JP3500791B2 - Switching power supply - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a switching power supply device having an output having a constant voltage characteristic and a constant current characteristic.

[0002]

2. Description of the Related Art In recent years, a battery-powered device such as a personal computer or a video camera has become popular because it can be carried to any place due to its cost reduction, size reduction and weight reduction. Along with this, cost reduction, size reduction, and weight reduction are also required for switching power supply devices that have output characteristics for charging batteries, that is, constant current output characteristics and output characteristics for supplying power to devices, that is, constant voltage output characteristics. There is.

An example of a conventional switching power supply device will be described below with reference to the drawings.

FIG. 5 is a circuit diagram of a conventional switching power supply device. In FIG. 5, 1 is a switching element composed of a bipolar transistor or a field effect transistor, 2 is a transformer for power conversion, 2a is a first primary winding, 2b is a second primary winding,
2c is a secondary winding, 3 is a DC power supply obtained by rectifying and smoothing a commercial AC power supply, 4 is an output circuit including a secondary rectifying diode 4a and a secondary smoothing capacitor 4b, and 5 is A load, which is a DC power supplied from the DC power supply 3 through the first primary winding 2a of the transformer 2, is switched by the switching element 1, and the switching output is switched from the first primary winding 2a of the transformer 2 to the secondary winding. Line 2
It is rectified and smoothed by the output circuit 4 connected to the secondary winding 2c, and is supplied to the load 5 as DC power.

Reference numeral 6 denotes an output voltage error detection circuit, which is a load 5
Is divided by resistors 6a and 6b into a first input terminal of an error amplifier 6c, and an error component with a reference voltage source 6d inputted into a second input terminal is amplified to a diode 6e. , To the pulse width control circuit 11 via the photo coupler 8.

Reference numeral 7 denotes an output current error detection circuit, which is a load 5
Is converted into a voltage signal by the resistor 7a and is input to the first input terminal of the error amplifier 7b.
The error component with respect to the reference voltage source 7c input to the input terminal is amplified and output to the pulse width control circuit 11 via the diode 7d and the photocoupler 8.

Reference numeral 9 is an overcurrent detection circuit. The output of the current detection circuit 10 for converting the current flowing through the switching element 1 into a voltage signal by the resistor 10a is input to the first input terminal of the comparator 9a, and the second input The reference voltage source 9b input to the input terminal is compared and the output is output to the pulse width control circuit 11.

The pulse width output circuit 11 outputs the voltage of the second primary winding 2b of the transformer 2 to the diode 12 when the DC power supply 3 or the pulse width control circuit 11 starts its operation.
The output of the rectifying / smoothing circuit 12 composed of a and the capacitor 12b is used as the power supply voltage. (1) When the output current value Io <the output constant current value Ioconst, Io × R (7a) <VREF (7c) 7 does not operate, the on / off time of the switching element 1 is determined by the output signal from the photocoupler 8 of the output voltage error detection circuit 6, and the output switches the switching element 1 via the drive circuit 13. Then, the on / off time of the switching element 1 is controlled so that the output voltage Vo becomes constant, that is, the constant voltage output characteristic is obtained.

However, R (7a) is the resistance value of the resistor 7a for detecting the output current Io in the output current error detection circuit 7, and VREF (7c) is the second input of the error amplifier 7b in the output current error detection circuit 7. It is the voltage value of the reference voltage source 7c connected to the terminal.

FIG. 6 is an output current-output voltage characteristic (hereinafter, IV characteristic) which is an output characteristic of the switching power supply device of FIG.
In this case, the characteristics are indicated by the thick line portion A in the figure.

(2) Next, when the output current value Io = the output constant current value Ioconst, Io × R (7a) = VREF (7c) and the output current error detection circuit 7 starts operating, so that the output voltage Vo decreases. The operation of the output voltage error detection circuit 6 stops, the on / off time of the switching element 1 is determined by the output signal from the photocoupler 8 of the output current error detection circuit 7, and the output is switched via the drive circuit 13. The on / off time of the switching element 1 is controlled so that the element 1 is switched and the output current value Io = the output constant current value Ioconst.

