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Driving 2-Phase Unipolar Stepping Motors

2023.03.29

Points of this article

・Compared with a 2-phase bipolar stepping motor driving circuit, the driving circuit for a 2-phase unipolar stepping motor is essentially similar in having 2 channels each of which has an input stage configuration, internal logic and control circuit, and driving circuit, but the output stage configuration is different.

・Whereas in 2-phase bipolar stepping motor driving two H bridge output stage channels are used, in 2-phase unipolar stepping motor driving 2 channels of 2 switches (MOSFETs) each are used.

・At the instant the output is turned off, a transient voltage appears ,and due to the structure, a voltage of (2 × VM) or more is generated due to transformer coupling, so that caution must be exercised where the driver withstand voltage rating is concerned.

table of contents

・Driving Circuit for 2-phase Unipolar Stepping Motors
・Driving 2-Phase Unipolar Stepping Motors:
2-phase excitation PWM driving waveforms

In succession to the “Driving 2-Phase Bipolar Stepping Motors” of the previous article, here “Driving 2-Phase Unipolar Stepping Motors” is explained. For the differences between bipolar and unipolar stepping motors, see this reference.

Driving Circuit for 2-phase Unipolar Stepping Motors

Compared with Driving circuit for 2-phase bipolar stepping motors, a driving circuit for 2-phase unipolar stepping motors is basically similar in using two channels each having an input stage configuration, internal logic and control circuit, and driving circuit. However, the output stage configuration is different. For driving a 2-phase bipolar stepping motor, two H bridge output stage channels are used, but to drive a 2-phase unipolar stepping motor, 2 channels each with 2 switches (MOSFETs) are used. In the circuit diagram example below, Q12 and Q14 are paired, as are Q22 and Q24. This is because a 2-phase unipolar stepping motor can be driven simply by passing current in one direction from the power source provided to the center taps of each of the coils. Current flows from the center taps to the OUT terminal which the switch (MOSFET) is turned on.

Driving 2-Phase Unipolar Stepping Motors:
2-phase excitation PWM driving waveforms

Below are examples of 2-phase excitation PWM driving waveforms for a 2-phase unipolar stepping motor. Please study the relationships between input signals and output voltages and currents.

In driving of a 2-phase unipolar stepping motor, attention must be paid to the possible occurrence of transient voltages at output turn-off and of back emfs due to structural transformer coupling (see the diagram below). Taking as examples the voltage waveforms of OUT11 and OUT12, at the instant of OUT11 turn-off, a transient voltage occurs that sends the OUT11 output voltage to above 2×VM (the motor power source voltage provided to the center taps), after which it stabilizes at 2×VM due to the transformer coupling with the OUT12 regeneration current. When OUT11 is turned off, OUT12 is deflected to below GND due to the OUT11 current regeneration, and when OUT11 is turned on, the OUT12 goes to 2×VM due to transformer coupling with the OUT11 current, , and the cycle is repeated. Consequently the motor driver withstand voltage rating must be considered carefully.

【Download Documents】 Basics of Stepping Motors and Driving Methods

Stepping motors are used in various devices and equipment, and there are many types of equipment that cannot function without stepping motors. This handbook presents the fundamentals of stepping motors, explaining their structure, principles of operation, characteristics, and driving methods.

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