JPH07135769A - Dc resonance converter - Google Patents

Abstract

PURPOSE:To avoid an unstable operation caused by the decline of an operation frequency over a whole load range by a method wherein a frequency control is performed in a heavy load range and a PWM control is performed in a light load range. CONSTITUTION:When a load resistance 28 is small, a voltage is controlled by a frequency control performed by switching. A resonance current is made to flow by the on-off operations of main switching devices 11-14. The resonance current is carried by transformers 19 and 20 and converted into the secondary currents of the respective transformers 19 and 20 which are carried by the load resistance 28 as a common load current. If the load resistance 28 is increased, the switching operation frequency is declined. When the frequency becomes close to an audio frequency range, the control is switched to a PWM control. That is, while the resonance current is carried continuously, auxiliary switching devices 29-32 are turned on to apply the resonance current to the primary windings of the transformers 19 and 20 for required periods. As a result, the on-periods are controlled and the voltage control by PWM is performed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a series resonant converter applied to a switching power supply or the like.

[0002]

2. Description of the Related Art FIG. 3 shows an example of a circuit configuration of a conventional series resonance converter. For example, two main switching elements 11 and 12 such as MOSFETs are connected in series to form a set of bridge circuits, and two main switching elements 1 are separately provided.
3, 14 are connected in series to form another set of bridge circuits, and each bridge circuit is connected to the DC power supply 10 to form a so-called full bridge circuit.

One ends of the primary windings of the transformers 19 and 20 are connected between the connection points of the main switching elements 11 and 12 and the connection points of the main switching elements 13 and 14, respectively.
A resonance inductance 21 is provided between the other ends of the transformers 19 and 20.
Is connected to the resonance capacitor 22.

At both ends of the resonance inductance 21 and the resonance capacitor 22, corresponding to two sets of bridge circuits, energy is fed back to the DC power supply 10 while the main switching elements 11, 12 and 13, 14 are OFF. A series circuit of the feedback diodes 15 and 16 and the feedback diode 17,
A series circuit of 18 is connected to the DC power supply 10.

Further, rectifying diodes 23, 24 and 25, 26 are respectively constructed as full-wave rectifying circuits in the secondary windings of the transformers 19, 20, each output is connected in parallel, and a smoothing capacitor 27 and a load resistor are connected. 28 is connected.

The prior art series resonant converter operates as follows. When the main switching element 11 and the main switching element 14 are turned on, the DC power supply 10-the main switching element 11-the primary winding of the transformer 19-resonance inductance 21-resonance capacitor 22-the primary winding of the transformer 20-the main switching element 14- A resonance current I ₁ as shown by the dotted line in FIG. 3 flows in the loop of the DC power supply 10.

The resonance current I ₁ continues to flow during the ON width period T _ON as shown in FIG. The resonance current I ₁ is applied to the transformer 1
Assuming that the ratio of the primary and secondary windings of 9 and 20 is N, a resonant current of 1 / N times flows in the secondary winding of each transformer, so this resonant current is used for rectification. Rectification is performed by the diodes 23, 24 and 25, 26, the smoothing capacitor 27 is charged, and a DC voltage is supplied to the load resistor 28.

In addition, OF of the main switching elements 11 and 14
In the period F, the charge charged in the resonance capacitor 22 with the illustrated polarity becomes the feedback current I ₂ shown by the dotted line in FIG. 3 and is fed back to the power supply, and the current is the feedback current period T in FIG. _Flow for ₁ period.

In this system, the main switching element 11,
The ON width of 14 is constant, and the OFF width, that is, the period of the pause period T _{2 in} FIG. 4 is varied to perform constant voltage control of the output. Therefore, when the load is light, the OFF period T
_{In the OFF state} , the quiescent period T ₂ is lengthened, the operating frequency is lowered, and then the audible frequency region around 20 KHZ is reached.

In the above, the half cycle operation is explained, but in the next half cycle, the main switching element 12,
When 13 is turned on, the same operation as described above is performed.

[0011]

However, the conventional technique has the following problems. That is, when the load is heavy, the ON width period, that is, T _ON is constant, so
In the FF width period, that is, T _{OF F,} the idle period T ₂ is short, and therefore stable operation is performed at an operating frequency of 30 to 60 KHZ. However, when the load becomes lighter, T _ON is constant, and therefore the period T ₂ of the pause period of T _{OF F} becomes longer. Therefore, the operating frequency falls to about 20 KHZ and the operation is performed in the audible frequency range. .

