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ON Semiconductor FAN7688

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LLC resonant converter design using FAN7688

The FAN7688 is an advanced Pulse Frequency Modulated (PFM) controller for LLC resonant converters with Synchronous Rectification (SR) that offers best in class efficiency for isolated DC/DC converters. It employs a current mode control technique based on a charge control, where the triangular waveform from the oscillator is combined with the integrated switch current information to determine the switching frequency. This provides a better control-to-output transfer function of the power stage simplifying the feedback loop design while allowing true input power limit capability. Closed-loop soft-start prevents saturation of the error amplifier and allows monotonic rising of the output voltage regardless of load condition. A dual edge tracking adaptive dead time control minimizes the body diode conduction time thus maximizing efficiency.


  • Advanced PFM with a charge current control
  • Adaptive Synchronous Rectification Control w/ dual edge tracking
  • NON ZVS prevention (NZP) by compensation cutback (frequency shift)
  • Hybrid control : PFM + PWM improves light load efficiency
  • Closed loop soft-start with for monotonic output voltage at startup
  • Wide operating frequency (39kHz ~ 690kHz)
  • Programmable dead time for Primary Side Switches and SR switches
  • Protection Functions with auto-restart for OCP; OSP; OLP; OTP


  • Desktop ATX, Desktop-Derived Server, Blade Server and Telecom Power Supplies
  • Intelligent 100 W-2 kW + Off-Line Power Supplies
  • High Efficiency Isolated DC-DC Converters
  • Large Screen Display Power
  • Industrial Power

Block diagram

ON Semi FAN7688 block diagram

Product table

Function ON Semiconductor product Part number(s)
LLC controler FAN7688  
Bridge rectifier DFB series DFB20100, DFB2560
PFC diode Stealth II rectifier FFB30S60S, FFP08S60SN
Isolated current sense Optocoupler sense OPAMP FODM8801C (high temp), FODM1008
NCS210, NCS199
Isolated primary sense OPAMP optocoupler gate driver  NCS210, NCS199
FOD8316, FOD8320, FOD8342
Driver  FAN3225C  
PFC controller NCP16xx  
Primary power FET SuperFET III 650V FCH023N65S3, FCP067N65S3, FCB199N65S3
Secondary power FET T8 - 40V
T8 - 60V
PTNG - 80V
PTNG - 100V
FDMC86180, FDMC86184
ON Semi FAN7688 product image

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