Switching power supply control circuit
View Patent ↗A comparator detects whether a feedback signal of an output voltage detecting circuit for a switching power supply circuit reaches a control voltage. A comparator detects an operating state of the switching power supply circuit, which is instructed by a switching instruction signal, by comparing the instruction signal with a reference voltage. The comparators are connected to a decision circuit which outputs to a control circuit a signal instructing a normal state until the feedback signal reaches the control voltage, and thereafter a signal instructing a normal state or a stand-by state that is instructed by the switching instruction signal. Thus, the control circuit makes the switching power supply circuit operate in a normal state until the output voltage reaches the control voltage. After the output voltage reaches the control voltage, the switching power supply circuit operated in an operation state instructed by the switching instruction signal to enable stable startup.
1. A switching power supply control circuit configured to operate a switching power supply circuit so as to be switched into a normal state or a stand-by state, comprising:
a first comparator which has a feedback signal of an output voltage detecting circuit inputted to compare the inputted feedback signal with a feedback reference voltage corresponding to a specified control voltage, the detecting circuit detecting an output voltage of the switching power supply circuit, and outputs a voltage decision signal indicating whether the output voltage of the switching power supply circuit reaches the specified control voltage or not;
a second comparator which has a switching instruction signal inputted to compare the inputted switching instruction signal with an operating state decision reference voltage signal, the switching instruction signal being produced by a signal generating unit operated by an output voltage of the switching power supply circuit, and outputs a switching decision signal indicating whether the switching instruction signal is a signal instructing a stand-by operation or not;
a decision circuit which is connected to the first comparator and the second comparator and, after the voltage decision signal outputted from the first comparator indicates that the output voltage of the switching power supply circuit reaches the specified control voltage, makes the switching decision signal outputted from the second comparator effective; and
a control circuit which is connected to the decision circuit to operate the switching power supply circuit in a normal state until the output voltage reaches the specified control voltage and, after the output voltage reaches the specified control voltage, operate the switching power supply circuit in an operation state based on the instruction of the switching instruction signal.
2. The switching power supply control circuit according to claim 1 , wherein the switching power supply control circuit is disposed at a primary side of the switching power supply circuit having the primary side and a secondary side connected with a load, divided by an insulating transformer, and wherein the signal generating unit that generates the switching instruction signal for instructing the normal state or the stand-by state according to a switching instruction from the external is disposed at the secondary side of the switching power supply circuit.
3. The switching power supply control circuit according to claim 1 , wherein the first comparator has a hysteresis allowing the voltage instruction signal to be stabilized.
4. The switching power supply control circuit according to claim 1 , further comprising a holding circuit holding an output signal state of the first comparator after the output voltage reaches the control voltage, and the feedback signal reaches the feedback reference voltage,
wherein the switching power supply control circuit determines the operation state based on the switching decision signal detected by the second comparator after the output voltage reaches the control voltage.
5. The switching power supply control circuit as claimed in claim 1 , wherein the switching determination signal is transmitted to the second comparator from the signal generating unit through a photo coupler or a pulse transformer.
6. The switching power supply control circuit as claimed in claim 1 , wherein the switching power supply circuit includes two stage converters of a PFC converter and a DC to DC converter.