The global demand for energy-saving and green energy has led to the continuous growth of motor variable frequency drives in industrial applications, and even expanded to civilian products and automotive fields. Therefore, in the past few years, the market demand for frequency converters and the corresponding output have been continuously increasing. With the continuous expansion of output and the maturity of technology, competition in the frequency converter market has become increasingly fierce, and the requirements for product cost performance have continued to increase.
Standard three-phase AC drive inverters use insulated gate bipolar transistors (IGBT) to realize 6 switches in the main circuit. Now, except for a small number of low-power and low-cost inverters that use discrete IGBT devices, general industrial inverters use modules IGBT (including IPM). The modular concept provides users with an insulated packaged and tested power switch assembly, thereby reducing design workload, improving system performance, and increasing the power level of the Inverter.
But even with IGBT modules, design challenges still exist. Due to the harsh working conditions of IGBT modules (switching and controlling large currents under high pressure and high temperature) and the inherent weaknesses of semiconductor devices, design engineers must ensure that the IGBT modules can work safely while exerting their maximum performance to achieve low-cost designs.
This article will first state the main challenges faced by inverter design engineers, then introduce innovative IGBT chips and packaging technologies and support tools from Infineon Technologies, and briefly state the advantages of these solutions.
Challenges facing inverter design
The safe operation of IGBT is the primary requirement of the application. There are two basic conditions for safe operation. Running beyond these two conditions can cause permanent damage to the device. The two conditions are:
1) Vce ≤ Vces, where Vce is the collector-emitter transient Voltage, and Vces is the blocking voltage of the IGBT chip (the data sheet specifies 600V/1200V/1700V/3.3kV/6.5kV)
2) Tj ≤ Tvj, op, max, where Tj is the instantaneous junction temperature of the IGBT chip, and Tvj, op, max (specified as 125°C or 150°C on the data sheet) is the maximum junction temperature allowed during switching
To follow these two conditions in the application, you must first understand how Vce and Tj are established.
First, in the inverter circuit structure, the IGBT half-bridge is powered by the DC side, and the DC side voltage Vdc is almost constant. Affected by the electromagnetic field and materials of the circuit, there is distributed inductance in the system (see Figure 1). When the IGBT turns off the current at a rate of di/dt, Vce is equal to the sum of Vdc and an induced voltage di/dt×Ls, namely Vce = Vdc + di/dt×Ls, where Ls is the distributed inductance of the loop formed by the DC side and the related half bridge. Vdc has been fixed by the power supply or battery voltage in the application, so di/dt and Ls must be limited to make Vce ≤ Vces.
have to be aware of is:
* di/dt is the technical characteristic of the IGBT chip, it represents the “softening degree” of the IGBT, which is affected by the gate resistance Rg, but not completely controlled by Rg.
* Ls can be divided into two parts, one part is outside the relevant half bridge (Ls1), and the other part is inside the half bridge (Ls2). Ls1 is determined by the layout structure of the half-bridge externally connected to the DC side, while Ls2 is determined by the layout structure of the half-bridge.
Therefore, one of the challenges faced by design engineers is how to adjust the di/dt when the IGBT is turned off and how to reduce Ls. Using a suitable snubber circuit can balance Ls1, but has no effect on Ls2.
Secondly, the Tj of the IGBT is determined by the power loss, thermal resistance (between the junction and the environment) and the ambient temperature of the IGBT, that is, Tj = P_loss×Rthja + Ta. If Rthja is divided into Rthjc (between the case), Rthch (between the tube case and the radiator) and Rthha (between the radiator and the environment), three formulas can be used to express Tj, where Tc is the IGBT tube The temperature of the shell, Th is the temperature of the radiator:
* Tj = P_loss×Rthjc + Tc
* Tc = P_loss×Rthch + Th
* Th = P_loss×Rthha + Ta
When the frequency converter works in sinusoidal pulse width modulation, the thermal impedance (Zthjc) model of the IGBT device is required to describe its overall thermal characteristics. Tj fluctuates, and its fluctuation range varies with the output frequency of the inverter.
However, Tj is difficult to measure in practical applications. In order to satisfy Tj ≤ Tvj, op, max and maximize the capability of the IGBT at the same time, it is necessary to accurately estimate Tj. To estimate Tj, you must first know the power loss of the IGBT. The power loss of IGBT is determined by IGBT chip technology, working conditions (ie Vdc, output current, switching frequency, modulation depth and load power factor) and gate drive conditions such as gate voltage and Rg. Therefore, in the case of sinusoidal pulse width modulation, IGBT The calculation of power loss is very complicated.
