AC-DC Power Supply Design Guide with On-Bright Controllers
Introduction to AC-DC Conversion
AC-DC power supplies convert alternating current from the mains to regulated direct current for electronic devices. The flyback topology is widely used for power levels up to 100W due to its simplicity, low cost, and ability to provide multiple outputs. On-Bright offers both PWM and quasi-resonant controllers for flyback applications.
Topology Selection
Select PWM controllers (OB2263) for cost-sensitive applications where standard efficiency is acceptable. Choose QR controllers (OB2273) for high-efficiency requirements where valley switching can reduce switching losses. Consider power level, efficiency targets, EMI requirements, and cost constraints when selecting topology.
Transformer Design
Transformer design is critical for flyback converter performance. Key parameters include turns ratio, magnetizing inductance, and leakage inductance. The turns ratio determines the voltage conversion and affects MOSFET voltage stress. Magnetizing inductance affects current ripple and operating mode (CCM vs DCM). Minimize leakage inductance to reduce voltage spikes and EMI.
Component Selection
Select power MOSFET based on voltage rating (typically 600V for universal input), current rating, and RDS(on). Choose output rectifier based on reverse voltage, forward current, and switching speed. Input capacitors should handle the rectified AC voltage with adequate ripple current rating. Output capacitors determine output ripple and transient response.
💡 FAE Insights
📋 Customer Cases
Challenge
Team needed to quickly develop 60W adapter meeting efficiency regulations
Solution
Followed On-Bright design guide and reference design for OB2273-based adapter
Results
First-pass design success, passed all efficiency and EMI requirements
Frequently Asked Questions
1. What is the difference between CCM and DCM operation?
In Continuous Conduction Mode (CCM), inductor current never reaches zero during the switching cycle. In Discontinuous Conduction Mode (DCM), inductor current falls to zero before the next cycle begins. DCM is simpler to control and better for low power; CCM offers lower ripple but more complex control.
2. How do I calculate transformer turns ratio?
Turns ratio Np/Ns = (Vdc_min * Dmax) / (Vout + Vf), where Vdc_min is minimum DC bus voltage, Dmax is maximum duty cycle (typically 0.45), Vout is output voltage, and Vf is diode forward voltage. Include margin for voltage drops and regulation.
3. What MOSFET voltage rating should I select?
For universal input (85-265VAC), use 600V or 650V MOSFETs. The drain voltage stress is approximately Vin_max + Vreflected + Vspike. With proper snubber design, 600V rating is usually sufficient for flyback converters up to 100W.
4. How do I reduce EMI in flyback converters?
Reduce EMI through proper PCB layout (minimize loop areas), input filtering (common-mode choke, X and Y capacitors), soft switching (QR operation), snubber circuits, and shielded transformers if needed. Frequency jittering in On-Bright controllers also helps spread EMI spectrum.
5. What is the purpose of the snubber circuit?
The snubber circuit absorbs energy from transformer leakage inductance, protecting the MOSFET from excessive voltage spikes. It also reduces EMI by damping high-frequency oscillations. RC snubbers are commonly used across the primary winding or MOSFET drain-source.