【Introduction】Inverters are constantly innovating in design. In order to improve power density and reduce costs, engineers are racking their brains. IGBT (Insulated Gate Bipolar Transistor) is a key device in the inverter, and its selection and loss are directly related to the size of the radiator, and also directly affect the performance, cost and size of the system.
Starting from the application characteristics of frequency converters, this paper introduces how the seventh-generation IGBTs help the application of frequency converters, combined with the characteristics of the seventh-generation IGBTs such as low saturation Voltage drop and maximum operating junction temperature.
This paper analyzes the loss composition of the inverter, compares the performance of the fourth generation IGBT and the seventh generation IGBT through thermal simulation, and finally verifies the conclusion through experiments. Under the same working conditions, the loss and junction temperature of IGBT7 are significantly lower than those of IGBT4, so that the volume of the inverter can be reduced or the output current can be increased while maintaining the same volume, so as to realize power skipping and improve the power density of the product.
Motors are widely used in industries such as home appliances, transmission, transportation, new energy and industrial robots. Electric motors drive our daily work and play. As the main product of driving motors, low-voltage inverters are widely used in elevators, fans, pumps, textiles, metallurgy and other industries because of their wide speed regulation range and simple operation, which can realize functions such as energy saving, soft start, and efficiency improvement.
2021 is the first year of the “14th Five-Year Plan”. China has finalized a roadmap for carbon neutrality, striving to achieve peak carbon dioxide emissions by 2030 and carbon neutrality by 2060. At present, China’s manufacturing industry is undergoing a new round of transformation and upgrading, which puts forward higher requirements for the performance of industrial equipment. Energy-saving and green driving methods will become the mainstream, which will promote the growth of the inverter market, especially the new generation of inverters. Products with high power density.
The low-voltage general-purpose inverter market is very competitive. In response to the current market demand, major manufacturers have increased investment in research and development to reduce product costs and improve product performance. The inverter circuit topology mainly adopts AC-DC-AC frequency conversion, the circuit topology is fixed, and the development is relatively slow. The development characteristics of frequency converter products are concentrated in the reduction of size, improvement of power density and cost reduction in hardware.
From the hardware point of view, the main features of low-voltage general-purpose inverters are:
● AC-DC-AC: no control rectification + braking unit + three-phase inverter;
● Low switching frequency: rated 4KHz~6kHz, if the switching frequency is increased, it will be derated;
● Short-time overload requirement: 150% overload/1 minute;
In view of these application characteristics of general-purpose inverters, Infineon has launched the seventh generation of IGBT modules. So what are the advantages of the seventh-generation IGBT modules compared to the fourth-generation IGBT modules mainly used in the current market in inverter applications? How to improve power density? This paper will explore through simulation and experiment.
02 IGBT7 chip technology
At present, the IGBT chip technology has developed to the level of the seventh generation. Taking Infineon’s IGBT chip technology as an example, from the initial PT technology, to the NPT plane gate, to the trench gate, and now to the seventh generation, also It is the micro trench gate (referred to as MPT, the same below) technology. IGBT7 adopts the IGBT structure based on MPT. At the bottom of the n-substrate, the collector region is realized by p+ doping. Between the n-substrate and the p+, a field-stop (FS) structure is achieved by n+ doping. It can drop the electric field sharply, while affecting the static and dynamic characteristics of the device.
The difference from IGBT4 is that in addition to the common edged gate, the trenches in IGBT7 also include emitter trenches and dummy gates. The latter two trenches are invalid trenches. These three trench cell types enable fine-tuning of IGBTs. By increasing the active gate density, the conduction channel per chip area can be increased.On the one hand, static losses can be reduced due to the steeper device output characteristic curve . Of course, there is also an increase in gate-emitter capacitance (CGE), which represents a change in its switching parameters.
Figure 1. Infineon chip technology
At the specific application level, the advantages of IGBT7 are summarized as follows:
● Lower turn-on saturation voltage drop Vcesat;
● The maximum short-time operating junction temperature can reach 175℃;
● Optimization of dv/dt characteristics for motor drive applications;
03 IGBT7 technology applied in inverter
The original intention of IGBT7 design is for motor drive applications. By reducing the total loss of power devices and increasing the maximum junction temperature under overload conditions to 175 ℃ to improve power density, reduce system size and ultimately achieve the purpose of reducing system cost. Why is IGBT7 suitable for inverter applications?
1. In inverter applications, under normal circumstances, the rated switching frequency range is 4KHz~6KHz. Under this condition, the conduction loss accounts for the largest proportion of the total loss. IGBT7 reduces conduction losses by lowering Vcesat. So as to achieve the purpose of reducing the total loss;
2. IGBT7 supports a maximum operating junction temperature of 175°C, effectively meeting the needs of inverter overload;
3. The IGBT7 PIM MODULE integrates a rectifier bridge, a braking unit and an inverter bridge, which is tailor-made for the inverter.
