Abstract:
The present invention discloses a power conversion apparatus for pulse current suitable for electrical discharge machining. The power conversion apparatus comprises a linear power conversion circuit and a switching power conversion circuit. The linear power conversion circuit offers a fast response to changes in output load and is used to adjust the output voltage. The magnitude or the waveform of the current through the workpiece is adjusted by the power resistor or the power transistor. Then, the switching power conversion circuit supplies the workpiece a stable power to make the power conversion apparatus output a stable pulse current to achieve the goal of high power conversion efficiency, high power density, and small size.

Description:
RELATED APPLICATIONS  
       [0001]     The present application is based on, and claims priority from, Taiwan Application Serial Number 93138844, filed Dec. 15, 2004, the disclosure of which is hereby incorporated by reference herein in its entirety.  
       FIELD OF THE INVENTION  
       [0002]     The present invention relates to a power conversion apparatus for pulse current, and more particularly, to a power conversion apparatus having a linear power conversion circuit and a switching power conversion circuit to improve the efficiency of power conversion, decrease the size of the apparatus, and supply stable voltages and currents to meet the demands of electrical discharge machining (EDM).  
       BACKGROUND OF THE INVENTION  
       [0003]     Electrical machining involves machining metal materials by melting them in electrolyte or by directly etching them by electricity. Electrical discharge machining (EDM) is a non-contact machining method; that is, there is always a gap between the tool and the workpiece such that there is no mechanical force on the workpiece. Thus, EDM is suitable for machining delicate, tiny workpieces and complexly shaped workpieces, thin walls of which are easily misshaped.  
         [0004]     At present, EDM is based on etching by electrical pulse discharging. The tool and the workpiece are electrically connected to each side of the pulse power, respectively. The difference between EDM and electrolytic machining is that the tool, one electrode, is consumed during machining. Therefore, the material used for the electrode and the machining parameters should be chosen carefully and the size of the electrode should also be designed reasonably to decrease machining inaccuracy caused by the consumption of the electrode.  
         [0005]     However, since the gap for electrical discharging needs to be very small and since the tool and the workpiece are constantly being etched away during machining, discharging quickly ceases because the gap widens. Therefore, the machining system needs to have a server control feeding system with high accuracy to constantly feed the electrode and to automatically keep the discharging gap at a best distance.  
         [0006]     Reference is made to FIGS.  1  to  3 , which illustrate conventional power conversion apparatuses for electrical discharge machining. Since the linear power converter is used as the main structure in the conventional power conversion apparatuses for electrical discharge machining, the output voltage and current are adjusted by using power resistor D or power transistor E working in the active region to make the voltage big enough to discharge to the workpiece A. Hence, these circuits each have low power conversion efficiency and a large circuit footprint; therefore, they are unfavorable in the industry.  
         [0007]     Reference is made to  FIG. 4 , which illustrates a circuit diagram of a switching power converter. If the switching power converter is used alone, although high power conversion efficiency and a smaller size can be achieved, desirable load current cannot be obtained, causing the slow dynamic response of the switching power converter.  
         [0008]     In summary, the power conversion efficiency of the linear power converters is not sufficient, so the size of the power conversion apparatus of the machine becomes quite big. Moreover, although using the switching power converter alone can improve the power conversion efficiency and decrease the size of the power conversion apparatus, it cannot effectively improve its dynamic response to a sudden change in load current.  
       SUMMARY OF THE INVENTION  
       [0009]     Therefore, one objective of the present invention is to provide a power conversion apparatus suitable for electrical discharge machining of which the output pulse current to the workpiece is more stable.  
         [0010]     Another objective of the present invention is to provide a power conversion apparatus with a linear power conversion circuit of which the dynamic response of the output current and voltage is better than that of the prior art.  
         [0011]     Still another objective of the present invention is to provide a power conversion apparatus with a switching power conversion circuit having higher power conversion efficiency, higher power density and smaller size than that of the prior art.  
