JPH03251079A - Series resonance converter - Google Patents

Abstract

PURPOSE:To reduce power loss under low power mode by performing pulse modulation with a fixed frequency or frequency modulation through switching elements under high power mode whereas performing pulse frequency modulation through the switching elements under low power mode. CONSTITUTION:Switches 20, 21 are switched, as shown on the figure, under high power mode and the difference between the load 5 current and a reference value is detected through an error amplifier 17. pulse width, i.e., the duty, of a pulse width modulation circuit 25 is then regulated and an inverter 2 is driven through AND gates 27, 28 thus controlling the load current. The switches 20, 21 are switched reversely under low power mode, and the frequency of a reference oscillator 22 is varied by the output from the error amplifier 17 through an LED 32 and a phototransistor 30 thus controlling the load current.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention provides an X-ray frE using a series resonant inverter.
This invention relates to series resonant converters such as power sources.

[Conventional techniques and inventions try to solve the problem! ! [Question] In recent years, inverter-type X-ray stacking using switching devices such as transistors and FETs has been put into practical use.
Problems have also arisen as the frequency of inverters increases. 6th
The figure is a configuration diagram showing an equivalent circuit of the step-up transformer of the inverter ancient X-ray power supply 19. 1 is ti current input power supply, 2
is an inverter.

3 is a step-up transformer represented by an equivalent circuit, 4 is a high-voltage rectifier, and 5 is a load such as an X-ray tube. )? The voltage transformer 3 has an excitation inductance. , leakage inductance LL, and winding distribution capacitance C2 converted to Sγ in a first-order example. When the frequency of the inverter 2 is increased, the leakage inductance Lt,'!
The J-line distribution capacity 1c affects the operation of the inverter 2, the inverter 2 has a leakage inductance, and the winding distribution capacity MI
CP series resonance 18'! li! Together with P+, the operation shifts to a series resonant inverter. In such an operating mode, one winding distributed capacitance CP only contributes to alternate voltage reversals at the operating frequency r8, and the circuit circulating current i3. which is not supplied to the load. [conversion], the reactive current will increase. This circulating electricity t
li i s causes switching loss and conduction loss of the switching element, and resistance loss of the winding of the step-up transformer 3. This resistance loss increases as the frequency of the circulating current i increases due to the surface covering effect of the second winding material. This circulation fr
By its nature, the L current i is proportional to the output voltage rather than the output power, and in an X-ray power supply, the higher the output voltage, that is, the tube voltage, the more it tends to increase.

By the way, in the case of an X-ray power supply, for example, 10 kW (tube voltage 100 kV) is required to perform normal X-ray photography.

A high power mode that supplies a tube current of 10 (lsA) for several seconds at a low duty cycle, and a high power mode that supplies a tube current of 10 (lsA) for several seconds at a low duty cycle, and 2, for example, 500 W (tube voltage 100 kV,
A low power mode is required that supplies tube current 5-^) continuously for a long time. In X-ray power supplies using series resonant inverters and in high power mode.

Inverter 2. The power loss of the step-up transformer 3 increases as the sum of the amount that depends on the output power and the amount that depends on the circulating ill flow, but since the duty of the operating time is small, the power loss of the inverter 2. Since the temperature rise of the step-up transformer 3 etc. is averaged out, the temperature rise of the inverter 2. The step-up transformer 3 and the like are designed to have less margin than normal designs, in order to make them smaller and more economical. However, the power loss in the low power mode depends on the output power and is small, but because the tube voltage is high, the amount due to circulating current is almost the same as in the high power mode, and i! +i rotation time becomes longer, so inverter 2. The temperature rise in the step-up transformer 3 can no longer be ignored.

[Means to solve the problem]

In order to eliminate the above drawbacks, the present invention connects a series resonant circuit consisting of a resonant inductance and a resonant angle capacitance equivalently to the output of an inverter consisting of at least one pair of switching elements, and the resonant In a series resonant converter that rectifies and outputs the voltage across an angular capacitance, the switching element is pulse width modulated or phase modulated at a substantially fixed frequency in the high power mode, and the switching element is pulsed in the low power mode. The present invention provides a series resonant converter characterized by frequency modulation.

[Function] According to such a series resonant converter, the switching element undergoes pulse width variation or phase modulation at a substantially fixed frequency in the high power mode, and pulse frequency modulation is performed on the switching element in the low power mode. In addition to being able to reduce the time pull, the effective value of the series resonant current in the low power mode can be lowered, and even in the low power mode, the power loss can be reduced.

〔Example〕

FIG. 1 is a diagram for explaining one embodiment of the present invention, and shows an embodiment in which the series resonant converter of the present invention is applied to an X-ray power source. In the figure, 1 is a battery or an Ishikawa AC power source. Indicates the rectified and smoothed DC input power supply 1 Usually D
C100V - DC300V. 2 is 4 (I[I's switching elements 6 to 9.

