Abstract:
A power converting apparatus with dynamical driving adjustment includes a rectifying unit, a power factor correction unit, a power conversion unit and a feedback unit. The rectifying unit rectifies an AC input power to generate and transfer a DC power to the power factor correction unit for performing power factor correction. A power factor correction power is generated and transferred to the power conversion unit. The feedback unit is electrically connected to the power conversion unit to form a closed control loop. A PWM driving controller of the power conversion unit performs an adjustment process to control a switching transistor based on a feedback signal from the feedback unit, and the power conversion unit converts the power factor correction power into an output power supplied to an external load. Thus, the margin for electromagnetic interference is increased, and both switching loss and conduction loss are considerably reduced.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the priority of Taiwanese patent application No. 104102393, filed on Jan. 23, 2015, which is incorporated herewith by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to a power converting apparatus, and more specifically to a power converting apparatus with dynamical driving adjustment for a power conversion unit using forward, full bridge, half bridge, boost or buck structure and for performing an adjustment process to reduce electromagnetic interference, switching loss and conduction loss by generating a PWM driving signal based on the dynamic state of the switch transistor. 
     2. The Prior Arts 
     Lately, power conversion efficiency has been a crucial topic for various electronic products, which need different voltage or current of electric power to normally operation. For instance, integrated circuits (ICs) need 5V or 3V, electric motors need 12V DC power, and lamps of LCD monitors need much higher voltage like 1150V. Thus, it is needed for power converters to meet the requirements of actual applications. 
     In the prior arts, the scheme of switching power conversion is one of the primary technologies of power conversion, and generally employs the pulsed width modulation (PWM) signal at high frequency to drive the switch transistor (or called driving transistor) to turn on so as to control the current of the inductors (or transformer) connected in series to the switch transistor. When the switch transistor is turned off, the current flowing through the inductor does not stop but gradually changes because the inductor has an effect of sustaining the current to avoid abrupt change. Thus, the inductor is charged or discharged, thereby attaining the purpose of changing the output voltage. 
     Please refer to  FIG. 1  showing the adjustment of driving capability for the switch transistor in the prior arts. The driving signal VD 1  is generated by the pre-driver to provide fixed driving capability through a source current/sink current architecture. To adjust driving capability of the switch transistor M 1 , the first gate resistor RG 1 , the second gate resistor RG 2 , the switch diode D 1  and the pull-low resistor RGG are used. The first gate resistor RG 1  and the second gate resistor RG 2  are connected in series, wherein first gate resistor RG 1  receives the driving signal VD 1  and the second gate resistor RG 2  drives the gate G of the switch transistor M 1 . Additionally, the switch diode D 1  and the second gate resistor RG 2  are parallel connected, and the pull-low resistor RGG is connected across the gate G of the switch transistor M 1  and the ground GND. Thus, to turn on the switch transistor M 1 , the driving signal VD 1  controls the driving current IG 1  to flow through the first gate resistor RG 1  and the second gate resistor RG 2  to the gate G of the switch transistor M 1 . At this time, the switch diode is reverse biased and turned off, and the voltage of the gate G is increased to turn on the switch transistor M 1 . To turn off. When the switch transistor M 1 , the driving signal VD 1  is reduced such that the voltage of the gate G drops because of the turn-off current IG 2 . Specifically, the switch diode is turned on due to forward biasing, and the turn-off current IG 2  flows through the switch diode D 1  and the second gate resistor RG 2 , instead of flowing through the first gate resistor Rg 1 . Additionally, the turn-off current IG 2  may flow to the ground GND through the pull-low resistor RGG. 
     For example, in the turn-off operation of the switch transistor M 1 , when the first gate resistor RG 1  is 0Ω (ohm) and the second gate resistor is 22Ω, the falling time for the drain-source voltage (Vds) of the switch transistor M 1  is about 80 ns, and the time for Miller plateau of the gate-source voltage (Vgs) of the switch transistor M 1  is about 200 ns. Alternatively, if the first gate resistor RG 1  and the second gate resistor are 100Ω (ohm) and 22Ω, respectively, the falling time is prolonged to about 104 ns, and the time for Miller plateau is increased up to about 300 ns. Thus, power conversion efficiency can be increased by reducing the first gate resistor RG 1  and the second gate resistor RG 2 , but EMI issue is still not improved. While EMI can be reduced by increasing the first gate resistor RG 1  and the second gate resistor RG 2  to prolong the falling time, Miller plateau extends too much and the effective turn-on resistance of the switch transistor M 1  can not fast decrease. As a result, power conversion efficiency is adversely affected. 