Therefore, another challenge faced by design engineers is how to calculate the power loss of the IGBT in consideration of various relevant conditions, and use the Zthjc model to estimate the instantaneous value of Tj.
Third, for applications such as electric vehicles (cars, buses), the reliability of the inverter is a problem that needs special consideration, and this is basically a problem of the reliability of the IGBT module used. The IGBT module designed for high reliability uses a special material process, so its cost is higher than that of a standard reliability module. Therefore, the challenge faced by design engineers in this area is how to keep the cost at an acceptable level while meeting the application’s reliability requirements.
Fourth, a special situation in China is that the increasingly fierce competition in the frequency converter market has made the time to market for frequency converter products more critical than at any time in the past, which ultimately leads to a very tight time for product development. At the same time, due to economic reasons, many domestic inverter manufacturers have very limited investment in research and development. Therefore, for Chinese design engineers, how to use limited R&D resources to complete the inverter design in a short time is a special challenge they face.
Multiple solutions help complete the design
In response to the design difficulties faced by inverter design engineers, Infineon Technologies provides the following solutions to support application design:
1. Innovative IGBT chip process
Following the third-generation trench field-stop IGBT (IGBT3: E3, T3), Infineon has now launched three versions of the fourth-generation 1200V IGBT (IGBT4), including
* High-power version (HiPo): Has better softness (lower di/dt when turned off) and lower Vcesat than E3
*Medium Power Version (MePo): The softening degree is the same as E3, but the speed is faster (lower Eoff)
* Low-power version (LoPo): faster than T3, and softer than T3
In addition, the di/dt of IGBT4 at turn-off can be fully controlled by Rg, which is another advantage of it. Based on the improvement of softening degree, IGBT4 reduces the design difficulty of inverters, especially high-power inverters, from the following perspectives:
* Allow higher DC side voltage (thus making better use of the blocking ability of IGBT)
* Simplified buffer circuit (thus reducing system cost)
* Under the same DC side voltage and safety margin, a lower Rg value can be used to achieve a faster switching speed (so that the switching loss remains unchanged)
2. Innovative packaging technology
We will soon launch a new module package called PrimePACK (see Figure 2).
PrimePACK adopts a half-bridge circuit structure and provides two package sizes (PrimePACK2 and PrimePACK3), corresponding to two current specifications of 400A-900A and 1400A, and two series of 1200V and 1700V. As a brand-new series of IGBT modules for medium and high power applications, the main feature of PrimePACK is to reduce the package inductance. Based on the improved power terminal layout and internal structure, PrimePACK has a 60% reduction in package inductance compared with the existing IHM 130×140 package, which significantly reduces the design difficulty of reducing loop distributed inductance (Ls).
The global demand for energy-saving and green energy has led to the continuous growth of motor variable frequency drives in industrial applications, and even expanded to civilian products and automotive fields. Therefore, in the past few years, the market demand for frequency converters and the corresponding output have been continuously increasing. With the continuous expansion of output and the maturity of technology, competition in the frequency converter market has become increasingly fierce, and the requirements for product cost performance have continued to increase.
Standard three-phase AC drive inverters use insulated gate bipolar transistors (IGBT) to realize 6 switches in the main circuit. Now, except for a small number of low-power and low-cost inverters that use discrete IGBT devices, general industrial inverters use modules IGBT (including IPM). The modular concept provides users with an insulated packaged and tested power switch assembly, thereby reducing design workload, improving system performance, and increasing the power level of the inverter.
But even with IGBT modules, design challenges still exist. Due to the harsh working conditions of IGBT modules (switching and controlling large currents under high pressure and high temperature) and the inherent weaknesses of semiconductor devices, design engineers must ensure that the IGBT modules can work safely while exerting their maximum performance to achieve low-cost designs.
This article will first state the main challenges faced by inverter design engineers, then introduce innovative IGBT chips and packaging technologies and support tools from Infineon Technologies, and briefly state the advantages of these solutions.
