TL494 is used in a forward power supply to dissipate infinite energy

This article is based on the dual-tube forward low-power power supply of the TL494 driver chip. Let's first talk about the forward conversion. The forward conversion has a simple topology and a wide range of step-up and step-down, so it is widely used in small and medium power supply conversion occasions. Forward conversion and TL494 are a perfect match. They have the same characteristics and are the protagonist I will introduce today - TL494. The driver chip TL494 is a low-cost, strong driving capability, controllable dead time, and has two error amplifiers. When the load changes, the voltage and current feedback PI adjustment is performed, which further enhances the stability of the power supply.

1 Two-Tube Forward Converter Circuit

The dual-switch forward converter circuit is shown in Figure 1.

This main circuit topology has three advantages:

(1) It overcomes the disadvantage of high switch voltage stress in single-ended forward converter.

(2) There is no need to use special flux resetting technology, avoid the complex demagnetization winding design and reduce the size of the high-frequency transformer, making the circuit simpler. There is no need to add RCD for remagnetization clamping, and the power supply can be fed, which improves efficiency.

(3) Compared with the full-bridge converter and the half-bridge converter, each bridge arm is composed of a diode and a switch tube in series. There is no problem of bridge arm direct conduction, and the reliability is high.

2 Characteristics of PWM driver chip TL494

TL494 is a typical fixed frequency pulse width modulation control integrated circuit, which contains all the functions required to control the switching power supply, and can be used as a control system for dual-tube forward, half-bridge, and full-bridge switching power supplies. Its operating frequency is 1~300 kHz, the input voltage is 40V, and the output current is 200mA. Its internal schematic diagram is shown in Figure 2.

TL494 has a linear sawtooth oscillator with an oscillation frequency of f = 1.1/(RC), which can be adjusted by two external components R and C (connected to pins 6 and 5 respectively). TL494 has two error amplifiers, which can form a voltage feedback regulator and a current feedback regulator, respectively controlling the stability of the output voltage and the protection of the output overcurrent; a 5V 1% voltage reference (pin 14) is set, and its dead time adjustment output can be single-ended or double-ended, generally used as a double-ended output type of pulse width modulation PWM. As a PWM control chip, TL494 has the following characteristics:

(1) The control signal is input from the outside of the IC, one to the dead time control terminal and one to the two error amplifier input terminals, also known as the PWM comparator input terminals.

(2) The dead time control comparator has an effective input offset voltage of 120 mV, which limits the minimum output dead time to approximately 4% of the sawtooth wave cycle time. Setting a fixed voltage (range 0~0.3 V) at the dead time control terminal can generate additional dead time on the output pulse.

(3) With OUTPUT CONTROL 13 connected to ground, this will give a maximum duty cycle of 96 % of the given output, while with OUTPUT CONTROL 13 connected to the reference level, the duty cycle will be 48 % of the given output.

(4) Pulse width modulation comparator and error amplifier can adjust the output pulse width.

Figure 4 is a sampling of the voltage output on the DC side, where the choice of optocoupler is crucial. We use TLP521, which has two optocouplers integrated in one chip. Their transmission characteristics are almost completely consistent. Based on the principle of equal current, high-precision DC high-voltage isolation sampling can be achieved.

TL494 has a linear sawtooth oscillator with an oscillation frequency of f = 1.1/(RC), which can be adjusted by two external components R and C (connected to pins 6 and 5 respectively). TL494 has two error amplifiers, which can form a voltage feedback regulator and a current feedback regulator, respectively controlling the stability of the output voltage and the protection of the output overcurrent; a 5V 1% voltage reference (pin 14) is set, and its dead time adjustment output can be single-ended or double-ended, generally used as a double-ended output type of pulse width modulation PWM. As a PWM control chip, TL494 has the following characteristics:

(1) The control signal is input from the outside of the IC, one to the dead time control terminal and one to the two error amplifier input terminals, also known as the PWM comparator input terminals.

(2) The dead time control comparator has an effective input offset voltage of 120 mV, which limits the minimum output dead time to approximately 4% of the sawtooth wave cycle time. Setting a fixed voltage (range 0~0.3 V) at the dead time control terminal can generate additional dead time on the output pulse.

(3) With OUTPUT CONTROL 13 connected to ground, this will give a maximum duty cycle of 96 % of the given output, while with OUTPUT CONTROL 13 connected to the reference level, the duty cycle will be 48 % of the given output.

