Analysis of Six Common Causes of MOSFET Tube Failure

Date：2022-12-09 14:40:55 Views：1243

MOS tubes are metal oxide semiconductor field effect transistors, or metal insulator semiconductor. The source and drain of MOS tube can be exchanged. They are both N-type regions formed in the P-type background. To correctly test and judge whether MOSFET fails, the key is to find the reason behind the failure and avoid making the same mistake again. This paper has collected and sorted out some data, hoping to be of great reference value to all readers.

Use a multimeter to simply check whether the MOS tube is in good condition

Use a pointer multimeter to test whether MOS is good or not. Select ohm R during the test × At 10K, the voltage can reach 10.5V. The red pen is negative potential and the black pen is positive potential.

Test steps:

The detection of MOS tube is mainly to judge the leakage, short circuit, open circuit and amplification of MOS tube. The steps are as follows:

1) Connect the red pen to the source electrode S of MOS and the black pen to the drain electrode of MOS tube. A good pointer indication should be infinite. If there is a resistance value, the MOS tube under test has leakage.

2) Connect a 100K Ω - 200K Ω resistor to the grid and source electrode, then connect the red pen to the MOS source electrode S, and connect the black pen to the drain electrode of the MOS tube. At this time, the value indicated by the meter needle is generally 0. At this time, the lower charge charges the MOS tube grid through this resistor to generate a grid electric field. As the electric field generates, the conductive channel leads to the conduction of the drain electrode and source electrode, so the multimeter pointer deflects. The deflection angle is large, and the discharge performance is better.

3) Remove the resistance connecting the grid electrode and the source electrode, and the red and black pens of the multimeter will remain unchanged. If the meter pin slowly returns to high resistance or infinity after removing the resistance, the MOS tube will leak, and if it remains unchanged, it will be intact.

4) Then a wire connects the gate and source of the MOS tube. If the pointer returns to infinity immediately, the MOS is intact.

Analysis of Six Reasons for MOSFET Failure

1、 Avalanche failure (voltage failure)

That is, we often say that the BVdss voltage between drain sources exceeds the rated voltage of MOSFET and reaches a certain capacity, which leads to MOSFET failure.

Simply put, MOSFET is a failure mode on the power board due to the superposition of bus voltage, transformer reflection voltage, leakage inductance peak voltage and other system voltages between MOSFET drain sources. In short, it is a common failure mode caused by the MOSFET drain source voltage exceeding its specified voltage and reaching a certain energy limit.

Preventive measures for avalanche failure:

Avalanche failure is ultimately voltage failure, so we focus on voltage prevention. For details, refer to the following methods:

1. Reasonably use the derating. At present, 80% - 95% of the derating in the industry is generally selected according to the company's warranty terms and circuit concerns;

2. Reasonable reflected voltage of transformer;

3. Reasonable RCD and TVS absorption circuit design;

4. The large current wiring shall adopt the thick and short layout structure as far as possible to minimize the parasitic inductance of the wiring;

5. Select reasonable grid resistance Rg;

6. In high-power power supply, RC damping or zener diode can be added appropriately as required for absorption.

2、 SOA failure (current failure)

SOA failure refers to the failure mode caused by abnormal large current and voltage superimposed on MOSFET at the same time when the power supply is running, resulting in instantaneous local heating. Or the chip, radiator and package can not reach the thermal balance in time, which leads to heat accumulation. The continuous heating makes the temperature exceed the limit of the oxide layer, which leads to the thermal breakdown mode.

1. Limited by the maximum rated current and pulse current;

2. RDSON limited to maximum temperature;

3. Limited by the maximum dissipation power of the device;

4. Limited to the maximum single pulse current;

5. Breakdown voltage BVDSS limit zone.

The MOSFET on our power supply can effectively avoid the power failure problem caused by MOSFET as long as the energy devices are within the upper limit area.

Preventive measures for SOA failure:

1. Ensure that all power limiting conditions of MOSFET are within the SOA limit line under the worst conditions;

2. The OCP function must be precise and detailed.

When designing OCP points, most engineers may take 1.1-1.5 times of current margin, and then start debugging RSENSE resistance according to IC protection voltage such as 0.7V. Some experienced people will take into account the actual impact of detection delay time and CISS on OCP. But at this time, there is a more noticeable parameter, that is, Td (off) of MOSFET.

3、 Body diode failure

In bridge, LLC and other topologies that use body diodes for freewheeling, the body diodes are damaged and cause failure.

In different topologies and circuits, MOSFETs play different roles. For example, in LLC, the speed of internal diodes is also an important factor in MOSFET reliability. It is difficult to distinguish the body diode failure and drain source voltage failure between drain sources, because the diode itself is a parasitic parameter. Although it is difficult to distinguish the cause of the body after the failure, the solutions to prevent the failure of the voltage and diode are quite different, which are mainly analyzed with their own circuits.

