Technical Article

New Gen 3 650V IGBT A Soft and Efficient Switch for Industrial Applications

November 22, 2017 by Masaharu Nakanishi

This article discusses the technological improvement that is presented by Gen 3 IGBTS from Rohm Semiconductor to be used for many industrial applications.

Recent developments of trench stop IGBTs has led to very high performance devices. However, high performance comes together with some challenges related to the occasional snappy behavior of the device. The new Gen 3 IGBTs from Rohm Semiconductor offer an optimal compromise between performance and ease of use. High system efficiency is combined with minimum effort in the PCB design and electromagnetic interference (EMI) filtering.

 

Rohm IGBT Introduction

Rohm Semiconductor started to manufacture IGBT devices in 2009. In its second generation, the light punch through (LPT) structure was introduced and is shown in Figure 1. LPT structures bring several benefits to IGBTs, like lower saturation voltage VCE(sat) and faster switching. This is obtained since the carrier concentration gradient in the drift layer is smaller than in conventional punch-through type, where the epitaxial layer is used. This way, the electron current density – majority carriers – is increased, whilst holes density – minority carriers - is decreased. [1]

 

Technology improvement of punch though IGBTs in RohmSemiconductor

Figure 1 – Technology improvement of punch though IGBTs in RohmSemiconductor

 

In the Gen 3 IGBTs, in addition to LPT structure, a significant shrink of the cell structure has been realized. This reduces the parasitic capacitances of the device, optimizing the dynamic behavior and reducing the driver's efforts. Finally, a wafer thinning of 15 % with respect to the former generation is achieved. This not only reduces the device losses in the conduction state, but also the dynamic losses, as less carriers have to be extracted during the turn-off process.

 

Product portfolio

The complete portfolio of Gen 3 is presented in Table 1. Device names follow the code depicted in Figure 2. As an important difference from other IGBT vendors, the current portion of the device name contains twice the rated current at TC=100°C. For each current class, there are two available devices: a single IGBT, or co-packed with fast recovery diode (FRD). In RGTV series, the co-packed FRD has the same rated current as the IGBT. In RGW series, the FRD is rated to a lower current than the IGBT. The rated current of FRDs is shown in brackets in Table 1.

In addition, Gen 3 portfolio is divided in two different series, namely:

  • RGTV, for fast switching and with shot circuit withstand time (SCWT) of 2 µs;
  • RGW, for even faster switching, in applications where SCWT is not required.

These series contain devices with different rated currents, from 30 A to 80 A at case temperature TC=100°C. They are packaged in both TO-247N (non-isolated) and TO-3PFM (isolated).

 

RGTV Series

650V TO-247N TO-3PFM
Single IGBT Co-packed w/ FRD
(diode rated current)
Single IGBT Co-packed w/ FRD
(diode rated current)
30A RGTV60TS65 RGTV60TS65D (30A) RGTV60TK65 RGTV60TK65D (30A)
50A RGTV00TS65 RGTV00TS65D (50A) RGTV00TK65 RGTV00TK65D (50A)
80A RGTVX6TS65 RGTVX6TS65D (80A) - -

RGW Series

650V TO-247N TO-3PFM
Single IGBT Co-packed w/ FRD
(diode rated current)
Single IGBT Co-packed w/ FRD
(diode rated current)
30A RGW60TS65 RGW60TS65D (20A) RGW60TK65 RGW60TK65D (20A)
40A RGW80TS65 RGW80TS65D (20A) RGW80TK65 RGW80TK65D (20A)
50A RGW00TS65 RGW00TS65D (30A) RGW00TK65 RGW00TK65D (30A)

Table 1 – Portfolio of Gen 3 IGBT

 

Rohm Semiconductor offers several IGBT series, each of them tailored to match certain application requirements, like short-circuit withstand time SCWT, saturation voltage VCE(sat) and dynamic losses. Figure 3 plots the different IGBT series from Rohm, according to the rated SCWT (x-axis), and operation switching frequency (y-axis).

 

Naming code for Rohm IGBTs. Product positioning

Figure 2 – Naming code for Rohm IGBTs. Product positioning

 

As shown in Figure 3, the newest Gen 3 is targeting high efficiency industrial applications such as single-phase power supplies, photovoltaic inverters, uninterruptable power supplies (UPS), battery chargers and welding machines. In these applications, very short or no short circuit withstand time is required. Instead, maximum IGBT performance is requested. This is what devices from Gen 3 are able to offer, with low static and dynamical losses, as is going to be presented in the next section.

 

Applications spectrum and available IGBT series from Rohm Semiconductor

Figure 3 – Applications spectrum and available IGBT series from Rohm Semiconductor.

