JPH07222440A - Switching power supply - Google Patents

Abstract

PURPOSE:To obtain a switching power supply provided with a three-terminal- type switching device for producing stepwise variable voltage by a method wherein a few components are connected to an external terminal for the switching device is which a voltage at the external terminal is controlled to a definite voltage by a control circuit. CONSTITUTION:In a package 10 of a switching device, a MOSFET 20 and a control circuit 30 are incorporated in the same chip, and three terminals, i.e., a power-supply input terminal 40, a drain terminal D and a source terminal S for the FET 20, are provided as external terminals. The anode of a Zener diode 70 is connected to the output terminal 40, a voltage which has rectified and smoothed a commercial power supply 50 via a starting resistance 53 is applied to the cathode, and a voltage which has rectified and smoothed a voltage obtained from a bias winding 63 for a transformer 60 is applied. Thereby, a voltage variable with a Zener voltage can be obtained as an output voltage OUT.

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 using a switching device having a MOSFET as a switching element.

[0002]

2. Description of the Related Art In recent years, a MOSF used as a switch element for reducing the number of parts of a switching power supply, improving a space factor, downsizing, cost reduction and the like.
A switching device in which ET and its control circuit are incorporated in a semiconductor integrated circuit of the same chip has been put into practical use.

By the way, in such a conventional switching device, since, for example, 20 or more pins are provided as external terminals, it is difficult to mount the device on a printed board, and the external device is difficult to mount. Since external parts corresponding to the number of terminals are required, there are restrictions in miniaturization. In addition, the cost also increases corresponding to the number of external parts.

In order to solve such a problem, the applicant of the present invention has filed a patent application for the title of the invention, "switching device", on the same date as the present application. According to this application, the external terminal is composed of three terminals, the drain terminal and the source terminal of the MOSFET, and the power input terminal of the control circuit. Therefore, the external terminal can be easily mounted on the printed board and the number of external parts can be reduced. There are features. However, the voltage of the power supply input terminal of the control circuit in the device of the same application is fixed,
The switching power supply configured using this device has a problem that only a constant output voltage can be obtained.

Before explaining the present invention, a circuit configuration diagram of the above-mentioned patent application "switching device" is shown in FIG. 1 as an example of the switching device used in the switching power supply of the present invention, and description will be given with reference to this figure. To do. In FIG. 1, reference numeral 10 denotes a package, which is a multi-source MO used as a switching element in the package.
SFET (hereinafter simply referred to as FET) 20 and this FE
A current control type control circuit 30 for controlling T is incorporated as a semiconductor integrated circuit of the same chip. Reference numeral 40 denotes a terminal to which a voltage Vin from the outside is input as a power supply voltage. In the package 10, the drain terminal 2 of the FET 20
There are three terminals, a source terminal 22, a power source terminal 22, and a power input terminal 40. That is, it is a three-terminal type package.

In the control circuit 30, 31 is an oscillation circuit,
32 is an OR gate, 32a is an inverter, and 33 is a latch circuit composed of an RS flip-flop. The oscillating circuit 31 generates a pulse (blanking pulse) having a constant cycle and outputs a sawtooth wave (or triangular wave) for generating the pulse. The blanking pulse is applied to the OR gate 32 and also input to the set terminal S of the latch circuit 33, and the Q-bar output of the latch circuit 33 is applied to the OR gate 32. 34 is a power input terminal 4
A reference voltage generation circuit that receives a voltage Vin applied from 0 to generate a reference voltage Vr, 35 is a low voltage detection circuit that detects a case where the voltage Vin is lower than a reference value, and 36 is conversely Vi.
A high voltage detection circuit that detects when n is higher than a predetermined value,
Both reference voltages of both detection circuits are obtained from the reference voltage generation circuit 34. 36a is a switch and 36b is a voltage dividing resistor. The power supply voltage Vin obtained via the switch 36a is applied to the high voltage detection circuit 36 after being divided by the voltage dividing resistor 36b.

37 is an abnormal heating detection circuit for detecting an abnormal temperature rise for chip protection, the output of which is a low voltage,
And the outputs of the high voltage detection circuits 35 and 36 are added to the OR gate 32. The output of the OR gate 32 is applied to the gate electrode of the FET 20 via the inverter 32a.

