JPH02133069A - Converter - Google Patents

Abstract

PURPOSE:To reduce switching loss by switching a second switching element by a current transformer and the keying signal of a first switching element and preventing inflow through the first switching element of the discharge currents of a capacitor for resonance connected in series with the current transformer when the first switching element is brought to a close state from an open state. CONSTITUTION:A second switching element 13 keeps an ON state during a time when charging currents are made to flow through a capacitor 4 for resonance by a current transformer 12. When the charging of the capacitor 4 for resonance is completed, the second switching element 13 is turned OFF, and the discharge of the capacitor 4 for resonance is started. A control circuit 11 controls the discharge currents of the capacitor 4 for resonance so that the discharge currents of the capacitor 4 connected in series with the current transformer 12 are not made to flow into a first switching element 3 when the first switching element 3 is brought to an OFF state from an ON state, fixes the ON time of the first switching element 3, changes the OFF time and keeps output voltage to load 8 constant. Accordingly, switching loss can be reduced.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a converter that performs DC-DC conversion,
In particular, it relates to improvements in the circuit system.

[Conventional technology]

FIG. 4 is a circuit diagram of this type of conventional voltage resonant converter, showing the circuit system of a forward type single-ended converter. In the figure, 1 is a DC power supply, 2 is a conversion transformer connected to this DC power supply 1, 3 is a first switching element such as a transistor connected to the primary winding 2a of this conversion transformer 2, and 4.5 is this switching element. 1st
A resonance capacitor and a diode are connected in parallel to the switching element 3, and the diode 5 is connected in anti-parallel. 6.7 is the secondary winding 2b of the conversion transformer 2
A diode 9.10 provided as a rectifying element that converts the alternating current generated in the current into direct current and supplies it to the load 8 is a stabilizing element.
A smoothing choke coil and a capacitor 11 are a control circuit that outputs an opening/closing signal to the first switching element 3 to control the output output to the load 8.

Next, the operation will be explained.

First, when the switching element 3 is turned on by a signal from the control circuit 11, energy is transmitted to the secondary side of the conversion transformer 2 during this on period. Next, the switching element 3 is turned off, but at this time, a resonant circuit is formed by the inductance of the primary winding 2a of the conversion transformer 2 and the capacitance of the capacitor 4 connected in parallel to the switching element 3, and the switching element 3 is turned off. The voltage waveform across the switching element 3 draws a reciprocal arc, and the switching element 3 is controlled so that the voltage becomes zero just before the next cycle is turned on. Here, in principle, the switching element 3 is turned on. There is no loss when transitioning off, that is, there is no switching loss.

When the switching element 3 is turned on. By turning off, an AC output is generated in the secondary winding 2b of the conversion transformer 2. This alternating current is converted into direct current by a diode 6.7, and is further supplied to a load 8 via a choke coil 9 and a capacitor 10. The control circuit 11 then compares the output voltage supplied to the load 8 with a preset reference voltage, and outputs a switching signal to the switching element 3 based on the comparison result so that the output voltage is constant. . At this time, the off time of the switching element 3 is fixed, and the on time is changed to control the operating frequency.

[Problem to be solved by the invention]

Conventional converters are configured as described above, and the off-time of the switching element is fixed, so when it is necessary to reduce the output energy, such as during light loads, when the on-time of the switching element falls below a certain value, the off-time of the switching element is fixed. There is a problem in that the switching element becomes conductive while voltage remains in the resonant capacitor, increasing switching loss and possibly destroying the switching element.

This invention was made in view of the above-mentioned problems, and aims to provide a converter in which switching loss is reduced and there is no risk of destruction of switching elements.

[Means for Solving the Problems] A converter according to the present invention includes a conversion transformer connected to a DC power source, a first switching element connected in series to the primary winding of the conversion transformer, and a first switching element connected in series to the primary winding of the conversion transformer. A resonant capacitor and a diode connected in parallel to the switching element, a rectifying element that converts the alternating current generated in the secondary winding of the conversion transformer into direct current and supplies it to the load, and a switching element connected to the first switching element. In the converter, the converter includes a control circuit that outputs a signal to control the output output to the load, and a current transformer and a second switching element are connected in series with a resonance capacitor connected in parallel to the first switching element. At the same time, a diode is connected in antiparallel to this second switching element, and the second switching element is opened and closed by the switching signal of the secondary winding of the current transformer and the first switching element. When the first switching element changes from open to closed, the discharge current of the resonance capacitor connected in series with the current transformer is controlled so as not to flow into the first switching element.

Further, in the converter according to the present invention, the open time of the first switching element is controlled to be short when the voltage of the DC power source is high and to be long when the voltage of the DC power supply is low.

[Effect]

In the converter of the present invention, opening and closing of the second switching element allows the converter to escape from the resonance state when the first switching element is off. Furthermore, by controlling the on-time of the first switching element according to the DC power supply voltage, the allowable range of the input voltage can be widened.

〔Example〕

FIG. 1 is a circuit diagram showing an example of the present invention. In the figure, 1 to 1l are the same constituent parts as the conventional one in FIG. 4, so redundant explanation will be omitted.

Reference numerals 12 and 13 denote a current transformer and a second switching element connected in series with the resonance capacitor 4 connected in parallel to the first switching element 3, respectively, and the current transformer 12 has a primary winding l.
it 1 2 a is connected to the resonance capacitor 4, and the second switching element 13 is connected to the secondary winding 12 of the current transformer 12.
b and the opening/closing signal of the first switching element 3. 14 is a diode connected in antiparallel with the second switching element 13; 15 is the second switching element 1;
3 on. The turn-off timing is set to the turn-on timing of the first switching element 3. This is a capacitor provided to offset the off timing. Note that one end of each of the first and second switching elements 3.13 is connected to the negative electrode side of the DC power supply 1.

