High-Performance Quasi-Resonant SMPS Controller: Design Considerations for the Infineon ICE1QS01

The relentless pursuit of higher efficiency, reduced electromagnetic interference (EMI), and compact form factors in switch-mode power supplies (SMPS) has driven the adoption of advanced control topologies. Among these, quasi-resonant (QR) operation stands out as a superior alternative to conventional fixed-frequency pulse-width modulation (PWM), particularly in applications like AC-DC adapters and TV power supplies. The Infineon ICE1QS01 is a dedicated QR current-mode controller IC that encapsulates the benefits of this technology. Designing a robust and high-performance SMPS with this controller requires careful attention to several critical aspects.

A fundamental advantage of the ICE1QS01 is its valley switching operation. Unlike hard-switching controllers that turn on the power MOSFET while the drain-to-source voltage (V_DS) is high, resulting in significant switching losses, the ICE1QS01 actively detects the valleys of the resonant V_DS ringing. By initiating turn-on at the moment of minimum V_DS (a valley), it achieves zero-voltage switching (ZVS) conditions. This dramatically reduces switching losses, lowers stress on the MOSFET, minimizes dV/dt, and consequently reduces conducted EMI. This allows for either higher switching frequencies (enabling smaller magnetics) or cooler operation at existing frequencies.

Key to enabling this valley switching is the proper design of the auxiliary (aux) winding on the main transformer. This winding provides a scaled-down replica of the MOSFET's drain voltage, which is fed back to the `ZCD` (Zero Current Detection) pin of the ICE1QS01. The IC uses this signal to determine the timing of the resonant valleys. The resistor divider network attenuating this signal to the `ZCD` pin must be calculated precisely to ensure the internal comparator is triggered correctly without being overdriven. Furthermore, a small RC filter is often necessary to suppress any high-frequency noise on this sensing line that could lead to false valley detection.

The control loop stability and protection features are equally vital design considerations. The ICE1QS01 employs current-mode control, which simplifies loop compensation and provides inherent cycle-by-cycle current limiting. The compensation network connected to the `COMP` pin must be tailored to the specific transformer and output filter characteristics to ensure a stable output voltage with good transient response across all load conditions.

The IC's comprehensive protection suite, including over-voltage protection (OVP), over-load protection (OLP), and over-temperature protection (OTP), must be correctly configured. For instance, the OVP threshold is set via a resistor divider on the `V_CONTROL` pin, which monitors the aux winding voltage. This setting must be chosen to protect the output circuitry from dangerous voltage spikes during fault conditions like an open feedback loop. Similarly, the leading-edge blanking (LEB) time on the current sense input (`IS`) prevents false triggering of the OCP during the MOSFET turn-on event, allowing the use of a smaller sense resistor for improved efficiency without compromising protection reliability.

Finally, the layout of the printed circuit board (PCB) is paramount for noise immunity and overall performance. The path containing the high-switching-current components—the MOSFET, transformer, and output rectifier—must be as short and direct as possible to minimize parasitic inductance and reduce EMI. The ground connections for the IC's small-signal section and the high-current section should be separated and connected at a single point to avoid noise corruption of the control signals. Decoupling capacitors for the `VCC` pin must be placed immediately adjacent to the IC.

ICGOODFIND: The Infineon ICE1QS01 QR controller is a powerful enabler for high-efficiency, low-EMI power supplies. A successful design hinges on meticulous attention to valley switching synchronization via the ZCD circuit, precise loop compensation for stability, rigorous configuration of integrated protection features, and an optimized low-noise PCB layout to unleash its full performance potential.

Keywords: Quasi-Resonant Controller, Valley Switching, Zero-Voltage Switching (ZVS), Current-Mode Control, EMI Reduction.