Brief Analysis of UIS and Avalanche Energy of MOSFET

　　In the data sheet of power MOSFET, there are usually parameters such as single pulse avalanche energy EAS, avalanche current IAR, repetitive pulse avalanche energy EAR, etc. However, many electronic engineers　　 rarely consider the relationship between these parameters and the application of the power system in the process of designing the power system, how to evaluate the impact of these parameters on it in actual application, and under what application conditions these parameters need to be considered. This article will discuss these issues and explore the working state of power MOSFET under non-clamped inductive switching conditions.

　　Definition and measurement of EAS, IAR and EAR

　　The avalanche energy of MOSFET is related to the thermal performance and working state of the device, and its final manifestation is the temperature rise, which is related to the power level and the thermal performance of the silicon chip package. The thermal response of power semiconductors to fast power pulses (time of 100 to 200μs) can be described by equation 1:

　　Where A is the silicon wafer area, and the K constant is related to the thermal performance of the silicon wafer. From formula (1), we get:

　　Where tav is the pulse time. When measuring avalanche energy at low current for a long time, the power consumed will increase the temperature of the device, and the failure current of the device is determined by the peak temperature it reaches. If the device is strong enough and the temperature does not exceed the maximum allowable junction temperature, the measurement can be maintained. During this process, the junction temperature usually increases from 25°C to TJMAX, the external ambient temperature is constant at 25°C, and the current is usually set at 60% of ID. The avalanche voltage VAV is approximately 1.3 times the rated voltage of the device. Avalanche energy is usually measured under unclamped inductive switching UIS conditions. There are two values ​​EAS and EAR. EAS is the single pulse avalanche energy, which defines the maximum energy that the device can consume in a single avalanche state; EAR is the repetitive pulse avalanche energy. Avalanche energy depends on the inductance value and the starting current value. Figure 1 shows the EAS measurement circuit and waveform of VDD decoupling. Among them, the driving MOSFET is Q1, the MOSFET to be measured is DUT, L is the inductor, and D is the freewheeling tube. The MOSFET to be measured and the driving MOSFET are turned on at the same time, the power supply voltage VDD is applied to the inductor, the inductor is magnetized, and its current rises linearly. After the conduction time tp, the inductor current reaches the maximum value; then the MOSFET to be measured and the driving MOSFET are turned off at the same time. Since the current of the inductor cannot change suddenly, it must maintain its original size and direction at the moment of switching, so the freewheeling diode D is turned on.

　　Since there is parasitic capacitance between DS of MOSFET, when D is turned on, the inductor L and CDS form a resonant circuit. The current of L decreases, causing the voltage on CDS to rise until the current of the inductor is 0. D is naturally turned off, and all the energy stored in L should be converted into CDS.　　

　　Such a high voltage value is impossible, so why does this happen? From the actual waveform, the DS region of the MOSFET is equivalent to an anti-parallel diode. Since the reverse voltage is applied to both ends of this diode, it is in the reverse working area. As the DS voltage VDS increases, when it increases to a value close to the clamping voltage of the corresponding voltage regulator, that is, V(BR)DSS, the VDS voltage will no longer increase significantly, but will remain basically unchanged at the V(BR)DSS value, as shown in Figure 1. At this time, the MOSFET works in the avalanche region, and V(BR)DSS is the avalanche voltage. For a single pulse, the energy applied to the MOSFET is the avalanche energy EAS.

　　At the same time, since the avalanche voltage has a positive temperature coefficient, when the temperature of some units inside the MOSFET increases, its withstand voltage value also increases. Therefore, those units with low temperatures automatically balance and flow more current to increase the temperature and thus increase the avalanche voltage. In addition, the measured value depends on the avalanche voltage, and during the demagnetization period, the avalanche voltage will change with the increase in temperature. In the above formula, there is a problem, that is, how to determine IAS? When the inductance is determined, is it determined by tp? In fact, for a MOSFET device, IAS must be determined first. In the circuit shown in Figure 1, after the inductance is selected, the current is continuously increased until the MOSFET is completely damaged, and then the current value at this time is divided by 1.2 or 1.3, that is, the rating is reduced by 70% or 80%, and the current value obtained is IAS. Note that after IAS and L are fixed, tp is also determined.　　

　　In the past, the traditional circuit diagram and waveform for measuring EAS are shown in Figure 2. Note that the final voltage of VDS does not drop to 0, but to VDD, which means that part of the energy is not converted into avalanche energy.

　　In the turn-off region, the area of ​​the triangle corresponding to Figure 2 (b) is the energy. Ignoring VDD, the demagnetization voltage is VDS, and the actual demagnetization voltage is VDS-VDD. Therefore, the avalanche energy is

　　For some low-voltage devices, VDS-VDD becomes very small, and the introduced error will be large, thus limiting the use of this measurement circuit in low-voltage devices.　　

　　At present, different companies have different standards for measuring inductance. For low-voltage MOSFET, most companies tend to use an inductance value of 0.1mH. It is usually found that if the inductance value is larger, although the avalanche current value will decrease, the final measured avalanche energy value will increase. The reason is that as the inductance increases, the current rise rate slows down, so the chip has more time to dissipate heat, so the final measured avalanche energy value will increase. There is a problem of dynamic thermal resistance and thermal capacitance, which will be discussed later.