Synchronous rectification

A synchronous rectifier controller for a switched mode power supply (SMPS) comprising:

• a synchronous rectifier, the SMPS having a power input, a switch, a transformer, an output capacitor and a power output, said transformer having a primary winding on a primary side of said SMPS coupled to said power input and said switch, and a secondary winding on a secondary side of said SMPS coupled to said power output and said output capacitor, said switch to switch power to said primary winding responsive to a drive signal, the SMPS further comprising sense circuitry on the primary side to sense a voltage on the secondary winding, the SMPS having a primary side sensing controller to control said drive signal according to at least the sensed voltage, the synchronous rectifier for blocking current flow from the output capacitor to the secondary winding, the synchronous rectifier controller comprising:
• a linear amplifier having an output to sink current from a control terminal of the synchronous rectifier dependent on a difference between a voltage across the synchronous rectifier and an amplifier reference value, said voltage across the synchronous rectifier being a voltage across a controllable conduction path for current of the secondary winding,
• wherein the linear amplifier is configured to, when the voltage across the synchronous rectifier differs from the amplifier reference value such that the linear amplifier is active, discharge the control terminal to the threshold voltage of the synchronous rectifier such that the voltage across the synchronous rectifier approaches the amplifier reference value,
• wherein the linear amplifier is configured to have high output impedance when the voltage across the synchronous rectifier is below the amplifier reference value, said high output impedance to inhibit discharge of the control terminal.

A method for controlling a synchronous rectifier of a switched mode power supply (SMPS), the SMPS having a power input, a switch, a transformer, an output capacitor and a power output, said transformer having a primary winding on a primary side of said SMPS coupled to said power input and said switch, and a secondary winding on a secondary side of said SMPS coupled to said power output and said output capacitor, said switch to switch power to said primary winding responsive to a drive signal, the SMPS further comprising sense circuitry on the primary side to sense a voltage on the secondary winding, the SMPS having a primary side sensing controller to control said drive signal according to at least the sensed voltage, the synchronous rectifier for blocking current flow from the output capacitor to the secondary winding, the method comprising:

• detecting turn off of the switch;
• in response to said detecting, supplying charge to a control terminal of the synchronous rectifier, to thereby charge the control terminal to a voltage beyond a threshold voltage of the synchronous rectifier to allow the synchronous rectifier to conduct current of the secondary winding;
• allowing the control terminal to remain charged at the voltage beyond the threshold voltage during a time interval following the charge supplying, while allowing current in said secondary winding to decline;
• detecting when voltage across the synchronous rectifier reaches an amplifier reference value, to thereby trigger discharge of the control terminal to the threshold voltage, said voltage across the synchronous rectifier being a voltage across a controllable conduction path for current of the secondary winding;
• coupling the control terminal to a reference voltage to further discharge the control terminal,
• wherein said discharge to the threshold voltage comprises controlling current to the control terminal such that the voltage across the synchronous rectifier approaches the amplifier reference value;
• and
• configuring a linear amplifier to have high output impedance when the voltage across the synchronous rectifier is below the amplifier reference value, said high output impedance to inhibit discharge of the control terminal, wherein the linear amplifier has an output to sink current from the control terminal dependent on the difference between the voltage across the synchronous rectifier and the amplifier reference value.