38 is an error amplifier, 38a is a switch, 3
8b is a voltage dividing resistor. The error amplifier 38 has a switch 3
The voltage Vin applied from the power input terminal 40 obtained via 8a is divided by the voltage dividing resistor 38b and added.
The error amplifier 38 compares this divided voltage with the reference value Vr,
The difference is amplified and output. This reference voltage Vr is also obtained from the reference voltage generating circuit 34 described above. The switch 38a and the switch 36a are both driven by the output of the low voltage detection circuit 35.

Reference numeral 39 is a current detection comparator for comparing the current proportional to the source current detected from the multi-source of the FET 20 with the output of the error amplifier 38. The output of the error amplifier 38 is added to its inverting input terminal. There is. FET2 detected by the resistor 23 at the non-inverting input terminal
A voltage proportional to the source current of 0 is converted into a current and added, and the sawtooth wave output from the oscillation circuit 31 is added as a bias. The output of this comparator is connected to the reset terminal R of the latch circuit 33. The sawtooth wave obtained from the oscillation circuit 31 is superposed in order to correct the system so as not to become unstable when the duty becomes 50% or more. 39a is a constant voltage element for limiting the maximum value of the voltage applied to the inverting input terminal of the current detection comparator 39.
Is designed to limit the maximum drain current.

The operation of the switching device shown in FIG. 1 will be described below with reference to the waveform chart of FIG. 3 is a truth table of the latch circuit 33 used in FIG.

The voltage Vi input from the power input terminal 40
As described above, n is applied as a power supply voltage to each circuit constituting the control circuit 30, and as a result, the reference voltage generation circuit 34 generates the reference voltage Vr. In this case, the input voltage Vin gradually increases after the power is turned on, but the switches 36a and 38a are off while the value is lower than the starting voltage determined by the reference value Vr of the low voltage detection circuit 35, but the starting voltage is Then, both switches are turned on by the output of the low voltage detection circuit 35, whereby the entire control circuit is put into operation.

When the control circuit 30 is activated, the oscillation circuit 31
Outputs the sawtooth wave shown in FIG. 2 (a) and generates a blanking pulse having a constant period shown in FIG. 2 (b) obtained by the sawtooth wave. The sawtooth signal is applied to the non-inverting input terminal of the current detection comparator 39, and the blanking pulse is applied to the input terminal of the OR gate 32 and the set terminal S of the latch circuit 32. On the other hand, when the switch 38a is turned on, the input voltage Vin is applied to the voltage dividing circuit 38a via this switch to be divided. This divided voltage is compared with the reference value Vr by the error amplifier 38,
The difference is amplified and applied to the current detection comparator 39 as the threshold current Ith at the inverting input terminal of this comparator. The level of this threshold current is Ith in FIG.
Shown as 1 to Ith3. Ith1 indicates a case where the difference between the divided voltage of the voltage dividing circuit 38a and the reference value Vr is small, and Ith3 indicates a case where the difference is large.

Here, FET2 detected by the resistor 21
The waveform of the source current of 0 is shown in FIG. This detection current rises at the fall of the blanking pulse shown in FIG. For example, when the blanking pulse falls at time t1, the detection current (source current) rises, the current gradually increases, and when the value reaches the threshold current Ith1 at time t2, the current detection comparator 39 causes the current detection comparator 39 shown in FIG. This is detected as shown in (c). This detection output is applied to the reset terminal R of the latch circuit 33. The detected current is the threshold current It
The output Q-bar of the latch circuit 33 is at the low level as shown in FIG. 2D during the period from t1 to t2 where it does not reach h1. Become. This high level output becomes low at the next rising edge of the blanking pulse.

When the input levels applied to the OR gate 32 are all low, the output of the OR gate 32 becomes low level as shown in FIG.
2a, the signal is inverted as shown in FIG. 2 (), and the high level signal is applied to the gate of the FET 20. F
The ET 20 is turned on while the output of the inverter 32a applied to this gate is at the high level. That is, in FIG. 2, the FET 20 is turned on during the period from t1 to t2,
The signal level applied to the gate of the FET 20 becomes low during the period from t2 to t3, and as a result, the FET 20 is off.

