Isolated switching power supply apparatus
A control IC including a full-bridge circuit is disposed on a primary side and a secondary side. Bidirectional communication is performed between the primary side and the secondary side in a state in which they are isolated. A control signal output from the primary side or the secondary side earlier is preferentially processed. As a result, the authority to control a switching element can be freely given to a primary-side control IC or a secondary-side control IC, and any control processing can be performed with software.
1. An isolated switching power supply apparatus comprising:
a direct-current input power supply;
a power transmission transformer including a primary winding and a secondary winding;
at least one main switching element arranged to perform switching control on a direct-current voltage applied to the primary winding of the power transmission transformer;
a rectification circuit including at least one rectification switching element and being connected to the secondary winding of the power transmission transformer;
a smoothing circuit connected to the secondary winding of the power transmission transformer; and
a control circuit including a primary control unit arranged to control the at least one main switching element, a secondary control unit arranged to control the at least one rectification switching element, and an isolated signal bidirectional transmission element connected between the primary control unit and the secondary control unit; wherein
an input/output conversion ratio is controlled in accordance with a duty ratio of the at least one main switching element;
the isolated signal bidirectional transmission element defines a signal transmission path via which a timing signal corresponding to a turn-on time or a turn-off time of the at least one main switching element can be transmitted between a primary side and a secondary side;
the primary control unit includes at least one primary drive switch arranged to drive the isolated signal bidirectional transmission element, a primary digital control unit arranged to control a time of driving of the at least one primary drive switch, and a primary detection unit arranged to detect an output of the isolated signal bidirectional transmission element;
the secondary control unit includes at least one secondary drive switch arranged to drive the isolated signal bidirectional transmission element, a secondary digital control unit arranged to control a time of driving of the at least one secondary drive switch, and a secondary detection unit arranged to detect an output of the isolated signal bidirectional transmission element; and
the primary detection unit and the secondary detection unit are arranged to turn on or turn off the at least one main switching element and the at least one rectification switching element, respectively, when detecting the timing signal, and the at least one main switching element or the at least one rectification switching element is turned on or turned off in response to the timing signal that has been transmitted earlier from the primary control unit or the secondary control unit.
2. The isolated switching power supply apparatus according to claim 1 , wherein
the timing signal includes an ON timing signal generated when the at least one main switching element is turned on and an OFF timing signal generated when the at least one main switching element is turned off;
one of the primary control unit and the secondary control unit which has transmitted the ON timing signal earlier determines a switching frequency;
one of the primary control unit and the secondary control unit which has transmitted the OFF timing signal earlier performs a feedback operation; and
the input/output conversion ratio is controlled in accordance with the duty ratio of the at least one main switching element.
3. The isolated switching power supply apparatus according to claim 2 , wherein one of the primary control unit and the secondary control unit which does not transmit the ON timing signal earlier includes a switching operation stop unit arranged to stop generation of the ON timing signal and terminate a switching operation.
4. The isolated switching power supply apparatus according to claim 3 , wherein the switching operation stop unit in the secondary control unit terminates the switching operation by shutting off driving power for the secondary control unit transmitted from the primary side to the secondary side via the isolated signal bidirectional transmission element.
5. The isolated switching power supply apparatus according to claim 1 , further comprising:
an output current detection unit arranged to directly or indirectly detect an output current; wherein
when the output current detection unit detects a light-load condition, a power saving operation is performed to set an ON timing signal generation cycle to be longer than that in a heavy-load condition, to reduce a switching frequency, and to reduce a fixed loss resulting from switching of each switching element.
6. The isolated switching power supply apparatus according to claim 5 , wherein
the at least one rectification switching element includes a synchronous rectifier; and
when the light-load condition is detected, an ON period of the synchronous rectifier is set to be shorter than that in the heavy-load condition, or the synchronous rectifier is maintained in an OFF state and backflow of a current to the synchronous rectifier is prevented.
7. The isolated switching power supply apparatus according to claim 1 , wherein the isolated signal bidirectional transmission element is a signal transmission transformer that is connected between the primary control unit and the secondary control unit and includes a primary winding and a secondary winding.
8. The isolated switching power supply apparatus according to claim 1 , wherein
the isolated signal bidirectional transmission element is a first capacitor connected between the primary control unit and the secondary control unit; and
a second capacitor is connected between a ground at the primary control unit and a ground at the secondary control unit and defines a return path for a current passing through the first capacitor.
9. The isolated switching power supply apparatus according to claim 1 , wherein the timing signal, which is bidirectionally transmitted between the primary side and the secondary side via the isolated signal bidirectional transmission element, is a substantially square wave signal including a fundamental wave of the switching frequency, and is switched between a high level and a low level when the at least one main switching element is turned on or turned off.
10. The isolated switching power supply apparatus according to claim 1 , wherein the timing signal, which is bidirectionally transmitted between the primary side and the secondary side via the isolated signal bidirectional transmission element, is an edge signal including a fundamental wave of a frequency greater than the switching frequency, and is generated when the at least one main switching element is turned on or turned off.