Constant voltage output control method and constant voltage output control device for switching power supply circuit

The present invention provides a constant voltage output control method and a constant voltage output control device for a switching power supply circuit that allow accurate execution of a constant voltage control of an output voltage, even if there is variation in a utilized circuit element or an integrated circuit. A set voltage V2aset is obtained based on equation (1),V2ASET=NP÷NS×V2BSET  (1)where, V2bset represents an output voltage of a secondary output winding that is subject to the constant voltage control, Np represents a number of turns of a primary winding, and Ns represents a number of turns of the secondary output winding; and the set voltage V2aset is compared with a flyback voltage V2a of the primary winding.

INCORPORATION BY REFERENCE

The present application claims priority under 35 U.S.C. §119 to Japanese Patent Application No. 2003-074368 filed on Mar. 18, 2003. The content of the application is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to a constant voltage output control method and a constant voltage output control device that executes a constant voltage control of an output voltage from a secondary side of a transformer of a switching power supply circuit.

BACKGROUND OF THE INVENTION

In a switching power supply circuit, an exciting current is fed to a primary winding of a transformer thus causing energy stored in the transformer to be discharged as an output of a secondary output winding. The switching power supply circuit offers a stable power supply that is compact, lightweight and highly efficient, and is therefore utilized in power supply circuits such as those in battery chargers, AC adapters, and the like.

Conventionally, in this type of switching power supply circuit, an output voltage and current of a rectifying smoothing circuit of a secondary side are monitored such that excessively high power is not output from the rectifying smoothing circuit of the secondary side. The monitoring results are transmitted to a primary side using an insulated signal transmission element such as a photocoupler. On the primary side, an oscillating switching device is controlled so as to switch to ON and OFF in accordance with the transmission signal. Constant voltage control of the output voltage is executed by controlling an ON period (an energized period) and an OFF period of the exciting current fed to the primary winding (as in Japanese Patent Laid-Open Publication No. 2002-136116).

Hereinafter, a constant voltage control of the output voltage executed by a conventional switching power supply circuit100, like the one described above, will be explained with reference to a circuit diagram shown inFIG. 8.

A direct current power supply1is an unstable power supply configured from a high voltage side terminal1aand a low voltage side terminal1b. A transformer2is configured from a primary winding2aand a secondary output winding2b. An oscillating switching device3is formed from a field effect transistor. Further, an Ip detection resistor22detects a primary winding current Ip that flows in the primary winding2a. The oscillating switching device3is connected between an end of the primary winding2a, and the low voltage side terminal1bvia the Ip detection resistor22. The oscillating switching device3is switched ON and OFF with a predetermined cycle, by a switching control circuit101that is connected to a gate. Accordingly, the entire switching power supply circuit100oscillates.

A rectifying diode4and a smoothing capacitor13, which are shown in a secondary side output of the transformer2, configure a rectifying smoothing circuit. The diode4and the smoothing capacitor13rectify and smooth an output of the secondary output winding2b, which is then output between the high voltage side output line20aand a low voltage side output line20b. An output monitoring circuit is provided between the output lines20aand20b. This output monitoring circuit monitors the output voltage and the output current and is configured from a voltage monitoring circuit and a current monitoring circuit. In the case that either the output voltage or the output current exceeds a respective predetermined reference voltage and reference current, the output monitoring circuit causes a photo coupler light-emitting device35a, shown in the drawing, to emit light.

In the voltage monitoring circuit, voltage dividing resistors30and31are connected in series between the high voltage side output line20aand the low voltage side output line20b. A divided voltage of an output voltage is obtained from an intermediate tap point32and is inputted to an inverted input terminal of a differential amplifier33a. Further, a voltage monitoring reference supply34ais connected between a non-inverted input terminal of the differential amplifier33aand the low voltage side output line20b, and inputs a first comparative voltage to the non-inverted input terminal for comparison with the divided voltage of the output voltage. A reference voltage is set to a selected value by changing respective resistance values of the voltage dividing resistors30and31, or the first comparative voltage of the voltage monitoring reference supply34a.

The photo coupler light-emitting device35ais connected to an output side of the differential amplifier33a. Further, the photo coupler light-emitting device35ais connected to the high voltage side output line20avia an electrical resistor36, and is supplied with current from the drive power supply.

Moreover, a current detection resistor43is disposed in the low voltage side output line20bin the current monitoring circuit, and one end of the current detection resistor43is connected to the inverted input terminal of the differential amplifier33, and the other end is connected to the non-inverted input terminal via a current monitoring reference supply34b.

Accordingly, an output current that flows in the low voltage side output line20bis indicated by a potential difference between both ends of the current detection resister43. It can be determined whether this output current exceeds the predetermined reference current by comparison with a second comparative voltage of the current monitoring reference supply34bin a differential amplifier33b. A reference current is set to a selected value by changing a resistance value of the current detection resistor43, or the second comparative voltage of the voltage monitoring reference supply34b. An output side of the differential amplifier33bis connected to a connection point of the output side of the differential amplifier33afor monitoring the output voltage and the photo coupler light-emitting device35a.

Furthermore, the resistor37aand the capacitor38a, and the resistor37band the capacitor38b, which are respectively connected in-series, act as alternating current negative feedback devices that cause operation of the differential amplifier33aand the differential amplifier33b, respectively, to be stable.

At the primary side of the transformer2, a photo coupler light-receiving device35bphoto coupling with the photo coupler light-emitting device35ais connected between the switching control circuit101and the low voltage side terminal1bof the direct current power supply1.

The switching control circuit101incorporates a variable reference supply101athat outputs a variable voltage in accordance with a collector current of the photo coupler light-receiving device35bthat is configured from a phototransistor; a comparator101b; an oscillator101c; and an AND gate101d.

An inverted input of the comparator101bis connected to a connection point of the oscillating switching device3and the Ip detection resistor22, and a non-inverted input of the comparator101bis connected to the variable reference supply101a. Accordingly, a voltage by the Ip detection resistor22arepresented current Ip which flows in the primary winding2aand a voltage output from the variable reference supply101arepresented light amount of a limit signal received by the photo coupler light-receiving device35bfrom the photo coupler light-emitting device35aare compared.

An output of the comparator101bis input to the AND gate101dalong with an output of the oscillator101c. Further, an output of the AND gate101dis connected to a gate of the oscillating switching device3.

With regard to the operation of the switching power supply circuit100configured in this way, when the variable reference supply101adoes not receive collector current from the photo coupler light-emitting device35a, namely, in a normal operating state where the output is stable, a reference voltage Vset set to a predetermined value from the variable reference supply101ais output to the non-inverted input of the comparator101b.

On the other hand, the voltage of the Ip detection resistor22that indicates the current Ip that flows in the primary winding2ais input to the inverted input of the comparator101b. The reference voltage Vset is compared to a primary winding current Ip that increases with the elapse of time once the oscillating switching device3has been switched to ON. Accordingly, the comparator101boutputs “H” until the voltage indicating the primary winding current Ip reaches the reference voltage Vset, and then outputs “L” once the reference voltage Vset has been exceeded.

The oscillator101coutputs a clock pulse that accords with an oscillation period T of the switching power supply circuit100to the AND gate101d. As a result, the AND gate101doutputs “H” when the clock pulse is “H” and the output of the comparator101bis “H”, namely, when the voltage that indicates the primary winding current Ip has not reached the reference voltage Vset, and controls the oscillating switching device3to switch ON.

In contrast to this, when the output current increases past the reference current due to load connected between the high voltage side line20aand the low voltage side line20b, the voltage input to the inverted input terminal of the differential amplifier33brises. Thus, the potential difference between this voltage and the second comparative voltage is inverted and amplified, and reaches a potential that exceeds a light-emitting threshold value of the photo coupler light-emitting device35a.

Furthermore, even when the output voltage increases past the reference voltage due to load connected between the high voltage side line20aand the low voltage side line20b, the divided voltage input to the inverted input terminal of the differential amplifier33balso rises. Thus, the potential difference between this voltage and the first comparative voltage is inversely amplified, and reaches a potential that exceeds the light-emitting threshold value of the photo coupler light-emitting device35a.

Accordingly, when either one of the output voltage and the output current exceeds the respective reference voltage or reference current, the photo coupler light-emitting device35aemits a limit signal of an emitted light amount to the photo coupler light-receiving device35b, in accordance with the respective exceeded amount.

When the photo coupler light-receiving device35breceives the limit signal from the photo coupler light-emitting device35a, the output voltage of the variable reference supply101areduces from the reference voltage Vset in accordance with the increase in the received light amount. Thus, the output of the comparator101bis rapidly switched to “L”, as compared to the normal operation in which the reference voltage Vset is output.

As a result, the oscillating switching device3is switched on, a time T1for which the primary winding2ais excited is made shorter, and the energy stored in the transformer2reduces within one oscillation period. Accordingly, the output voltage or the output current, which respectively exceed the reference voltage or the reference current, spontaneously reduce, and become equal to or less than the reference voltage or the reference current.

