Texas Instruments TL494 is a voltage-driven production of PWM control IC, are used in a variety of switching power supply. This article describes it with the corresponding input and output circuit together form a single loop controller.
Switch IC TL494 internal schematic diagram:
1, TL494 pin configuration and function
TL494's internal circuitry from the reference voltage generating circuit, an oscillator, intermittent period of adjustment circuit, two error amplifier, PWM comparator and output circuit. Figure 1 is its pin diagram, which is 1,2 feet in phase I, the error amplifier and the inverting input; 3 feet is the phase correction and gain control; 4 feet for the intermittent period of conditioning, which add to 0 ~ 3.3V voltage When the deadline can pregnant change from 2% to 100% line; 5,6 feet were used for external oscillation resistor and capacitor oscillator; 7 feet to ground; 8,9 and 11,10 feet within two feet were TL494 the final output transistor collector and emitter; 12-pin power supply terminal; 13 pin output control terminal when the ground pin parallel single-ended output, then 14 push-pull output mode when the foot; 14 pin 5V reference voltage output, maximum output current 10mA; 15,16 feet and RP II error amplifier inverting input.
2, the loop controller works
Loop controller block diagram shown in Figure 2. Controlled variables (such as pressure, flow, temperature, etc.) is exchanged through the sensor's signal 0 ~ 5V, as a closed loop feedback signal, through a simple second-order active low pass filter circuit to smooth, remove clutter evacuation to the error amplifier TL494 I, inverting input IN +. Set the input signal is a 5V voltage reference TL494 by a precision multi-turn potentiometer divider, the potentiometer fixed side by a simple second-order low-pass active filter circuit TL494 access to the error amplifier inverting input IN-I, end. Set the signal through the feedback signal and the error amplifier TL494 comparison I zoom in, then control the pulse width, the pulse duty cycle and after changes in the output rectifier and filter circuit constituted by an integrated operational amplifier to smooth the isolation amplifier and amplification process, output one with the pulse width directly proportional to the change in the range of 0 ~ 10V DC voltage. This is the required output voltage control voltage, and use it to control the implementation of the circuit, to adjust the amount to be controlled so that the controlled quantity is always consistent with the set value, the formation of closed single-loop control.
TL494 achieved by a single loop controller of the circuit diagram shown in Figure 3.
2.1 Input Circuit
Two operational amplifiers IC1A, IC1B are connected into a simple second-order low-pass active filter circuits, respectively, set as a feedback signal input and the input signal processing circuit. In circuit design, the two input circuit to take the form of completely symmetrical. The simple second-order low-pass active filter circuit is designed to cut-off frequency fp 4Hz, according to a simple second-order low-pass active filter circuits fp = 0.37f0 (f0 characteristic frequency of the filter) to select C1 and C2 as 1μF, R1 and R2 then be regarded as a 16kΩ. This filter leads the sensor long distance brought the input frequency clutter interference and smoothing fluctuations in the sensor signal itself, so that added to the TL494 pin 1 is the error amplifier inverting input I IN + signal as far as possible to smooth and relatively stable. In the simple second-order low-pass active filter circuit with the error amplifier inverting input I IN + 10kΩ indirectly limit the flow of isolated resistance. TL494 14-pin to 5V output voltage reference, with a 3.3kΩ precision multi-turn potentiometer W1 partial pressure as a set input signal, and processing the sensor feedback signal through the same circuit, into the TL494 pin 2, which is error the inverting input of the amplifier I IN-end. It was found, R19, R20 isolated from these two limiting resistor is necessary. Otherwise, TL494 I error amplifier input potential of the two will interact. In addition, the experimental data also show that, TL494 error amplifier's two input terminals at low voltage linear track is not very good, so there will be two input op amp is taken as 2 magnification to improve the feedback signal and set the signal Tracking linear.
2.2 pulse width modulation circuit
In this controller uses only the error amplifier TL494 I, II, so the error amplifier IN + (16 feet) Ground, IN-(15 feet) high then. TL494 for the protection of the output transistor, the R13 and R10 divider, added nearly 4 feet of intermittent adjustment of the voltage 0.3V. R9, R12 and C5 form the phase correction and gain control network. In experiments, oscillations in the controller and the oscillating capacitor resistor 200kΩ were collected and 0.1μF. Output is and take way from the emitter. 12V single-supply power to take machine.
2.3 Output Circuit
In order to change the pulse width converted square wave signal changes in the size of the DC signal, by switching diode D1, the rectifier filter capacitor C8. R15 as a whole wave filter output load, the C8 is also provided for the period ending pulse discharge circuit, the voltage on C8 and the output pulse width proportional to the TL494. To further smooth the output voltage and improve the load capacity and the output voltage varies between 0 ~ 10V, has joined a second voltage-controlled voltage source of low-pass filter circuit. Device parameters shown in the figure, the maximum DC output voltage is 10V, IC3A 10V output termination voltage regulator diode is to ensure that the case of an accident, so that the output voltage is not greater than 10V.
3, the working process
When the feedback signal is greater than the set value, by TL494 pulse width modulation, the 9-pin and 10 pin parallel output signal of the pulse width decreases, the output signal is then filtered by the rectifier circuit and output circuit isolation and amplification, so that the final DC output voltage control signal corresponding decline. DC control signal through the control circuit by the implementing agencies (such as electric motors, electric control, etc.) to be controlled decline, and then further reduced by the sensor so that the feedback signal to form a single loop closed-loop control. When the feedback signal is less than the set value, the control process is reversed. In addition, the system can be controlled according to the specific circumstances, to adjust the second-order low-pass filter the input capacitor size, the control process timely, accurate and stable. Again, the control process is intuitive and should be added to set the amount of volume and controlled display (instructions) circuit. Remove from two input signals, respectively, through the isolation amplifier and then (as with the composition of the op amp voltage follower) to the header instruction. Meter multi-function digital electronic form can be finished or the direct use of the first full-scale mechanical 5V said.
4, the measured data analysis
Table 1 The data in Table 3 10KΩ load resistor in the output termination of the open-loop conditions by DT9102A digital multimeter measured. Feedback signal and set the signal which were multi-turn potentiometer with a precision 5V reference to the standard partial pressure to simulate, and measure the point from IC1A and that the output of IC1 IC1B 1 foot and 7 feet, 1 output from IC3A feet. All units are volts.