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Posted on 04 April 2019

1200V Generation 8 IGBT for Low Frequency Industrial Motor Drive Applications

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With the ever increasing demand on energy, governments are now facing the problem of supplying electricity whilst trying to minimize the environmental impacts of pollution.

By Wibawa Chou, Llewellyn Vaughan-Edmunds and Andrea Gorgerino,
International Rectifier Corporation, El Segundo, CA

One of the largest utilizers of electricity are electrical motor drives, which consume approximately 45% of all the total global electricity consumption.

Currently the majority of motor drives in the industry run at fixed frequency speed. By moving to a Variable-Speed Drive system, as much as 30% of energy can be saved. In Europe alone, by transferring to electronic variable speed drive control and energy-efficient motors, Europe's CO2 annual emissions could be reduced by 69 million tons (CEMEP).

Governments around the world have been introducing regulations that require an increase of efficiency in applications such as industrial pumps, fans and various motor drives. In Europe, the regulation is IEC 60034-30 for motors, which divides efficiency levels into 3 classes of efficiency; IE1 (standard), IE2 (high) and IE3 (premium). IE1 and IE2 are very similar to the USA’s EPAct NEMA standard and IE3 similar to the NEMA PremiumTM. Specific motors sold after June 2011 must meet IE2 standards, whilst IE3 will become mandatory from January 2015 (7.5 to 375 kW), or will have to meet the IE2 efficiency and be equipped with a variable speed drive. The same IE3 standard applies in January 2017 to cover the complete power range (0.75 to 375 kW), or again will have to meet the IE2 efficiency and be equipped with a variable speed drive.

With the pressure now on the motor manufacturers and electronic variable speed architecture, reducing power losses are the governing factor to increase efficiency.

In industrial motor drives systems, IGBT modules are commonly used to drive the motor. These modules must be able to deliver excellent performance with the lowest power losses. Typically these motors will switch below 10 kHz, therefore the IGBT silicon within the module must be optimized for this condition. At these lower switching frequencies; conduction losses in the IGBT govern the total power losses, as the switching losses are proportional to the switching frequency.

A New Approach

The recent introduction of Generation 8, 1200V IGBTs from International Rectifier are targeted for industrial motor drive applications. These applications require low voltage drop, low voltage overshoot and robust short circuit capability from the IGBTs. These requirements along with trench gate technology have allowed motor drive designers to increase current density from an existing power module which can potentially increase reliability or reduce cost of the system. In this paper, we will discuss characteristics of Gen 8 1200V IGBT for its static and dynamic behaviors. A simulation is also presented at the conclusion of the paper to highlight the benefits of Gen 8 IGBT for low frequency industrial motor drive applications.

Power Module Applications for Gen 8 IGBT

The majority of industrial motor applications rely on using power modules for its power electronic assembly. These modules are available in various standard configurations such as 3-phase, half bridge, low side chopper, high side chopper and single switch. The half bridge configuration is the most widely used and most versatile since a half bridge forms the basic building block of most power electronic systems.

Half bridge module schematic representation and typical module

A typical half bridge power module is presented in Figure 1. This module consists of two switches and two anti-parallel diodes in one single package. The power connections (Positive, Output and Negative) are made using screw connections (typically M5 screws). The gate and emitter sense connections of the IGBTs are brought out through soldered terminals. This type of module is commonly referred to as a 34 mm module in reference to its width. With previous IGBT generations, based on planar-gate technologies, a 34 mm module (shown on Figure 1) can only accommodate up to 1200V/150A IGBT dies. With the development of Gen 8 1200V IGBTs, which utilizes Trench Field Stop technology, it is now possible to develop a 1200V/200A half bridge module in the same 34mm package. This achievement is due to Gen 8 IGBTs offering a higher current density than previous generation IGBTs with a much smaller die size. The novel Gen 8 design has also been designed to be highly robust, therefore this reduction in die size does not compromise the Gen 8 short circuit and reverse bias operating capability demanded by industrial motor drive application.

Forward Voltage Drop

One of the requirements for industrial motor drive applications is lowest voltage drop across the IGBTs when they are conducting. Conduction loss is the most dominant loss for motor drive applications operating at 6 kHz or less.

Vce(on) comparison

Figure 2 shows VCE(ON) comparisons of 34 mm 1200V/100A half bridge power modules using IR Gen 8 IGBT dies and two of the main IGBT competitors in this market. These measurements are observed at the terminals of the power modules. The measurement takes into account the voltage drop across the die, the module substrate, wirebonds and the power terminals. It can be seen that IR Gen 8 IGBT has the lowest voltage drop for a given die area. The VCE(ON) of Gen 8 IGBT at 150°C is 24% lower than Competitor A while the die is 7% smaller than Competitor B.

An important consideration is when the system is running at low load, which means the IGBT is running below its rated current. In this situation, the system will run at a different efficiency due to the characteristics of the IGBT. At lower than rated current, Gen 8 IGBT has consistently lower VCE(ON) compared to Competitor A. Figure 3 shows curve tracer plot of all the 34mm IGBT modules at VGE = +15V. Two curves are plotted on each chart with different color intensities for Tj = 25°C (Darker) and Tj = 150°C (Lighter).

Terminal voltage

Switching Characteristics

The other necessary requirement is for the IGBTs to have a low voltage overshoot at turn-off, to minimize conducted and radiated EMI, as well as to prevent IGBT over-voltage failure. Large motor drive systems typically consist of long bus bars with a lot of parasitic inductance which generates unwanted over voltages. In order to minimize voltage overshoot due to system’s parasitic inductance, the IGBTs must have low di/dt during turn-off events.

