Design scheme based on multi-channel single-ended flyback switching power supply (Part 2)

3.5 Output Rectification and Filtering Circuit

It is composed of a rectifier diode, a filter capacitor and a smoothing inductor. The high-frequency square wave voltage of the secondary winding is converted into a pulsating DC voltage, and then the high-frequency ripple is filtered out through the output filter circuit, so that a stable DC voltage is obtained at the output end. The Schottky diode has a small forward conduction loss and a short reverse recovery time. It has obvious performance advantages in reducing reverse recovery losses and eliminating ripples in the output voltage. Therefore, the Schottky diode is selected as the rectifier diode. The parameters are selected according to the maximum reverse peak voltage VR. At the same time, the rated current of the diode should be at least 3 to 5 times the maximum output current. The reverse peak voltage VSM of the secondary winding is:

Where: Iout is the rated current of the output terminal, in A; Dmin is the estimated minimum duty cycle under high input voltage and light load (estimated value is 0.3); V(PK-PK) is the maximum output voltage ripple peak-to-peak value, in mV. After calculation, the threshold C6 is taken as 100 μF/10 V, and C8 is taken as 220 μF/35 V.

The second stage is filtered by LC to filter the voltage that does not meet the ripple requirements again. The output filter capacitor should not only consider whether the output ripple voltage can meet the requirements, but also consider the suppression of load current changes. Here, C7 can be selected to be 22 μF/10 V, C9 to be 10 μF/35 V. C5 takes the empirical value of 0.1 μF/25 V. The output filter inductor is 2.2~4.7 μH based on experience. Using a 3.3 μH through-hole inductor can actively suppress the generation of switching noise.

To reduce common-mode interference, connect a common-mode suppression capacitor C15 between the output ground and the high-voltage side ground.

3.6 Feedback loop design

There are four types of feedback circuits for switching power supplies: basic feedback circuit, improved basic feedback circuit, optocoupler feedback circuit with voltage regulator, and optocoupler feedback circuit with TL431. This design uses an adjustable precision shunt regulator TL431 with high voltage regulation accuracy and a linear optocoupler PC817A to form a feedback loop.

TL431 detects the change in output voltage ΔU through the circuit sampling resistor, and then sends the sampled voltage to the input control terminal of TL431 to compare it with the 2.5 V reference voltage of TL431. The output voltage UK also changes accordingly, causing the working current of the light-emitting diode in the linear optocoupler to change linearly, and the optocoupler outputs current.

The external error amplifier composed of the optocoupler and TL431 adjusts the current IC of the control terminal C of TOP223Y and adjusts the duty cycle D (IC is inversely proportional to D), thereby changing the output voltage and achieving the purpose of stabilizing the output voltage.

For the feedback part of the circuit, the switching power supply feedback circuit only draws the feedback signal from one output loop, and the rest are not added with feedback circuit. In this way, when the load current of the 5 V output changes, it will definitely affect the stability of the 12 V output.

The solution is to add a feedback circuit to the 12 V output. In addition, C10 in the circuit is the frequency compensation capacitor of TL431, which can improve the transient frequency response of TL431. R5 is the current limiting resistor of the optocoupler, and the size of R5 determines the gain of the control loop. Capacitor C13 is a soft-start capacitor, which can eliminate the voltage overshoot generated by the chip when the power is just started.

The following is mainly to determine the value of R4~R8:

According to the application requirements, the 5 V power supply is required to be higher, but the 12 V power supply must also be taken into consideration. The feedback amount is weighed and the feedback weights of R7 and R8 are set to 0.6 and 0.4 respectively. The stability of each output is guaranteed and improved.

When only the 5 V output has feedback, if R4 and R7 are both 10 kΩ, the current IR7 = 250 μA. After weighting, R7 gets 150 μA and R8 gets 150 μA. According to the characteristics of TL431, the following relationship exists between Vo, VREF, R7, R8, and R4:

Where: VREF is the reference voltage of TL431, which is 2.5 V; Vo is the output voltage of TL431. According to the current distribution relationship (unit: kΩ):

Where: VF is the forward voltage drop of the optocoupler diode. According to the PC817 technical manual, the typical value is 1.2 V. First, take R5=390 Ω, then R6=139 Ω, and take the nominal value of 150 Ω.

3.7 Control loop

It is composed of capacitor C7 and resistor R12 in series. C9 is used to filter the peak voltage of the control end and determine the automatic restart timing, and together with R12, set the main pole of the control loop for feedback control loop compensation. According to the data sheet, C9 selects a 47 μF/25 V electrolytic capacitor. When C9 = 47 μF, the automatic restart frequency is 1.2 Hz, that is, every 0.83 s, it detects whether the regulation out-of-control fault has been eliminated. If it is confirmed that it has been eliminated, it automatically restarts the switching power supply to resume normal operation. R12 is 6.2 Ω.

4 Experimental results of the scheme

According to the design methods and specifications of the above scheme, a flyback switching power supply with dual output of +5 V/3 A, +12 V/1 A based on TOP223Y was designed. Its performance was tested under a wide input range of 85~265 VAC, as shown in Table 1.

From the experimental data selected above, it can be concluded that the output voltage regulation rate of +5 V/3 A (feedback weight 0.6, load 500 Ω) is SV = ±0.18%, the output ripple voltage is 39 mV, and the maximum output current is 3.2 A; the output voltage regulation rate of +12 V/1 A (feedback weight 0.4, load 750 Ω) is SV = ±0. 3%, the output ripple voltage is 68 mV, and the maximum output current is 1.10 A.

When the power supply is fully loaded, the power can reach 27.6 W, the maximum duty cycle is 0.60, and the power efficiency is 83.1%. The switching power supply has good performance and meets the application requirements.

5 Conclusion

The switching power supply solution designed in this paper has a highly integrated chip and a simple peripheral circuit design. The performance of the power supply can still be improved through parameter adjustment. The dual-output dual-feedback weighted design makes the switching power supply more practical and flexible. The design of different protection circuits makes the power supply more safe and reliable. The power supply designed in this solution performs well in practical applications.