Electric bicycles are generally powered by lead-acid batteries, available in 24V, 36V, and 48V specifications. However, the lights, audio equipment, anti-theft alarms, mileage and speed displays, and motor controllers of electric bicycles generally only require 12V DC voltage. This article introduces a step-down DC-DC switching power supply based on the TL494PWM chip technology. Through closed-loop control, the output voltage is stabilized. The system's overcurrent protection and phase compensation design for stable system operation have good effects.
Electric bicycles are generally powered by lead-acid batteries, which come in 24V, 36V, and 48V specifications. However, the lights, audio system, anti-theft alarm, mileage and speed display, and motor controller of electric bicycles generally only require 12V DC voltage.
An electric vehicle controller is a core component used to control the starting, running, forward and reverse movement, speed, and stopping of the electric vehicle's motor , as well as other electronic devices. It's like the brain of the electric vehicle, a crucial part of the system. Currently, electric vehicles mainly include electric bicycles, electric two-wheeled motorcycles, electric tricycles, electric three-wheeled motorcycles, electric four-wheeled vehicles, and battery-powered vehicles. Electric vehicle controllers also vary in performance and characteristics depending on the different vehicle models.
Faced with exhaust pollution from internal combustion engine vehicles and energy depletion, electric vehicles are gradually becoming a part of people's lives. Major automakers worldwide are heavily investing in the research and development of electric vehicle control technologies and have recently launched their own electric vehicles at auto shows. The motor controller is the most critical component of an electric vehicle control system, and its development has a profound impact on the popularization and promotion of electric vehicles. Currently, mainstream electric vehicle controllers are mainly composed of power electronic devices, with the motor controller acting as its heart.
The principle of electric vehicle motor controller
The electric vehicle described in this paper uses a separately excited DC brushed motor, and the power battery consists of six lead-acid batteries connected in series. Each battery has a rated voltage of 6V, and the total rated output voltage is 36V. The separately excited DC motor has a rated voltage of 36V, a rated power of 3kW, a rated output torque of 10.84Nm, a rated speed of 2600r/min, a magnetizing current of 10A, and a maximum load current of 110A.
In this controller design, the armature control section adopts a two-quadrant DC chopper PWM system, and the excitation control section adopts a bipolar reversible DC chopper PWM system. The controller uses the TL494 chip, which has advantages such as simple structure, reliable function, low price, and good stability. The TL494 integrates a 5V±5% reference power supply, two error amplifiers, a PWM generation comparator, and adjustable dead time control.
Two-quadrant DC chopper PWM system
The schematic diagram of a two-quadrant DC chopper is shown in Figure 1. It employs a half-bridge structure, allowing the current Ia flowing through the motor to be either positive or negative. Switch S2 only functions during braking, enabling the system to operate in both quadrants. Although this control scheme only allows the motor to operate in the first and second quadrants, it provides energy regenerative braking, is easy to control, uses fewer electronic components (resulting in a smaller controller size), and offers high system reliability and low cost.
Figure 1
Bipolar reversible DC chopper PWM system
The schematic diagram of a bipolar reversible DC chopper is shown in Figure 2. This type of DC chopper allows the motor to operate in four quadrants, meaning that the armature voltage Va and current Ia can be either positive or negative.
Figure 2
This control scheme enables the motor to operate in all four quadrants. The circuitry and control are not complex. A small vibration current exists when the motor stops, eliminating the static friction dead zone. At low speeds, the drive pulses for each power switch remain relatively wide, ensuring reliable conduction. The current flowing through the excitation circuit of a separately excited DC motor is small, and the excitation circuit does not need frequent switching; it only needs to operate in the first and third quadrants.
Overall Scheme of Separately Excited DC Brushed Motor Controller
In the final controller design, a two-quadrant DC chopper is used for the armature circuit, and a four-quadrant DC chopper is used for the excitation circuit. Since electric vehicles must be able to operate in various road conditions and weather conditions (high and low temperatures), the controller must possess certain levels of shock resistance, waterproofing, and electromagnetic interference resistance. To adapt to the harsh operating environment of this controller, a purely hardware-based design is adopted.
The overall block diagram of the separately excited DC brushed motor controller is shown in Figure 3.
Figure 3
To improve controller stability, several protection functions were added, such as overheat protection, excitation detection protection, overcurrent protection, battery undervoltage protection, and reverse connection protection. This design uses two TL494 chips as the core controllers of the entire system, with one TL494 dedicated to controlling the excitation section.
