Introduction Permanent magnet brushless DC motors are a new type of motor with novel principles, processes, and methods. They are mechatronic systems composed of three parts: the permanent magnet brushless DC motor body (BLDCM), the rotor position sensor (RPS), and the controller (CU). This type of motor overcomes many drawbacks of brushed motors, and therefore has developed rapidly in recent years, finding applications in many fields. The control power supply primarily provides power to the various chips in the controller. Generally, the external input voltage is converted into multiple voltage outputs by a high-frequency DC/DC isolated switching power supply before powering the controller chips. The control power supply has relatively low power requirements but demands simplicity, reliability, and good stability. Traditional switching power supplies use discrete components, resulting in complex circuitry, low efficiency, and poor reliability. The TOPSwitch-II series of dedicated integrated chips for switching power supplies from PI (Power Integrations) in the United States effectively solves these problems. Operating at 100kHz, it features simple peripheral circuitry, low electromagnetic interference, and low cost, effectively reducing the size and weight of the controller and enhancing system reliability. Therefore, the TOP224Y chip is selected as the controller power supply to form a single-ended flyback switching power supply. 1. Basic working principle of single-ended flyback converter [1] The single-ended flyback converter is also called the inductor energy storage converter. Its transformer has the dual functions of energy storage and isolation. Figure 1 is its circuit schematic diagram. The so-called single-ended means that the transformer core only works on one side of its hysteresis loop. When the high voltage switch S1 is turned on, the DC input voltage VI is applied to both ends of the primary winding LP and stores energy in the transformer primary inductor coil. Since the phase of the secondary winding is negative at the top and positive at the bottom, the diode D is reverse biased and cut off. No current flows through the secondary circuit. At this time, the power energy is converted into magnetic energy and stored in the inductor. When S1 is cut off, the polarity of the primary voltage is reversed, which reverses the polarity of the secondary voltage. Thus, the diode D is turned on, and the energy stored in the transformer is transferred to the load. At the same time, the output capacitor C is charged. At this time, the magnetic energy is converted into electrical energy and released. When the switching transistor is turned on again, the load current is provided by the capacitor C, and the primary side of the transformer re-stores energy, and so on. From the above circuit analysis, it can be seen that when S1 is turned on, there is no current in the secondary circuit; when S1 is turned off, there is current in the secondary circuit, which is the meaning of "flyback". Figure 1 Schematic diagram of the flyback converter circuit 2. Circuit principle and design 2.1 Main characteristics of TOP224Y[2] TOP224Y is one of the TOPSwitch-domain series integrated chips. It is a typical three-terminal device. The three pins are the control terminal C, the source terminal S and the drain terminal D, respectively. Its internal MOSFET withstand voltage is as high as 700V. It has a wide voltage input range (AC input voltage can reach 85～265V) and AC/DC conversion efficiency can reach 90%. It integrates the power switching transistor and its control circuit into one chip and has functions such as automatic reset, overheat protection and overcurrent protection. Due to its high integration and comprehensive protection circuitry, switching power supplies constructed using it require fewer external components, have a smaller size, and higher reliability, making them ideal for designing low-power auxiliary power supplies. Figure 2 shows its internal structure block diagram. When the system is powered on, the drain D becomes high, the internal current source begins supplying power to the C terminal, and the on-chip switch is in position 0, charging the external capacitor connected in parallel between the C and S terminals (C2 in Figure 3). When the voltage reaches 5.7V, the automatic restart circuit shuts down, the on-chip switch jumps to position 1, and the TOPSwitch enters normal operation, outputting a PWM wave to drive the internal MOS transistor. Afterward, the IC is powered by the feedback circuit. The control voltage UC is divided by the ZC, P-channel MOSFET, and resistor RE to obtain the feedback voltage Uf, which is applied to the inverting input of the error amplifier. The error amplifier compares Uf with the 5.7V reference voltage and outputs an error current If. When If flows through resistor RE, an error voltage is formed on it, which is compared with the sawtooth wave voltage to adjust the pulse duty cycle. From the above analysis, it can be seen that TOPSwitch-Ⅱ is a current-controlled switching power supply. The bias voltage is provided by the control terminal voltage UC, and the duty cycle is adjusted by the control terminal current IC. [align=center] Figure 2 Internal structure block diagram of TOP224Y[/align] 2.2 Main circuit working principle Figure 3 shows the auxiliary power supply circuit diagram of the flyback controller based on TOP224Y designed in this paper. The input voltage is DC 160~220V, and the output is one +5V voltage and two mutually isolated +15V voltages. The designed power is 5W. [align=center] Figure 3 Single-ended flyback power supply circuit composed of TOP224Y[/align] In the circuit, D1 is a TVS (transient voltage suppressor), D2 is an ultra-fast recovery diode, and D1 and D2 form a clamping protection circuit, which is used to clamp and absorb the peak voltage generated by the leakage inductance of the high-frequency transformer, thereby protecting the power MOSFET. The secondary voltage, after rectification and filtering by D3 and C3, outputs +15V to power the pulse width modulation chip. This voltage is then stepped down by the LM7805 linear regulator to output +5V, powering the logic synthesis chip. Using the LM7805 not only eliminates the need for an additional +5V secondary winding but also provides stable output