Research on DC/DC DC Converter Based on SG3525

**Introduction** With the advancement of power conversion technology, power MOSFETs have become essential components in switching power supplies. To effectively drive these devices, Silicon General Semiconductor Corporation from the United States introduced the SG3525, a highly integrated PWM control chip designed specifically for N-channel power MOSFETs. The SG3525 is known for its excellent performance, comprehensive features, and versatility. It is easy to use, reliable, and provides strong driving capability with a push-pull output structure. The chip also includes important protection functions such as undervoltage lockout, soft start, and overcurrent protection. Additionally, it allows for adjustable frequency and limits the maximum duty cycle, making it ideal for a wide range of applications. The key performance characteristics of the SG3525 include: 1) A wide operating voltage range from 8V to 35V, allowing flexibility in different power supply configurations. 2) An internal reference voltage source of 5.1V ± 1.0%, ensuring stable operation under varying conditions. 3) An on-chip oscillator that can operate between 100Hz and 400kHz, offering a broad range of frequency options. 4) The ability to synchronize the oscillator externally, providing additional control over timing. 5) Adjustable dead time, which helps prevent cross-conduction in power switches and improves efficiency. The final stage uses a push-pull configuration, enabling faster switching speeds and delivering up to 400mA of sink or source current. 6) A built-in undervoltage lockout circuit that disables the chip when the input voltage drops below 8V, reducing power consumption to less than 2mA. 7) A soft start function that gradually increases the output voltage, preventing inrush currents and protecting the system during startup. The soft-start capacitor connected to pin 8 charges through a constant current source, and the time to reach 2.5V is calculated as t = (2.5V / 50μA) × C, allowing the duty cycle to increase smoothly from 0% to 50%. 8) A built-in PWM latch that eliminates noise and oscillations from the comparator, ensuring clean and stable output signals. The system only resets in the next clock cycle, enhancing overall reliability. **1. Pulse Width Modulator SG3525 Introduction** **1.1 Block Diagram** The SG3525 is a fixed-frequency PWM controller housed in a standard 16-pin DIP package. Its pin configuration and internal block diagram are illustrated in Figures 1 and 2, respectively.

**1.2 Pin Function Description** When the power supply voltage Vs is applied to pin 15, it is split into two paths: one is sent to the NOR gate, and the other is used to generate a stable reference voltage. Pin 5 is connected to an external capacitor CT, while pin 6 is connected to an external resistor RT, determining the oscillator frequency. The oscillator output is divided into two parts: one serves as a clock signal for the flip-flop and NOR gates, while the other is used as a sawtooth wave for the comparator. The non-inverting input of the comparator receives this sawtooth wave, while the inverting input is connected to the output of the error amplifier. The comparator then generates a variable-width square wave based on the error voltage. This signal is fed into one of the inputs of the NOR gate, while the other two inputs receive the flip-flop output and the sawtooth wave. The flip-flop outputs complementary signals, which drive transistors VT1 and VT2, producing two PWM signals with a 180° phase difference. The sawtooth wave ensures a proper dead time, preventing both transistors from turning on simultaneously, thus improving system safety and efficiency.

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