Stock Code:800328

MOS damage mystery


The working state of MOS in the controller circuit: turn-on process (transition process from turn-off to turn-on), turn-on state, turn-off process (transition process from turn-on to turn-off), turn-off state, and the main loss of MOS also corresponds to this. Several states, switching losses (turn-on and conduction), conduction losses, turn-off losses, and avalanche energy losses. As long as these losses are controlled within the tolerance specification of the MOS, the MOSwill work normally, and if it exceeds the tolerance range, damage will occur.


The damage of MOS tube mainly revolves around five aspects: avalanche damage, device heat damage, built-in diode damage, damage caused by parasitic oscillation, gate surge, and electrostatic damage. Next, the editor will make the following brief introduction to the damage of the MOS tube.


1. Avalanche damage


If a surge voltage exceeding the rated VDSS of the device is applied between the drain and the source, and the breakdown voltage V(BR)DSS (the value varies according to the breakdown current) is applied, and a certain energy is exceeded, the damage will occur.


Avalanche damage can be caused by the flyback voltage generated when the switching operation of the dielectric load is turned off or the spike voltage generated by the leakage inductance exceeding the drain rated withstand voltage of the power MOS and entering the breakdown region.


2. The device is damaged by heat


This damage is caused by heating beyond the safe area. The causes of heat generation are divided into two types: DC power and transient power.


(1) The DC power is the heat generated by the loss caused by the applied DC power. The losses include the RDS(on) loss of the on-resistance (the RDS(on) increases at high temperature, resulting in an increase in power consumption under a certain current), and the leakage Losses due to current IDSS (very small compared to other losses).


(2) Transient power is the heating caused by the loss caused by the external one-shot pulse, loss of load short circuit, switching loss (on, off, related to temperature and operating frequency), trr loss of the built-in diode (up and down The short-circuit loss of the bridge arm is related to temperature and operating frequency).


Overcurrent caused by load short-circuit that does not occur during normal operation of the device, resulting in instantaneous local heating and damage. In addition, when the chip cannot be dissipated normally due to the mismatch of heat or the switching frequency is too high, the continuous heat generation makes the temperature exceed the channel temperature, resulting in the destruction of thermal breakdown.


3. The built-in diode is damaged


When the parasitic diode formed between the DS terminals operates, the parasitic bipolar transistor of the power MOS operates during the flyback, resulting in a mode in which the diode is destroyed.


4. Damage caused by parasitic oscillations


This kind of damage is very easy to occur in parallel connection. The gate parasitic oscillation that occurs when the power MOS is connected in parallel without inserting the gate resistance and connecting directly. This parasitic oscillation occurs on the resonant circuit formed by the gate-drain capacitance Cgd (Crss) and the gate pin inductance Lg when the drain-source voltage is repeatedly turned on and off at high speed. When the resonance condition (ωL=1/ωC) is established, a vibration voltage much larger than the driving voltage Vgs(in) is applied between the gate and the source, and the gate is destroyed due to exceeding the rated voltage between the gate and the source, or The oscillation voltage when the drain-source voltage is turned on and off is superimposed on the gate-drain capacitance Cgd and Vgs waveforms, causing forward feedback, which may cause oscillation destruction due to malfunction.


5. Grid surge, electrostatic damage


Mainly due to the damage caused by voltage surge and static electricity between the gate and the source, that is, gate overvoltage damage and static electricity at both ends of the GS in the power-on state (including the charging of installation and measurement equipment) resulting in gate damage.


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