JPH0662571A - Switching power circuit - Google Patents

Abstract

PURPOSE:To provide a switching power circuit having a switching snubber circuit which requires no complicated timing circuit. CONSTITUTION:A switching power circuit has a transformer T wherein a primary winding N1 is connected to a source of DC voltage source VIN through a switching element SW. A snubber circuit is constituted of a constant current circuit composed of a transistor Q which is driven by the constant voltage developed across a specific terminal of the transformer T and a resistor R inserted into an emitter of the transistor Q and a capacitor C for absorbing a voltage noise. The voltage noise absorbed by the capacitor C through a base collector junction of the transistor Q and the charging charge is regenerated to the transformer T through the transistor circuit and then the transistor Q is automatically turned off when the regenerative current is saturated.

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 circuit, and more particularly to a switching power supply circuit improved with a snubber circuit for removing voltage noise generated when a switching element is turned off.

[0002]

2. Description of the Related Art The basic structure of a flyback type switching power supply circuit is shown in FIG. Switching element SW for controlling ON / OFF at a predetermined frequency for the DC voltage source VIN
A transformer T is connected to the transformer T through an output circuit, and an output circuit including a diode D for rectifying and smoothing and an output capacitor COUT is provided on the secondary side thereof. In this switching power supply circuit, when the switching element SW is turned off, the magnetic energy stored in the self-inductance of the transformer T is released to the secondary side and becomes output power. In this type of switching power supply circuit, the energy stored in the leakage inductance LS has no load to release it, so that a large peak voltage noise occurs when the switching element SW is turned off, and this causes a stray capacitance or a voltage resonance capacitor (see FIG. 9). (Represented by the capacitance Cs) at this time and ringing occurs. The situation is shown in FIG.

As shown in FIG. 10, the switching element S
When the terminal voltage VS generated when W is off is ringing, it becomes (a) a source of radiation noise, and (b) an instantaneous voltage applied to the switching element SW becomes high, which causes element breakdown and reliability deterioration. , And (c) Output current IOU
As shown in the figure, T also causes ringing, which is very harmful.

In order to remove such ringing noise, various snubber circuits have been conventionally considered.
An example thereof is shown in FIG. The snubber circuit in Fig. 11 (a) is
A resistor R and a capacitor C are connected between the primary windings of the transformer T. As a result, the extra energy of the transformer T is stored in the capacitor C and consumed by the resistor R.
In this method, the energy accumulated in the leakage inductance is consumed by the resistor R, which causes heat generation and lower efficiency.
FIG. 11B shows that the peak portion of the ringing voltage is stored in the capacitor C via the diode D to suppress the ringing peak value. When the diode D becomes conductive, the capacitor Cs, which determines the resonance frequency of the ringing, enters in series with the capacitor Cs, so that the resonance frequency decreases, and in reality, not only the upper half of the ringing but also the whole becomes smaller, which is effective. is there. Further, since the capacitance of the capacitor C does not cause a problem because it is too large, the capacitor C is set to a larger value than in the case of FIG. 11 (a). However, the capacitor C
The energy stored in must also be consumed, in parallel with which a resistor R is provided. Therefore, heat generation and efficiency reduction are inevitable.

FIG. 11 (c) is a modification of FIG. 11 (b), in which the power of the capacitor C is not consumed by the resistor, and a switching element S provided separately from the main switching element SW1 is provided.
It is configured to be regenerated into a transformer T by W2 and is called a so-called switching snubber circuit. If a large amount of energy regeneration is set, this circuit will be used as a partial resonance type. Two switching elements SW1,
SW2 is alternately turned on and off. First, when the switching element SW1 is turned off, the capacitor C is charged through the diode D by the stored energy of the transformer T. When the charging current becomes zero, the diode D is turned off, but the switching element SW2 is turned on during this period, and the accumulated charge of the capacitor C is generated by the switching element S2.
Backflow into the transformer T via W2. After an appropriate amount of charge has been returned to the transformer T, the switching element SW2 is turned off. In this way, the capacitor C is charged and discharged without generating heat, and the snubber effect is maintained.

