Abstract: A novel two-phase hybrid stepper motor driver was developed, using the L297 single-chip stepper motor controller as the control core, a discrete power drive circuit composed of transistors and power MOSFETs, and a high-frequency switching power supply circuit based on the SI9114A single-chip current-mode pulse width modulation (PWM) controller. The overall structure of the system and the working principle and implementation of its key circuits were analyzed and studied. Through practical testing and use, it was demonstrated that the system has the characteristics of wide-range single power input, strong versatility, high reliability, and low cost, and can be widely used in small mechatronics equipment. Keywords: stepper motor; power drive; PWM; switching power supply Abstract: A low-cost two-phase hybrid stepper motor driver is designed. The driver consists of a control circuit based on L297, a power drive circuit consisting of discrete transistors and power MOSFETs, and a high-frequency switching power supply circuit using SI9114A. The whole system structure is analyzed. The principle and implementation of the main circuits are studied. Experimental and application results show that the system has the characteristic of single wide-range power supply input, good stability, and low cost. It may be widely used in small mechano-electronic devices. Keyword: stepper motor; power drive; PWM; switching power supply 0 Introduction A stepper motor is a special synchronous motor controlled by pulses. For each power supply pulse, it generates a constant amount of step motion, which can be angular or linear displacement. Stepper motors can realize signal conversion and are widely used actuators in digital control systems. It has certain open-loop control accuracy, step error does not accumulate over a long period of time, easy to start, stop, reverse and speed change, and simple and convenient interface with host computer. With the development of power electronics technology, control technology and motor body, stepper motors have been more widely used in many fields such as office automation, industrial automation machines, and CNC machinery [1]. At the same time, stepper motor drivers have also been greatly developed and improved [2,3,4]. However, in current industrial applications, most drivers are large in size, not very versatile, often require multiple external power supplies, and are not cheap. Therefore, this paper proposes a design scheme for a two-phase hybrid stepper motor driver with single power input, wide voltage, wide current, and low cost, and verifies its strong versatility, simple control and high reliability through experiments and actual use. 1 Stepper Motor Driver System Structure The main requirements of this drive system are: (1) low cost and small size; (2) wide range single power input: 25～85Vdc; (3) maximum output current per phase: 7A. [align=center]Figure 1 Block Diagram of Stepper Motor Drive Control System[/align] Based on the system requirements, the block diagram of the designed stepper motor drive control system is shown in Figure 1. In this system, the single-chip stepper motor controller integrated circuit L297 serves as the control core. Its integrated circuitry, including a pulse distributor, PWM chopper, and output logic control unit, makes system control more convenient and simple. A discrete power drive circuit composed of transistors and power MOSFETs is used instead of the integrated circuit, reducing system cost and meeting the requirements for driving stepper motors with wide voltage and high current. A high-frequency DC-DC converter circuit designed with the single-chip current-type PWM controller integrated circuit SI9114A as the core solves the system's need for multiple power supplies, reducing system power consumption, decreasing the size of the power supply circuit, and lowering costs. The system receives the stepping pulse signal CP, direction signal DIR, offline signal FREE, and operating mode selection signal MODE. These signals are input to the L297 via optocoupler isolation. The L297 then distributes the signals according to a specific pattern and outputs logic control signals. These signals are amplified by the power drive circuit and applied to the input terminals of each phase of the stepper motor to drive it to operate according to commands. The system also features phase current selection, half-current locking, and overcurrent protection. 