Controlling Inductive Loads With a Microcontroller

 Basic Circuit to Control the Inductive Load

 
  • Problems with this basic circuit.
    • In this circuit the MOSFET acts as a switch. Just like the demonstration in the previous class we know huge voltage spikes will build up across the MOSFET (switch) as it is turned on and off. These voltage spikes can be great enough to burn out the thin metal oxide electrical insulation between the gate and the substrate of the MOSFET.
    • There is a floating gate between the microcontroller and the gate of the MOSFET. This is a BAD design.
      • Wires connected to the gate of a MOSFET should never be left at an undefined state. Magnetic and or electric fields can induce voltages in the line and spontaneously turn on or off the MOSFET (example during microcontroller startup) .
      • In addition the metal gate separated from the substrate in the MOSFET by the metal oxide insulating layer creates a capacitor which will tend to pass alternating (changing) voltages. The high voltages created across the MOSFET when the the inductive load is switched on and off can pass into the gate circuit by this capacitance and my burn out the output circuit inside the microcontroller.
    • The voltage spikes created in this circuit (and any circuit with fast switching and/or inductive circuits) can pass through signal or power lines to other parts of the circuit causing malfunctions and or failure.

Circuit Solution to Switching Inductive Loads

  1. Recirculating Diode (also called clamping diode) placed around the inductive load.
    • This is almost always done. In fact many relays come with this diode built right in.
    • When the magnetic field of the inductor collapses (when the circuit is switched off), it creates a voltage opposite in polarity to the voltage which was applied to it. (If + was at the top of the inductor when power was applied then a + will be created at the bottom of the inductor when the magnetic field collapses.) The recirculating diode is reverse biased when the inductor is operating but will conduct when the inductor is shut off because it will be forward biased. This has the effect of reducing or virtually eliminating the voltage spike by providing a current path across the inductor when the switch is shut off.
    • As a rule of thumb the current capability of this diode should be at least as great as the current required by the load. (If there is a 1 A coil then use a 1A diode. In many cases a general purpose power diode can be used. (1N4001 is a 1A general purpose power diode, 1N4002 is 2A and so on)
    • As seen in the previous lesson demonstration, the voltage spikes created can be very short in time duration. In many cases general purpose diodes actually switch too slowly to correct the problem. Ideally a type of diode which switches faster called a Schottky diode is used for recirculating and clamping functions
  2. Clamping Diode placed across the MOSFET.
    • During normal circuit operations this diode is reverse biased. This diode provides a current path if the inductive load creates an electrical potential (voltage) lower than ground.
    •  Because of the construction of many power mosfets this diode is often actually built into the devise. (see the data sheet for an IRF520N) It is not a good idea to count on this internal diode, and it is suggested that an additional diode be added externally.
    •  see notes in #1 about type and current rating of diode.
  3.  Snubber Capacitor
    • The capacitor is often added with or without a series resistor, and in addition to or instead of a recirculating diode.
    • The idea is that the capacitor will give a current path (to charge up the capacitor) between the ends of the inductive load, when the inductor has been switched off. The effect is to greatly reduce the magnitude of the voltage spike.
    • It is suggested that a 0.1 to 1 uF polyester type capacitor is connected across a motor. (Braga)
  4. Pull Down Resistor
    • A Pull Down Resistor should always be connected to the gate circuit so that in absence of a signal the gate circuit is off. (In some cases a pull up resistor may be used.)
    • The magnitude of the pull down resistor is not critical, a suggested range is between 100k to 1M.
    • In the event that voltage is induced (from magnetic or electric fields) the resistor provides a path to ground. (Note: for this reason it is a good idea to keep wires connected to the gate of mosfets short)
  5.  Zener Diode
    • A zener diode is a type which will conduct in reverse (reverse breakdown) at a very specific voltage.
    • As shown the zener will have no effect on the circuit (since it is reverse biased) until this reverse voltage is exceeded. If we chose for example a 6 volt zeener, then it would limit the voltage in the gate circuit to 6 volts. If 6 volts were exceeded the zeener will conduct to ground.
  6. Series Resistor
    • A series resistor of suggested values between 1k to 10k can limit the current in the gate circuit. This can crreate a slight delay in operation of the gate which need to be considered for high speed switching. (Since the gate acts as a small capacitor, this resistor effectively makes an rc circuit.) Adding this resistor can eliminate a condition called latch up. Latch up is a condition where the mosfet transistor stays on and will not turn off. (The cause is complicated except to say under certain conditions the mosfet acts as an PNPN devise called a silicon controlled rectifier or SCR) One suggestion to prevent latch up is to limit the current in the gate circuit to less than 20 mA.
  7. Protection diode
    •  On rare ocassion a diode will be introduced into the gate circuit. This would prevent any positive pulses from feeding back into the microcontroller.

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