[Abstract]: With the development of integration, the complexity of switching power supply circuit structure design has relatively decreased, making the design of high-frequency transformers more important. This article introduces the VIPer22A device and the working principle of the switching power supply it is made from, and then focuses on the design steps and debugging methods of the high-frequency power transformer in the converter, using examples. [Keywords]: Power converter, inductor, energy storage converter, magnetic induction intensity 1. Introduction In the past two years, the monolithic switching power converter VIPer22A has been increasingly used due to its small package, simple peripheral circuit, and high stability and reliability. The VIPer22A chip uses a current-controlled PWM modulator, which is suitable for standby power supplies of battery chargers, power adapters, televisions or monitors, and especially suitable for auxiliary power supplies of various control circuits. Further analysis shows that its peripheral circuit is easy to master, but its power transformer parameters are often neglected. This article focuses on the design, debugging procedures and specific methods of the high-frequency transformer of the monolithic switching power converter VIPer22A, which may be helpful. 2. VIPer22A Functional Overview 2.1 Internal Circuit Structure Block Diagram of the VIPer22A Device The VIPer22A monolithic switching power converter features an internal high-voltage power MOSFET with a drain-source breakdown voltage exceeding 730V; a limiting current between 0.56 and 0.84A (typically 0.7A); and an on-state resistance of 15Ω. It can output 12W of power when the input voltage fluctuates between 85 and 265VAC. It has overcurrent, overvoltage, and overheat protection functions with hysteresis characteristics, resulting in excellent stable and reliable operation. As long as the transformer design is correct, almost no debugging is required; simply connecting the circuit will allow for normal operation. It is packaged in a DIP-8 package. FB is the output voltage feedback terminal, with a voltage range between 0 and 1V. UDD is the power supply terminal of the control circuit. Upon power-up, current from the high-voltage current source charges the UDD terminal, including the external capacitors. When the voltage at the UDD terminal reaches 14.5V, the high-voltage current source is automatically turned off; when UDD drops to 8V, the high-voltage current source is automatically turned on again. After the power MOSFET starts working, the auxiliary winding of the power transformer continues to supply power, ensuring that the device is always in normal operating condition. 3. Overall Circuit Working Principle The circuit schematic of the 12V switching power supply made using the VIPer22A single-chip switching power converter is shown in Figure 2. During the startup process, upon power-on, the high-voltage current source in the VIPer22A converter starts operating and automatically starts the power supply. When the UDD voltage reaches the turn-on voltage UDD(ON) = 14.5V (typical value), the high-voltage current source is turned off, the power MOSFET starts working, and the auxiliary winding begins to supply power to the chip after the high-voltage current source is turned off. Thus, the VIPer22A converter completes the startup procedure. A current flows through the primary winding of transformer T, generating a magnetic flux in the transformer core. This flux induces an electromotive force (EMF) in each winding, the direction of which is indicated by the terminal symbols. The induced EMF in the secondary winding also provides the VIPer22A with its power supply voltage VDD, instantly putting the converter into normal operation. The terminal symbols for each winding indicate that when the power MOSFET is on, the rectifier diode VD7 is reverse-biased; when the power MOSFET is off, VD7 is on—hence the name single-ended flyback converter, also known as an inductor-based energy storage converter—charging capacitors C9 and C10. Due to the high operating frequency of up to 60kHz, even a small capacitance can meet the load's ripple voltage requirements. A smoothing inductor L of tens of microhenries is sufficient to meet the ripple voltage requirements, and in some cases, it may even be unnecessary, requiring only an increased filter capacitor. 3.2 Automatic Adjustment Process: When the external voltage fluctuates, the voltage at pin 1 of the integrated adjustable reference regulator N3 changes accordingly, causing a reverse change in the voltage at pin 3 of its output (opposite to the change in pin 1 of N3, the same below). This, in turn, causes a reverse change in the voltage at pin 3 (FB) of the integrated power converter N1 via optocoupler N2. Consequently, the gate pulse width of the power MOSFET in N1 changes in reverse, ultimately causing a reverse change in the output voltage, thus restoring the output voltage to its value before the external voltage fluctuation as much as possible. When the load current at the output changes, the output voltage also changes accordingly, causing a change in the voltage at the input of N3. The change at the output of N3, through optocoupler N2, causes a change in the FB voltage of N1, thereby causing a reverse change in the gate pulse width of the power MOSFET in N1, ultimately causing a reverse change in the output voltage, thus maintaining the output voltage as stable as possible. Typically, the source voltage regulation of a regulator is better than its load regulation, especially for switching regulators. A voltage regulator made using the VIPer22A device has a lower load regulation than source voltage regulation, so it is generally used in applications where the output current is fixed or does not change much. 4. Estimation of High-Frequency Power Transformer Anyone who has studied electronic circuits is familiar with switching power supplies, and may even find them simple; however, once built, they are often