IGBT turn-off characteristics

Switching characteristics of the power modules are measured with bus voltage, VCE(ON) = 600V, at Tj = 150°C with external R g = 0. Figure 4 shows the turn off characteristics at ICE = 100A. From these waveforms, it can be seen that Gen 8 IGBT has the lowest overvoltage among the IGBTs tested. The measured overvoltage for Gen 8 IGBT is less than 750V.

Turn on characteristics for the modules are also measured. The turn on behaviors depend on the ability of the IGBT to deliver needed di/dt to recover the anti parallel diodes. The turn on loss of the IGBTs is a combination of IGBT loss and reverse recovery loss of the anti-parallel diodes.

IGBT turn-on characteristics

Figure 5 shows the turn on characteristics of the IGBTs at ICE = 100A. Gen 8 IGBT is able to deliver higher di/dt than competitor IGBTs and allow it to recover its anti-parallel diode much quicker resulting in lower turn on loss. Having high di/dt capability along with high peak current makes Gen 8 IGBT turn on loss linearly increases with collector current. This is a desirable feature since the losses can be managed easily as the device is operated in overload conditions. As it can be seen from Figure 5, Competitor A IGBT cannot deliver high enough peak current and di/dt making its turn on loss much larger than IR or Competitor B IGBTs. This IGBT will have large power loss during overload, therefore increases the case and junction temperatures of the module.

In order to observe the switching loss characteristics over a wide operating range from low load to overload, switching loss measurements are taken between 25A to 200A. Figure 6 shows turn on and turn off energy loss measurement results.

IGBT switching loss vs collector current

The turn off energy loss of IR Gen 8 IGBT is not as low as that of Competitor A however for low frequency industrial motor drive applications the turn off energy difference will not result in significantly different power loss. The turn on energy loss of Competitor A increases significantly as the collector current is increased past its nominal current of 100A. This was previously explained by the fact that this IGBT does not have high enough di/dt to recover the diode. Competitor A IGBT turn on energy loss increases non linearly which will result in very high turn on power loss during overload conditions.

Short Circuit Characteristics

Short circuit robustness is the main requirement for IGBTs used in industrial motor drive applications. IR Gen 8 IGBT is designed to have a minimum of 10 μsec short circuit withstand time at VCE = 600V with starting Tj = 150°C.

Using the same gate driver board and resistors, all samples of 34 mm power modules are subjected to a minimum of 10 μsec short circuit. Figure 7 shows short circuit waveforms of these modules. Channel 1 shows voltage drop across the IGBT at 200V /division. Channel 2 shows short circuit current at 200A/division. IR Gen 8 IGBT has more current overshoot at the beginning due to its higher di/dt capability compared to the competitors. This is consistent with the previous observation of turn on characteristics of Gen 8 which allows higher di/dt and peak reverse recovery current of the anti-parallel diode, which helps in reducing the recovery time of the diode. Competitor A IGBT has a peak current of approximately 300A. This creates a problem in which the IGBT cannot deliver enough current to quickly recover the anti-parallel diode resulting in large turn on energy loss as can be seen previously on Figure 6.

Short circuit waveforms

It can also be observed that the overvoltage across Gen 8 IGBT when the short circuit current is terminated is one of the smallest at approximately 850V. This is important in order to minimize the need for a separate gate drive voltage or soft shutdown circuitry when a short circuit event occurs. With the information gathered so far on VCE(ON), turn-on and turn-off energy losses, and short circuit measurements, we are now ready to simulate the performance of these IGBTs under real operating conditions. Using IR proprietary simulation software, the power losses of the 34 mm power modules will be shown in the next section.

Three Phase Motor Drive Design using 34 mm, 1200V/100A Power Modules

A 3 phase motor drive design consisting of three 34 mm, 1200V/100A power modules is simulated to study the power loss of the system at switching frequency of 4 kHz. The simulation is based on measurements taken on the actual power modules. It is assumed that a 1200V/100A power module would be operated on a system with nominal output current of 50 Arms and overload current of 100 Arms.

Figure 8 shows power loss breakdown of the IGBTs due to their conduction, turn on and turn off losses. At 4 kHz switching frequency, conduction loss is a dominant component of the total loss. Therefore it is preferred to have IGBTs with the lowest voltage drop. At 50 Arms nominal output current, Gen 8 IGBT shows 20% lower power dissipation compared to competitor A. At 100 Arms overload output current, the nonlinear behavior of Competitor A’s turn on characteristics result in turn on loss that is significantly higher than IR Gen 8 IGBT.

IGBT power loss calculation

The total loss of Competitor A at the overload condition is 40% higher than that of Gen 8 IGBT. This makes the design of the cooling requirement of the system using Competitor A power modules to be proportionally larger due to the higher case temperature of the module.

Summary

The design example in this paper using a 34 mm, 1200V/100A half bridge power module, shows that the latest Gen 8 IGBT silicon technology offers the lowest overall losses for motor drive applications operated at 4 kHz both at nominal current and overload conditions.

It also highlights the voltage drop, switching and short circuit characteristics are superior and excellent in industrial applications. International Rectifier’s best-in-class Gen 8 IGBTs have been created to target the industrial motor drives market, where there is a significant potential to reduce power consumption globally. As demand for electricity rises, we must ensure that it is utilized efficiently. We must keep our focus on reducing our carbon footprint, whilst understanding how to improve system efficiencies of next generation motor drive systems.

 

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