Then, the photo coupler light-emitting device35astops emitting light and the photo coupler light-receiving device35bno longer receives the limit signal. Accordingly, the oscillating switching device3once again repeats oscillation that is controlled according to the reference voltage Vset, and a stable output that accords with the power supplied to the load can be obtained.

In order for a voltage to be controlled to a constant by the constant voltage output control method, the switching power supply circuit100is provided with voltage dividing resistors30and31and a voltage monitoring reference supply34ain a voltage monitoring circuit; a variable reference supply101athat outputs a reference voltage Vsetin a switching control circuit101; and an Ip detection resistor22that is in-series with a primary winding2a. However, as a result of variation of circuit constants of these circuit devices, variation of the integrated circuit itself when the switching control circuit101acts as an integrated circuit, and the like, a problem arises since stable and simple mass production of products having highly accurate constant voltage output characteristics is difficult.

Further, in case various output voltage characteristics of the switching power supply circuit are required, it becomes necessary to set each of the aforementioned circuit constants, and the like, or necessary to exchange circuit components, to adjust the characteristics. Accordingly, costs are increased due to factors such as an increase in time spent on intricate design and circuit component adjustment.

Moreover, an output voltage detection circuit is provided at a secondary side of a transformer2. As a result, the number of components in the circuit is increased, thereby causing the overall circuit to become larger.

In addition, increase in the output voltage detected by the output voltage detection circuit of the secondary side of the transformer2is adjusted by control of the primary side. Accordingly, it is necessary to provide a photocoupler light-emitting device35a, a photocoupler light-receiving device35b, and so on, which leads to an increase in cost, as well as the circuit configuration becoming more complicated.

SUMMARY OF THE INVENTION

In light of the aforementioned circumstances, it is an object of the present invention to provide a constant voltage output control method and a constant voltage output control device for a switching power supply circuit. This constant voltage output control method and device are capable of accurately executing constant voltage control of an output voltage, even if there is variation of a utilized circuit element or an integrated circuit.

In addition, a further object of the present invention is to provide a constant voltage output control method and a constant voltage output control device for a switching power supply circuit that allow mass production of switching power supply circuits having different output voltage specifications, using the same circuit components.

Moreover, a further object of the present invention is to provide a constant voltage output control method and a constant voltage output control device for a switching power supply circuit that execute a constant voltage control of a constant voltage using only a primary side of a circuit. Accordingly, an output voltage detection circuit on the secondary side, an optically coupled device, and the like, do not need to be provided.

In order to address the above described problems, according to the present invention, even if there is variation of circuit constants of respective circuit devices, it is possible to execute constant voltage control such that an output voltage V2bof a secondary output winding or an output voltage V2oof a rectifying smoothing circuit are accurately set to predetermined set output voltages V2bsetand V2oset, respectively, simply by adjusting a set voltage V2aset, or V2oset,. Accordingly, it is possible to mass produce a switching power supply circuit which can easily accommodate specifications changes, and which has consistent quality.

Moreover, a value of primary winding current Ip is indicated by a voltage drop Vipthat is a voltage. Accordingly, it is easily possible to execute comparison with the set voltage V2aset, or the like, that is subject to the constant voltage control by using a comparator circuit including a comparator, without executing calculation processing.

In addition, it is possible to execute constant voltage control of the secondary side using just the primary side. Accordingly, an output voltage detection circuit, an optically coupled device, or the like, do not need to be provided at the secondary side of the transformer.

According to an aspect of the present invention, a constant voltage output control method for a switching power supply circuit having a transformer provided with a primary winding and a secondary output winding; an oscillating switching device which is connected in-series with the primary winding to a direct current power supply that excites the primary winding; a switching control circuit that executes ON/OFF control of the oscillating switching device; and a rectifying smoothing circuit that rectifies and smoothes an output of the secondary output winding; this constant voltage output control method executing a constant voltage control of an output voltage V2bthat is generated in the secondary output winding by changing an ON time T1of the oscillating switching device of the switching power supply circuit, comprising the steps of:

deriving a set voltage V2asetbased on equation (1),
V2aset=Np÷Ns×V2bset(1)

where, V2bsetrepresents an output voltage of the secondary output winding that is subject to the constant voltage control, Np represents a number of turns of the primary winding, and Ns represents a number of turns of the secondary output winding;

comparing the set voltage V2asetwith a flyback voltage V2aof the primary winding; and

executing at least one of a time shortening control and a time lengthening control such that, in the case that the flyback voltage V2ais more than the set voltage V2aset, the time shortening control is executed that shortens the ON time T1of the oscillating switching device in accordance with a difference voltage dV between the set voltage V2asetand the flyback voltage V2a, and in the case that the flyback voltage V2ais less than the set voltage V2aset, the time lengthening control is executed that lengthens the ON time T1of the oscillating switching device in accordance with the difference voltage dV.

If the output voltage V2bsetof the secondary output winding that is subject to the constant voltage control is substituted into Equation (1), the set voltage V2asetfor setting the output voltage V2bto a constant voltage is obtained.

In the case that a flyback voltage V2aof the primary winding that is actually generated is more than the set voltage V2aset, an ON time T1is shortened in accordance with a difference voltage dV there between, whereby energy stored in the transformer is reduced during one time cycle such that the flyback voltage V2ais reduced so as to approach the set voltage V2aset.

Moreover, in the case that the flyback voltage V2aof the primary winding is less than the set voltage V2aset, the ON time T1is lengthened in accordance with the difference voltage dV there between, whereby energy stored in the transformer is increased during one time cycle such that the flyback voltage V2ais increased so as to approach the set voltage V2aset.

As a result, the output voltage V2bof the secondary output winding is controlled so as to become the set output voltage V2bset.

According to another aspect of the present invention, a constant voltage output control method for a switching power supply circuit having a transformer provided with a primary winding and a secondary output winding; an oscillating switching device which is connected in-series with the primary winding to a direct current power supply that excites the primary winding; a switching control circuit that executes ON/OFF control of the oscillating switching device; and a rectifying smoothing circuit that rectifies and smoothes an output of the secondary output winding; this constant voltage output control method executing a constant voltage control of an output voltage V2oof the rectifying smoothing circuit by changing an ON time T1of the oscillating switching device of the switching power supply circuit, comprising the steps of:

deriving a set voltage V2aset′ based on equation (2),
V2aset′=V2oset×Np÷Ns÷(1−T2×k÷Ls)  (2)where, V2osetrepresents an output voltage of the rectifying smoothing circuit that is subject to the constant voltage control, Np represents a number of turns of the primary winding, Ns represents a number of turns of the secondary output winding, Ls represents an inductance of the secondary output winding, T2represents an output time for which output is generated in the rectifying smoothing circuit within an oscillation time cycle Tc, and k represents a proportionality constant provided dividing a forward voltage drop of a diode of the rectifying smooth circuit by an output current;

comparing the set voltage V2aset′ with a flyback voltage V2aof the primary winding; and

executing at least one of a time shortening control and a time lengthening control such that, in the case that the flyback voltage V2ais more than the set voltage V2aset′, the time shortening control is executed that shortens the ON time T1of the oscillating switching device in accordance with a difference voltage dV between the set voltage V2aset′ and the flyback voltage V2a, and in the case that the flyback voltage V2ais less than the set voltage V2aset′, the time lengthening control is executed that lengthens the ON time T1of the oscillating switching device in accordance with the difference voltage dV.

If the output voltage V2osetof the rectifying smoothing circuit that is subject to the constant voltage control is substituted into Equation (2), the set voltage V2aset′ for setting the output voltage V2oto a constant voltage is obtained.

In the case that a generated flyback voltage V2aof the primary winding is more than the set voltage V2aset′, an ON time T1is shortened in accordance with a difference voltage dV there between, whereby energy stored in the transformer is reduced during one time cycle such that the flyback voltage V2ais reduced so as to approach the set voltage V2aset′.

Moreover, in the case that the flyback voltage V2aof the primary winding is less than the set voltage V2aset′, the ON time T1is lengthened in accordance with the difference voltage dV there between, whereby energy stored in the transformer is increased during one time cycle such that the flyback voltage V2ais increased so as to approach the set voltage V2aset′.

As a result, the output voltage V2oof the rectifying smoothing circuit is accurately controlled so as to become the set output voltage V2oset.

According to a further aspect of the present invention, a constant voltage output control method for a switching power supply circuit having a transformer provided with a primary winding and an auxiliary winding on a primary side and a secondary output winding on a secondary side; an oscillating switching device which is connected in-series with the primary winding to a direct current power supply that excites the primary winding; a switching control circuit that executes ON/OFF control of the oscillating switching device; and a rectifying smoothing circuit that rectifies and smoothes an output of the secondary output winding; this constant voltage output control method executing a constant voltage control of an output voltage V2bthat is generated in the secondary output winding by changing an ON time T1of the oscillating switching device of the switching power supply circuit, comprising the steps of:

deriving a set voltage V2csetbased on equation (3),
V2cset=Nt÷Ns×V2bset(3)where, V2bsetrepresents an output voltage of the secondary output winding that is subject to the constant voltage control, Nt represents a number of turns of the auxiliary winding, and Ns represents a number of turns of the secondary output winding;

comparing the set voltage V2csetwith a flyback voltage V2cof the auxiliary winding; and

executing at least one of a time shortening control and a time lengthening control such that, in the case that the flyback voltage V2cis more than the set voltage V2cset, the time shortening control is executed that shortens the ON time T1of the oscillating switching device in accordance with a difference voltage dV between the set voltage V2csetand the flyback voltage V2c, and in the case that the flyback voltage V2cis less than the set voltage V2cset, the time lengthening control is executed that lengthens the ON time T1of the oscillating switching device in accordance with the difference voltage dV.