Next, combined with the 5.5KW inverter, the advantages of IGBT7 in inverter applications are verified through simulation and experiments.
First we can evaluate the junction temperature and loss distribution of IGBT7 in inverter application by simulation. PLECS involves the electrical circuits, magnetic components, heat dissipation circuits and mechanics of the power conversion system, as well as its control parts, which can provide fast simulation. This paper uses Icepak and PLECS hybrid thermal simulation experiments, and calculates losses and junction temperature.
Figure 2. Simulation and experimental flow chart
3.1 Establishing the PLECS device model
(1) Double pulse test
Although there is switching loss data in the device datasheet, bus voltage, junction temperature, stray inductance of the main power loop, parasitic inductance and parasitic resistance of the gate loop all contribute to switching loss. The switching loss of IGBT7 and the reverse recovery loss of the diode can be obtained through the double-pulse test. Of course, the dynamic parameters including the peak value of each voltage and current, and the slope change value can also be obtained. In this experiment, the double-pulse test is directly performed on the main power circuit board of the whole machine, so that the measured data is more realistic.
In this test, four different temperatures of room temperature, 35°C, 75°C and 125°C were selected to obtain the turn-off loss and turn-on loss of IGBT7.Because the seventh-generation IGBT uses MPT technology, the drive resistance can be selected to be smaller while maintaining a lower dv/dtso this double-pulse test drive resistance (Rg) selects 10 ohms and 15 ohms, as shown in Figure 3 and Figure 4.
Figure 3. Measured data of IGBT7 turn-off loss when Rg=15Ω
Figure 4. Measured data of IGBT7 turn-on loss when Rg=15Ω
(2) Create a device model
Based on the data in the specification, the output characteristic curves of Vcesat and Ic (collector current) are imported into the PLECS device model, and the switching loss obtained before can be used to obtain the loss model of the IGBT and the anti-parallel diode. Finally, enter the fourth-order transient thermal resistance, and the PLECS thermal model of IGBT7 can be obtained, as shown in Figure 5.
Figure 5. IGBT7 device model
3.2 3D and PLECS joint thermal simulation results
The purpose of using 3D and PLECS joint thermal simulation is to improve the accuracy of the simulation results. Most of the heat generated by the IGBT and diode chips is transferred to the environment through the vertical thermal resistance in Figure 6; only a small fraction of the heat is transferred laterally and can be ignored in this paper. It can be seen from the thermal equivalent network that the accuracy of Rth,ch (heat sink thermal resistance, the same below) directly affects the estimation of the junction temperature of the IGBT chip. The advantage of PLECS is that the loss and chip junction temperature can be obtained through simulation, and the advantage of 3D thermal simulation is that the thermal resistance value of the heat sink can be obtained. The purpose of using 3D and PLECS joint thermal simulation is to improve the accuracy of the simulation results.
Figure 6. IGBT thermal equivalent network
Based on the seventh generation IGBT FP25R12W2T7, after using PLECS simulation to calculate the loss, import it into the 3D thermal simulation to get the thermal resistance of the radiator, and then import the thermal resistance of the radiator into the PLECS iteration and re-simulate, you can get the IGBT and diode wafers junction temperature, see Table 1 for specific results. The thermal resistance in the “Simulation Mode” column in Tables 1 and 2 refers to the heat sink thermal resistance Rth,ch.
The simulation conditions are as follows:
● Bus voltage Vdc=540V;
● The modulation ratio is 1;
● The output frequency is 50Hz;
● The time constant of the radiator is 67s;
● The output power factor is 0.85;
Table 1. IGBT7 thermal simulation results
Then use the same method to simulate the fourth-generation IGBT FP25R12W2T4 of 25A to obtain its loss. As shown in Table 2 below:
Table 2. Thermal Simulation Results of IGBT4
The above simulations are based on the operating parameters of the actual operating conditions. From the comparison of the simulation results, the loss of IGBT7 is significantly lower than that of IGBT4 under the same operating conditions; and with the increase of current or the increase of switching frequency, the gap between losses is larger, such as shown in Table 3.
Table 3. Thermal Simulation Loss Comparison
04 IGBT7 and IGBT4
Performance comparison in inverter applications
4.1 Construction of experimental platform
In this paper, Flextronics AC310 series 5.5KW inverter is used to build a test platform. The IGBT modules use specially treated FP25R12W2T7 of the seventh generation 25A with thermocouples attached to the chip and FP25R12W2T4 of the fourth generation with the same 25A. Because the two modules are in the same package and the same pins, they can be tested on the same machine; in addition, the positions of the chips on the temperature rise lines to which the two modules are attached are the same, and the junction temperature on the chip at the same position can be directly read. This is convenient to directly compare the chip junction temperature of IGBT4 and IGBT7.
