Quasi-Resonant Operation and Valley Switching Guide
Principles of Quasi-Resonant Operation
Quasi-resonant flyback converters switch the MOSFET at the minimum drain voltage point (valley) after transformer demagnetization. This valley switching reduces switching losses and EMI by switching when both voltage and current are minimized. The resonant frequency is determined by the primary inductance and MOSFET output capacitance.
Benefits of Valley Switching
Valley switching provides several benefits: reduced switching losses leading to higher efficiency, lower EMI generation, reduced MOSFET stress, and simplified EMI filtering. At light loads, frequency reduction maintains high efficiency across the entire load range.
Transformer Design for QR
QR transformer design requires attention to leakage inductance and winding capacitance. Lower leakage inductance provides cleaner valley detection. The auxiliary winding must provide accurate voltage feedback for proper valley detection. Core selection should consider the variable frequency operation.
Frequency Foldback and Light Load Operation
At light loads, QR controllers reduce switching frequency to minimize losses while maintaining regulation. This frequency foldback extends high efficiency to very light loads, important for meeting modern efficiency standards. The minimum frequency is typically limited to avoid audible noise.
💡 FAE Insights
📋 Customer Cases
Challenge
Needed to achieve >90% efficiency in 65W adapter for notebook application
Solution
Implemented OB2273 QR controller with synchronous rectification
Results
Achieved 92% peak efficiency, passed all DoE and CoC requirements
Frequently Asked Questions
1. What is valley switching and how does it work?
Valley switching turns on the MOSFET when the drain voltage reaches its minimum point (valley) during the resonant ring after transformer demagnetization. This minimizes switching losses because switching occurs at minimum voltage and current.
2. How does QR compare to traditional PWM in efficiency?
QR operation typically provides 2-4% efficiency improvement over PWM at full load due to reduced switching losses. The benefit is greater at light loads where frequency reduction further reduces losses. However, QR requires more careful transformer design.
3. What causes missed valleys in QR operation?
Missed valleys are usually caused by excessive ringing, poor auxiliary winding coupling, or insufficient delay in the valley detection circuit. Ensure clean voltage feedback and proper transformer construction.
4. How do I prevent audible noise in QR converters?
Audible noise can occur at light loads when switching frequency enters the audible range. Most QR controllers include minimum frequency clamping to prevent this. Ensure proper PCB layout and transformer construction to minimize mechanical vibration.
5. Can QR operation be used with synchronous rectification?
Yes, QR operation works well with synchronous rectification. The SR controller (like OB2203) replaces the output diode with a MOSFET, reducing conduction losses. This combination provides the highest efficiency for flyback converters.