The operating point of the switching snubber lies in the setting and control of the ON time of the switching element SW2. If the ON time of the switching element SW2 is short, the snubber effect or the regenerative effect becomes small, and if it is too long, the regenerative current becomes large and the reverse saturation or efficiency of the transformer is reduced. Since the ON time of the main switching element SW1 is variably controlled in order to stabilize the output voltage, ideally the regenerative current is constant unless the ON time of the switching element SW2 of the snubber circuit is changed accordingly. do not become. If the ON time of the switching element SW2 is fixed, the fluctuation of the operating frequency becomes small, but the regenerative current increases and the efficiency decreases when the load is light. In order to improve the efficiency, it is necessary to make the regenerative current constant, and for that purpose, it is necessary to control the ON time while detecting the current of the switching element SW2.

[0007]

As described above, the conventional switching snubber circuit not only has a complicated structure, but also controls the switching element in order to exert the snubber effect without reducing the efficiency. There is a problem that the timing circuit becomes complicated. This invention
An object of the present invention is to provide a switching power supply circuit having a switching snubber circuit which does not require a complicated timing circuit.

[0008]

SUMMARY OF THE INVENTION A switching power supply circuit according to the present invention is a transformer connected to a DC voltage source through a switching element whose ON / OFF is controlled, and a rectifying / smoothing smoothing provided on the secondary side of the transformer. And a snubber circuit that absorbs voltage noise when the switching element is turned off. The snubber circuit has a capacitor having one end set to a reference potential for absorbing excess energy of the transformer, and a capacitor other than the capacitor. A collector is connected to the end, an emitter is connected to a predetermined terminal on the primary side of the transformer through a resistor, and a constant voltage obtained on the primary side of the transformer when the switching element is turned off is applied to the base as a bias. And a constant current circuit configured as described above. The constant current circuit regenerates the energy stored in the capacitor to the transformer via the bipolar transistor, and the regenerative current is saturated, so that the bipolar transistor is automatically turned off.

[0009]

A snubber circuit according to the present invention uses a bipolar transistor as a switching element and is driven by a transformer winding voltage so that the bipolar transistor is automatically turned off when a regenerative current reaches a certain value. Is configured. That is, a resistor is inserted in the emitter of the bipolar transistor to form a constant current circuit driven by a constant voltage obtained between predetermined terminals of the transformer when the main switching element is turned off.
For example, excess energy of the transformer is stored in the snubber capacitor as a charging current via the base-collector junction of the bipolar transistor. The charge stored in the capacitor then flows backward and is discharged through a constant current circuit composed of a bipolar transistor and a resistor and returned to the transformer as a regenerative current. This regenerative current gradually increases, but does not exceed a certain value determined by the circuit constant of the constant current circuit. Then, when the regenerative current is saturated, the electromotive force of the transformer winding driving the bipolar transistor is lowered, and the bipolar transistor is automatically turned off. As described above, according to the present invention, a switching power supply circuit having a constant regenerative current type switching snubber circuit can be obtained without requiring a complicated timing circuit.

[0010]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows the configuration of a flyback type switching power supply circuit according to an embodiment of the present invention. The DC voltage source VIN is provided with a transformer T to which a primary winding N1 is connected via a switching element SW which is on / off controlled at a predetermined frequency, and a diode D and an output capacitor COUT are provided on the secondary side of the transformer T. A rectifying / smoothing circuit composed of

A switching snubber circuit that absorbs voltage noise due to the leakage inductance of the transformer T is formed by an auxiliary power supply winding Nf whose one end is connected to one end of the primary winding N1 of the transformer and a voltage generated at this winding Nf. It is composed of a driven NPN transistor Q, a resistor R inserted in the emitter of this transistor Q, and a capacitor C for absorbing voltage noise. The operation of absorbing the excess energy of the transformer T when the switching element SW is turned off is performed by charging the capacitor C through the base-collector junction of the transistor Q by the voltage obtained in the auxiliary power supply winding Nf. Done.