     It is obvious that the adjustment function for driving capability in the above traditional scheme is implemented by changing the first gate resistor RG 1  and the second gate resistor RG 2  to control the turn-off speed for the switch transistor M 1 . However, one drawback in the prior arts is that the first gate resistor RG 1  and the second gate resistor RG 2  can not be dynamically changed during switching operation to control the driving signal VD 1  to adjust the turn-on time and the turn-off time for the switch transistor M 1 . While it is possible to reduce switching loss, EMI issue is not solved. In other words, during the turn-on process of the switch transistor M 1 , when the original state of the switch transistor M 1  is turn-off and the turn-on current is zero or approximately zero, fast rising the driving signal VD 1  dose not improve switching loss issue, but causes EMI to get worse. Alternatively, when the switch transistor is partly or fully turned on, the turn-on current is considerable, and at this time, slowing down the rising speed and the falling speed of the driving signal VD 1  may result in larger power consumption at switching transition. 
     Therefore, it is greatly needed for the power control apparatus with dynamical adjustment of driving capability, which employs the feedback signal to perform the adjustment process to dynamically adjust the PWM driving signal based on the operation state of the switch transistor and consideration of EMI and switching loss, thereby overcoming the above problems in the prior arts. 
     SUMMARY OF THE INVENTION 
     The primary objective of the present invention is to provide a power converting apparatus with dynamical driving adjustment comprising a rectifying unit, a power factor correction unit, a power conversion unit and a feedback unit. The rectifying unit receives and rectifies an AC input power to generate and transfer a DC power to the power factor correction unit for performing a process of power factor correction, and then a power factor correction power is generated and transferred to the power conversion unit. Further, the feedback unit is electrically connected to the power conversion unit to form a closed control loop such that a PWM driving controller of the power conversion unit performs an adjustment process based on the feedback signal from the feedback unit to control a switching transistor, and the power conversion unit converts the power factor correction power into an output power supplied to an external load. 
     Specifically, the power conversion unit comprises a transformer, a pulsed width modulation (PWM) driving controller, a switch transistor, an auxiliary diode, an output inductor, an output diode and an output capacitor. The transformer comprises a first side coil and a second side coil, and the switch transistor is connected to the first side coil. The PWM driving controller controls the switch transistor. The second side coil is connected to the output diode and the output capacitor coupled in series. The output capacitor is connected to the external load in parallel so as to generate the output power, which is supplied to the external load. 
     The feedback unit comprises a first resistor, a second resistor, a third resistor, a thyristor and a photo coupler. The first, second and third resistors are sequentially connected in series. A connection point of the first and second resistors is connected to a connection point of the output diode and the output capacitor for receiving the output power. A connection point of the second and third resistors is connected to a gate end (G end) of the thyristor, and a positive end (A end) of the thyristor is connected to the output capacitor. The photo coupler is connected between a negative end (K end) of the thyristor and the first resistor such that the photo coupler generates and transfers the feedback signal to the power converting unit. 
     More specifically, the PWM driving controller performs an adjustment process based on the feedback signal to generate a PWM driving signal for controlling the switch transistor to turn on and turn off. 
     The switch transistor of the power conversion unit is an N type switch element like NMOS (N-channel Metal-Oxide Semiconductor) or NPN Bipolar Transistor). The PWM driving controller is implemented by an electrical circuit formed of discrete electronic elements, or by a central processing unit (CPU) or a microcontroller (MCU) executing a software program or firmware program. In other words, the PWM driving controller can operate by means of analog or full digital. In particular, the adjustment process performed by the PWM driving controller comprises the following steps. 
     At the beginning, since the initial current is considerably small in a CCM (continuous conduction mode), or zero in a DCM (discontinuous conduction mode), EMI is the first priority such that the first rising period when the driving voltage generated by the PWM driving controller from 0V up to about 5V needs to be appropriately prolonged. 
     Next, to reduce conduction loss when the transient phase of the voltage and current of the switch transistor finishes, the driving voltage is fast increased up to more than 8V such that the switch transistor is surely in the saturation state and the conduction current is kept as small as possible. That is, the second rising time when the driving voltage increases from 5V to more than 8V is shortened. 
