https://www.analog.com/cn/technical-articles/dcdc-converter-capacitor-charger-takes-inputs-from-475v-to-400v.html
The LT3751 operates in boundary-mode, between continuous conduction mode and discontinuous conduction mode. Boundary-mode operation allows for a relatively small transformer and an overall reduced PCB footprint. Boundary-mode also reduces large signal stability issues that could arise from using voltage-mode or PWM techniques.
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