Challenges facing inverter design
The safe operation of IGBT is the primary requirement of the application. There are two basic conditions for safe operation. Running beyond these two conditions can cause permanent damage to the device. The two conditions are:
1) Vce ≤ Vces, where Vce is the collector-emitter transient voltage, and Vces is the blocking voltage of the IGBT chip (the data sheet specifies 600V/1200V/1700V/3.3kV/6.5kV)
2) Tj ≤ Tvj, op, max, where Tj is the instantaneous junction temperature of the IGBT chip, and Tvj, op, max (specified as 125°C or 150°C on the data sheet) is the maximum junction temperature allowed during switching
To follow these two conditions in the application, you must first understand how Vce and Tj are established.
First, in the inverter circuit structure, the IGBT half-bridge is powered by the DC side, and the DC side voltage Vdc is almost constant. Affected by the electromagnetic field and materials of the circuit, there is distributed inductance in the system (see Figure 1). When the IGBT turns off the current at a rate of di/dt, Vce is equal to the sum of Vdc and an induced voltage di/dt×Ls, namely Vce = Vdc + di/dt×Ls, where Ls is the distributed inductance of the loop formed by the DC side and the related half bridge. Vdc has been fixed by the power supply or battery voltage in the application, so di/dt and Ls must be limited to make Vce ≤ Vces.
have to be aware of is:
* di/dt is the technical characteristic of the IGBT chip, it represents the “softening degree” of the IGBT, which is affected by the gate resistance Rg, but not completely controlled by Rg.
* Ls can be divided into two parts, one part is outside the relevant half bridge (Ls1), and the other part is inside the half bridge (Ls2). Ls1 is determined by the layout structure of the half-bridge externally connected to the DC side, while Ls2 is determined by the layout structure of the half-bridge.
Therefore, one of the challenges faced by design engineers is how to adjust the di/dt when the IGBT is turned off and how to reduce Ls. Using a suitable snubber circuit can balance Ls1, but has no effect on Ls2.
Secondly, the Tj of the IGBT is determined by the power loss, thermal resistance (between the junction and the environment) and the ambient temperature of the IGBT, that is, Tj = P_loss×Rthja + Ta. If Rthja is divided into Rthjc (between the case), Rthch (between the tube case and the radiator) and Rthha (between the radiator and the environment), three formulas can be used to express Tj, where Tc is the IGBT tube The temperature of the shell, Th is the temperature of the radiator:
* Tj = P_loss×Rthjc + Tc
* Tc = P_loss×Rthch + Th
* Th = P_loss×Rthha + Ta
When the frequency converter works in sinusoidal pulse width modulation, the thermal impedance (Zthjc) model of the IGBT device is required to describe its overall thermal characteristics. Tj fluctuates, and its fluctuation range varies with the output frequency of the inverter.
However, Tj is difficult to measure in practical applications. In order to satisfy Tj ≤ Tvj, op, max and maximize the capability of the IGBT at the same time, it is necessary to accurately estimate Tj. To estimate Tj, you must first know the power loss of the IGBT. The power loss of IGBT is determined by IGBT chip technology, working conditions (ie Vdc, output current, switching frequency, modulation depth and load power factor) and gate drive conditions such as gate voltage and Rg. Therefore, in the case of sinusoidal pulse width modulation, IGBT The calculation of power loss is very complicated.
Therefore, another challenge faced by design engineers is how to calculate the power loss of the IGBT in consideration of various relevant conditions, and use the Zthjc model to estimate the instantaneous value of Tj.
Third, for applications such as electric vehicles (cars, buses), the reliability of the inverter is a problem that needs special consideration, and this is basically a problem of the reliability of the IGBT module used. The IGBT module designed for high reliability uses a special material process, so its cost is higher than that of a standard reliability module. Therefore, the challenge faced by design engineers in this area is how to keep the cost at an acceptable level while meeting the application’s reliability requirements.
Fourth, a special situation in China is that the increasingly fierce competition in the frequency converter market has made the time to market for frequency converter products more critical than at any time in the past, which ultimately leads to a very tight time for product development. At the same time, due to economic reasons, many domestic inverter manufacturers have very limited investment in research and development. Therefore, for Chinese design engineers, how to use limited R&D resources to complete the inverter design in a short time is a special challenge they face.
