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The flyback converter is one of the most widely used topologies in power electronics, particularly for low to medium power applications. Figure 1 illustrates a basic flyback converter configuration that includes various parasitic components such as primary leakage inductance, MOSFET's stray capacitance (Coss), and the junction capacitance of the secondary diode. These parasitic elements play a significant role in the performance and stability of the converter.
The flyback topology is derived from a buck-boost converter by replacing the filter inductor with a coupled inductor, typically a transformer with an air gap. When the main switching device, a 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. Due to the need for energy storage during the on-time, the transformer core must include an air gap to prevent saturation.
During normal operation, when the MOSFET is turned off, the primary current (Id) charges the MOSFET’s Coss (which includes Cgd and Cds) in a short period. As the voltage across Coss exceeds the input voltage plus the reflected output voltage (Vin + nVo), the secondary diode turns on, clamping the primary inductor voltage to nVo. This causes the primary leakage inductance (Lk) to resonate with Coss, leading to high-frequency voltage spikes that may cause instability or damage to the MOSFET.
The flyback converter can operate in two modes: Continuous Conduction Mode (CCM) and Discontinuous Conduction Mode (DCM). In CCM, the secondary diode remains conducting until the next switch turn-on, which introduces a reverse recovery current that adds to the primary current, causing a large spike at turn-on. 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 and Figure 3 show the waveforms for CCM and DCM, respectively. In DCM, the measured MOSFET voltage and current waveforms reveal significant voltage and current changes during turn-on and turn-off, along with oscillations and current spikes.
Figure 4 illustrates the DCM operation of the flyback converter, highlighting the dynamic behavior of the MOSFET during switching. The waveform analysis shows that the MOSFET experiences both voltage and current transients, which are critical to understanding the converter’s performance and potential issues.
Figure 5 presents the equivalent circuit analysis of the flyback converter operating in DCM. It breaks down the switching process into four stages: turn-on instant, turn-on phase, turn-off instant, and turn-off phase. During the turn-on instant, the voltage across the stray capacitance Cp starts to change, generating a discharge current. As the MOSFET turns on, the supply voltage Vin charges the stray capacitance, creating a current spike through the switch.
During the turn-on phase, the secondary diode is off, and the primary inductor current rises linearly. When the MOSFET turns off, the primary current charges the Coss, and if the voltage exceeds Vin + nVo, the secondary diode turns on, clamping the primary inductor voltage to nVo. This leads to resonance between the leakage inductance and the Coss, resulting in high-frequency oscillations.
In the turn-off phase, the secondary coil releases energy to the load, and the primary inductor voltage is clamped to nVo. As the primary inductor discharges, it resonates with the Coss and Cp, reducing the voltage on Cp.
Analyzing the current waveform (Is) from the source of the flyback switching MOSFET reveals important insights into the converter’s behavior. For example, a spike at the front end of the Ids current waveform is often caused by parasitic capacitances in the primary winding. To reduce this spike, increasing the coupling between windings, using single-layer winding, or optimizing the transformer design can help.
Another common issue is a depression in the Is current waveform during the MOSFET turn-off. This is due to the fact that Is is not equal to Id; it includes a reverse current component from the gate-source capacitance (Cgs). Adjusting the MOSFET model or improving the layout can help mitigate this issue.
Understanding these phenomena is crucial for designing reliable and efficient flyback converters, especially in applications like battery chargers, adapters, and consumer electronics. Proper handling of parasitic elements and careful waveform analysis can significantly improve the performance and reliability of the converter.