JPH0797901B2 - Switching power supply - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a single-stone forward switching power supply. More specifically, it relates to a reset circuit for releasing flyback energy, in which a three-terminal control element is connected between terminals of a first winding to which a switching element is connected, and a control electrode side of the three-terminal control element is connected. By connecting a reference voltage source to the switching element and suppressing the voltage applied to the switching element during the reset operation to a constant value by the reference voltage source regardless of the fluctuation of the power supply voltage, The ET product allowable value that is effectively used.

Conventional technology The basic structure of a single-stone forward switching power supply is
As shown in FIG. 6, a power conversion transformer 1 and a DC input Vin applied through a first winding 101 of the transformer 1.
Is configured to include a switching element 2 that turns on and off, and an output circuit 3 that rectifies and smoothes the switching output extracted to the second winding 102 of the transformer 1. The switching element 2 comprises a bipolar transistor or a field effect transistor. Reference numeral 4 is a load, 5 is a control circuit including a pulse width modulation circuit, and 6 is a DC power source obtained by rectifying and smoothing a commercial AC power source.

The output circuit 3 includes a forward rectifying diode 31 and
A commutation diode 32 in the flywheel direction is provided, and the rectified output is smoothed by a choke input type smoothing circuit composed of a choke coil 33 and a capacitor 34, and a DC output V ₀ is supplied to the load 4. .

7 is a reset circuit. The reset circuit 7 has a configuration in which a reset winding 103 is connected to one end of the first winding 101 of the transformer 1 and a diode 71 is connected between one end of the reset winding 103 and the ground. By releasing the flyback energy generated in the transformer 1 through the diode 71, the transformer 1 is reset and magnetic saturation is prevented.

When the switching element 2 is turned off, the first winding 10
At 1, a flyback voltage Vf that is determined by the power supply voltage Vin and the turn ratio of the reset winding 103 to the first winding 101 is generated. For example, the first winding 101 and the reset winding
When the turns ratio with 103 is 1: 1, the flyback voltage Vf
Becomes approximately equal to the power supply voltage Vin. In this case, the voltage on the anode side (a) of the switching element 2 is the sum of the power supply voltage Vin and the flyback voltage Vf when the ground is used as a reference, so that the switching element 2 has the power supply voltage Vin and the flyback voltage. A breakdown voltage larger than the sum of the voltage Vf is required. Therefore, conventionally, the turn ratio of the reset winding 103 and the first winding 101 is set so that the sum of the expected maximum value of the power supply voltage Vin and the flyback voltage Vf does not exceed the withstand voltage of the switching element 2. Had been set.

The problem to be solved by the invention is that in this type of switching power supply, as is well known, the energy stored when the switching element 2 is turned on is almost completely released as flyback energy during the next off period. To work. In other words, the voltage-time product when the transformer winding is on (hereafter referred to as the ET product)
And the ET products when off are equal. Therefore, in the reset circuit shown in FIG. 6, when the power supply voltage Vin increases and the ON time Ton ₁ decreases, the flyback energy release time Tf ₁ also decreases as shown in FIG. 7.

On the contrary, when the power supply voltage Vin decreases and the ON time Ton ₂ increases, the flyback energy release time Tf ₂ increases as shown in FIG.

For this reason, there is a problem that the utilization efficiency of the ET product allowable product given by the breakdown voltage of the switching element 2 and the period T becomes low, and the waste increases. For example, when the power supply voltage Vin is maximum and the device is operated at a full breakdown voltage as shown in FIG. 7,
When the half period (T / 2) of the cycle T becomes the rest period and the power supply voltage Vin becomes the minimum, as shown in FIG.
Only half of the breakdown voltage is used.

On the other hand, when a field effect transistor is used as the switching element 2, it is necessary to increase the duty in order to reduce the ON resistance loss. Next, this point will be described.

The average value of the current flowing through the first winding 101 of the transformer 1 and the switching element 2 formed of a field effect transistor is calculated as
As shown in the figure, Iav Becomes Therefore, the on-resistance Ron of the field effect transistor
The loss due to That is, the on-resistance loss is inversely proportional to the duty a. In the case of the conventional example shown in FIG. 6, the duty cannot be increased as is apparent from the waveform charts shown in FIGS. 7 and 8. Moreover, the switching element 2 needs to have a withstand voltage that is about twice the power supply voltage.

For this reason, in the conventional switching power supply shown in FIG. 6, there is a problem that the heat generation of the switching element 2 becomes large and the switching element 2 having a high breakdown voltage is required, resulting in high cost. Furthermore, transformer 1
In addition, there is a problem in that the dedicated reset winding 103 is required, which leads to an increase in size and cost of the transformer 1.

As a means for solving the above problems, as shown in FIG. 10, a Zener diode is provided in parallel with the switching element 2.
A reset circuit in which VZ is connected has been proposed. According to this conventional example, the voltage applied to the switching element 2 is suppressed to the Zener voltage Vz regardless of the fluctuation of the power supply voltage Vin, and when the power supply voltage Vin becomes high, the flyback energy release time is extended, and When the power supply voltage vin becomes low, the breakdown voltage and the cycle can be effectively used by shortening the flyback energy release time.

However, in this conventional example, since the exciting current of the transformer 1 flows to the ground level through the Zener diode Vz, there is a problem that heat loss becomes very large.

