H Bridge Control of a Motor – Low Side Switch


H-bridges are used in robotics applications, motion control systems, and other applications where the current to the load needs to be controlled.  The H-bridge allows you to control both the direction and amount of current to your load.   In the image above the red lines indicate the path of current through the motor.  This blog post goes into some details regarding the low side switch element.

n_channel_mosfetWhen controlling a motor (the load) with an H-bridge it is common to pulse-width-modulate, or PWM,  the low side switches (S3 and S4, those connected between the load and ground).  N-channel MOSFETs are often used for the low-side switches because of their low “on resistance”(Rds(on); resistance drain-to-source during the on-state). They are also capable of relatively high switching speeds that minimize power dissipation in the MOSFET.  The schematic to the left is useful in describing some basic functionality of a MOSFET low side switch.  In this case the “load” is the resister and the LED connected between the positive battery terminal and the drain of the MOSFET.

When the pushbutton switch in the schematic is pressed the MOSFET’s gate voltage goes from ground to the battery voltage level.  For N-channel MOSFETs applying a gate voltage higher than the source can turn on the MOSFET.  There is a specific voltage threshold (which depends on the MOSFET you use) that turns the MOSFET all the way on (Vgs(th)).  When all the way on the MOSFET’s  Rds(on) as low as possible, causing the part to act like a switch.  Turning on the  MOSFET allows current to flow through it from the drain to the source.  Pressing and releasing that switch in the schematic super fast is the equivalent of PWMing the MOSFETs gate. The amount of time the switch is pressed over a period of time can be used to vary the average voltage across the load.

For example, if you held the switch in the schematic down for 5ms out of every 10ms you would be providing a 50% duty cycle (the MOSFET is turned on 50% of the time).  This would light the LED in the schematic.  If you pressed and held the switch down forever the LED would be at its brightest.  If you held it down for 1ms out of 10ms it would be dimmer, as its only lit 10% of the time.  Replacing the LED and resister with a motor would give you control over how much the motor is on, and therefore how fast it is running.  Obviously a single MOSFET can only control current flowing in a single direction, you need the full H bridge to change direction.

I ran across the animated gif below that describes pretty well what the MOSFET gate voltage does to the drain-source connection.  The graph with the moving dot shows the voltage applied to the gate.  The graphic on the right shows the electron density, but also visually describes the state of the MOSFET as a switch.  When the gate voltage exceeds about 0.5V the switch is all the way on.  Below about 0.4V the switch is all the way off.  In between 0.4-0.5V the switch is in an active region where it’s kind-of-on and sort-of-off.  The Vgs(th) (gate voltage turn on threshold) on the graph is not typical for commonly used MOSFETs.   You’ll usually find logic level MOSFETs that turn on somewhere above 2V, and other MOSFETs that may need 7V+ to begin turning on.

One interesting application of the active region where the MOSFET is somewhere between on and off is a voltage controlled load.  By adjusting Vgs you can control the load resistance anywhere from Rds(on) to infinite.   You have to have fine control of Vg and deal with power dissipation issues, but we’ve found it to be a good way to provide a variable load during tests.

As an example of an N-channel MOSFET I’ve included a ratings page for the Microsemi COOLMOS part.  You can see the entire datasheet here.  It’s is a pretty stout part, not cheap, and not what you would normally use to control a small DC motor.  These parts cost about $30 each.  The COOLMOS part is rated for 650V operation and 60A at 100 degrees C.  One thing to be clear on though is that the part can’t really handle that much power on its own.  The Rds(on) is rated at 35 milliohms.  So at 60A the power dissipation would be P=I*I*R => 3600*0.035 = 126 watts.  You’d need heat sinks, fans, and might even want to parallel MOSFETs to share the load.

As an example of a more commonly used MOSFET here’s one that rated for 60V, 7A, and costs about $1 each.

That’s it for today, but I’ll cover more on H bridges in coming posts.


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