An integrated power supply primary circuit typically features a switching IC, a primary-side MOSFET, a transformer, and feedback/control circuitry to regulate output without secondary-side optocouplers.

Primary Circuit Overview

The primary side of an integrated power supply is responsible for converting the input voltage (AC or DC) into a controlled high-frequency signal that drives the transformer. Key components include:

• Switching Device: Usually a MOSFET integrated into the IC (e.g., LM5180) that switches the input voltage at high frequency .
• Controller IC: Provides primary-side regulation (PSR), monitoring the voltage across the primary winding to maintain output voltage without secondary feedback .
• Transformer: Steps down or isolates the voltage. In PSR designs, the secondary voltage is inferred from primary-side sensing.
• Snubber/Clamp Circuit: Protects the MOSFET from voltage spikes caused by transformer leakage inductance.
• Input Filter: Capacitors and inductors smooth the input voltage and reduce EMI.

Typical Operation

1. Input Voltage Application: The AC or DC input is filtered and applied to the primary winding.
2. Switching Control: The IC turns the MOSFET on and off in boundary conduction mode (BCM) or continuous mode, controlling energy transfer to the transformer .
3. Primary-Side Feedback: The IC senses the reflected voltage from the secondary through the transformer and adjusts the duty cycle to regulate output voltage, eliminating the need for optocouplers .
4. Output Rectification: On the secondary side, diodes or synchronous rectifiers convert the high-frequency AC to DC, which is then filtered by capacitors.

Example Block Diagram

A simplified primary-side regulated flyback circuit includes:

• AC Input → EMI Filter → Bridge Rectifier → Bulk Capacitor → Primary MOSFET (controlled by IC) → Transformer → Secondary Rectifier → Output Capacitor
• Primary Feedback Loop: Voltage sense resistor or auxiliary winding feeds back to the IC to regulate switching duty cycle .

Design Considerations

• Load Regulation: PSR designs can achieve ±1% regulation without secondary feedback components .
• Isolation: Transformer provides galvanic isolation between input and output.
• Protection: Overvoltage, overcurrent, and thermal shutdown are integrated in modern ICs.
• Efficiency: Boundary conduction mode and integrated MOSFETs reduce switching losses and component count .

References for Practical Circuits

• LM5180 Flyback PSR Design: Provides a compact primary-side regulated solution with integrated MOSFET and high-density transformer design .
• SMPS Power Supply Circuits: Various AC-DC and DC-DC topologies with primary-side regulation and integrated ICs for hobbyist and industrial applications .
• Linear Power Supply Examples: For simpler low-power applications, series voltage regulators with primary-side control can be used . This structure allows designers to implement compact, efficient, and isolated power supplies without requiring secondary-side optocouplers, making it ideal for modern integrated power supply designs.