Means for Solving the Problems In order to solve the conventional problems, the present invention provides a transformer, and one of the main electrodes is connected to one end of a first winding of the transformer. A three-terminal switching element for turning on and off a DC input given through the output circuit, an output circuit for rectifying and smoothing the switching output taken out to the second winding of the transformer, and releasing flyback energy generated in the transformer. In a switching power supply including a reset circuit, the reset circuit has a three-terminal control element in which a main electrode is connected between terminals of the first winding, and one end is connected to a control electrode side of the three-terminal control element, The other end is provided with a reference voltage source connected to the other side of the main electrode of the switching element.

Action The reset circuit configured as described above, when the voltage applied to the switching element reaches a certain level with respect to the reference voltage source, the three-terminal control element conducts, and the voltage applied to the switching element reaches the certain level. It can be suppressed. Therefore, when the power supply voltage becomes high, the flyback energy release time operates so as to be long, and conversely, when the power supply voltage decreases, the flyback energy release time becomes short. become. As a result, the breakdown voltage and the cycle of the switching element can be effectively used, the utilization efficiency of the ET product allowable value is improved, and waste is eliminated.

Further, since the exciting current is returned to the power supply side by the conduction of the three-terminal control element, heat loss is reduced and efficiency is improved as compared with the case where a Zener diode is used.

Furthermore, since the duty can be extended, the on-resistance loss when using a field effect transistor can be reduced and the efficiency can be increased.

Embodiment FIG. 1 is an electric circuit diagram of a switching power supply according to the present invention. In the figure, the same reference numerals as those in FIG. 6 denote the same components. Reference numeral 8 is a reset circuit. In this embodiment, the switching element 2 is composed of a field effect transistor (hereinafter referred to as FET), and the collector of the transistor 81 as a three-terminal control element is provided between the terminals of the first winding 101 connected to the drain D side thereof. C and the emitter E are connected, and a series circuit of a resistor 82 and a Zener diode 83 as a reference voltage source is connected between the base B of the transistor 81 and the ground.

Reference numeral 84 is a diode that prevents the reverse voltage of the transistor 81.

In the reset circuit configured as described above, the voltage between the anode D of the EFT2 and the ground (source S) is changed by the transistor 81 due to the flyback voltage Vf generated in the first winding 101.
When it reaches a level at which the Zener diode 83 connected to the base B side of is made conductive, the Zener diode 83 becomes conductive. When the Zener diode 83 becomes conductive, a current flows through the base B of the transistor 81 and becomes conductive, and the voltage on the drain D side of the EFT2 becomes the Zener voltage V of the Zener diode 83.
It is kept approximately equal to z ₁ and cannot be higher than that.

For this reason, for example, the power supply voltage Vin becomes high, and the flyback voltage Vf that has been correspondingly high in the prior art can be suppressed as shown in FIG. And the ET product B when the flyback energy is released and the ET product B when ₁₂ is on
_The flyback energy release time Tbf ₁ extends to be equal to ₁₁ . When the power supply voltage Vin decreases, the flyback energy release times Tbf ₂ and Tbf ₃ are shortened as shown in FIGS.

As a result, as shown in FIGS. 2, 3, and 4, the duty is large and the range of the power supply voltage Vin can be widened as compared with the conventional waveforms of FIGS. 7 and 8.

FIG. 5 shows another embodiment of the switching power supply according to the present invention. In this embodiment, the source of the EFT 2 is connected to the first winding 101 of the transformer 1.

EFFECTS OF THE INVENTION As described above, according to the present invention, the withstand voltage and the cycle of the switching element can be effectively used, the utilization efficiency of the ET product allowable value is high, there is no waste, and the heat loss is low and the efficiency is high. When a field effect transistor is used as a switching element, the ON resistance loss can be reduced and the efficiency can be increased, and a switching circuit having a reset circuit suitable for downsizing and cost reduction of the power conversion transformer can be provided. A power supply can be provided.

[Brief description of drawings]

FIG. 1 is an electric circuit diagram of a switching power supply according to the present invention,
2 to 4 are voltage waveform diagrams applied to the switching element when the power supply voltage changes, and FIG. 5 is an electric circuit diagram of another embodiment of the switching power supply according to the present invention.
FIG. 6 is an electric circuit diagram of a conventional switching power supply, FIGS. 7 and 8 are waveform diagrams of a voltage applied to a switching element when the power supply voltage is changed, and FIG. 9 is an ON resistance loss of the switching element. Waveform diagram for
The figure is an electric circuit diagram of another conventional example of the switching power supply. 1 ... transformer, 101 ... first winding 102 ... second winding 2 ... EFT as switching element 3 ... output circuit, 8 ... reset circuit 81 ... as three-terminal control element Transistor 83 ... Zener diode as reference voltage source

Claims (2)

[Claims] 1. A transformer, and a three-terminal switching element, one of the main electrodes of which is connected to one end of a first winding of the transformer to turn on and off a DC input provided through the first winding. In a switching power supply comprising an output circuit for rectifying and smoothing a switching output extracted to the second winding of the transformer, and a reset circuit for discharging flyback energy generated in the transformer, the reset circuit comprises:
A three-terminal control element having a main electrode connected between the terminals of the first winding, one end connected to the control electrode side of the three-terminal control element, and the other end connected to the other side of the main electrode of the switching element. And a reference voltage source that has been set. 2. The switching power supply according to claim 1, wherein the switching element is a field effect transistor.