In the IV characteristic of FIG. 6, the characteristic is the thin line portion B. (3) Further, when the switching power supply device is abnormal (for example, the secondary winding 2c of the transformer 2 in FIG. 6 is short-circuited or the output current error detection circuit 7 is faulty), protection of the switching power supply device and overcurrent to the load 5 are caused. For protection, the output V10 of the current detection circuit 10 which converts the current flowing through the switching element 1 connected in series to the switching element 1 into a voltage signal and outputs the voltage signal is V10 = R (10a) · I (1) limit = V9REF Then, the overcurrent detection circuit 9 operates and the overcurrent detection circuit 9
The switching element 1 is turned off by the output of the switching element 1 and the switching element 1 is turned off via the drive circuit 13.

However, R (10a) is the resistance value of the resistor 10 in the current detection circuit 10, I (1) limit is the current value flowing through the switching element 1 when the overcurrent detection circuit 9 operates, and V9REF is the overcurrent. It is the voltage value of the reference voltage source 9b connected to the second input terminal of the comparator 9a of the detection circuit.

However, assuming that the current value flowing through the switching element 1 required at the maximum output power is I (1) max, I (1) 1imit> I (1) max, and therefore the dotted line portion C in FIG. Is the output overcurrent protection current value Iolimit characteristic.

[0015]

In order to have the constant voltage characteristic and the constant current characteristic in the output characteristics of the switching power supply device as described above, (1) the output voltage error detection circuit 6 and the output current error detection circuit 7 are provided. Therefore, many error amplifiers and relatively expensive parts for the reference voltage are required, and accordingly, the number of parts constituting the input means to the error amplifier increases. (2) Since the resistor 7a is required as a means for detecting the current supplied to the load 5 in the output current error detection circuit 7, the loss due to the resistor 7a is large and the loss cannot be reduced. (3) Furthermore, for overcurrent protection of the switching power supply device, the current flowing through the switching element 1 is always I (1) 1imit> I (1) max, which is the characteristic of the dotted line portion C of the IV characteristic of FIG. For,
The switching element 1, the secondary rectifying diode 4a, the transformer 2 and the like must be designed with the output overcurrent protection current value Iolimit, and those having a rating higher than necessary will be used. (4) The above-mentioned output overcurrent protection current value I for thermal design
Since it is necessary to design with olimit, there are problems that it is difficult to reduce the cost and size of the switching power supply device.

The present invention has been made to solve the above-mentioned problems. It eliminates the output current error detection circuit, and the output current value Io <
At the output overcurrent protection current value Iolimit, the output voltage error detection circuit and the pulse width control circuit are used to control the on / off of the switching element so that the constant voltage output characteristic is obtained, and the output current Io = the output overcurrent protection current value Iolimit Then, by using the voltage of the second primary winding of the transformer proportional to the secondary winding of the transformer and the overcurrent detection circuit and the pulse width control circuit, the output current value Io = output constant current value Ioconst = output overcurrent protection. The switching element 1 has a constant current output characteristic of the current value Iolimit.
By controlling the on / off of the power supply, it is possible to reduce the number of parts, reduce loss, optimize the ratings of the parts used, optimize the thermal design, and lower the switching power supply with constant voltage output characteristics and constant current output characteristics. Cost reduction and miniaturization are possible.

[0017]

In order to achieve this object, the present invention provides a switching element and a current which flows through the switching element via a first primary winding of a transformer and converts the current into a voltage signal for output. Connect the detection circuit in series,
The output of the current detection circuit is connected to the first input terminal of the comparator, and the second input terminal of the comparator produces a voltage proportional to the secondary winding voltage of the transformer. The output of the means for converting the voltage obtained by rectifying and smoothing the winding into a current signal is connected , and the
A series connection of a diode and a capacitor between the two primary windings
The first one of the transformers having a configuration in which paths are connected in parallel.
A current proportional to the DC input voltage applied to the next winding
The output of the means for converting into a signal is connected, and the output of the comparator is connected to a pulse width control circuit for controlling ON / OFF of the switching element.

[0018]

Further, in the above structure, the switching element is a MOSFET, and the MOSFET has a first source electrode to which a majority of the unit cells are connected and a second source electrode to which a part of the unit cell is connected. The second source electrode is connected in series to the current detection circuit.

[0020]

With this configuration, the output current error detection circuit is unnecessary, the number of parts can be reduced, and the output current detection resistor for detecting the current supplied to the load is not required, so that the loss can be reduced and the output characteristics can be reduced. In the above, the constant voltage characteristic is the same as the conventional one, and with the constant current characteristic, the output current Io = the output constant current value Ioconst = the output overcurrent protection current value Iolimit can be set, and the parts used and the thermal design can be optimized.

