Abstract: This paper introduces a switching power supply designed using the TOP227Y intelligent integrated chip. The performance characteristics of this chip are described in detail. Based on the characteristics of the TOP227Y, the design method of the switching power supply is given, the design of the peripheral circuit is described in detail, and the performance test results of the switching power supply are presented. Keywords: Switching power supply; TO227Y; Pulse Width Modulation (PWM) Introduction The TOPSwitch monolithic switching power supply chip is a new type of high-frequency switching power supply chip launched by PI Corporation in the mid-1990s. It is an abbreviation for Three Terminal Offline PWM Switch and is hailed as a "top-tier switching power supply". Its characteristic is that it integrates the PWM controller and MOSFET power switch in a high-frequency switching power supply onto the same chip, making it a two-in-one device. Its first-generation product was the TOP100/200 series launched in 1994; the second-generation product was the TOPSwitch-II series launched in 1997. These products, upon their introduction, demonstrated strong vitality and were widely used in various fields such as instruments, displays, switching power supplies, VCD/DVD players, and mobile phone chargers, forming a new type of high-efficiency, low-cost switching power supply. Furthermore, their simple design method makes the TOOPSwitch chip particularly easy to use. Switching power supplies designed using this chip have advantages such as high efficiency, small size, and simple peripheral circuitry. It is foreseeable that monolithic switching power supplies designed using the TOOPSwitch series chips will be applied in a wider range of fields. 1 TOP227Y Chip Performance Characteristics and Internal Block Diagram 1.1 TOP227Y Chip Performance Characteristics The TOP227Y belongs to the second generation of the TOP series. Its power switching transistor has a withstand voltage of up to 700V and has the following significant characteristics. 1) The entire function of the pulse width modulation control system is integrated into a three-terminal chip, including a pulse width modulator, power switching MOSFET, automatic bias circuit, protection circuit, high-voltage start-up circuit, and loop compensation circuit. A high-frequency transformer completely isolates the output from the power grid, truly achieving a transformerless, isolated, flyback switching power supply with monolithic integration. 2) It is an open-drain output power supply that uses the power supply to linearly adjust the duty cycle for AC/DC conversion; it is a current-controlled switching power supply. 3) It has a wide range of input AC voltage and frequency. 4) It has only three leads. It can be used to construct a transformerless flyback switching power supply in the simplest way. Its control terminal is a multi-functional lead, capable of performing various control, bias, and protection functions. It has both continuous and discontinuous operating modes, and four basic types of feedback circuits, enabling the construction of various general-purpose and precision switching power supplies. 5) The typical switching frequency is 100kHz, with an allowable range of 90–110kHz, and the duty cycle adjustment range is 1.7%–67%. 6) The peripheral circuit is simple, requiring only a rectifier filter, high-frequency transformer, drain clamping protection circuit, feedback circuit, and output circuit. 7) Due to the chip's very low power consumption, the power efficiency is high, reaching approximately 80%, and up to 90%. 8) If paired with a low-dropout linear integrated voltage regulator, it can form a new type of composite switching power supply, retaining the advantages of small size and high efficiency of switching power supplies while possessing the excellent characteristics of good stability and low ripple voltage of linear voltage regulators. 9) Using this chip can reduce electromagnetic interference generated by the switching power supply. 10) Its operating temperature range is 0–70℃, and the chip's maximum junction temperature Tom = 135℃. 1.2 TOP227Y Internal Block Diagram The internal block diagram of the TOP227Y is shown in Figure 1, mainly including a control voltage source, a bandgap reference voltage source, an oscillator, a parallel regulator/error amplifier, a gate driver stage and an output stage, a pulse width modulator, an overcurrent protection circuit, a shutdown/automatic restart circuit, a power-on reset circuit, and an overheat protection circuit. The basic working principle of TOP227Y is to use the feedback circuit's Ic to adjust the duty cycle, D, thereby achieving voltage regulation. For example, when the output voltage Vo↓, the optocoupler feedback circuit causes Ic→D→Vo↑, ultimately keeping Vo constant. For details on the working principle, please refer to the references. 2 Design Example According to the technical requirements, a high-power switching power supply with an input of AC 220 V and an output of DC 5 V and 20A was designed. The basic circuit structure block diagram of this switching power supply is shown in Figure 2. Due to the high integration of TOPSwitch, the design work mainly focuses on the design of the peripheral circuits. The peripheral circuits consist of five parts: an input rectifier and filter circuit, a clamping protection circuit, a transformer, an output rectifier and filter circuit, and a feedback circuit. The circuit schematic is shown in Figure 3. 