Five Factors of MOS Tube Failure and Treatment Measures

Date：2022-12-28 16:15:30Views：644

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. In most cases, the two regions are the same, and even if the two ends are switched, the performance of the semiconductor device will not be affected. Such devices are considered symmetrical. At present, consumer electronic power adapter products rank first in market applications. In the second place are computer motherboards, NB, computer adapters, LCD monitors and other products. The third is in the fields of network communication, industrial control, automotive electronics and electrical equipment. These products are in great demand for MOS transistors. Here are five major reasons for MOS tube failure.

1、 Avalanche failure (voltage failure)

Avalanche failure means that the BVdss voltage between drain sources exceeds the rated voltage of MOSFET and reaches a certain capacity, which leads to MOSFET failure. What is avalanche failure? Simply put, MOSFET is a failure mode caused by bus voltage, transformer reflection voltage, leakage inductance peak voltage and other system voltages superimposed between MOSFET drain sources on the power board. 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, please 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 warranty terms and circuit concerns of the enterprise.

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、 Grid voltage failure

There are three main reasons for abnormally high grid voltage: static electricity during production, transportation and assembly; High voltage resonance caused by parasitic parameters of equipment and circuits in power system operation; In the process of high-voltage impact, the high-voltage is transmitted to the power grid through Ggd (in the lightning stroke test, the fault caused by this reason is more common).

Preventive measures for grid voltage failure:

Overvoltage protection between grid and source: If the impedance between grid and source is too high, the sudden change of voltage between drain and source will be coupled to the grid through interelectrode capacitance, resulting in very high UGS voltage overshoot, which will lead to grid overshoot. The oxide layer is permanently damaged. If it is the UGS transient voltage in the positive direction, the equipment may also be connected incorrectly. To this end, the impedance of the grid drive circuit should be appropriately reduced, and a damping resistor or a voltage regulator with a voltage stabilization of about 20V should be connected in parallel between the grid and the source. Special attention must be paid to prevent door opening.

3、 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: Limited to RDSON at 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. What effect does it have? Let's take a look at the FLYBACK current waveform.

When the current waveform is approaching the current peak, there will be a drop and a rise time after the drop point. The essence of this time is that the MOSFET itself will start to turn off after the IC detects the overcurrent signal to turn off. However, due to the switching delay of the device itself, the current will have a secondary rise platform. If the secondary rise platform is too large, then when the transformer margin design is insufficient, It is very likely to cause a current surge of magnetic saturation or a failure of current exceeding the device specification.

3: The reasonable thermal design margin is not much to say. Each enterprise has its own derating specifications, which can be strictly implemented. If not, add radiators.

4、 Electrostatic failure

The basic physical characteristics of static electricity are: attractive or repulsive; There is electric field and potential difference with the earth; Generate discharge current. 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 due to the effect of electric field or current, making the element unable to work (completely damaged).

3. Components are damaged due to transient soft breakdown of electric field or overheating of current. Although it can still work, its life has been damaged.

Preventive measures for electrostatic failure:

The current tolerance of the protective diode at the input of the MOS circuit when energized is 1 mA. When excessive instantaneous input current (greater than 10mA) may occur, input protection resistance shall be connected in series. At the same time, 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 invalid. Therefore, during welding, the soldering iron must be reliably grounded to prevent leakage of the equipment input terminal. In general, after the power is cut off, the residual heat of the soldering iron can be used for welding, and the grounding pin should be welded first.

5、 Body diode fault

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, MOS transistors have different functions. For example, in LLC, the speed of the body diode is also an important factor affecting the reliability of MOS transistors. As the diode itself is a parasitic parameter, it is difficult to distinguish the drain source diode fault from the drain source voltage fault. The solution of diode fault is mainly analyzed by combining its own circuit.

The above is the MOS tube failure related content compiled by the wound core detection team. I hope it will be helpful to you. Our company has a team of professional engineers and industry elites, and has built three standardized laboratories with an area of more than 1800 square meters, which can undertake a variety of test projects such as electronic component test verification, IC authenticity identification, product design material selection, failure analysis, functional testing, factory incoming material inspection and tape weaving.