When the next blanking pulse falls at time t3, the detection current shown in FIG. 2 (g) rises. In this case, the value of the power supply voltage Vin is from time t1 to t3.
If the threshold current of the current detection comparator 39 is Ith2, the detected current is Ith2.
Up to time t4, the Q-bar output of the latch circuit 33 does not become high level. That is, the period from t3 to t4 in which the Q bar output of the latch circuit 33 is low is longer than the period from t1 to t2, and the FET 20 is on during the period from t3 to t4. Further, from the time t3 to t5, the power supply voltage V
When the value of in is small and the threshold current of the current detection comparator 39 is Ith3, t3 to t6 as shown in periods t5 to t6.
The FET 20 is on for a period longer than the period of t4. Thus, the voltage V applied to the power input terminal 40
If there is a difference between in and the reference value Vr, FE is changed according to the difference.
The value of the input voltage Vin is controlled so that the detection current has a constant value by controlling the on-time of T20.

In addition to the blanking pulse, a low voltage detection circuit 35 for detecting the case where the voltage Vin is lower than the reference value Vr, a high voltage detection circuit 36 for detecting this when the voltage Vin is higher than the reference value, and a circuit. The outputs of the abnormal heating detection circuit 37 for detecting abnormal heating are applied to the OR gate 32, respectively. When even one of the outputs of these circuits becomes high level, the output of the OR gate 32 becomes high level, which is inverted by the inverter 32a and added to the gate of the FET 20. As a result, the FET 20 is turned off.

As described above, the switching device shown in FIG. 1 has three external terminals, and can be made into a small and inexpensive switching device.

FIG. 4 is a circuit diagram of an example of a switching power supply configured by using the device of FIG. In FIG.
Reference numeral 10 denotes the package described in FIG. 1, in which a multi-source MOSFET 20 and a control circuit 30 for controlling the FET are incorporated as a semiconductor integrated circuit on the same chip. 40 is a power input terminal. Figure 1
As described above, the power input terminal 40 is used as the external terminal.
Also, there are three drain terminals D and source terminals S of the FET 20.

Reference numeral 50 is a commercial power supply input terminal, 60 is a transformer, and a primary winding 61, a secondary winding 62, and a bias winding 63.
Has become Reference numeral 51 is a full-wave rectifying circuit for full-wave rectifying the commercial power supply voltage, and 52 is a smoothing circuit. The output end of the rectified and smoothed commercial power supply voltage is connected to the power supply input terminal 40 of the package 10 via the starting resistor 53, and is also applied to the drain terminal D of the FET 20 via the primary winding 61 of the transformer 60. There is. 62a and 63a are diodes, 62b and 63b are capacitors, and the diode 6
2a and the capacitor 62b are the secondary winding 62 of the transformer 60.
The voltage induced in the bias winding 62 is rectified and smoothed by the diode 63a and the capacitor 63b. The DC voltage obtained from the bias winding 62 is applied to the power input terminal 40.

The AC voltage applied to the commercial power input terminal 50 is converted into a DC voltage by the full-wave rectifying circuit 51 and the smoothing circuit 52, and the DC voltage is applied to the primary winding 61 of the transformer 60 and used for starting. It is applied to the control circuit 30 from the power supply input terminal 40 as the power supply voltage Vin via the resistor 53, whereby the control circuit 30 as described in FIG.
Will start. FET2 is provided on the primary winding 61 of the transformer 60.
The drain / source terminals D and S of 0 are connected in series, and the current flowing through the source terminal S is detected by the resistor 21. As described with reference to FIG. 1, the ON time of the FET 20 is controlled so that the value of the detected current becomes constant. FE
When T20 is turned on / off, the current flowing through the primary winding 61 of the transformer 60 is turned on / off. As a result, the secondary winding 62 and the bias winding 63 of the transformer 60 are
A voltage is induced in the rectified and smoothed by the diodes 62a and 63a and the capacitors 62b and 63b, respectively. The DC voltage obtained from the secondary winding 62 is taken out as the output voltage OUT, and the DC voltage obtained from the bias winding 63 is applied to the power supply input terminal 40 as the power supply voltage Vin.

The power supply voltage Vin is divided by the voltage dividing resistor 38b as described with reference to FIG. The divided voltage is compared with the reference value Vr in the error amplifier 38, and the power supply voltage applied to the external terminal 40 so that the difference becomes small, that is, the source current of the FET 20 flowing through the detection resistor 21 becomes a constant value. The value of Vin is controlled to a constant value. In this case, the value of the voltage Vin when controlled to a constant value and the voltage generated in the bias winding 63 of the transformer 60 are almost equal, so that the turn ratio between the bias winding 63 and the secondary winding 62 should be made appropriate. As a result, the desired DC voltage OUT can be obtained from the rectifying / smoothing circuit of the secondary winding 62. in this way,
In the power supply of FIG. 4 using FIG. 1, a DC voltage isolated from the commercial power supply can be obtained. However, in the power supply of FIG. 4, the voltage across the capacitor 63b connected to the bias winding 63 is always controlled to the magnitude of the power supply voltage Vin applied to the terminal 40. There is a problem that only a constant DC output voltage can be obtained from the secondary winding 62.

