Electronic Design

Are MOSFETs truly self-protecting?

Often, some concern is felt regarding the survival of MOSFETs during handling and soldering. Whereas this fear is well-grounded for small-signal devices, most power MOSFETs are observed as being inherently quite robust. This is despite the fact that the absolute maximum gate-to-source voltage specified in data sheets is generally only ±20 V.

The reason for such renewed confidence is that the gate/source structure behaves as a substantial capacitance, on the order of 1000 pF for an IRF530. If a soldering iron using an isolating transformer is employed, its metal tip behaves as a voltage source with series capacitance (see the figure, a). When the tip is applied to the gate, with the source and drain grounded, it’s clear that the maximum ac seen by the gate is about a third of the 12 Vrms available (i.e., less than ±6 Vpk, which is perfectly safe). If, instead, the source and drain are floating, the peak gate-source voltage is reduced considerably, since the coupling to ground is now minimal—via stray capacitance only.

In another scenario, the tip is applied to the drain; it’s assumed that the source is grounded, with the gate at virtually the same potential as the source so that the MOSFET is “off” and hence vulnerable (see the figure, b). The drain presents a variable capacitance with respect to the source, reducing in value as Vds rises. At +10 V, it’s approximately 500 pF. Negative bias isn’t a concern since the voltage is clamped by the internal source-to-drain “body” diode. This diode causes the circuit to behave as a form of voltage pump. Because of the drain’s capacitance, the driving voltage is attenuated, with the drain going up to about +16 V at the crest of the 50-Hz waveform. This is absolutely safe for the IRF530, which has a rated Vdss of 100 V. The voltages were checked on an oscilloscope with a 10-Meg probe, which unfortunately has a fairly significant loading effect. Also, the MOSFET had more capacitance than its specification sheet values: For Fig 1a, ±3.5 V was seen, and for Fig. 1b, +9 V pk.

Static electricity is more troublesome however. It’s possible for a person to become charged up to several kilovolts simply by walking across a carpet. The human body capacitance is between 500 and 1000 pF, and, if the gate terminal is touched while the source is grounded, then Q= CV indicates that the device will be damaged long before voltage equilibrium is reached. Thus, it’s advisable to ground oneself before handling MOSFETs. Unless nylon or similar synthetic clothing is worn, it normally isn’t necessary to use a grounded wristband or mat. In any case, it’s preferable to have an insulated work surface when testing most circuitry.

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