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Engineering
Electrical Engineering
Getting a High Speed Synchronous N-Channel MOSFET Driver working right
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[QUOTE="Baluncore, post: 6900104, member: 447632"] The boost cap will be charged to +5V, but the low voltage terminal will switch between 0V to +24V as the output MOSFETs switch. The higher voltage terminal of the boost cap will switch between +5V and +29V. The boost capacitor can be seen as a charge pump or a floating local power supply for the high-side gate drive. The capacitor is referenced to the source of the high side MOSFET. That boost capacitor charges through the diode to supply voltage while the output is low. The cap then provides positive power to the high-side MOSFET gate during output high. Maybe we need a circuit diagram with values for the components. I am using "Figure 4. NCP3420 Example Circuit" from the data sheet. Vcc for the NCP3420 can be between +5V and +12V. The positive supply to the MOSFETs, let's call it Vmm, can be higher. But (Vcc + Vmm) ≤ 35 volts, in order to meet the SW terminal absolute maximum voltage specification. For your circuit; Vcc=5; Vmm=24; You get 29 volts which is well within spec. Maybe the gate driver needs to be disabled by holding the OD signal low while the power supplies become stabilised, and the boost capacitor is initially charged. Only then can the high-side MOSFET be driven. During testing, I would use a couple of 12V filament lamps in series with the 24V MOSFET supply. Select the lamp power to glow dull-red when operating, while lighting and protecting the good MOSFET, when one MOSFET fails. [/QUOTE]
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Getting a High Speed Synchronous N-Channel MOSFET Driver working right
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