In this frequency range, the oscillating sound during operation becomes harsh and causes a noise problem. In addition, the OFF width period (T _OFF ) becomes infinitely long as the load is reduced, and the operation becomes unstable.

Therefore, in order to overcome the problems of the prior art as described above, the object of the present invention is as follows. That is, while maintaining the characteristic that the noise due to the surge voltage and current of the series resonant converter is small,
ON width period control, that is, PWM (PULSE WIDTH MODULA
TION) Control operation is also used to perform operation so that the OFF width period does not become longer than necessary. In this way, it is an object to provide a series resonance converter in which the operating frequency does not drop to the audible frequency range.

[0014]

That is, according to the present invention, two sets of bridge circuits, in which two main switching elements are connected in series, are connected to a DC power source, and two series main switching elements of the two sets of bridge circuits are connected. One end of the primary winding of each transformer is connected to the connection point of, and a series circuit of a resonance inductance and a resonance capacitor is connected between the other ends of the primary windings of the transformers. Furthermore, from the both ends of the series circuit of resonance inductance and resonance capacitor, corresponding to the two series main switching elements for each of the two sets of bridge circuits,
A series circuit of two feedback diodes is connected to two sets of DC power sources in the direction of returning energy to the power source.
A full-wave rectifier circuit and a smoothing capacitor are connected to the secondary winding of each transformer, and a series resonant converter is configured to supply power to the load.

Then, two sets of 2 of this series resonance converter are used.
When each auxiliary switching element in the forward direction is connected in parallel to the power source and the operating frequency of the main switching element is lower than the audible frequency range, 2 is connected to each of the series feedback diodes.
It is characterized in that the two auxiliary switching elements of the set are configured to operate so as to perform conduction width control (PWM) within the conduction period of each main switching element.

[0016]

The series resonant converter having such means is
When the load is heavy, the OFF width period control is performed in exactly the same way as the conventional circuit, but when the load decreases and the operating frequency drops to near the audible frequency, it switches to the ON width period control (PWM control) and becomes audible. Control so that the frequency does not drop.

That is, when the load becomes light, the auxiliary switching element corresponding to each main switching element becomes conductive within the conduction period of the main switching element, and the transformer 1
The ON width period control is performed by diverting the resonance current that has been flowing as the secondary current to the auxiliary switching element so that the transformer primary current does not flow.

[0018]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows an embodiment of the present invention, in which parts corresponding to those of FIG. Further, FIG. 2 shows a current waveform of a main part according to the present invention.
In the present invention, auxiliary switching elements 29, 30, 31, 32 are added to the prior art so that the respective feedback diodes 15, 16,
17 and 18 are connected in parallel.

The operation of the series resonant converter of the present invention is as follows. When the load is heavy, that is, when the load resistance 28 is small, as described in the prior art, the main switching elements 11 and 14 are turned on, so that the resonance current I as shown by the dotted line in FIG.
Flow ₁ This resonance current I ₁ flows through the transformers 19 and 20 during the half cycle period as in FIG.
It is converted to the next winding and flows as a load current through the load resistor 28.

After half a cycle, the resonance capacitor 22
The stored charge flows as the feedback current I ₂ and then the rest period T ₂ is continued, which is exactly the same as the waveform of FIG. 4 of the prior art.

Next, when the load becomes light and the load resistance 28 becomes large, by constant voltage control, within the OFF width period T _OFF ,
The quiescent period T ₂ becomes longer and the operating frequency lowers. Then, when it approaches the audible frequency region of about 20 KHZ, it is switched to constant voltage control by ON width period control, that is, PWM (PULSE WIDTH MODULATION) control.

The operation after the PWM control is the portion related to the present invention. That is, when approaching the audible frequency range,
PWM control is performed by the following operation. When the main switching elements 11 and 14 are turned on and the auxiliary switching elements 29 and 32 are turned on while the transformer primary current I ₁ which is a resonance current is flowing as shown by the dotted line in FIG. Since the primary winding voltage due to the transformer primary current I ₁ is generated in the primary winding of the transformer 20, the resonance current (transformer primary current) I ₁ is commutated from the main switching element to the auxiliary switching element side.

That is, as shown in FIG. 2, the transformer primary current I ₁
When the auxiliary switching elements 29 and 32 are turned on during the PWM ON width period T _A , the transformer 1
Next current I ₁ is DC power supply 10-auxiliary switching element 29
-Resonance inductance 21-Resonance capacitor 22-Auxiliary switching element 32-Commuted into the loop of the DC power supply 10 and switched to the bypass resonance current I ₁ 'as shown by the alternate long and short dash line in FIG.