Next, the operation will be explained using the operation waveform diagram shown in FIG. Second
The figure shows the operating waveforms of each part in Figure 1, where Vds is the voltage across the first switching element 3 (drain-source voltage), and 11 is the current flowing through the first switching element 3 (drain current). , ic represents the charging/discharging current of the resonance capacitor 4, iD represents the waveform of the current flowing through the diode 5, and S, , S2 represents the on/off of the first and second switching elements 3.13, respectively. (Of
It shows the timing of f).

First, in periods 1 and -12 shown in FIG. 2, the first switching element 3 is turned on, and energy is transmitted to the secondary side of the conversion transformer 2. At this time, the second switching element 13 is also turned on a little later than the first switching element 3 due to the influence of the capacitor 15. Next, t 2
When entering the period t3, the first switching element 3 is turned off, but the second switching element 13 is kept on by the capacitor 15. Therefore, a resonant circuit is formed by the inductance of the primary winding 2a of the conversion transformer 2 and the capacitor 4, resulting in a voltage resonance state. The second switching element 13 remains on while the charging current is flowing through the resonance capacitor 4 by the current transformer 12.

Thereafter, the period 13-14 begins, and when the charging of the resonance capacitor 4 is completed, the second switching element 13
is turned off, and the resonance capacitor 4 starts discharging through the diode 14.

Next, in the period 14-15, when the discharge of the resonance capacitor 4 is completed, the excitation energy stored in the conversion transformer 2 during the voltage resonance III is regenerated through the diode 5. Then, in the period t s ta, when the current flowing through the diode 5 becomes zero, the first switching element 3 and the second switching element 1
3 are both off.

Thereafter, an AC output is generated in the secondary winding 2b of the conversion transformer 2 by repeating the above-described cycle operation according to the opening/closing signal from the control circuit 1l.

Then, after this alternating current is rectified by diode 6.7,
It is supplied to the load 8 through the choke coil 9 and the capacitor 10.

Here, the control circuit 11 is configured such that when the first switching element 3 changes from on (open) to off (closed), the discharge current of the resonance capacitor 4 connected in series with the current transformer 12 flows through the first switching element 3. The on-time of the first switching element 3 is fixed and the off-time is varied to keep the output voltage to the load 8 constant. Therefore, switching loss is reduced and there is no fear that the switching element 3 will be destroyed.

Note that when controlling the second switching element 13 described above, the period indicated by the hatched portion in FIG. 2 may be either on or off. Further, the second switching element 13 only needs to be connected in series with the resonance capacitor 4, and the second switching element 13 and the resonance capacitor 4 in FIG. 1 may be connected interchangeably. However, it is easier to control by connecting as shown in FIG.

FIG. 3 is a circuit diagram showing another embodiment of the invention. In this embodiment, the control circuit 11 detects the human power voltage from the DC power supply 1, and the on-time of the first switching element 3 is determined by the control circuit 11.
The length is shortened when the input power supply voltage is high, and lengthened when the input power supply voltage is low.

In this way, by controlling the on-time of the first switching element 3 according to the level of the input voltage, it is possible to escape from the resonance state during the off-time and reduce switching loss, and at the same time, it is possible to reduce the switching loss within the allowable range of the human voltage. can be widened.

〔Effect of the invention〕

As described above, according to the present invention, a current transformer and a second switching element are connected in series with a resonance capacitor, and a first switching element is connected in series with a resonance capacitor.
Switching loss is reduced because it is possible to escape from the simultaneous shooting state during the closing time of the switching element.
This has the effect that there is no fear that the switching element will be destroyed. Moreover, by controlling the open time of the first switching element according to the input voltage, the allowable range of the input voltage is widened.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing one embodiment of the present invention, FIG. 2 is an operation waveform diagram showing the operation of each part of FIG. 1, FIG. 3 is a circuit diagram showing another embodiment of the invention, and FIG. The figure is a circuit diagram of a conventional converter. 1...DC power supply 2...Conversion transformer 2 a =...Primary winding 2 b -------Secondary winding 3 --------First switching Element 4 -----
---Resonance capacitor 5... Diode 6, 7... Diode (rectifier) 8 ---
---Load 1l...Control circuit 12...Current transformer 12a...Primary winding 12b...Secondary winding 13-- -Second switching element 1 4-...
. . . Diode 15 --- Capacitor Note that the same reference numerals in the drawings indicate the same or corresponding parts. Figure 1

Claims (2)

[Claims] (1) A conversion transformer connected to a DC power source, a first switching element connected in series to the primary winding of this conversion transformer, and a resonant capacitor and diode connected in parallel to this first switching element. and a rectifying element that converts the alternating current generated in the secondary winding of the conversion transformer into direct current and supplies it to the load, and outputs a switching signal to the first switching element to control the supply output to the load. In the converter, a current transformer and a second switching element are connected in series with a resonance capacitor connected in parallel to the first switching element, and in antiparallel with the second switching element. A diode is connected, and the second switching element is opened and closed by the secondary winding of the current transformer and the opening/closing signal of the first switching element, and the current transformation occurs when the first switching element changes from open to closed. A converter characterized in that the converter is controlled so that a discharge current of a resonance capacitor connected in series with the converter does not flow into the first switching element. (2) The converter according to claim 1, wherein the open time of the first switching element is controlled to be short when the voltage of the DC power supply is high and to be long when the voltage of the DC power supply is low.