The flyback pulse-width modulated (PWM) DC-DC power converter is a very important circuit in switching mode power supply (SMPS) converters for low power applications. The main drawback of the conventional single-switch flyback converter is the high turn-off voltage stress suffered by the switch. The high voltage transients are caused by the resonant behavior of the transformer leakage inductance and the transistor output capacitance, resulting in ringing superimposed on the steady-state switch voltage level. This requires a transistor with higher voltage rating. However, a transistor with higher voltage rating has higher on-resistance causing higher conduction loss. The high voltage ringing also increases the switching loss. In addition, the switch voltage stress is not easily predictable because it is difficult to determine the magnitude of ringing during the design stage. The two-switch flyback DC-DC converter is an extended version of the single-switch flyback converter. The circuit arrangement with an addition of a power transistor and two clamping diodes to the conventional single-switch flyback converter leads to the two-switch flyback PWM DC-DC converter, which effectively reduces the switch overvoltage and eliminates the uncertainty of its value. The clamping diodes in the two-switch flyback converter clamps the voltage across each switch to the DC input voltage and also provide a path to return most of the energy stored in the transformer leakage inductance to the DC input source.
In the first part of this research, detailed steady-state analyses of the two-switch flyback PWM DC-DC converter for continuous conduction mode (CCM) and discontinuous conduction mode (DCM) are performed. The transistor output capacitance and the transformer leakage inductance are included in the analyses. Design equations for both CCM and DCM operation modes are derived. Furthermore, by incorporating an active clamp circuit into the hard-switching two-switch flyback converter, a new soft-switching two-switch flyback converter, namely, zero-current transition (ZCT) two-switch flyback converter is proposed. The principle of circuit operation, steady-state analysis, equivalent circuits, converter steady-state waveforms, and design procedure of the proposed ZCT two-switch flyback converter is presented. The key features of the proposed soft-switching converter are 1) the voltage stresses of the main switches are reduced to DC input voltage VI, and 2) all the semiconductor devices are turned off under zero-current (ZC) switching condition. Clamping of the switch overvoltages and reduction in switching loss are achieved in the proposed ZCT two-switch flyback converter. Saber Sketch simulation and experimental results of the hard-switching and the proposed ZCT soft-switching two-switch flyback converters are presented to validate the theoretical analyses.
High frequency (HF) transformers used in PWM converters, such as flyback transformers conduct periodic nonsinusoidal currents, which give rise to additional winding losses due to harmonics. In the second part of this research, a theory is developed to find the harmonic winding loss in an HF transformer conducting periodic nonsinusoidal current. Dowell's equation is used to determine the winding resistances due to eddy currents as a function of frequency. Both skin and proximity effects are taken into account. Fourier series of the primary and secondary current waveforms in a two-winding flyback transformer and the primary and secondary winding resistances are used to determine the primary and secondary winding power losses at various harmonics for both CCM and DCM cases, respectively. The harmonic winding loss factors FRph and FRsh are introduced. The theory is illustrated by the case study of flyback converter for both CCM and DCM operations. Using the equations developed to find the winding losses due to harmonics, detailed methodology and step-by-step procedures to design two-winding flyback transformers for CCM and DCM operations, respectively, are given. Examples illustrating the design of two-winding flyback transformer for CCM and DCM operations are presented. Computed characteristics of the designed flyback transformer for a wide range of operating conditions of the flyback converter in CCM and DCM modes are presented.