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请问,LM3414用PWM调光为什么效率很低,通过改变Riadj电阻调光效率很高

Other Parts Discussed in Thread: LM3414

请问,LM3414用PWM调光为什么效率很低,输入28V,1.5A,输出端串联了8个灯,输出电压为24v左右, 通过调Riagd 电阻,电流在350mA到1A驱动效率能达95%,当设置LM3414固定输出为1A,然后用PWM调光,电流从350mA到1A,效率从65%到80%变化,请问为什么效率低了这么多,而且随占空比的增大而改变。

频率500Khz,电感47UH