Figure 7. IGBT module with die-attached temperature rise line
Drill holes on the radiator, drill holes directly under the U-phase lower bridge IGBT, V-phase upper bridge diode and rectifier bridge diode, and insert the thermocouple into the hole, close to the side of the radiator substrate and flush with the surface of the radiator , used to measure the temperature of the radiator.
Figure 8. Perforated Heatsink
4.2 Experimental results
Corresponding to the previous simulation, the temperature rise test is still tested according to these four working conditions: 13A/2kHz, 13A/4kHz, 13A/6kHz and 17A/2kHz. Monitor and record respectively: the junction temperature of the IGBT chip, the temperature of the radiator directly under the U-phase lower bridge IGBT chip, the temperature of the radiator directly under the V-phase upper bridge diode chip, and the temperature of the radiator directly under the rectifier bridge diode chip; monitor and record the input current; Monitor output current. Record and compare the experimental results, as shown in Table 4.
Table 4. Thermal test comparison of IGBT7 and IGBT4
4.3 Analysis of experimental results
The experimental results show that the chip junction temperature of IGBT7 is significantly lower than that of IGBT4 under the same working conditions; and as the output current increases, the difference between the temperature rises of IGBT7 and IGBT4 increases. It can also be seen from the experimental results that as the switching frequency increases, the difference between the temperature rises of IGBT7 and IGBT4 is also increasing. This is because engineers currently use IGBT4 with a larger driving resistance value, which It can be seen from the simulation results of IGBT4 that it is consistent.
4.4 Experimental Error Analysis
The two modules in this experiment are both thermocouple modules and use the same heat sink. The final result of the test is the difference in temperature rise, which is a relative value, which conforms to the univariate principle.
However, considering that the T7 and T4 modules in this test have built-in thermocouples, this is a difference from the standard modules; coupled with the drilling of the radiator, the thermal resistance of the radiator will increase. The difference between these two points on the absolute value of the experimentally obtained temperature needs to be further explored.
The following two sets of experiments were designed to explore the influence of the above two points on the absolute value of temperature rise.
Group 1: Use the same module, different heat sinks
Table 5. Radiator Error Comparison Test
The second group: using the same heat sink, different modules
Table 6. Module Error Comparison Test
The two sets of tests show that with the worsening of the working conditions, the greater the error. This also explains why as the loss increases, the difference between the simulated IGBT junction temperature Tvj and the measured thermocouple temperature becomes larger and larger.
4.5 Design Summary
At present, 5.5KW inverters generally use fourth-generation 35A IGBT modules such as Infineon’s FP35R12W2T4, or other modules with similar current levels and packages. According to the above simulation and experimental verification, IGBT7 can help the inverter to improve the power density from two ideas: First, the 5.5KW inverter can replace the module with the seventh-generation IGBT module with the same package of 35A, which can directly convert the 5.5KW inverter. The power is raised to 7.5KW and the size of the whole machine is kept unchanged, thereby improving the power density of the whole machine, which is equivalent to increasing the power density by 40%~70%, as shown in Figure 9; Replacing the 35A Easy2B IGBT module with the seventh-generation 25A Easy1B module with a smaller volume can directly reduce the overall size of the inverter by 25% to 40%.
Figure 9. IGBT7 assists the inverter power to jump
Through a series of simulations and tests, it can be clearly seen that Infineon’s seventh-generation IGBT reduces the total inverter loss and reduces the chip junction temperature by reducing Vcesat. In addition, the operating junction temperature can be as high as 175°C. , which brings a large design margin to the engineer, thereby helping the inverter power to skip gears. It can not only reduce the size of the inverter, improve the power density, and reduce the cost; it can also keep the original inverter volume unchanged, increase the output current, and jump the power to achieve the goal of increasing the power density and reducing the cost.
 Infineon AN2018-14 TRENCHSTOP™ 1200 V IGBT7 Application Note
 CR Mueller., “New 1200 V IGBT and Diode Technology with Improved Controllability for Superior Performance in Drives Application”, PCIM Europe, Nuremberg, Germany, 2018
 Paper | 1200V IGBT7 and Emcon7 have better controllability and help improve the performance of the inverter system (Part 1)? February 19, 2019
 Infineon Industrial Semiconductors Double Pulse Test Basic Series: Why is my loss different from the datasheet? April 22, 2021
 Infineon Industrial semiconductor Breakthrough – High Power Density Design Example for IGBT7, 13 May 2020
 VEICHI official website https://www.veichi.com/
Source: Infineon, Original: Zhao Kai, Chen Huikai, Zhang Hao, etc.