2.1 Input Rectifier and Filter Circuit Design The rectifier and filter circuit includes three parts: input AC filtering, rectification, and capacitor voltage regulation. The AC filter uses a Type II filter circuit with the following parameters: Co, C1, and C1 for removing common-mode interference are 10nF; G2 and G3 for removing differential-mode interference are 1μF; 10mH, with dual-wire parallel winding. An uncontrollable rectifier bridge is selected for the rectifier circuit. Under the current power supply conditions, the capacitance value of capacitor G4 can be determined based on the output power, with 1 μF per W. Assuming the diode conduction time in the rectifier bridge is tc = 3ms, the minimum withstand voltage (minimum DC input voltage) of the capacitor can be obtained as follows: VACmin is the minimum AC mains voltage; Po is the total output power; η is the system efficiency, which can be selected as 80%; f is the AC mains frequency. 2.2 Clamping Protection Circuit Design During each switching cycle, the turn-off of the TOPSwitch will cause a voltage spike in the transformer leakage inductance. The clamping circuit composed of VR1 and VD1 prevents this voltage from damaging the TOPSwitch. The selection of VR1 and Vo1 is determined by the reflected voltage VOR. VOR is generally chosen as 135 V. The VR1 clamping voltage VCLD can be obtained from the empirical formula VCLO = 1.5VOR. The withstand voltage of VD1 should be greater than the maximum DC input voltage Vmas, and a fast recovery diode should be selected. 2.3 Transformer Design 1) Core Type: To meet the 100 kHz operating frequency of the TOP227Y chip, a manganese-zinc ferrite core is recommended. This design selects the EE-42 type ferrite core. 2) Maximum Duty Cycle Dmax: Where: VOR is the reflected voltage from the secondary to the primary, which can be selected as 135 V; VDS is the on-state voltage of TOP227Y, which can generally be selected as 10Y. 3) Transformer Primary Self-Inductance Lp: Where: fs is the switching frequency of TOP227Y, which is selected as 100 kHz. 4) Wire Diameter: At a switching frequency of 100 kHz, the penetration depth of copper core wire is 0.20~0.22 mm, and the diameter of round copper core wire is twice the penetration depth. 40~0.44mm, and then increase the thickness of the polyester insulation outer layer by 0.06mm, so the measured insulation outer diameter of the wire is 0.46~0.50mm. Here we choose a wire diameter of 0.5mm. 5) The voltage value per turn is proportional to the voltage value per turn when operating in flyback mode. The voltage value per turn must be determined before determining the number of turns N of each winding. 6) The number of turns of the transformer primary and secondary windings The number of transformer turns can be determined by selecting the number of turns of the secondary winding. For a circuit with an input voltage of AC 220V, a secondary winding of 0.6T/V is sufficient. The number of turns of the primary winding is then determined according to the turns ratio. 2.4 Output rectifier and filter circuit design The output rectifier and filter circuit consists of rectifier diodes and filter capacitors. The switching loss of the output rectifier diode accounts for 1/6 to 1/5 of the system loss and is the main factor affecting the efficiency of the switching power supply. It includes forward conduction loss and reverse recovery loss. Since the forward voltage drop of the Schottky diode is low when it is conducting, its forward conduction loss is low. Furthermore, Schottky diodes have a short reverse recovery time, offering significant advantages in reducing reverse recovery losses and eliminating ripple in the output voltage. Therefore, Schottky diodes are selected as the rectifier diodes. The Schottky diode is selected based on the maximum reverse peak voltage. The maximum reverse peak voltage of the secondary winding is given by the formula: VSM is the maximum reverse peak voltage output from the secondary winding; VS is the output voltage of the secondary winding; Np is the number of turns in the primary winding; Ns is the number of turns in the secondary winding; and VAcmax is the maximum value of the primary input voltage of the transformer. 2.5 Feedback Circuit Design The feedback circuit is determined based on the output voltage accuracy. This power supply uses a linear optocoupler + TL431 scheme, which can control the output voltage accuracy within ±1%. The voltage feedback signal is introduced to the Ret terminal of the TL431 via a voltage divider network, converted into a current feedback signal, and then input to the control terminal of the TOP227Y after optocoupler isolation. The optocoupler operates in linear mode, providing isolation. If the upper limit of the current amplification of the selected optocoupler exceeds 200%, it is easy to cause the TOP227Y overvoltage protection to trip. Conversely, if the lower limit of the current amplification is less than 40%, the duty cycle D will not decrease with the increase of feedback current, thus leading to overcurrent. Therefore, an optocoupler with a current amplification range close to 100% should be selected. In this case, Siemens' CNY17-2 (current amplification of 63% to 125%) was selected. 3 Power Supply Performance Test and Result Analysis Based on the above design, the performance of a switching power supply with an output voltage of 5 V and a current of 20 A using the TOP227Y was tested. The measured results show that when the switching power supply is working at full load, the maximum duty cycle is 0.42, the power supply efficiency is 84%, and the ripple voltage control, voltage regulation accuracy, and power supply efficiency all exceed those of previous switching power supplies with separate control circuits and power switching transistors. 4. Conclusion Because the TOP227Y chip integrates a PWM controller, a power MOSFET switch, and various protection circuits, the switching power supply designed using this chip features small size, light weight, low cost, simple peripheral circuitry, high efficiency, and high reliability, thus having broad application prospects in electronic devices. The switching power supply designed in this paper has been applied to a circuit, and through operational observation, its performance is good, achieving excellent application results.