Luca Bruno
EDN
A traditional method of driving a low- to medium-power permanent-magnet dc motor involves using four MOSFET or bipolar transistors in an H-bridge configuration. For example, in Figure 1, the motor connects between collector pairs C1 and C2 and C3 and C4. Turning on diagonally opposite transistor pairs Q1 and Q3 or Q2 and Q4 steers current through the motor and allows for reversal of its direction of rotation. However, this method requires that each of the four transistors receive its own control input. Depending on the motor’s voltage requirements, the upper two drive signals may require electrical isolation or a level-shifter circuit to match the microcontroller’s output-voltage limitations.
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| Figure 1. | In an H-bridge output-driver stage, diagonally opposed transistor pairs conduct to energize a dc motor. The circuit requires four control signals. |
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This Design Idea describes an alternative circuit that drives only the H bridge’s two low-side switching transistors. In a standard bipolar-transistor H bridge for bidirectional motor control, Q1’s and Q4’s bases connect to Q3’s and Q2’s collectors through resistors R3 and R4 (Figure 2). Inputs VINA and VINB each control a pair of switches. When Q2 turns on, resistor R4 and diode D6 pull Q4’s base low, saturating Q4 and pulling current through the motor and Q2. Similarly, turning on Q3 pulls Q1 into saturation and drives the motor in the opposite direction. Diode D5 ensures that Q1 remains off when Q4 conducts, and D6 performs the same function for Q4 when Q1 conducts. Resistors R1, R2, R7, and R8 increase the switching speed of their associated transistors, and resistors R5 and R6 limit base-current drain from the microcontroller’s 5 V high-logic-level outputs to approximately 15 to 20 mA. Resistors R3 and R4 set Q1’s and Q4’s saturation base currents. Their value depends on the motor-supply voltage and Q1’s and Q4’s dc current-gain according to the following equation:
For best performance, select bipolar-junction transistors with low collector-emitter saturation voltages, VCE(SAT), and high values of dc-current gain, hFE. Currently available medium-power transistors compete favorably with MOSFETs by offering these characteristics in combinations that minimize collector-power dissipation and require little base drive.
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| Figure 2. | This improved H-bridge-driver circuit uses transistors in complementary pairs and requires only two low-level control signals. |
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Discrete devices such as ON Semiconductor’s NSS40200LT1G PNP and NST489AMT1 NPN bipolar transistors work well in the circuit in Figure 1. For a more compact implementation, you can select an integrated H bridge, such as Zetex’s ZHB6790, which operates at power-supply voltages as high as 40 V, with 2 A continuous and 6 A peak pulse-current collector ratings. Its minimum current gain of 500 at a collector current, IC, of 100 mA can decrease to 150 at IC of 2 A. At a worst-case collector current of 2 A in Q2 and Q3, achieving a saturation voltage of 0.35 V or less requires a base current of 13 to 20 mA. Fortunately, many microcontrollers’ outputs can source or sink as much as 25 mA and thus directly drive the H bridge independently of the motor’s power-supply voltage. To further reduce drive current or to use a standard CMOS or TTL IC as a drive source, you can buffer Q2’s and Q3’s inputs with small-signal transistor inverters. As an option, you can connect fractional-ohm resistors between the emitters of Q2 and Q3 and ground. This approach can provide analog voltages proportional to motor current, allowing the microcontroller to detect a stalled or overloaded motor.
