Abstract : This paper introduces a DC motor control system based on the TL494 and employing PWM technology. The PWM speed control principle, interface circuit design, and H-bridge power drive principle and circuit design of the DC motor are detailed. Experiments show that the TL494-based H-bridge DC motor control system simplifies the circuit structure, provides strong driving capability, low power consumption, convenient control, and stable performance.
Keywords : TL494 PWM H-bridge DC motor
Due to their excellent starting, braking, and speed regulation performance, DC motors have been widely used in various fields such as industry and aerospace. With the development of power electronics technology, pulse width modulation (PWM) speed regulation technology has become a commonly used speed regulation method for DC motors, featuring high speed regulation accuracy, fast response speed, wide speed regulation range, and low power consumption. Power drive circuits using H-bridge circuits as drivers can easily achieve four-quadrant operation of DC motors, including forward rotation, forward braking, reverse rotation, and reverse braking, and are widely used in modern DC motor servo systems.
1. Principle of DC Motor PWM Speed Control
As is well known, the formula for the speed of a DC motor is [1] :
n=(U-IR)/Kφ
Where U is the armature terminal voltage, I is the armature current, R is the total resistance of the armature circuit, φ is the magnetic flux per pole, and K is the motor structural parameter.
DC motor speed control can be divided into excitation control and armature voltage control. Excitation control is rarely used; armature voltage control is employed in most applications. With advancements in power electronics technology, armature voltage can be altered through various methods, among which pulse width modulation (PWM) is a commonly used speed control method. This method adjusts the DC motor's armature voltage U by changing the ratio of the motor's armature voltage on-time to the energizing cycle (i.e., the duty cycle), thereby controlling the motor speed.
The core component of PWM is the voltage-pulse-width converter, which modulates the pulse width according to the control command signal so as to use the pulse signal with the width changing with the command to control the conduction time of the high-power transistor and realize the control of the voltage across the armature winding.
The voltage-to-pulse-width converter (VPWM) structure, as shown in Figure 1, consists of a triangular wave generator, an adder, and a comparator. The triangular wave generator produces a triangular wave UT at a specific frequency. This triangular wave is added to the input command signal UI by the adder to generate the signal UI + UT , which is then fed into the comparator. The comparator is an operational amplifier operating in open-loop mode, possessing extremely high open-loop gain and limiting switching characteristics. Even a slight change in the signal difference between the two input terminals will cause the comparator to output a corresponding switching signal. Generally, the negative input terminal of the comparator is grounded, and the signal UI + UT is input from the positive terminal. When UI + UT > 0, the comparator outputs a full-amplitude positive level; when UI + UT < 0, the comparator outputs a full-amplitude negative level.
The modulation process of the signal waveform by the voltage-to-pulse-width converter is shown in Figure 2. Due to the limiting characteristic of the comparator, the amplitude of the output signal US remains unchanged, but the pulse width changes with the change of UI . The frequency of US is determined by the frequency of the triangular wave.
When the command signal UI = 0, the output signal US is a rectangular pulse with equal width for both positive and negative pulses.
When UI > 0, the positive pulse width of US is greater than the negative pulse width.
When UI < 0, the positive pulse width of US is smaller than the negative pulse width.
When UI ≥ UTPP / 2 ( UTPP is the peak value of the triangular wave), US is a positive DC signal; when UI ≤ UTPP / 2, US is a negative DC signal.
2. Overall Flowchart of DC Motor Drive Control
The DC motor drive control circuit is divided into control signal circuit, pulse width modulation circuit, drive signal amplification circuit, H-bridge power drive circuit, etc. The overall control flow is shown in Figure 3.
As shown in Figure 3, the microcontroller first sends out motor logic control signals, mainly including the motor direction signal Dir, the motor speed control signal PWM, and the motor braking signal Brake. Then, the TL494 performs pulse width modulation, and its output signal drives the H-bridge power circuit to drive the DC motor. The H-bridge is composed of four high-power enhancement-mode MOSFETs, and its function is to change the direction of the motor and amplify the drive signal.
3. TL494 Pulse Width Modulation Circuit
3.1 Functions of each pin of TL494.
In the circuit implementing PWM control of the motor, this system uses the TL494 chip, whose internal circuitry consists of a reference voltage generation circuit, an oscillation circuit, an interval adjustment circuit, two error amplifiers, a pulse width modulation comparator, and an output circuit, etc. It has a total of 16 pins, and its functional structure is shown in Figure 4.
The TL494 chip is widely used in single-ended forward dual-transistor, half-bridge, and full-bridge switching power supplies. Its on-chip resources include [2]:
It integrates all pulse width modulation circuits.
The chip has a built-in linear sawtooth wave oscillator, and only two external oscillation components (a resistor and a capacitor).
