N MOSFET high side vs. low side drive

586

10:08:32

circuit on the left has the NMOS sourcing current to the inductive load, high side driving.  the circuit on the right has the NMOS sinking current from the load, low side driving.

N channel devices can be switched on in two ways:
1.  the gate terminal voltage goes Volts Turn On (Vto) Volts above the source terminal voltage. 
2. the source terminal goes Vto volts below the gate voltage.

notice in the left side circuit that switching the gate on does not turn the mosfet completely on - the little blue indicator is not vertical.  the fet is now operating in its pinch region and has a high resistance, so the resistance between the drain and source terminals (Rds) is quite high.  the voltage drop across the drain and source terminals behaves the same as a regular resistor; power is wasted to create heat.  to fully saturate the fet and get the Rds to the lowest possible value, the gate voltage also has to also go Vto Volts above the drain terminal as well.  Lower resistance means lower voltage drop and thus less power wasted. 

the next problem in the left hand circuit is the coil itself.  when current through a coil of substantial inductance is switched off, the emf pulls the voltage low.  If the voltage on the gate is 0V, and the coil pulls the voltage on the source terminal Vto Volts below the gate pin, the fet will switch back on - we've lost control of the fet.  The right circuit does not have this problem as the source is plugged directly to ground; it can't go below 0.  the voltage on the drain terminal can vary up or down, but it won't turn the fet on.

published 9 years ago

Sine_eyed

9 years ago

That's good stuff- bookmarked..

bilben08

9 years ago

Indeed

lenzrulz

8 years ago

Although the MOSFET is a four-terminal device with source (S), gate (G), drain (D), and body (B) terminals, the body (or substrate) of the MOSFET is often connected to the source terminal, making it a three-terminal device like other field-effect transistors. Because these two terminals are normally connected to each other (short-circuited) internally, only three terminals appear in electrical diagrams. The MOSFET is by far the most common transistor in both digital and analog circuits, though the bipolar junction transistor was at one time much more common. The main advantage of a MOSFET over a regular transistor is that it requires very little current to turn on (less than 1mA), while delivering a much higher current to a load (10 to 50A or more). However, the MOSFET requires a higher gate voltage (3-4V) to turn on. In enhancement mode MOSFETs, a voltage drop across the oxide induces a conducting channel between the source and drain contacts via the field effect. The term "enhancement mode" refers to the increase of conductivity with increase in oxide field that adds carriers to the channel, also referred to as the inversion layer. The channel can contain electrons (called an nMOSFET or nMOS), or holes (called a pMOSFET or pMOS), opposite in type to the substrate, so nMOS is made with a p-type substrate, and pMOS with an n-type substrate (see article on semiconductor devices). In the less common depletion mode MOSFET, detailed later on, the channel consists of carriers in a surface impurity layer of opposite type to the substrate, and conductivity is decreased by application of a field that depletes carriers from this surface layer. this is what happens when we become bored, we post drivel on each other's circuits, faceblast started this trend and I'm continuing it...enjoy!

eb2

8 years ago

I think I've stumbled across the Wikipedia of electronics with practical demonstrations. Brilliant!!

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