JPH06141541A - Method and circuit for controlling series resonance convertors - Google Patents

Abstract

PURPOSE:To supply a method and circuit for controlling series resonance convertors so that their efficiency does not lower even on a light load. CONSTITUTION:In a series resonance converter, a series resonance circuit consisting of circuit components equivalent to a resonance inductance 12 and resonance capacitor 13 is connected to the output of an inverter 2 comprising at least a pair of switching elements 3-6. The series resonance convertor rectifies the voltage across the capacitor 13 and outputs the rectified voltage. The switching elements 3-6 are controlled by pulse width modulation control and pulse frequency modulation control by applying the output signal of an error amplifier 19 that compares the output parameter with the reference value to a pulse width modulation circuit 29 and voltage controlled oscillator 23 that provides signals for the pulse width modulation circuit.

Description

Detailed Description of the Invention

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a control method and control circuit for a series resonant converter.

2. Description of the Related Art FIG. 2 is a diagram for explaining the principle of a series resonant converter. In the figure, 1 is a DC input power source, 2 is a bridge type inverter composed of four FETs, switching elements 3 to 6 such as IGBTs, and diodes 7 to 10 connected in antiparallel to each switching element 3 to 6. . The diodes 7 to 10 may be FETs or parasitic diodes. Reference numeral 11 denotes a transformer, a resonance inductance 12 of which is connected in series to a primary winding thereof, and a resonance capacitance 13 of which is connected in parallel to a secondary winding thereof. The resonance inductance 12 and the resonance capacitance 13 form a series resonance circuit. Transformer 11
The input of the rectifier 14 is connected to the secondary winding of the rectifier 1.
A capacitor 15 and a load 16 are connected to the output of 4.
In this converter, depending on whether the operating frequency fs of the switching elements 3 to 6 is higher, lower, or the same as the resonant frequency fr of the resonance inductance 12 and the resonance capacitance 13, Although the operation mode is different, here, the operation frequency fs is selected as a frequency lower than the resonance frequency fr. Conventionally, as a control method and control circuit of this operation mode, the switching elements 3 to 6 are pulse-width modulated,
Controlling the output voltage or current is being done.

However, in such a conventional control method and control circuit for a series resonant converter, when the output voltage and current are rated and the input voltage is high, for example, the rated input ± At 10% or the like, there is a problem that the resonance current, that is, the peak value of the circulating current in the circuit increases and the efficiency decreases. In addition, even when the output voltage is rated and the output current is small, that is, when the load is light, the resonance capacitance operates at the rated voltage and rated frequency. ，
There is a problem that the efficiency is lowered due to a large circuit loss due to this current.

According to the present invention, in order to eliminate the above drawbacks, the output of an inverter composed of at least a pair of switching elements is equivalently connected in series with a resonance inductance and a resonance capacitance. In a series resonance converter that connects a resonance circuit and rectifies and outputs the voltage across the resonance capacitance, the output signal of the error amplifier that compares the output parameter with the reference value is output to the pulse width modulation circuit and the pulse width modulation circuit. Provided is a control method and a control circuit for a series resonant converter, characterized in that the switching element is controlled by pulse width modulation control and pulse frequency modulation control by applying the signal to a modulation circuit and a voltage controlled oscillator. It is a thing.

