JPH02106164A - Series resonance type dc-dc converter - Google Patents

Abstract

PURPOSE:To prevent a switching(SW) frequency from changing steeply relative to an input-output variation by connecting a discharging resistor in parallel with a resonance capacitor via switching(SW) element and by turning ON the SW element within the OFF period of an SW circuit at the time of a light load. CONSTITUTION:A discharging resistor 14 is connected in parallel with a resonance capacitor 8 via switching element 15. In the state of a light load, DC output voltage Io tends to decrease while DC output voltage Vout tends to rise. A load discriminator circuit 16 gives ON signal to the element 15 when the DC output voltage Io is under a set current. When the element 15 is closed, the electric charge of the capacitor 8 is discharged through the resistor 14. Therefore, when transistors 2, 2 are turned ON subsequently, the voltage of the capacitor 8 is reduced so that a resonance current decreases and the DC output voltage Vout lowers. Consequently, a frequency control circuit 19 increases a switching frequency fs to adjust an output frequency.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a series resonant DC-DC converter.

[Conventional technology]

FIG. 4 is a circuit diagram of a series resonant DC-DC converter disclosed in, for example, Japanese Patent Laid-Open No. 58-98076. In the figure, 1.2 is a switching element (transistor in this example), 3.4 is a flywheel diode connected anti-parallel to transistor 1.2, 5.6 is a base drive circuit of transistor 1.2, and 7 is a resonance reactor, 8 is a resonance capacitor, 9
is a transformer, IO is a rectifier circuit, 11.12 is a rectifier diode, and 13 is a smoothing capacitor.

Vs is the DC input voltage. Note that an on/off command is given to the base drive circuit 5.6 from a voltage control circuit (not shown).

Next, the operation of this DC-DC converter will be explained with reference to the waveform diagram in FIG.

When a base signal (Fig. 5(a)) is applied from the base drive circuit 5.5 to the base of the transistor 11, both transistors 1 are turned on, and a current i flows through the path shown by the dotted line.
(resonant current) flows. During the period when both transistors 1 are on (region I in FIG. 5), the resonant current i becomes a positive sine wave and has the magnitude shown below (112).

+ i (to) cos ωt −−・−−−・−
・(1) However, Vc(to): Capacitor voltage VT when t = t o, ll: Transformer-next side voltage f = 2π π below (2) This resonance The current i is applied to the rectifier circuit 10 through the transformer 9, converted into a DC current 1out by the rectifier circuit 10, smoothed by the smoothing capacitor 13, and has a DC output voltage across the capacitor 13 as shown in FIG. 5 +1141. Vout appears. In region (3), both transistors l are off, and resonant current i flows through both diodes 3. In region ■, the transistor 2.2 is connected from the base drive circuit 6 to FIG.
It is turned on in response to the signal indicated by bl, and the resonant current i flows through the path indicated by the - dotted chain line. During this period when both transistors 2 are on, the resonant current l becomes a negative sine wave, and during the period when both transistors 2 are off (region ■),
A resonant current i flows through both diodes 3.

The above operations are repeated to obtain the DC output voltage Vout, but the magnitude of the DC output current IO is as shown in (3) below.
It becomes as shown by the formula.

ro=2ip' L+ ,/yr (t+ +τ
)N... (3) However, N: Number of turns of transformer iP: Maximum value of resonant current t,: Period of half-wave bone of ip As is clear from this equation (3), transistor 1.2
If the period τ during which is off is increased, the DC output current 1o
decreases. From this, when the DC input voltage Vs or the load fluctuates and the output voltage Vout fluctuates, constant voltage control can be performed by changing the switching frequencies of the transistors 1 and 2.

[Problem to be solved by the invention]

In conventional series resonant DC-DC converters, the output voltage is controlled by controlling the switching frequency of transistors I and 2. Therefore, when performing constant voltage control, the switching frequency depends on the increase or decrease in load resistance. Since it is almost inversely proportional to was there.

This invention was made to solve the above problem.
It is an object of the present invention to provide a series resonant DC/DC converter that does not require significant changes in switching frequency in response to input/output fluctuations.

[Means to solve the problem]

In order to achieve the above object, the present invention connects a discharging resistor as a switching element valve in parallel to the resonance capacitor, and also sends a signal to turn on the switching element during the off period of the Swissotig circuit when the load is light. This system is equipped with a load condition determination circuit that generates a load condition.

[Effect]

In this invention, when the load is light, the charging voltage of the resonance capacitor decreases, so the DC output voltage tends to decrease when the switching circuit is next turned on. To adjust this, the switching frequency is increased and the frequency characteristics of the switching frequency are improved.

