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UC3842 Application in Voltage Feedback Circuit (3 Circuits Comparison)

  • Contents

This article first introduced the three commonly used stable output voltage circuits of UC3842, analyzed their respective advantages and disadvantages, and designed a new voltage feedback circuit on this basis. Experiments prove that this new circuit has a good voltage stabilizing effect.

Catalog

I. Common Voltage Feedback Circuits

1.1 Direct voltage division of the output voltage as the input of the error amplifier

1.2 Auxiliary power supply output voltage division as the input of the error amplifier

1.3 Change the input error voltage of the error amplifier using linear optocoupler

II. Experiments and Results

2.1 Change the gain of error amplifier by using linear optocoupler

2.2 Experimental results

III. Conclusion


I. Common Voltage Feedback Circuits

Generally, the PWM type switching power supply uses 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 cycle of the switching signal is adjusted to achieve the stability of the output voltage. But different voltage feedback circuits have different output voltage stability accuracy. The internal circuit diagram of UC3842 is shown as in Figure 1.

uc3842 internal circuitUC3842 Internal Circuit

 

1.1 Direct voltage division of the output voltage as the input of the error amplifier

As shown in Figure 2, the output voltage Vo is divided by R2 and R4 and then used as a sampling signal and input to the UC3842 pin 2 (the reverse input of the error amplifier). The forward input of the error amplifier is connected to the reference voltage of 2.5V inside the UC3842. When the sampling voltage is less than 2.5V, the voltage difference between the forward and reverse outputs of the error amplifier is amplified by the amplifier to adjust the output voltage, making the duty cycle of the output signal of the UC3842 larger, the output voltage rises, and finally the output voltage is stabilized at the set voltage value.R3 and C1 are connected in parallel to form a current-type feedback.

 

The advantage of this circuit is that the sampling circuit is simple, but the disadvantage is that the input voltage and output voltage must share the same ground and cannot be electrically isolated. This is easy to cause difficulty in power supply wiring, and the power supply works in a high-frequency switching state, which is easy to cause electromagnetic interference, which will inevitably bring difficulties in circuit design, so this method is rarely used.

Figure 2 Sampling diagram for direct voltage division of output voltage

Figure 2 Sampling diagram for direct voltage division of output voltage

 

1.2 Auxiliary power supply output voltage division as the input of the error amplifier

As 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, and the voltage is rectified, filtered and regulated by D2, D3, C15, C14, C13 and R15 to obtain a DC voltage to power the UC3842. At the same time, the voltage is divided by R2 and R4 as a sampling voltage and sent to the UC3842's pin 2. After comparing with the reference voltage, it is amplified by the error amplifier, so that the duty cycle of pin 6 output pulse becomes smaller and the output voltage drops to achieve the purpose of voltage regulation. Similarly, when the output voltage decreases, the duty cycle of the output pulse of pin 6 becomes larger and the output voltage rises, finally stabilizing the output voltage at the set value.

Figure 3 Sampling Diagram for auxiliary power supply division of output voltage

 

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, which is easy to wire.

 

The disadvantage is that the sampling voltage is not obtained directly from the secondary winding, and the voltage stabilization effect is not that positive. It was found in the experiment that the voltage stabilization basically cannot be achieved when the load of the power supply varies greatly. The circuit is suitable for the case of a fixed load.

 

1.3 Change the input error voltage of the error amplifier using linear optocoupler

As shown in the figure 4, the voltage sampling circuit of the switching power supply has two circuits: one is the voltage of the auxiliary winding through D1, D2, C1, C2, C3, R9 rectification, filtering and voltage regulation to obtain 16V DC voltage to the UC3842 power supply, in addition, the voltage through R2 and R4 voltage division to obtain a sampling voltage, the sampling voltage mainly reflects the change of the DC bus voltage.