dissatisfied. The reason is often that the parameters of the high-frequency power transformer were not selected correctly. The following example, using a single-ended flyback converter, illustrates the estimation method for the transformer T. 4.1. Core Estimation: A single-chip switching power supply converter, the VIPer22A, is used. The given output power is 8W, efficiency is 0.8, and input power is 10W; the core material is Mn-Zn power ferrite R2K5D. Referring to the book "New Switching Power Supplies and Applications", substituting the selected values ​​into the calculation formula yields: [img=315,77]http://www.chuandong.com/uploadpic/THESIS/2007/12/2007120517383939523M.jpg[/img] Selecting an EI25 type magnetic core, the core cross-sectional area Sc is approximately 0.43 cm², and the window cross-sectional area So is approximately 0.78 cm². 0.43 × 0.78 = 0.34 cm⁴ >> 0.07 cm⁴, therefore the magnetic core meets the requirements. On the one hand, the magnetic induction intensity should not be too high. If it is too high, the magnetic core will enter the saturation region, the switching power supply will not work properly, and may even damage the components. On the other hand, the magnetic induction intensity should not be too conservative. Taking a conservative value will inevitably increase the number of turns and the air gap, which means increasing the leakage inductance and the peak voltage. Excessive peak voltage increases noise voltage, and most importantly, reduces operational reliability and lifespan, making the VIPer22A integrated circuit prone to damage. The reason for choosing a larger magnetic core is solely for ease of trial production. After trial production, a smaller magnetic core, such as EI22, can be used. 4.2 Estimation of the Number of Turns: The number of turns in the secondary winding is calculated based on 300V after rectification and filtering of AC220V. Referring to the formula in the book "New Switching Power Supplies and Applications", we get: NP = 240.6T, taking 240T as the number of primary winding turns. 0.17mm enameled wire is used. The calculation of the primary winding turns is roughly based on the formula in the book "New Switching Power Supply and Applications": H = 348A/m. It is known that the H value of R2K3D ferrite is 1194A/m, which is much larger than the calculated value of 348A/m, so it is considered to meet the requirements. Secondary winding turns [img=315,70]http://www.chuandong.com/uploadpic/THESIS/2007/12/20071205173903952357.jpg[/img] Where: ————- The secondary output voltage of the transformer, considering various voltage drops, should be 1.5 times the rated DC output voltage, that is, 1.5×12＝18V; ————- The primary winding of the transformer, considering the lowest possible input voltage, is taken as 127V. Substituting into formula (3), we get N＝34T. The conductor is 0.47 enameled wire. The feedback winding provides a VDD voltage of 8–40V. Since the current supplied is very small, 0.11mm enameled wire can be used. A value of 240T is chosen, with an inductance of 8.6mH (three layers of printing paper are inserted between the magnetic cores), and = 34T. When the output is DC 16.1V 0.32A, the pulse width is 5μS/17μS/58.4KHz, and the peak reverse voltage is 44V; the feedback voltage is DC 18.5V. When the output is DC 12V, the pulse width is 4μS, the peak reverse voltage is 44V, and the feedback voltage is DC 12.3V. The voltage waveform of this set of data has no voltage spikes and the leading and trailing edges are very steep, making it worthwhile to use. If the feedback voltage is below 10V, the number of turns in NF can be appropriately increased. 4.3 Estimation of Wire Diameter For small high-frequency transformers below 10W, especially under air-cooled conditions with good heat dissipation, the cross-sectional area of ​​the winding conductor can be selected based on the effective value of the current flowing through it (3~4) A/mm2; of course, if the window area allows, it is better to select (2~3) A/mm2. When the wire diameter is less than 1mm, the influence of the skin effect of high-frequency current can be ignored or neglected; otherwise, several thin wires can be wound together as a single strand. The effective value and the maximum value of the current have the following relationship [img=310,50]http://www.chuandong.com/uploadpic/THESIS/2007/12/2007120517392023038E.jpg[/img] 4.4 Measurement and Adjustment of Inductance Although switching power supplies also belong to the category of electronic circuits, and ordinary testing instruments can meet the requirements, the inductance of the primary winding of the high-frequency transformer must be measured with a high-precision inductance meter. It cannot be exempted from testing simply because the number of turns is the same and the magnetic core brand is the same. Generally, several pieces of paper are always placed between the magnetic cores of a high-frequency transformer to ensure a certain magnetic gap. This serves three purposes: 1. To prevent the magnetic flux density of the core from entering the saturation region, ensuring stable and reliable operation; 2. To adjust the inductance to meet the output power requirements; 3. To reduce the inductance while meeting the output power requirements, reducing peak voltage and increasing safety and reliability. Therefore, this step cannot be omitted. 5. Conclusion Although the extreme values ​​of various parameters of the VIPer22A monolithic switching power converter are quite considerable, as is well known, from a safety and reliability design perspective, in practical applications, its voltage, current, power, and other extreme parameters should be derated—multiplied by a safety factor of 0.5. The so-called extreme values ​​are discontinuous, instantaneous maximum values ​​and cannot be considered as a steady state for continuous operation. Only in this way can a switching power supply completed according to the prescribed design procedures and manufacturing processes operate stably and reliably.