voltage performance and lower ripple. Since the accuracy requirement for the output voltage is not very high, an optocoupler feedback circuit with a Zener diode is used. The circuit utilizes the change in the LED current If caused by the change in the output voltage to control the control electrode current IC of the TOP224Y, thereby adjusting the duty cycle D, changing the PWM width, and achieving a stable output voltage. For example, if UO↑ for some reason, the current If of the optocoupler LED will increase. After transmission through the optocoupler, the current ICE of the receiving tube will increase, so the IC of TOP224Y will increase. Since IC is inversely proportional to the duty cycle D, D↓, which leads to UO↓, thus achieving voltage regulation. Conversely, UO↓→If↓→ICE↓→IC↓→D↑→UO↑, which also achieves voltage regulation. The output voltage of the feedback winding is rectified and filtered by D4 and C4 to provide bias voltage to the receiving end of the optocoupler, and at the same time, it serves as another +15V voltage output to power the dedicated driver chip. C2 in the circuit is a bypass capacitor, which has three functions: filtering out the spike voltage on the control terminal; determining the automatic restart frequency; and forming a compensation circuit for the control loop with R1. 2.3 Design of high frequency transformer [3] Since there are few peripheral components, the key to the design is the transformer. The single-ended flyback transformer operates in the first quadrant of the hysteresis loop. The core is simultaneously supplied with both AC and DC current. The change in magnetic flux density ΔB of the transformer core is very small. To prevent core saturation, an air gap is typically added, which increases the difficulty of transformer design. The calculation method for transformer parameters in this design is given below. This flyback converter uses discontinuous conduction mode (DCM). The maximum duty cycle Dmax = 0.4 is taken. The transformer uses a manganese-zinc ferrite R2KB core with a permeability of up to 2000 Ω·cm and a saturation magnetic flux density BS of 480 mT (at 25℃). Calculations show that an EI-22 core is selected, with an effective cross-sectional area of ​​42 mm², and ΔB = 0.15 T is taken. 2.3.1 Calculation of the maximum primary current Ip: Where: Po is the output power; η is the converter efficiency; Vin(min) is the minimum input DC voltage; Dmax is the maximum duty cycle. 2.3.2 Calculate the primary inductance Lp: Where ton is the on-time of the switching transistor, ton = DT. The operating frequency of TOP224Y is 100 kHz, so T = 1/f = 10 μs. 2.3.3 Calculate the air gap length lg: Where Ac is the effective cross-sectional area of ​​the core (mm²); Bm is the maximum magnetic flux density (T). 2.3.4 Calculate the number of turns in the primary, secondary, and feedback windings: Primary winding turns Np, secondary winding turns Ns, feedback winding turns: NF = Ns = 16. All winding turns are rounded values. 2.3.5 Verify the core's ΔB: Where σ₀ is the permeability of free space, taken as 4π × 10⁻⁷ H/m. Then: Bmax = 1/2ΔB + B = 225 mT. Comparison shows: Bmax < 1/2Bs, therefore the previously selected core is suitable. 2.3.6 Selection of Conductors and Transformer Winding Due to the small primary and secondary currents and considering ease of winding, Φ0.31 mm enameled wire was selected for the transformer winding based on calculations. To reduce leakage inductance, the transformer windings should be coaxially distributed, and a sandwich winding method should be used: half primary winding 52 turns (inner layer) + secondary winding 16 turns + the other half primary winding 53 turns + feedback winding 16 turns (outer layer). Insulating tape is sandwiched between each layer, and after winding, the outermost layer is wrapped with insulating tape. Epoxy resin is used to firmly bond the magnetic core and frame. 2.4 Determination of Feedback Loop Parameters To achieve linear adjustment of the duty cycle, the control pin current IC should be between 2 and 6 mA. IC is controlled by the optocoupler LED current If. Since PC817 is a linear optocoupler, when the diode forward current If is around 3 mA, the transistor collector current Ice is around 4 mA, and the collector-emitter voltage varies linearly over a wide range. Therefore, the forward current If of the PC817 LED is generally taken as 3mA. In this design, D8 in the feedback circuit uses a Zener diode IN4743 with a breakdown voltage of 13V. Since the forward voltage drop of the LED in the optocoupler PC817 is Uf≈1.2V, the typical value of the stable current IZ of IN4743 is 20mA. The R2 branch can only supply about 3mA of current. Therefore, resistor R3 is used to provide another current of about 17mA, and at the same time, it serves as part of the dummy load to improve the voltage regulation performance under light load. Therefore , the resistance value of R3 can be calculated as 3. Experimental Results and Analysis Based on the above analysis and calculation, a prototype was manufactured and tested. Figures 4 and 5 show the output voltage waveforms of +5V and +15V when the input voltage is 160V, respectively. The ripple voltage is less than 3%. Figures 6 and 7 show the drain voltage Ud waveforms of the TOP224Y at an output power of 4.5W when the input voltage is 160V and 89.5V, respectively. It can be seen that the system operates across both discontinuous and continuous modes when the input voltage varies widely, and the measured output voltage remains stable. Experimental results show that the power supply achieves an efficiency of 81% under full load, and the voltage regulation, load regulation, and ripple meet the control circuit's requirements for the power supply voltage, indicating stable system operation. 4. Conclusion Brushless DC motors are mechatronic products, and the controller is crucial for their normal operation. It determines the motor's electronic commutation law, reversible forward/reverse operation, and effective power flow control. Therefore, the design of the regulated power supply for the controller is particularly important. The low-power auxiliary power supply developed using the TOPSwitch integrated chip in this paper has been tested and shows that it has stable performance, high reliability, and strong anti-interference capabilities.