JPH07106064B2 - DC-DC converter - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a DC-DC converter (DC-DC converter) used in a stabilized DC power supply device or the like.
It is about.

[0002]

2. Description of the Related Art A switching transistor is connected in series to a primary winding of a transformer and is turned on / off to intermittently flow a direct current to rectify and smooth a voltage induced in a secondary winding. A DC-DC converter that obtains a direct current output by using a known method is already widely known.

As one of the devices of this type, there is an on / off type self-excited DC-DC converter having a simple circuit configuration. Although this type of circuit has the simplest configuration, it can reduce the number of parts, but the peak value of the switched current is large compared to the output capacitance, and the voltage applied to the transformer is unidirectional and the utilization factor is poor. It is mainly used for small output because it is necessary to be careful not to cause excessive current to flow due to magnetic saturation.

FIG. 10 is a circuit diagram showing the structure of a conventional DC-DC converter. In FIG. 10, 1 and 2 are DC power supply terminals, and 24 and 25 are output terminals for connecting a load. T1 is a switching transformer, N1 is a primary winding,
N2 is a secondary winding, and NB is a feedback winding (base winding). Q11 is a switching transistor, and R12 is a bias resistor for supplying a starting current to the base at the start of oscillation. R13 is a resistor that applies the voltage of the base winding NB to the base while limiting the base current.

The transistor Q12, the resistor R14 and the light receiving transistor 3-1 constitute a control circuit for bypassing (shunting) the base current and controlling the output voltage. Here, the light receiving transistor 3-1 is a photodiode 3-2.
And a photocoupler 3 are formed, which receives light from the photodiode 3-2 to control the output voltage, changes the resistance value thereof, and controls the base current flowing through the transistor Q12. The resistor R14 is a resistor for limiting the maximum value of this current and operating the circuit stably.

The capacitor C1, the resistor R1 and the diode CR1 form a snubber circuit which absorbs an abnormal back electromotive force generated due to the coupling loss of the switching transformer T1 and the rectification loss on the secondary side. Here, the diode CR1 becomes conductive when an abnormal voltage is generated, and functions as a switching diode for absorbing the abnormal voltage in the absorbing capacitor C1. The resistor R1 is a resistor for discharging the electric charge accumulated in the capacitor C1.

The diode CR22 is a transistor Q1.
1 is a rectifying diode for rectifying the counter electromotive voltage on the secondary side that occurs when the capacitor C22 is a rectifying diode CR.
It is a smoothing capacitor for smoothing the rectified current from 22 and outputting a stable direct current.

U1 is a variable shunt regulator for controlling the output voltage, and the output voltage V _out is a resistor R2.
6, the voltage is divided by R27 and is applied to the control terminal, so that the amount of current flowing changes as the output voltage V _out changes. The variable shunt regulator U1 is composed of an IC such as TL431 manufactured by Texas Instruments Incorporated. When a current flows through the variable shunt regulator, the light emitting diode 3-2 emits light and the resistance value of the light receiving transistor 3-1 changes. The base current of the transistor Q11 is controlled by the change in the resistance value. By controlling the base current of the transistor Q11 in this way, the output voltage can be stabilized. The resistor R25 is a resistor for limiting the current flowing through the light emitting diode 3-2 and for operating the circuit stably.

Next, the operation of the conventional example configured as described above will be described.

First, a slight base current flows through the bias resistor R12 to the base of the switching transistor Q11, and the transistor Q11 is turned on. For current flows in the primary winding N1 transistor Q11 is turned on, the base winding NB is feedback winding flows through the base current I _B is the transistor Q11 saturates.

When the transistor Q1 is on, the current I _C (I _C = V _C ) determined by the primary side inductance L of the transformer T1 and the voltage V _in of the input current power supply.
_in · t / L) increases and I _C = H _FE · I _B (H _FE
Is the amplification factor of the transistor) and the current stops increasing. In the above period, the base current I _B remains at the transistor Q
It is controlled by a control circuit composed of 12, 3-1 and a resistor R14.

When the current I _C stops increasing, the base winding N
The voltage of B begins to disappear, the base current I _B also decreases, and the transistor Q1 turns off rapidly. The counter electromotive voltage (flyback voltage) generated in the secondary winding N2 during this off period is rectified by the diode CR22 and smoothed by the capacitor C22, and at the same time, direct current is output to the output terminals 24 and 25. further,
During this off period, the base winding NB reverse biases the transistor Q11 to maintain the off state. When the back electromotive force decreases and the current I _D flowing through the diode CR22 becomes zero, the base winding NB becomes the switching transistor Q.
Since it becomes impossible to maintain 11 in the off state, the transistor Q11 is turned on again and oscillation continues.