On the other hand, the charge of the capacitor C is discharged by
This is done through the transistor Q and the resistor R while the switching element SW is off, and this discharge path constitutes a constant current circuit. That is, in the auxiliary power supply winding Nf, a voltage VNf having a polarity that forward biases the transistor Q is generated when the switching element SW is off and energy is being emitted to the secondary side. This voltage VNf is determined by the winding ratio with the secondary winding N2, but since the swing element SW is controlled so that the output of the secondary winding N2 is substantially stabilized, the voltage VNf is also a substantially constant voltage. . The collector current Ic that can be driven by the voltage VNf and can flow in the transistor Q is Ic = (VNf-VBE) / R (1) where VBE is the base-emitter voltage of the transistor Q. In this way, the circuit of the auxiliary power supply winding Nf, the transistor Q, and the resistor R operates as a constant current circuit when the capacitor C is discharged by the action of negative feedback by the constant bias and the resistor R, and the discharge current is the transformer T. It becomes the regenerative current to.

Next, the operation of the snubber circuit of this embodiment will be described in detail with reference to FIGS. 2 and 3. FIG. 2 shows voltage / current waveforms of each part, and FIG. 3 shows a state of charging / discharging current by the snubber circuit. The relationship between the primary winding N1 of the transformer T of FIG. 1 and the auxiliary power supply winding Nf is equivalently expressed as shown in FIG. When the switching element SW is turned off, a counter electromotive force is generated at the terminal of the primary winding N1, and at the same time, a voltage having a polarity for forward biasing the transistor Q is generated in the auxiliary power supply winding Nf. The voltage of the winding Nf causes the charging current I2 to flow into the snubber capacitor C as shown in FIGS. 2 and 3 (a). This current flows through the base-collector junction of transistor Q. This is the noise absorption operation.

After the charging of the capacitor C is completed, the energy stored in the primary winding N1 is discharged to the secondary side, and the current I4 flows to the secondary side. At this time, the transistor Q has a winding Nf
Since it is forward-biased by the voltage of, the capacitor C can be discharged through the transistor Q. This becomes the regenerative current I3 to the transformer T as shown in FIGS. 2 and 3 (b). This regenerative current increases with time due to the inductance of the transformer T, but its upper limit is limited by the value of the formula (1) determined by the electromotive force of the winding Nf as described above. The electromotive force itself of the winding Nf sharply decreases due to the saturation of the regenerative current, and the transistor Q is momentarily turned off.

As described above, the switching and
The transistor Q of the snubber circuit has its base-collector junction functioning as a diode in a conventional switching snubber circuit, and also has a function as a switching element for discharging the electric charge of the capacitor C to regenerate it. Moreover, since the transistor circuit forms a constant current characteristic by the emitter resistance and the constant base bias, the transistor Q is automatically turned off when the regenerative current reaches this constant current value, and no special timing circuit is required. .

FIG. 4 shows a switching power supply circuit according to another embodiment of the present invention. The basic structure is the same as that of the embodiment of FIG. 1, and the portions corresponding to those of FIG. 1 are designated by the same reference numerals as those of FIG. 1 and their detailed description is omitted. In this embodiment, as a charging path for the snubber capacitor C, a diode Da is provided separately from the transistor circuit so as to be in parallel with the base-collector junction. With the addition of this diode Da, a diode Db is inserted in the collector to turn off the charging path to the capacitor C via the base-collector junction of the transistor Q. The transistor circuit constitutes a constant current circuit, and the transistor Q is automatically turned off by a constant regenerative current, as in the previous embodiment.

This embodiment is preferable for a switching power supply having a large capacity. This is because in the large-capacity switching power supply, the charging current to the snubber capacitor C becomes large, which may exceed the maximum rated current of the transistor Q. On the other hand, it is possible to use a transistor having a large maximum rated current as the transistor Q, but a smaller transistor Q and a diode D are used.
The combination of a can reduce the total cost. The reason why the transistor Q may be small is that the charging current to the capacitor C has a large peak current at the rising edge, whereas the regenerative current has a triangular wave shape whose rise is limited by the inductance and the current peak is small.

FIG. 5 shows an embodiment in which the construction of the embodiment of FIG. 4 is slightly modified. In this embodiment, a diode Db having the same polarity as the base junction is inserted in the base of the transistor Q. Also in this embodiment, the charging current to the capacitor C does not pass through the transistor Q but bypasses the diode Da. Therefore, the same operation as that of the embodiment of FIG. 3 becomes possible.

