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What's the purpose of the diode between VCC and VB?

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    \$\begingroup\$ When the bottom MOSFET is on, the bootstrap capacitor connected between Vb and Vs is charged up to a voltage equaling Vcc. Then when the bottom MOSFET is turned off and the top MOSFET has to be turned on, Vb is switched trough to the top MOSFET's gate. As the source voltage rises, the bootstrap capacitor rises along with it, so the gate is always kept above the source, this causes Vb to rise above Vcc, hence the diode. \$\endgroup\$ Dec 31, 2018 at 23:20

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Vb is a charge boost converter to raise Vb above Vcc.

Thus the low side must use PWM to create a clock drive thru D to charge up C above Vcc so Vgs (high side) is positive for high side Nch switch.

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