High Side Switch Using N Channel Mosfet

Posted by crimsonpisang on   0 comments   0 trackback

I've built up a prototype board for using N channel mosfet as high side switch. Usually, P channel mosfets were used in such application. However, they have higher RDS(on) resistance as compared to N mos and they also cost slightly more. Another downside of using N channel as high side switch is that it requires a form of bootstrapping its Gate voltage as it requires >Vs + Vth in order to fully turn on.

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As the control circuitry is sitting on the low side of the power, the ground, driving low side switches is usually easier than driving high side switches. Another reason for using a low side switch is when one side of the load is already connected to the positive supply beyond our control. The only choice we have is to leave the low side of the load floating to turn off the load, or connect it to ground to turn it on.

low side

Though in some cases the load does care. If the load has other ground-referenced signals it has to connect to, then we will usually keep its ground node connected to ground. In that case, we will have to switch the positive power to the load. Again, this is usually slightly complicated than driving a low side switch, but not overly so that it requires great lengths to avoid.

high side

I came up with a simple bootstapping circuit design using a diode and a capacitor to lift up the Gate voltage above Source voltage. The Enable signal is active high and can be powered via the supply line. A quick test so far shows about 100mV dropped across the mosfets at 5A.

schematic.jpg

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