JPH0783592B2 - Series resonant converter - Google Patents

Description

The present invention relates to a series resonant converter applied to, for example, a switching power supply device.

[Conventional technology]

A circuit that converts a DC voltage into a DC voltage with a different polarity or a DC voltage with a different voltage value is small and lightweight.
DC-DC converters are used with the aim of improving efficiency. In the DC-DC converter, it is possible to reduce the size and weight of the transformer and output filter by increasing the conversion frequency.

Currently, the most frequently used circuit type of the DC-DC converter is a forward type or flyback type in which the current flowing through the switching element is forcibly cut off by turning off the switching element. These circuit types have the following problems when increasing the frequency.

(A) Since a spike-shaped current due to the recovery current of the rectifying diode flows when the switching element is turned on, loss and noise at the time of turn-on are large.

(B) Since the load when the switching element is turned off is inductive due to the leakage inductance of the transformer and the inductance component distributed in the circuit, the loss in the switching element and the snubber circuit is large.

Due to these problems, at present, even if MOS-FET is used as a switching element, the conversion frequency is up to 100 W even if the output capacity is about 100 W.
About 500kHz, 100kHz is the limit above 1kW. Furthermore, in order to increase the conversion frequency, there are many items to be considered, such as improving the characteristics of switching elements and rectifying elements, manufacturing transformers with less leakage inductance, mounting components with less stray inductance, and reducing the loss of snubber circuits. For this reason, the DC-DC converter that can reduce the loss in the switching element by making the waveform of the current flowing through the switching element sinusoidal with the capacitor and the inductor has been actively studied. The circuit type of this DC-DC converter is called a series resonance converter. FIG. 3 shows a circuit configuration of a series resonance converter which is generally considered. Two switching elements such as MOS-FET
A series circuit of 11,12, a series circuit of two resonance capacitors 13,14, and a series circuit of two feedback diodes 15,16 are connected to a DC power supply 17, and switching elements 11,12 are connected. A series circuit of the primary winding of the transformer 18 and the resonance inductor is connected between the connection point and the connection point of the resonance capacitors 13 and 14, and the rectifying diodes 21 and 2 are connected to the secondary winding of the transformer 18.
2 is connected to form a full-wave rectifier circuit, and a smoothing capacitor 23 and a load resistor 24 are connected to the output side of the full-wave rectifier circuit.

Next, the operation of this conventional series resonance converter will be described.
When the switching element 11 is turned on, the DC power supply 17
-Switching element 11-Primary winding of transformer 18-Resonance inductor 19-Resonance capacitor 14-DC power supply 17 loop and resonance capacitor 13-Switching element 11-Primary winding of transformer 18-Resonance inductor 19- A resonance current flows through the loop of the resonance capacitor 13. When the switching element 12 is turned on, the DC power supply 17-resonance capacitor 13-resonance inductor 19-transformer 18 primary winding-
A resonance current flows through the loop of the switching element 12-the DC power supply 17 and the resonance capacitor 14-the resonance inductor 19-the primary winding of the transformer 18-the switching element 12-the resonance capacitor 14 loop. If the ratio of the number of primary windings to the number of secondary windings of the transformer 18 is n, the secondary winding of the transformer 18 has n.
Since a double resonance current flows, this resonance current is rectified by the rectification diodes 21 and 22, the smoothing capacitor 23 is charged, and a DC voltage is supplied to the load resistor 24.

When the voltage of the DC power supply 17 is E, the resonance capacitors 13 and 14
If the charging voltage of is less than 3E / 2, no current flows through the feedback diode 15 or 16. This area will be called mode I. In mode I, the output voltage is constant even if the load resistance 24 is changed, and is given by E / (2n). The charging voltage of the resonance capacitors 13 and 14 increases as the load resistance 24 is reduced, and when it reaches 3E / 2, the resonance capacitor 14-resonance inductor 19-transformer 18 primary winding-feedback diode 15
-DC power supply 17-Loop of resonance capacitor 14, or resonance capacitor 13-DC power supply 17-Feedback diode 16-
A feedback current begins to flow in the loop of the primary winding of the transformer 18, the resonance inductor 19 and the resonance capacitor 13. This area is called Mode II. In mode II, the output current is constant even if the load resistance 24 is changed, and is given by 8fC ₀ nE.
However f is the operating frequency, and the resonance frequency is f ₀ f _0>
This is the case for 2f. C ₀ is the capacitance of each of the resonance capacitors 13 and 14. Switching element 11 or 12 in mode I
The resonance current i ₀ flowing in the equation (1) is given by the equation (1) when t = 0 when the switching element is turned on.

i ₀ = [I ₀ sin {t / (2L ₀ C ₀ ) ^1/2 }] / 4nf (2L ₀ C ₀ ) ^1/2 (1) where I ₀ is the current flowing through the load resistor 24. Resonant current i ₀ flowing through switching element 11 or 12 in mode II
Becomes equation (2).

