JPH05300735A - Voltage-resonant switching power supply - Google Patents

Abstract

PURPOSE:To make zero-voltage switching of a main switching element possible over the wide range of input voltage to contrive to improve a power efficiency by connecting the reset winding of a saturable reactor between the output ends of rectifier circuit and smoothing circuit and by supplying a reset current by means of a constant voltage determined by output voltage. CONSTITUTION:In a voltage-resonant switching power supply using a saturable reactor SR, the positive electrode side of the reset winding Lr of the saturable reactor SR is connected with the butt terminal of diodes D1 and D2 so that a reset current is supplied. A reset current-supply circuit is constituted from a diode D4 and current-limiting resistor R4 and the anode side of the diode D4 is connected with a smoothing capacitor Co. Thus, a reset voltage is maintained almost at a constant value corresponding to output voltage regardless of input voltage so that the reset current is constant even if the input voltage increases and therefore zero-voltage switching is made possible over the wide range of input voltage.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a voltage resonance type switching power supply, and more particularly to improvement of a structure in which a main switch performs zero voltage switching by a saturable inductor.

[0002]

2. Description of the Related Art A voltage resonance type switching power supply using a saturable inductor is known as, for example, "CONSTANT FREQUENCY, FORWAR".
D CONVERTER WITH RESONANT TRANSITION "HFPC ・ JUNE 19
91 PROCEEDINGS P.282. FIG. 7 is a circuit diagram of the conventional device disclosed in this document. In the figure, a DC voltage supplied from a DC power source Vin is converted into a DC output of a voltage Vo by a converter unit having a transformer T. As a feedback circuit for stabilizing the output voltage, an error amplifying section 20 for calculating an error voltage by comparing the output voltage Vo with a predetermined reference voltage, and a photo isolation circuit for insulating the error signal between the primary side and the secondary side of the transformer T. It has a coupler 30 and a PWM (pulse width) control circuit 40 for sending a control signal in the direction of reducing the error signal sent to the primary side to the switching element of the converter section.

The converter section includes a primary winding n of the transformer T.
1 is applied with a DC voltage Vin and is turned on / off by the main switching element Q1. A reset circuit including a snubber capacitor Cs and an auxiliary switching element Q2 is connected in parallel with the main switching element Q1. The main switching element Q1 and the auxiliary switching element Q2 are turned on / off in a complementary manner (complementary logic s).
(witch), there is a dead time between the two switching. Since the drive potentials of the two are different, the control signal from the PWM control circuit 40 sent to the auxiliary switching element Q2 is DC-insulated by the insulating transformer Tr.

Since a switching signal is induced in the secondary winding n2 of the transformer T, it is rectified by a rectifying circuit. Here, the forward diode D is provided on the positive side of the secondary winding n2.
1 is connected to the flywheel diode D on the common side
2 are connected and the cathode sides of these diodes D1, D2 are butted. The smoothing circuit is a filter circuit that smoothes the current rectified by the rectifying circuit, and the choke coil L1 removes high-frequency components to form a capacitor C.
It is smoothed by o, and the DC output voltage Vo is supplied to the load side. The saturable reactor SR is mounted between the secondary winding n2 and the anode terminal of the diode D1.
The reset winding Lr is for resetting the saturable reactor SR, and has one end with the secondary winding n2 and the saturable reactor SR.
And the other end is connected to the common side of the secondary winding n2 via the diode D3 and the resistor R1.

FIG. 8 is a diagram showing the relationship between the reset voltage V _B of the saturable reactor SR and the input voltage Vin. The reset winding L2 uses the flyback voltage V _B of the transformer T to reset the core of the saturable reactor SR.
On the other hand, since the flyback voltage V _B is inversely proportional to the input voltage Vin, the reset voltage V _B also decreases in inverse proportion to the input voltage Vin. However, the voltage Vsr applied when the saturable reactor SR is saturated increases in proportion to the input voltage Vin. Therefore, when the input voltage is low, the saturable reactor S
Although R is sufficiently reset, the reset voltage becomes insufficient at a high input voltage and the reset becomes insufficient.