Preventive measures for body diode failure:

In fact, the D and S of MOS tubes are essentially symmetrical structures, just two contacts of the channel. However, since the opening and closing of the channel involves the electric field between the grid and the substrate, it is necessary to give the substrate a certain potential. Since the MOS transistor has only three pins, the substrate needs to be connected to one of the other two pins. Then the pin connected to the substrate is S, and the pin not connected to the substrate is D. When we use it, the potential of S is always stable. In integrated circuits, such as CMOS or analog switches, since the chip itself has power pins, the substrate of those MOS transistors is not connected to the pins, but directly connected to the VCC or VEE of the power supply. At this time, D and S are no different.

4、 Resonance failure

Grid parasitic oscillation occurs when the power MOSFETs are connected in parallel without inserting a grid resistor. When the drain source voltage is turned on and off repeatedly at high speed, this parasitic oscillation occurs on the resonant circuit formed by the grid drain capacitor Cgd (Crss) and the grid pin inductance Lg. When resonance condition（ ω L=1/ ω C) When it is true, a vibration voltage much greater than the drive voltage Vgs (in) is applied between the grid and the source. The grid is damaged due to exceeding the rated voltage between the grid and the source, or the vibration voltage when the drain source voltage is turned on or off passes through the grid drain capacitor Cgd and Vgs waveform overlap, resulting in positive feedback. Therefore, the vibration damage may be caused by misoperation.

Preventive measures for resonance failure:

Resistance can inhibit oscillation because of the effect of damping. But connecting a small resistor in series with the grid is not a solution to the problem of oscillation damping. It is mainly due to the impedance matching of the drive circuit and the reason for adjusting the switching time of the power transistor.

5、 Electrostatic failure

The basic physical characteristics of static electricity are: it has the power of attraction or repulsion; There is electric field and potential difference with the earth; Discharge current will be generated. These three conditions have the following effects on electronic components:

1. The element absorbs dust, changes the impedance between lines, and affects the function and life of the element;

2. The insulation layer and conductor of the element are damaged by electric field or current, making the element unable to work (completely damaged);

3. The component is injured due to transient electric field soft breakdown or current overheating. Although it can still work, its service life is damaged.

Preventive measures for electrostatic failure:

The current tolerance of the protective diode at the input end of MOS circuit is generally 1mA when it is turned on. When excessive transient input current (more than 10mA) may occur, the input protection resistance should be connected in series. Since no protective resistor was added in the initial design, this is also the reason why MOS tubes may be broken down, and this failure can be prevented by replacing a MOS tube with an internal protective resistor. Also, because the instantaneous energy absorbed by the protection circuit is limited, too large instantaneous signal and too high static voltage will make the protection circuit ineffective. Therefore, the electric soldering iron must be reliably grounded during welding to prevent leakage from striking the input end of the device. In general, the residual heat of the electric soldering iron can be used for welding after the power is cut off, and the grounding pin can be welded first.

6、 Grid voltage failure

There are three main reasons for the abnormal high voltage of the grid:

1. Static electricity during production, transportation and assembly.

2. High voltage resonance generated by parasitic parameters of devices and circuits when the power system is working.

3. During high voltage impact, high voltage is transmitted to the grid through Ggd (failure caused by this reason is more common during lightning test).

As for PCB pollution level, electrical gap and other phenomena such as high voltage breakdown into the IC and entering the grid, we will not explain too much.

Preventive measures for grid voltage failure:

Overvoltage protection between gate sources, that is, if the impedance between gate sources is too high, the sudden change of the voltage between drain sources will be coupled to the gate through interelectrode capacitance, resulting in a very high UGS voltage overshoot. This voltage will cause permanent damage to the gate oxide layer. If the UGS transient voltage is in the positive direction, it will also lead to misleading of the device. For this reason, the impedance of the grid drive circuit should be properly reduced, and the damping resistance or the voltage regulator with a voltage stabilization value of about 20V should be connected in parallel between the grid sources. Special attention shall be paid to prevent grid from open circuit operation.

The second is overvoltage protection between drain electrodes. If there is inductive load in the circuit, when the device is turned off, the sudden change of drain current (di/dt) will produce a much higher drain voltage overshoot than the power supply voltage, resulting in device damage. Protective measures such as voltage regulator clamping, RC clamping or RC suppression circuit shall be taken.

The above is the MOSFET tube failure related content compiled by the wound core detection team, and I hope it will be helpful to you. Chuangxin Testing is a professional testing agency for electronic components. At present, it mainly provides integrated circuit testing services such as capacitance, resistance, connector, MCU, CPLD, FPGA, DSP, etc. Specializing in electronic component function testing, electronic component incoming appearance testing, electronic component dissection testing, acetone testing, electronic component X-ray scanning testing, ROHS component analysis testing. Welcome to call, we will serve you wholeheartedly!