Flyback Switching Power Supply Waveform Analysis
Two Modes: CCM and DCM
1. CCM (Continuous Conduction Mode): In a switching cycle, the inductor current never reaches zero. In other words, the inductor never "resets", meaning the magnetic flux in the inductor never returns to zero during the switching cycle. When the power switch is closed, current still flows through the coil.
2. DCM (Discontinuous Conduction Mode): In a switching cycle, the inductor current always reaches zero, meaning the inductor is properly "reset", i.e., when the power switch is closed, the inductor current is zero.
Differences in Waveforms Between the Two Modes
The primary current of the transformer: in CCM mode, it is a trapezoidal wave, while in DCM mode, it is a triangular wave.
The secondary rectifier current waveform: in CCM mode, it is a trapezoidal wave; in DCM mode, it is a triangular wave.
The Vds waveform of the MOS: in CCM mode, before the next cycle turns on, Vds remains on the platform of Vin+Vf. In DCM mode, before the next cycle turns on, Vds drops from the Vin+Vf platform and undergoes damped oscillation. (Vf is the secondary voltage reflected to the primary side). Therefore, we can easily determine from the waveform whether the flyback power supply operates in CCM or DCM mode.
Influence of parasitic parameters on the waveform at the moment of MOSFET turn-on and turn-off
(1) DCM (Vds, Ip)
When the MOS is turned off, the Vds waveform shows that the voltage across the MOS far exceeds Vin+Vf. This is because the transformer primary has leakage inductance. The energy of the leakage inductance cannot be coupled to the secondary through the magnetic core. During the MOS turn-off process, the leakage inductance current cannot change abruptly. The current change in the leakage inductance also generates an induced electromotive force, which cannot be clamped by the secondary coupling, so the voltage can spike very high. To prevent the MOS from being damaged by voltage breakdown, we add an RCD snubber circuit on the primary side to store the leakage energy in a capacitor and then dissipate it through the resistor R.
When the secondary inductor current drops to zero, it means the energy in the magnetic core has been completely released. Since the current through the diode drops to zero, the diode automatically turns off, and the secondary is effectively open-circuited, so the output voltage is no longer reflected back to the primary. At this time, because the MOS Vds voltage is higher than the input voltage, under the voltage difference, the MOS junction capacitance and the primary inductance resonate. The resonant current discharges the MOS junction capacitance, and the Vds voltage begins to drop. After a quarter of the resonant period, it starts to rise again. Due to the presence of the RCD clamp circuit and other parasitic resistances, this oscillation is a damped oscillation with decreasing amplitude.
f1 is much larger than f2 (as can be seen from the waveform), because the leakage inductance is generally relatively small; at the same time, since the impedance of the circuit where f1 is located is relatively small, the resonant current is larger, so it can be quickly dissipated on the equivalent resistance. This is why the resonance in the circuit where f1 is located ends quickly! (The specific resonance time can be estimated by solving the equivalent model using a second-order differential equation.)
(2) CCM (Vds, IP)
(3) Other Waveform Analyses (Secondary Output Voltage Vs, Is, Vds)
Whether in CCM or DCM mode, at the moment of MOSFET turn-on, there is oscillation on the secondary side of the transformer. The main reason is the resonance caused by the leakage inductance between primary and secondary, the junction capacitance of the output Schottky (or fast recovery) diode, and the output capacitance. In CCM mode, it is also related to the reverse recovery current of the Schottky diode. Therefore, it is common to connect an RC snubber in parallel with the output Schottky diode to absorb this oscillation and reduce the stress on the Schottky diode.
Whether in CCM or DCM mode, at the moment of MOSFET turn-off, the secondary current IS waveform also has some oscillation. The main reason is the resonance between the secondary inductance, the Schottky junction capacitance, and the output capacitance.
(4) Influence of RCD Snubber Circuit on Vds
When the MOS is turned off, the Vds waveform shows that the voltage across the MOS far exceeds Vin+Vf! This is because the transformer primary has leakage inductance. The energy of the leakage inductance cannot be coupled to the secondary through the magnetic core. During the MOS turn-off process, the leakage inductance current cannot change abruptly. The current change in the leakage inductance also generates an induced electromotive force, which cannot be clamped by the secondary coupling, so the voltage can spike very high. To prevent the MOS from being damaged by voltage breakdown, we add an RCD snubber circuit on the primary side to store the leakage energy in a capacitor and then dissipate it through the resistor R.