[0006]

According to the conventional circuit, the saturable reactor S by the reset winding Lr at high input voltage is used.
As a result of not being able to reset R, there is a problem in that zero voltage switching cannot be performed and the efficiency of the power supply at a high input voltage decreases. Further, when the input voltage is low, the reset amount of the saturable reactor is too large and the duty ratio is widened, so that there is a problem that the input voltage range cannot be widened from the viewpoint of controllability, stress of the element, and the like.

The present invention has been made to solve the above problems, and an object of the present invention is to provide a voltage resonance type switching power supply capable of performing zero voltage switching of a main switching element over a wide input voltage range and having high power supply efficiency.

[0008]

SUMMARY OF THE INVENTION The present invention which achieves the above object includes a transformer T having a primary winding to which a direct current is supplied, as a converter unit, and a main unit for turning on / off a current flowing through the primary winding. Switching element Q1, snubber capacitor Cs and auxiliary switching element Q2 connected in parallel with the main switching element or primary winding.
A reset circuit, a rectifier circuit that rectifies a switching signal induced in the secondary winding of the transformer, a smoothing circuit that smoothes the current rectified by the rectifier circuit and outputs a direct current of a predetermined voltage, It has a saturable reactor SR mounted between this secondary winding and the rectifier circuit.

Further, as an output voltage stabilizing circuit, an error amplifier circuit for inputting the output voltage of the smoothing circuit and comparing it with a reference voltage and outputting an error signal, and an on / off control signal for decreasing the error signal are provided. In a voltage resonance type switching power supply having a pulse width control circuit which supplies an on / off control signal complementary to this on / off control signal to an auxiliary switching element while being supplied to a main switching element,
It has the following configuration.

That is, a reset winding L wound around the saturable reactor, one end of which is connected to the output end of the rectifier circuit.
r, and a circuit that is mounted between the other end of the reset winding and the output end of the smoothing circuit and that supplies a reset current to the reset winding.

In addition, instead of the reset current supply circuit,
A beak voltage holding circuit that holds the peak voltage of the rectifier circuit and a circuit that supplies the holding voltage of the peak voltage holding circuit to the reset winding may be used.

[0012]

If the auxiliary switching element is turned on while the main switching element is turned off and the parasitic diode of the auxiliary switching element is turned on, zero voltage switching is realized and no turn-on loss occurs. Next, when the auxiliary switching element is turned off, resonance occurs due to the exciting inductance of the transformer and the output capacitance of the main switching element, the voltage across the main switching element is reduced to zero, and the main switching element is turned on. At this time, the saturable reactor blocks the flow of current from the secondary winding in the rectifier circuit until the main switching element is turned on, thereby realizing zero voltage switching and preventing loss due to turn-on. The reset winding and the reset current supply circuit send the reset current to the saturable reactor to secure the damming time regardless of the input voltage.

[0013]

The present invention will be described in detail below with reference to the drawings. FIG. 1 is a circuit diagram showing an embodiment of the first invention.
In FIG. 1, components having the same functions as those in FIG. 7 are designated by the same reference numerals and the description thereof will be omitted. In the figure, the positive side of the reset winding Lr is connected to the abutting terminals of the diodes D1 and D2, and the reset current is sent from the common side. The reset current supply circuit includes a diode D4 and a current limiting resistor R4, and the anode side of the diode D4 is connected to the capacitor Co.

The operation of the thus constructed apparatus will be described below. 2A and 2B are explanatory diagrams of resonance waveforms. FIG. 2A is a gate-source voltage Vgs of the main switching element Q1, and FIG. 2B is a gate-source voltage Vgs of the auxiliary switching element Q2.
(C) is the drain-source voltage Vds of the main switching element Q1, (D) is the current _IQ1 flowing between the drain and source of the main switching element _Q1 , (E) is the current _IQ2 flowing through the auxiliary switching element Q2, and (F). Is the magnetic flux Φ of the transformer T, (G) is the switching voltage V _T applied to the primary winding of the transformer, (H) is the voltage V _D2 generated in the flywheel diode D2, and (I) is the voltage V _D1 generated in the forward diode D1. , And the flyback voltage V _B, and (J) is the voltage Vsr applied to the saturable reactor.