i ₀ = [(nV ₀ + E / 2) sin {t / (2L ₀ C ₀ ) ^1/2 }] / (L ₀ / 2C ₀ ) ^1/2 (2) Feedback current flowing through the feedback diode 15 or 16 i ₁ is given by equation (3) when the time when the feedback current i ₁ starts to flow is T ₀ .

i ₁ = - _{[(- nV 0 + E / 2} ) sin {(t-T 0) / (2L 0 C 0) 1/2} ] _{/ (L 0 / 2C 0)} 1/2 (3) where V ₀ is the output voltage, L0 is the output voltage, L ₀ is the inductance of the resonance inductor 19. FIG. 4 shows current waveforms flowing through the feedback diode 15 or 16 and the switching element 11 or 12 in the mode I and the mode II.

In mode I, as can be seen from equation (1), when the load resistance 24 is reduced, the peak value of the resonance current increases and the load resistance 24 increases.
The voltage of 24 (output voltage) becomes constant. In mode II, when the load resistance 24 decreases, the peak value of the resonance current decreases from the equation (2) and the peak value of the feedback current increases from the equation (3) i ₀
And the average value of i ₁ is constant, the output current is constant. That is, the output voltage decreases.

[Problems to be solved by the invention]

In this series resonance converter, the reverse blocking capability recovery time of the feedback diode becomes a problem as the resonance frequency is increased. In other words, if the switching element turns on before the reverse blocking capability of the feedback diode is restored, a mode in which the DC power supply is short-circuited occurs and there is the problem of increased loss and noise, and in some cases the switching element may be destroyed. There is. Furthermore, when this series resonant converter is used at f ₀ <2f, the switching element is turned on when the feedback current is flowing through the primary winding of the transformer, so a current with a steep rise flows through the switching element. In addition, since a large spike-shaped current flows through the switching element until the reverse blocking function of the feedback diode is restored, problems such as increased loss and noise and increased stress on the switching element occur. For these reasons, in the conventional series resonant converter, the operating frequency must be used at 1/2 or less of the resonant frequency, and there is a problem that the smoothing capacitor becomes large.

The object of the present invention is to connect the feedback diode so that the feedback current can be fed back to the DC power supply without passing through the primary winding of the transformer, so that the current rise of the switching element is steep even when used with f ₀ <2f. Another object of the present invention is to provide a series resonant converter in which a spike-shaped current does not flow even if the reverse blocking function of the feedback diode is not restored.

[Means for solving problems]

According to the present invention, the resonance inductor is divided into two, one of which is inserted in series with the primary winding of the transformer between the connection point of the two switching elements and the connection point of the two feedback diodes, as in the conventional case. The other half of the resonance inductor divided into two is connected between the connection point of the two feedback diodes and the connection point of the two resonance capacitors.

In this way, when the feedback current does not flow in the primary winding of the transformer, and the operating frequency is twice as high as the resonance frequency, even if the reverse blocking function of the feedback diode is not restored, the switching element operates as a switching element. No spike-shaped current flows.

〔Example〕

FIG. 1 shows an embodiment of the present invention, and parts corresponding to those in FIG. 3 are designated by the same reference numerals. According to the present invention, the resonance inductor is divided into resonance inductors 25 and 26, and one of the resonance inductors 25 is the transformer 18 as in the conventional case.
It is connected in series with the primary winding of the switching element 11 and 12 and the connection point of the feedback diodes 15 and 16. The other resonance inductor 26 is a resonance capacitor.
It is connected between the connection point of 13,14 and the connection point of the feedback diodes 15,16.

