Currently, countries worldwide are researching 48VDC automotive power supply systems, and the European Community plans to adopt 48VDC power supply systems starting in 2008. How to achieve compatibility with 12VDC electronic devices under a 48VDC power supply system has become a challenge. Using a linear regulated power supply to achieve the 48VDC/12VDC conversion results in significant power loss, a clear drawback. This paper proposes a switching power supply design scheme for an automotive power supply system with overload and short-circuit protection. This scheme uses a single-ended flyback structure to achieve the 48VDC/12VDC conversion, resulting in stable output voltage, low ripple, uninterrupted operation, reliable performance, and low power loss. UC3842 Protection Circuit Design 1 Typical Application of UC3842 UC3842 is a high-performance single-ended output current-controlled pulse-width modulation (PWM) chip. Its typical application circuit is shown in Figure 1. Figure 1 Typical Application Circuit of UC3842 2 Overload Protection Principle Analysis When an output short circuit occurs, the output voltage will drop, and the feedback winding powering the UC3842 will also experience an output voltage drop. When the input voltage is below 10V, the UC3842 stops working, and the switching transistor is turned off. After the short circuit disappears, the power supply restarts and automatically resumes normal operation. However, due to the high peak voltage that occurs during high-frequency turn-off, even with a small duty cycle, the input voltage at pin 7 may not drop low enough. The overload protection circuit may not always effectively respond to overload conditions, negatively impacting the overall system performance and posing a safety hazard. 3. Overcurrent Protection Principle Analysis When the voltage at pin 3 of the current sampling terminal exceeds the voltage at the negative terminal of the current detection comparator, a reset signal can be input to the pulse width modulation latch, and the switching transistor is turned off. This peak detection circuit limits the maximum output current, providing some protection. However, as the switching frequency increases, the power supply may operate in continuous mode, meaning the primary inductor current increases from a certain amplitude in each switching cycle, resulting in subharmonic oscillations. This instability is unrelated to the closed-loop characteristics of the voltage regulator; it is caused by the simultaneous operation of a fixed frequency and peak current sampling. Figure 2 illustrates this phenomenon. Figure 2 shows the current waveform before compensation. At time t0, the switch is turned on, and the primary coil current rises with a slope of m1, which is a function of the input voltage and inductance. At time t1, the current sampling input reaches the threshold of the current sensing comparator, causing the switch to turn off, and the current decays with a slope of m2 until the next switching cycle. If a disturbance is applied to the threshold voltage of the current sensing comparator, generating a small ΔI (as shown by the dashed line in Figure 2), instability will occur. Within a fixed oscillation cycle, the current decay time decreases, and the minimum current rises by ΔI + m2/m1 at the time the switch is turned on (t2). The minimum current decreases to (ΔI + m2/m1)·(m2/m1) in the next cycle (t3). In each subsequent switching cycle, this disturbance is multiplied by (m2/m1), alternating between increasing and decreasing the primary coil current over several switching cycles. Perhaps after several switching cycles, the current will decrease to zero, causing the process to restart. If m²/m¹ is greater than 1, the system will be unstable. 4. Improvements to the Protection Circuit As shown in Figure 3, this design makes the following improvements to the overcurrent and overload protection circuit of a typical UC3842 application circuit. A resistor is connected in series with the rectifier diode circuit of the feedback winding. It and capacitor C2 form an RC filter network, which filters out the peak voltage at the moment the switching transistor turns on. Thus, due to the reduction of the peak voltage, when a short circuit occurs, the voltage output of the feedback winding will be effectively reduced, and the UC3842 will stop working until the short circuit is resolved. Slope compensation is applied to the overcurrent protection circuit. The compensation slope is generated from the RT and CT oscillators and applied to the voltage feedback terminal to improve the slope compensation of the error amplifier output. As shown in Figure 3, the output of the error amplifier is a slope with an m³ slope. After passing through two diodes, it is divided by a resistor and then input to the negative terminal of the current detection comparator as the control voltage of the overcurrent protection circuit. This configuration of the current detection comparator and pulse width modulation latch ensures that only one single pulse appears at the output terminal in any oscillator cycle. When overload or output voltage sampling loss occurs, the internal comparison threshold is limited to 1V, preventing circuit misalignment. Figure 3 shows the block diagram of the switching power supply. Figure 4 illustrates how adding an artificial ramp synchronized with the pulse width modulation clock to the control voltage effectively suppresses instability caused by ΔI disturbances in subsequent switching cycles. The slope (m³) of this compensation ramp must be equal to or greater than m²/2 for stability. Through the compensation of the m³ slope, the primary coil current is suppressed by the control voltage, closely following its amplitude. Figure 4 shows the current waveform after slope compensation. Experimental results: Table 1 shows the output voltage fluctuation when the input voltage fluctuates between 30 and 50V. Table 2 shows the output voltage fluctuation when the load current varies between 10 and 500mA. From the data in Table 1, the voltage regulation Sv < 0.3%. From the data in Table 2, the output resistance Ro < 0.4Ω. Conclusion: This paper proposes a simple and stable single-ended flyback switching power supply design. Thanks to the adoption of a "slope-compensated" overcurrent protection method, the performance is more stable and reliable, with low voltage regulation, low output resistance, low ripple, low power loss, and a high system safety factor. This enables the successful supply of power to the vehicle's power system, significantly improving the overall performance of the vehicle. This design has been successfully applied to the power supply system of a vehicle-mounted brushless DC motor controller designed by the Institute of Intelligent Information Systems at Wuhan University of Technology. Furthermore, the DC/DC scheme proposed in this paper is also applicable to the design of other DC power supplies. Due to its stable performance and low ripple, it offers valuable insights for the design of power supplies for digital control systems using microcontrollers.