Quasi-square-wave-resonant converters, also known as QR (quasiresonant) converters, allow the design of flyback-type SMPSs (switch-mode power supplies) with a reduced EMI (electromagnetic-interference) signature and improved efficiency. You can achieve so-called QR operation by authorizing the turn-on of the switching MOSFET when the drain voltage reaches its minimum—hence, the name valley switching operation. The circuit usually externally detects the minimum drain voltage of an auxiliary winding, which delivers a voltage image of the core's internal flux activity. The circuit in Figure 1 offers a solution that incorporates core-reset detection with the aid of an auxiliary winding. As you can see, the auxiliary winding solely performs the function of core-reset detection. To further simplify this schematic, you can remove the auxiliary winding and use the drain signal itself to generate the demagnetization signal that Pin 1 of the NCP1207 requires. Figure 2 shows this arrangement. Thanks to its use of high-voltage technology, On Semiconductor's (www.onsemi.com) NCP1207 QR controller can derive its power directly from the rectified mains via its "dynamic-self-supply" feature.

Capacitor C₁ removes the dc component of the drain signal. R₁, together with the internal resistor on the NCP1207 demagnetization pin (Pin 1), creates a resistor divider. The divider safely limits the voltage you apply on the controller when the drain swings high. C₂ delays the signal to detect exactly the drain signal valley. Compared with Figure 1, where R₁ and C₂ were present, the only addition is C₁ (in replacement of the auxiliary winding. Because capacitor C₁ touches the MOSFET drain, it must sustain at least the same maximum voltage: A 220-pF, 1-kV or 1-nF, 1-kV ceramic capacitor perfectly fills the bill. The internal resistance of NCP1207's demagnetization pin is 28 kΩ. The value of R₁ ranges from 1 to 2 MΩ if you want to create a 5V signal with a maximum drain voltage of 600 and 900V. The value of capacitor C₂ depends on the frequency of the resonating network comprising the primary inductance, L_PRI, and the total capacitance of the drain node. You adjust the values of R₁ and C₂ directly on the board to reach the best valley detection possible. Because R₁ has a relatively high value, it is essential that the component resides close to the controller's Pin 1. The Figure 3 waveforms show the final application results. The waveforms are captured on a single-output, 30W SMPS delivering 16V. In this application, C₁=220 pF/1 kV, R₁=1.5 MΩ, and C₂=100 pF. By properly adjusting the time constants, you can obtain perfect valley switching.

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