If the output voltage V2bsetof the secondary output winding that is subject to the constant voltage control is substituted into Equation (3), a set voltage V2csetfor setting the output voltage V2bto a constant voltage is obtained.

In the case that a generated flyback voltage V2cof an auxiliary winding is more than the set voltage V2cset, an ON time T1is shortened in accordance with a difference voltage dV there between, whereby energy stored in the transformer is reduced during one time cycle such that the flyback voltage V2cis reduced so as to approach the set voltage V2cset.

Moreover, in the case that the flyback voltage V2cof the auxiliary winding is less than the set voltage V2cset, the ON time T1is lengthened in accordance with the difference voltage dV there between, whereby energy stored in the transformer is increased during one time cycle such that the flyback voltage V2cis increased so as to approach the set voltage V2cset.

As a result, the output voltage V2bof the secondary output winding is controlled so as to become the set output voltage V2bset.

Accordingly, in the case that the auxiliary winding is provided in the transformer it is possible to execute constant voltage output control of the secondary side by monitoring a potential V2cof the auxiliary winding, and causing this potential V2cto equal a set potential V2cset.

According to another aspect of the present invention, a constant voltage output control method for a switching power supply circuit having a transformer provided with a primary winding and an auxiliary winding on a primary side and a secondary output winding on a secondary side; an oscillating switching device which is connected in-series with the primary winding to a direct current power supply that excites the primary winding; a switching control circuit that executes ON/OFF control of the oscillating switching device; and a rectifying smoothing circuit that rectifies and smoothes an output of the secondary output winding; this constant voltage output control method executing a constant voltage control of an output voltage V2othat is generated in the rectifying smoothing circuit by changing an ON time T1of the switching power supply circuit, comprising the steps of:

deriving a set voltage V2cset′ based on equation (4),
V2cset′=V2oset×Nt÷Ns÷(1−T2×k÷Ls)  (4)where, V2osetrepresents an output voltage of the rectifying smoothing circuit that is subject to the constant voltage control, Nt represents a number of turns of the auxiliary winding, Ns represents a number of turns of the secondary output winding, Ls represents an inductance of the secondary output winding, T2represents an output time for which output is generated in the rectifying smoothing circuit within an oscillation time cycle Tc, and k represents a proportionality constant provided dividing a forward voltage drop of a diode of the rectifying smooth circuit by an output current;

comparing the set voltage V2cset′ with a flyback voltage V2cof the auxiliary winding; and

executing at least one of a time shortening control and a time lengthening control such that, in the case that the flyback voltage V2cis more than the set voltage V2cset′, the time shortening control is executed that shortens the ON time T1of the oscillating switching device in accordance with a difference voltage dV between the set voltage V2cset′ and the flyback voltage V2c, and in the case that the flyback voltage V2cis less than the set voltage V2cset′, the time lengthening control is executed that lengthens the ON time T1of the oscillating switching device in accordance with the difference voltage dV.

If the output voltage V2osetof the rectifying smoothing circuit that is subject to the constant voltage control is substituted into Equation (4), a set voltage V2cset′ for setting the output voltage V2oto a constant voltage is obtained.

In the case that a generated flyback voltage V2cof an auxiliary winding is more than the set voltage V2cset′, an ON time T1is shortened in accordance with a difference voltage dV there between, whereby energy stored in the transformer is reduced during one time cycle such that the flyback voltage V2cis reduced so as to approach the set voltage V2cset.

Moreover, in the case that the flyback voltage V2cof the auxiliary winding is less than the set voltage V2set′, the ON time T1is lengthened in accordance with the difference voltage dV there between, whereby energy stored in the transformer is increased during one time cycle such that the flyback voltage V2cis increased so as to approach the set voltage V2cset′.

As a result, the output voltage V2oof the rectifying smoothing circuit is accurately controlled so as to become the set output voltage V2oset.

Accordingly, in the case that the auxiliary winding is provided in the transformer it is possible to execute constant voltage output control of the secondary side by monitoring a potential V2cof the auxiliary winding, and causing this potential V2cto equal a set potential V2cset.

According to a further aspect of the present invention, a constant voltage output control device for a switching power supply circuit having a transformer provided with a primary winding and a secondary output winding; an oscillating switching device which is connected in-series with the primary winding to a direct current power supply that excites the primary winding; a primary side current detection portion that indicates a primary winding current Ip flowing in the primary winding based on a voltage drop Vipof an Ip detection resistor, this Ip detection resistor having a resistance value ripand being connected in-series with the primary winding; a switching control circuit that executes ON control of the oscillating switching device with a predetermined time cycle, and executes OFF control of the oscillating switching device when the primary winding current Ip reaches a maximum current Ipmaxwhich is taken to occur when the voltage drop Vipreaches a maximum voltage ViMaxthat is the multiple of a maximum current Ipmaxand the resistance value rip; and a rectifying smoothing circuit that rectifies and smoothes an output of the secondary output winding; this constant voltage output control device executing a constant voltage control of an output voltage V2bthat is generated in the secondary output winding by changing an ON time T1of the oscillating switching device, comprising:

a voltage monitoring portion that detects a flyback voltage V2aof the primary winding;

a difference voltage generation circuit which obtains a set voltage V2asetbased on equation (1),
V2aset=Np÷Ns×V2bset(1)where, V2bsetrepresents an output voltage of the secondary output winding that is subject to the constant voltage control, Np represents a number of turns of the primary winding, and Ns represents a number of turns of the secondary output winding, and which outputs a difference voltage dV expressed by equation (5),
dV=V2a−V2aset(5)

which is based on the set voltage V2asetand the flyback voltage V2aof the primary winding; and

an adder circuit that sets a correction voltage drop Vip′ by adding the difference voltage dV to the voltage drop Vip, wherein

the switching control circuit sets the correction voltage drop Vip′ as the voltage drop Vipthat is compared with the maximum voltage Vimax, and executes at least one of a time shortening control and a time lengthening control such that, in the case that the flyback voltage V2ais more than the set voltage V2aset, the time shortening control is executed that shortens the ON time T1of the oscillating switching device in accordance with the difference voltage dV, and in the case that the flyback voltage V2ais less than the set voltage V2aset, the time lengthening control is executed that lengthens the ON time T1of the oscillating switching device in accordance with the difference voltage dV.

If the output voltage V2bsetof the secondary output winding that is subject to the constant voltage control is substituted into Equation (1), the set voltage V2asetfor setting the output voltage V2bto a constant voltage is obtained.

In the case that the flyback voltage V2aof the primary winding is more than the set voltage V2aset, a difference voltage dV obtained from Equation (5) becomes a positive value, and a correction voltage drop Vip′ becomes larger than a voltage drop Vipthat indicates a current Ip actually flowing in the primary winding while ON control of the oscillating switching device, and quickly reaches a maximum voltage ViMax. As a result, an ON time T1is only shorted by an increase portion of the difference voltage dV, and the flyback voltage V2ais reduced so as to approach the set voltage V2aset.

Further, in the case that the flyback voltage V2aof the primary winding is less than the set voltage V2aset, the difference voltage dV becomes a negative value, and the correction voltage drop Vip′ becomes smaller than the voltage drop Vipthat indicates the current Ip actually flowing in the primary winding while ON control of the oscillating switching device, and leisurely reaches the maximum voltage ViMax. As a result, the ON time T1is only lengthened by the increase portion of the difference voltage dV, and the flyback voltage V2ais increased so as to approach the set voltage V2aset.

Accordingly, it is possible to accurately execute the constant voltage control such that the output voltage V2bof the secondary output winding is set to the predetermined set output voltage V2bset, while also executing OFF control of the oscillating switching device when the voltage drop Vipreaches the maximum voltage Vimax, which is the multiple of a maximum current Ipmaxand a resistance value rip, so that electric power generated in each oscillation is approximately constant value.

