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UCC28740: Npa匝比和VS电压设置问题疑问与求助,谢谢!

Part Number: UCC28740
Other Parts Discussed in Thread: PMP20396, , PMP20432

我在参考PMP20396和PMP20432两个设计时发现,变压器的原边绕组和辅助绕组的匝比为45:8,也就是5.625,而在上述两个设计中UCC28740芯片的VS引脚的分压电阻从上往下分别是68.1k和17.8k也就是约0.2的分压比。如果输入电压是300V,那么VS分得的电压将会是:300/5.625 * 0.2 = 10.666V,远超过了VS触发保护的阈值电压4.6V,那么理论上就不能正常启动了。有谁能帮忙解释一下这样设计的原因吗?