JPH05316722A - Switching power source circuit - Google Patents

Abstract

PURPOSE:To suppress an abnormal voltage at a transient time by outputting a voltage proportional to a voltage of an input power source, and limiting a conduction angle of an IC for the power source. CONSTITUTION:A DC input voltage is supplied to a MOSFET 11. An alternating voltage is generated from a secondary winding 17 of a main transformer 9 by turning ON, OFF the MOSFET 11, rectified, and smoothed to obtain a predetermined DC output voltage at output terminals 23, 24. An erroneous voltage of the output voltage is detected by error detecting resistors 25, 26, a resistor 47 and a variable shunt regulator 40, insulated by a photocoupler 38 to be output, and input to an IC 27 for a power source through resistors 42, 42a. A pulse signal is obtained by a predetermined conduction angle from the input voltage and a sawtooth voltage. ON, OFF times of the MOSFET 11 are controlled by using transistors 44, 45 according to the pulse signal, and so controlled as to always become a predetermined voltage. Thus, an abnormal voltage at a transient time can be suppressed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a pulse width control type switching power supply circuit.

[0002]

2. Description of the Related Art Generally, a pulse width controlled switching power supply circuit has a primary winding of a transformer and a main switching element connected in series on the power input side, and a rectifier on the secondary winding of the transformer. An output terminal is provided via a filter, an error detection signal from the output side is fed back to a power supply IC for pulse width control, etc., and the output of this power supply IC controls ON / OFF of the main switching element. Is configured to.

However, in such a forward converter or the like, when an accident such as a sudden change in load or a short circuit occurs, an abnormal transient voltage is generated in the main switching element. But,
Conventionally, a bipolar transistor having a sufficient spare capacity with respect to the rated value of the withstand voltage capacity has been used as the main switching element, so that the transient voltage hardly causes damage.

[0004]

[Problems to be Solved by the Invention] However, recently, several hundred
Due to the demand for higher frequency of Hz, a field effect transistor (hereinafter referred to as MOSFET) has come to be used for this main switching element. This MOSFET has a small reserve capacity with respect to the rated value of the withstand voltage capacity as compared with the conventionally used bipolar transistor and the like, and may be damaged by the instantaneous transient voltage.

First, the mechanism of transient abnormal voltage generation at the time of sudden load change will be described in detail. MO as main switching element
There are three levels of voltage and current during operation of the SFET as shown in FIGS. 2 (a) (b) (c). That is, when the input side voltage is constant, Fig. 2 (a) shows a light load, Fig. 2 (b) shows a heavy load, Fig. 2 (c) shows no control, and the conduction angle τ ₃ reaches the limit. These are characteristics of the drain current ID and the drain-source voltage VDS at each time. As you can see from these figures,
The peak value of the drain-source voltage VDS is approximately proportional to the conduction angle τ.

FIG. 3 shows the drain-source voltage VDS of the MOSFET when the load suddenly changes from a light load to a heavy load.
Shows the transient state of the drain current ID. In FIG. 3, the stage a is a steady state under light load, and the stage b is a steady state under heavy load, but when the load suddenly changes when shifting from stage a to stage b, the feedback system that follows this change is used. There always exists a c-state in which the conduction angle τ is fully opened by the action. Here, the respective states of FIGS. 3A, 3B, 3C are shown in FIGS.
Corresponds to (c).

The state c in FIG. 3 exists not only when the load suddenly changes, but also when the load is suddenly short-circuited or when the short-circuit is released. Therefore, the c state is c
It is necessary to suppress abnormal voltage at the time of transition and prevent destruction of the MOSFET by improving the state to “”. Of course, even if the main switching element is a transistor or the like other than the MOSFET, it is desirable to suppress the abnormal voltage during the transition.

[0008]

According to the present invention, a primary winding 10 of a transformer 9 and a main switching element 11 are provided on the power input terminals 7 and 8 side.
Are connected in series, and the output terminals 23 and 24 are connected to the secondary winding 17 of the transformer 9 via the rectifier 18 and the filter 22 to output the error detection signal from the output side to the photocoupler 3
8 is insulated and taken out, and is input to a power supply IC 27 for pulse width control or the like provided on the primary side of the transformer 9,
The third winding 33, the diode 34, and the capacitor 3 provided in the
An output pulse signal corresponding to the conduction angle is obtained by coupling the auxiliary power source 15 composed of 5 to the power source IC 27 and comparing the error detection signal with the reference voltage by the voltage dividing resistors 28 and 29 coupled to the power source IC 27. In the switching power supply circuit for controlling on / off of the main switching element by this output pulse signal, the power supply input terminal 7 on the primary side of the transformer 9
The resistors 36 and 37 are directly connected to each other between 8 to form a detection circuit 32 for extracting a voltage proportional to the input power supply voltage, and this detection circuit 32 is for dead angle control for limiting the conduction angle of the power supply IC 27. It is connected to the terminal. FIG. 4 shows an outline of the inside of the MB3759 type, which is a typical element of the power supply IC for controlling the pulse width. For the purpose of the present invention, a TL494 type IC
Can be used as well.

