avoid problems caused presence signals different frequencies inside sa

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AND8187/D Synchronizing NCP1207 (NCP1377/B, NCP1378)
avoid problems caused presence signals different frequencies inside same apparatus, sometimes necessary synchronize switching frequency power supply (for tuner compatibility, instance). Instead designing specific controller those kinds applications, NCP1207 easily used inherent turn-ON control capability. This application note fully applicable other controllers family, i.e., NCP1377 NCP1378.
Demag Signal 7.00
Synchronization NCP1207 meant quasi-resonant controller, which turning dictated demagnetization (pin long voltage above stays low, when voltage crosses threshold, goes high (Figure minimum TOFF duration always ensured prevent switching frequency high.
5.00 Possible Restarts 3.00
1.00 -1.00
Figure NCP1207 Switches when Voltage Crosses Down Threshold
thus possible this synchronize turning with external signal. This signal must high maintain state, then authorize turn-ON. This signal must fulfill three major requirements: state duration must shorter than internal blanking time TBLANK (8.0 typically), avoid working fixed TOFF mode. this will happen
state duration longer than TBLANK, duration shorter than TBLANK (8.0 will safe. state voltage must always lower than even noisy environment; safer would slightly negative value. high state voltage must lower than overvoltage threshold Vref (7.2 typically).
Semiconductor Components Industries, LLC, 2004
November, 2004 Rev.
Publication Order Number: AND8187/D
AND8187/D
Based these assertions, define best synchronization signal apply (Figure allows reducing high state voltage needed (Figure dividing voltage applied internal impedance pin.
SYNC State
High State
Figure This Circuit Allows Using More Practical Positive SYNC Signal
Figure Best Synchronization Signal NCP1207
However, practical generate negative voltage, recommend using positive signal inserting small circuit (made capacitor parallel resistor series shift additional series resistor,
With this arrangement, synchronization signal have state above high state above signal applied will shifted down, ensuring slightly negative state, high state value adjusted changing value needed maximum voltage SYNC lower than signal finally applied NCP1207 seen Figure From DRAIN waveform, that power supply running synchronized mode.
DRAIN
DRAIN
Figure Switching Frequency Forced SYNC Signal Different Load Conditions http://onsemi.com
AND8187/D
Synchro interesting switch synchronization. When synch OFF, controller must keep switching, asynchronously. solutions possible: Variable TON, Fixed TOFF Mode connecting demagnetization ground, NCP1207 running fixed TOFF variable mode. TOFF duration given internal blanking time TBLANK, dictated current mode. This mode very interesting external components needed (Figure
SYNC
simplest switch between modes always apply SYNC signal pin, ground this when synchronization needed. voltage must lower than possible bipolar transistor, VCEsat usually greater than thus recommend small MOSFET like 2N7000 (Figure connect emitter bipolar slightly negative voltage.
SYNC_OFF
2N7000
Figure Simple MOSFET Allows Switching from Free-Running Synchronized Mode
Quasi-Resonant Mode
quasi-resonant mode operation typical which NCP1207 been designed. (pin used monitor auxiliary winding detect core reset transformer (see data sheet details).
SYNC
Switching from synchronized quasi-resonant mode more complicated requires splitting resistors order insert bipolar transistors. Those transistors will switches open close other path (see possible implementation Figure
Demagnetization Detection
SYNC_OFF
Figure Possible Switch from Synchronized Mode Operation
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AND8187/D
Conclusion NCP1207 other controllers from same family (NCP1207A, NCP1377, NCP1377B NCP1378) offer cheap easy synchronize switching frequency power supply without using dedicated controller. simple synchronization enough, even possible implement more advanced features (like switching between different modes operation) with additional components.
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AND8187/D

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