     To turn off the switch transistor, the driving voltage should fall down, and since conduction loss is increased if the falling time for the driving voltage is too long, it needs to shorten the falling time. That is, the first falling time when the driving voltage falls from more than 8V down to about 5V is shortened. 
     Finally, the second falling time when the driving voltage falls from more than 5V down to zero is also suitably shortened. 
     Another objective of the present invention is to provide a power converting apparatus with dynamical driving adjustment comprising a rectifying unit, a power factor correction unit, a power conversion unit and a feedback unit. In particular, the power conversion unit comprises a full bridge structure, a half bridge structure, a boost structure or a buck structure, and a PWM driving controller of the power conversion unit performs the above-mentioned adjustment process to control the switch transistor so as to achieve the object of dynamically controlling the driving capability. 
     Thus, the PWM driving controller generates a PWM driving signal based on the feedback signal from the feedback unit to drive and control the switch transistor to turn on and off. As a result, dynamically controlling the driving capability of the switch transistor is implemented. Overall speaking, the EMI effect is improved by prolonging the first rising period, and the switching loss is reduced by shortening the second rising period, the first falling period and the second falling period. The present invention greatly improves the whole power conversion efficiency and is applicable to the practical field of power conversion which is quite related to both EMI issue and power conversion efficiency. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be apparent to those skilled in the art by reading the following detailed description of a preferred embodiment thereof, with reference to the attached drawings, in which: 
         FIG. 1  is an illustrative view showing the adjustment of driving capability for the switch transistor in the prior arts; 
         FIG. 2  is a view of the power converting apparatus with dynamical driving adjustment according to the first embodiment of the present invention; 
         FIG. 3  is a waveform diagram showing the first increasing period, the second increasing period, the first decreasing period and the second decreasing period according to the first embodiment; 
         FIG. 4  is a waveform diagram showing the turn-on process of the driving voltage according to the first embodiment; 
         FIG. 5  is a waveform diagram showing the turn-off process of the driving voltage according to the first embodiment; 
         FIG. 6  is a view of the power converting apparatus with dynamical driving adjustment according to the second embodiment of the present invention; 
         FIG. 7  is a view of the power converting apparatus with dynamical driving adjustment according to the third embodiment of the present invention; 
         FIG. 8  is a view of the power converting apparatus with dynamical driving adjustment according to the fourth embodiment of the present invention; 
         FIG. 9  is a view of the power converting apparatus with dynamical driving adjustment according to the fifth embodiment of the present invention; 
         FIG. 10  is a waveform diagram showing the first increasing period, the second increasing period, the first decreasing period and the second decreasing period for the PMOS according to the fifth embodiment; 
         FIG. 11  is a view of the power converting apparatus with dynamical driving adjustment according to the sixth embodiment of the present invention; 
         FIG. 12  is a view of the power converting apparatus with dynamical driving adjustment according to the seventh embodiment of the present invention; 
         FIG. 13  is a view of the power converting apparatus with dynamical driving adjustment according to the eighth embodiment of the present invention; 
         FIG. 14  is a view of the power converting apparatus with dynamical driving adjustment according to the ninth embodiment of the present invention; and 
         FIG. 15  is a view of the power converting apparatus with dynamical driving adjustment according to the tenth embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention may be embodied in various forms and the details of the preferred embodiments of the present invention will be described in the subsequent content with reference to the accompanying drawings. The drawings (not to scale) show and depict only the preferred embodiments of the invention and shall not be considered as limitations to the scope of the present invention. Modifications of the shape of the present invention shall too be considered to be within the spirit of the present invention. 
     Please refer to  FIG. 2  showing the power converting apparatus with dynamical driving adjustment according to the first embodiment of the present invention. As shown in  FIG. 2 , the power converting apparatus of the first embodiment comprises a rectifying unit  10 , a power factor correction (PFC) unit  20 , a power conversion unit  30  and a feedback unit  40  for converting an AC (alternative current) input power with an input voltage Vin into an output power with an DC (direct current) output voltage Vo, which is supplied to an external load RL. 