Even if the output current error detection circuit is omitted, the output current Io = output constant current value Ioconst = output overcurrent in the output constant current characteristic regardless of the DC input voltage applied to the first primary winding of the transformer. The current protection current value Iolimit can be used.

Furthermore, loss of resistance in the current detection circuit for detecting the current flowing through the switching element can be reduced, and many circuits can be integrated on the same semiconductor substrate as the switching element. Therefore, the cost and size of the switching power supply device can be reduced.

[0023]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIGS.

FIG. 1 is a circuit configuration diagram of the switching power supply device of the present invention. 1 that are the same as those in FIG. 5 are denoted by the same reference numerals. In FIG. 1, 1 is a switching element composed of a bipolar transistor or a field effect transistor, 2 is a transformer for power conversion, 2a is a first primary winding, 2b is a second primary winding, 2c. Is a secondary winding, 3 is a DC power source obtained by rectifying and smoothing a commercial AC power source, 4 is an output circuit including a secondary rectifying diode 4a and a rectifying and smoothing capacitor 4b, and 5 is a load. Yes, the switching device 1 switches the DC power supplied from the DC power supply 3 through the first primary winding 2a of the transformer 2 and outputs the switching output from the first primary winding 2a to the secondary winding 2c of the transformer 2. Is rectified and smoothed by the output circuit 4 connected to the secondary winding 2c and supplied to the load 5 as DC power.

Reference numeral 6 denotes an output voltage error detection circuit, which is used for the load 5
The output voltage Vo supplied to the input terminal is divided by the resistors 6a and 6b and is input to the first input terminal of the error amplifier 6c, and is input to the second input terminal of the error amplifier 6c.
The error component between and is amplified and output to the pulse width control circuit 11 via the photocoupler 8.

Reference numeral 9 is an overcurrent detection circuit. The output of the current detection circuit 10 for converting the current flowing through the switching element 1 into a voltage signal by the resistor 10a is input to the first input terminal of the comparator 9a, and the comparator 9a. The second input terminal of the transformer 12 has a reference voltage source 9b consisting of a constant current source 9ba and a resistor 9bb, and a voltage of the second primary winding 2b of the transformer 2 proportional to the secondary winding 2c of the transformer 2 , A resistor 14a and a resistor 14 which are rectified and smoothed by a rectifying and smoothing circuit 12 including a capacitor 12b and connected in parallel to the rectifying and smoothing circuit 12.
The output of the output voltage monitor circuit 14 for converting the voltage of the rectifying / smoothing circuit 12 composed of b into a current signal is input, and the comparator 9a compares the signals input to the first and second input terminals and outputs the output. Pulse width control circuit 1 as output signal V14
Output to 1.

The pulse width output circuit 11 outputs the voltage of the second primary winding 2b of the transformer 2 which is proportional to the secondary winding 2c of the transformer 2 when the DC power supply 3 or the pulse width control circuit 11 starts its operation. Diode 12a, capacitor 1
The output of the rectifying / smoothing circuit 12 composed of 2b is the power supply voltage, and (1) the output current value Io <output constant current value Ioconst = output overcurrent protection current value Iolimit, the output through the photocoupler 8 by the voltage error detection circuit 6 The input signal V11 of the error amplifier 11a is changed by the error amplifier 11a by the reference voltage 11b and the input signal V11.
Are compared and amplified to output an output signal V12 to the PWM comparator 11c, and the PWM comparator 11c outputs the error amplifier 1
The output signal V12 of 1a and the triangular wave 11d are compared, and the output signal V
13 is output to the NOR circuit 11e, and the NOR circuit 11e outputs the output signal V14 of the overcurrent detection circuit 9 and the PWM comparator 1
The output signal V13 of 1c is logically operated and the output signal V15 is R-
The RS flip-flop 11f outputs the output signal V16 determined by the clock pulse 11g connected to the S terminal and the output signal V15 of the NOR circuit 11e to the R terminal of the S flip-flop 11f via the drive circuit 13 and the switching element. By outputting the signal to 1, the on / off time of the switching element is controlled so that the output voltage Vo becomes constant.