[0013]

As described above, in the conventional self-excited DC-DC converter, the PNP junction type transistor or the NPN junction type transistor is used as the switching element for the current flowing in the primary winding. . Here, a state in which these transistors are turned off is described below.

First, at the time of transition from the on state to the off state, a current having a value close to the peak value defined by I _C = H _FE · I _B flows in the transistor Q11. When the increase in collector current I _C slows down in this state, the voltage induced in the base winding NB also decreases and the base current of the transistor Q11 decreases. Then, when the induced voltage of the base winding NB further decreases and the base current cannot maintain the ON state of the transistor Q11, the transistor Q11 shifts from the ON state to the OFF state.

As described above, in the conventional device, when the peak value current flows in the collector, the base current decreases and the on-resistance value of the transistor increases, so that the on-state changes from the on-state to the off-state. A large power loss occurred in the transistor at the time of transition.

Further, these transistors are MOS type F
Since the switching speed is slower than that of ET (Field Effect Transistor), the on-state time cannot be shortened to a certain extent or more. For this reason, the minimum value of the magnetic energy stored in the transformer T1 is restricted by this ON minimum time, so that when a light load is connected to the conventional device, the switching operation is intermittent and the operation of the device is unstable. There was also a problem that became.

Further, in the conventional device using these transistors having a slow switching speed, the utilization rate of the transformer T1 is low because the oscillation frequency cannot be increased to a certain extent or more.

Here, if the transistor of the conventional device is replaced with a MOS type FET having a high switching speed, all the above-mentioned problems are solved, but the conventional device in which the transistor is turned off by reducing the base current is eliminated. In the circuit, it was not possible to use a MOS type FET which is a voltage control element as a switching element.

An object of the present invention is to use an FET as a switching element for the current flowing in the primary winding and operate the FET at a high speed to reduce the loss during switching, increase the utilization factor of the transformer, and further reduce the weight. It is to provide a DC-DC converter that operates stably even under load.

[0020]

In order to achieve the above object, the present invention is induced in a secondary winding by intermittently supplying a current from a DC power supply to the primary winding of a transformer. In a DC-DC converter that rectifies and smoothes a voltage and supplies it to a load, an electric field that is connected in series to the primary winding with respect to the DC power supply and has a gate connected to a positive electrode of the DC power supply via a resistor. An effect transistor, a switching means that is provided between the negative electrode of the DC power supply and the gate, and is in a conductive state when the voltage input to the control terminal is a predetermined value or more, and an output voltage detection that detects the voltage supplied to the load. Means, a feedback winding provided in the transformer, a first capacitor having one end connected to the gate of the field effect transistor and the other end connected to one end of the feedback winding, and one end connected to the switch. Is connected to the control end of the etching unit,
A second capacitor having the other end connected to the other end of the feedback winding; and a second capacitor provided between the one end of the second capacitor and the one end of the first capacitor. According to the output, the first capacitor to the second capacitor
Switching control means having a current control element for controlling the amount of current flowing to the second capacitor, and a negative phase supplying a voltage having a phase opposite to the voltage generated at the one end of the feedback winding to the second capacitor. And a voltage supply means.

[0021]

According to the present invention constructed as described above, first, a voltage is applied to the gate of the field effect transistor to turn it on, and a current flows through the primary winding. In this state, an induced voltage is generated across the feedback winding. This induced voltage is applied to the gate of the field effect transistor via the first capacitor of the switching control means, and the field effect transistor is completely turned on.

In this ON state, a current of an amount corresponding to the output voltage detected by the output voltage detecting means flows into the second capacitor from the first capacitor in the switching control means, and the current between both ends of the second capacitor is increased. The voltage rises. Along with this, the voltage at the control end of the switching means connected to one end of the second capacitor also rises, and when the voltage reaches a predetermined value, the switching means connects the gate of the field effect transistor to the negative electrode of the power supply and the field effect transistor. Instantly turns off.