According to another aspect of the present invention, a constant voltage output control device for a switching power supply circuit having a transformer provided with a primary winding and a secondary output winding; an oscillating switching device which is connected in-series with the primary winding to a direct current power supply that excites the primary winding; a primary side current detection portion that indicates a primary winding current Ip flowing in the primary winding based on a voltage drop Vipof an Ip detection resistor, this Ip detection resistor having a resistance value ripand being connected in-series with the primary winding; a switching control circuit that executes ON control of the oscillating switching device with a predetermined time cycle, and executes OFF control of the oscillating switching device when the primary winding current Ip reaches a maximum current Ipmaxwhich is taken to occur when the voltage drop Vipreaches a maximum voltage Vimaxthat is the multiple of a maximum current Ipmaxand the resistance value rip; and a rectifying smoothing circuit that rectifies and smoothes an output of the secondary output winding; this constant voltage output control device executing a constant voltage control of an output voltage V2oof the rectifying smoothing circuit by changing an ON time T1of the oscillating switching device, comprising:

a voltage monitoring portion that detects a flyback voltage V2aof the primary winding;

an output time detection portion that detects an output time T2for which output is generated in the rectifying smoothing circuit during an oscillation time cycle Tc;

a difference voltage generation circuit which obtains a set voltage V2aset′ based on equation (2),
V2aset′=V2oset×Np÷Ns÷(1−T2×k÷Ls)  (2)where, V2osetrepresents an output voltage of the rectifying smoothing circuit that is subject to the constant voltage control, Np represents a number of turns of the primary winding, Ns represents a number of turns of the secondary output winding, Ls represents an inductance of the secondary output winding, T2represents the output time for which output is generated in the rectifying smoothing circuit within the oscillation time cycle Tc, and k represents a proportionality constant provided dividing a forward voltage drop of a diode of the rectifying smooth circuit by an output current, and which outputs a difference voltage dV expressed by equation (6)
dV=V2a−V2aset′  (6)

which is based on the set voltage V2aset′ and the flyback voltage V2aof the primary winding; and

an adder circuit that sets a correction voltage drop Vip′ by adding the difference voltage dV to the voltage drop Vip, wherein

the switching control circuit sets the correction voltage drop Vip′ as the voltage drop Vipthat is compared with the maximum voltage ViMax, and executes at least one of a time shortening control and a time lengthening control such that, in the case that the flyback voltage V2ais more than the set voltage V2aset′, the time shortening control is executed that shortens the ON time T1of the oscillating switching device in accordance with the difference voltage dV, and in the case that the flyback voltage V2ais less than the set voltage V2aset′, the time lengthening control is executed that lengthens the ON time T1of the oscillating switching device in accordance with the difference voltage dV.

If the output voltage V2osetof the rectifying smoothing circuit that is subject to the constant voltage control is substituted into Equation (2), the set voltage V2aset′ for setting the output voltage V2oto a constant voltage is obtained.

In the case that the flyback voltage V2aof the primary winding is more than the set voltage V2set′, a difference voltage dV obtained from Equation (5) becomes a positive value, and the correction voltage drop Vip′ becomes larger than the voltage drop Vipthat indicates the current Ip actually flowing in the primary winding while ON control of the oscillating switching device, and quickly reaches the maximum voltage ViMax. As a result, the ON time T1is only shortened by an increase portion of the difference voltage dV, and the flyback voltage V2ais reduced so as to approach the set voltage V2aset′.

Further, in the case that the flyback voltage V2aof the primary winding is less than the set voltage V2set′, the difference voltage dV becomes a negative value, and the correction voltage drop Vip′ becomes larger than the voltage drop Vipthat indicates the current Ip actually flowing in the primary winding while ON control of the oscillating switching device, and leisurely reaches the maximum voltage ViMax. As a result, the ON time T1is only lengthened by the increase portion of the difference voltage dV, and the flyback voltage V2ais increased so as to approach the set voltage V2aset′.

Accordingly, it is possible to accurately execute the constant voltage control such that the output voltage V2oof the rectifying smoothing circuit is set to the predetermined set output voltage V2oset, while also executing OFF control of the oscillating switching device when the voltage drop Vipreaches the maximum voltage Vimax, which is the multiple of the maximum current Ipmaxand the resistance value rip, so that electric power generated in each oscillation is approximately constant value.

According to a further aspect of the present invention, a constant voltage output control device for a switching power supply circuit having a transformer provided with a primary winding and a secondary output winding; an oscillating switching device which is connected in-series with the primary winding to a direct current power supply that excites the primary winding; an oscillator circuit that generates a pulse control signal with a fixed time cycle Tc, a single cycle of the fixed time cycle Tc being formed by respective output periods of a first level and a second level continuous with the first level; a switching control circuit that executes ON control of the oscillating switching device while the pulse control signal is the first level, and executes OFF control of the oscillating switching device while the pulse control signal is the second level; and a rectifying smoothing circuit that rectifies and smoothes an output of the secondary output winding; this constant voltage output control device executing a constant voltage control of an output voltage V2bthat is generated in the secondary output winding by changing an ON time T1of the oscillating switching device, comprising:

a primary side current detection portion that indicates a primary winding current Ip flowing in the primary winding based on a voltage drop Vipof an Ip detection resistor, this Ip detection resistor having a resistance value ripand being connected in-series with the primary winding;

a voltage monitoring portion that detects a flyback voltage V2aof the primary winding;

a difference voltage generation circuit which obtains a set voltage V2asetbased on equation (1),
V2aset=Np÷Ns×V2bset(1)where, V2bsetrepresents an output voltage of the secondary output winding that is subject to the constant voltage control, Np represents a number of turns of the primary winding, and Ns represents a number of turns of the secondary output winding, and which outputs a difference voltage dV expressed by equation (5),
dV=V2a−V2aset(5)

which is based on the set voltage V2asetand the flyback voltage2aof the primary winding; and

a pulse width modulation circuit that executes pulse width modulation of a pulse width of the first level of the pulse control signal using the difference voltage dV, wherein

the switching control circuit executes at least one of a time shortening control and a time lengthening control such that, in the case that the flyback voltage V2ais more than the set voltage V2aset, the time shortening control is executed that shortens the ON time T1of the oscillating switching device in accordance with the difference voltage dV, and in the case that the flyback voltage V2ais less than the set voltage V2aset, the time lengthening control is executed that lengthens the ON time T1of the oscillating switching device in accordance with the difference voltage dV.

If the output voltage V2bsetof the secondary output winding that is subject to the constant voltage control is substituted into Equation (1), the set voltage V2asetfor setting the output voltage V2bto a constant voltage is obtained.

A pulse width of a first level that indicates an ON time T1of an oscillating switching device is pulse width modulated using a difference voltage dV obtained from Equation (5). In the case that the flyback voltage V2aof the primary winding is more than the set voltage V2aset, the pulse width is made smaller in accordance with an increase portion of the difference voltage dV, and the ON time T1is controlled to become shorter. Accordingly, the flyback voltage V2ais reduced so as to approach the set voltage V2aset.

Further, in the case that the flyback voltage V2aof the primary winding is less than the set voltage V2aset, the pulse width is made larger in accordance with a decrease portion of the difference voltage dV, the ON time T1is made longer, and the flyback voltage V2ais increased so as to approach the set voltage V2aset. Accordingly, the output voltage V2bof the secondary output winding is controlled so as to become the set output voltage V2bset.

Therefore, it is possible to adjust the ON time T1in accordance with the difference voltage dV of the voltage V2aof the primary winding and the set voltage V2aset, using a simple circuit including a PWM circuit.

According to a further aspect of the present invention, a constant voltage output control device for a switching power supply circuit having a transformer provided with a primary winding and a secondary output winding; an oscillating switching device which is connected in-series with the primary winding to a direct current power supply that excites the primary winding; an oscillator circuit that generates a pulse control signal with a fixed time cycle Tc, a single cycle of the fixed time cycle Tc being formed by respective output periods of a first level and a second level continuous with the first level; a switching control circuit that executes ON control of the oscillating switching device while the pulse control signal is the first level, and executes OFF control of the oscillating switching device while the pulse control signal is the second level; and a rectifying smoothing circuit that rectifies and smoothes an output of the secondary output winding; this constant voltage output control device executing a constant voltage control of an output voltage V2oof the rectifying smoothing circuit by changing an ON time T1of the oscillating switching device, comprising:

a primary side current detection portion that indicates a primary winding current Ip flowing in the primary winding based on a voltage drop Vipof an Ip detection resistor, this Ip detection resistor having a resistance value ripand being connected in-series with the primary winding;

a voltage monitoring portion that detects a flyback voltage V2aof the primary winding;

an output time detection portion that detects an output time T2for which output is generated in the rectifying smoothing circuit during an oscillation time cycle Tc;

a difference voltage generation circuit which obtains a set voltage V2aset′ based on equation (2),
V2aset′=V2oset×Np÷Ns÷(1−T2×k÷Ls)  (2)where, V2osetrepresents an output voltage of the rectifying smoothing circuit that is subject to the constant voltage control, Np represents a number of turns of the primary winding, Ns represents a number of turns of the secondary output winding, Ls represents an inductance of the secondary output winding, T2represents the output time for which output is generated in the rectifying smoothing circuit within the oscillation time cycle Tc, and k represents a proportionality constant provided dividing a forward voltage drop of a diode of the rectifying smooth circuit by an output current, and which outputs a difference voltage dV expressed by equation (6),
dV=V2a−V2aset′  (6)

which is based on the set voltage V2set′ and the flyback voltage2aof the primary winding; and

a pulse width modulation circuit that executes pulse width modulation of a pulse width of the first level of the pulse control signal using the difference voltage dV, wherein

the switching control circuit executes at least one of a time shortening control and a time lengthening control such that, in the case that the flyback voltage V2ais more than the set voltage V2aset′, the time shortening control is executed that shortens the ON time T1of the oscillating switching device in accordance with the difference voltage dV, and in the case that the flyback voltage V2ais less than the set voltage V2aset′, the time lengthening control is executed that lengthens the ON time T1of the oscillating switching device in accordance with the difference voltage dV.

