JPS6260912B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention uses a switching transistor driven by an output from an oscillation circuit to turn on and off the primary circuit of a transformer, thereby obtaining a high DC voltage from the secondary circuit of the transformer. The present invention relates to a high voltage power supply circuit configured such that the DC high voltage is made variable by changing the primary side voltage of the transformer.

BACKGROUND ART High voltage power supply circuits used in devices such as scanning electron microscopes are required to have a wide high voltage output voltage range as the performance of the devices improves, and strict conditions are required in circuit design. FIG. 1 is a connection diagram showing the configuration of a conventional separately excited oscillation type high voltage power supply circuit. In the figure, TR ₁ is a transistor that operates as a series regulator, and its collector is supplied with a power supply voltage V, its base is connected to a variable power supply Ei, and its emitter is
One end of the primary winding of the step-up transformer _T1 is connected. OSC is an oscillator with an oscillation frequency f ₀ and its output is given to the base of transistor TR ₂ . On the other hand, the collector of the transistor _TR2 is connected to the primary winding of the transformer _T2 , and the power supply voltage V is applied to the other end of the primary winding.
The secondary winding of the transformer _T2 is connected between the base and emitter of the switching transistor _TR3 via a resistor. Further, the other end of the primary winding of the transformer _T1 is connected to the collector of the transistor _TR3 . RC is a voltage doubler rectifier circuit that receives the secondary output of the transformer _T1 and converts it into a high voltage DC voltage. FC is a filter that converts the output of the voltage doubler rectifier circuit RC into a ripple-free DC voltage. The output V ₀ of is the output voltage of the circuit shown in FIG.

The operation of the conventional circuit configured as described above is summarized as follows. The output of the frequency f ₀ of the oscillator OSC turns the transistor TR ₂ on and off, and the change in the primary side current of the transformer T ₂ due to this on/off is transmitted to the secondary side of the transformer T ₂ , and the secondary side current turns the transistor TR 2 on and off. TR ₃ is driven. On the other hand, a voltage E ₀ is applied to the primary side of the transformer T ₁ by the variable power supply Ei. This transformer _T1 is
The above transistor is switched at frequency f ₀
Since it serves as the collector load of TR ₃ , changes in excitation current caused by turning on and off the transistor TR ₃ are transmitted to the secondary side of the transformer T ₁ . The voltage generated on the secondary side of the transformer _T1 is rectified and smoothed by the subsequent voltage doubler rectifier circuit RC and filter FC, and becomes a high voltage output _V0 . The level of this output V ₀ can be varied within a range of several hundreds of volts to several tens of kilovolts by changing the output voltage of the variable power supply Ei.

In this type of conventional circuit, the oscillation frequency f ₀ of the oscillator OSC is generally designed so that the circuit efficiency is maximized at the high voltage maximum output load. However, the optimum oscillation frequency f ₁ at the maximum high voltage output of this high voltage power supply circuit and the optimum oscillation frequency f ₂ at the minimum high voltage output
It is different from Therefore, in the conventional circuit, resonance could not be achieved at the time of high-voltage minimum output, and power was wasted in the switching transistor _TR3 , causing the transistor _TR3 to generate heat and be thermally destroyed.
Further, since this state is an unstable oscillation state, there are cases where stable high voltage output cannot be obtained.

In addition, as the primary side voltage changes, the transformer
The reason why the resonant frequency of T ₁ changes is because the capacitance of a capacitor used in a voltage doubler rectifier (RC) etc. connected to the secondary side of the transformer T ₁ changes depending on the secondary side voltage. This is because it changes over time.

The present invention has been made in view of these points, and by providing an oscillation circuit whose oscillation frequency changes according to the primary side voltage of the transformer and driving the switching transistor _TR3 with this output, two This realizes a highly reliable high-voltage power supply circuit that has good circuit efficiency whether the next-side high-voltage output is maximum, minimum, or any value in between, and therefore does not become unstable in oscillation.

Hereinafter, the present invention will be explained in detail with reference to the drawings.

FIG. 2 is a diagram showing the operating principle of an embodiment of the present invention.
The same parts as in FIG. 1 are indicated with the same reference numerals. In the figure, LD is a level detection circuit that detects the output level of the primary side voltage E ₀ of the transformer T ₁ and outputs a signal according to the detected level. or,
OC is an oscillation circuit whose oscillation frequency is controlled by the output of the level detection circuit LD, and this oscillation circuit
By the output of OC, the switching transistor
TR ₃ is now being driven.