[0009]

FIG. 5 shows the waveform of the pulse width control part inside the MB3759 type power supply IC. The solid line sawtooth wave M is the output between the 8th and 9th pins, and the dotted line sawtooth wave N is the 10th pin. This is the output between the 11th pin. V ₀ is a voltage obtained by amplifying the error between the output voltage and the reference value. When this voltage V ₀ is compared with the sawtooth wave M, the level of this voltage V ₀ causes the conduction during the time corresponding to the pulse output. The angles τ ₁ , τ ₂ , τ ₃ etc. are obtained. Note that T is one cycle.

Next, FIG. 6 is a diagram showing a dead time adjusting function by the fourth pin of the power supply IC. In this figure, the reference voltage VR of the 14th pin is set to V ₁ by resistors 1 and 2.
And V ₂ . At this time, the higher the voltage V ₁ , the smaller the conduction angle τ. Now, let us say that a voltage proportional to the input voltage is Vin, this voltage Vin is divided by resistors 3 and 4 to be the voltage by resistor 4, and Vf is applied to pin 4 to limit the conduction angle τ. It becomes possible.

That is, the conduction angle τ ₁ in FIG.
The conduction angle τ _{2 in} (b) corresponds to τ ₁ and τ ₂ in FIG. 5, respectively, and this limiting voltage Vf is applied to the 4th pin. Then,
Only the feedback system tries to fully open up to the conduction angle τ _3, but is limited to the conduction angle τf by the limiting voltage Vf. Therefore, the state of the characteristic c in FIGS. 2C and 3 can be suppressed like the c ′ characteristic in FIG. Although FIG. 6 shows a specific embodiment, it is possible in principle to use only the resistors 1 and 3 as shown in FIG.

As described above, in the conventional circuit, the transient abnormal voltage becomes extremely dangerous when the input voltage is high.
The circuit of the present invention suppresses it and functions extremely effectively. That is, when the input voltage is high, the required conduction angle τ ₂ has a lower value, but if the limiting voltage Vf is increased in proportion to the input voltage and τf is set to be slightly larger than τ ₂ , the input voltage is high. Even at this time, the state of the characteristic c in FIGS. 2C and 3 can be suppressed very effectively. Note that FIG.
In, the Zener diode 5 is for improving the characteristic of the voltage Vin proportional to the input voltage, and the diode 6 is added for facilitating the adjustment work, and is not an essential condition of the present invention. In the above embodiment, only the 8th and 9th pins of the output pins of the power supply IC MB3759 are used. In this case, FIG. 7 is effective, and when the 10th and 11th pins are used, 6 is effective in practice.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described with reference to FIG.
In FIG. 1, the power supply IC 27 is provided on the primary side of the transformer 9, and the error signal of the output voltage on the secondary side is detected by the photocoupler 38.
I try to insulate it and take it out. More specifically, 7 and 8 are input power terminals, and one terminal 7
To the primary winding 10 of the main transformer 9, M as the main switching element
It is coupled to the other terminal 8 via the drain source of OSFET 11. In addition, a series circuit of a diode 12 and a capacitor 13 is connected in parallel between the drain and source,
In addition, the resistor 14 is coupled between the coupling point of the diode 12 and the capacitor 13 and the terminal 7.

The transformer 9 is provided with an auxiliary winding 33, and the tertiary winding 33 includes a diode 34 and a capacitor 3.
5, the resistor 43 constitutes the auxiliary power supply 15. Further, the gate of the MOSFET 11 is coupled to the emitters of two transistors 44 and 45 inserted in series between the auxiliary power sources 15. The bases of these transistors 44 and 45 are coupled to the 9th pin of the power supply IC 27 provided on the primary side of the transformer 9, and the 8th pin is coupled to the auxiliary power supply 15 via the resistor 46.

The secondary winding 17 of the transformer 9 comprises a rectifier 1
8, through the commutation diode 19, and further through the filter 22 composed of the reactor 20 and the capacitor 21, the output terminal 2
3 and 24. Error detection resistors 25 and 26 are coupled between the output terminals 23 and 24, and a series circuit of a light emitting diode 39 and a resistor 47 of a photocoupler 38 and a variable shunt regulator 40 is inserted in parallel with the error detection resistors 25 and 26. Then, the connection point of the resistors 25 and 26 is connected to the variable shunt regulator 40. Further, the resistor 3 coupled to the 4th pin of the power supply IC 27 provided on the primary side
6, 37 are directly coupled to the input terminals 7, 8. This resistance 3
Reference numerals 6 and 37 constitute a detection circuit 32 for detecting the voltage Vin proportional to the input power supply voltage. Further, the first pin is connected to the connection point of the resistors 42 and 42a in series with the light receiving transistor 41 of the photocoupler 38.

