MOSFET UIS and avalanche energy analysis - Power Circuit - Circuit Diagram

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In the data sheet of the power MOSFET, parameters such as single-pulse avalanche energy EAS, avalanche current IAR, and repetitive pulse avalanche energy EAR are usually included, and many electronic engineers rarely consider these parameters and the power system during the design of the power system. What kind of connection is applied, how to evaluate the impact of these parameters on the actual application, and under which application conditions should be considered. This article will address these issues and explore the operating conditions of power MOSFETs under unclamped inductive switching conditions.

Definition and measurement of EAS, IAR and EAR
The avalanche energy of a MOSFET is related to the thermal performance and operating state of the device. The final performance is the rise in temperature, which is related to the power level and the thermal performance of the silicon package. The thermal response of a power semiconductor to a fast power pulse (time 100-200 μs) can be illustrated by Equation 1:
(1)
Among them, A is the area of ​​the silicon wafer, and the K constant is related to the thermal performance of the silicon wafer. From formula (1):
(2)
Among them, tav is the pulse time. When measuring avalanche energy at low currents 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 robust enough that the temperature does not exceed the maximum allowable junction temperature, the measurement can be maintained. During this process, the junction temperature typically increases from 25°C to TJMAX, the external ambient temperature is constant at 25°C, and the current is typically set at 60% of the ID. The avalanche voltage VAV is approximately 1.3 times the device rated voltage.

Avalanche energy is typically measured under non-clamped inductive switch UIS conditions. Among them, there are two values ​​EAS and EAR, EAS is a single pulse avalanche energy, which defines the maximum energy that the device can consume in a single avalanche state; EAR is the repeated pulse avalanche energy. The avalanche energy depends on the inductance value and the initial current value.

Figure 1 shows the EAS measurement circuit and waveform for VDD decoupling. Among them, the driving MOSFET is Q1, the MOSFET to be measured is DUT, L is inductance, and D is a freewheeling tube. The MOSFET to be measured and the driving MOSFET are simultaneously turned on, the power supply voltage VDD is applied to the inductor, the inductor is excited, and its current rises linearly. After the conduction time tp, the inductor current reaches a maximum value; then the MOSFET to be measured and the driving MOSFET are simultaneously Turn off, because the current of the inductor can not be abrupt, at the moment of switching, the original size and direction should be maintained, so the freewheeling diode D is turned on.

Figure 1 EA decoupled EAS measurement diagram

Since there is parasitic capacitance between the DS of the MOSFET, the inductor L and the CDS form a resonant loop when the D is turned on, and the current of the L decreases, so that the voltage on the CDS rises until the current of the inductor is 0, and D is naturally turned off. The energy stored in L should be fully converted to CDS.

If the inductance L is 0.1mH, IAS=10A, CDS=1nF, theoretically, the voltage VDS is
CDSVDS2=LIAS2 (3)
VDS=3100V

Such a high voltage value is impossible, so why is this happening? From the actual waveform, the DS region of the MOSFET is equivalent to an anti-parallel diode. Since the reverse voltage is applied across the diode, it is in the reverse working area. As the voltage VDS of the DS increases, it increases to a clamp voltage close to the corresponding Zener diode, that is, V(BR)DSS, VDS The voltage will not increase significantly, but will remain at the V(BR) DSS value, as shown in Figure 1. At this point, the MOSFET operates in the avalanche region, and the V(BR)DSS is the avalanche voltage. For a single pulse, the energy applied to the MOSFET is the avalanche energy EAS:

EAS=LIAS2/2 (4)

At the same time, since the avalanche voltage is a positive temperature coefficient, when the temperature of some cells inside the MOSFET increases, the withstand voltage value also increases. Therefore, those cells with low temperature are automatically balanced, and more current flows to increase the temperature to increase the avalanche voltage. In addition, the measured value depends on the avalanche voltage, and during demagnetization, the avalanche voltage will vary with increasing 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 first be determined. In the circuit shown in Figure 1, after the inductor 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 derating is 70% or 80%. The current value is IAS. Note that after IAS and L are fixed, tp is also determined.

In the past, the circuit diagram and waveform of the traditional measurement EAS are shown in Figure 2. Note that the final voltage of the VDS does not drop to zero, but VDD, that is, some of the energy is not converted into avalanche energy.

Figure 2 Traditional EAS measurement chart

In the turn-off area, the area of ​​the triangle corresponding to Figure 2(b) is energy, regardless of VDD, the demagnetization voltage is VDS, and the actual demagnetization voltage is VDS-VDD, so the avalanche energy is
(5)
For some low voltage devices, VDS-VDD becomes very small and introduces a large error, thus limiting the use of this measurement circuit in low voltage devices.

Different companies currently have different standards for measuring the inductance used. For low-voltage MOSFETs, most companies are beginning to use 0.1mH inductors. It is generally found that if the inductance value is larger, although the current value of the avalanche will decrease, the final measured avalanche energy value will increase because the inductance increases and the current rises slowly, so that the chip has more time to dissipate heat. The final measured avalanche energy value will increase. There are problems with dynamic thermal resistance and heat capacity, which will be discussed later.