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UCC28070: UCC28070 SS脚被接低 不断复位重启 ?

Part Number: UCC28070


ucc28070 在220VAC输入 升压到500V  轻载运行  SS脚会在起动几个周波后被拉低(如下图),然后周而复始的复位 重新起动  输出也无法达到500V的设定置(无法输出大于500V【420-480V波动】的直流)。假如我将电压设到600V,也无法稳定 但电压可以超过500V直流的。我想问一下,除了输入电源UVLO,VSENSE低于0.6V/0.75V的PFC启动阀值及芯片温度外,芯片内还有什么因素拉低SS脚及引起复位的。