1W CMOS Power Amplifier# Technical Documentation: FAN7021MX High-Voltage Current-Mode PWM Controller
Manufacturer : FAIRCHILD (ON Semiconductor)
Component : FAN7021MX
Description : High-Voltage, Current-Mode PWM Controller for Offline Quasi-Resonant Flyback Converters
---
## 1. Application Scenarios
### 1.1 Typical Use Cases
The FAN7021MX is specifically engineered for offline switch-mode power supplies (SMPS) utilizing a quasi-resonant (QR) flyback topology. Its primary function is to serve as the core control IC in AC-DC power conversion stages where high efficiency, reduced electromagnetic interference (EMI), and compact design are critical.
* Primary Application : Controlling the main power switch (typically a MOSFET) in a flyback converter. It generates a pulse-width modulated (PWM) signal whose duty cycle is adjusted based on feedback to regulate the output voltage.
* Key Operation : The controller operates in Quasi-Resonant (Valley-Switching) mode . It detects the valley point of the drain voltage waveform on the primary-side power MOSFET and triggers the next switching cycle at this minimum voltage. This minimizes switching losses and reduces turn-on noise, leading to higher efficiency.
### 1.2 Industry Applications
The FAN7021MX is a versatile component found in a wide range of consumer, industrial, and computing power systems requiring robust and efficient AC-DC conversion.
* Adapter/Charger Power Supplies : Ideal for notebook PC adapters, tablet chargers, and smartphone fast chargers (up to ~65W) due to its high efficiency across load ranges and good standby power performance.
* Auxiliary (Standby) Power Supplies (AUX) : Used in main power supplies for televisions, monitors, and home appliances to provide the always-on low-voltage rail (e.g., 5VSB).
* Open-Frame Switching Power Supplies : Common in industrial control systems, networking equipment (routers, modems), and LED lighting drivers where a compact, reliable, and cost-effective offline power solution is needed.
* Consumer Electronics : Power modules for set-top boxes, gaming consoles, and audio equipment.
### 1.3 Practical Advantages and Limitations
Advantages:
* High Efficiency : Valley-switching operation significantly reduces switching losses, especially at medium to light loads, improving overall power supply efficiency and helping meet energy standards like ENERGY STAR and CoC.
* Low EMI : Switching at the valley of the drain voltage minimizes voltage and current spikes, reducing conducted and radiated electromagnetic interference. This simplifies EMI filter design and compliance testing.
* Integrated High-Voltage Startup : The built-in HV startup circuit connects directly to the rectified high-voltage DC bus, eliminating the need for an external startup resistor and reducing standby power consumption.
* Comprehensive Protection Features : Includes cycle-by-cycle current limiting (OCP), over-voltage protection (OVP), over-load protection (OLP), and internal thermal shutdown (TSD), enhancing system reliability.
* Frequency Reduction at Light Loads : Improves light-load efficiency by reducing switching frequency, which lowers switching losses.
Limitations:
* Topology Specific : Optimized for quasi-resonant flyback converters. It is not suitable for forward, half-bridge, or LLC resonant topologies without significant external circuitry.
* Power Range : Best suited for low to medium power applications (typically up to 100W, though practical designs often target ≤65W for optimal performance). For higher powers, other topologies or controllers may be more appropriate.
* Minimum Load Considerations : In QR mode, the switching frequency increases at very heavy loads and decreases at light loads. At no-load or extremely light load, the frequency may drop into the audible range