By the way, as mentioned above, in this type of circuit,
The optimum value of the oscillation frequency f ₀ for obtaining a predetermined high-voltage power V ₀ changes depending on the value of V ₀ , in other words, depending on the value of E ₀ that is proportional to V ₀ . FIG. 3 is a characteristic diagram showing the relationship between the primary voltage E ₀ of the transformer T ₁ and the optimum frequency f ₀ , where the horizontal axis is E ₀ and the vertical axis is f ₀ . This figure 3 shows that when the primary voltage required to maximize the high voltage output V ₀ is E ₀₁ , the frequency f ₀ that maximizes the circuit efficiency is f ₁ , and the secondary The primary side voltage required to minimize the side high voltage output is
When E ₀₂ , the frequency that maximizes circuit efficiency is f ₀
shows that f ₂ . i.e. high voltage output
It is shown that as V ₀ increases, the optimum value of the oscillation frequency f ₀ also increases. The oscillation circuit OC in FIG. 2 is connected to the level detection circuit in FIG. 2 so that it oscillates at the frequency shown in the characteristic diagram in FIG.
Controlled by LD.

Therefore, in the above embodiment, the switching transistor _TR3 does not generate excessive heat and is not thermally destroyed. This improves the reliability of the circuit and stabilizes the output.

FIG. 4 is an electrical connection diagram of a specific example of a high-voltage power supply circuit based on the operating principle shown in FIG. 2. In the figure, a diode D connected to the primary voltage E ₀
is a level detection diode, and this diode D and the resistor _R1 connected in series thereto constitute the level detection circuit LD. Furthermore, the portion surrounded by a broken line in the figure is the oscillation circuit OC. The oscillation circuit OC shown here is a blocking oscillation circuit composed of a coil L, a capacitor C, a resistor _R2 , a transistor _TR4 , and the like. Note that the output of the level detection circuit LD is the resistance of the oscillation circuit OC.
It is added to the connection point A between R ₂ and the coil L.

In this high voltage power supply circuit, the level detection circuit
The oscillation frequency f of the oscillation circuit OC when no control signal is received from the LD side (when the diode D is non-conductive) is expressed by the following equation (1).

f=k ₁ /CR ₂ ...(1) On the other hand, the oscillation circuit OC when receiving the control signal
The oscillation frequency f' is given by the following equation (2).

f'=k ₁ /CR ₂ +k ₂ E ₀ /R ₁ ...(2) However, in equations (1) and (2), k ₁ and k ₂ are positive constants, and the resistances R ₁ and R ₂ And as the values of capacitor C, R ₁ , R ₂ and C were used as they were.

Therefore, the frequency f shown in equation (1) is made to correspond to the frequency f ₂ (see Figure 3) when the secondary side high voltage output is the minimum, and when E ₀ = E ₀₁ , f' = f If k ₂ and R ₁ are determined so that the value becomes ₁ , the operation of the embodiment shown in FIG. 2 can be performed.

In the above embodiment, the level detection circuit LD has one
Although the next-side voltage E ₀ itself is given, a voltage related to E ₀ , for example, the output voltage of the variable power source Ei, etc. may be given. Also, frequency correction
It is not necessary to perform it continuously as in the above example, and f ₁
It is also possible to change it step by step between and _f2 .

As explained above in detail, according to the present invention, the oscillation frequency of the oscillation circuit changes depending on the primary side voltage of the transformer, so that the oscillation frequency of the oscillation circuit changes from the minimum output to the maximum output of the secondary high voltage output. , it is possible to maintain good circuit efficiency and realize a highly reliable high voltage power supply circuit.

[Brief explanation of the drawing]

Fig. 1 is an electrical connection diagram showing a conventional high-voltage power supply circuit, Fig. 2 is a diagram of the operating principle of an embodiment of the present invention, Fig. 3 is a frequency characteristic diagram, and Fig. 4 is a specific diagram of the embodiment of Fig. 2. It is a configuration diagram. RC...Voltage doubler rectifier circuit, FC...Filter,
LD... Level detection circuit, OC... Oscillator circuit, T ₁ ,
T ₂ ...Transistor, TR ₁ to _{TR 4} ...Transistor,
OSC...Oscillator, _R1 , _R2 ...Resistor, D...Diode, C...Capacitor, Ei...Variable power supply.

Claims (1)

[Claims] 1. A high-voltage power supply circuit configured to turn on and off a primary circuit of a transformer using a switching transistor driven by an output from an oscillator circuit to obtain a high DC voltage from a secondary circuit of the transformer, the circuit comprising: In a high-voltage power supply circuit in which the DC high voltage is made variable by changing the primary side voltage of the transformer, the oscillation circuit changes in accordance with the change in the resonant frequency of the transformer due to the change in the primary side voltage. A high-voltage power supply circuit characterized by having a circuit for automatically changing the oscillation frequency.