In the embodiments of FIGS. 4 and 5, the diode Db is provided so that the charging current does not pass through the transistor Q, but the diode Db can be omitted. In this case, the charging current to the capacitor C will pass through the base Da and collector junction of the transistor Q and at the same time through the diode Da. Since this also disperses the charging current passing through the transistor Q, it can be applied to a large-capacity switching power supply in which a charging current equal to or higher than the maximum rated current of the transistor Q flows. Further, the transistor Q can be downsized even if the configuration of FIG. 11C is added to the circuit configuration according to the present invention as an auxiliary bypass path. FIG. 6 shows an example thereof.

Although an embodiment focusing on only the snubber circuit section has been described above, an embodiment in which the main switching element section and its control section are embodied will be described below.
FIG. 7 shows a switching power supply circuit of that embodiment, which is a flyback type self-excited switching power supply circuit. Q1
Is an NPN transistor which is the main switching element, and is connected in series to the primary winding N1 of the transformer T via the resistor Re at the emitter. This transistor Q1
It is the auxiliary winding N3 that drives. Transistor Q
A starting capacitor C3, a current limiting inductance element L and a resistor Rb are inserted in the base of 1. An initial starting resistor Rs is provided between the base of the transistor Q1 and the positive terminal of the DC power source VIN.

The NPN transistor Q2 provided on the base side of the transistor Q1 is an auxiliary transistor for turning off the main transistor Q1 for stabilizing the output. This transistor Q2 uses the intermediate terminal voltage of the auxiliary winding N3, the diode D2 and the capacitor C2 as an auxiliary power source, and the phototransistor P. Base drive is performed using Tr as a switching element. Phototransistor P. The photo diode PDi optically coupled to Tr is connected in series with the load resistance RL on the secondary side of the transformer T together with the Zener diode ZD. When the output voltage exceeds a certain value, Zener diode Z
D conducts, which drives the photodiode PDi. By the output light of the photo diode PDi,
Phototransistor P. When Tr is turned on, the auxiliary transistor Q2 is turned on, which causes feedback control that the main transistor Q1 is turned off.

A capacitor C6 and a resistor Rx are placed between the collector and emitter of the transistor Q1 which is the main switch.
A differentiation circuit consisting of is formed. This differentiator circuit
This is to accelerate the turn-off of the transistor Q1 and reduce the power loss at the turn-off. Transformer T
The capacitor C5, the NPN transistor Q3, and the resistor RT provided between the positive side terminal of the DC winding of the primary winding N1 and the intermediate terminal form a switching snubber circuit. A part of the primary winding N1 on the transistor Q1 side is used as an auxiliary power supply winding for driving the base of the transistor Q3 of the snubber circuit.

The operation of the switching power supply circuit of this embodiment will be described with reference to the operation waveforms of FIG. When the circuit is turned on, current is supplied from the input DC power source to the base of the main transistor Q1 via the starting resistor Rs.
At this time, since there is the capacitor C3 between the base of the transistor Q1 and the auxiliary winding N3, no current flows in the auxiliary winding N3. When the transistor Q1 turns on,
A current starts to flow in the winding N1 of the transformer T, and a current also starts to flow in the auxiliary winding N3, and an electromotive force that forward biases the transistor Q1 is generated in the auxiliary winding N3. Due to this positive feedback loop, the current in the primary winding N1 gradually increases. During this period, the transistor Q3 of the snubber circuit is off and the capacitor C5 is not charged.

During this period, the size of the inductance element L is set so that the base current waveform supplied to the transistor Q1 becomes similar to the collector current waveform that gradually increases due to the action of the inductance element L. . As a result, useless base current does not flow in the initial stage when the transistor Q1 is on. On the secondary side of the transformer T, since the diode D1 is cut off, no current flows in the secondary winding N2.