         [0012]     Still another objective of the present invention is to provide a switching DC/AC converter converting the AC power into DC power supplied by an outer power supply.  
         [0013]     Still another objective of the present invention is to provide a power conversion apparatus for pulse current that responds quickly to load changes.  
         [0014]     According to the aforementioned objectives, the present invention provides a power conversion apparatus for pulse current, comprising a linear power conversion circuit and a switching power conversion circuit. The linear power conversion circuit, electrically connected to an outer power supply, supplies a load current and strengthens the dynamic response to sudden changes in load current. The switching power conversion circuit, electrically connected to the outer power supply, strengthens the load current by phase-shifting to make the load current have better dynamic response during the sudden changes in the load current.  
         [0015]     According to the preferred embodiment of the present invention, the outer power supply is a DC power supply that can be an input power supply for the power conversion apparatus. The linear power conversion circuit is composed of a single circuit or is composed of a plurality of parallel circuits. The linear power conversion circuit can change the amount of the parallel circuits according to the load current. The switching power conversion circuit is selected from the group consisting of a buck power conversion circuit, a boost power conversion circuit, a buck-boost power conversion circuit, a forward power conversion circuit, a flyback power conversion circuit, a push-pull power conversion circuit, a half-bridge power conversion circuit, an asymmetrical half-bridge power conversion circuit, and a full-bridge power conversion circuit.  
         [0016]     According to another objective, the present invention provides a power conversion apparatus for pulse current, comprising a linear power conversion circuit, a switching power conversion circuit, and a switching DC/AC converter. The linear power conversion circuit, electrically connected to an outer power supply, supplies a load current and strengthens the dynamic response to sudden changes of load current. The switching power conversion circuit, electrically connected to the outer power supply, strengthens the load current by phase-shifting to make the load current have better dynamic response during sudden changes in the load current. The switching DC/AC converter, used for supplying input power, converts the AC power supplied by the outer power supply into DC power.  
         [0017]     According to the preferred embodiment of the present invention, the outer power supply is an AC power supply that can be an input power supply for the power conversion apparatus.  
         [0018]     According to another objective, the present invention provides a power conversion apparatus for pulse current, comprising a synchronous-rectifying buck converter with four phases, a buck converter with four phases, a plurality of diodes and a linear power converter. The synchronous-rectifying buck converter with four phases, electrically connected to an outer power supply, modulates the output voltage. The buck converter with four phases, electrically connected to a discharging electrode, modulates the output voltage. The diodes, electrically connected between the synchronous-rectifying buck converter with four phases and the buck converter with four phases, feed back the extra inductor voltage. The linear power converter, electrically connected to the discharging electrode, supplies the power to a load.  
         [0019]     The property of the linear power conversion circuit, the fast response to changes in the output load, is used to adjust the output voltage. The magnitude or the waveform of the current through the workpiece is adjusted by the power resistor or the power transistor. Then, the switching power conversion circuit supplies the workpiece a stable power to make the power conversion apparatus output a stable pulse current to achieve the goal of high power conversion efficiency, high power density, and small size. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]     The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:  
         [0021]      FIG. 1  illustrates the conventional linear power conversion circuit with power resistors controlling the output current;  
         [0022]      FIG. 2  illustrates the conventional linear power conversion circuit with power transistors controlling the output current;  
         [0023]      FIG. 3  illustrates the conventional linear power conversion circuit with power resistors and power transistors controlling the output current;  
         [0024]      FIG. 4  illustrates the switching power conversion circuit of the conventional electrical discharge machine;  
         [0025]      FIG. 5  is a block diagram of the power conversion apparatus of the present invention;  
         [0026]      FIG. 6  is a block diagram of the power conversion apparatus of the present invention with an AC power supply;  
         [0027]      FIG. 7  illustrates a power conversion apparatus for pulse current according to another preferred embodiment of the present invention;  
         [0028]      FIG. 8  illustrates the linear parts in this preferred embodiment of the present invention;  
         [0029]      FIG. 9  illustrates the buck converter with four phases according to the preferred embodiment of the present invention;  
         [0030]      FIG. 10  illustrates the waveform of the output pulse current with frequency being 50 kHz; and  
         [0031]      FIG. 11  illustrates the waveform of the output pulse current with frequency of 250 kHz.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0032]     In order to make the illustration of the present invention more explicit and complete, the following description is stated with reference to FIGS.  5  to  11 .  
