The stable operation of electronic devices relies on a consistent and reliable DC power supply. Most external power sources are AC, typically generated through thermal, hydro, nuclear, or wind energy. This AC power is then converted into the various DC voltages required by electronic systems using DC regulators. When grid conditions or load variations occur, DC regulated power supplies ensure a stable output voltage with minimal ripple. Over the past half-century, the technology behind these power supplies has matured significantly. In the last two decades, integrated switching power supplies have evolved in two main directions: one is the integration of the control circuitry within the switching power supply, and the other is the monolithic integration of medium and small power switching supplies.
Today, the market offers a wide range of regulated power supply products that are known for high efficiency, stable output, and reliability. Many use high-frequency transformers, which can be costly. The DC voltage regulator in this design employs a Sepic and Buck configuration as the main circuit, while the auxiliary power supply uses a high-frequency transformer. This setup takes full advantage of modern power electronics to deliver a stable DC voltage when AC input varies, all while maintaining cost-effectiveness, ease of debugging, and repairability.
**1. System Principle Design**
The high-voltage DC generated from rectification and filtering of the AC input voltage, after passing through the input protection circuit, serves as the input for the auxiliary power supply, providing the working voltage for the chip. It also acts as the input for the DC-DC/DC-DC converter, which is a core component of the switching power supply. The output voltage of the DC-DC/DC-DC converter is sampled via a feedback circuit and compared with a reference voltage to adjust the PWM duty cycle, ensuring a stable output. An overvoltage and overcurrent protection circuit is also included, which turns off the PWM signal when the voltage or current exceeds safe limits. A block diagram of the system is shown in Figure 1.
**2. Input Protection Circuit Design**
The input protection section includes a varistor RV, a thermistor RT, a fuse, a filter coil L0, a rectifier bridge, and a filter capacitor C16. The varistor protects against overvoltage surges, while the filter coil and capacitor help reduce EMI and noise. The rectifier bridge must withstand high reverse voltages and handle inrush currents up to 7–10 times the rated current. The NTC thermistor RT limits the initial charging current of C16, reducing stress on the components during startup. After startup, its resistance decreases, minimizing power loss. The circuit is illustrated in Figure 2.
**3. Auxiliary Power Supply Design**
To support the PWM control chip and the isolated driver, the auxiliary power supply uses a single-ended flyback topology with PI’s TOPSwitch II series. This device integrates control, protection, and a 700V MOSFET, offering low EMI and cycle-by-cycle current limiting. The TOP221Y is chosen for its 7W output capability, suitable for low-power applications. The auxiliary power supply circuit is shown in Figure 2, and its operation involves rectification, filtering, and a clamp circuit to manage leakage voltage. Feedback is achieved using a photocoupler and a TL431, ensuring stable output voltage regulation.
**4. Main Circuit Module Design**
The DC chopper circuit adjusts the DC voltage to a fixed or variable level, depending on application needs. The designed power supply falls into the latter category, stabilizing the input voltage (183–425 V) to produce a 24 V output. The circuit uses closed-loop feedback to maintain stability despite input or load changes. Both input and output stages include filtering, and RC snubber circuits protect the power switch from voltage spikes. The schematic is shown in Figure 3.
**5. PWM Control Circuit Design**
The PWM control circuit plays a critical role in the performance of the DC-DC/DC-DC converter. The SG3525 from Silicon General is used, featuring undervoltage lockout, soft start, error amplification, and adjustable deadband. Its peripheral circuit includes a feedback network, an accelerating capacitor, and a shutdown pin. The circuit is illustrated in Figure 4, showing how the reference voltage and feedback loop regulate the duty cycle.
**6. Optocoupler Isolation Drive Circuit Design**
An optocoupler-isolated drive circuit using the 6N137 high-speed optocoupler and a PNP transistor ensures fast response to PWM signals. The circuit includes decoupling capacitors and current-limiting resistors to enhance performance. The isolation prevents noise coupling and improves system reliability. The peripheral circuit is shown in Figure 5.
**7. Feedback Loop and Protection Circuit Design**
The feedback loop uses a magnetic amplifier isolator and sampling resistors to monitor and stabilize the output voltage. A hysteresis comparator provides overvoltage protection, while an overcurrent protection circuit uses a precision resistor and a comparator to detect and respond to excessive current. These circuits ensure the system remains safe under abnormal conditions.
**8. Experimentation**
The Sepic DC voltage regulator was tested on a prototype board, divided into an auxiliary power circuit and a main chopper circuit. Components were directly inserted for easy testing, troubleshooting, and replacement. The PWM signal was measured and recorded to evaluate performance.
**9. Conclusion**
The non-isolated design of the Sepic DC voltage regulator allows for efficient and simple driving of the MOSFET, with a reliable and cost-effective structure. The regulator meets the requirements for stable and reliable DC output, making it a practical solution for various applications.
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