them to Silicon-based (Si) power modules. Whereas SiC switches have lower overall dynamic losses, they show higher static losses due to their lack of conductivity modulation and to limitations in chip size due to their high base material price.

By Zhang Xi, Daniel Domes, Roland Rupp, Infineon Technologies

 

The demand for devices with low switching loss, low conduction loss, and high temperature tolerance is a major driving force of technology development in power semiconductors. In recent years, SiC-based Schottky diodes have been introduced into the market as discrete devices in standard TO-packages [1]. These new diodes have superior performance compared to Si-based devices, mainly with respect to switching losses and thermal performance, and are well-established in hard switching applications up to 600 V in high-end power supplies [2]. In addition to this market entry point for SiC power devices, this emerging technology is also being considered [1] by industry and research institutes as an ideal candidate for highpower module applications.

The main goal of this article is to offer insights regarding the trade-off between static and dynamic losses when moving from highly costeffective Si-based solutions to high-performance but costly SiCbased solutions. In this article, the following switch configurations are compared:

a) Si IGBT (Infineon IGBT4) vs. Si free-wheeling diode (Emitter Control4)
b) Fast Si IGBT (Infineon IGBT2) vs. SiC free-wheeling diode (Infineon 1200-V SiC Schottky diodes)
c) SiC JFET cascode switches (1200 V normally on SiC JFET vs. 40-V OptiMOS™2 Si MOSFET)

The SiC JFET is used due to the superior maturity and reliability of this switch compared to the SiC MOSFET [3].

IGBT power modules with SiC free-wheeling diodes

SiC Schottky diodes in discrete packages were introduced to the market in 2001. The main advantages of these diodes are welldescribed in [4].

The first commercially available high-power module containing SiC Schottky diodes as free-wheeling diodes is a 600-A, 1200-V PrimePACK™2 IGBT power module with a type designation FF600R12IS4F from Infineon Technologies.

The 1200-V SiC diodes used are bare Schottky diodes with a p+/p- JTE edge termination structure and a Ti-Schottky barrier providing a barrier height of 1.27 eV, and allowing nearly the same threshold voltage as Si-based PIN diodes. Each individual chip has a current rating of up to 15 A. The required current rating of the free-wheeling diode is achieved by paralleling of multiple chips, which can be done easily due to the positive temperature coefficient of these devices. The total current rating of the diodes used per IGBT chip is 60% of the nominal IGBT current rating, thanks to the superior switching and thermal performance of these diodes.

Figure 1 shows the typical switching behavior of SiC Schottky diodes in combination with Infineon’s fastest IGBT chip, the S4 chip based on IGBT2 technology.

Typical turn-on behavior of 1200-V, 600-A IGBT S4 Chip

Due to the absence of a reverse-recovery charge of SiC Schottky diodes, the turn-on gate resistor of the IGBT can be reduced dramatically to reduce turn-on losses (in the example, 0.5 Ω is used). The reverse-recovery current, as seen in Figure 1, is reduced dramatically in comparison to a Si-based freewheeling diode. Figure 2 shows the typical switching loss of the module:

Typical switching loss of PrimePACK™ 2 module

SiC JFET power modules

The conductivity of Si-based MOSFETs drops sharply when the blocking voltage of the switch is higher than 1000 V. IGBTs are appropriate choices for switches with blocking voltages > 1000 V. However, due to the tail current while switching off, switching losses have certain physical limits. The need for a faster switch with low conduction loss has driven the development of a new switch based on SiC material, the SiC-based Junction Field-Effect Transistor (JFET, see e.g. [7]).

Infineon’s SiC JFET is normally on with a pinch-off voltage of ~ -15 V. For compatibility with standard applications, it is optional to provide switches that are normally off (cascode configuration). These are formed by a 40-V low-voltage Si-MOSFET (OptiMOS™) in series with a 1200-V SiC JFET.

The first prototype of a power module containing SiC JFET switches is an EasyPACK 2B module with H-bridge configuration. Each switch in the module contains 6 SiC JFETs in parallel to achieve an Rds_on of approximately 70 mΩ at room temperature.

Figure 3 shows the static behavior of SiC JFET in combination with a low-voltage OptiMOS™2 in cascode configuration as described before.

Static characteristic of SiC JFET in cascode configuration at 25°C, 125°C

Dynamic tests have been done with different gate resistor values (39 Ω …82 Ω). The results are shown in Figures 4 and 5. The test conditions were: Tj = 125°C, Id = 40 A, Vdc = 600 V, inductive load.

Dynamic test waveforms of SiC JFET module turn-on at 125°C

Dynamic test waveforms of SiC JFET module turn-off at 125°C

A comparison of dynamic losses to standard IGBT module is shown in Figure 6:

Comparison of switching losses of the JFET module vs. a 25-A, 1200-V standard module with IGBT4

The results indicate that the turn-on losses are quite low due to the low reverse-recovery charge of the body diode. The turn-off losses are very low due the absence of minority carriers in JFETs, similar to MOSFETs. Another advantage is that the di/dt slope during turn-off can be fully controlled simply by varying the gate resistor.