 

Device performance

The structure of Gen 3 IGBTs has features that enable a better tradeoff between VCE(sat) and turn-off losses. It is possible to optimize these parameters and to obtain thus devices with both lower static and lower dynamic losses.

 

Static and dynamic comparison between IGBT devices from Gen 2 and Gen 3 from Rohm

Figure 4 – Static and dynamic comparison between IGBT devices from Gen 2 and Gen 3 from Rohm.

 

Figure 4 contains the comparison between the same current rated devices from RGTH series – Gen 2 – and from RGTV series – Gen 3. On the left side graph, the VCE(sat) of both devices is plotted as a function of collector current, for room temperature as well as for maximum junction temperature. The VCE(sat) of the new device is 0.1 V (6%) lower at Tj = 25°C, and 0.25 V (12%) at Tj = 175°C. In the same way, the right graph of Figure 4 shows the waveforms of both devices during turn-off, at the same environmental conditions. It is possible to observe the effects of the optimized structure of Gen 3. In the RGTV60TS65D, the collector current rapidly goes to zero, with minimum tail current. As a result, 10% less turn-off losses Eoff are achieved.

The devices from the RGW series are very fast IGBTs. This is proved by the waveforms from Figure 5, which compares the turn-off of a 50A rated IGBT from RGW series, compared to an equivalent device from RGTV series. Under the same conditions, the RGW device has an additional 30% reduction in the Eoff.

 

Comparison between 50A rated IGBTs from RGTV and RGW series

Figure 5 – Comparison between 50A rated IGBTs from RGTV and RGW series.

 

The fast recovery diode (FRD) technology that is co-packed with Gen 3 IGBT has also been improved. The new Gen 6 FRD technology presents a thinner wafer and field stop structure. This results in both lower forward voltage VF and lower reverse recovery charge QRR. These are important parameters in inverter applications, in order to reduce losses not only in the diode itself but also in the counterpart IGBT. At the same time, Gen 6 FRD has a very smooth response during its turn-off. This guarantees a fast but soft commutation, avoiding oscillations in the FRD as well as in the IGBT. This is presented in the waveforms of Figure 6, where the turn-off of the former Rohm Gen 3 FRD, the new Gen 6 and a competitor part are compared. Gen 6 presents almost no oscillations in the current and voltage. In addition, it is also the tested device whose QRR increases the least with temperature.

 

Simplified Gen 6 FRD technology (left), and turn-off of 30A rated FRDs at Tj =25°C and Tj =125°C

Figure 6 – Simplified Gen 6 FRD technology (left), and turn-off of 30A rated FRDs at Tj =25°C and Tj =125°C.

 

Benefits in DC-AC Inverters

Systems like photovoltaic (PV) inverters and uninterruptable power supplies (UPS) contain at least one inverter stage, which converts DC into AC voltage. The resulting sinusoidal energy will be injected into the grid in the case of PV inverters, or used to feed an AC load in case of the UPS. Typically used topologies are half-bridge (HB) and full-bridge (FB) in case of single-phase systems, and 3-level, neutral point clamped (NPC) topologies in case of 3-phase systems. IGBT discretes and modules are widely used in all these topologies. They commutate in hard-switching both for turn-on and turn-off. Therefore,

the performance of the co-packed anti-parallel diode plays an important role. As mentioned before, the co-packed Gen 6 FRDs have low forward voltage and reduced reverse recovery charge.

In order to evaluate the performance of Gen 3 IGBTs in hard switched operation, a single-phase HB inverter has been used. Figure 7 presents on its left side its simplified schematic, while the right side contains a table with main electrical parameters.

 

Simplified schematic of the inverter used for tests (left) and its main electrical parameters (right)

Figure 7– Simplified schematic of the inverter used for tests (left) and its main electrical parameters (right).

 

For comparison in the inverter circuit, devices from Gen 2 (RGTH80TS65D) and Gen 3 (RGW80TS65D) have been used. Additionally, devices from „Comp C“, today’s benchmark device available on the market, have been tested for comparison. The gate resistor Rg,off has been selected in order to avoid an excessive VDS voltage spike during turn-off. The inverter loop inductance is quite low, around 50 nH, therefore small gate resistors can be used. For both Gen 2 and Gen 3 IGBTs, a Rg,off = 5 Ω resulted in a maximum voltage spike during turn-off of 520 V, i.e. 20% below breakdown voltage. For Comp. C, Rg,off was increased to 10 Ω in order to reach 20% margin. The ON gate resistor was Rg,on= 5 Ω for all three devices. With the above-defined gate resistors, the efficiency of the inverter has been measured for different load conditions. The values are plotted in Figure 8. They include the losses in the IGBTs, as well as in the output filters, cables and connectors.

