[Easy to understand] Quickly determine the parameters of the feedback loop - switching power supply!

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[Easy to understand] Quickly determine the parameters of the feedback loop - switching power supply! [Copy link]

Introduction The feedback loop of a switching power supply is mainly composed of devices such as optocouplers (such as PC817) and precision adjustable voltage shunt regulators (such as TL431). To study how to design a feedback loop, we must first understand the basic parameters of these two most important components. 1. Optocouplers The basic parameters of PC817 are as follows: The basic parameters of PC817 are as follows: 2. Adjustable shunt regulator From the equivalent circuit diagram of TL431, we can see that Uref is an internal 2.5V reference source connected to the inverting input of the op amp. According to the characteristics of the op amp, only when the voltage at the REF terminal (non-inverting terminal) is very close to Uref (2.5V), a stable non-saturated current will flow through the transistor, and with the slight change of the REF terminal voltage, the current through the transistor VT will change from 1 to 100mA. Of course, this diagram is by no means the actual internal structure of TL431, so this combination cannot simply replace it. But if you are designing, analyzing and applying the circuit of TL431, this module diagram is very helpful to open up your thoughts and understand the circuit. As mentioned earlier, the TL431 contains a 2.5V reference voltage inside, so when the output feedback is introduced at the REF end, the device can control the output voltage through a wide range of shunting from the cathode to the anode. As shown in the circuit in Figure 2, when the resistance values of R1 and R2 are determined, the two introduce feedback to the voltage division of Vo. If Vo increases, the feedback amount increases, and the shunting of TL431 also increases, which in turn causes Vo to decrease. Obviously, this deep negative feedback circuit must be stable when Uref is equal to the reference voltage, at which time Vo=(1+R1/R2)Vref. Figure 2 By choosing different values of R1 and R2, any voltage output in the range of 2.5V to 36V can be obtained. In particular, when R1=R2, Vo=5V. It should be noted that when selecting resistors, the necessary condition for the operation of TL431 must be guaranteed, that is, the current through the cathode must be greater than 1 mA. After understanding the basic parameters of TL431 and PC817, let's look at the actual circuit: Figure 3 The feedback loop mainly focuses on the values of R6, R8, R13, R14, and C8."] Let's first look at R13. R13 and R14 are the voltage-dividing resistors of TL431. First, the value of R13 should be determined, and then the value of R14 should be calculated according to the formula Vo=(1+R14/R13)Vref. 1. Determine the values of R13 and R14 To determine the value of R13, consider the following two conditions: 1. TL431 The current at the reference input end is generally about 2uA. In order to avoid the current at this end affecting the voltage divider ratio and avoiding the influence of noise, the current flowing through resistor R13 is generally taken to be more than 100 times the reference segment current, so this resistance should be less than 2.5V/200uA=12.5K. 2. Considering the standby power consumption and transient response, if the value is too small, the current passing through is large. According to the P=I2R formula, the standby power consumption is large; if the value is too large, the current passing through is small, and the transient response of the feedback loop will be affected. Therefore, R13 should try to take the middle value or greater than the middle value when condition 1 is met. This design is for a 5V/1.5A adapter. R13 is 5.6K. Theoretically, to get a 5V output, R13 and R14 can be equal. However, considering the line loss in actual application of the adapter, R14 is slightly larger than R13, and is 6.2K. Calculation yields: Vo=(1+6.2/5.6)*2.5=5.26V. Combined with the output line specifications and line loss used, a 5V voltage can be obtained at the end of the line when the output is fully loaded. 2. Determine the values of R6 and R8 Since the output is 5V, the voltage at point a is slightly higher than 5V, so take 5.3V Figure 4 is the internal circuit diagram of TL431. It can be seen from the figure that the K terminal and the R terminal differ by a PN node (that is, when the transistor works in saturation state, the K terminal will be 0.7V higher than the R terminal voltage (silicon tube)). When the switching power supply is working, Q1 in the figure below will work in the amplification mode. According to the amplification characteristics of the transistor, the K terminal voltage will be at least 0.7V higher than the R terminal voltage. According to experience, the K terminal voltage is 1.5V~1.7V higher than the R terminal voltage, that is, the voltage at point c in Figure 3 is 1.5V~1.7V higher than the voltage at point d. The voltage at point d is the TL431 reference voltage, which is 2.5V, and the voltage at point c is 4V~4.2V. Figure 4 From the optocoupler parameter table, we can know that the forward voltage drop of the light-emitting diode is 0.8~1.4V (take 1V, IF is 3~5mA), so the voltage at point b is 5V~5.2V. Based on the above conditions, we have calculated that the voltage at point a in Figure 2 is 5.3V; the voltage at point b is 5~5.2V (take 5.1V); The voltage at point c is 4~4.2V (4.1V is taken); the voltage at point d is 2.5V; From the parameters of the light-emitting diode, we know that IF<50mA. According to experience, IF is generally 3mA. The R8 resistor is designed to provide dead-zone current for TL431. By consulting the parameters of TL431, we know that to ensure normal operation, the Ika of TL431 needs to be greater than 1mA and less than 100mA, and is generally 3~5mA. Calculated R6=(5.3V-5.1V)/3~5mA =40Ω~67Ω. This design uses 56R. R8<(1.2V/1mA)=1.2K. According to experience, 1K or 470Ω is generally used. 3. Determine the value of C8 In some circuit designs, in order to improve the low-frequency gain, a resistor and a capacitor are connected in series to the TL431 control terminal and the output terminal to suppress the low-frequency (100Hz) ripple and improve the output regulation rate, that is, the static error. The purpose is to improve the phase. It should be placed in front of the bandwidth frequency to increase the phase margin. The specific position depends on the phase of the remaining power part at the design bandwidth. The lower the frequency of the resistor and capacitor, the higher the phase improvement. Of course, the maximum is only 90 degrees, but when the frequency is very low, the low-frequency gain will also decrease. It is generally placed at the beginning of 1/5 of the bandwidth, which improves the phase by about 78 degrees. According to calculations, 104 capacitors or 104 capacitors connected in series with 1K resistors are generally selected. (The specific calculation is more complicated) The above data is only theoretical calculations, and should be fine-tuned according to actual test conditions.

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