Advantages of Secondary Side Control over Primary Side Control in Flyback Converter Solutions – KBA227974

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    Author: rajathb_01           Version: **


    Translation - Japanese: フライバック コンバータ ソリューションにおける1次側制御に対する2次側制御の利点 - KBA227974 - Community Translated (JA)


    Question:  What are the advantages of using Secondary Side Control (SSC) over Primary Side Control (PSC) in Flyback converter solutions?


    Answer: PSC has been used in low cost Flyback converter solutions and has proved to be sufficient for legacy power adapter systems. But with the arrival of USB-PD and other fast charging protocols, higher output power and efficiency demands and modern control schemes, SSC has become the best choice for most solutions. Here are the advantages of SSC over PSC:

    • PSC makes use of an opto-coupler as an isolation device in the analog feedback path. The photodiode – phototransistor pair in this model has limited bandwidth, poor accuracy, and suffers degradation over time and temperature. Improved models come at significantly increased cost.
    • SSC uses a pulse edge transformer for directly coupling the PWM control signal from the secondary to the primary, which is a cheaper and robust option. Pulse transformer has higher bandwidth compared to opto-isolator, making it work better at high PWM frequencies.
    • SSC uses Voltage Mode Control for regulation. This supports regulation over a wide load range as compared to Current Mode Control in PSC.
    • SSC can improve efficiency – Cypress SSC PAG1S can choose the best efficiency operating point for widely varying load conditions. It measures both voltage on VBUS and current (IOUT) instead of one parameter; thus effectively measuring the output power POUT and operating for the best efficiency.
    • Design for SSC is relatively simpler due to the absence of an additional pole from the opto-isolator. Designers must now compensate only for one pole. Design is further simplified due to more integration on the secondary: PAG1S integrates Synchronous Rectifier (SR), Regulation, and Power Delivery (PD) controller on a single secondary side controller.
    • SSC gives the option to achieve better transient load response since bandwidth-limiting component like opto-coupler in the analog-feedback loop is absent. This allows operating at higher frequencies, where the difference becomes significant.