Multiple solutions help complete the design
In response to the design difficulties faced by inverter design engineers, Infineon Technologies provides the following solutions to support application design:
1. Innovative IGBT chip process
Following the third-generation trench field-stop IGBT (IGBT3: E3, T3), Infineon has now launched three versions of the fourth-generation 1200V IGBT (IGBT4), including
* High-power version (HiPo): Has better softness (lower di/dt when turned off) and lower Vcesat than E3
*Medium Power Version (MePo): The softening degree is the same as E3, but the speed is faster (lower Eoff)
* Low-power version (LoPo): faster than T3, and softer than T3
In addition, the di/dt of IGBT4 at turn-off can be fully controlled by Rg, which is another advantage of it. Based on the improvement of softening degree, IGBT4 reduces the design difficulty of inverters, especially high-power inverters, from the following perspectives:
* Allow higher DC side voltage (thus making better use of the blocking ability of IGBT)
* Simplified buffer circuit (thus reducing system cost)
* Under the same DC side voltage and safety margin, a lower Rg value can be used to achieve a faster switching speed (so that the switching loss remains unchanged)
2. Innovative packaging technology
We will soon launch a new module package called PrimePACK (see Figure 2).
PrimePACK adopts a half-bridge circuit structure and provides two package sizes (PrimePACK2 and PrimePACK3), corresponding to two current specifications of 400A-900A and 1400A, and two series of 1200V and 1700V. As a brand-new series of IGBT modules for medium and high power applications, the main feature of PrimePACK is to reduce the package inductance. Based on the improved power terminal layout and internal structure, PrimePACK has a 60% reduction in package inductance compared with the existing IHM 130×140 package, which significantly reduces the design difficulty of reducing loop distributed inductance (Ls).
The improved package design also brings two other advantages to PrimePACK:
1) Reduce thermal resistance by improving chip layout and substrate design. Compared with the IHM 130×140 package, PrimePACK reduces the thermal resistance by 30% while reducing the mounting area by 14%
2) By improving the wire bonding process, Tvj, op, max can be defined at 150°C, which is 25°C higher than most existing package indicators
The innovation of packaging technology not only improves the heat dissipation capacity of the IGBT module, but also improves its reliability in terms of power cycling (PC) and thermal cycling (TC) capabilities. With the improvement of the wire bonding process, IGBT modules with 150°C Tvj, op, max can provide higher PC capacity at the same Tj, or maintain the same PC capacity at a higher Tj. In addition, with the innovation of ceramic substrates and substrate materials, the TC capabilities of IGBT modules have also been improved while keeping costs within acceptable levels. All of these help to solve the problem faced by design engineers in the field of electric vehicle applications, that is, how to choose standard cost IGBT modules to achieve the required reliability.
3. Calculation program for device selection
Infineon Technologies provides an Excel-based calculation program called IPOSIM. IPOSIM uses the data sheet to calculate the power loss and temperature of the IGBT and freewheeling diode according to the working conditions set by the user, the Zthjc model and the working principle of sine pulse width modulation. According to the upper limit of Tj set by the user and the RBSOA limit specified for each IGBT module, IPOSIM can list the IGBT modules that meet the above limit under given working conditions. IPOSIM can also calculate the maximum output current that the selected module can provide under different working conditions, and help users determine the required heat sink thermal resistance specifications and the maximum allowable ambient temperature. In addition, the program also gives the calculation results in the form of graphs, which is convenient for users to analyze. It can even calculate a group of continuously changing operating points. Advanced users can use the IPOSIM database to create new models in IPOSIM, or perform calculations based on actual conditions when the actual conditions are different from the data sheet test conditions. The latest version of IPOSIM also provides a new function that allows users to compare the relationship between the current output capability of four different types of IGBT modules and the switching frequency.
IPOSIM frees design engineers from heavy calculation work and helps them choose IGBT module types reasonably. Although it is only a theoretical calculation program, it can bring great convenience to design optimization and quantitative analysis. The program can be downloaded for free from the Infineon website.
4. Evaluation board
The evaluation board is an application circuit designed and tested by the module manufacturer for the IGBT module to be evaluated. It has functions such as gate drive and IGBT protection. The evaluation board is provided for two main purposes:
1) Promote and accelerate the user’s testing process of IGBT modules
2) Provide module peripheral circuit reference design
In addition to the evaluation board, we also provide users with a full set of design documents, so that when inverter design engineers face tight development time and limited R&D resources, they can have a set of effective solutions.
in conclusion
To deal with a series of design challenges such as di/dt, parasitic inductance, loss and temperature calculation, reliability requirements, tight development time and available resources, we provide a variety of solutions, including the use of innovative IGBT modules at the device level Technology, provide multi-function calculation programs at the software level and provide evaluation boards at the system level.
The Links: 1MBI600NN-060 LJ64H052
