Why mosfets fail

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Often, the gate oxide will be taken into the diffusion, too, causing a short betweem all three terminals. Only if the short circuit current after this first mode of failure is high enough to blow the bond wires or the entire transistor, there is an open circuit.

Shorted gate to drain is a very common and easily tested failure mode. It'll often be a dead short or 10s of ohms. Mosfets failed in this way also tend to destroy whatever IC was driving them. When suspecting dead mosfets that's the first thing I look for. Sign up to join this community.

The best answers are voted up and rise to the top. Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Learn more. Sometimes you can also see a Zener diode from gate to source, e. The driver has to supply the necessary current through the additional R gs and possibly C gs and this increases the power losses. In principle, the following applies to all circuit parts: either high impedance and susceptible to interference or low impedance and high loss.

Figure 6: The green dotted box shows the output stage of the gate driver IC as complementary output stage. Figure 6 shows the output stage of the gate driver, which basically consists of two transistors. Sure thing - where's the problem? In reality, this is not possible. Both transistors must not conduct simultaneously, otherwise, there is a "hot branch", i. So there are always switching points where the driver output is high-impedance! In the datasheets of the drivers, this is unfortunately mostly not well specified.

Hints are given by "delay time", "rise time" and "fall time". Let's take an example with a switching frequency of kHz and a switching time of ns each. Again, only a resistor from gate to source helps to minimize the risk that the switching transistor does not accidentally switch on at the wrong time due to a disturbance.

If there is not only one switching transistor, but two, then the driving topic is even more critical. And if a switching transistor is floating in a half-bridge, i. Many of my customers believe that they find development errors through intensive endurance tests in the temperature cabinet with various load cycles and on-off profiles.

Unfortunately, this is not the case. If an "inexplicable" failure occurs during the endurance test, it is assumed that the test was too hard and that this does not correspond to the real case. One is already finished with the development, the necessary approvals are finally there and the brochures are printed. Nobody dares to raise their hand and order a further investigation, let alone a redesign. In addition, it is usually the case that such failures only occur after years in the field, due to disturbances in the environment, when the components age or due to component tolerances.

In a large production plant, there were power supply failures, very rare, but the consequence was very expensive because then everything stood still and it took a long time until the production ran properly again. During the failures the switching transistor was broken, the fuse was open and partly also the gate driver IC or the shunt resistor was blown up, once also a conductor track melted.

My analysis showed that the customized power supply was not so bad for us Swabians this means good. MOSFETs boast a high input gate resistance while the current flowing through the channel between the source and drain is controlled by the gate voltage. However, if not appropriately handled and protected, the high input impedance and gain can also lead to MOSFET damage caused by over voltage or too-high current.

Obviously, V gs and V ds must both be within limits. The same for current, I d. There is also a power limit given by the maximum junction temperature. But it turns out, other thermal limits can apply. But there are a variety of conditions that may cause high thermal gradients that may lead to expansion and cracking of the MOSFET die.

One factor to consider in this regard is that MOSFET thermal resistance is an average; it applies if the whole die is at a similar temperature. But MOSFETs designed for switch-mode power supplies can experience a wide temperature variation over different areas of their die. In this case, only the source impedance of the power source limits the peak current. A common outcome of a direct short is a melting of the die and metal, eventually opening the circuit.

Gates rated at 12 V will likely succumb at about 15 V or so; gates having a V rating typically fail at around 25 V.