The flyback converter running in discontinuous conduction mode can be operated in hard switching or quasi resonant (or valley switching, or ZVS) mode regarding the primary side switch. The difference between a hard switching and quasi resonant flyback converter is the turn on time point of the primary switch. In a hard switching mode the turning on of the MOSFET is not synchronized with the drain-source voltage value. This type of converters runs mainly in fixed frequency mode.
电感电流断续工作的反激式变换器一次侧开关可工作于硬开关或准谐振（或谷值开关或零电压开关）模式。硬开关和准谐振反激变换器之间的差异在于一次侧开关的开启时间点。在硬开关里场效应晶体管的开启波形拐点并不和漏源极电压值同步。这种变换器大体上运行于固定频率模式。 In a quasi resonant mode the resonant circuit determined by the output capacity of the MOSFET and the inductance of the transformer will be utilized to switch on at lowest possible value of the drain-source voltage. This circuit starts to oscillate at the end of the current flow through the secondary side of the transformer, hence at the end of the flyback phase. The MOSFET will be turned on at the minimum of this oscillation. The quasi resonant approach uses this oscillation to achieve minimum voltage switchi
ng during turn on for the MOSFET. This operation mode runs at a variable frequency.
在准谐振模式里，由变压器电感和场效应晶体管输出电容引起的谐振促使开关的开通时刻发生在漏源极电压的最小值上。这种电路在电流从变压器二次侧流尽以后（反激回扫过程结束）开始振荡。场效应晶体管将在振荡幅值的最小值开启（谷值开通）。这种运行模式工作在可变的频率上。 Higher amplitude of the oscillation results in lower drain source voltage level at which the MOSFET turns on correspondingly lower switching losses and higher efficiency of the system. 更高幅值的振荡导致场效应晶体管更低的漏源极开通电压幅值来产生更低的开关损耗和更高的系统效率。 To achieve high oscillation peaks, the design of the transformer has to be set to high reflected voltage. This increase of the reflected voltage results in a higher drain-source voltage blocking MOSFET and longer duty cycles.
要达到比较高的振荡电压峰值，变压器的反射电压必须设置的比较高。增加的反射电压导致使用更高漏源极击穿电压的场效应晶体管和更大的开关占空比。
Comparison of three different flyback solutions has been made. All of them have been operation at 300 kHz, bus voltage of 400 V, output power of 120 W, output voltage of 16 V. These de
sign included different modes of operation and different values of reflected voltage, resulting in different MOSFET’s voltage ratings: 比较现有的三种反激变换器。它们都工作在３００千赫兹，直流母线电压４００伏特，输出功率１２０瓦特，输出电压１６伏特。这些设计包含不同的运行模式和反射电压等级，因此使用不同电压等级的场效应晶体管：
? Hard switching flyback with CoolMOS 600V, reflected voltage of 100V
? 硬开关反激变换器使用６００伏特CoolMOS，１００伏特反射电压
? Quasi resonant flyback with CoolMOS 600V, reflected voltage of 100V
? 准谐振反激变换器使用６００伏特CoolMOS，１００伏特反射电压
? Quasi resonant flyback with CoolMOS 800V, reflected voltage of 390V
? 准谐振反激变换器使用8００伏特CoolMOS，39０伏特反射电压
The clamping snubber circuit was set to the rated breakdown voltage of the MOSFET (600 V and 800 V respectively).
钳位缓冲电路被设定在场效应晶体管的额定击穿电压上（分别为６００伏特和８００伏特）。
Flyback in hard switching mode with 600V MOSFET 使用６００伏特场效应晶体管的硬开关反激变换器
The hard switching approach (as shown in Fig. 26) doesn’t consider the minimum drain-source voltage. The MOSFET will be turned on hard, in this case at a voltage level of 500 V (at time point 3.3 μs). The discharge of circuits’ parasitic capacitances leads to a high current spike during turning on.
硬开关（图２６所示）几乎不考虑漏源极电压的最小值。场效应晶体管开通应力大，在这个例子里，开通电压在５００伏特（在３.３微秒的时间点）。由寄生电容引起的泄放电流在开通时产生很高的电流尖刺。
Fig. 26 Drain-source voltage and drain current of hard switching
600V flyback
图２６ ６００伏特硬开关反激变换器的漏源极电压和漏
极电流
Flyback in quasi resonant mode with 600 V MOSFET 使用６００伏特场效应晶体管的准谐振反激变换器
The drain-source voltage (Fig. 27) starts oscillating at the end of the flyback phase and reaching the minimum of 300 V when the MOSFET turns on.
漏源极电压（图２７）在反射过程结束后并减小到３００伏特时场效应晶体管导通。
The duty cycle is lower compared to an 800 V solution due to a lower reflected voltage of 100V. Shorter duty cycle for the same output power results in higher peak currents on the primary side.
因为１００伏特的反射电压，比较８００伏特解决方案它有更小的占空比。小占空比实现同样的功率输出必须使用更高的一次侧峰值电流。
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