The stable operation of electronic devices relies on a consistent and reliable DC power supply. Most external power sources today are AC, generated through thermal, hydro, nuclear, or wind energy. This AC power is then converted into the various DC voltages required by electronic systems via DC regulators. When grid conditions or load changes occur, these regulated power supplies maintain a stable output voltage with minimal ripple. Over the past half-century, the technology behind DC power supplies has matured significantly. In the last 20 years, integrated switching power supplies have evolved in two main directions: first, the integration of control circuits within the power supply, and second, the monolithic integration of medium and small power switching power supplies.
There are numerous types of regulated power supplies available on the market, most of which offer high efficiency, stable output, and high reliability. Many use high-frequency transformers, which can be relatively expensive. The DC voltage regulator in this design uses a Sepic and Buck circuit as its main configuration, while the auxiliary power supply employs a high-frequency transformer. This approach leverages modern power electronics to provide a stable DC output from a wide range of AC inputs, ensuring reliability, stability, and a lower cost, making it easier to debug and maintain.
**1. System Principle Design**
The high-voltage DC generated by rectifying and filtering the AC input voltage through the input protection circuit serves as the input for the auxiliary power supply, providing the necessary 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 by a feedback circuit, compared with a set reference voltage, and the PWM duty cycle is adjusted accordingly to maintain a stable output. An overvoltage and overcurrent protection circuit is included, which turns off the PWM signal when the voltage or current exceeds safe limits. A system block diagram is shown in Figure 1.
**2. Input Protection Circuit Design**
The input protection section includes components such as a varistor (RV), a thermistor (RT), a fuse, a filter coil (L0), a rectifier bridge, and a filter capacitor (C16). The varistor provides overvoltage protection for the AC input, absorbing surges. The filter coil L0 and capacitor C0 help suppress EMI and reduce noise interference. When selecting the rectifier bridge, it should be able to withstand high reverse voltage and handle inrush currents greater than 7–10 times the rated current. The thermistor RT, a negative temperature coefficient (NTC) resistor, limits the peak charging current of C16 during startup, and after startup, its resistance decreases, minimizing power consumption. The circuit diagram is shown in Figure 2.
**3. Auxiliary Power Supply Design**
To ensure proper operation of the designed Sepic DC regulator, the system requires two independent power supplies for the PWM control chip and the isolated driver. The auxiliary power supply uses a single-ended flyback topology, with the control chip being the PI company's TOPSwitch II series. This device integrates control, protection, and a 700V MOSFET switch. It features leading-edge blanking, automatic restart, low EMI, and cycle-by-cycle current limiting. Since the PWM control chip and isolation chip operate at lower power, the TOP221Y is used, delivering 7W across a wide input voltage range, which meets the system’s requirements. The auxiliary power supply circuit is shown in Figure 3.
**4. Main Circuit Module Design**
The DC chopper circuit is responsible for converting one DC voltage into another, either fixed or adjustable. Applications include variable DC output or constant DC output under varying input conditions. This design falls into the latter category. The high-voltage DC input (183–425 V) is stabilized by the Sepic circuit, acting as a Buck circuit. By setting an appropriate duty cycle, a stable 24V DC output is achieved. The chopper circuit uses closed-loop feedback, allowing the output voltage to remain stable despite variations in input voltage or load. Both input and output stages include filtering, resulting in reduced voltage ripple. RC snubber circuits are used across the drain and source of the power transistor to absorb voltage peaks and protect the switch.
**5. PWM Control Circuit Design**
The PWM control circuit plays a crucial role in the performance of the DC-DC/DC-DC converter. The SG3525 from Silicon General was selected due to its undervoltage lockout, soft start, reference voltage source, error amplifier, and output current limit features. Its totem-pole output stage, oscillator, and adjustable deadband make it suitable for power supply designs. The peripheral circuit of the SG3525 is shown in Figure 4, with detailed analysis provided.
**6. Optocoupler Isolated Drive Circuit Design**
The regulator operates at up to 100kHz and uses a high-speed optocoupler (6N137) and a transistor to form an isolated drive circuit that responds quickly to PWM signals. The optocoupler isolates the signal and ensures safety. The output must be pulled up with a resistor, and the LED inside the optocoupler requires a current-limiting resistor. A decoupling capacitor is placed near the power supply pins of the optocoupler to improve stability. The circuit is shown in Figure 5.
**7. Feedback Loop and Protection Circuit Design**
The feedback loop consists of a magnetic amplifier isolator and sampling resistors. The sampled voltage is isolated and amplified, then used as a feedback signal for the SG3525. It is also used in the overvoltage protection circuit, which triggers when the output voltage exceeds a set threshold. The overcurrent protection circuit uses a precision sampling resistor and a magnetic isolation amplifier, similar to the overvoltage protection mechanism.
**8. Experiment**
The Sepic DC voltage regulator’s prototype board is divided into an auxiliary power circuit and a main chopper circuit. Components are directly inserted for easy testing, troubleshooting, and replacement. The PWM signal was tested and recorded for further analysis.
**9. Conclusion**
The main circuit of the Sepic DC voltage regulator is non-isolated, yet it provides sufficient driving current for the MOSFETs, with a simple and reliable driver structure. The regulator meets the requirements for stable and reliable output voltage, and its overall cost is relatively low.
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