description:
Usually, the PWM type switching power supply uses the sampling of the output voltage as the feedback voltage of the PWM controller. After the feedback voltage is passed through the error amplifier inside the PWM controller, the duty ratio of the switching signal is adjusted to stabilize the output voltage. However, the stability of the output voltage is different for different voltage feedback circuits.
1 Overview
In this paper, the three stable output voltage circuits commonly used in the current-type pulse width controller UC3842 (internal circuit diagram shown in Figure 1) are introduced, and their respective advantages and disadvantages are analyzed. Based on this, a new voltage feedback is designed. The circuit proves that this new circuit has a good voltage regulation effect.
2 UC3842 commonly used voltage feedback circuit
2.1 Output voltage direct voltage division as the input of the error amplifier
As shown in Figure 2, the output voltage Vo is divided by R2 and R4 and used as a sampling signal. Input UC3842 pin 2 (inverting input of the error amplifier). The positive input of the error amplifier is connected to the 2.5V reference inside the UC3842. When the sampling voltage is less than 2.5V, the voltage difference between the forward and reverse output terminals of the error amplifier is amplified by the amplifier, and the output voltage is adjusted, so that the duty ratio of the output signal of the UC3842 becomes larger, the output voltage rises, and finally the output is made. The voltage is stable at the set voltage value. R3 is connected in parallel with C1 to form current-type feedback.
The advantage of this circuit is that the sampling circuit is simple, and the disadvantage is that the input voltage and the output voltage must be common ground and cannot be electrically isolated. It is bound to cause difficulties in power supply wiring, and the power supply operates in a high-frequency switching state, which easily causes electromagnetic interference, which inevitably brings difficulties in circuit design, so this method is rarely used.
2.2 Auxiliary power supply output voltage divider as the input of the error amplifier is shown in Figure 3. When the output voltage rises, the induced voltage generated on the auxiliary winding of the single-ended flyback transformer T also rises. This voltage passes through D2, D3. The rectified, filtered, and regulated networks consisting of C15, C14, C13, and R15 receive a DC voltage to power the UC3842. At the same time, the voltage is divided by R2 and R4 and used as the sampling voltage. It is sent to pin 2 of UC3842. After comparison with the reference voltage, it is amplified by the error amplifier, so that the duty ratio of the output pulse of the pin 6 becomes smaller, and the output voltage drops. The purpose of voltage regulation. Similarly, when the output voltage is lowered, the duty ratio of the output pulse of the pin 6 is increased, the output voltage is increased, and finally the output voltage is stabilized at the set value. The advantage of this circuit is that the sampling circuit is simple, and there is no electrical path between the secondary winding, the primary winding and the auxiliary winding, and the wiring is easy. The disadvantage is that the sampling voltage is not directly obtained from the secondary winding, and the voltage stabilization effect is not good. It is found in the experiment that when the load of the power supply changes greatly, the voltage regulation cannot be basically achieved. This circuit is suitable for situations with certain fixed loads.
2.3 Using linear optocoupler to change the input error voltage of the error amplifier
As shown in Figure 4, the voltage sampling circuit of the switching power supply has two paths: First, the voltage of the auxiliary winding is rectified, filtered, and stabilized by D1, D2, C1, C2, C3, and R9 to obtain a DC voltage of 16V. The UC3842 is powered. In addition, the voltage is divided by R2 and R4 to obtain a sampling voltage. The sampling voltage of the circuit mainly reflects the change of the DC bus voltage. The other is the photocoupler and the three-terminal adjustable voltage regulator Z and R4. R5, R6, R7, R8 voltage sampling circuit, the voltage reflects the change of the output voltage; when the output voltage rises, the reference voltage of the input Z is also increased after the voltage is divided by the resistors R7 and R8, the voltage regulator The regulation value increases, the current flowing through the LED in the optocoupler decreases, the current flowing through the phototransistor in the optocoupler decreases accordingly, and the input feedback voltage of the error amplifier decreases, causing the UC3842 pin 6 to output the drive signal. The duty cycle becomes smaller, and the output voltage drops, achieving the purpose of voltage regulation.
Because of the optocoupler, the circuit achieves isolation of output and input, isolation of weak current and strong electricity, reduces electromagnetic interference, strong anti-interference ability, and is sampling the output voltage, which has good voltage regulation performance. The disadvantage is that the number of external components increases, which increases the difficulty of wiring and increases the cost of the power supply.
3 Linear optocoupler changes the error amplifier gain voltage feedback circuit and experimental results
3.1 Using a linear optocoupler to change the gain of the error amplifier
As shown in FIG. 5, the voltage sampling and feedback circuit is composed of R2, R5, R6, R7, R8, C1, a photocoupler, and a three-terminal adjustable voltage regulator Z. When the output voltage rises, the sampling voltage obtained by dividing the output voltage by R7 and R8 (ie, the reference voltage of Z) also rises, and the voltage regulation value of Z also rises, and the current flowing through the light-emitting diode in the optocoupler decreases. Small, the current flowing through the phototransistor is reduced, and the resistance of the variable resistor connected in parallel with C1 becomes larger (the resistance of the equivalent resistor is controlled by the current flowing through the LED), and the gain of the error amplifier becomes larger. As a result, the duty ratio of the output driver signal of the UC3842 pin 6 is reduced, and the output voltage is lowered to achieve the purpose of voltage regulation. When the output voltage is lowered, the gain of the error amplifier becomes smaller, the duty ratio of the output switching signal becomes larger, and finally the output voltage is stabilized at the set value. Because the voltage feedback input pin 2 of the UC3842 is grounded, the input error of the error amplifier is always fixed, and the gain of the error amplifier is changed (the phototransistor in the linear optocoupler can be regarded as a variable resistor). The equivalent circuit diagram is shown in Figure 6.
The circuit changes the duty cycle of the switching signal by adjusting the gain of the error amplifier instead of adjusting the input error of the error amplifier to change the output of the error amplifier. This topology structure not only has fewer external components, but also uses a three-terminal adjustable voltage regulator in the voltage sampling circuit, so that the output voltage does not change substantially when the output voltage changes greatly. The experiment proves that the circuit has a good voltage stabilization effect compared with the above three feedback circuits.
3.2 Experimental results
This new voltage feedback circuit that uses a linear optocoupler to change the gain of the error amplifier for a 48V/12V single-ended flyback DC/DC switching power supply (maximum output current of 5A) shows that the output voltage of the power supply is stable. Strong load capacity. Figure 7(a)-(h) shows the output voltage and drive waveform when the load is 100Ω, 25Ω, 10Ω, 3Ω, respectively. As can be seen from the waveform, when the load current is gradually increased, the duty of the drive signal The ratio increases accordingly, but the output voltage is always stable at 12.16V.
4 Conclusion
In the single-ended isolated PWM type power supply, the current-mode pulse width modulator UC3842 has a wide range of applications. Based on the analysis of three commonly used voltage feedback circuits, this paper designs a new linear optocoupler to change UC3842. Voltage feedback circuit for error amplifier gain. Experiments show that the new voltage feedback circuit makes the voltage regulation high and the load adaptability.
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