Synchronous rectifier circuit and switching power supply apparatus
Based on a control voltage outputted by a secondary-side control circuit and a drain voltage of a second transistor that has a drain electrode connected to a primary winding of a transformer and performs switching operations based on a gate voltage, a control voltage generating circuit of a synchronous rectifier circuit generates the gate voltage of a first transistor. The gate voltage turns off the first transistor irrespective of the control voltage at timing where the drain voltage falls from a first value to a second value.
1. A synchronous rectifier circuit comprising:
a first transistor that has a first terminal, which is connected to a secondary winding of a transformer included in an isolated synchronous rectification-type switching power supply apparatus, and a second terminal, to which a first control voltage is applied, and performs a switching operation based on the first control voltage;
a control circuit that detects a first voltage of the first terminal and outputs, based on the first voltage, a second control voltage that controls the switching operation of the first transistor; and
a control voltage generating circuit that generates the first control voltage based on the second control voltage and one of a second voltage of a third terminal of a second transistor and a third voltage that changes at a same timing as the second voltage, the third terminal being electrically connected to a primary winding of the transformer, the second transistor performing a switching operation based on a third control voltage applied to a fourth terminal of the second transistor,
wherein the first control voltage changes and turns the first transistor off irrespective of the second control voltage at a same timing as the second voltage changes from a first value to a second value which is lower than the first value; and
wherein, when the switching power supply apparatus is operating in continuous current mode, at the same timing as the second voltage changes from the first value to the second value, the control voltage generating circuit changes the first control voltage before the second control voltage changes.
2. The synchronous rectifier circuit according to claim 1 ,
wherein the control voltage generating circuit generates the first control voltage based on a logical AND calculated for the second control voltage and one of the second voltage and a fourth voltage produced by inverting a logic level of the third voltage.
3. The synchronous rectifier circuit according to claim 1 ,
wherein the third voltage is the third control voltage.
4. The synchronous rectifier circuit according to claim 1 ,
wherein the third voltage is the first voltage.
5. The synchronous rectifier circuit according to claim 1 ,
wherein the control voltage generating circuit is connected to an auxiliary winding provided in the transformer and detects changes in the second voltage via the auxiliary winding.
6. The synchronous rectifier circuit according to claim 5 ,
wherein the control voltage generating circuit includes:
a first attenuator with a first input terminal connected to one end of the auxiliary winding;
a second attenuator with a second input terminal connected to an output terminal of the control circuit that outputs the second control voltage;
a logical AND circuit that calculates a logical AND of a first output voltage of the first attenuator and a second output voltage of the second attenuator; and
an amplifier that generates the first control voltage by amplifying an output voltage of the logical AND circuit.
7. An isolated synchronous rectification-type switching power supply apparatus comprising:
a transformer including a primary winding and a secondary winding;
a first transistor that includes a first terminal and a second terminal, the first terminal being electrically connected to the primary winding, the first transistor performing a switching operation based on a first control voltage applied to the second terminal;
a second transistor that includes a third terminal, which is connected to the secondary winding, and a fourth terminal, to which a second control voltage is applied, and performs a switching operation based on the second control voltage;
a control circuit that detects a first voltage of the third terminal and outputs, based on the first voltage, a third control voltage that controls the switching operation of the second transistor; and
a control voltage generating circuit that generates the second control voltage based on the third control voltage and one of a second voltage of the first terminal of the first transistor and a third voltage that changes at a same timing as the second voltage,
wherein the second control voltage changes and turns the second transistor off irrespective of the third control voltage at a same timing as the second voltage changes from a first value to a second value which is lower than the first value; and
wherein, when the switching power supply apparatus is operating in continuous current mode, at the same timing as the second voltage changes from the first value to the second value, the control voltage generating circuit changes the second control voltage before the third control voltage changes.