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Summary of problems related to single-ended flyback switching power supply circuit debugging
**Debugging Conclusions for Single-Ended Flyback Switching Power Supply Circuit**
After completing the debugging of our single-ended flyback switching power supply circuit, we have addressed several critical issues that arose during the process. Below are the conclusions we’ve drawn from these efforts:
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**Issue 1: Excessive Temperature Rise of the MOS Tube**
One of the primary challenges was the overheating of the MOS tube. During testing, the MOS tube reached temperatures as high as 45°C. To resolve this, we began by altering the switching frequency of the TOP246YN chip from its default 132kHz to 66kHz. However, this adjustment only yielded minimal improvements. We then replaced the MOS tube with the lower RDS version, the TOP247YN, which reduced the temperature rise significantly. Additionally, we adjusted the external current limit point to further mitigate the heating effect. Finally, we increased the external heat sink's surface area, bringing the MOS tube’s temperature rise down to approximately 25°C.
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**Issue 2: High Temperature of the Output Rectifier**
The output diode also exhibited excessive temperature rise, reaching 42°C. Initially, we used the MUR1060 diode in a TO220 package, which had a conduction voltage drop of around 1.1V. After switching to the MBR10200 diode, which has a lower voltage drop of 0.7–0.8V, we noticed an immediate improvement. Further optimization involved replacing the standard TO220 heat sink with a wider body heat sink, which ultimately brought the output rectifier temperature rise down to around 22°C.
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**Issue 3: Overshoot Voltage Upon Power-Up**
During initial testing, we observed a significant overshoot in the output voltage upon power-up, which exceeded the safe operating range of downstream modules. This was likely due to the sluggish dynamic response of the feedback loop. By introducing TL431-based feedback, we fine-tuned the loop’s poles and zeros, significantly reducing the overshoot. With these adjustments, the output voltage smoothly rose to the desired level without any noticeable overshoot, improving overall stability.
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**Issue 4: Large 100Hz Power Frequency Ripple**
Testing revealed a substantial 100Hz ripple in the output voltage. Initial attempts to address this included increasing the capacitance of the output electrolytic capacitors and reducing their Equivalent Series Resistance (ESR). While these steps provided limited improvement, connecting a 103-sized Y capacitor between the output voltage ground and the chassis ground (which was already grounded) led to a dramatic reduction in the ripple. The ripple was successfully controlled to within 0.5–1% of the output voltage.
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**Issue 5: Transformer Overheating**
During testing, the transformer’s surface temperature reached 50°C, posing a challenge in environments where the ambient temperature could reach 85°C. To mitigate this, we experimented with different materials, opting for PC44 instead of PC40, which showed slight improvement. Increasing the number of strands in the windings helped reduce the skin effect. Additionally, enlarging the copper surface area of the transformer’s soldering pins and slotting the PCB beneath the transformer enhanced airflow, though these measures only provided marginal benefits. Overall, combining these changes reduced the transformer temperature rise to around 40°C.
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**Issue 6: Improving Power Supply Efficiency**
While the initial efficiency of the power supply was 85.3%, we aimed to optimize performance even further. Adjustments included replacing the feedback resistors, eliminating the power output indicator circuit, and fine-tuning the loop’s DC gain. We also swapped the output diode for a low-recovery, high-reverse-voltage fast recovery diode while removing its RC absorption circuit. Additionally, we optimized the RCD clamp circuit of the primary winding to minimize unnecessary energy losses. These modifications boosted the efficiency to 87.6%.
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**Issue 7: High-Frequency Transformer Whistling**
Upon completing the initial assembly, we encountered a high-frequency whistling sound emanating from the transformer during operation. To troubleshoot this, we adjusted the feedback loop parameters and tested the MOS tube’s DS waveform and output waveform, confirming that the design parameters were correct. Replacing some ceramic capacitors with through-hole decoupling capacitors eliminated the piezoelectric effect-induced noise. Furthermore, switching between dipped and non-dipped transformers showed that the dipped transformer resolved the issue entirely. Our conclusion was that the whistling was caused by high-frequency oscillations in the transformer coils.
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This concludes our debugging journey, highlighting the importance of systematic troubleshooting and iterative improvements in achieving optimal performance in power supply circuits.
**• End •**