This is a pre-Fuji type inverter consisting of feedback diodes IO-13 connected in antiparallel to each switching element 6-9. Reference numeral 3 denotes a step-up transformer for boosting the AC output voltage of the inverter 2 to a required voltage 3, for example, 100 kV. The leakage inductance Lt of the four-voltage transformer 3 forms a series resonant circuit with the secondary winding distributed capacitance C5 together with the resonant angular inductance 14 provided as necessary. The secondary winding of the step-up transformer 3 is grounded at the center point 5 and is connected to a full-wave height positive regulator 4.
connected to. The DC output with high voltage rectification = 4 is connected between the anode and filament of the X-ray tube, which is the load 5.
Although the filament power supply circuit for the wire tube is not directly related to the present invention and is therefore omitted, the filament is normally replaced and the rJf4 adjustment of the filament power is performed in the high power mode and the low power mode. Resistors 15 and 16 are voltage dividers that detect the x-ray tube voltage.

The output iif pressure of the voltage divider is ! In the i difference amplification W17,
It is compared with the voltage of the tube voltage setting reference power source I8 to generate an error signal. here! The entire 4-control circuit can be switched between a high power mode and a low power mode by interlocking mode changeover switches 20 and 21, and the connection of the mode changeover switches in FIG. 1 is on the high power mode side. Reference oscillator 22
The oscillation frequency is determined by the time constant of the capacitor 23 and the combined resistor. If fN.

When the resistor 24 is selected by the mode selector switch 21, the inverter operating frequency is 40kHz, which is twice the 20kHz.
Oscillates at z. The output of the reference oscillator 22 is transmitted to a pulse width modulation circuit 25. The signal is supplied to the M large pulse width generation circuit 26.

The pulse width modulation circuit 25 has a pulse width modulated signal of 40kllz in a well-known manner, for example, by comparing an error signal with a triangular wave.
generates a pulse. The pulse width control signal is mode 9J.
Selected by changeover switch 20, AND circuit 27.28
added to. The AND circuits 27 and 28 are further supplied with the maximum pulse width signal of the maximum pulse width generation circuit 26 and the 40k
Two-phase signals are added from the flip-flop 29, which generates a two-phase signal of 20kllz from the signal of llz. By these signals.

The outputs of the AND circuits 27 and 28 are alternately 2-phase 20k.
112 and a maximum pulse width of, for example, 20 ps is generated. These ON signals are applied to the 'tAm poles of each of the switching elements 6 to 9 via a pulse transformer (not shown) or a signal insulating means such as an optical isolator.
The switching element is turned on at a fixed frequency of 20k11z.

First, tII8N in the high power mode will be explained.

In high power mode, with this 4-control circuit configuration,
Based on the detection voltage of tube voltage! 1! The tube voltage is compared with the voltage of the electric current 11218 by the error amplifier 17, and the tube voltage is made constant by setting the pulse width of the ON signal, that is, the duty, to 11]. In such a high power mode, the computer simulation results at 10kW output are shown in Figure 3.The zero conditions are: DC input power voltage is 250V DC, resonance inductance is 15μH, resonance capacitance is 1.9μF (series resonance frequency 30KLLZ). , the on-pulse width is 20n. The horizontal axis in Figure 2 is time, 200r, ~500
The US simulator 2 period is displayed. waveform l
is a positive two-phase pulse width control signal.

Waveform diagram shown in negative polarity, waveform 2 is a waveform diagram of high voltage rectified current 1°, waveform 3 is a waveform diagram of resonant current 11 flowing through the primary side of step-up transformer 3, waveform 4 is the output voltage of inverter 2 ■1
In the lower frame of Fig. 2, which shows the waveform diagrams of
) and the simulation period (200Irs~500p
The average value of high voltage rectified current 1° of s) is 102m8,
Also, the effective value of the resonance type *l+ shows m'alco to 101 (tier), I! llchi, 100kV, 10
The effective value of the primary current of the step-up transformer 3 at the time of output to the step-up transformer 2- is 10IA. In this high power mode.

Since it operates at a fixed frequency of 20 kllz, the output voltage ripple can be made sufficiently small.

Next, control in the low power mode will be explained.

In the low power mode, the mode changeover switches 2o and 21 are connected in the opposite direction as shown. That is, the pulse width control signals to the AND circuits 27 and 28 are switched to the +V level by the mode changeover switch 20 and no longer function. By switching the mode selector switch 21, the resistor 24 is connected to the phototransistor 30 as a variable resistor and the highest frequency rJN! i! Resistor 3
1 into a series circuit. Phototransistor 30
is urn by the light emitting diode 32 driven by the error signal voltage of the error amplifier 517 and the current determined by the resistor 33.
be done. When the voltage of the error signal increases, the one light emitting diode IF current increases, the resistance value of the phototransistor 30 decreases, and the oscillation frequency of the reference oscillation S22 increases. That is,
Standard issue 11522 is.