         [0033]     Reference is made to  FIG. 5 , illustrating a power conversion apparatus for pulse current according to the preferred embodiment of the present invention. The power conversion apparatus for pulse current according to the preferred embodiment of the present invention comprises a linear power conversion circuit  3  and a switching power conversion circuit  4  wherein the linear power conversion circuit  3  and the switching power conversion circuit  4  are set in parallel. The linear power conversion circuit  3 , electrically connected to an outer power supply  2 , supplies a load current and strengthens the dynamic response to sudden changes in the load current. The switching power conversion circuit  4 , electrically connected to the outer power supply  2 , strengthens the load current by phase-shifting to make the load current have better dynamic response during the sudden change in the load current. The switching power conversion circuit  4  is selected from the group consisting of a buck power conversion circuit, a boost power conversion circuit, a buck-boost power conversion circuit, a forward power conversion circuit, a flyback power conversion circuit, a push-pull power conversion circuit, a half-bridge power conversion circuit, an asymmetrical half-bridge power conversion circuit, and a full-bridge power conversion circuit.  
         [0034]     When a DC current is input from the outer power supply  2 , the DC current is input simultaneously to the linear power conversion circuit  3  and the switching power conversion circuit  4 . With the fast response of the linear power conversion circuit  3 , the pulse current of the load is supplied to a workpiece  1  in a very short time, and another part of the current is compensated by the switching power conversion circuit  4 . The power is supplied to the load current, the dynamic response to sudden changes in the load current is strengthened, and the output voltage is clamped to output a pulse current to be applied variously for different loads during power conversion of different instruments. It is suitable for the power conversion apparatus such as the electrical discharge machine or the voltage regulator module (VRM) owing to the fast response and high power conversion efficiency, especially when the output current is pulse current.  
         [0035]     The linear power conversion circuit  3  may be composed of a single circuit or composed of a plurality of parallel circuits to supply the load current. Furthermore, the linear power conversion circuit  3  can change the amount of the parallel circuits according to the load current.  
         [0036]     Reference is made to  FIG. 6 , illustrating a power conversion apparatus for pulse current according to another preferred embodiment of the present invention. When the outer power supply  2  is an AC power supply, a switching DC/AC converter  5  can be electrically connected to the outer power supply  2  before the power is supplied to the linear power conversion circuit  3  and the switching power conversion circuit  4 . The switching DC/AC converter  5  can convert the AC power supplied by the outer power supply  2  into DC power. Then, with the combination of the linear power conversion circuit  3 , having fast response to sudden changes in load current, and the switching power conversion circuit  4 , having high power conversion efficiency and high power density, a stable pulse current is output to the workpiece  1 .  
         [0037]     Reference is made to  FIG. 7 , illustrating a power conversion apparatus for pulse current according to another preferred embodiment of the present invention. The power conversion apparatus for pulse current according to this preferred embodiment of the present invention comprises a synchronous-rectifying buck converter with four phases  6 , a buck converter with four phases  7 , a plurality of diodes T 1 B˜T 4 B and a linear power converter Q 1 . The synchronous-rectifying buck converter with four phases  6  is electrically connected to an outer power supply  2 . The synchronous-rectifying buck converter with four phases  6  is composed of transistors S 1 ˜S 8  and inductors L 1 ˜L 4 . The synchronous-rectifying buck converter with four phases  6  modulates the output voltage. The phase angle of the four phases of the synchronous-rectifying buck converter with four phases  6  is 90 degrees.  