SUMMARY OF THE INVENTION The present invention has been made in order to solve such a problem, and its purpose is to connect a small number of parts to an external terminal so that the output voltage can be stepped. It is to obtain a switching power supply equipped with a three-terminal switching device that can be varied in a variable manner.

[0022]

The present invention provides a MOSFE
T and a control circuit for controlling the on / off of the MOSFET are configured as a semiconductor integrated circuit of the same chip, and the M
A switching device in which the drain terminal and the source terminal of the OSFET and the power input terminal of the control circuit are used as external terminals, and the voltage of the external terminal is controlled to a constant voltage by the control circuit, and the commercial power supply is rectified and smoothed, and its smoothed output Is connected to the power source input terminal of the control circuit via a starting resistor, the drain terminal of the MOSFET is connected in series, and a voltage obtained by rectifying and smoothing the commercial power source is applied to the primary winding and the output voltage is extracted. A switching power supply comprising a secondary winding and a bias winding, rectifying and smoothing the voltage generated in the bias winding and applying the smoothed output to the power input terminal It is characterized in that a variable means for varying the value of the output voltage to be taken out is provided.

[0023]

In the present invention as described above, the external terminals are three terminals and the output voltage is variable.

[0024]

FIG. 5 is a circuit diagram of an embodiment of the switching power supply according to the present invention. 5 that are the same as those in FIG. 4 are assigned the same reference numerals as in FIG. 4 and their re-explanation is omitted. In FIG. 5, 70 is a Zener diode, and the anode pole of this Zener diode element is the external terminal 40.
And a voltage obtained by rectifying and smoothing the commercial power supply 50 is applied to the cathode electrode via the starting resistor 53, and a voltage obtained by the bias winding 63 of the transformer 60 is also rectified and smoothed. . As described with reference to FIG. 4, in this switch power supply, the value of the power supply voltage Vin applied to the external terminal 40 is controlled to a constant value, and the DC output voltage OUT corresponding to the value of Vin can be obtained. .

In the switching power supply shown in FIG. 5,
The voltage across the capacitor 63b of the bias winding 63 is always the voltage of the external terminal 40 plus the voltage of the Zener diode 70. That is, by selecting the diode 70 having an appropriate Zener voltage, the output voltage O
As the UT, it is possible to obtain a voltage having a magnitude that is stepwise changed according to the Zener voltage.

FIG. 6 is a circuit diagram of another embodiment of the switching power supply according to the present invention. In the switching power supply of FIG. 5, the Zener diode 70 is used in order to obtain output voltages of different magnitudes, but in FIG. 6, the voltage which is offset to the terminal 40 is changed by using the shunt regulator 80 and the resistors R1 and R2 instead of the Zener diode. I made it. The voltage between the anode and the cathode of the shunt regulator 80 = (R1 + R2) / Vref.
The shunt regulator 80 has less variation in Zener voltage than the Zener diode 70 and has good temperature characteristics. As a result, the switching power supply of FIG. 6 can obtain a switching power supply with higher accuracy than the power supply of FIG.

FIG. 7 is a circuit configuration diagram of still another embodiment of the present invention. In FIG. 7, 80 is a shunt regulator, 81 to 86 are resistance elements, 87 is a capacitor, and 91.
Is a phototransistor forming a photocoupler, and 92 is a light emitting diode. One end of the resistor 81 is the device 10
Is connected to the external terminal 40 provided at the other end, and the other end is connected to the point A (the connection point between the starting resistor 53 and the diode 63a). A phototransistor 91 is connected in parallel to the resistor 81.

The resistance elements 84 to 86 form a voltage dividing circuit for dividing the output voltage Vout taken out from the secondary winding 62 of the transformer 60, and the shunt regulator 80 is connected to this voltage dividing circuit. The light emitting diode 92 forming the photo coupler is connected to the secondary winding 6 via the resistor 82.
2 and the cathode electrode of the shunt regulator 80.