2. Key Points in System Design 2.1 Stepper Motor Control Circuit Design The stepper motor control circuit is based on the L297. An internally integrated pulse distributor generates three phase sequence signals, corresponding to three different operating modes: half-step mode; basic step distance, single-phase excitation mode; and basic step distance, two-phase excitation mode. The pulse distributor internally contains a 3-bit reversible counter. Combined with some combinational logic, it generates an 8-beat Gray code timing signal per cycle, which is the timing signal for the half-step mode. In the basic step distance mode, it generates a 4-beat timing signal per cycle. Another important component of the L297 is the use of two PWM choppers to control the phase winding current, achieving constant current chopping control and obtaining excellent torque-frequency characteristics. Each chopper consists of a comparator, an RS flip-flop, and an external sampling resistor, and includes a general-purpose oscillator that provides trigger pulse signals to both choppers; the pulse frequency is determined by an external RC network. The L297's CONTROL pin is used to select the chopping signal control, setting it low to apply the chopping signal to INH1 and INH2, while the A, B, C, and D pins control the winding's on/off state and polarity. The peak phase winding current is determined by the VREF pin. Constant current chopping control technology is currently the mainstream technology for stepper motor control, using PWM and other methods to keep the phase winding current essentially constant regardless of whether it operates at low or high frequencies. Since the motor's electromagnetic torque is only related to the phase winding current, constant current chopping control technology can ensure that the average value of the motor's output torque remains essentially constant. Simultaneously, the motor's high-frequency response is improved, and resonance is reduced. This two-phase hybrid stepper motor driver fully utilizes the functions of the L297 and employs a constant current chopper drive control method. By comparing the feedback phase winding current value with the set peak phase winding current value through a sampling resistor, a drive control signal is generated, causing the power supply voltage to operate in a switching state, thereby keeping the winding current fluctuating around the set value. Since the power supply voltage does not continuously supply power to the winding, but only through narrow pulses, the total input energy is the integral of the voltage and current product of each pulse duration. The energy drawn from the power supply is significantly reduced, lowering heat generation and resulting in higher efficiency. 2.2 Implementation of Half-Current Lockout Function During stepper motor operation, to output larger torque and achieve rapid response, the winding current should be kept at its rated value and not allowed to decrease. However, in the locked state of the motor, it is usually unnecessary to output large torque. To reduce motor heat generation, improve system efficiency, and reduce the burden on the driver, the winding current can be appropriately reduced in the locked state. The circuit for the half-lockout current function designed in this driver is shown in Figure 2. In the figure, CLK is the step pulse signal CP after optocoupler isolation. The circuit uses a repeatable triggerable monostable multivibrator 74LS123, whose output pulse width TW is: (1) When the period of CLK is less than or equal to TW, the transistor is always in the off state and there is no lock-in time. When the period of CLK is greater than TW or in the stationary state, the transistor is turned on, and the resistor R1 is connected in parallel to the reference voltage terminal, so that the reference voltage is halved, that is, the function of halving the current is realized. [align=center] Figure 2 Half-current lock-in function circuit diagram[/align] 2.3 Design of discrete power drive circuit The typical design of the power drive circuit of stepper motor is generally to use integrated circuits, such as the dual H-bridge high voltage and high current power integrated circuit L298, IR company's MOSFET driver integrated circuit, etc. However, for L298, although it is simple and convenient, it can only drive stepper motors with a bus power supply voltage of 46V and a current of less than 2A per phase, so its power input range is relatively narrow and has great limitations; while for IR company's MOSFET driver integrated circuit, it has strong versatility, but the price is relatively expensive and it is not suitable for low-cost drivers. To avoid the shortcomings of the aforementioned integrated circuits, the power drive circuit in this stepper motor driver is implemented using discrete components. The power circuit employs a high-power dual H-bridge circuit, with the upper half using a P-channel power MOSFET IRF9540 and the lower half using an N-channel power MOSFET IRF540. This meets the requirements for driving a stepper motor with a bus power supply voltage of 85V and a phase current of 7A. Furthermore, this structure simplifies the design of the drive circuit power supply, as multiple isolated drive power supplies are no longer needed, allowing the bus power supply and drive power supply to share a common ground. The gate drive of the upper bridge P-channel power MOSFET uses a complementary drive circuit composed of NPN and PNP transistors, resulting in very low resistance in the MOSFET input capacitor charging and discharging circuit, accelerating the switching of the power transistor. A 13V Zener diode is connected in parallel to clamp the MOSFET's gate-source drive voltage when the bus voltage is high, preventing it from exceeding the gate-source breakdown voltage. For the gate drive of the lower-bridge N-channel power