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1、-Power MOSFET in High-Side Operating Modes, Possible Failure Modes, and Failure SignaturesPower MOSFETs in high-side application can fail under any one of the following modes of operation:(a) High-impedance gate drive(b) Electro-static discharge (ESD) e*posure(c) Electrical over-stressed (EOS) opera
2、tionIn most cases, failure analysis of the damaged MOSFET reveals a signature that can point towards a possible cause of failure.(a) High-Impedance Gate DriveA generic schematic configuration for a high-side driver with inductive load is shown in figure 1. A typical gate drive design based on an app
3、lication-specific integrated circuit (ASIC) caters to the primary gate charge requirements of the MOSFET, which are on the order of a few tens to hundreds of nanocoulombs. An effort to achieve high efficiency and optimize the integration of the ASIC results in low-current ( 1 M). This basically sati
4、sfies the total gate-charge requirements of the MOSFET.However, to avoid overloading the ASIC, the e*ternal gate resistor (R2) in such a design tends to have a high ohmic value (ranging from tens of ohms to several kiloohms). The feedback resistor (R3) also usually has a high ohmic value (ranging fr
5、om several kiloohms to 1 M). As a result, the source (S) of the power MOSFET (U1) is virtually floating.The effect is magnified during turn-off and recirculation of the energy stored in the inductor (L1).Fig. 1 - Typical High Side DriverThis approach fully satisfies the requirements for steady-state
6、 operation, where small amount of current, on the order of a few microamps, is all that is required to maintain the on state. But it can be inadequate for dynamic (transient) turn-on and turn-off operation of MOSFET. In this latter case, the MOSFET switching operation can easily occur in linear mode
7、, which is an unstable and non-characterized area of MOSFET operation (figure 2).Fig. 2 - Floating Source and Slow Turn-OffDuring turn-on and turn-off, switching power losses are likely to dissipate from a very small area of the active die. The resulting failure signature is high-power burn-out, wit
8、h a random amount of damage located in the active area of the die. Invariably such failure is labeled as EOS.Another subtle failure evidenced during the analysis of the failed MOSFET (with a virtually floating gate circuit configuration) is in the gate-source area. A DC parameter test of the gate-so
9、urce area shows out of spec readings. The failure mode is subtle insofar as the leaky device recovers even during the DC parameter test. This failure mode remains under investigation to obtain a full e*planation.A failure e*ample of a DPAK power MOSFET is shown in the figures 3, 4, and 5.Fig. 3 - EO
10、S Damage LocationFig. 4 - Damage on BPSGFig. 5 - Damage on Gate Poly(b) Electro-Static Discharge (ESD) E*posuresA second possible scenario arising from a virtually floating gate condition caused by the high impedance of an ASIC driver and/or high gate resistor is increased susceptibility of the gate
11、-source area for damage from ESD or a similar event. In addition to a typical ESD event, high-energy, high-voltage transients of short duration can be generated from energy re-circulation and/or energy interruption frominductive ponents. With reference to figure 6, these include the low-side load (L
12、1, R1), other parallel connected load (L2, R4), and series feed-in/harness impedance (L3, R5).Fig. 6 - L1, L2, L3 Sources of TransientsFailure signatures are usually located near the gate termination and in the gate-source area. An e*ample of a DPAK power MOSFET failure is shown in figures 7, 8 and
13、9.Fig. 7 - ESD Damage near the Gate-SourceFig. 8 - ESD on the Gate TerminationFig. 9 - Close-up of ESD Damage(c) Electrical Over Stressed (EOS) OperationHigh-energy or high-voltage short-duration transients appearing on the drain-source termination of the MOSFET (figure 6) can result in damage with
14、a variety of signatures.Transients that are faster than the breakdown time of the body diode may produce punctures anywhere in the active area of the drain-source. Such punctures have the appearance of a pin-hole at the damage site. They may be attributed to an ESD event or an ESD-like event arising
15、 from other circuit ponents or parasitics.Where energy/voltage values are not too high but of a sufficient duration to break down the body diode, avalanche results. Avalanche results in e*tensive damage and in fact destroys the evidence that failure analysis would need to determine the root cause. T
16、hese are true EOS events.EOS can also result from a mismatch of thermal management and continuous power dissipation in the device, even when the device is fully turned on. When heat-sinking is inadequate, heat can build up in the junction during continuous operation. Eventually a thermal runaway occurs. The device is pletely destroyed and also classified as an EOS.D2PAK e*ample: figures 10 to 12 are fa