If the output voltage V2osetof the rectifying smoothing circuit that is subject to the constant voltage control is substituted into Equation (2), the set voltage V2aset′ for setting the output voltage V2oto a constant voltage is obtained.

A pulse width of a first level that indicates an ON time T1of an oscillating switching device is pulse width modulated using a difference voltage dV obtained from Equation (5). In the case that the flyback voltage V2aof the primary winding is more than the set voltage V2aset′, the pulse width is made smaller in accordance with an increase portion of the difference voltage dV, and the ON time T1is controlled to become shorter. Accordingly, the flyback voltage V2ais reduced so as to approach the set voltage V2aset′.

Further, in the case that the flyback voltage V2aof the primary winding is less than the set voltage V2aset′, the pulse width is made larger in accordance with a decrease portion of the difference voltage dV, the ON time T1is made longer, and the flyback voltage V2ais increased so as to approach the set voltage V2aset′. Accordingly, the output voltage V2oof the rectifying smoothing circuit is accurately controlled so as to become the set output voltage V2oset.

Therefore, it is possible to adjust the ON time T1in accordance with the difference voltage dV of the voltage V2aof the primary winding and the set voltage V2aset′, using a simple circuit including a PWM circuit.

According to further forms of some of the above aspects, it is possible to detect the output time T2by monitoring the potential of the primary winding. The time T2for which output is generated in the rectifying smoothing circuit is a discharge time of energy stored in the transformer. The time T2is equal to the time from when flyback voltage is generated in the primary winding when the oscillating switching device is turned off, until a time when a polarity of the primary winding reverses as a result of the start of natural oscillation following reduction of the flyback voltage. Accordingly, it is possible to detect the output time T2from the primary side of the transformer by monitoring the potential of the primary winding, without having to monitor the output of the rectifying smoothing circuit.

Accordingly, it is not necessary to provide a transmission element for transmitting detection results of the secondary side to the primary side, and it is possible to execute the constant voltage control using just the primary side of the circuit.

According to further forms of some of the above aspects, it is possible to detect the output time T2by monitoring the potential of the auxiliary winding of the primary side of the transformer.

The time T2for which output is generated in the rectifying smoothing circuit is equal to a time from when flyback voltage is generated in the auxiliary winding, until a time when a polarity of the auxiliary winding reverses. Accordingly, it is possible to detect the output time T2from the primary side of the transformer by monitoring the potential of the auxiliary winding of the primary side of the transformer, without having to monitor the output of the rectifying smoothing circuit.

Accordingly, it is not necessary to provide a transmission element for transmitting detection results of the secondary side to the primary side, and it is possible to execute the constant voltage control using just the primary side of the circuit.

According to further forms of some of the above aspects of the present invention, the maximum value of the difference voltage dV is set at a permitted maximum difference voltage dVLIM. Accordingly, it is possible to set the permitted maximum difference voltage dVLIMso that the correction voltage drop Vip′ does not reach the maximum voltage ViMax—before the primary winding current Ip is the minimum necessary current for execution of the constant voltage control of the switching power supply circuit.

Accordingly, it is possible to ensure that, during each oscillation time cycle, the primary winding current Ip beyond minimum necessary current for execution of the constant voltage control of the switching power supply circuit.

According to a further form of the certain aspects, in the case that the voltage drop Vipdoes not reach a minimum potential Viminexpressed by equation (7),
Vimin=Vimax−dVLIM(7)
ON control of the oscillating switching device is temporarily stopped.

In the case that the voltage drop Vipdoes not reach a minimum voltage drop Vimin, the primary winding current Ip that is flowing is small enough that the correction voltage drop Vip′ does not reach the maximum voltage ViMaxeven if the maximum voltage ViMaxis added. Thus, it is possible to inhibit generation of over-voltage by temporarily stopping ON control.

According to a further form of the some aspects, a maximum voltage Vimax′ is derived from equation (8),
ViMax′=ViMaxδt×Vcc÷Lp×rip(8)

where, δt is a time difference between detection of the drop voltage Vip, to which the difference voltage dV has been added, reaching the maximum voltage Vimaxand stopping of ON control of the oscillating switching device, Vcc represents a power supply voltage of the direct current power supply, and Lp represents an inductance of the primary winding, and

the switching control circuit sets this maximum voltage Vimax′ as the maximum voltage Vimaxthat is compared with the correction voltage drop Vip.

Following turn off of the oscillation switching device, the primary winding current Ip increases almost proportionally to a power supply voltage Vcc÷Lp. Thus, δt×Vcc÷Lp×ripof Equation (8) expresses a voltage converted increase portion of the current Ip caused by a delay δt between operations of the switching control circuit and the oscillating switching device.

Thus, the switching control circuit is able to accurately execute the constant voltage control even if there is delay caused by circuit elements, since a maximum voltage Vimax′ including delay of the circuit elements is set to the maximum voltage Vimaxwhich is a reference potential that executes turn off of the oscillating switching device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, an embodiment of the present invention will be described with reference toFIGS. 1 to 3. It should be noted that, in the figures, structural members that are the same as those in the conventional switching power supply circuit100shown inFIG. 8, will be denoted with the same reference numerals.

FIG. 1is a circuit diagram of a switching power supply circuit10according to the first embodiment of the present invention. As is apparent through comparison with the conventional switching power supply circuit100shown inFIG. 8, the configuration of the switching power supply circuit10does not utilize a voltage monitoring circuit on the output side, an optically coupled device, or the like.

InFIG. 1, an unstable direct current power supply1that allows variation in voltage has a high voltage side terminal1aand a low voltage side terminal1b. Further, a transformer2is provided with a primary winding2aand a secondary output winding2b. A field effect transistor (FET)3acts as an oscillating switching device (hereinafter referred to as a “switching device”). The switching device3is, in this case, a MOS (insulated gate type) FET. The switching device3has a drain connected to one end of the primary winding2a, and a source connected to the low voltage side terminal1bvia an Ip detection resistor22. A gate of the switching device3is connected to a switching control circuit5that executes ON/OFF control of the switching device3.

The switching control circuit5is configured from an AND gate6which has an output terminal Vg connected to the gate of the switching device3; an oscillator7that generates a clock pulse that is equal to a frequency cycle of the switching power supply circuit10with a fixed time cycle Tc, and outputs this clock pulse to the AND gate6; and a comparator8having an output connected to another input of the AND gate6.

A non-inverting input of the comparator8is connected to a first reference power supply12set at a potential of a maximum voltage Vimax, described hereinafter, via a subtracter11, and the maximum voltage Vimaxis input to the comparator8. Further, the inverting input of the comparator8connects an input terminal Id to a connection point of the Ip detection resistor22and the switching device3via the adder9, such that a voltage drop Vipof the Ip detection resistor22, resulting from a primary winding current Ip flowing in the primary winding2a, is input to the comparator8. Accordingly, the voltage drop Vipis obtained by multiplying the primary winding current Ip by a resistance value ripof the Ip detection resistor22that is a constant, and indicates the value of the primary winding current Ip.

Moreover, the maximum voltage Vimaxthat is input to the other side of the comparator8is determined by an electric power P generated by the switching power supply circuit10.

In other words, the switching power supply circuit10is configured to act as a converter in which the electric power P is generated in the primary winding2a, and then transmitted to the secondary winding2b. The electric power P is generated in accordance with Equation (9) below:
P=½×Lp×Ipmax2÷Tc(9)

where, an inductance of the primary winding2ais Lp, and a maximum current flowing in the primary winding2awhen the switching device3is switched OFF (hereinafter referred to as “turn off”) is Ipmax. If the inductance Lp of the primary winding2aand the frequency cycle Tc are taken to be constants, this electric power P is proportional to the square of the maximum current Ipmax.

Accordingly, at first, the electric power P that appropriate accords with the magnitude of a load connected between output lines20aand20bof the secondary side is obtained. Then, the electric power P is substituted into Equation (9), the maximum current Ipmaxis obtained, and set the maximum voltage Vimaxby multiplying the value of the maximum current Ipmaxand the resistance value rip.

Voltage conversion of this type using multiplication of the resistance value ripis executed so as to compare the primary winding current Ip and the maximum current Ipmaxat the same amplification. This can be executed because the voltage drop Vipinput to the other side of the comparator8is equal to the primary winding current Ip multiplied by the resistance value rip.