In the figure, time t0 is the main switching element Q1.
Off time, time t1 on the time of the auxiliary switching element Q2, time t2 current time I _Q2 is inverted from positive to negative, the time t3 off time of the auxiliary switching element Q2 of
Time t4 is the time when the voltage Vds becomes zero, time t5 is the on time of the main switching element Q1, and time t6 is the time when the current _IQ1 starts to flow. The period td is a time delay from the time t3 until the diode D2 is turned on, and the saturable reactor S
This corresponds to the time required to saturate R. Further, regarding the value of the ON voltage, the voltage Vds is {Vin ² / (Vin-nV
o)}. The voltage V _T is Vin on the positive side and {nVo on the negative side.
/ (Vin-nVo)} · Vin. The voltage V _D2 is V
in / n. Finally, the voltage V _D1 is Vo / (Vin-nV
o) ・ Vin. Here, n is the turn ratio n1 / n2 of the primary winding and the secondary winding.

The main switching element Q1 and the auxiliary switching element Q2 are alternately turned on and off with a dead time in between. When the main switching element Q1 is turned off at time t0, the exciting current flowing through the main switching element Q1 flows through the capacitor Cs and the parasitic diode of the auxiliary switching element Q2. If the auxiliary switching element Q2 is turned on at any time t1 during the period when the parasitic diode is on (time t0 to t2), the auxiliary switching element Q2
Zero voltage switching is realized at and no turn-on loss occurs. At time t2, the exciting current of the transformer commutates,
It is regenerated to the input side through the auxiliary switching element Q2. When the auxiliary switching element Q2 is turned off at time t3, the exciting current moves to the main switching element Q1 again,
The electric charge stored in the output capacitance of the main switching element Q1 is extracted to reduce the voltage across the main switching element Q1 to zero. At times t3 to t4, resonance occurs due to the exciting inductance of the transformer and the output capacitance of the main switching element Q1. Here, in order to bring the voltage Vds to zero, a certain amount of exciting current is required, so the main inductance of the transformer T is selected to be smaller than that of a transformer used in a normal forward power supply.

The saturable reactor SR has a function of blocking the load current from the time t3 to t6 so that the main switching element Q1 can be turned on at zero voltage by ensuring a sufficient time for resonance. If the saturable reactor SR does not exist, the diode D1 is turned on and the load current starts flowing before the voltage Vds becomes zero.
Zero voltage switching becomes impossible. From time t3 to t6, saturable reactor SR is in an unsaturated state, and time t6
After that, it saturates and the load current flows. Since the magnetic flux of the transformer is excited in the positive and negative directions by the action of the capacitor Cs and the auxiliary switching element Q2, it is not necessary to provide the reset winding Lr if the input voltage is constant on the rating.

Next, the reset current supply circuit will be described. The saturable reactor SR is saturated in a shorter time as the voltage applied to the winding is higher. After the auxiliary switching element Q2 is turned off, the voltage applied to the saturable reactor SR is Vin / n, which is proportional to the input voltage. FIG. 3 is an operation explanatory diagram of the saturable reactor on the BH curve. After being reset with a certain reset current I _R , the voltage Vi
When n / n is applied, the saturable reactor is next time t
d Delay and saturate. td = n _SR · Δφ / (Vin / n) (1) where n _SR is the number of turns of the saturable reactor. This delay time td corresponds to the times t3 to t6 in FIG. In the conventional circuit, as the input voltage increases, the reset current I _R decreases and the magnetic flux change amount Δφ also decreases, so that the delay time td at a high input voltage cannot be sufficiently taken as is apparent from the equation (1).

FIG. 4 is an operation explanatory diagram of the reset current supply circuit of FIG. The reset voltage is held at a substantially constant value corresponding to the output voltage regardless of the input voltage. Therefore, since the reset current I _R is constant even if the input voltage is increased, the delay time td at a high input voltage can be taken sufficiently as compared with the conventional example. Therefore, zero voltage switching can be performed over a wide input voltage range.