MOSFET, a simple NPN transistor drive amplifier circuit is used. This improves the MOSFET turn-on process and reduces the power of the drive power supply. A diode is connected in anti-parallel to the base and emitter of the transistor, providing a discharge path for the input capacitor and accelerating the turn-off process of the power transistor. When the drive circuit directly drives the power MOSFET, it causes rapid turn-on and turn-off of the driven power MOSFET, which may cause oscillations in the drain-source voltage of the driven power MOSFET. This can lead to radio frequency interference and may also cause the power MOSFET to break down due to excessive voltage. To solve this problem, a 15Ω non-inductive resistor is connected in series between the gate of the driven power MOSFET and the output of the drive circuit. The specific upper and lower half-bridge drive circuits are shown in Figures 3 and 4, respectively. The L297 outputs INH1 and INH2, carrying chopping signals, are appropriately combined with the timing logic signals A, B, C, and D through logic gate circuits to generate PWM1 and PWM2 signals, which serve as the chopping signal input terminals of the drive circuit. [align=center] Figure 3 Upper half-bridge drive circuit[/align] [align=center] Figure 4 Lower half-bridge drive circuit[/align] 2.4 Implementation of high-frequency switching power supply circuit The switching power supply system adopts the form of a forward converter in terms of structure[5], as shown in Figure 5. Using the SI9114A chip of VISHAY as the control core, a DC-DC converter circuit with a power of 12W, [align=center] Figure 5 Forward converter topology[/align] operating frequency of 100kHz, input DC voltage range of 25～85V, and output DC voltage of 12V was designed. SI9114A adopts a constant frequency current control mode with a duty cycle of less than 50%. By increasing the conversion frequency, the size of the energy storage element can be further reduced, the power consumption of the system can be reduced, and the structure of the distributed power supply can be simplified. The system operating frequency is set to 100kHz by using a simple external resistor ROSC and capacitor COSC in conjunction with the internal oscillation circuit and the frequency divider circuit. To address the startup problem caused by the large voltage difference between the DC bus voltage and the chip control circuit voltage, the SI9114A employs a low-power BiC/DMOS circuit and a high-voltage depletion-type MOSFET, resolving startup delays and the need for large capacitors. Since the leading edge of the detected current waveform often contains noise levels, an external RC network-based low-pass filter circuit suppresses glitches, preventing waveform distortion. 100nF ceramic capacitors are also connected in parallel at each power supply terminal for high-frequency bypass. The output drive uses a complementary N-channel and P-channel output stage, directly driving the power MOSFET. It also features soft-start and overvoltage/overcurrent protection. 3. Experimental Results The driver was connected to two hybrid stepper motors (models 86BYG200 and 90BYG200) for torque-frequency characteristic tests and long-term operation tests under rated load. The results show that the driver achieves the same technical specifications as dedicated drivers, and the motors operate stably over a wide frequency range with low electromagnetic noise and heat generation. This driver also possesses many advantages that dedicated drivers lack, such as: single power input, eliminating the need for external control and drive power supplies; adaptability to a wide input power range of 25-85V; smaller size and lower cost, saving approximately 20% in cost compared to dedicated drivers. 4. Conclusion The author's innovation lies in using the L297 single-chip stepper motor controller as the control core, employing a discrete power drive circuit composed of transistors and power MOSFETs, and a high-frequency switching power supply circuit based on the SI9114A single-chip current-mode pulse width modulation (PWM) controller to construct and implement a versatile, simple, and low-cost two-phase hybrid stepper motor driver. Practical application in a packaging machine control system further demonstrates the reliability, high efficiency, and good torque-frequency characteristics of this stepper motor driver. It can be widely used in small mechatronics equipment, effectively reducing costs and better meeting requirements. References [1] Liu Baoting, Cheng Shukang. Stepper motor and its drive control system [M]. Harbin: Harbin Institute of Technology Press, 1997. [2] Guo Weina, Deng Hong. Design of coaxial connection and drive device for dual stepper motors [J]. Microcomputer Information, 2005, 21 (4): 103-104. [3] Wang Yulin. A practical drive circuit for three-phase reactive stepper motors [J]. Power Electronics Technology, 2005, 39 (3): 71-72. [4] Yang Jianning. Power drive circuit for stepper motor composed of PMM8713 and SI-7300A [J]. Small and Medium Motors, 2004, 31 (5): 43-49. [5] Ding Daohong. Power Electronics Technology [M]. Beijing: Aviation Industry Press, 1999.