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I want to create a DC-chopper to drive a motor. I found the following schematic online: MOSFET driver circuit However i don't understand how the MOSFETs are driven. On top there is a P-channel MOSFET with source connected to Vcc, between 48V and 60V in the case of this example. However, according to the datasheet of this MOSFET, Vgs is -20V max. Since source is at 60V when the battery is full, the gate voltage must be above 40V. But the MOSFET driver is fed at 15V, and according to its datasheet the output voltage will be (almost) the same as the feeding voltage. So, Vgs is -45V, well above the rated Vgs. But, according to the video, the circuit works just fine. Another question: doesn't the gate have to be at 60V to conduct and below around 56V to stop conducting? According to the schematic I think the gate voltage is being switched between 0V and 15V, so even if the MOSFET would be able to handle this, it should be powered off all the time? What am I missing?

Links:

P-channel MOSFET datasheet: https://www.infineon.com/dgdl/irf5305pbf.pdf?fileId=5546d462533600a4015355e370101993

MOSFET Driver: https://www.mouser.be/datasheet/2/268/21422c-73495.pdf

Instructable (with video): https://www.instructables.com/id/Make-Your-Own-Sensored-ESC/

Thanks!

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  • \$\begingroup\$ @Brent Thierens "with source connected to Vcc, between 48V and 60V in the case of this example" TC4427 maximum operating voltage is 18V, Vcc of circuit is around 15 volt! \$\endgroup\$ – M KS Nov 17 '18 at 14:01
  • \$\begingroup\$ @MKS Vcc is not specified in the schematics, but according to the OP it is between 40V and 60V (from what he is saying, I suppose it is a battery pack of some sort and the voltage depends on the SOC of the pack). Note that the TC4427 is not powered from Vcc, but from a 15V rail obtained from a 7815 regulator, so it is powered correctly. \$\endgroup\$ – Lorenzo Donati Nov 17 '18 at 14:20
  • \$\begingroup\$ Indeed, tre whole schematic is powered from a battery pack ranging from 60V (full) to 48V (empty). The drivers are fed at 15V from the regulator, but the P mosfet has its source at Vcc, so ranging from 60V-48V. \$\endgroup\$ – Brent Thierens Nov 17 '18 at 14:43
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    \$\begingroup\$ @MKS You are correct. That design is somewhat questionable also for other aspects. I.e. the regulators are linear regulators powered from Vcc. They are going to dissipate a lot of power, since the min voltage drop across their input-output is 40V-5V=35V for the 7805 and 40V-15V=25V for the 7815. If the current they provide is just 100mA, they have to dissipate 3.5W and 2.5W respectively (more with full battery). This will make them run hot w/o an heat sink (as in the video). To be sure one should make some calculations since their output current is not continuous, but spiky, especially 7812. \$\endgroup\$ – Lorenzo Donati Nov 17 '18 at 15:03
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    \$\begingroup\$ BTW, I just checked TI datasheeet, the design is broken for other reasons: the regulators maximum input voltage cannot go over 35V(max). So unless he uses some high-voltage parts the fact that they still work at 60V it's just matter of luck (they are driven terribly out of specs!). \$\endgroup\$ – Lorenzo Donati Nov 17 '18 at 15:08

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