Switching power supply and electronic device using the same
A switching power supply includes: transformer 60 which includes a primary winding and a secondary winding and outputs, through the secondary winding, an AC voltage based on the current supplied to the primary winding; switching element 61 which controls current supply to the primary winding; rectifying/smoothing circuit 65 which converts the AC voltage output from the secondary winding into a DC voltage; voltage detecting circuit 62 which detects the DC voltage converted by rectifying/smoothing circuit 65 ; feedback amplifier 63 which is supplied with the DC voltage detected by voltage detecting circuit 62 as one input and which is supplied with a reference voltage as the other input, so as to output a difference between the input voltage values; and control circuit 64 which controls switching element 61 , so as to eliminate the difference detected at feedback amplifier 63 . Upon receipt of an instruction signal from the outside, voltage detecting circuit 62 supplies to feedback amplifier 63 , a voltage higher than the voltage output before receipt of the instruction signal.
1. A switching power supply comprising:
a transformer including a primary winding and a secondary winding, so that an AC voltage based on a current supplied to the primary winding is output through the secondary winding;
a switching element which controls current supply to the primary winding;
a rectifying/smoothing circuit which converts the AC voltage output from the secondary winding to a DC voltage;
a voltage detecting circuit which detects the DC voltage converted by said rectifying/smoothing circuit;
a feedback amplifier which is supplied with the DC voltage detected by said voltage detecting circuit as one input and which supplied with a reference voltage as the other input, so as to output a difference between values of these input voltages; and
a control circuit which controls said switching element, so as to eliminate the difference detected at said feedback amplifier,
wherein upon receipt of an instruction signal from an external apparatus, said voltage detecting circuit supplies a voltage higher than a voltage output before receipt of the instruction signal, to said feedback amplifier, and
wherein said voltage detecting circuit includes a series circuit in which a variable-resistance circuit and a resistive element whose resistance value is fixed are connected in series, the series circuit is connected in parallel between output lines connected to the secondary winding, a divided voltage according to a ratio between the respective resistance values of the variable-resistance circuit and the resistive element is supplied to one input of said feedback amplifier, and the ratio between the respective resistance values varies upon receipt of the instruction signal.
2. The switching power supply according to claim 1 , wherein the variable-resistance circuit is connected to a positive polarity-side line of the output lines, a first resistance value is set during a period in which the instruction signal is received, and a second resistance value larger than the first resistance value is set during periods other than that period.
3. The switching power supply according to claim 1 , wherein the variable-resistance circuit is connected to a negative polarity-side line of the output lines, a first resistance value is set during a period in which the instruction signal is received, and a second resistance value smaller than the first resistance value is set during periods other than that period.
4. The switching power supply according to claim 1 , further comprising:
at least one further secondary winding which outputs an AC voltage based on a current supplied to the primary winding; and
another rectifying/smoothing circuit which converts the AC voltage output from said further secondary winding to a DC voltage.
5. The switching power supply according to claim 2 , further comprising:
at least one further secondary winding which outputs an AC voltage based on a current supplied to the primary winding; and
another rectifying/smoothing circuit which converts the AC voltage output from said further secondary winding to a DC voltage.
6. The switching power supply according to claim 3 , further comprising:
at least one further secondary winding which outputs an AC voltage based on a current supplied to the primary winding; and
another rectifying/smoothing circuit which converts the AC voltage output from said further secondary winding to a DC voltage.
7. An electronic device, comprising:
a switching power supply;
an operating part; and
a first control circuit which supplies a standby mode command signal to said switching power supply when receiving, from said operating part, a signal that indicates a transition from normal operation to standby mode in which power consumption is smaller than the normal operation,
wherein said switching power supply includes:
a transformer including a primary winding and a secondary winding, so that an AC voltage based on a current supplied to the primary winding is output through the secondary winding;
a switching element which controls current supply to the primary winding;
a rectifying/smoothing circuit which converts the AC voltage output from the secondary winding to a DC voltage;
a voltage detecting circuit which detects the DC voltage converted by said rectifying/smoothing circuit;
a feedback amplifier which is supplied with the DC voltage detected by said voltage detecting circuit as one input and which is supplied with a reference voltage as the other input, so as to output a difference between values of these input voltages; and
a second control circuit which controls said switching element, so as to eliminate the difference detected at said feedback amplifier,
wherein upon receipt of a standby mode command signal from an external apparatus, said voltage detecting circuit supplies a voltage higher than a voltage output before receipt of the standby mode command signal, to said feedback amplifier, and
wherein said voltage detecting circuit includes a series circuit in which a variable-resistance circuit and a resistive element whose resistance value is fixed are connected in series, the series circuit is connected in parallel between output lines connected to the secondary winding, a divided voltage according to a ratio between respective resistance values of the variable-resistance circuit and the resistive element is supplied to one input of said feedback amplifier, and the ratio between the respective resistance values varies upon receipt of the standby mode command signal.
8. The electronic device according to claim 7 , wherein the variable-resistance circuit is connected to a positive polarity-side line of the output lines.
9. The electronic device according to claim 7 , wherein a first resistance value is set during a period in which the standby mode command signal is received, and a second resistance value larger than the first resistance value is set during periods other than the period.
10. The electronic device according to claim 7 , wherein the variable-resistance circuit is connected to a negative polarity-side line of the output lines.
11. The electronic device according to claim 7 , wherein a first resistance value is set during a period in which the standby mode command signal is received, and a second resistance value smaller than the first resistance value is set during periods other than the period.