It should be noted that direct comparison of the primary winding current Ip and the maximum current Ipmaxas current values is possible; however, in this case, conversion to voltage values makes it possible to easily execute adding in the calculation circuit or comparison processing using the comparator10, and the like.

Next, a basic operation of the switching power supply circuit10in which other values are not input to the adder9, the subtracter11will be briefly explained. While “L” is output from the oscillator7, the output terminal Vg of the AND gate6becomes “L”, and the switching device3is OFF controlled.

In this state, the primary winding current Ip does not flow, and thus the voltage drop Vipis “0”, and the output of the comparator8that is compared with the maximum voltage Vimaxbecomes “H”.

When the output of the oscillator7turns the level to “H”, because of the output of the comparator8level is also “H”, the output terminal Vg of the AND gate6switches to “H”. Then a forward bias voltage is applied to the gate of the switching device3and the switching device3turns on. As a result, an exciting current Ip begins to flow in the primary winding2athat is connected in series with the direct current power supply1, and an induced electromotive force is generated in each of the windings2aand2bof the transformer2.

InFIG. 2(a) shows a current Ip flowing in a primary winding2aof a transformer2, (b) shows a current is flowing in a secondary output winding2bof the transformer2, and (c) shows a voltage V2aof the primary winding2aof the transformer2.

As shown inFIG. 2, the primary winding current Ip increases in proportion to an elapsed time t following turn on, and when it reaches the maximum current Ipmaxafter time T1, the voltage drop Vipreaches maximum voltage Vimaxand the output of the comparator8switches to “L”.

Accordingly, the output terminal Vg of the AND gate6becomes “L”, and the switching device3is controlled to turn off, consequently the current flowing in the primary winding2abecomes substantially interrupted. Note that, in order to continue OFF control, the output of the comparator8keeps “L” level until, at the least, the clock pulse output from the oscillator7is switched from “H” to “L”.

As a result of the switching device3turns off, a so-called flyback voltage is generated in each of the windings2aand2bof the transformer2. The flyback voltage generated in the secondary output winding2bis rectified and smoothed by rectifying smoothing circuit4and13, configured from the rectifying diode4and the smoothing capacitor13, and then output as the electric power P supplied to a load connected between the output lines20aand20b.

When electric energy stored in the secondary output winding2bas a result of the induced electromotive force is entirely discharged, oscillation begins due to series resonance of stray capacity of the primary winding2aand the switching device3, and so on, with inductance of the primary winding2a, as shown by a voltage (V2a) waveform of the primary winding2ainFIG. 2(c). This oscillation gradually reduces in magnitude.

After the voltage generated in each of the windings2aand2bfalls, the clock pulse output by the oscillator7switches from “L” to “H” once again, and ON control of the switching device3is executed. Accordingly, in this manner, a series of oscillation operations are repeated with the fixed time cycle Tc.

During this oscillation operation, the flyback voltage generated in each of the windings2aand2bof the transformer2is proportional to a respective number of turns thereof. Accordingly, the flyback voltage V2aof the primary winding2a, and a flyback voltage V2bof the secondary output winding2bare expressed by Equation (10) below:
V2a=Np÷Ns×V2b(10)

where, Np is the number of turns of the primary winding2a, and Ns is the number of turns of the secondary output winding2b.

The number of turns Np and Ns are constants determined by circuit elements. Accordingly, if the output voltage V2bin Equation (10) is assumed to be a set output voltage V2bsetof the second output winding2bthat is subject to the constant voltage control, control is able to be executed at the primary side of the transformer2such that the flyback voltage V2aof the primary winding2aequals a set voltage V2asetobtained from Equation (1) below:
V2aset=Np÷Ns×V2bset(1)

In order to set the flyback voltage V2aof the primary winding2ato the set voltage V2aset, in this embodiment, the switching power supply circuit10is provided with an differential amplifier14that monitors the flyback voltage2aof the primary winding2a; a sample hold circuit15connected to an output of the differential amplifier14; an differential amplifier18with a non-inverting input connected to the differential amplifier14, and a inverting input connected to a variable second reference power supply16that normally outputs a potential of the set voltage V2aset; a clamp circuit19; and the aforementioned adder9.

The non-inverting input of the differential amplifier14is connected to the input terminal Vd connected to the low voltage side end portion of the primary winding2avia a resistor23, and the inverting input is connected via a resistor21to the high voltage side terminal1aof the direct current power supply1that is a potential of the high voltage side end portion of the primary winding2a. Accordingly, the differential amplifier14outputs the flyback voltage V2agenerated in the primary winding2a.

As shown inFIG. 2, the voltage V2agenerated in the primary winding2ais not stable during time T4after the switching device3turns off. Accordingly, the sample hold circuit15starts to sample a peak value of the voltage V2agenerated in the primary winding2a, following time T1has elapsed, from a time point after a delay of time t4generated by a delay circuit, not shown and at a time point following a predetermined detection time T5has elapsed and before an output time T2has elapsed (which is detected by a comparator27, as will be explained later), acts as a trigger for a sampling value, namely, the flyback voltage V2a, to be output to the differential amplifier18.

To explain a basic operation, the variable second reference power supply16outputs the set voltage V2asetwithout change to the differential amplifier18, and the differential amplifier18outputs a difference voltage dV (refer toFIG. 2(c)) indicated by Equation (5) below to the clamp circuit19.
dV=V2a−V2aset(5)

The clamp circuit19outputs the difference voltage dV to one side of the adder9by which the difference voltage dV have been added to the voltage drop Vipinput from the input terminal Id, and the result is output to the comparator8. Accordingly, the comparator8sets a correction voltage drop Vip′ to which the difference voltage dV has been added as the voltage drop Vip, and then executes comparison with the maximum voltage Vimax.

The primary winding current Ip that indicates the voltage drop Vipis expressed by Equation 11:
Ip=V2a÷Lp×t(11)

where, the voltage of the primary winding2ais V2a, and the inductance of the primary winding2ais Lp. The primary winding current Ip increases in proportion with the voltage added to the primary winding2afollowing turn on of the switching device3.

Note that, as compared to the voltage V2aapplied to the primary winding2aduring an ON time of the switching device3, other voltage drop elements resulting from exciting current flowing in the circuit are very little and are able to ignored, the voltage V2acan be replaced with a power supply voltage Vcc of the direct current power supply1. As shown byFIG. 3, following turn on of the switching device3, the voltage drop Vipthat indicates the primary winding current Ip increases with respect to the power supply voltage Vcc÷Lp×ripin an almost proportional manner. Once the voltage drop Vipreaches the maximum voltage Vimaxat time T1, the switching device3turns off.

On the other hand, in the case that the detected flyback voltage V2ahas exceeded the set voltage V2athat is subject to the constant voltage control, and the difference voltage dV is a positive value, the correction voltage drop Vip′ reaches the maximum voltage Vimaxat time T1′, which is prior to elapse of time T1. Accordingly, an ON time T1is shortened.

As a result, the electric energy generated in the transformer2is reduced, and the secondary output winding voltage V2bapproaches the output voltage V2bsetthat is subject to the constant voltage control.

On the other hand, in the case that the detected flyback voltage V2ais less than the set voltage V2asetthat is subject to the constant voltage control, and the difference voltage dV is a negative value, the correction voltage drop Vip′ reaches the maximum voltage Vimaxafter the time T1has elapsed. Accordingly, the ON time T1is lengthened, as a result, the electric energy generated in the transformer2is increased, and in this case as well, the secondary output winding voltage V2bapproaches the output voltage V2bsetthat is subject to the constant voltage control.

Following this, by repetition of the method described above, it is possible to execute the constant voltage control in which the output voltage V2bof the secondary output winding2bis output at the set voltage V2bset.

It should be noted that, in this embodiment, when the detected flyback voltage V2ais less that the set voltage V2asetthat is subject to the constant voltage control, a large load is connected to the secondary side output line, and it is conceivable that excessive current may be generated in each of the windings2aand2b. Accordingly, under the ON time T1is controlled to be not lengthened, constant current control is executed by a circuit which is not shown. Accordingly, when the difference voltage dV is a negative value, the difference voltage dV is not output from the clamp circuit19in this embodiment.

Moreover, the clamp circuit19sets an upper limit of the difference voltage dV at a permitted maximum difference voltage dVLIM. Thus, when the difference voltage dV has a positive value that exceeds the permitted maximum difference voltage dVLIM, the permitted maximum difference voltage dVLIMis output from the adder9.

The permitted maximum difference voltage dVLIMis set at a value derived from Equation (12):
dVLIM≦Vimax−Ipmin×rip(12)

where, a minimum current that must flow in the primary winding2ain order to execute the oscillation operation of the switching power supply circuit10is Ipmin.

Accordingly, prior to when the current Ip flowing in the primary winding2aactually reaches the minimum current Ipmin, the correction voltage drop Vip′ doesn't reach the maximum voltage Vimaxto turn off, and thus it is possible to reliably continue the oscillation operation.

