switching loads with MOSFETs

Posted on Feb 7, 2014

The biggest issue is that the RDS_on of the used MOSFETS is, driven with 5V gate voltage, too low. The other issue is that the the reverse voltage that occurs, when the MOSFET switches from on to off state kills the transistor. This revese voltage is caused by the parasitic series inductance of the heatbed, hotend heater cartride or resistor. This article tries

switching loads with MOSFETs
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to generate understanding of what is happening. -> Thermal power loss of the MOSFET device: 12. 5A ² * 40mOhm = 6. 25W -> Theoretical resulting device case temperature 6. 25W * 80 °K/W + 25 °C (ambient temp. ) = 525 °C This definitely kills the MOSFET to survive add a heat spreader with at least 15 °K/W which results in a device temperature of approx. 120 °C at room temperature. -> Thermal power loss of the MOSFET device 12. 5A ² * 10mOhm = 1. 6W -> Theoretical resulting device case temperature 1. 6W * 40 °K/W + 25 °C (ambient temp. ) = 90 °C The two diagrams on the top left shows a single switching sequence from heatbed off - on - off The leftmost shows the voltage, the picture next to this the Drain current. You can see that the highest voltages occur when switching from on state to off state. All other pictures show a zoomed view of the drain-source voltage (Vds) at this event. You can see that in the top rightmost diagram a voltage of approx. 250V(!) volts occures at the drain of the IRF7936. The device is rated only with 30V Vds. This voltage applys only 100ns and will not kill the device instantaneous but over time this overvoltage will lead to malfunction of the device. The leftmost three diagrams in the second row show the Vds voltage for the PSMN7R0-60YS MOSFET used on RUMBA in different applications: Heated bed, Hotend with 40W heater cartrige and Hotend with Vishay heater resistor. As you can see the Vds voltage...

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