Built-in error amplifier.
Internal stop 5V reference voltage source.
Dead time can be adjusted.
The built-in power transistor can provide a drive capability of 500mA.
It offers two output modes: push or pull.
3.2 Brief Description of Working Principle
The TL494 is a fixed-frequency pulse width modulation circuit with a built-in linear sawtooth wave oscillator. The oscillation frequency can be adjusted by an external resistor and a capacitor, and its oscillation frequency is as follows:
The width of the output pulse is controlled by comparing the positive sawtooth wave voltage across capacitor CT with two other control signals. Power output transistors Q1 and Q2 are controlled by NOR gates. The bistable trigger is activated only when the clock signal is low, meaning it is activated only when the sawtooth wave voltage is greater than the control signal. As the control signal increases, the width of the output pulse decreases.
The control signal is input externally to the integrated circuit, with one path to the dead-time comparator and the other to the input of the error amplifier. The dead-time comparator has an input compensation voltage of 120mV, which limits the minimum output dead time to approximately 4% of the sawtooth wave period. When the output is grounded, the maximum output duty cycle is 96%, while when the output is connected to a reference level, the duty cycle is 48%. Connecting a fixed voltage (between 0 and 3.3V) to the dead-time control input generates an additional dead time on the output pulse.
This chip features strong anti-interference capabilities, simple structure, high reliability, and low price.
3.3 Circuit Design Based on TL494 Push-Pull Output
The specific implementation circuit of the control system is shown in Figure 5. The system power drive uses a MOSFET, which has a high input impedance and can be directly driven by a transistor. Pin 13 of the TL494 is used to control the output mode. In this system, when this pin is set to low, the internal flip-flops Q1 and Q2 of the TL494 are inactive, the two outputs are identical, their frequency is the same as the oscillator frequency, and the maximum duty cycle is 98%.
4. H-bridge power drive principle and circuit design
After pulse width modulation by the TL494, the drive signal is used in DC motor control, where an H-bridge circuit is commonly used as the power drive circuit for the driver. This drive circuit can easily achieve four-quadrant operation of the DC motor, corresponding to forward rotation, forward braking, reverse rotation, and reverse braking. Since power MOSFETs are voltage-controlled components, they have characteristics such as high input impedance, fast switching speed, and no secondary breakdown, meeting the requirements of high-speed switching. Therefore, power MOSFETs are commonly used to form the arms of the H-bridge circuit. The four power MOSFETs in the H-bridge circuit are of N-channel and P-channel types, respectively. P-channel power MOSFETs are generally not used for driving the motor in the lower arm; instead, the upper and lower arms use two P-channel power MOSFETs and two N-channel power MOSFETs, respectively. The circuit diagram is shown in Figure 6.
In the diagram, VCC is the motor power supply voltage. A small capacitor is connected in parallel at the output to reduce voltage spikes generated by the inductive motor. The four diodes are freewheeling diodes, providing a freewheeling path for the coil windings. When the motor is running normally, the drive current flows through the main switch. When the motor is braking, it operates in a generating state, and the rotor current must flow through the freewheeling diodes; otherwise, the motor will overheat and may even burn out. US is the output of the TL494, and -US can be obtained by inverting US. When US > 0, VT1 and VT4 are on; when US < 0, VT2 and VT3 are on.
Depending on the different control commands, the power amplifier circuit and the DC servo motor it drives can have the following four operating states:
1) When UI=0, the positive and negative pulse widths of US are equal, the DC component is zero, the conduction time of VT1 and VT4 is equal to the conduction time of VT2 and VT3, the average current through the armature winding is zero, and the motor does not rotate.
2) When UI > 0, the positive pulse width of US is greater than the negative pulse width, the DC component is greater than zero, the conduction time of VT1 and VT4 is greater than the conduction time of VT2 and VT3, the average current through the armature winding is greater than zero, the motor rotates in the forward direction, and the speed increases with UI.
3) When UI < 0, the DC component of US is less than zero, the conduction time of VT1 and VT4 is less than the conduction time of VT2 and VT3, the average current through the armature winding is less than zero, the motor reverses, and the reverse speed increases as UI decreases.
4) When UI≤UTPP/2 or UI≤-UTPP/2, US is a positive or negative DC signal, VT1 and VT4 or VT2 and VT3 are always on, and the motor rotates forward or reverse at the highest speed.
Conclusion
The DC motor speed control system described in this article uses the TL494 as its core, forming an H-bridge bipolar PWM DC motor speed control system. This system effectively controls the speed of the DC motor and offers advantages such as high precision, fast response, and good stability. In practical applications, the TL494, used for PWM speed control of DC motors, not only demonstrates excellent performance but is also economical and reliable, thus possessing significant practical value.