(4) Pulse width modulation comparator and error amplifier can adjust the output pulse width.

Figure 4 is a sampling of the voltage output on the DC side, where the choice of optocoupler is crucial. We use TLP521, which has two optocouplers integrated in one chip. Their transmission characteristics are almost completely consistent. Based on the principle of equal current, high-precision DC high-voltage isolation sampling can be achieved.

From the circuit diagram, we can see the input-output ratio:

When the feedback voltage at pin 3 changes from 0.5V to 3.5V, the output pulse width decreases from the maximum on-time determined by the dead-time control input to zero.

3 Power supply circuit

3.1 Main power circuit

As can be seen from Figure 3, the circuit structure is simple and easy to implement, and a Hall sensor is added to the MOSFET bridge arm to ensure the requirements of the output feedback current loop. In order to increase the versatility of the circuit, the designed circuit board adds the function of dual-channel output. As long as the design of the transformer is changed, multiple outputs can be completed. When the two main power switch tubes are cut off, the voltage polarity of the primary winding is opposite, so that the two diodes of the other bridge arm are turned on, and the voltage is clamped at the input voltage value. Therefore, the voltage borne by the switch tube is the same as the input voltage. When the maximum input voltage is lower than 350 V, the switch tube only needs to select a withstand voltage value of 450 V. Here we choose N-channel MOSFET, IRF830 (4.5 A/500 V).

3.2 DC side voltage sampling

As long as the parameter value of the resistor is selected reasonably, the output voltage on the high-voltage side can be reduced to the required sampling voltage value.

3.3 Current sampling of the main circuit switch tube

In Figure 5, 4R1 is connected to the Hall sensor on the main circuit, which effectively avoids the phenomenon that the power switch tube of the main circuit may be burned out due to overcurrent of the primary side current of the transformer. To this end, the pulse current flowing through the MOSFET switch tube must be sampled. When overcurrent occurs, the system should be able to respond quickly and take corresponding protective measures. The pulse current flowing through the MOSFET is converted into a voltage signal by the open-loop Hall current sensor, and then the required current sampling value is obtained through a simple RC filter and a common-mode proportional amplifier.

3.4 Main control circuit

The main control chip circuit is shown in Figure 6. The 13th pin of TL494 is connected to a high level and operates in push-pull output mode. The 10th pin is used as the drive signal output interface, and the drive current can reach 500 mA. The 4th pin peripheral circuit is the soft start part. Since the amplifier composed of the 15th, 16th, and 3rd pins of the TL494 internal amplifier constitutes an overcurrent protection circuit, once the current overcurrent is detected, the 3rd pin outputs a high level to close the amplifier composed of the 1st, 2nd, and 3rd pins. At the same time, the PWM output duty cycle is reduced to ensure the safety of the main circuit switch tube.

The voltage closed loop is formed by an amplifier inside the LM324 of the PI adjustment part of the feedback voltage. The internal amplifier formed by the 1, 2, and 3 pins of the TL494 forms a current closed loop. When the output voltage is high, after passing through the voltage closed loop circuit, ULOOP becomes smaller, after passing through the current closed loop, the FB port voltage becomes larger, the output PWM pulse width becomes smaller, and the output voltage is lowered. When the primary current of the transformer increases, after passing through the current closed loop, the FB port voltage becomes larger, the output PWM pulse width becomes smaller, and the current value decreases. It can be seen that the dual-loop system can operate stably.

3.5 MOSFET drive circuit

The two MOSFET switches in the main circuit are required to be turned on and off at the same time. The control signal from the main control chip TL494 needs to be divided into two to drive the MOSFET. The drive signal passes through the push-pull circuit and then through the pulse transformer to easily obtain a pair of control signals with the same phase.

4 Test waveform

Input 110 V AC voltage at the power supply end through the voltage regulator, so that the system can work stably under 30 V, 1 A load. Observe the drive signal waveform output by the TL494 power chip, the MOSFET switch tube Ugs, Uds, the waveform of the load during normal operation, and the sudden loading and sudden load drop conditions. The test waveform is shown in Figure 7.

5 Conclusion

The most important parts of a switching power supply are the DC-DC converter and the control circuit. This paper tests the prototype to prove that the circuit is practical, reliable and stable. Of course, there is still room for improvement in technical details, and further research will be conducted on PFC and soft switching design in the future.