The small system comprised of TL494 chips in the controller's excitation section begins operation after the electric vehicle is powered on, ensuring an uninterrupted excitation signal during operation and effectively protecting the motor. After acquiring signals from the forward and reverse selection buttons, it selects the direction of the excitation current in the excitation circuit to control the electric vehicle's direction of travel. The controller's armature control section uses another TL494 chip. This system acquires throttle pedal signals to adjust the duty cycle of the TL494's output PWM wave for speed regulation. By detecting the current signal in the excitation circuit, it activates the armature circuit after confirming that the excitation current is normal. By detecting the armature circuit signals, it promptly shuts off the TL494 pulse output when the motor is stalled, preventing excessive current from burning out power devices and the motor.
Hardware Design
The PWM regulation in the TL494 peripheral circuit design is implemented using the TL494CN chip, and its circuit schematic is shown in Figure 4. The main power devices adopt a configuration of multiple MOSFETs and multiple diodes connected in parallel. To achieve smooth speed regulation of the DC brushed motor, pin 13 of the TL494 is grounded, enabling the TL494 to operate in single-ended output mode and achieving continuous adjustment of the PWM duty cycle from 0 to 96%.
Figure 4
To increase the output drive current of the TL494, improve its drive capability, and protect its output terminals (pins 9 and 10), two high-speed diodes are connected in parallel to output a maximum current of 500mA, significantly enhancing the output drive capability. A Darlington transistor push-pull output is used at the output terminal to drive multiple parallel MOSFETs in the armature circuit. When multiple MOSFETs are connected in parallel, attention must be paid to current sharing and heat dissipation.
Overheat protection
The core components of the overheat protection circuit are mainly the 65°C normally open temperature control switch R40 and the high-speed operational amplifier LM358. By changing the voltage at the inverting input terminal of the operational amplifier, the high and low levels of the operational amplifier output are controlled, thereby controlling the dead time of the TL494. The overheat protection circuit diagram is shown in Figure 5.
Figure 5
One end of R40 is grounded, and the other end is connected between R5 and R6, forming a differential operational amplifier circuit. When the temperature exceeds 65°C and the temperature control switch is closed, the output of the LM358 operational amplifier is high (greater than 3.3V). When the dead-time voltage of the TL494 is greater than 3.3V, the duty cycle of the output PWM wave drops to 0, and the output is low, preventing the switching transistor from conducting and cutting off the power supply to the motor.
When the controller is working normally, the temperature of the heat sink will not exceed 65°C, and the temperature control switch will always be in the open state. At this time, the dead-time voltage of the TL494 is a bias voltage of 120mV, and the output PWM duty cycle is controlled by the output voltage of the TL494 op-amp.
Battery undervoltage protection
Electric vehicles are powered by lead-acid batteries. The battery capacity is limited, and it needs to be charged in time when the power is low. In order to prevent the electric vehicle battery from over-discharging, protect the battery, and improve the battery's lifespan, undervoltage protection should be set up. The battery undervoltage protection circuit diagram is shown in Figure 6.
When the motor stalls, the battery output current increases rapidly. Since the battery cannot provide such high power, this causes a sharp drop in battery voltage. Based on battery characteristics, a 36V lead-acid battery, under normal operating conditions, will not output voltage lower than 30V. Setting the minimum protection voltage of the battery to 30V protects the battery and prevents the motor from stalling.
The undervoltage protection of the battery is implemented using an LM393 voltage comparator. The 36V battery voltage is divided and compared with a reference voltage. When the battery voltage is lower than 30V, the voltage at the inverting input terminal of the LM393 is lower than the voltage at the non-inverting input terminal, and the output is high. This causes the dead-zone voltage of the TL494 to reach its maximum value, shutting off the pulse output of the TL494 and stopping the motor.
Through long-term experimental operation and vehicle testing, the electric vehicle motor drive system designed in this paper has been verified to have low energy loss, wide application range, and meet the actual needs of electric vehicles. By adjusting the motor excitation and armature, the speed regulation, forward and reverse rotation, and energy feedback of the separately excited DC brushed motor can be achieved. Furthermore, multiple signal communication interfaces are reserved in the design to input key status signals into the onboard ECU, facilitating overall vehicle coordination and upgrades.
