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The flyback converter is one of the most commonly used topologies in power supply design due to its simplicity and efficiency. As shown in Figure 1, it features a basic structure that includes various parasitic components such as the primary leakage inductance, the MOSFET's Coss (which consists of Cgd and Cds), and the junction capacitance of the secondary diode. These elements play a significant role in the behavior of the circuit during switching transitions.
The topology is derived from a buck-boost converter, but instead of using a single inductor, it uses a coupled inductor—typically a transformer with an air gap. When the main switching device, the MOSFET, is turned on, energy is stored in the magnetic core of the transformer. When the MOSFET turns off, this energy is transferred to the output through the secondary winding. Because the transformer must store energy during the on-time, an air gap is essential to prevent saturation.
Flyback converters are well-suited for low-to-medium power applications such as battery chargers, adapters, and DVD players. However, their performance is limited by the parasitic elements present in the circuit, especially when operating at high frequencies.
During normal operation, when the MOSFET is turned off, the primary current (Id) charges the MOSFET’s Coss. As the voltage across Coss increases, it exceeds the input voltage plus the reflected output voltage (Vin + nVo), causing the secondary diode to turn on. At this point, the voltage across the primary inductor Lp is clamped to nVo. The primary leakage inductance Lk (Lkp + n²×Lks) then resonates with Coss, leading to high-frequency voltage spikes that can cause issues with the MOSFET.
Flyback converters can operate in either continuous conduction mode (CCM) or discontinuous conduction mode (DCM). In CCM, the secondary diode remains conducting until the next switch-on, which leads to a reverse recovery current that adds to the primary current and causes a large spike. In DCM, the secondary current drops to zero before the end of the switching cycle, allowing resonance between the primary inductor and the MOSFET’s Coss.
Figure 2 shows the waveform in CCM, while Figure 3 illustrates the DCM mode. In both cases, the MOSFET experiences voltage and current changes during switching, which can result in oscillations and current spikes.
As shown in Figure 4, the measured MOSFET voltage and current waveforms in DCM reveal significant variations during turn-on and turn-off. These spikes are caused by the interaction between the parasitic capacitances and inductances in the circuit.
Figure 5 presents the equivalent analysis circuit of the flyback converter in DCM mode, showing the behavior of the MOSFET at different stages: turn-on, turn-on phase, turn-off, and turn-off phase. During turn-on, the voltage across the stray capacitance Cp starts to change, generating a discharge current. As the MOSFET turns on, the power supply voltage charges the stray capacitance, creating a current spike. Similarly, the Cds capacitor discharges, forming another spike within the MOSFET itself.
When the MOSFET turns off, the primary current charges the Coss, and once the voltage exceeds Vin + nVo, the secondary diode turns on, clamping the primary inductor voltage to nVo. This results in resonance between the leakage inductance and the Coss, producing high-frequency oscillations.
Understanding these phenomena is crucial for optimizing the performance of a flyback converter. Engineers often encounter challenges when analyzing the measured waveforms, particularly when dealing with current spikes and voltage dips.
For example, if a current spike appears at the front of the Ids waveform, it is typically caused by the distributed parameters of the primary winding. To reduce this, increasing the inter-winding spacing, using single-layer windings, or choosing a transformer with a larger Ae value can help minimize the coupling and reduce the peak current.
Another common issue is a dip in the Is current waveform during the MOSFET turn-off. This is due to the difference between the drain current (Id) and the source current (Is), as Is includes a negative current component from the Cgs discharge. Adjusting the MOSFET model or improving the layout can help mitigate this issue.
In summary, the flyback converter is a versatile and widely used topology, but its performance depends heavily on the management of parasitic elements. Understanding the switching behavior and waveform characteristics is essential for designing efficient and reliable power supplies.