On the other hand, when a load with a high resistance value is connected to the secondary side, there is a possibility that the detected flyback voltage V2awill become an over-voltage that is significantly larger than the set voltage V2aset. In this state, the maximum current Ipmaxof the primary winding2ais low, and even if the permitted maximum difference voltage dVLIMis added to the voltage drop Vip, the sum doesn't reach the maximum voltage Vimax.

In other words, in this case the voltage drop Vipdoes not reach the minimum potential Viminderived from Equation (7).
Vimin=Vimax−dVLIM(7)

Accordingly, an over-voltage protection circuit configured from the comparator25and a third reference power supply26shown inFIG. 1operates such that a state of over-voltage is avoided.

The non-inverting input of the comparator25is connected to the third reference power supply26that is set at a potential that is slightly lower than the potential of the minimum potential Vimin. Moreover, the inverting input is connected to the input terminal Id such that the voltage drop Vipthat indicates the primary winding current Ip flowing in the primary winding2ais input.

The oscillator7that is connected to the output of the comparator25, as described previously, generates the clock pulse with the fixed time cycle Tc. Input of “L” level from the comparator25is taken as a prerequisite for generation of the clock pulse for the next cycle. While “L” level is not input, oscillation of the clock pulse stops and after some period (for example, 10 to 20 msec), the clock pulse for the next cycle is generated.

With the over-voltage protection circuit configured in this manner, in the state of over-voltage in which, as described previously, the voltage drop Vipdoes not reach the minimum potential Vimin, the output of the comparator8does not change to “L”. However, the switching device3turns off as a result of clock pulse of the oscillator7turns “L” from “H”. On the other hand, the output of the comparator25does not turn to “L”. Accordingly, in the oscillator7, oscillation of the clock pulse is stopped for some fixed period (for example, 10 to 20 msec), and the clock pulse does not turn to “H”. As a result, the switching device3does not turn on, and the electric energy stored in the transformer2is gradually reduced as it is consumed by the load of the secondary side. Accordingly, the over-voltage state is removed.

In addition, in a normal operation state in which the correction voltage drop Vip′ reaches the maximum voltage Vimax, when the voltage drop Viphas reached the minimum potential Vimin, the output of the comparator25turns to “L”, and following elapse of the fixed time cycle Tc of the oscillator7, the next clock pulse is generated. Accordingly, the clock pulse is generated continuously without interruption.

The above described embodiment executes the constant voltage control such that the output voltage V2bof the secondary output winding2bis set to the set voltage V2bset. However, it is not sufficiently the case that an output voltage V2oof the rectifying smoothing circuit4and13is accurately set to a constant voltage.

In other words, the output voltage V2oof the rectifying smoothing circuit4and13is expressed by Equation (13):
V2o=V2b−Vf(13)

where, a forward voltage drop portion of the diode4of the rectifying smoothing circuit is Vf. The forward voltage drop portion Vf is proportional to a passed current value, namely, a current Is of the secondary output winding2b, and thus the output voltage V2odoes not become a constant voltage even if the constant voltage control of the output voltage V2bis executed.

During the oscillation operation of the switching power supply circuit10, a maximum current Ismaxgenerated in the secondary output winding2bis expressed by Equation (14) below:
Ismax=V2b÷Ls×T2(14)

where, the output voltage of the secondary output winding2bis V2b, an inductance of the secondary output winding2bis Ls, and the output time for which output is generated in the rectifying smoothing circuit4and13is T2.

Using the Equation (14), Equation (13) can be expressed to Equation (15) below:
V2o=V2b(1−k÷Ls×T2)  (15)

Where, a proportionality constant as a result of dividing the forward voltage drop of the diode4by the maximum current Ismaxis k.

Moreover, if output voltage V2bis substituted with the voltage V2ausing Equation (10):
V2a=Np÷Ns×V2b(10)

it is possible to obtain Equation (16):
V2a=V2o×Np÷Ns÷(1−T2×k÷Ls)  (16)

Accordingly, by means of setting the output voltage V2oof Equation (16) to the output voltage V2osetof the rectifying smoothing circuits4and13that is subject to the constant voltage control, control is executed at the primary side of the transformer2such that the flyback voltage V2aof the primary winding2aequals a set voltage V2aset′ obtained from Equation (2) below:
V2aset′=V2oset×Np÷Ns÷(1−T2×k÷Ls)  (2)

Note that, Np, Ns, Ls and k are constants determined by circuit elements and the output time T2for which output is generated in the rectifying smoothing circuits4and13is able to be detected.

Thus, it is possible to execute the constant voltage control such that the output voltage V2oof the rectifying smoothing circuits4and13is set to the output voltage V2osetand control the flyback voltage V2aof the primary winding2aequal to the set voltage V2aset′ obtained by substituting V2osetand detected T2value into Equation (2).

With the switching power supply circuit10ofFIG. 1, if the set voltage V2aset′ obtained from Equation (2) is input to the inverting input of the differential amplifier18, it is possible to execute the constant voltage control of the output voltage V2oof the rectifying smoothing circuits4and13using the above described method. For this purpose, the comparator27, a time T2time-voltage conversion circuit28, and a sampling hold circuit29are additionally provided in the switching power supply circuit10.

With regard to the detection of the output time T2used in Equation (2), it is easily possible to obtain the output time T2by measurement of the time for which current flows in the diode4of the secondary side of the transformer2. However, in this embodiment, the output time T2is detected using the comparator27so that the constant voltage control is executed using only the primary side circuit of the transformer2. The comparator27has an non-inverting input that is connected to the input terminal Vd connected to the low voltage side end portion of the primary winding2avia the resistor23, and an inverting input that is connected via the resistor21to the high voltage side terminal1aof the direct current power supply1that is the potential of the high voltage side end portion of the primary winding2a. The comparator27compares the potentials at the both ends of the primary winding2a.

The output time T2for which the output current is generated at the secondary side of the transformer2is a discharge time of the energy stored in the transformer2. As shown inFIG. 2, this time is equal to the time from when the oscillating switching device3turns off and flyback voltage is generated in the primary winding2a, until a time when the potential fluctuates centering on the voltage Vcc applied to the transformer2as a result of reversal of the polarity of both ends of the primary winding2adue to reduce the flyback voltage generated in the primary winding2aand start of natural oscillation.

Accordingly, the output time T2is detected from the time when the polarity of the primary winding2areverses following output of an OFF control signal for turn off the switching control circuit5, to the time when the polarity reverses once again following the start of natural oscillation. In other words, the output time T2is detected based on the period for which “H” is output from the comparator27.

Note that, following turn off of the switching control circuit5, the time until the waveform of the primary winding voltage V2areaches its initial minimum value due to the natural oscillation approximates to the time taken for the polarity of the primary winding2ato reverse. Accordingly, the output time T2may also be detected by estimating the time from which the OFF control signal is output until when the initial minimal value.

The output of the comparator27is connected to an input of the T2time-voltage conversion circuit28. The T2time-voltage conversion circuit28takes the period during which “H” is input from the comparator27as the output time T2, and calculates a control voltage Vk by voltage dividing the value of Np÷Ns÷(1−T2×k÷Ls) using the output time T2, and then outputs the control voltage Vk to the sample hold circuit29. The sample hold circuit29outputs this control voltage Vk to the variable second reference power supply16until the comparator27detects the output time T2of the next oscillation cycle period.

When the variable second reference power supply16receives the control voltage Vk, it outputs the potential of the set voltage V2aset′ obtained from Equation (2) to the inverting input of the differential amplifier18. Accordingly, the ON time T1is controlled such that the flyback voltage V2abecomes the set voltage V2aset′.

Thereafter, it is possible to execute the constant voltage control in which the output voltage V2oof the rectifying smoothing circuits4and13is controlled to the set voltage V2osetthat takes into account the voltage drop portion Vf of the diode4, by detecting the output time T2for each oscillation cycle period and repeating the same control.

In either of the embodiments described above, as well, delay which is natural to the circuit elements, such as the adder9, the comparator8, the AND gate6, and the switching device3, is generated between the time of input of the primary winding current Ip that allows the correction voltage drop Vip′ to reach the maximum voltage Vimax, and the time when the switching device3is actually turned off.

As described previously, the primary winding current Ip is expressed by Equation (11):
Ip=V2a÷Lp×t(11)

where, V2ais the voltage of the primary winding2a, and Lp is the inductance of the primary winding2a. This primary winding current Ip increases in proportion to the voltage applied to the primary winding2afollowing turn on of the switching device3.

As compared to the voltage V2aapplied to the primary winding2aduring the ON time of the switching device3, other voltage drop elements resulting from exciting current flowing in the circuit are very little and are able to ignored, the voltage V2acan be substituted with the power supply voltage Vcc of the direct current power supply1. The primary winding current Ip can be expressed using Equation (17) below in which the voltage V2ais substituted with the power supply voltage Vcc.
Ip=Vcc÷Lp×t(17)

Accordingly, if a sum of the delay times resulting from the aforementioned circuit elements is taken as δt, based on Equation (17), an increase portion δIp thereof is expressed by Equation (18):
δIp=Vcc÷Lp×δt(18)

As shown inFIG. 4, the correction voltage drop Vip′ on the occasion when the switching device3turns off, is increased by Vcc÷Lp×rip×δt, as compared to when it reaches the maximum voltage Vimax.