FIG. 1 is a diagram for explaining one embodiment of the present invention, showing an embodiment in which the control method and control circuit for a series resonant converter of the present invention is applied to an X-ray power source. In the figure, 1 indicates a DC input power source obtained by rectifying and smoothing a battery or a commercial AC power source, which is usually ± 1 with respect to the rated voltage.
There is a variation of 0%. 2 is four switching elements 3 to 6
And the feedback diodes 7 to 10 connected in anti-parallel to the switching elements 3 to 6, respectively. 11 is the required AC output voltage of the inverter 2,
For example, it is a transformer that steps up to 100 kV. A series resonance circuit is configured by the resonance inductance 12 including the leakage inductance of the transformer 11 and the resonance capacitance 13 including the secondary winding distributed capacitance. Transformer 1
The secondary winding of 1 is grounded at the midpoint, and full-wave high-voltage rectifier 14
Connected to. The DC output of the rectifier 14 is connected between the filament of the X-ray tube, which is the load 16, and the filament. X
The filament power supply circuit of the wire tube is omitted because it is not directly related to the present invention. The resistors 17 and 18 are voltage dividers that detect the X-ray tube voltage. The output voltage of this voltage divider is compared with the voltage of the tube voltage setting reference power source 20 in the error amplifier 19 to generate an error signal. This error signal is input to the input terminal 24 of the voltage controlled oscillator 23 via the diode 21 and the resistor 22. Reference numeral 23 is a voltage controlled oscillator that determines the operating frequency fs of the inverter 2 and has an input terminal 24
It oscillates at a frequency proportional to the voltage of. The voltage divider composed of the resistors 25 and 26 supplies a bias voltage to the input terminal 24, and has a minimum frequency of the operating frequency fs, for example, 16 kHz.
To decide. The maximum frequency of the operating frequency fs is limited by locking the input voltage with the Zener diode 27. Assuming that the maximum frequency is the rated frequency and a frequency lower than the resonance frequency fr, for example, 20 kHz, the voltage controlled oscillator 23 is twice the frequency of 20 kHz, that is, 40 kHz.
It oscillates at kHz. Here, the voltage controlled oscillator 23 is 20
The reason for oscillating at twice the kHz is that the frequency is divided in half by the flip-flop 28. The output of the voltage controlled oscillator 23 is supplied to the flip-flop 28, the pulse width modulation circuit 29, and the maximum pulse width generation circuit 30.
The pulse width modulation circuit 29 is, for example, 40 kH pulse-width modulated by a known method by comparing an error signal and a triangular wave.
A pulse width modulated signal that is a pulse of z is generated. This pulse width modulation signal is applied to AND circuits 31 and 32. Further, to the AND circuits 31 and 32, the maximum pulse width signal of the maximum pulse width generating circuit 30 and the two-phase distribution signal from the flip-flop 28 that generates a two-phase signal of 20 kHz from the signal of 40 kHz are added, respectively.
Due to these signals, two-phase ON signals having 20 kHz and a maximum pulse width of, for example, 20 μs are alternately generated at the outputs of the AND circuits 31 and 32. These ON signals are applied to the control poles of the switching elements 3 to 6 via a pulse transformer (not shown) or a signal insulating means such as an optical isolator, and the switching elements are fixed at a fixed frequency 20.
Turn on at kHz. First, the control of a rating with a relatively large load current will be described. When the input voltage is low, for example, −10% of the rated input voltage, the detected voltage of the tube voltage is compared with the voltage of the reference power source 20 by the error amplifier 19, and by adjusting the pulse width of the ON signal, that is, the duty, The tube voltage is made constant. In such a relatively high power mode, the output of the error amplifier 19 rises to widen the pulse width. Due to this raised error signal, the control voltage of the voltage controlled oscillator 23 also rises and operates at the rated frequency, for example 20 kHz. Input voltage rises, for example +1
When it reaches 0%, the output of the error amplifier 19 drops to lower the high voltage output. For this reason, the pulse width is narrowed, but the operating frequency is lowered, and even if the peak value of the resonance current rises, the frequency is low, so the effective value does not increase and the circuit loss does not increase so much. Next, the case of a light load will be described. Since the output is reduced at a light load, the output of the error amplifier 19 is reduced and the pulse width of the pulse width modulation circuit 29 is reduced. At the same time, the control voltage of the voltage controlled oscillator 23 decreases, and the operating frequency fs decreases, for example, 16k.
Becomes Hz. However, since the control voltage does not drop below the voltage determined by the resistors 25 and 26, it does not drop below the lower limit frequency. Usually, the lower limit frequency should be higher than the audible frequency. In this embodiment, since the frequency of the reactive current for charging and discharging the resonance capacitance 13 is lowered, the power loss is reduced and the efficiency is improved. Since the ripple voltage is inversely proportional to the frequency, the ripple voltage increases as the frequency decreases, but the operating frequency fs does not fall below the lower limit frequency, so that the increase in ripple voltage does not pose a practical problem. In the above embodiments, the case of the full bridge inverter has been described, but the same can be applied to the half bridge inverter and the center tap type inverter. Further, in the above embodiments, the case where the output voltage is detected as the output parameter to make the voltage constant is described, but the same can be applied to the case where the output current is detected as the output parameter to make the current constant.

As described above, according to the present invention, the series resonance circuit equivalently composed of the resonance inductance and the resonance capacitance is connected to the output of the inverter composed of at least a pair of switching elements, and In a series resonant converter that rectifies and outputs a voltage across a resonance capacitance, an output signal of an error amplifier that compares an output parameter with a reference value is used as a pulse width modulation circuit and a voltage that gives the pulse width modulation circuit a signal. A control method and a control circuit for a series resonant converter, characterized in that the switching element is controlled by pulse width modulation control and pulse frequency modulation control by being applied to a controlled oscillator. Since the present invention has such characteristics, it is possible to reduce the effective value of the series resonance current when the load is light. Therefore, when the load is light, it is possible to deal with a small power loss, and the efficiency does not drop when the load is light as in the conventional case, so that the inverter, the transformer, etc. can be designed compactly and economically.

[Brief description of drawings]

FIG. 1 is a diagram for explaining an embodiment of the present invention.

FIG. 2 is a diagram for explaining a principle diagram of a series resonance converter.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... DC input power supply 2 ... Inverter 3-6 ... Switching element 7-10 ... Diode 11 ... Transformer 12 ... Resonance inductance 13 ... Resonance capacitance 14 ... Rectifier 15 ... Capacitor 16 ... Load 17, 18 ... Resistor 19 ... Error amplifier 20 ... Reference power supply 21 ... Diode 22 ... Resistor 23 ... Voltage controlled oscillator 24 ... Voltage controlled oscillator input terminal 25, 26 ... Resistor 27 ... Zener diode 28 ... Flip-flop 29 ... Pulse width modulation circuit 30 ... Maximum pulse width generation circuit 31 , 32 ... AND circuit

Claims (2)

[Claims] 1. A series resonance circuit equivalently composed of a resonance inductance and a resonance capacitance is connected to the output of an inverter formed by at least a pair of switching elements, and the voltage across the resonance capacitance is rectified. In a series resonant converter that outputs, the switching signal is output by applying an output signal of an error amplifier that compares an output parameter and a reference value to a pulse width modulation circuit and a voltage controlled oscillator that supplies a signal to the pulse width modulation circuit. A control method for a series resonant converter, characterized in that an element is controlled by pulse width modulation control and pulse frequency modulation control. 2. A series resonance circuit equivalently composed of a resonance inductance and a resonance capacitance is connected to the output of an inverter composed of at least a pair of switching elements, and the voltage across the resonance capacitance is rectified. In the output series resonant converter, an error amplifier that compares the output parameter with the reference value, a pulse width modulation circuit that inputs the output signal of this error error amplifier, and the pulse signal that receives the output signal of the above error error amplifier. A control circuit for a series resonant converter, comprising: a voltage controlled oscillator for applying a signal to a width modulation circuit.