 

Efficiency comparison between 40A rated IGBTs from Gen 2, Gen 3 and Comp. C

Figure 8– Efficiency comparison between 40A rated IGBTs from Gen 2, Gen 3 and Comp. C.

 

It is possible to observe in Figure 8 the improvement of Gen 3 with respect to Gen 2. For low power conditions, up to 1.4% improvement in efficiency is achieved. For middle and high power conditions the improvement is 0.4%. The difference between Comp. C and RGW80TS65D (Gen 3) is inside the accuracy of the measurements, and can be therefore neglected.

 

Behavior during Turn-Off

Besides performance, an important characteristic of the IGBT is its VCE characteristics during turn-off. This characteristic is related to the oscillations which can be generated in VCE, and eventually be reflected to gate voltage.

Figure 9 shows a comparison between the RGW80TS65D (left side) and a competitor device (right side). This is a fast version of Comp. C, and has been labeled as Comp. B. The waveforms have been obtained during the tests in a portable welding machine, with relatively high loop stray inductance, above 100nH.

In the right upper corner of Figure 9, Comp. B was tested with Rg,off =10 Ω. Compared with the waveforms from RGW80TS65D (left), one can see that not only the overshoot of VCE of Comp. B is twice as high – 244V against 120V, - but also that a series of further oscillations occur after the first peak. In the RGW80TS65D, there is a single overshoot, after which the VCE is reaching the DC link voltage smoothly. It is thus expected that a much lower level of electromagnetic emission (EMI) comes from RGW80TS65D. Additionally, the feedback capacitance between collector and gate causes oscillations in VCE to be reflected in VGE. This generates positive peaks much higher than the threshold voltage of the IGBT. This leads to the risk of parasitic turn-on and leg shoot through, with consequent destruction of the entire machine.

 

IGBT waveforms of VCE (yellow) and VGE (light blue) during turn-off in tested in a portable welding machine

Figure 9– IGBT waveforms of VCE (yellow) and VGE (light blue) during turn-off in tested in a portable welding machine.

 

In order to reduce the oscillations in the IGBT, the turn-off gate resistor can be increased. This is proved by the waveforms in the lower right corner of Figure 9, for Rg,off =33 Ω. However, even if the peaks are now reduced, the oscillation in VCE in Comp. B will occur during a much longer time than in RGW80TS65D with Rg,off =10 Ω. In addition, the peak in VGE is still higher than the IGBT threshold voltage.

 

Summary

Gen 3 IGBTs from Rohm Semiconductor represent a remarkable technology improvement. The two available versions RGTV and RGW offer IGBT devices rated up to 50A (RGW) and 80A (RGTV), in a standard TO-247N package. The performance of Gen 3 matches the needs of many industrial applications, such as: single-phase power supply, welding machines, photovoltaic inverters, UPSs and battery chargers. Experimental tests of Gen 3 devices in DC/AC inverter proved that their performance is comparable to the market benchmark. Differently to other high-speed IGBTs in the market, though, Gen 3 has a soft and oscillation free turn-off. This guarantees safe operation even if small values of external gate resistance are used. In combination with the new Gen 6 co-packed fast recovery diode technology, Gen 3 IGBTs offer an optimal compromise between performance, design simplicity and filtering effort.

 

About the Authors

Masaharu Nakanishi works as a Product Marketing Manager in power devices at Rohm Semiconductors, an electronic parts manufacturer company with its main branch based in Kyoto, Japan. He has been responsible for Product Marketing Performance Components since 2013. He has a degree in Pseudo-Physics from Kobe University in Japan. Since joining the company, he has worked in the area of simulation technology for power components as well as in the product marketing of power components at Rohm in Japan and Rohm Semiconductor in Germany. He is currently responsible for the development of new SiC power construction elements.

Vladimir Scarpa is working as a Field Application Engineer at ROHM Semiconductor, an electronic parts manufacturer company with its main branch based in Kyoto, Japan. He was born in Uberlândia, Brazil, in 1980. He received the B.Sc. and M.Sc. degrees in electrical engineering from the Federal University of Uberlândia, Uberlândia, in 2003 and 2005, respectively. His research interests include power electronics applied to renewable energy sources and digital control of switching power supplies.

 

Reference

  • S.Hondo, Y.Enomoto, Y.Kawamoto, A.Hikasa, K.Ino, “High efficient and soft IGBT technology”, PCIM 2017