In this off state, the counter electromotive voltage generated in the secondary winding of the transformer is rectified, smoothed and supplied to the load.
Further, in this OFF state, the feedback winding reversely biases the field effect transistor, and the negative phase voltage supply means continues to supply a voltage of a predetermined value to the control end of the switching means via the second capacitor of the switching control means. Keep the lever off. Then, when the counter electromotive voltage decreases and the anti-phase voltage supply means cannot maintain the voltage of the above-mentioned predetermined value, the switching means is turned off and the field effect transistor is turned on.

Thereafter, the above operation is repeated.

In this way, at the control end of the switching means,
By applying a voltage having the opposite phase to the gate of the field effect transistor, that is, the same phase as the drain, through the capacitor, the on / off operation of the field effect transistor can be performed reliably and at high speed to achieve stable operation and low loss. .
Further, by controlling the charging current of the second capacitor and controlling the on-time of the field effect transistor, magnetic saturation of the transformer is prevented, overcurrent does not flow, and the output voltage is stabilized. Further, since the field effect transistor is used, the switching speed can be increased, the oscillation frequency can be set high, and the transformer utilization rate can be increased.

[0026]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail below with reference to the drawings.

FIG. 1 is a circuit diagram showing a first embodiment of the present invention, in which the same reference numerals as those in FIG. 10 designate the same components. In addition to the primary winding N1 and secondary winding N2, a feedback winding is provided in the switching transformer T1. This feedback winding is divided into two parts at the position where it is connected to the negative pole of the power supply. Of these, NG is a gate winding, and NB is a voltage of the opposite phase to the gate winding NG at the base of the gate control transistor Q2. Is a base winding for adding.

Q1 is a MOS FET switching transistor, R2 is a bias resistor for applying a voltage to the gate at the start of oscillation, and C2 and R5 are a capacitor and a resistor for applying the voltage from the gate winding NG to the gate. The Zener diode CR2 configures a clamp circuit that uses a reverse diode characteristic that protects the gate from being applied with an unnecessarily high voltage, and functions as a normal diode to increase the voltage utilization rate of the gate winding NG, and further I try not to apply negative voltage to the gate. Q
Reference numeral 2 is a gate control transistor for shifting the transistor Q1 from the on state to the off state.
C3 is a capacitor that applies the voltage of the feedback winding (base winding NB) to the base, resistors R3 and R4, and the light-receiving transistor 3-1 are resistors that flow a charging current to the capacitor C3, and control the on-time of the switching transistor Q1. To do. Here, the light receiving transistor 3-1 constitutes a photo coupler with the light emitting diode 3-2, and functions to keep the output voltage constant. R4 is a resistor for stabilizing the oscillation operation.

Next, the operation of the present embodiment configured as described above will be described with reference to FIG. 2 which shows the waveform of each part.

When a voltage is applied to the gate of the switching transistor Q1 through the bias resistor R2, and the gate voltage reaches a threshold voltage at which a drain current flows, the transistor Q1 turns on and a drain current flows. When the drain current flows, the gate voltage is applied from the gate winding NG which is the feedback winding through the capacitor C2 and the resistor R5, and the transistor Q1 is completely turned on. Further, in this state, the gate control transistor Q2 is in a completely off state because the feedback voltage having a phase opposite to that of the gate winding NG is supplied through the capacitor C3. When the transistor Q1 is on, a current I _C (I _C = V _in · t / L) determined by the primary inductance L of the transformer T1 and the voltage V _in of the input linear power supply flows through the transistor Q1, and with time, It will gradually increase.

On the other hand, the gate control transistor Q
Since a charging current flows into the capacitor C3 that applies a negative voltage to the base of the transistor 2 through the resistors R3 and R4 and the light receiving transistor 3-1, the base voltage of the transistor Q2 turns on the voltage V _BE ( ≒ 0.6
V) approaches. Then, when this voltage is reached, the transistor Q2 is turned on. As a result, the gate voltage of the transistor Q1 drops, Q1 is turned off, and the current flowing through the primary winding N1 of the transformer T1 sharply decreases. In this state, the base winding NB of the feedback winding completely turns on the transistor Q2, and the output voltage of the gate winding NG also becomes negative, so that the transistor Q1 is completely turned off at high speed. When the transistor Q2 turns on, the capacitor C3 discharges the electric charge accumulated through the resistors R3 and R4.

The counter electromotive voltage (flyback voltage) generated in the secondary winding N2 during this off period is rectified by the diode CR22, smoothed by the capacitor C22, and a DC voltage is output to the output end.