When the collector current Ic of the transistor Q1 reaches a predetermined value and the terminal voltage of the resistor Re reaches a value capable of turning on the auxiliary transistor Q2, the transistor Q2 turns on. At this time, the differentiation circuit composed of the capacitor C6 and the resistor Rx catches the collector potential variation of the transistor Q1 and accelerates the base drive of the transistor Q2, so that the turn-on of the transistor Q2 is accelerated. When the transistor Q2 turns on, the main transistor Q1 turns off. The transistor Q2
As described above, the feedback control is also performed by the output.
That is, when the output voltage is about to exceed the specified value, the phototransistor P. Tr turns on and transistor Q
Turn on 2. Therefore, the control is performed such that the lighter the load, the earlier the turn-off timing of the main transistor Q1. The collector current peak of the transistor Q1 is the emitter resistance Re considering the maximum rated current.
Set by.

When the transistor Q1 is turned off, the loop of the primary winding N1 of the transformer T is substantially opened, and the magnetic energy stored in the primary winding N1 is released to the secondary side. The electromotive force generated in the secondary winding N2 is diode D
It has the polarity to turn on 1, and the capacitor C is connected by the inductance of the secondary winding N2 and the integrating circuit of the capacitor C1.
The power is stored in 1 and the output voltage increases. The switching snubber circuit operates when the transistor Q1 is off. In the initial stage when the transistor Q1 is turned off, a part of the energy stored in the primary winding N1 is charged in the capacitor C5 through the base-collector junction of the transistor Q3. After the charging is completed, the charging energy is released through the path of the transistor Q3 and the resistor RT and is regenerated to the transformer T from the intermediate terminal of the primary winding N1. As described above, the regenerative current reaches the saturation value and the transistor Q3 is automatically turned off.

When the energy stored in the primary winding N1 of the transformer T is exhausted to the secondary side, the diode D on the secondary side is discharged.
1 is turned off and the secondary winding N2 is opened. As a result, an electromotive force is generated in the secondary winding N2, which is transmitted to the feedback winding N3 to turn on the main transistor Q1 again. Thereafter, the transistor Q1 is repeatedly turned on and off by the same operation.

Although the flyback type switching power supply has been described above exclusively, the present invention is not limited to this, and is similarly applicable to a forward side switching power supply circuit. .

[0029]

As described above, according to the present invention, there is provided a switching power supply circuit having a constant regenerative current type switching snubber circuit capable of automatically turning off the switching element of the snubber circuit without requiring a complicated timing circuit. can get.

[Brief description of drawings]

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

FIG. 2 is a diagram showing operation waveforms of the same embodiment.

FIG. 3 is a diagram for explaining the operation of the snubber circuit according to the same embodiment.

FIG. 4 is a diagram showing a switching power supply circuit according to another embodiment.

FIG. 5 is a diagram showing a switching power supply circuit of still another embodiment.

FIG. 6 is a diagram showing a switching power supply circuit of still another embodiment.

FIG. 7 is a diagram showing a switching power supply circuit according to a specific embodiment.

FIG. 8 is an operation waveform diagram of the circuit of the embodiment.

FIG. 9 is a diagram showing a basic configuration of a switching power supply.

FIG. 10 is a waveform diagram illustrating ringing of the switching power supply.

FIG. 11 is a diagram showing a conventional snubber circuit configuration.

[Explanation of symbols]

T ... Transformer, Nf ... Auxiliary power supply winding, SW ... Switching element, D ... Diode, COUT ... Output capacitor, Q ... NPN transistor, R ... Resistor, C ... Capacitor.

Claims (1)

[Claims] 1. A transformer connected to a DC voltage source via a switching element controlled to be turned on and off, a rectifying / smoothing means provided on a secondary side of the transformer, and a voltage noise generated when the switching element is turned off. In a switching power supply circuit having a snubber circuit that absorbs, the snubber circuit includes a capacitor whose one end for absorbing excess energy of the transformer is set to a reference potential, and a collector connected to the other end of the capacitor, and an emitter. A constant current circuit connected to a predetermined terminal on the primary side of the transformer via a resistor, and a constant voltage circuit obtained by applying a constant voltage obtained on the primary side of the transformer as a bias to the base when the switching element is turned off; The energy stored in the capacitor is A switching power supply circuit characterized in that the bipolar transistor is automatically turned off by being regenerated by the transformer through a transistor and saturating a regenerative current.