UCC28740: Application issues

Hello, when designing the QR quasi-resonant flayback in UCC28740, in section 7.4.2 of the datasheet, the maximum value of Dmagcc is limited to 0.425, I would like to ask a few questions:

1. Is the restriction of Dmagcc triggered only in CC mode? Is there such a restriction in CV mode?

2. Under what conditions will CC mode be entered? Whether the switching frequency reaches the maximum value of 100KHZ will enter CC mode

3. If Dmagcc has a maximum value limit in CV mode, then if we want to increase the power of flayback, due to the limitation of Dmagcc, in QR mode, only the turns ratio or primary side shutdown current can be increased, then the secondary side loss of the transformer in QR mode is greater than the loss in DCM mode, and the QR mode will not reflect the advantage. (Input voltage range: 500V~1000VDC, output power: 85W (double winding output 12V&24V, 12V winding: 35W, 24V winding: 50W); QR parameters: shutdown current 1.9A, transformer turn ratio 10:1:2, Lm=650uH; DCM parameters: transformer turn ratio: 27:2:4, excitation inductance 700uH. )

4. Is there any other QR quasi-resonant control chip, there is no Dmagcc limit or Dmagcc is limited to about 0.7.

中文：

你好，在UCC28740设计QR准谐振flayback时，在手册7.4.2节，Dmagcc的最大值限制在0.425，我想咨询一下几个问题：

1、是否只有在CC模式触发Dmagcc的限制？在CV模式是否会有该限制？

2、什么条件下会进入CC模式？是否是开关频率达到最大值100KHZ才会进入CC模式？

3、如果Dmagcc在CV模式有最大值限制，那如果我们要把flayback的功率做大，由于Dmagcc的限制，在QR模式下只能将匝比或者原边关断电流增大，那QR模式的变压器副边损耗要比DCM模式的损耗还要大，QR模式就体现不出优势。（输入电压范围：500V~1000VDC，输出功率：85W（双绕组输出12V&24V，12V绕组：35W，24V绕组：50W）；QR参数：关断电流1.9A，变压器匝比10:1:2，Lm=650uH；DCM参数：变压器匝比：27:2:4，励磁感量700uH。）

4、是否有其他的QR准谐振控制芯片，没有Dmagcc的限制或者Dmagcc限制在0.7左右。

Links：

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