“The development direction of chip technology is to reduce conduction loss and dynamic loss. The development direction of packaging reduces the parasitic inductance, allowing the chip to switch quickly without oscillation; improve the reliability of the packaging process, improve the power cycle and temperature cycle, that is to say, while increasing the junction temperature of the device, it must also ensure the life of the device. Improve heat dissipation and reduce junction-to-case thermal resistance Rthjc.
Ziying Chen, Infineon Industrial semiconductor
Power semiconductors are destined to withstand large power losses, high temperatures and temperature changes. Improving the power density of devices and systems is an important design goal of power semiconductors. We pursue the output current capability per unit chip area all the way, and the realization method is as follows:
1. Reduce conduction loss and dynamic loss
2. Reduce parasitic inductance and give full play to the switching speed of the chip
3. Increase the maximum allowable operating junction temperature
4. Reduce junction-to-case thermal resistance Rthjc
The development direction of chip technology is to reduce conduction loss and dynamic loss. The development direction of packaging reduces the parasitic inductance, allowing the chip to switch quickly without oscillation; improve the reliability of the packaging process, improve the power cycle and temperature cycle, that is to say, while increasing the junction temperature of the device, it must also ensure the life of the device. Improve heat dissipation and reduce junction-to-case thermal resistance Rthjc.
It can be seen in the formula that technological progress has increased Tvj and reduced Rthjc, which allows the device to withstand a greater loss Vce*Ic, which means that the heat generation on the chip is allowed to be greater.
Let’s make an interesting comparison with the Sun’s specific power density.
Infineon’s appearance is the star product:
900A 1200V IGBT7.
Power Density of FF900R12ME7_B11
The first working condition:
Solve the power density of the FF900R12ME7_B11 IGBT module at a case temperature of 80 degrees. Transform the above formula:
The allowable power consumption of a 900A 1200V chip is 1549 watts when the case temperature is 80 degrees. If the dynamic switching loss is not considered in the case of DC, Ptot=Vcesat*Ic, since the typical value of the saturation voltage drop is 1.7V, at this time Device collector current (without switching losses) is around 911A.
Since the chip area of 900A IGBT is about 6cm², the power density is: 2.6*10⁶W/m². At this time, the chip power density of IGBT7 is one order of magnitude higher than that of a match flame and 9 orders of magnitude higher than that of an electric iron! ! !
The second working condition is short circuit:
The IGBT is connected to the 900V DC bus, and the first type of short-circuit experiment is carried out. The bus voltage is 900V during short-circuit, and within 8us, the short-circuit current can reach more than 3200A, and the instantaneous power is as high as P=900V*3200A=2.88MW!!!
In the same way, it is calculated that the power density of the chip is as high as 4.8*10⁹ W/m², which is 2 orders of magnitude higher than the power density of the solar surface of 5.0*10⁷W/m²! ! !
1. The mass of a match is about 0.065g, and the calorific value of wood is about 1.2×107J/kg. Assuming that the cross-sectional area of the flame is 100mm2, the match will burn out in 15 seconds.
2. The median value of human exercise fever is 200W, and the surface area of the human body is based on Xu Wensheng’s formula: body surface area (m2) = 0.0061 × height (cm) + 0.0128 × weight (kg) – 0.1529
In the system design, the working junction temperature of the IGBT is generally 100°C higher than the boiling point of water, the design target is 150°C, and the transient is as high as 175°C.In the cooling water pump of the hydrogen fuel cell, the cooling liquid temperature of the IGBT in the driver may be 95 degrees. Under such harsh working conditions, the driving kilometers and service life of the vehicle must also be met, and the reliability and service life of the IGBT are highly required.
The challenge of high power density
Due to the very high requirements on the operating temperature and power density of power semiconductors in the design of power Electronic systems, it is a great challenge for the design and production of chip technology and packaging technology.
High temperature and large case temperature change will cause the mechanical fatigue of the module solder layer to separate, which will increase the thermal resistance Rthjc from the junction to the case, and then fail.
Only after the comparison can we know that the power density of the IGBT chip is so high, now let’s study the design specification and current density of the bonding wire.
There is a less noticeable parameter in the data sheet of the module, the module lead resistance, that is, the resistance value RCC’+EE’ from the terminal to the chip, this resistance value does not seem to be too much loss for small current modules, but this The current density of the bonding wire is as high as 254A/mm², which is much higher than the current density of 6A/mm² of the copper wire in the household power distribution specification. If the lead wire of the 900A module is designed according to the aluminum wire current density of 2.5A/mm, the lead wire of the 900A module needs to be 360mm², which will be an aluminum row with a cross-section of 60*60mm.
The repeated flow of such a high-density current through the bonding wire will cause mechanical stress on the bonding wire, resulting in mechanical damage such as cracking of the bonding wire.
One end of the bonding wire is connected to the metallization layer of the IGBT chip, which is a 3.2um thick AlSiCu material. This connection point is also a weak link that is prone to mechanical fatigue. Large junction temperature changes will cause another failure mechanism. Alignment is off.
Datasheet for the IGC193T120T8RM 200A 1200V chip
The lead resistance of the module, that is, the resistance value RCC’+EE’ from the terminal to the chip, will cause the loss, which is not a small value for medium and high power modules.
EconoDUAL™3 FF900R12ME7 module lead resistance, the resistance value from terminal to chip is 0.8mΩ, the voltage drop is 0.72V at 900A, and the power consumption is as high as 648W.
FF900R12ME7 Current and Lead Loss
If the PrimePACK™ package is selected, its maximum specification is 2400A half bridge, and the lead resistance of such a module is much lower, because the terminal adopts a copper bar structure. FF900R12IE4, the resistance value from the 900A 1200V module terminal to the chip is 0.3mΩ, the voltage drop at 900A is 0.27V, and the power consumption is only 243W, which is only 38% of the EconoDUAL™3 FF900R12ME7.
Therefore, when selecting a device, it is necessary to consider the characteristics of different packages to meet the system requirements.
From this point of view, the main problem caused by high power density is to cause mechanical fatigue of the device and affect the life of the device. Fortunately, these life mechanisms are known and can be described by power cycles and temperature cycles. The life of the device and system can be designed.
In order to evaluate the life of the device in the system in the application fields of wind power generation, electric vehicle and locomotive traction, etc. with large load changes, it is necessary to further understand the life mechanism and design method of the device. Infineon provides a fee-based service for life simulation.
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