FIG. 2 shows the IV characteristic of the output of the switching power supply device of FIG. 1, and the IV characteristic at this time becomes the characteristic of the thick line portion A in FIG. 2 and becomes the constant voltage characteristic. (2) Next, when the impedance of the load 5 decreases and the output current value Io increases, and the output current value Io = output constant current value Ioconst = output overcurrent protection current value Iolimit, the current I (1 ) Becomes larger, V10 = I (1) ・ R (10a) = {R (14a) ・ (1 / R (9bb) + 1 / R (14a) + 1 / R (14b))} ・ {NB / When NP ・ Vo + Icc ・ R (14a)} = V9REF (1), the overcurrent detection circuit 9 operates and the output signal V14 is changed to N
The NOR circuit 11e outputs the OR signal to the OR circuit 11e and outputs the output signal V15 to the R terminal of the RS flip-flop 11f. The RS flip-flop 11f outputs the clock pulse 11g connected to the S terminal and the NOR circuit 11e. Since the output signal V16 determined by the output signal V15 is output to the switching element 1 via the drive circuit 13, the output voltage Vo decreases, and the output error detection circuit 6 eliminates the signal via the photocoupler 8 and the photo signal disappears. Since the voltage V11 of the coupler 8 becomes higher than the reference voltage 11i, the comparator 11 turns off the switch 11j and the output V13 of the PWM comparator becomes low level. Therefore, the output V15 of the NOR circuit 11e is determined by the output signal V14 of the overcurrent detection circuit 9. Will be.

However, V9REF is a voltage value input to the second input terminal of the comparator 9a in the overcurrent detection circuit 9, and in the above equation (1), R (10a) is in the current detection circuit 10. Is the resistance value of the resistor 10a, R (14a) and R (14b) are the resistance values of the resistors 14a and 14b in the output voltage monitor circuit 14, and R (9bb) is the overcurrent detection circuit 9. Resistance 9
is the resistance value of bb, Icc is the constant current value of the constant current source 9ba in the overcurrent detection circuit 9, NP is the number of windings of the first primary winding 2a of the transformer 2, and NB is , The number of windings of the second primary winding 2b of the transformer 2.

The output current Io at this time is Io = k · Lp · I (1) ² / Vo · f = k · Lp · {V9REF / R (10a)} ² / Vo · f (2) = Ioconst Becomes

However, k is a proportional constant, Lp is the inductance value of the first primary winding of the transformer 2, and f is the oscillation frequency of the switching element 1.

It is point B in the IV characteristic of FIG.
Further, even if the impedance of the load 5 is reduced and the output voltage Vo is reduced, the output current value Io = the output constant current value Ioconst = the output overcurrent protection current value Iolimit. Circuit 9
Voltage value 9R input to the second input terminal of the internal comparator 9a
The output current value Io = the output constant current value Ioconst = the output overcurrent protection current value Iolimit due to the EF being lowered by the output voltage Vo.

Further, the current I (1) flowing through the switching element 1 is also reduced by the above equation (1). Output current Io
Becomes the above-mentioned formula (2).

The output I- of the switching power supply of FIG.
In the V characteristic, it is from point B to point C.

When the impedance of the load 5 further decreases and the output voltage Vo decreases, the output current value Io = the output constant current value Ioconst = the output overcurrent protection current value Iolimit. Circuit 9
Voltage value 9R input to the second input terminal of the internal comparator 9a
As the EF decreases, the output current value Io = the output constant current value Ioconst = the output overcurrent protection current value Iolimit.

The current I flowing through the switching element 1
(1) becomes even smaller. The output current Io is given by the above equation (2).

The output I- of the switching power supply of FIG.
The V characteristic changes from the C point to the D point, and when the overcurrent detection circuit 9 starts to operate, in the IV characteristic of FIG. 2, the characteristic of the dotted line portion, that is, the output constant current characteristic can be obtained. Figure 2
In the output constant current characteristic portion of the dotted line portion, since the on / off of the switching element 1 is controlled by the output of the overcurrent detection circuit 9 input to the pulse width control circuit 11, the output current value Io = the output constant current value Ioconst = output overcurrent protection current value Iolimit.

FIG. 3 shows another embodiment of the present invention. FIG. 3 is a circuit configuration diagram of another switching power supply device of the present invention. In FIG. 3, the same parts as those in the first part are designated by the same reference numerals. Reference numeral 15 denotes an input voltage monitor circuit for detecting the voltage value E of the DC power supply 3 which is composed of a series circuit of a diode 15a and a capacitor 15b, which is connected in parallel to the second primary winding 2b of the transformer 2 and the output of which is a resistor 16 It is connected to the second input terminal of the comparator 9a in the overcurrent detection circuit 9 via. The operation of the switching power supply device shown in FIG. 3 is the same as the operation of the switching power supply device shown in FIG.