The output voltage supplied to the load is the resistance R2.
Variable shunt regulator U divided by 6 and R27
Added to 1. This variable shunt regulator U1
Controls the current flowing in the light emitting diode 3-2 according to the applied voltage to control the charging current of the capacitor C3 flowing in the light receiving transistor 3-1. The output voltage is stabilized as described above.

CR2 is a Zener diode,
In addition to limiting the gate voltage applied to the transistor Q1, during the off period when a negative voltage is applied to the gate, it operates as a normal diode and keeps the gate potential constant. Therefore, when the next ON period starts, the voltage of the capacitor C2 charged in the OFF period is added to the voltage of the gate winding NG and output to the gate. Thus, when the on period starts, the gate winding NG is almost at the peak −
Since the peak value is output, the voltage utilization rate of the gate winding NG is increased and the allowable range of the input DC power supply voltage can be widened.

As described above, according to the present embodiment, the switching transistor has a MOSFE excellent in switching characteristics.
Since T is used, the switching speed can be increased,
Switching loss can be reduced. Further, the oscillation frequency can be increased to improve the utilization efficiency of the transformer, and the energy stored in the transformer at the time of turning on can be reduced, which is a drawback of the on-off type self-excited DC-DC converter. It is possible to considerably reduce the instability caused by intermittent oscillation when the load is light (no load).

Further, unlike the conventional example using the NPN or PNP junction type transistor, the saturation phenomenon of the collector current defined by the product of the base current and the current amplification factor is not used at the time of conversion from the ON period to the OFF period. Since the gate control transistor Q2 is turned on and the switching transistor Q1 is turned off by the voltage change when discharging the electric charge charged in the capacitor, there is no influence of the variation of the current amplification factor of the switching transistor unlike the conventional example. Therefore, it is possible to obtain stable operation without variation.

Since a voltage having a phase opposite to that of the gate of the transistor Q1 is applied to the base of the gate control transistor Q2, the rising and falling edges of the gate signal become steep and the switching speed of the MOSFET is increased to increase the switching loss. Can be made smaller.

FIG. 3 is a circuit diagram showing the configuration of the second embodiment of the present invention, and FIG. 4 is a waveform diagram showing the waveform of each part.
In FIG. 3, the same reference numerals as those in FIG. 1 indicate the same elements, and R8 is a resistor and CR4 is a Zener diode. The difference between the first embodiment shown in FIG. 1 and this embodiment is that instead of the voltage generated in the base winding NB, a resistor R8 is connected in series to the gate winding NG and both ends of this resistor R8 are connected. That is, the generated voltage is applied to the base of the transistor Q2. Zener diode CR
4 limits the voltage applied across the resistor R8 to limit the base applied voltage.

The operation of this embodiment will be described below with reference to FIGS.

FIG. 5 is an explanatory diagram for explaining the operation of this embodiment, and FIG. 6 is a schematic diagram showing the states of the voltage of the gate winding NG, the gate voltage of the transistor Q1 and the base applied voltage of the transistor Q2. It is a figure.

The direction of the current flowing through the gate winding NG changes depending on whether the switching transistor Q1 is turned on or off. That is, since the current i ₂ flows during the ON period of the transistor Q1, the base-emitter of the gate control transistor Q2 is reverse-biased by the Zener voltage, and the transistor Q2 maintains the OFF state. Further, during the off period of the switching transistor Q1, the current i ₁
Flow through the gate control transistor Q2
The voltage across the Zener diode CR4 and the voltage of the capacitor C3 are added and applied to the base of the gate of the gate control transistor Q2 to keep the gate control transistor Q2 in the ON state.

The other operation of this embodiment is the same as that of the first embodiment, and therefore its explanation is omitted.

FIG. 7 is a circuit diagram showing the configuration of the third embodiment of the present invention, and FIG. 8 is a waveform diagram showing the waveform of each part.

The difference between the second embodiment described above and this embodiment is that the gate control transistor portion of the second embodiment is constructed by a circuit in which two transistors are connected in a thyristor. That is, the gate control is performed by a circuit composed of two transistors Q2 and Q3 connected in thyristor which connect the bases of the NPN junction type transistor and the PNP junction type transistor to the other collector.

With this configuration, the time required for the transistor Q2 to transition from OFF to ON is extremely shortened, and the switching speed of the switching transistor Q1 can be further shortened. In this case, if the direction of the Zener diode CR4 is set as shown in FIG. 7, this switching operation can be performed more reliably as compared with the second embodiment. Other operations are similar to those of the second embodiment.

