Please note the transient voltage generated by the mechanical switch in parallel with the relay control. Do not connect the mechanical switch to the MOSFET when driving an inductive load.
When the relay is turned off, a high transient voltage (dv/dt) is generated due to the short start-up time of the relay, which is usually accumulated due to the large amount of dv/dt generated.
potential problems:
When the MOSFET is off, the opening of the switch can cause damage to the MOSFET: due to the short time of the mechanical switch, a high dvdt activates the parasitic transistor contained in the MOSFET. The energy of the surge will be concentrated on this parasitic transistor; this can destroy the parasitic bipolar transistor and destroy the MOSFET itself. The MOSFET is damaged by an absolute short circuit between the drain source and the source.
Note: Due to the vibration of the relay, the MOSFET can also be destroyed when the switch is turned off.
Precaution:
1. Use a bipolar transistor with a Zener diode protection to limit the oscillating voltage, which is lower than VCEmax. The 39vzener voltage is the recommended value for accepting the load dump.
2. Use a zener protection diode whose voltage is lower than the second breakdown voltage of the MOSFET (the second breakdown voltage can be as low as 50% of VDSmax) to select the MOSFET (the second breakdown voltage is 39V).
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