I’ve recently posted about H-bridge motor driver construction, with a focus on the low side switching element (see part 1 here and part 2 here). Part 1 focused on the N-channel MOSFET and you can reference that blog post if some of the terms here aren’t familiar to you. Part 2 focused on driver circuits that lie between a microcontroller pin and a high current MOSFET used as a low side switch (S3 or S4 in the h-bridge diagram). In this post I’m going to focus some attention on the high side switch elements (S1 and S2).
Although high side switches can be configured from P-channel MOSFETs, it is more common to use N-channel MOSFETs. There are a couple of reasons that I think this is the case. First, early on (a decade ago) it seemed that the N-channel MOSFETs had lower Rds(on) specifications than P-channel MOSFETs. This made them more appropriate for high current handling applications, like H-bridges. Second, using 4 identical N-channel switch components should reduce the per board parts cost since you’d buy a higher volume ( = lower price) of one part versus half as many of two different parts ( = higher price). Machine assembly cost would be lower as well since you’d reduce the number of different parts on the board.
So what’s necessary to use an N-channel MOSFET as a high side switch element (S2 in the h-bridge diagram above)? The biggest hurdle is getting the turn on threshold voltage above the source voltage, and keeping it there. This is typically handled by a high side driver chip. An example of the a driver chip is the Micrel MIC5018. These parts are tiny, under a dollar in 100’s, and work as both low and high side drivers. A down side might be the operating voltage of these parts, but there are a slew of high-side and low side drivers from a variety of manufacturers. Some of these will meet the specifications of your design.
In many applications the high side of the h-bridge is not PWM’ed. This reduces switching power dissipation in the high side element and reduces the current source needs of the high side driver. Using the diagram at the head of this blog you would turn on S2 (close the switch) and PWM S3 to control a motor’s speed in the forward direction. To do the same in reverse you turn on S1 and PWM S4.
When the high side switch (MOSFET) is turned on the source voltage, connected to the load, is at the supply voltage. In the images above this would place +5V at the source (load), and +5V at the drain (power supply). In order for the MOSFET to completely turn on the gate-to-source voltage must exceed MOSFET’s threshold voltage (Vgs(th)). If the drain and source both equal +5V then the gate must be at least Vgs(th) +5V to turn on the MOSFET. Therefore, the gate voltage must exceed the supply voltage. This is what the high side driver does. Using an internal charge pump the high side driver places 15V at the gate, making the Vgs (source to gate voltage) higher than the turn-on threshold.
There are MOSFET driver ICs with high and low side switch drivers built in. Here is a Zetex part that works up to 180V and costs less than the MIC5018 that will drive both the low side and high side MOSFETs. And there are MOSFET drivers IC’s designed to drive an entire H-bridge (two low side and two high side drivers) such as the one below.