However, when the overcurrent detection circuit 9 operates and outputs the output signal V14 to the NOR circuit 11e, the output V10 of the current detection circuit and the voltage of the second input terminal of the comparator 9a in the overcurrent detection circuit 9 V9REF is V10 = I (1) ・ R (10a) = (R (14a) ・ (1 / R (9bb) + 1 / R (14a) + 1 / R (14b) + I / R (16)) } ・
{NB / NP ・ Vo + R (14a) / R (16) ・ VBB + Icc ・ R (14a)} = V9REF …… (3).

However, VBB is the input voltage monitor circuit 15
Output voltage V, VBB = -NB / NP · E, NB is the number of windings of the second primary winding 2b of the transformer 2, and NP is of the first primary winding 2a of the transformer 2. The number of windings, E is the voltage value of the DC power supply 3, and R (16) is the resistance 16
Is the resistance value of.

The output current Io and the output characteristic are represented by the equation (2) and FIG. 2, respectively, similarly to the embodiment of FIG. 1, and the output current value Io = the output constant current value Ioconst = the output overcurrent protection current value Iolimit can be set. .

FIG. 4 shows still another embodiment of the present invention.
FIG. 4 is a circuit configuration diagram of another switching power supply device of the present invention. In FIG. 4, the same components as those in FIG. 1 are designated by the same reference numerals. FIG. 4 shows a case where the power MOSFET 17 is used as the switching element 1 of FIG.
The SFET 17 has a first source electrode S1 composed of a majority of unit cells and a second source electrode S1 composed of a small number of unit cells.
1 has the same configuration as that of FIG. 1 except that the second source electrode S2 is connected to the current detection circuit 10.
Since the operation is the same as that of the switching power supply device shown in FIG.

However, when the overcurrent detection circuit 9 operates and outputs the output signal V14 to the NOR circuit 11e, the output V10 of the current detection circuit and the voltage of the second input terminal of the comparator 9a in the overcurrent detection circuit 9 are detected. V9REF is V10 = I (S2) ・ R (10a) = n2 / n1 ・ I (S1) ・ R (10a) = n2 / n1 ・ I (1) ・ R (10a) = (R (14a) ・ ( 1 / R (9bb) + 1 / R (14a) + 1 / R (14b))} ・ {NB / NP ・ Vo + Icc ・ R (14a} = V9REF ... (4)

However, n1 is a power MOSFET connected to the first source electrode S1 of the power MOSFET 17.
N2 is the number of unit cells of the power MOSFET 17
Power MOSFET connected to the second source electrode S2 of
Is the number of unit cells of T, and I (S1) is the power MOSFET
17 is a current value flowing through the first source electrode S1 of
(S2) is the second source electrode S of the power MOSFET 17.
It is the current value flowing through 2, and I (S2) = n2 / n1 · I (S1).

Since n2 / n1 is usually set to about 1 to 0.1%, the current I (1) flowing through the first switching element 1 and the current I (S1) flowing through the first source electrode S1 of the power MOSFET 17 are set. Are almost equal.

The output current Io and the output characteristic are represented by the equation (2) and FIG. 2, respectively, similarly to the embodiment of FIG. 1, and the output current value Io = the output constant current value Ioconst = the output overcurrent protection current value Iolimit can be set. .

Further, as is clear from the equation (4), R (10
a) is multiplied by n1 / n2 or resistors R (14a), R (9b), and R (14b) are n1 / n2.
If set to double, it becomes the same as the formula (1) of the embodiment of FIG. 1, but compared with the embodiment of FIG. 1. With a very small current n2 / n1 times the current I (1) flowing through the switching element 1. Since the overcurrent detection circuit 9 can be operated, the loss of the resistor 10 in the current detection circuit 10 can be reduced and the switching power supply device can be downsized.

The power MOSFET 17, the current detection circuit 10, the overcurrent detection circuit 9, the pulse width control circuit 11, and the output voltage monitor circuit 14 can be integrated on the same semiconductor substrate. From the above points, the size and cost can be further reduced as compared with the embodiment of FIG.