The operation of the series resonance converter shown in FIG. 1 will be described below. When the switching element 11 is turned on, the DC power supply 17-the switching element 11-the primary winding of the transformer 18-the resonance inductor 25-the resonance inductor 26-the resonance capacitor 14-the DC power supply 17 loop and the resonance capacitor
A resonance current flows through 13-the switching element 11-the primary winding of the transformer 18, the resonance inductor 25, the resonance inductor 26, and the loop of the resonance capacitor 13. Switching element
When the 12 is turned on, the DC power supply 17-resonance capacitor 13-resonance inductor 26-resonance inductor 25-transformer 18 primary winding-switching element 12-DC power supply 17
Loop and resonance capacitor 14-resonance inductor 26
A resonance current flows through the resonance inductor 25, the primary winding of the transformer 18, the switching element 12 and the loop of the resonance capacitor 14. If the ratio of the number of primary windings to the number of secondary windings of the transformer 18 is n, then n times the resonance current flows in the secondary windings, so this resonance current is rectified by the rectifying diodes 21 and 22,
The smoothing capacitor 23 is charged and a DC voltage is supplied to the load resistor 24. Assuming that the voltage of the DC power supply 17 is E, if the charging voltage of the resonance capacitors 13 and 14 is equal to or lower than E, no current flows through the feedback diode 15 or 16. Mote I in this area
I will call it. In mode I, the output voltage is constant even if the load resistance 24 is changed, and is given by E / (2n). The charging voltage of the resonance capacitors 13 and 14 increases as the load resistance 24 is reduced, and when it reaches E, the resonance capacitor 13-DC power supply 17-feedback diode 16-resonance inductor 26-resonance capacitor 13 loop, or Resonant capacitor 14
-Resonance inductor 26-Feedback diode 15-DC power supply
17-A feedback current begins to flow in the loop of the resonance capacitor 14. This area is called Mode II. In mode II, the output current is constant and is given by 4fC ₁ nE even if the load resistance 24 is changed. f is the operating frequency and the resonance frequency is f ₀
This is the case when f ₀ > 2f. C ₁ is the capacitance of the resonance capacitors 13 and 14. Switching element 11 in mode I or
The resonance current i ₀ flowing through 12 is given by the equation (4) when t = 0 when the switching element is turned on.

i ₀ = [I ₀ sin [t / {2C ₁ (L ₁ + L ₂ )} ^1/2 }] / 4fn {2C
₁ (L ₁ + L ₂ )} ^1/2 (4) The resonance current i ₀ flowing through the switching element 11 or 12 in mode II is given by equation (5).

i ₀ = [Esin [t / {2C ₁ (L ₁ + L ₂ )} ^1/2 ]] / 2 {(L ₁ + L ₂ ) / 2C ₁ } ^1/2 (5) Flow through the feedback diode 15 or 16 The feedback current i ₁ is given by the equation (6) when the time when the feedback current i ₁ starts to flow is T ₀ .

i ₁ = - _{[(- nV 0 + E / 2} ) sin {(t-T 0) / (2C 1 L 2) 1/2} ] _{/ (L 2 / 2C 1)} 1/2 (6) wherein L ₁ is the inductance of the resonance inductor 25, and L ₂ is the inductance of the resonance inductor 26. FIG. 2 shows current waveforms flowing through the feedback diode 15 or 16 and the switching element 11 or 12 in the mode I and the mode II.

In mode I, when the load resistance 24 is reduced as can be seen from the equation (4), the peak value of the resonance current increases and the output voltage becomes constant. In mode II, when the load resistance 24 becomes smaller, the peak value of the feedback current increases from the equation (6), but the peak value of the resonance current becomes constant from the equation (5). The output current is constant because it is n times the average value of i ₀ . That is, the output voltage decreases. While the feedback current is flowing, the connection point of the feedback diodes 15 and 16 is the power supply voltage E, and even if the switching element 11 or 12 is turned on in this state, resonance occurs between the feedback diode and the switching element. Since there is the inductor 25 for use, the rise of the current when the switching element is turned on is given by Et / L ₁ (t is time). Therefore, even if the reverse blocking capability of the feedback diode is not restored, a spike-shaped current does not flow, and loss, noise, stress on switching elements, etc. can be reduced as compared with the conventional series resonant converter. In addition, if a switching element such as a MOS-FET that theoretically has no storage time is used, the operating frequency can be increased to the resonance frequency, and the smoothing capacitor can be downsized. It goes without saying that the resonance inductor 25 can utilize the leakage inductance of the transformer 18.

〔The invention's effect〕

As described above, in the series resonance converter according to the present invention, even when the reverse blocking capability of the feedback diode is not restored, the current at the turn-on of the switching element rises gently by the resonance inductor, so the conventional series resonance converter is used. Compared to converters, higher frequencies are possible, and there is the advantage that loss and noise do not increase even when the operating frequency is raised to the resonance frequency.

[Brief description of drawings]

FIG. 1 is a connection diagram showing an embodiment of the present invention, and FIG.
FIG. 3 is a current waveform diagram flowing through the switching element and the feedback diode in the figure, FIG. 3 is a connection diagram showing a conventional series resonance converter, and FIG. 4 is a current waveform diagram flowing through the switching element and the feedback diode in FIG.

Claims (1)

[Claims] 1. A series circuit of two switching elements, and 2
A series circuit of resonance capacitors and a series circuit of two feedback diodes are connected to a DC power supply, and a transformer is provided between the connection point of the switching element series circuit and the connection point of the feedback diode series circuit. A series circuit of the primary winding and the first resonance inductor is connected, and a second resonance inductor is provided between the connection point of the feedback diode series circuit and the connection point of the series circuit of the resonance capacitor. A series resonance converter in which a full-wave rectifier circuit and a smoothing capacitor are connected to the secondary winding of the transformer.