 

The other is the photocoupler, three-terminal adjustable regulator Z and R4, R5, R6, R7, R8 voltage sampling circuit, the road voltage reflects the change in the output voltage; when the output voltage rises, the reference voltage of the input Z also rises after the resistor R7 and R8 divides the voltage, the regulator voltage value of the regulator rises, the current flowing through the light-emitting diode in the photocoupler decreases, the current flowing through the phototransistor in the photocoupler also correspondingly reduced, the error amplifier input feedback voltage is reduced, resulting in the UC3842 pin 6 output drive signal duty cycle becomes smaller, so the output voltage drops, to achieve the purpose of voltage regulation.

Figure 4 Auxiliary power sampling and optocoupler sampling synthesis

Figure 4 Auxiliary power sampling and optocoupler sampling synthesis

 

Since the circuit uses an opto-coupler, it achieves the isolation of output and input, the isolation of weak and strong power, reducing electromagnetic interference, thus its anti-interference ability is improved, and it is to the output voltage sampling, with good voltage regulation performance.

 

The disadvantage of the circuit is the increase in external components, which increases the difficulty of wiring and adds the cost of the power supply.

 

II. Experiments and Results

2.1 Change the gain of error amplifier by using linear optocoupler

 

As shown in Figure 5, the voltage sampling and feedback circuit consists of R2, R5, R6, R7, R8, C1, opto-coupler, and three-terminal adjustable regulator Z. When the output voltage rises, the output voltage is divided by R7 and R8 to obtain the sampling voltage (i.e., Z's reference voltage) also rises, Z's regulator value also rises, the current flowing through the light-emitting diode in the photocoupler decreases, resulting in a reduction in the current flowing through the phototransistor.

 

This is equivalent to C1 parallel variable resistor resistance value becomes large (the equivalent resistance value by the flow of light-emitting diode current control), the gain of the error amplifier becomes large, resulting in the UC3842 pin 6 output of the drive signal duty cycle becomes small, the output voltage drops to achieve the purpose of voltage regulation.

 

When the output voltage decreases, the gain of the error amplifier becomes smaller and the duty cycle of the output switching signal becomes larger, which eventually stabilizes the output voltage at the set value. Because the voltage feedback input pin 2 of UC3842 is grounded, so the input error of the error amplifier is always fixed, and what changes is the gain of the error amplifier (the phototransistor in the linear optocoupler can be regarded as a variable resistor), and its equivalent circuit diagram is shown in Figure 6.

Figure 5 Varying the gain of the error amplifier using an optocoupler

Figure 5 Varying the gain of the error amplifier using an optocoupler

Figure 6 Equivalent circuit for changing the gain of the error amplifier

Figure 6 Equivalent circuit for changing the gain of the error amplifier

 

The circuit changes the output of the error amplifier by adjusting the gain of the error amplifier instead of adjusting the input error of the error amplifier, thus changing the duty cycle of the switching signal. This topology not only has fewer external components, but also employs a three-terminal adjustable regulator in the voltage sampling circuit, making the output voltage essentially unchanged when the load changes significantly. The experiment proves that the circuit has a very good voltage regulation effect compared with the above three feedback circuits.

 

2.2 Experimental results

This new voltage feedback circuit using a linear optocoupler to change the gain of the error amplifier is used in a 48V/12V single-ended flyback DC/DC switching power supply (maximum output current of 5A), which shows that the output voltage of the power supply is stable and has a strong load carrying capability. Figure 7(a)-(h) gives the output voltage and drive waveforms when the load is 100Ω, 25Ω, 10Ω, and 3Ω, respectively. From the waveforms, it can be seen that when the load current gradually increases, the duty cycle of the drive signal increases accordingly, but the output voltage is always stable at 12.16V.

Figure 7 Output voltage and drive waveform at different loads

Figure 7 Output voltage and drive waveform at different loads

 

III. Conclusion

In the single-ended isolated PWM power supply, the current-mode pulse width modulator UC3842 has a wide range of applications. This article summarizes the voltage feedback circuit design that uses a linear optocoupler to change the gain of the UC3842 error amplifier. And it is proved by experiments that the new voltage feedback circuit has high voltage regulation accuracy and strong load adaptability.

 

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