JPS62290353A - Switching power source - Google Patents

Abstract

PURPOSE:To decrease turn-off loss or the like, by a method wherein circuit constant is selected so that secondary circuit current becomes oscillatory, and ON-period of a switch is made a half period of the oscillation period or more. CONSTITUTION:A switching power source is composed of a DC power source 1, a transformer 2, a controllable switch 8, a control circuit 11, a diode 5, a capacitor 6 and the like. Stable output is supplied to a magnetron 7A as load, and when the controllable switch 8 is turned on by leakage inductance of the transformer 2 and capacitors 6A, 6B, current flowing through secondary circuit of the transformer 2 is made oscillatory and the switch current waveform at the turn-off time is made small. In this constitution, reduction of the switching loss and the applied voltage at the turn-off time of the controllable switch 8 can be attained.

Description

Detailed Description of the Invention 3. Detailed Description of the Invention [Field of Industrial Application] The present invention relates to a switching power supply, and in particular to reducing the turn-off loss of a controllable switch in a power supply that drives a load such as a magnetron. The present invention relates to a switching power supply suitable for.

[Conventional technology]

As described in Japanese Patent No. 41212, the oscillation output of the magnetron can be changed by changing the input frequency applied to the high voltage transformer using a frequency converter. However, no consideration was given to switching loss when switching elements are used in frequency converters.

[Problem that the invention seeks to solve]

In a forward converter, energy is stored in the excitation inductance of the transformer during the ON period of the controllable switch and supplies power to the load, and this stored excitation energy is supplied to the load during the OFF period of the controllable switch. If , the current flowing through the controllable switch becomes the sum of the excitation current and the load current, which increases the turn-off loss, and the voltage applied to the controllable switch when turned off becomes excessive.

An object of the present invention is to reduce the turn-off loss of a controllable switch and to keep the voltage applied to the controllable switch low during turn-off.

[Means for solving problems]

The current flowing through the controllable switch during the on-period of the controllable switch is the sum of the transformer's excitation current and the secondary circuit current.

The current in the secondary circuit is determined by the leakage inductance of the transformer and the constants of the secondary side capacitor and load, but it is possible to make the secondary circuit current oscillating by appropriately selecting the values of the leakage inductance and capacitor. be. Therefore, the above object is achieved by selecting the circuit constants so that the secondary circuit current becomes oscillatory, and by making the ON period of the controllable switch longer than half the oscillation period.

[Effect]

By making the secondary circuit current during the on-period of the controllable switch into an oscillating waveform, the current that conventionally flows through the controllable switch during turn-off can be reduced, thereby reducing the turn-off loss of the controllable switch and the controllable switch's turn-off loss. Reduce the applied voltage.

〔Example〕

An embodiment of the present invention is shown in FIG. 1. In FIG. 0, 1 is a DC power source, 2 is a transformer, and 3 is a controllable switch.

4 is a control circuit, 5 is a diode, 6 is a capacitor, and 7 is a load. The operation of the embodiment is shown in the equivalent circuit shown in Fig. 2 and in Fig. 3.
This will be explained using the operating waveforms shown in Figure 4.5.

In FIG. 2, a DC voltage of 1 is applied during the ON period of transistor 8.
is applied to the excitation inductance 9, an excitation current IL flows, a load current ION flows through the capacitor 6, the leakage inductance 10, and the load 7 that constitute the secondary side series resonant circuit, and the sum of the excitation current Em and the resonance current ION is A transistor current It flows through the transistor 8 and can be expressed by the following equation.

Ir=It, +IoN FIGS. 3 and 4 show operating waveforms of the excitation current IL and the transistor current It when the power supply voltage fluctuates.

However, this is a case where the current flowing through the load 7 is not vibrating. If the on period is constant, the DC i! ! When the source voltage increases (dashed line I), the excitation current also increases (dashed line ■),
When the current decreases (dashed line ■), the excitation current also decreases (dashed line ■).
)Become.

The transistor current IT is the excitation current IL and the resonance current IO.
If N is added and the excitation current is large at turn-off, the transistor current also becomes large.

Regarding the transistor current, it will be explained with reference to the operating waveforms of FIG. 5 that by shaping the waveform of the load current ION, which is an added portion of the excitation current, the transistor current at the time of conventional turn-off can be reduced. The waveforms are shown when the load current is made to oscillate and is maximized at turn-off as shown in the figure, or when it is minimized during the on-period. By changing the load current to a waveform that minimizes the on-period flux, the transistor current at turn-off can be made smaller than the conventional value.

As shown in Fig. 6, when the on/off duty of the transistor 8 is controlled, for example, when the voltage of the DC power supply 1 increases (break gT), the duty of the transistor 8 is reduced and the excitation current becomes small (break ml). ), but the above-described matters also hold true when duty control is performed in this way.

A specific embodiment of the present invention is shown in the circuit shown in FIG. As shown in FIG. 2, the current flowing through the time-varying positive secondary circuit when the controllable switch is turned on is made oscillatory, and the transistor current waveform at turn-off is made smaller than before.

〔Effect of the invention〕

According to the present invention, in a high frequency switching power supply that obtains a high voltage output such as a magnetron using a forward converter, it is possible to reduce the switching loss and applied voltage when turning off the controllable switch, so it is possible to reduce the switching loss and the applied voltage with a smaller capacity than before. A switch can be used.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram of an embodiment of the present invention, Fig. 2 is an equivalent circuit diagram of the circuit shown in Fig. 1, Figs. 3 and 4 are operational waveform diagrams of Fig. 1, and Figs. 5 and 6. 2 is an operating waveform diagram of FIG. 2, FIG. 7 is a circuit diagram of another embodiment of the present invention, and FIG. 8 is an operating waveform diagram of FIG. 7. 1...Skin left thunder food 9...亦；鼎 3...Kaka 1
Example 4: Control circuit, 5: Diode,
6... Capacitor. 7...Load, 9...Exciting inductance, 10...
・Leakage inductance.

Claims (1)

[Claims] 1. A switching power supply in which a DC power supply, the primary winding of a transformer, and a controllable switch are connected in series, and the primary side of the transformer is composed of a diode, a capacitor, and a load,
A switching power supply characterized in that the leakage inductance and capacitor of the transformer cause the current waveform flowing in the secondary circuit of the transformer to become oscillatory during the ON period of the controllable switch.