How to design a more efficient, lower EMI quasi-resonant adapter

Quasi-square wave resonant converters, also known as quasi-resonant (QR) converters, are widely used in power adapters. The key feature of quasi-square wave resonance is that the metal oxide semiconductor field effect transistor (MOSFET) is turned on when the drain-to-source voltage (VDS) reaches its minimum value, thereby reducing switching losses and improving electromagnetic interference (EMI) signals.

When the quasi-resonant converter operates in discontinuous conduction mode (DCM), VDS must be reduced from the sum of the input voltage (Vin) and the reflected voltage (Vreflect) to Vin. The transformer primary inductance (Lp) and the node capacitance (Clump, which is the combined value of all capacitances surrounding the MOSFET drain node, including MOSFET capacitance and transformer parasitic capacitance, etc.) form a resonant network. Lp and Clump oscillate with each other, and the oscillation half-period is calculated by the formula.

However, the switching frequency of the self-oscillating quasi-resonant converter increases when the load decreases; thus, under light load conditions, if the switching frequency is not limited, the loss will be high, affecting the power supply efficiency; therefore, the switching frequency must be limited.

There are two ways to limit the switching frequency. The first is the frequency clamping method with frequency foldback used by traditional quasi-resonant converters, which limits the switching frequency by frequency clamping. However, under light load conditions, when the system switching frequency reaches the frequency clamping limit value, multiple valley frequency jumps in the audible noise range occur, resulting in signal instability.

To solve this problem, the second method, valley locking, is used. When the load decreases, it remains locked at a certain valley until the output power drops significantly, and then changes the valley. When the output power drops to a certain value, it enters the voltage-controlled oscillator (VCO) mode, as shown in Figure 1. Specifically, the feedback (FB) comparator selects the valley and passes the information to the counter, and the hysteresis characteristics of the FB comparator lock the valley. This method provides natural switching frequency limitation when the system load decreases, without valley frequency hopping noise and without reducing energy efficiency.