Design Description

This design utilizes the LM5020 Current Mode PWM Controller configured in a dual output Flyback converter.  This isolated design operates at 300 kHz and uses a custom transformer to convert the input voltage, via flyback action, to an output voltage level of 12V at 2.1A, as well as produce an isolated 5V@0.2A rail.  The relative low output power of this design, and the fact that V_cc is not required to power additional IC’s, allows the primary bias winding to be connected to V_in.  The LM5020 has a minimum input voltage of 13V so a start-up bias circuit was added to apply an external voltage to V_cc until the input voltage reached the proper threshold level.

With total potential output power less than 30W, a custom EFD20 core was chosen for this design.  Because of the input voltage range, a CCM to DCM transition tradeoff was made to minimize losses associated with DCM operation.   Radiated EMI requirements necessitated the use of snubbers and other filtering elements to minimize high energy signals from interfering with adjacent circuitry.

 The 12V rail also fed a 5V@0.5A rail, as well as an optional 3.3V@0.5A rail.  The secondary side feedback components are selected appropriately to ensure the voltage regulation remains solid and that proper gain and phase margins are maintained for stable operation and quick transient response.

•	RD-136 Reference Design Document
•	Design file (schematic)

Additional Resources

Board Layout

• Internal Start-up Bias Regulator
• Error Amplifier
• Precision Voltage Reference
• Programmable Softstart
• A Peak Gate Driver
• Maximum Duty Cycle Limiting (80% for LM5020-1 or 50% for LM5020-2)
• Programmable Line Under Voltage Lockout (UVLO) with Adjustable Hysteresis
• Cycle-by-Cycle Over-Current Protection
• Slope Compensation (LM5020-1)
• Programmable Oscillator Frequency with Synchronization Capability
• Current Sense Leading Edge Blanking
• Thermal Shutdown Protection