Voltage controlled by error signal 1) Acts as a moderator. The resistor 31 becomes 1 when the phototransistor 30 is saturated.
This determines the upper limit of the oscillation frequency, and in this example,
The switching frequency f, is the series resonant frequency f, , = 30
The oscillation frequency is set to 36kllz or less, and the switching frequency in high power mode is 20kllz or less.
Select k llz. This 36 kllz pulse is divided by the signal, and sent to the switching element 8.9 at a maximum of 18 kllz below the series resonant frequency f.
It becomes a signal. ii During large pulses, the series resonant frequency rP
Approximately half a period to one period is desirable, and in this embodiment, one period of the series resonance frequency r, , is approximately 33.3 μs, so it is set to about 20 fs, which is the same as in the high power mode. With such a control circuit in the small power mode, the inverter 2 performs constant voltage control in the frequency modulation mode. When the output voltage is lower than the set voltage, the frequency is increased to compensate for this, and conversely, when the output voltage is higher than the set voltage, the frequency is lowered by one level.

In such a series resonant converter, the relationship between switching frequency and output voltage is as shown in Figure 3.The peak value of the output voltage is before and after the series resonance point, and the output voltage decreases as you move away from that resonance point. is well known. In this embodiment, the inverter 2 is operated in the low power mode at a series resonance frequency f or less. Even if it is operated at such a low frequency, the 5 output tri current is small, so the output voltage rise and pull voltage will not increase. Figure 0, which shows the simulation results, is viewed in the same way as Figure 2, but the 1-time axis is 0.70m5 to 1.5m5. It is ooss.

The effective value of the primary side resonant current is 36. It is IA. On the other hand, Figure 5 shows the results of a simulation in which almost the same output is generated with a 20kllz pulse width of 14m. In this example, the output is 1ookV4.66mA (the effective value of the primary current is .1 A is more than twice that of the present invention, which proves the effect of the present invention.As a vI control method in still higher power mode, switching elements 6 to
The on-pulse width of switching elements 6 and 7 is constant, and the on-pulse width of switching elements 6 and 7 is
The on-phase difference between the pair of switching elements 8 and 9 and the on-phase difference between the pair of switching elements 8 and 9 are controlled, and the output! It is also possible to control 4. Also, when the output is relatively large, frequency modulation is performed,
When it becomes smaller, fix it to the lowest frequency and change the pulse width t14.
In this method, the lowest frequency is determined, so the output ripple voltage can be limited.

As explained above, according to the present invention, it is possible to lower the effective value of the series resonant current in the low power mode, and it is possible to cope with the low power mode with less power loss, thereby improving efficiency. Inverters, transformers, etc. can be designed compactly and economically. Furthermore, the present invention can be applied not only to full-bridge inverters but also to half-bridge inverters and center-tapped inverters.

〔Effect of the invention〕

As described above, the present invention connects a series resonant circuit consisting of a resonant inductance and a resonant capacitance equivalently to the output of an inverter composed of at least one pair of switching elements, and calculates the voltage across the resonant capacitance. In the series resonant converter that outputs U current, the switching element is pulse width modulated or phase modulated at a substantially fixed frequency in a high power mode, and the switching element is pulse frequency modulated in a low power mode. Since the present invention, which is a series resonant converter, has such characteristics, it is possible to lower the effective value of the series resonant current when in two small power modes. Therefore, it is possible to cope with the low power mode with less power loss, thereby improving efficiency and making it possible to design inverters, transformers, etc. in a compact and economical manner. In particular, the X
Useful as a line power source.

[Brief explanation of drawings]

1 to 1715 are diagrams for explaining an embodiment of the present invention, and FIG. 6 is a diagram for explaining a conventional series resonant converter. ■...DC input TsFj 2...Inverter 3-...Correction transformer 4...High voltage rectifier 5...
・Loads 6 to 9... Switching elements 10 to 13... Sweeping diode 14... Resonant inductance 15, 16.24.31.33... Resistor 17... Error amplifier 18... Reference Power supply 20, 21...
Mode cut-off switch 22...Reference source 1. s 23... Capacitor 25... Pulse width modulation circuit 26... Maximum pulse width generation circuit 27° 28... AND circuit 29... Flip-flop 30... Phototransistor 32... Light emitting diode

Claims (1)

[Claims] A series resonant circuit equivalently consisting of a resonant inductance and a resonant capacitance is connected to the output of an inverter composed of at least one pair of switching elements, and the voltage across the resonant capacitance is rectified. In a series resonant converter that outputs an output, the switching element is pulse width modulated or phase modulated at a substantially fixed frequency in a high power mode, and the switching element is pulse frequency modulated in a low power mode. shape converter.