A brief discussion on how to design the voltage feedback circuit in the current-mode switching power supply

Abstract: In traditional voltage-type control, there is only one loop, and the dynamic performance is poor. When the input voltage is disturbed, the duty cycle changes slowly through the voltage loop feedback. Therefore, the voltage-type control mode is not ideal when the transient error of the output voltage is required to be small. To solve this problem, the current-type control mode can be used. The following will introduce the design of the voltage feedback circuit in the current-type switching power supply.

　　1 Circuit category and main function description

　　The circuit shown in the figure below is a voltage feedback circuit. When the output voltage changes, it is fed back to the control chip through this feedback circuit, thereby adjusting the output voltage and making the output voltage stable. The circuit is as follows:

　　Voltage feedback circuit

　　1.2 Working Principle Analysis

　　When the output voltage changes, the voltage at the inverting input of U15 changes through the voltage divider R27 and R28. The voltage is compared with the fixed voltage at the non-inverting input of U15, and the output voltage is amplified by the operational amplifier. The current at the light-emitting diode of the optocoupler changes, which is then transmitted to the output of the optocoupler's transistor and input to the control chip. The control chip then adjusts the output voltage to achieve output voltage stability.

　　2 Introduction to UC3842

　　Figure 1 is the internal structure block diagram of the UC3842 PWM controller. Its internal reference circuit generates a +5V reference voltage as the internal power supply of UC3842. After attenuation, the 2.5V voltage is used as the reference of the error amplifier and can be used as the power supply of the circuit to output 5V/50mA. The oscillator generates square wave oscillations. The oscillation frequency depends on the external timing element. The resistor R connected between pins 4 and 8 and the capacitor C connected between pins 4 and ground jointly determine the oscillation frequency of the oscillator, f=1.8/RC. The feedback voltage is connected from pin 2 to the inverting end of the error amplifier. Pin 1 is connected to an external RC network to change the closed-loop gain and frequency characteristics of the error amplifier. The square wave that drives the switch tube at pin 6 is the totem pole output. Pin 3 is the current detection terminal, which is used to detect the current of the switch tube. When the voltage at pin 3 is ≥1V, UC3842 turns off the output pulse to protect the switch tube from overcurrent damage. The UC3842 PWM controller is equipped with an undervoltage lockout circuit with a turn-on threshold of 16V and a turn-off threshold of 10V. Because of this, it can effectively prevent the circuit from oscillating when operating near the threshold voltage.

　　1.1 UC3842 has the following features:

　　1. Small number of pins, simple peripheral circuit and low price;

　　2. The voltage regulation rate is very good;

　　3. The load regulation rate is significantly improved;

　　4. Good frequency response characteristics and large stability range;

　　5. It has over-current limiting, over-voltage protection and under-voltage lockout functions.

　　1.2 UC3842 has good linear regulation

　　Because the change of input voltage Vi is immediately reflected as the change of inductor current, it can change the output pulse width in the comparator without passing through any error amplifier. Adding a level of output voltage Vo to the control of the error amplifier can make the linear regulation rate better; it can significantly improve the load regulation rate, because the error amplifier can be used specifically to control the output voltage change caused by load changes, especially to greatly reduce the voltage increase at light load. The external circuit compensation network of the error amplifier is simplified, the stability is improved and the frequency response is improved, with a larger gain-bandwidth product. The current limiting circuit is simplified. Since the peak inductor current is induced on the resistor, a pulse-by-pulse limiting circuit can be naturally formed. As long as the level on Rs reaches 1V, the PWM will be immediately turned off, and this peak inductor current detection technology can sensitively limit the maximum output current.

　　3 UC3842 commonly used voltage feedback circuit

　　3.1 Output voltage is directly divided as the input of the error amplifier

　　The output voltage Vo is divided by two resistors as a sampling signal and input into UC3842 pin 2 (the reverse input terminal of the error amplifier). See Figure 2.

　　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 will inevitably cause difficulties in power wiring, and the power supply works in a high-frequency switching state, which is easy to cause electromagnetic interference and inevitably brings difficulties to circuit design, so this method is rarely used.

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

　　The induced voltage generated on the auxiliary winding of the single-ended flyback transformer T increases as the output voltage increases. After the voltage is rectified, filtered and stabilized, it is converted into a DC voltage to power UC3842. At the same time, the voltage is divided by two resistors and used as a sampling voltage to be sent to pin 2 of UC3842.

　　When UC3842 starts, if the feedback winding cannot provide enough UF, the circuit will start continuously and hiccup will occur. In addition, according to experience, if UF is greater than 17.5V, it will also cause UC3842 to work abnormally, resulting in a smaller output pulse duty cycle and a lower output voltage. Therefore, the selection of the number of turns of the feedback winding and its winding are very important. Generally, it can be designed according to 13~15V, so that when UC3842 works normally, the voltage of pin 7 is maintained at about 13V.

　　The advantage of this circuit is that the sampling circuit is simple, there is no electrical path between the secondary winding, the primary winding and the auxiliary winding, and it is easy to wire. 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 stabilization can basically not be achieved. This circuit is suitable for a certain fixed load.