Comparison between Si-based power modules and power modules utilizing SiC devices

The two possibilities described above for utilization of SiC-based devices in power modules will now be compared to pure Si-based configurations by calculating inverter losses based on [5]. The data for pure Si-based modules was taken from the datasheet for a 25-A, 1200-V IGBT4 module [6]. The measurements with IGBT and SiC Schottky diodes were performed on a 25-A, 1200-V IGBT4 module together with the 15-A, 1200-V SiC Schottky diodes previously described. The measurement of SiC JFET in cascode configuration was performed with the SiC JFET EasyPACK 2B module. The conditions for calculation were: Tj = 125‹C, Vdc = 600 V, Irms = 21.2 A, cos. = 0.8. The gate resistor value for each configuration was chosen to have the lowest possible switching losses. The results are shown in Figure 7.

Benchmark of total losses between IGBT module, SiC JFET module, and IGBT+SiC diode module

The calculated results in this example clearly demonstrate the advantages of SiC-based devices:

• For applications, whose first priority is efficiency, the efficiency of the converter at fsw = 20 kHz can be increased by 1.1% by utilizing an SiC diode with an IGBT if the same output power is maintained. Utilizing a SiC JFET can increase the efficiency by up to 1.3%.
• For applications whose first priority is power density, the output power the converter at fsw = 20 kHz can be increased by 31% by utilizing a SiC diode if the same semiconductor losses are maintained. Utilizing an SiC JFET can increase the output power by 28%. Under these conditions, the IGBT/SiC diode combination outperforms the JFET due to lower conduction losses of the IGBT compared to the unipolar JFET device.
• Utilizing a SiC diode with an IGBT can increase the switching frequency of the converter from 20 kHz to 38 kHz if the same semiconductor losses are maintained. With a SiC JFET, the switching frequency can be even increased to 70 kHz with the same losses. The increase of switching frequency can then decrease the size and cost of the output filter. However, the exact degree of size or cost reduction depends on several other factors.

The utilization of SiC Schottky diodes or SiC JFETs can decrease the switching losses dramatically. The configuration of IGBTs together with SiC diodes as free-wheeling diodes combines the superior conduction performance of IGBT chips with the ultra-low reverse-recovery losses of SiC Schottky diodes. Even lower switching losses can be achieved with SiC JFETs. Due to the missing conductivity modulation in the unipolar component, the conduction losses of the SiC JFET, however, are slightly higher than with IGBTs due to a trade-off between chip area and cost. Depending on the switching frequency or the application requirements, one can choose among these different configurations.

Conclusions

This article describes modules utilizing SiC devices (Schottky diodes, JFETs). The performance of these modules is compared to Si-based power modules. The results clearly show the benefits of utilizing SiC devices:

• With the same converter design, the efficiency of the whole system can be increased. Smaller heat sinks or passive cooling systems can be used.
• With the same thermal design, the output power of the converter can be increased. The power density of the system can also be increased.
• Increasing the switching frequency makes it possible to reduce the size of the output filter and thus reduce the system cost.

The first applications that can benefit from these advantages are those that need high efficiency (for example, solar converters) or contain an output filter (for example, medical equipment or UPS). In the long run, depending on the cost and diameter development of silicon carbide base material, silicon carbide-based power semiconductors are thought likely to become the next generation of power semiconductors and bring a new wave of innovation into this field.

 

References:

[1] Zverev, I; Treu, M; Kapels, H; Hellmund, O; Rupp, R: SiC Schottky rectifiers: Performance, reliability and key application, Proc. 9th Conf. Power Electronics and Applications 2001, p. DS2.1-6.
[2] Rupp, R.; Miesner, C.; Zverev, I.: Combination of new generation CoolMOS and thinQ! Silicon Carbide Schottky diodes: new horizons in hard switching applications, Proc. of PCIM 2001 Power Electronics Conference, 2001, p.210-20.
[3] Treu, M; Rupp, R; Blaschitz, P; Rüschenschmidt, K; Sekinger, Th.; Friedrichs, P; Elpelt, R; Peters, D: Strategic considerations for unipolar SiC switch options: JFET vs. MOSFET, Proc. IAS 2007, published on CD.
[4] Miesner, C; Rupp, R; Kapels, H; Krach, M; Zverev, I: thinQ!™ SiC Schottky Diodes:An SMPS Circuit Designer’s Dream Comes True!
[5] Schütze, T: IPOSIM user manual www.infineon.com/highpower
[6] Datasheet for Infineon Technologies power module FP25R12W2T4 www.infineon.com/highpower
[7] Friedrichs, P; and Reimann, T: Behavior of high voltage SiC VJFETs under avalanche conditions, CD-Proceedings of the APEC 2006, 19-23 March 2019, Dallas.

 

 

VN:F [1.9.17_1161]
Rating: 0.0/6 (0 votes cast)

This post was written by:

- who has written 155 posts on PowerGuru - Power Electronics Information Portal.


Contact the author

Leave a Response

You must be logged in to post a comment.