In the above-mentioned structure, the DC input voltage changes to 2 of the main transformer 9 by turning on and off the MOSFET 11.
An alternating voltage is generated in the next winding 17, and this is rectified and smoothed by the rectifier 18, the filter 22, the commutation diode 19, etc., and a predetermined DC output voltage is obtained at the output terminals 23 and 24. The error voltage of this output voltage is
26, the resistor 47, the variable shunt regulator 40, and the photocoupler 38 insulates and takes out the resistor 4
It is input to pins 1 and 2 of the power supply IC 27 via 2, 42a. From the input voltage V ₀ and the sawtooth voltage M, a pulse signal can be obtained with a predetermined conduction angle τ shown in FIG. The pulse signal causes the transistor 44,
The on / off time of the MOSFET 11 is controlled via 45 so that the MOSFET 11 is always controlled to have a predetermined voltage.

Here, the input voltage is divided by the resistors 36 and 37 constituting the detection circuit 32 to become the limiting voltage Vf, which is 4
Type in pin number. This limiting voltage Vf is a conduction angle τ obtained from the detection voltage V ₀ of the output voltage and the sawtooth wave M, and τf
As described above, the above restrictions are restricted.
Therefore, even if a transient abnormal voltage is generated, its magnitude is greatly suppressed.

[0018]

Since the present invention is configured as described above, the so-called feedforward operation is performed by using the power supply IC, and not only the safety of controlling the abnormal voltage during the transient but also the disturbance. It also has an effective function in terms of prevention. Further, since the power supply IC is provided on the primary side of the transformer, there is no need for an insulating means for coupling with a signal system or the like, or a simple configuration is sufficient, and the circuit configuration is simple.
And it becomes cheaper. Therefore, it is indispensable for the switching power supply circuit of the main switching device such as MOSFET.

[Brief description of drawings]

FIG. 1 is an electric circuit diagram showing an embodiment of a switching power supply circuit according to the present invention.

2 (a), (b), and (c) are characteristic diagrams of VDS and ID when the MOSFET is lightly loaded, heavily loaded, and uncontrolled, respectively.

[Fig. 3] MOSFE in the case of sudden change from light load to heavy load
It is a characteristic view of VDS and ID of T.

FIG. 4 is a schematic diagram of a power supply IC (MB3759).

FIG. 5 is an output waveform diagram of a power supply IC.

FIG. 6 is a diagram showing a dead time adjustment function of a power supply IC.

FIG. 7 is a principle diagram of FIG.

[Explanation of symbols]

1, 2, 3, 4 ... Resistance, 5 ... Zener diode, 6 ...
Diodes, 7, 8 ... Input terminals, 9 ... Main transformer, 10 ...
Primary winding, 11 ... MOSFET, 15 ... Auxiliary power supply, 17
... secondary winding, 18 ... rectifier, 22 ... filter, 23, 24
... output terminal, 27 ... power supply IC, 32 ... detection circuit, 33
... tertiary winding, 34 ... rectifier, 35 ... capacitor, 36,
37 ... Voltage dividing resistor, 38 ... Photo coupler.

Claims (1)

[Claims] 1. A transformer 9 is provided on the power input terminals 7 and 8 side.
The secondary winding 10 and the main switching element 11 are coupled in series, and the secondary winding 17 of the transformer 9 is coupled to the output terminals 23 and 24 via the rectifier 18 and the filter 22 from the output side. The error detection signal of is isolated by the photocoupler 38, is taken out, and is provided on the primary side of the transformer 9 for power supply IC for pulse width control and the like.
27, and the auxiliary power supply 15 including the tertiary winding 33, the diode 34, and the capacitor 35 provided in the transformer 9 is coupled to the power supply IC 27, and is coupled to the error detection signal and the power supply IC 27. In the switching power supply circuit that compares the reference voltage of the voltage dividing resistors 28 and 29 with the reference voltage to obtain the output pulse signal corresponding to the conduction angle, and controls the ON / OFF of the main switching element by this output pulse signal, Resistors 36 and 37 are directly coupled between the power source input terminals 7 and 8 on the primary side to form a detection circuit 32 for taking out a voltage proportional to the input power source voltage.
7. A switching power supply circuit characterized by being connected to a dead angle control terminal for limiting the conduction angle of 7.