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  • Contents

I Description

This blog introduces a pulse width modulation technology with TL494 as the control core. And it is applied to DC motor control system. The working principle of the system, the realization circuit and the structure and specific application of the PWM control chip are analyzed in detail.

Catalog

I Description

II Working Principle and Realization of Control System

2.1 How System Works

2.2 Selection of PWM Control Chip

2.3 Realization Circuit of System

III Simulation Results

IV Conclusion

FAQ

Ordering & Quantity

II Working Principle and Realization of Control System

2.1 How System Works

The basic design idea of this control system is to use a step-down chopper circuit as shown in Figure 1.

Figure 1. Step-down Chopper Circuit and its Waveform

In the figure, the DC power supply is Ud, and the load is a motor (M). When the switching device (VT) is triggered and turned on, the DC voltage is applied to the motor for a duration of t1. When the field switching device is turned off, the voltage on the load is zero and lasts for t2 time. If we define the duty cycle T=t1+t2, and the duty cycle k=t/T, the waveform diagram and the principle of the DC chopper circuit are as follows:

  • The average value (Uo) of the system output voltage is:

  • The effective value (U) of its output voltage is:

The pulse width modulation (PWM) working mode adopted by this system keeps T unchanged and t1 changes.

The principle block diagram of the control system is shown in Figure 2.

Control System Block DiagramFigure 2. Control System Block Diagram

The power supply in this system makes the motor work through the power drive circuit. The on-off of the power drive circuit is controlled by the PWM control chip. The system samples the motor current feeds it back to the PWM control chip and compares it with the current value of the current comparison circuit. To control the PWM signal output to achieve the purpose of speed regulation. The system also samples the power supply voltage and feeds it back to the PWM control chip. In this way, it can be compared with the voltage value of the voltage comparison circuit to control the PWM signal output and achieve the effect of Undervoltage protection.

We use the common BUCK circuit for the power drive circuit and Power MOSFET for the switch tube. The regulation of its output voltage is realized by controlling the turn-on time of the device. Considering the influence of the inductance of the motor, the output current is relatively stable and the energy consumption is low.

2.2 Selection of PWM Control Chip

In the control circuit of the motor PWM control system, the TL494 chip is selected. TL494 chip has the features of strong anti-interference ability, simple structure, high reliability and low price.

The internal circuit of TL494 (Figure 3) consists of the following parts:

  • The reference voltage generating circuit;
  • Oscillation circuit;
  • Intermittent adjustment circuit;
  • Two error amplifiers;
  • Pulse width modulation comparator;
  • Output circuit;
  • ...

tl494 internal structure

Figure 3. TL494 Internal Structure

Among them:

  • For pins 1 and 2, they are the non-inverting and inverting input terminals of error amplifier 1.
  • For pin 3, it is phase correction and gains control.
  • For pin 4, it is an intermittent period of conditioning, and the cut-off time can be changed from 2% to 100% when a voltage of 0~3.3V is applied to it.
  • For pins 5 and 6, they are used to connect external oscillation resistor RT and oscillation capacitor CT to determine the frequency fosc of the sawtooth wave generated by the oscillator.

Where:

  • The value range of RT and CT: RT=5~100kQ, CT=0.001~0.1μF.
  • Pin 7 is the ground terminal;
  • For pins 8, 9 and 11, 10, they are the collector and emitter of the two final output transistors inside TL494;
  • For 12 feet, it is the power supply terminal;
  • For pin 13, it is the output control terminal. When this pin is grounded, it is a parallel single-ended output mode. When pin 14 is connected, it is a push-pull output mode;
  • For 14 feet, it is the 5V reference voltage output terminal, the maximum output current is 10mA;
  • For 15 and 16 pins, they are the inverting and non-inverting input terminals of the error amplifier 2.

2.3 Realization Circuit of System

The concrete realization circuit of this control system is shown in Fig. 4. The system uses current negative feedback to track the speed of the motor. At the same time, through the negative feedback of the voltage of the power supply, the system has the function of undervoltage protection.

tl494 circuit

Figure 4. System Implementation Circuit

The motor current is detected and fed back to pin 1 of the error amplifier 1 of the TL494, compared with the current reference signal of pin 2 to control the PWM output of TL494. So as to realize the function of regulating the speed of the motor. By adjusting the resistance value of the adjustable resistor (RES1). That is, change the size of the set current reference signal to adjust the duty cycle of the PWM output signal. So as to achieve the purpose of adjusting the motor speed.

By sampling the voltage of the power supply, it is fed back to pin 15 of the internal error amplifier 2 of TI494. Then compare with the voltage reference signal of pin 16 to control the PWM output of TL494. So as to realize the undervoltage protection function of the system.

The system power is driven by Power MOSFET, which has a high input impedance and can be directly driven by a transistor. Pin 13 of TI494 is used to control the output mode. In this system, choose to input this end as low level. At this time, the flip-flops Q1 and Q2 in the TL494 do not work, the two outputs are the same, the frequency is the same as the oscillator frequency, and the maximum duty cycle is 98%.