　　3.3 Using Linear Optocoupler to Change the Input Error Voltage of the Error Amplifier

　　As shown in FIG3 , the voltage sampling circuit of the switching power supply has two paths: one is that the voltage of the auxiliary winding is rectified, filtered and stabilized by D1, D2, C1, C2, C3 and R9 to obtain a 16V DC voltage to power UC3842. In addition, the voltage is divided by R2 and R4 to obtain a sampling voltage. The sampling voltage mainly reflects the change of the DC bus voltage. The other is a voltage sampling circuit composed of a photoelectric coupler, a three-terminal adjustable voltage regulator Z and R4, R5, R6, R7 and R8. The voltage of this path reflects the change of the output voltage. When the output voltage increases, the reference voltage of the input Z also increases after the voltage is divided by resistors R7 and R8, the voltage value of the voltage regulator increases, the current flowing through the light-emitting diode in the optocoupler decreases, the current flowing through the phototransistor in the optocoupler also decreases accordingly, and the input feedback voltage of the error amplifier decreases, resulting in a smaller duty cycle of the output drive signal of UC3842 pin 6, so that the output voltage decreases, achieving the purpose of voltage regulation.

　　Because this circuit uses a photocoupler, it can achieve isolation between output and input, isolation between weak current and strong current, reduce electromagnetic interference, have strong anti-interference ability, and sample the output voltage, so it has good voltage stabilization performance. The disadvantage is that the number of external components increases, which increases the difficulty of wiring and the cost of power supply.

　　3.4 Circuit using optocoupler and voltage reference for feedback control

　　In order to meet the power supply requirements when the load changes greatly. To improve the stability of the output voltage, a circuit is designed to sample from the output end of the secondary winding for feedback control. The circuit is shown in Figure 4: The voltage sampling and feedback circuit is composed of an optocoupler PC8I7, a TL431 and a resistor-capacitor network connected to it. The control principle is as follows: The output voltage is divided by RIJ and R? to obtain a sampling voltage, which is compared with the 2.5 V reference voltage provided by TL431. When the output voltage is normal (5 V), the sampling voltage is equal to the 2.5V reference voltage provided by TL431, and the K pole potential of TL431 remains unchanged. The current flowing through the optocoupler diode remains unchanged, and the current flowing through the optocoupler CE remains unchanged. The potential of pin 1 of UC3842 is stable, the duty cycle of the output drive remains unchanged, and the output voltage is stable at the set value. When the output 5 V voltage is too high for some reason, the voltage is divided by the voltage divider resistors RIJ and R? The voltage divider value will be greater than 2.5 V, then the K-pole potential of TL431 will drop, the current flowing through the optocoupler diode will increase, and the current flowing through the optocoupler CE will increase. The potential of pin 1 of UC3842 will drop, the duty cycle of the output drive pulse of pin 6 will decrease, and the output voltage will decrease, thus completing the feedback voltage regulation process. When using UC3842 to control the duty cycle of the switching power supply, the conventional usage is to add an R network between pins 1 and 2 of UC3842, use optocouplers and TL431 and other components to form the feedback control loop of the power supply, and connect the C-pole of the optocoupler to pin 2 of UC3842 as the feedback of the output voltage. The circuit shown in Figure 3 does not use this connection method, but directly connects the C-pole of the optocoupler to pin 1 of UC3842 as the output voltage feedback, and pin 2 is directly grounded. Pin 2 of UC3842 is the reverse input terminal of its internal error amplifier, and pin 1 is the output terminal of the error amplifier. This connection method skips the amplifier inside UC3842. This is because the amplifier has its transmission time when used for signal transmission. The output and input are not established at the same time, so the internal amplifier of UC3842 is not used. The advantage is that the transmission time of the feedback signal is shortened by the transmission time of one amplifier, so that the dynamic response of the power supply is faster. In addition, TL431 itself has a high-gain error amplifier, but it is isolated from the high-voltage side. Therefore, the feedback signal directly controls the output end (pin 1) of the internal error amplifier of UC3842 after passing through the amplifier and optocoupler in TL431, and its control accuracy will not be reduced. When the internal error amplifier of UC3842 is used, the feedback signal passes through two high-gain error amplifiers continuously, which increases the transmission time. This circuit samples at the output end and then feeds back to pin 1 of UC3842 through optoelectronic isolation, skipping the internal amplifier of UC3842, shortening the transmission time and making the dynamic response of the power supply faster. At the same time, the high-gain error amplifier inside TL431 is used to ensure high control accuracy. This circuit has a simple topology, few external components, and uses a three-terminal adjustable voltage reference in the voltage sampling circuit, so that the output voltage remains basically unchanged when the load changes significantly. Experiments have shown that this circuit has a good voltage stabilization effect.

　　4 Advantages of current-mode control method

　　Current-mode control retains the output voltage feedback of voltage-mode control and adds inductor current feedback; this current feedback is used as the ramp function of the PWM control converter, so the sawtooth generator is no longer needed, making the system performance significantly superior. The characteristics of the current-mode control method are as follows:

　　1. The system has fast input and output dynamic response and high stability;

　　2. High output voltage accuracy;

　　3. Have the inherent ability to control the power switch current;

　　4. Good parallel operation capability. Since the change rate of the feedback inductor current didt directly changes with the changes of the input voltage and the output voltage. In the voltage feedback loop, the output of the error amplifier is used as a current given signal, which is compared with the feedback inductor current to directly control the duty cycle of the power switch on and off. Therefore, voltage feedback is a very important issue in the design of current-type power supplies. This article introduces the design of the voltage feedback circuit when using the current-type control chip uc3842.

　　5 Conclusion

　　Different feedback methods can be selected according to specific requirements. However, for multi-channel output feedback circuits, since different output applications require different output accuracy, the proportion of each positive polarity output terminal in the feedback is also different. It is necessary to design according to specific requirements to meet application requirements. For example, when two positive voltages of +5v +12v are required to be output, since the former is often used in situations with higher accuracy, the proportion in the feedback is relatively large, which can be taken as 60%, while the latter is taken as 40%. Since there are multiple outputs, superposition technology can be used in the secondary winding to reduce the number of turns of the transformer winding.