Note that, if this increase portion is taken into account, the maximum voltage Vimax′ is derived using Equation (8):
ViMax′=ViMax−δt×Vcc÷Lp×rip(8)

If this maximum voltage ViMax′ is set as the maximum voltage Vimaxthat is compared with the correction voltage drop Vipin the comparator8, it is possible to turn off the switching device3at the timing that the correction voltage drop Vip′ reaches real maximum voltage Vimax.

Note that, with regard to this error resulting from delay of the circuit elements, it is possible to add the increase portion (Vcc÷Lp×rip×δt) to the correction voltage drop Vip′ and compare the result to the maximum voltage Vimax.

In the switching power supply circuit10shown inFIG. 1, the delay correction circuit17is connected via the resistor21to the high voltage side terminal1aof the direct current power supply1that is the potential of the high voltage side end portion, and the power supply voltage Vcc is input thereto. The delay correction circuit17generates the increase portion (Vcc÷Lp×rip×δt) due to time delay and outputs to the subtracter11, and then adding it to the maximum voltage Vimaxof the first reference power supply12for the above described correction processing is executed.

FIG. 5shows a circuit configuration of a switching power supply circuit50of another embodiment according to the present invention in which an auxiliary winding2cis provided at the primary side of the transformer2; in this case, a flyback voltage V2cof the auxiliary winding2cis monitored, a voltage difference dV obtained, and an output time T2detected.

The switching power supply circuit50, as compared to the switching power supply circuit10shown inFIG. 1, only differs with respect to the fact that the auxiliary winding2cis additionally provided in the transformer2, and the input terminal Vd is connected to a low voltage side portion of the auxiliary winding2cvia the resistor24.

During the oscillation operation of the switching device power supply circuit50, the respective flyback voltages generated in each of the windings of the above described transformer2are proportional to respective numbers of turns thereof. Accordingly, the flyback voltage V2cof the auxiliary winding2cis also proportional to the flyback voltage V2bof the secondary output winding2b; thus, both can be expressed by Equation (19):
V2c=Nt÷Ns×V2b(19)

where, a number of turns of the auxiliary winding2cis Nt, and the number of turns of the secondary output winding2bis Ns.

The number of turns Nt and Ns are constants determined by circuit elements. Accordingly, in the switching power supply circuit50, if the output voltage V2bof Equation (19) is set to an output voltage V2bsetof the auxiliary winding2bthat is subject to the constant voltage control, control is executed at the primary side of the transformer2such that the flyback voltage V2cof the auxiliary winding2cis equal to a set voltage V2csetobtained from Equation (3) below:
V2cset=Nt÷Ns×V2bset(3)

Accordingly, as compared to the switching power supply circuit10, the non-inverting input of the differential amplifier14is connected to one side end portion of the auxiliary winding2cvia the resistor24, and the inverting input is connected to the other side of the auxiliary winding2cthat is the grounded side. The differential amplifier14outputs the flyback voltage V2cgenerated in the auxiliary winding2c.

The flyback voltage V2cis input to one side of the differential amplifier18. At the other side of the differential amplifier18, the set voltage V2csetfrom the variable second reference power supply16is input, and a difference voltage dV of the flyback voltage V2cand the set voltage V2csetis output.

In addition, in the case that the constant voltage control of the output voltage V2oof the rectifying smoothing circuits4and13is executed, taking into account the drop portion of the diode4in the same manner as Equation (2), a set potential V2cset′ obtained from Equation (4):
V2cset′=V2oset×Nt÷Ns÷(1−T2×k÷Ls)  (4)

V2cset′ is set as the set potential V2csetthat is compared with the flyback voltage V2c, and output to the inverting input of the differential amplifier18.

In Equation (4), the output time T2for obtaining the set potential V2cset′ can also be detected from the voltage V2cof the auxiliary winding2cwith respect to the voltage V2bof the secondary output winding2b, which is proportional to the turns ratio. Accordingly, in this case, as with the differential amplifier14, both ends of the auxiliary winding2care respectively connected to the pair of inputs of the comparator27. The output time T2is detected based on the time from turn off of the switching device3until when the polarity of the voltage V2cof the auxiliary winding2creverses.

With regard to the remaining configuration of this embodiment, it is the same as that of the above described first embodiment. Accordingly, an explanation will be omitted here.

FIG. 6shows a circuit configuration of a constant voltage output control device of a switching power supply circuit60according to another embodiment of the present invention.

The switching power supply circuit60shown inFIG. 6pulse width modulates a pulse width of a pulse control signal that directly executes ON control of the switching device3using the difference voltage dV output from the differential amplifier18, and uses the difference voltage dV to adjust the ON time T1. Accordingly, the explanation presented here will focus on the configuration of the switching power supply circuit60that is different to that of the previously described embodiments. Structural members which are the same as those of the previous embodiments are denoted with the same reference numerals, and an explanation is omitted.

InFIG. 6, a PWM circuit63is configured from an integration circuit61that obtains an integrated waveform by integrating the clock pulse of the fixed time cycle Tc output from the oscillator7; a comparator62having a inverting input to which the output of the integration circuit61is input, and a non-inverting input to which the difference voltage dV output from the clamp circuit19is input; the oscillator7, and the AND gate6.

The differential amplifier18outputs the difference voltage dV to the clamp circuit19in the same manner as in the above described embodiments. Accordingly, in the case that the constant voltage control is executed for the output voltage V2bthat is generated in the secondary output winding2b, dV is expressed by Equation (5) below.
dV=V2a−V2aset(5)

In the other case that the constant voltage control is executed for the output voltage V2oof the rectifying smoothing circuits4and13, the voltage difference dV is expressed by Equation (6) below.
dV=V2a−V2aset′  (6)

InFIG. 7(a) shows a clock pulse output from an oscillator7, (b) shows an integrated signal output from an integrated circuit61, (c) showing an input signal of a comparator62, (d) shows an output signal from the comparator62, and (e) shows a pulse control signal output from an and gate6.

The oscillator7, as in the above described embodiments, outputs the clock pulse with the fixed time cycle Tc as shown inFIG. 7(a). This clock pulse is output to one input of the AND gate6and the integration circuit61.

The clock pulse input to the integration circuit61is deformed to the integrated waveform ofFIG. 7(b) and input to the inverting input of the comparator62. The comparator62compare the integrated waveform with the difference voltage dV input to the non-inverting input via the clamp circuit19, as shown inFIG. 7(c).

As a result, the pulse waveform shown inFIG. 7(d) output from the comparator62has a pulse width outputting “H” that varies depending on the value of the difference voltage dV.

The AND gate6outputs, as a pulse control signal shown inFIG. 7(e), the logical AND of the clock pulse of the fixed time cycle TC output from the oscillator7and the pulse waveform output from the comparator62. Due to the pulse waveform that varies depending on the value of the difference voltage dV, the pulse width of the “H” pulse control signal also lengthens or shortens depending on the value of the difference voltage dV.

While the “H” pulse control signal is being output, ON control of the switching device3is executed. Accordingly, the ON time T1is controlled in accordance with the value of the difference voltage dV. In other words, if a positive difference voltage dV is input, the pulse width of the “H” pulse control signal shortens in accordance with the value thereof, and the ON time T1is controlled to be shorter. On the other hand, if a negative difference voltage dV is input, the pulse width of the “H” pulse control signal lengthens in accordance with the value thereof, and the ON time T1is controlled to be longer.

In each of the above described embodiments, in the case that the constant voltage control is executed for the output voltage V2oof the rectifying smoothing circuits4and13, as of the output time T2is not constant from beginning of the oscillation operation until stable operation of the entire switching power supply circuit, the set voltage V2aset′ of the Equation (2) cannot be detected. Accordingly, until stable operation occurs, it is preferable that the ON time T1is fixed, without adding the difference voltage dV to the voltage drop Vip.

In addition, the above described constant voltage control does not necessarily have to be executed for each cycle of the time cycle Tc, and may be executed with a time cycle that is different, for example, longer, than the oscillation time cycle Tc.

Furthermore, according to the present invention, in the case that the output voltage exceeds a set reference voltage value, the voltage is reduced. However, the present invention is not limited to this, and if the output voltages of the secondary output winding2band the rectifying smoothing circuits4and13are set as V2band V2o, respectively, the set output voltage can be obtained. Accordingly, it is possible to apply and utilize the present invention with switching power supply circuits which have an output detection circuit on the secondary side of the transformer2, and which are provided with a feedback circuit that transmits the detection signal thereof to the primary side via an insulated signal transmission element such as a photocoupler, or the like, in a similar manner to the conventional example.

Further, the elements enclosed within the dashed line U shown in the figures for the above described switching power supply circuits10,50and60can be integrated as a single chip circuit component that executes input and output via analogue input terminals Vcc, Vd and Id.