III Simulation Results

In order to verify the effectiveness of the system, we can simulate the circuit of the above-mentioned DC motor control system based on PWM technology by computer. We set the system's power supply voltage Ucc=12V, DC motor rated parameters: Un=12V, In=15A, 4 poles, armature resistance Ra=0.21Q, moment of inertia J=0.57kg㎡. Through changing the adjustable resistance RES1 of the control circuit to realize the different duty ratio of the system output. When the output duty ratio is 0.2, 0.65/0.8, the voltage and current waveforms of the motor are shown in Figure 5.

tl494 motor voltage and current waveformFigure 5. Motor Voltage and Current Waveform

It can be seen from Figure 5 that when outputting different duty ratios, the voltage and current waveforms of the motor are stable, which is conducive to the long-term stable operation of the motor. This shows that the TL494-based PWM control technology introduced in this article is feasible in practical applications and runs stably.

IV Conclusion

This blog summarizes a PWM control technology based on the TL494 chip. The simulation results show that the system has a simple structure, low energy consumption, and stable operation. And the characteristics of the TL494 chip have been fully utilized so that the system has the advantages of undervoltage protection.


FAQ

  • What is TL494?

TL494 is a PWM controller IC used for power electronics circuits. It comprises of on-chip two error amplifiers an oscillator with adjustable frequency feature, an output flip-flop having pulse steering control, and an output control circuit with feedback.

  • What is the detailed description of TL494?

The TL494 device incorporates all the functions required in the construction of a pulse-width-modulation (PWM) control circuit on a single chip. Designed primarily for power-supply control, this device offers the flexibility to tailor the power-supply control circuitry to a specific application. The TL494 device contains two error amplifiers, an on-chip adjustable oscillator, a dead-time control (DTC) comparator, a pulse-steering control flip-flop, a 5-V, 5%-precision regulator, and output-control circuits. The error amplifiers exhibit a common-mode voltage range from –0.3 V to VCC – 2 V. The dead-time control comparator has a fixed offset that provides approximately 5% dead time. The on-chip oscillator can be bypassed by terminating RT to the reference output and providing a sawtooth input to CT, or it can drive the common circuits in synchronous multiple-rail power supplies. The uncommitted output transistors provide either common-emitter or emitter-follower output capability. The TL494 device provides for push-pull or single-ended output operation, which can be selected through the output-control function. The architecture of this device prohibits the possibility of either output being pulsed twice during push-pull operation.

  • What are TL494 product features?
    • Complete PWM Power-Control Circuitry
    • Uncommitted Outputs for 200-mA Sink or Source Current
    • Output Control Selects Single-Ended or Push-Pull Operation
    • Internal Circuitry Prohibits Double Pulse at Either Output
    • Variable Dead Time Provides Control Over Total Range
  • What is PWM IC?

The TL494 fixed frequency PWM Controller can be used for DC to DC conversion regardless of buck or boost topology. ... This IC feature an output control circuit, a flip flop, a dead time comparator, two different error amplifiers, a 5V reference voltage, an oscillator, and a PWM comparator.

  • How does PWM IC work?

As its name suggests, pulse width modulation speed control works by driving the motor with a series of “ON-OFF” pulses and varying the duty cycle, the fraction of time that the output voltage is “ON” compared to when it is “OFF”, of the pulses while keeping the frequency constant.

  • Which IC is better for a buck converter, TL494 or UC3843?

They mainly differ in type of control…
TL494 => voltage mode control (One loop control) ….
while UC3843 uses current mode control (Nested loop control, with a inner/fast current loop and another outer/slower voltage loop)…
Typically voltage mode are used in multiple output converters with good cross-regulation. Current mode when you want to parallel multiple converters to make a single converter with higher current rating…
TL494 is a very popular IC. If you have simple requirements… TL494 is recommended…

  • How do I properly set the feedback pin on a TL494 SMPS IC?

The feedback pin is the output of both error amplifiers, used in comparing and adjusting the output pulse width to the DC control voltage.
On various circuits I have looked up, the op-amp connected to pins 2 & 3 are used to set the gain of the feedback loop, using 2 resistors with one resistor connecting to 2.5V potential divider on 5V reference voltage. With the other connecting to the output (via suitable isolation)
The gain appears to be set at 101, using a 51k feedback with 510 ohms to the 2.5V reference. It is used to control the gain of the feedback voltage. No literature I have yet found, gives an indication on how this gain be set, except a graph showing an open loop gain of 1000, presumably the gain is set for the best stability, although there will also be a time constant.

  • Why is there no frequency compensation required in TI's TL494 example buck regulator design (operational amplifier, buck phase, shift phase, margin, TL494, electronics)?

It's a fixed frequency PWM controller with internal dead time timer. Frequency compensation is not required. Take a look at the datasheet.

  • How to use TL494?

 

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