         [0038]     The buck converter with four phases  7  is electrically connected to a discharging electrode. The buck converter with four phases  7  is composed of transistors S 1 A˜S 4 A, transistors S 1 B˜S 4 B and coupling transformers T 1 A˜T 4 A. The buck converter with four phases  7  modulates the output voltage. The diodes T 1 B˜T 4 B are used to feed back the extra inductor voltage. The linear power converter Q 1 , electrically connected to the discharging electrode, is used to supply the power to the load.  
         [0039]     As mentioned above, when the outer power supply  2  outputs a voltage Vin, the output voltage Vo 1  is modulated by the synchronous-rectifying buck converter with four phases  6  to supply the workpiece  1  with currents of different pulses. The buck converter with four phases  7  then outputs the voltage to the linear power converter Q 1 . The linear power converter Q 1  supplies the power to the load to speed up the reaction, to strengthen the dynamic response, to increase the conversion efficiency, and further to meet the demands of the workpiece  1 . The diodes T 1 B˜T 4 B comprise a feedback path. They are used to feed the extra inductor voltage supplied to the load back to Vo 1  to protect the components within the normal rated range.  
         [0040]     Reference is made to  FIG. 7  and  FIG. 8 , wherein  FIG. 8  illustrates the linear parts in this preferred embodiment of the present invention. The linear power converter Q 1  is used to control the magnitude of the output current. The diodes T 1 B˜T 4 B clamp the voltage to the workpiece  1  to prevent the voltage from being too high. The switching power conversion apparatus uses phase-shifting by single-phase or multi-phase parallel circuitry to quicken response.  
         [0041]     Reference is made to  FIG. 9 , illustrating the synchronous-rectifying buck converter with four phases according to the preferred embodiment of the present invention. The synchronous-rectifying buck converter with four phases  6  composed of transistors S 1 ˜S 8  and inductors L 1 ˜L 4  is used to improve the conversion efficiency and the reaction speed.  
         [0042]     The IsSpice simulation program can be used to verify the feasibility and the result of the present invention. Referring to the circuit diagram in  FIG. 7 , when Vin=48V, Vo 1 =24V, L 1 ˜L 4 =20 uH, the ratio of the primary winding and the secondary winding of the coupling transformers is 2, the magnetizing inductance is 20 uH, and the current through the workpiece is high frequency pulse current of  12 A. These results are plotted in  FIG. 10  and  FIG. 11  and illustrate that the present invention quickly supplies the workpiece with currents of different frequencies.  
         [0043]     According to the aforementioned description, the present invention has various advantages. The present invention combines the advantages of the linear power conversion circuit with the switching power conversion circuit to power the electrical discharge machining process. Thus, the response time is shortened for when the output current changes, the power conversion efficiency is increased, the power density is improved, and the physical size is reduced.  
         [0044]     Furthermore, the parallel circuits of the linear power conversion circuit  3  and the parallel phases and angles of the switching power conversion circuit  4  can be controlled by analog circuits or by a single-chip controller or DSP controller. They can also be controlled by an PWM IC combined with analog circuits or by an PWM IC combined with a microprocessor, or controlled even by a microprocessor alone.  
         [0045]     The present invention combines the linear power conversion circuit having fast response to sudden changes in load current with the switching power conversion circuit having high power conversion efficiency and high power density.  
         [0046]     The power conversion apparatus of the present invention outputs a fixed pulse current or a program controllable pulse current. It can be applied variously for different loads in power conversion of different instruments. It is suitable for a power conversion apparatus such as in an electrical discharge machine or a voltage regulator module (VRM) due to its fast response and high power conversion efficiency, especially when the output current is pulse current.  
         [0047]     As is understood by a person skilled in the art, the foregoing preferred embodiments of the present invention are illustrative of the present invention rather than limiting of the present invention. It is intended that various modifications and similar arrangements are included within the spirit and scope of the appended claims, the scope of which should be accorded the broadest interpretation so as to encompass all such modifications and similar structure.