## Overview of the VIPER31 The **VIPER31** is a high-voltage converter from STMicroelectronics belonging to the ViperPlus family. It is an integrated Offline Switcher IC designed for Switched-Mode Power Supplies (SMPS). It combines an 800 V avalanche-rugged MOSFET with a PWM controller in a single package (typically SO-16 or DIP-7). --- ### Key Technical Specifications | Feature | Specification | | :--- | :--- | | **Drain-Source Voltage ($V_{DSS}$)** | 800 V | | **Typical $R_{DS(on)}$** | 3.5 $\Omega$ | | **Max Output Power** | Up to 18W (Wide range) / 30W (High range) | | **Switching Frequency** | 30 kHz, 60 kHz, or 132 kHz (fixed options) | | **Supply Voltage ($V_{CC}$)** | 8 V to 30 V | | **Protections** | OVP, OCP, OTP, Short Circuit, Brown-out | --- ### Main Electronic Components (Internal Architecture) The VIPER31 integrates several critical sub-systems into one chip to reduce external component count: 1. **High-Voltage MOSFET:** * An 800 V power transistor that handles the switching of the primary transformer winding. Its high voltage rating provides a safety margin against voltage spikes without needing complex snubber circuits. 2. **High-Voltage Startup Current Source:** * This allows the IC to start directly from the rectified AC mains. Once the device starts switching, the internal startup source turns off to save power, enabling ultra-low standby consumption (less than 10 mW to 30 mW). 3. **PWM Controller:** * The "brain" of the chip. It manages the switching frequency and duty cycle based on the feedback received from the output side (usually via an optocoupler). 4. **Error Amplifier:** * Includes a high-gain error amplifier for voltage regulation, making it suitable for both isolated and non-isolated topologies. 5. **Protection Logic:** * **Pulse-by-pulse current limiting:** Prevents transformer saturation. * **Hysteretic Thermal Shutdown:** Shuts the device down if it overheats. * **Brown-out Protection:** Monitors the input voltage to ensure the device only operates within safe AC mains levels. --- ### Typical Application Circuit Parts To build a power supply using the VIPER31, you generally need the following external parts: * **Transformer:** Specifically designed for flyback topology. * **Input Rectifier & Filter:** A bridge rectifier and a high-voltage electrolytic capacitor (Bulk Cap). * **Feedback Network:** Typically a **TL431** voltage reference and an **Optocoupler** (e.g., PC817) to provide isolation. * **VCC Capacitor:** A small electrolytic capacitor to provide stable power to the IC controller. * **Snubber Circuit:** Usually a diode, resistor, and capacitor (RCD) to protect the MOSFET from leakage inductance spikes. --- ### Advantages for Designers * **Jittered Frequency:** The switching frequency is modulated (jittered) to spread the EMI energy, making it easier to pass electromagnetic compliance tests with smaller filters. * **Massive Integration:** Replaces roughly 10-15 discrete components, saving PCB space and increasing reliability. * **Robustness:** The 800V rating is significantly higher than standard 600V or 650V MOSFETs, offering better durability against unstable power grids.