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UCC28056轻载保护问题

规格:400V OUT,满载 80W,85~265VAC IN,负载功率要求可调(1%~100%)。

遇到的问题:如原理图,过零检测ZCD分压电阻按照数据手册要求需要并联电容,根据测试结果得出:C12取值越大,电路在正弦半波峰值处越容易触发OCP1,使开关管提前关断。通过减小C12取值可以改善这种现象,但是轻载下情况无明显改善,电容已经减小至47p。如果去掉该电容,电路无法启动。(暂时没有高压电容,所以C11未接)

请问这种问题如何处理,另外zcd分压电阻为什么一定要并联电容才能正常工作,谢谢。

(后面再给出一张测试波形:1-驱动VGS,3-桥后电压波形Vbus,4-电感电流)