This bypass resonance current I ₁ 'is applied to the transformer 1
As it does not flow in the primary winding of 9 and 20, PWM as shown in Fig. 2
Since the transformer primary current I ₁ becomes zero during the OFF width period T _B and the bypass resonance current I ₁ ′ flows through the bypass loop, the resonance current continues to flow continuously for a half cycle.

Therefore, while the resonance current continues to flow continuously, the auxiliary switching element is turned on in the primary windings of the transformer 19 and the transformer 20, so that the resonance current (transformer primary current) I _{1 is} maintained only for a necessary period. As a result, the ON width period T _ON is controlled and constant voltage control is performed.

After the resonance current has finished flowing, the charge stored in the resonance capacitor 22 becomes the feedback current I ₂ ,
The mode in which the power is fed back to the power supply during the feedback current period T ₁ and the mode during the idle period T ₂ are exactly the same as those in the conventional technique.

Therefore, in FIG. 2, O when the load is heavy
Since the N width period is T _A + T _B , the operation duty (DUT
Y) is (T _A + T _B ) / T, and the ON duty period at light load is T _A, so the duty of operation (DUTY) is T _A / T. In this way, frequency control is performed under heavy load, and PWM control is performed under light load, so that constant voltage control can always be performed stably.

Further, although not shown, a leakage inductance specific to the transformer 19 and the transformer 20, or a resonance inductance 21 separately provided in series with the transformer, or a new resonance inductance 21 provided in series from the resonance inductance 21 to the feedback circuit is provided. , It is possible to prevent troubles during commutation. For example, when commutating from the resonance current I ₁ to the feedback current I ₂ , short circuit current is prevented from occurring during the recovery time of the feedback diode, or switching loss occurs due to switching for a certain time at each commutation. And switching noise can be prevented by providing the above-mentioned inductance.

[0029]

Since the present invention has the structure as described above, it has the following effects. That is, by using the frequency control at the time of heavy load and the PWM control at the time of light load together, it is possible to prevent the unstable operation due to the lowering of the operating frequency in the entire range from the full load to the no load,
Further, since the operating region at the audible frequency is avoided, it also exhibits the effect of preventing noise. Furthermore, the circuit of the present invention can be realized without losing the characteristics of the conventional series resonant converter that can prevent the generation of switching loss and switching noise due to the abrupt switching of the commutation current, so that a more useful switching power supply can be provided. It is a thing.

[Brief description of drawings]

FIG. 1 is a series resonant converter of the present invention.

FIG. 2 is a main part current waveform of the present invention.

FIG. 3 is a conventional series resonant converter.

FIG. 4 is a conventional main part current waveform.

[Explanation of symbols]

10 DC power supply 11, 12, 13, 14 Main switching element 15, 16, 17, 18 Feedback diode 19, 20 Transformer 21 Resonance inductance 22 Resonance capacitor 23, 24, 25, 26 Rectification diode 27 Smoothing capacitor 28 Load resistance 29, 30, 31 and 32 the auxiliary switching element I ₁ transformer primary current (resonance current) I ₁ 'bypass resonance current I ₂ feedback current T 1 cycle period T _ON ON width period T _OFF OFF width period T _A PWM ON width period T _B PWM OFF width period T ₁ Feedback current period T ₂ Pause period

Claims (1)

[Claims] 1. Two sets of bridge circuits, in each of which two main switching elements are connected in series, are connected to a DC power supply, and transformers are respectively provided at connection points of the two series main switching elements of the two sets of bridge circuits. One end of the primary winding of is connected,
A series circuit of a resonance inductance and a resonance capacitor is connected between the other ends of the primary windings of the respective transformers. A series circuit of two feedback diodes is connected to two sets of DC power sources in the direction of returning energy to the power source corresponding to the switching elements, and a full-wave rectification circuit is connected to the secondary winding of each transformer. In a series resonance converter to which a smoothing capacitor is connected and which supplies power to a load, a forward auxiliary switching element is connected in parallel to a power source to each of the two sets of two series feedback diodes, and the main switching element When the operating frequency is lower than the audible frequency range, the two sets of two auxiliary switching elements are electrically connected to the respective main switching elements. Series resonant converter, characterized in that configured so as to operate as performing the conduction width control (PWM) in between.