In the switching power supply of FIG. 7 having such a configuration, the external terminal 40 of the switching device 10 is controlled so as to have a constant voltage Vin as described above. Therefore, the voltage at the point A in the figure is a voltage obtained by adding the voltage across the resistor 81 to Vin, and the voltage obtained by multiplying the voltage at the point A by the turn ratio of the bias winding 63 and the secondary winding 62 is the output voltage Vout. Taken out. Here, the voltage across the resistor 81 is controlled by comparing the output voltage Vout divided by the voltage dividing circuit including the resistance elements 84 to 86 by the shunt regulator 80 with the reference voltage Vref. ing. That is, if the output voltage Vout rises and the comparison voltage becomes higher than the reference voltage Vref, the current flowing into the cathode of the shunt regulator 80 increases, and the forward current of the light emitting diode 92 also increases, so that the collector current of the phototransistor 91 increases. Increase, and as a result, the voltage across resistor 81 decreases,
The output voltage Vout will decrease.

As described above, in the circuit of FIG. 7, the output voltage Vout can be changed by changing the voltage dividing ratio of the voltage dividing circuit, and the output on the secondary side can be detected.
Since the voltage at the point is controlled,
There is an advantage that the output voltage Vout with higher accuracy can be obtained than the circuit of FIG.

[0031]

According to the present invention, it is possible to obtain an inexpensive switching power supply which has three external terminals and uses a switching device which can be easily mounted on a printed board and which can change the output voltage stepwise. You can

[Brief description of drawings]

FIG. 1 is a configuration diagram of an example of a switching device for explaining the present invention.

FIG. 2 is a waveform diagram for explaining the operation of FIG.

FIG. 3 is a truth table of the latch circuit used in FIG.

4 is a configuration diagram of an example of a switching power supply configured by using the device of FIG.

5 is a circuit configuration diagram of an embodiment of a switching power supply according to the present invention configured by using the device of FIG.

FIG. 6 is a circuit configuration diagram of another embodiment of the switching power supply according to the present invention configured by using the device of FIG.

7 is a circuit configuration diagram of still another embodiment of the switching power supply according to the present invention configured by using the device of FIG.

[Explanation of symbols]

 10 package 20 MOSFET 30 current control circuit 31 oscillation circuit 32 OR gate 33 latch circuit 36a switch 38 error amplifier 38a switch 39 current detection comparator 40 power input terminal 50 commercial voltage input terminal 60 transformer 70 Zener diode 80 shunt regulator 91 phototransistor 92 light emitting diode

Claims (4)

[Claims] 1. A MOSFET and the on-state of this MOSFET.
A control circuit for controlling off is configured as a semiconductor integrated circuit of the same chip, and the drain terminal and the source terminal of the MOSFET and the power input terminal of the control circuit are external terminals, and the voltage of the external terminal is controlled to a constant voltage by the control circuit. The switching device, the means for rectifying and smoothing the commercial power source, and applying the smoothed output to the power source input terminal of the control circuit through the starting resistor; and the MOSFET.
The drain terminal of is connected in series and consists of a primary winding to which a voltage obtained by rectifying and smoothing the commercial power source is applied, a secondary winding that extracts the output voltage, and a bias winding. The voltage generated in this bias winding is rectified and smoothed. A switching power supply comprising a transformer adapted to apply the smoothed output to the power supply input terminal, and a variable power supply having a varying means for varying a value of an output voltage taken out from the secondary winding. 2. A Zener diode is used as a varying means for varying the value of the output voltage, the Zener diode is connected to a power supply input terminal of the control circuit, and the voltage generated in the bias winding of the transformer is applied to the Zener diode. 2. The switching power supply according to claim 1, wherein the smoothed output of 1. 3. A shunt regulator is used as a varying means for varying the value of the output voltage, the shunt regulator is connected to a power supply input terminal of the control circuit, and the voltage generated in the bias winding of the transformer is applied to the shunt regulator. 2. The switching power supply according to claim 1, wherein the smoothed output of 1. 4. A resistance element connected to a power supply input terminal of the control circuit as a variable means for varying the value of the output voltage, a divided voltage of the output voltage taken out from the secondary winding of the transformer, and a reference voltage. 2. The switching power supply according to claim 1, wherein a shunt regulator for comparing with a shunt regulator and a photocoupler controlled by the shunt regulator are used, and the value of the resistance element is changed by the photocoupler.