Designing Isolated Flyback Converters with the Microchip HV9112NG-G High-Voltage SMPS Controller

Isolated power supplies are fundamental to modern electronics, providing safety, noise isolation, and voltage flexibility in applications ranging from industrial controls to consumer appliances. Among the various topologies, the flyback converter remains a dominant choice for low-to-medium power requirements due to its simplicity, cost-effectiveness, and inherent isolation capabilities. Designing a robust and efficient flyback converter, however, requires a sophisticated controller that can handle high voltages and complex control loops. The Microchip HV9112NG-G is a high-voltage Switch-Mode Power Supply (SMPS) controller engineered specifically to meet these challenges.

At its core, the HV9112NG-G is a current-mode PWM controller integrated into a high-voltage process. This allows it to operate directly from rectified line voltages up to 450V, significantly simplifying the start-up circuit and enhancing overall system reliability. Unlike controllers that require a low-voltage auxiliary supply for start-up, the HV9112NG-G self-initiates, drawing its initial operating power directly from the high-voltage DC bus. Once running, it seamlessly transitions to a lower-voltage supply generated by an auxiliary winding on the main transformer, improving efficiency.

A key advantage of current-mode control, as implemented in this IC, is its inherent cycle-by-cycle current limiting. This provides superior line regulation and simplified feedback loop compensation compared to voltage-mode controllers. The device features a programmable oscillator, allowing designers to set the switching frequency with a single external resistor, typically in the range of 100 kHz to 500 kHz. This flexibility enables optimization for size (higher frequency) or efficiency (lower frequency).

The design process for a flyback converter with the HV9112NG-G involves several critical steps:

1. Transformer Design: The flyback transformer (or more accurately, a coupled inductor) is the heart of the converter. Its design dictates the turns ratio, which affects the output voltage range and the voltage stress on the primary-side switch. Proper calculation of the primary inductance is crucial as it determines the converter's operating mode (Continuous Conduction Mode - CCM or Discontinuous Conduction Mode - DCM) and the peak primary current.

2. Selecting the Power MOSFET: The controller drives an external N-channel MOSFET. The selection of this switch is critical; its voltage rating must withstand the maximum drain voltage, which is the sum of the input voltage, the reflected output voltage from the transformer, and any leakage inductance spike. The integrated high-voltage start-up regulator of the HV9112NG-G eliminates the need for a separate high-voltage transistor, simplifying the BOM.

3. Feedback and Isolation: For primary-side regulation (PSR), the output voltage is sensed via the auxiliary winding. This method provides a cost-effective solution without an optocoupler. For higher accuracy, a secondary-side error amplifier and an optocoupler can be used to close the feedback loop, with the signal fed into the controller's feedback (FB) pin.

4. Protection Circuitry: The HV9112NG-G offers robust protection features. The cycle-by-cycle current limiting protects the MOSFET and transformer from overcurrent conditions. Additional external circuitry can be implemented for over-voltage protection (OVP) and under-voltage lockout (UVLO), ensuring safe operation under fault conditions.

The performance of the HV9112NG-G is characterized by its excellent line and load regulation, making it suitable for a wide array of applications. It is particularly well-suited for auxiliary power supplies (AUX PSUs) in major appliances, industrial systems, and smart meters, where reliability and isolation from noisy mains power are non-negotiable.

ICGOODFIND: The Microchip HV9112NG-G stands out as a highly integrated and robust high-voltage SMPS controller that simplifies the design of isolated flyback converters. Its ability to start up directly from the high-voltage rail, combined with current-mode control and flexible frequency programming, provides engineers with a powerful platform to build efficient, reliable, and cost-effective isolated power supplies for demanding environments.

Keywords: Isolated Flyback Converter, Current-Mode Control, High-Voltage Start-Up, Primary-Side Regulation, SMPS Controller.