Synchronous rectifier scheme for continuous conduction mode in primary side controlled fly-back converter

A Primary Side Controlled (PSC) fly-back converter comprising:

• a transformer having a primary winding coupled to a rectified AC input through a power switch (PS), and a secondary winding coupled to a DC output through a synchronous rectifier field effect transistor (SR_FET), the SR_FET having a drain coupled to the secondary winding and a source coupled to the DC output;
• and
• a fly-back-controller implemented as an integrated circuit (IC) comprising:
• a primary-side-controller operable to control a duty cycle of the PS based on power supplied to the DC output;
• and
• a secondary-side-controller operable to turn OFF the SR_FET when the PS turns ON in continuous conduction mode (CCM), the secondary-side-controller including a CCM zero-crossing-detector (ZCD) comparator having a first input coupled to the drain of the SR_FET through a capacitor to receive a first voltage, and a second input coupled to the drain of the SR_FET through a voltage divider including both an external resistor (Rext) external to the IC of the fly-back-controller and an internal resistor (Rint), the CCM ZCD comparator operable to detect a change in a voltage on the drain of the SR_FET during CCM when the PS turning ON causes a change in secondary current, and to output a signal to a gate drive circuit to turn OFF the SR_FET.

A method of operating a Primary Side Controlled (PSC) fly-back converter, the method comprising:

• turning ON a power-switch (PS) in a primary side of a transformer while maintaining a synchronous-rectifier (SR) field effect transistor (SR_FET) in a secondary side of the transformer OFF causing a SR drain voltage to rise to a maximum voltage;
• turning OFF the PS;
• turning ON the SR_FET causing the SR drain voltage to rise at a first rate followed by a continued rise at a second rate, lower than the first rate, as power is drawn from a DC coupled to the secondary side of the transformer through the SR_FET;
• turning ON the PS before current in the transformer is completely discharged causing the PSC fly-back converter to operate in a CCM;
• sensing through a SR capacitor coupled to the SR drain a resultant rise in the SR drain voltage and coupling the sensed SR drain voltage to a first input of a CCM zero crossing detector (ZCD) comparator;
• comparing the sensed SR drain voltage to a voltage coupled to a second input of the CCM ZCD comparator;
• and
• when the SR drain voltage coupled to the first input exceeds the voltage coupled to the second input by a predetermined amount for a predetermined time, outputting a CCM ZCD signal, which causes the SR_FET to turn OFF, eliminating cross-conduction between the PS and the SR_FET when the PSC fly-back converter is operating in CCM.