FIG. 5 is a circuit diagram of an embodiment of the second invention. A peak voltage holding circuit and a reset current supply circuit are provided as circuits for supplying a reset current to the reset winding Lr. In the peak voltage holding circuit, the anode terminal of the diode D5 is connected to the abutting point of the diodes D1 and D2, and the cathode terminal is connected to the capacitor C5, and the peak voltage of the rectified pulsating current is held in the capacitor C5. In the reset current supply circuit, the anode terminal of the diode D6 is connected to the capacitor C5, and the cathode terminal is connected to the reset winding Lr via the current limiting resistor R6.

In the circuit thus constructed, the peak voltage holding circuit stores the voltage Vin / n proportional to the input voltage. Therefore, since the reset amount of the saturable reactor SR is proportional to the input voltage, the reset current I _R increases and the magnetic flux change amount Δφ also increases as the input voltage increases, so that the delay time td at a high input voltage becomes sufficient. Therefore, the zero voltage switching can be performed in a wider input voltage range as compared with the embodiment of FIG.

FIG. 6 is a circuit diagram of a modified embodiment of the present invention. Compared with the circuit diagram of FIG. 1, the reset circuit is different in that it is connected in parallel with the primary winding n1 instead of the main switching element Q1. Even if the connection is made in this way, the same operation as the circuit of FIG. 1 is performed.

[0023]

As described above, according to the first aspect of the invention, since the reset current is sent at a constant voltage determined by the output voltage, the delay time of the saturable reactor can be sufficiently secured even if the input voltage is high, Since the zero voltage switching is possible, the efficiency of the power supply is increased. Further, since the number of parts is small, it is possible to realize at low cost.
Furthermore, according to the second aspect of the invention, since the reset current is increased in proportion to the input voltage, a power supply with high efficiency can be obtained for a wider range of input voltage.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of a first invention.

FIG. 2 is an explanatory diagram of a resonance waveform.

FIG. 3 is an operation explanatory diagram of a saturable reactor on a BH curve.

FIG. 4 is an operation explanatory diagram of the reset current supply circuit of FIG.

FIG. 5 is a circuit diagram of an embodiment of the second invention.

FIG. 6 is a circuit diagram of a modified example of the present invention.

FIG. 7 is a circuit diagram of a conventional device.

FIG. 8 is a relationship diagram between a reset voltage and an input voltage of a conventional device.

[Explanation of symbols]

 20 Error amplification circuit 30 Photocoupler 40 PWM control circuit Q1 Main switching element Q2 Auxiliary switching element SR Saturable reactor

Claims (2)

[Claims] 1. A transformer T having a primary winding to which a direct current is supplied, a main switching element Q1 for turning on / off a current flowing through the primary winding, and a main switching element or a primary winding connected in parallel. A reset circuit having a snubber capacitor Cs and an auxiliary switching element Q2, a rectifier circuit that rectifies a switching signal induced in the secondary winding of the transformer, and a current rectified by the rectifier circuit is smoothed to obtain a predetermined voltage. A smoothing circuit that outputs a direct current, a saturable reactor SR mounted between this secondary winding and a rectifier circuit, and the output voltage of this smoothing circuit are input, and an error signal is output by comparing with the reference voltage. And an on / off control signal for reducing the error signal to the main switching element. In a voltage resonance type switching power supply having a pulse width control circuit for supplying a complementary on / off signal to an auxiliary switching element, a reset winding wound around the saturable reactor, one end of which is connected to an output end of the rectifier circuit. A voltage resonance type switching power supply, comprising: Lr, and a circuit which is mounted between the other end of the reset winding and the output end of the smoothing circuit and which supplies a reset current to the reset winding. .. 2. A beak voltage holding circuit for holding the peak voltage of the rectifier circuit in place of the reset current supply circuit according to claim 1, and a circuit for supplying the holding voltage of the peak voltage holding circuit to the reset winding. A voltage resonance type switching power supply characterized by comprising: