Differences between Bipolar Stepper Motors and Unipolar Stepper Motors

Compared with the driving circuit of the two-phase bipolar stepper motor, the driving circuit of the two-phase unipolar stepper motor is basically the same in terms of input section configuration, internal logic and control circuit and the use of dual channels in the driving circuit, but the configuration of the output section is different. The two-phase bipolar stepper motor is driven by a dual-channel H-bridge, while the two-phase unipolar stepper motor is driven by two switches (MOSFET) of dual channels.

In the circuit diagram example below, Q12 and Q14, Q22 and Q24 are paired. This is because a two-phase unipolar stepper motor can be driven by making the current flow in a certain direction from the power supply supplied to the center tap of each coil. The current flows from the center tap to the OUT pin where the switch (MOSFET) is in the on state.

Two-phase unipolar stepper motor drive:
two-phase excitation PWM drive waveform

Below is an example of a two-phase excitation PWM drive waveform for a two-phase unipolar stepper motor. Please see the relationship between the input signal and the output voltage/current.

It should be noted that when driving a two-phase unipolar stepper motor, a transient voltage is generated when the output is OFF; due to its structure, transformer coupling generates back electromotive force. Taking the voltage waveforms of OUT11 and OUT12 as an example, at the moment OUT11 is turned off, a transient voltage is generated in which the output voltage fluctuates by more than 2×VM (the motor voltage power supply supplied to the center tap), and falls back to 2xVM due to the transformer coupling of the regenerative current of OUT12. When OUT11 is turned off, OUT12 fluctuates below GND due to the current regeneration of OUT11; when OUT11 is turned on, the current of OUT11 becomes 2×VM through transformer coupling, and the above process is repeated. Therefore, it is necessary to consider the withstand voltage of the motor driver.