Also in the embodiment of FIG. 3, the switching element 1 has the first source electrode S1 composed of the majority unit cells of the power MOSFET and the second element composed of a small number of unit cells as in the embodiment of FIG. The power MOSFET having the source electrode S2 also operates, the output current Io and the output characteristic are the same, and the input voltage monitor circuit 15 and the resistor 16 can be integrated on the same semiconductor substrate, and the size is further reduced as compared with the embodiment of FIG. The cost can be reduced.

[0050]

As described above , according to the present invention, (1) the output current error detection circuit 7 becomes unnecessary and the number of parts is reduced.
Output that can be reduced and detects the current supplied to the load 5.
Since the current detection resistor 7a is unnecessary, loss can be reduced.
At the same time, the output characteristics have the same constant voltage characteristics as before.
With the constant current characteristic, the output current value I _o = the output constant current value I
_Oconst = used can be the output overcurrent protection value I _Olimit
Parts and thermal design can be optimized.

[0051]

[0052]

(2) Even if the output current error detection circuit 7 is eliminated, the output current Io = output constant current value Ioconst = in the output constant current characteristic regardless of the DC input voltage applied to the first primary winding of the transformer. The output overcurrent protection current value Iolimit can be set.

(3) Further, the loss of the resistor 10a in the current detection circuit 10 for detecting the current flowing through the switching element 1 can be reduced, and many circuits can be integrated on the same semiconductor substrate as the switching element 1 for switching. It is possible to reduce the number of primary side parts of the power supply device. Therefore, it is possible to provide a switching power supply device having a constant voltage output characteristic and a constant current output characteristic, which can achieve cost reduction and size reduction.

[Brief description of drawings]

FIG. 1 is a circuit configuration diagram of a switching power supply device showing an embodiment of the present invention.

FIG. 2 is an output characteristic diagram of a switching power supply device according to an embodiment of the present invention.

FIG. 3 is a circuit configuration diagram of a switching power supply device showing another embodiment of the present invention.

FIG. 4 is a circuit configuration diagram of a switching power supply device showing another embodiment of the present invention.

FIG. 5 is a circuit configuration diagram of a conventional switching power supply device.

FIG. 6 is an output characteristic diagram of a conventional switching power supply device.

[Explanation of symbols]

1 Switching Element Composed of Bipolar Transistor or Field Effect Transistor 2 Transformer 2a First Primary Winding 2b of Transformer 2 Second Primary Winding 2c of Transformer 2 Secondary Winding 3 of Transformer 2 DC Power Supply 4 Output Circuit 4a Secondary rectifier diode 4b Secondary smoothing capacitor 5 Load 6 Output voltage error detection circuits 6a, 6b, 7a, 9bb, 10a, 14a, 14b,
16 resistors 6c, 7b, 11a error amplifiers 6d, 7c, 9b, 11b, 11i reference voltage sources 6e, 7d, 12a, 15a diode 7 output current error detection circuit 8 photocoupler 9 overcurrent detection circuits 9a, 11h comparator 9ba constant Current source 10 Current detection circuit 11 Pulse width control circuit 11c PWM comparator 11d Triangle wave 11e NOR circuit 11f RS flip-flop 11g Clock pulse 11j Switch 12 Rectification smoothing circuit 12b, 15b Capacitor 13 Drive circuit 14 Output voltage monitor circuit 15 Input voltage monitor Circuit 17 Power MOSFET

Continuation of front page (58) Fields surveyed (Int.Cl. ⁷ , DB name) H02M 3/28

Claims (2)

(57) [Claims] 1. A switching element and a current detection circuit for converting a current flowing through the switching element into a voltage signal and outputting the voltage signal via a first primary winding of a transformer are connected in series, and an output of the current detection circuit is connected. Rectifying and smoothing the second primary winding voltage of the transformer, which is connected to the first input terminal of the comparator and generates a voltage proportional to the secondary winding voltage of the transformer at the second input terminal of the comparator. thereby connecting the output of means for converting the voltage obtained by the current signal, the
A diode and a capacitor between the second primary winding of the transformer
The above-mentioned transformer having a configuration in which the series connection circuits of
Proportional to the DC input voltage applied to the first primary winding of the
A switching power supply device connected to the output of the means for converting the voltage into a current signal, and the output of the comparator is connected to a pulse width control circuit for controlling the on / off of the switching element. 2. The switching power supply device according to claim 1, wherein the switching element is a MOSFET, and a part of a unit cell of the MOSFET is a second source electrode, and the second source electrode and a current detection circuit are connected in series.