FIG. 9 shows a fourth embodiment which is an improvement of the third embodiment. The difference from the third embodiment is that the positions of the capacitor C2 and the resistor R5 are changed so that the capacitor C2 is connected via the diode CR5. That is, the voltage is applied to the resistor R3.

[0047] With such a configuration, when the voltage the voltage V _in of the input power is induced in the gate winding NG rises, the higher the voltage applied to the resistor R3 along with the change for, it is possible to follow the charging time of the capacitor C3 to a voltage V _in the power supply. That is, when the voltage V _in of the power supply is high, can be made to follow thereto to shorten the ON period of the switching transistor Q1,
The range of power supply voltage that can be used can be expanded. Other operations are similar to those of the third embodiment.

[0048]

As described above, according to the present invention, the current flowing through the primary winding of the transformer can be switched at high speed, the loss during switching is small, the utilization factor of the transformer is high, and It is possible to obtain a DC-DC converter that operates stably even under a light load.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a configuration of a first exemplary embodiment of the present invention.

FIG. 2 is a waveform diagram showing a waveform of each part of the first embodiment.

FIG. 3 is a circuit diagram showing a configuration of a second exemplary embodiment of the present invention.

FIG. 4 is a waveform diagram showing a waveform of each part of the second embodiment.

FIG. 5 is an explanatory diagram for explaining the operation of the second embodiment.

FIG. 6 is an explanatory diagram for explaining the operation of the second embodiment.

FIG. 7 is a circuit diagram showing a configuration of a third exemplary embodiment of the present invention.

FIG. 8 is a waveform diagram showing waveforms at various portions of the third embodiment.

FIG. 9 is a circuit diagram showing a configuration of a fourth exemplary embodiment of the present invention.

FIG. 10 is a circuit diagram showing a configuration of a conventional device.

[Explanation of symbols]

 1, 2 Power supply terminal 3 Photo coupler 3-1 Light receiving transistor 3-2 Light emitting diode 24, 25 Output terminal C1, C2, C3, C22 Capacitor CR1 to CR5 Diode N1 Primary winding N2 Secondary winding NB Base winding NG Gate winding Q1 Field effect transistor Q2, Q3 Transistors R1 to R5, R8, R9, R25 to R27 Resistor T1 Transformer U1 Shunt regulator

Claims (5)

[Claims] 1. A DC-DC converter for rectifying and smoothing a voltage induced in a secondary winding by supplying a current from a DC power supply intermittently to a primary winding of a transformer and supplying the rectified voltage to a load. A field effect transistor connected in series with the primary winding with respect to the DC power supply and having a gate connected to a positive electrode of the DC power supply via a resistor; and a field effect transistor provided between the negative electrode of the DC power supply and the gate. A switching means that is in a conductive state when the voltage input to the control terminal is equal to or higher than a predetermined value; an output voltage detecting means that detects a voltage supplied to the load; a feedback winding provided on the transformer; Connected to the gate of the field effect transistor,
A first capacitor having the other end connected to one end of the feedback winding, and a second capacitor having one end connected to the control end of the switching means and the other end connected to the other end of the feedback winding. Is provided between the one end of the second capacitor and the one end of the first capacitor, and determines the amount of current flowing from the first capacitor to the second capacitor according to the output of the output voltage detection means. A switching control unit having a current control element for controlling; and a negative-phase voltage supply unit for supplying the second capacitor with a voltage having a reverse phase to the voltage generated at the one end of the feedback winding. A DC-DC converter characterized by: 2. The DC-DC converter according to claim 1, wherein the negative-phase voltage supply means is a connection line that connects a point between both ends of the feedback winding to a negative electrode of the DC power supply. . 3. The anti-phase voltage supply means is a resistor interposed between the other end of the feedback winding and the negative electrode of the DC power supply. DC-D
C converter. 4. The thyristor-connected PNP connecting the one base to the other collector of the switching means.
A DC-DC converter according to claim 1, wherein the DC-DC converter comprises a N-type transistor and a NPN-type transistor. 5. The switching control means includes the first
A series circuit including a first resistor and a constant voltage element provided between the one end of the capacitor and the negative electrode of the DC power supply, and between the one end of the first capacitor and the one end of the second capacitor. And a second resistor inserted in the first resistor, and the one end of the first capacitor is connected to the current control element and the gate via the first resistor. The DC according to claim 1,
-DC converter.