〔Example〕

An embodiment of the present invention will be described below with reference to the drawings.

In FIG. 1, reference numeral 14 denotes a discharge resistor, which is connected in parallel to the resonance capacitor 8 via a switching element 15. Reference numeral 16 denotes a load state determination circuit which takes in the detection signal of the current detection circuit 18 that detects the output current IO, compares it with the set current Iref, and turns on the switching element 15 when 10<Iref. Generates an on signal S (described later). Reference numeral 19 denotes a voltage control circuit that generates an on/off command, which takes in the deviation between the DC output voltage Vout detected by the voltage detection circuit 17 and the set voltage Vref, and generates an on/off command so that the deviation becomes zero. Adjust the frequency (switching frequency) fs. Since the other configurations are the same as those of the conventional device, the same components are given the same reference numerals.

Next, the operation of this DC-DC converter will be explained with reference to FIGS. 2 and 3.

In a heavy load state where the load resistance is small, the voltage control circuit 19 increases the switching frequency Is so that the DC output voltage Vout once falls to the set voltage Vref. At this time, the DC output current 1o increases, so the load state determination circuit 16 determines that the DC output current 0 is larger than the set current 1ref, and gives an OFF command to the switching element 15. is off.

The capacitor 8 is charged by the resonant current i, and the charging voltage is 2Vs, assuming that the circuit loss is very small.

In a light load state, since the load resistance is large, the DC output current IO decreases and the DC output voltage Vout tends to increase. The load discrimination circuit 16 has a DC output current ■0 is the set current 1
When it becomes less than ref, it is determined that the load is light, and an on signal is sent to the switching element 15 (Fig. 2 (d)
)) occurs. This on signal is transmitted to the frequency control circuit 19
This signal is delayed by Δt1 with respect to the output of , and has a width of Δt2. When the switching element 15 closes in response to this ON signal, the electric charge of the capacitor 8 is discharged through the resistor 14 during Δt2, so that the voltage Vc of the capacitor 8 becomes the value shown in (4) 2 below.

Δt2 C Vc(Δt2)=Vc(to)・ε・・・・・・(4
) However, R: resistance value of resistor 14 C: capacitance of capacitor 8 Therefore, next time transistor 2.2 is turned on, (1
) The voltage Vc (to) of the capacitor 8 expressed by the formula is ε
Therefore, the value of the resonant current i becomes small as shown in FIG. adjust.

In other words, the switching frequency fs at heavy load (see Fig. 3) is
The region a) is the same as that of the conventional converter, but when the load is light (the region indicated by bl in FIG. 3), the frequency is higher than that of the conventional converter, and the frequency characteristics are improved.

Note that in the above embodiment, the discharge resistor 14 for the capacitor 8
However, instead of the resistor 14, a voltage limiting element such as a varistor may be used.

Further, although the discharge resistor is not controlled, if the voltage of the capacitor 8 is detected and the closing time of the switching element 15 is controlled, the device becomes even more accurate.

〔Effect of the invention〕

As explained above, this invention, even under light load conditions,
Since the output voltage can be adjusted without significantly changing the switching frequency compared to the conventional method, switching losses can be reduced, noise can be reduced, and audible noise can be reduced.

[Brief explanation of drawings]

Fig. 1 is a circuit diagram showing an embodiment of the present invention, Fig. 2 is a waveform diagram for explaining the operation of the above embodiment, Fig. 3 is a diagram showing switching frequency characteristics in the above embodiment, and Fig. 4 is a diagram showing switching frequency characteristics in the above embodiment. FIG. 5, a circuit diagram of a conventional DC-DC converter, is a waveform diagram for explaining the operation of this conventional example. In the figure, 2-1 transistor, 8-resonant codenser, 9-transformer, 14-discharge resistor, 15 switching element, 16-load state determination circuit, 19-voltage control circuit. In addition, in the figures, the same reference numerals indicate the same or corresponding parts.

Claims (1)

[Claims] A switching circuit that obtains single-phase alternating current from a direct current input, a resonant capacitor, a resonant reactor, and a transformer inserted between the output terminals of the switching circuit, a rectifier circuit connected to the secondary side of the transformer, and this rectifier circuit. In a DC-DC converter that includes a smoothing circuit that smoothes the output of the switching circuit and adjusts the output voltage by controlling the switching frequency of the switching circuit, a discharging resistor connected in parallel to the capacitor via a switching element; A series resonant DC-D, characterized in that it includes a load state determination circuit that generates a signal that turns on the switching element when the load is light, and that the signal is generated within an off period of the switching circuit.
C converter.