Basic drive circuit
The drive circuit is used in applications that use certain types of controllers and require speed control. The purpose of the drive circuit is to provide the controller with a means of changing the winding current in the BDC motor. The drive circuit discussed in this section allows the controller to pulse width modulate the supply voltage of the BDC motor. In terms of power consumption, such a speed control method is much more efficient than a conventional analog control method in changing the speed of a BDC motor. Traditional analog control requires an additional varistor in series with the motor windings, which reduces efficiency. There are many ways to drive a BDC motor. Some applications only require the motor to operate in one direction. Figures 6 and 7 show the circuit for driving the BDC motor in one direction. The former uses low-end drivers and the latter uses high-end drivers. The advantage of using a low-end driver is that you don't have to use a FET driver. The purpose of the FET driver is to:
1. Convert the TTL signal of the driving MOSFET to the level of the supply voltage.
2. Provide enough current to drive the MOSFET (1)
3. Provide level shifting in half bridge applications.
Note 1: For most PIC chipper applications, the second point is usually not applicable because the I/O pin of the PIC microcontroller can provide 20mA of current.
Note that in each circuit, a diode is connected across the motor to prevent the BackElectromagnetic Flux (BEMF) voltage from damaging the MOSFET. BEMF is generated during the rotation of the motor. When the MOSFET is turned off, the windings of the motor are still energized and a reverse current is generated. D1 must have a suitable rating to be able to consume this current.
The resistors R1 and R2 in Figures 6 and 7 are important for the operation of each circuit. R1 is used to protect the microcontroller from current spikes. R2 is used to ensure that Q1 is turned off when the input pin is tri-stated.
Bidirectional control of a BDC motor requires a circuit called an H-bridge. The H-bridge is named for its schematic appearance, which allows the current in the motor winding to move in both directions. To understand this, the H-bridge must be divided into two parts, or two half-bridges. As shown in Fig. 8, Q1 and Q2 constitute one half bridge, and Q3 and Q4 constitute another half bridge. Each half bridge can control the conduction and turn-off of one end of the BDC motor to make its potential supply voltage or ground potential. For example, when Q1 is turned on and Q2 is turned off, the left end of the motor will be at the potential of the supply voltage. Turning on Q4, keeping Q3 off will ground the opposite end of the motor. The IFWD labeled with an arrow shows the flow of current in this configuration.
Note that there is a diode (D1-D4) across each MOSFET. These diodes protect the MOSFET from current spikes caused by BEMF when the MOSFET is turned off. These diodes are only needed if the diode inside the MOSFET is not sufficient to consume the BEMF current. Capacitors (C1-C4) are optional. These capacitors are typically no more than 10 pF and are used to reduce the RF radiation generated by the commutator arching.
Table 1 shows the different drive modes for the H-bridge circuit. In the forward and backward modes, one end of the bridge is at ground potential and the other end is at VSUPPLY. In Figure 8, the IFWD and IRVS arrows depict the circuit paths for the forward and backward modes of operation, respectively. In the Coastal mode, the terminals of the motor windings remain suspended and the motor coasts until it stops. The Brake mode is used to quickly stop the BDC motor. In brake mode, the motor terminals are grounded. When the motor rotates, it acts as a generator. Short-circuiting the leads of the motor is equivalent to having an infinite load on the motor, which can cause the motor to stop quickly. IBRK arrow depicts this
When designing an H-bridge circuit, a very important consideration must be considered. When the input to the circuit is unpredictable (such as during microcontroller startup), all MOSFETs must be biased to the off state. This will ensure that the MOSFETs on each of the half bridges of the H-bridge will never turn on at the same time. Turning on the MOSFET on the same half bridge at the same time will cause a short circuit in the power supply, which will eventually damage the MOSFET and render the circuit inoperable. A pull-down resistor on the input of each MOSFET driver will do this.