     More specifically, the rectifying unit  10  is configured to receive and rectify the AC input power so as to generate and transfer a DC power to the power factor correction unit  20 , which performs a process of power factor correction to generate and transfer a power factor correction power based on the received DC power. Then, the power conversion unit  30  receives the power factor correction power and performs a process of power conversion for converting the output power supplied the external load RL. The feedback unit  40  is electrically connected to the power conversion unit  30  for receiving and converting the output voltage Vo of the output power into a feedback signal FB, which is transferred back to the power conversion unit  30 , such that power conversion unit  30  obtains the state of the external load RL and dynamically adjusts the driving capability. As a result, the overall efficiency of power conversion is greatly improved. 
     The above-mentioned rectifying unit  10 , power factor correction unit  20  and feedback unit  40  can be easily implemented by traditional skills, and are not the key feature of the present invention. Therefore, only brief description about their electrical operation is explained for reference. 
     As shown in  FIG. 2 , the rectifying unit  10  is composed of four diodes as a bridge structure, and the power factor correction unit  20  generally comprises a PFC controller  21 , which works with an inductor  22 , a diode  23 , a capacitor  24  and a transistor  25  to exhibit the PFC function. 
     The power conversion unit  30  comprises a transformer TR 1 , a pulsed width modulation (PWM) driving controller  31 , a switch transistor  32 , an output diode Do and an output capacitor Co. The transformer TR 1  consists of the first side coil and the second side coil. The output diode Do is connected to the output capacitor Co in series. The switch transistor  32  is connected to one end of the first side coil. The PWM driving controller  31  performs the adjustment process to generate a PWM dicing signal for controlling the operation of turning on and off the switch transistor  32 . In addition, the second side coil is connected to the output diode Do and the output capacitor Co in series. The output capacitor Co is connected to the external load RL in parallel and generates the output power. 
     The PWM driving controller  31  is preferably implemented by a single semiconductor chip like microcontroller (MCU) or central processing unit (CPU), or alternatively, an electrical circuit formed of discrete electrical elements. Thus, the PWM driving controller  31  can operate by means of digital. The switch transistor  32  is an N type switch element like NMOS (N-channel Metal-Oxide Semiconductor) or NPN bipolar transistor. 
     Furthermore, the feedback unit  40  may comprise the first resistor R 1 , the second resistor R 2 , the third resistor R 3 , a thyristor DD and a photo coupler PH. The first resistor R 1 , the second resistor R 2  and the third resistor R 3  are connected sequentially in series. The connection point of the first resistor R 1  and the second resistor R 2  is connected to the connection point of the output diode Do and the output capacitor Co for receiving the input power. The connection point of the second resistor R 2  and the third resistor R 3  is connected to a gate end (G end) of the thyristor DD, a positive end (A end) of the thyristor DD is connected to the output capacitor Co, and the photo coupler PH is connected between a negative end (K end) of the thyristor DD and the first resistor R 1 . Thus, the photo coupler PH generates the feedback signal FB, which is transferred back to the PWM driving controller  31 . In particular, the PWM driving controller  31  obtains the loading state of the external load RL from the feedback signal FB, like light loading or heaving loading. The adjustment process is performed to generate the PWM driving signal VD with an appropriate waveform for controlling the switch transistor  32  to turn on or off, thereby reducing EMI and switching loss. 
     To explain the feature of reducing EMI and switching loss, please refer to  FIGS. 4 and 5  showing the waveforms for turning on and off, respectively. More specifically, the adjustment process of the PWM driving controller  32  comprises the following steps. 
     At the beginning, the driving voltage of the PWM driving signal VD generated by the PWM driving controller  31  is configured to increase from the lowest voltage VL of 0V up to the first voltage V 1  during the first rising period T 1 . 
     Next, the driving voltage of the PWM driving signal VD is configured to increase up to the second voltage V 2  larger than the first voltage V 1  and further attain to the highest voltage VH exceeding the second voltage V 2  during the second rising period T 2 . Thus, the drain-source voltage (Vds) of the switch transistor  32  to turn on the switch transistor  32  is lowered. 
     The present state is sustained for a preset period of time. 
     Then, the driving voltage of the PWM driving signal VD falls from the highest voltage VH exceeding the second voltage V 2  down to the first voltage V 1  during the first falling period T 3 . Finally, the driving voltage of the PWM driving signal VD falls from the first voltage V 1  down to the lowest voltage VL of 0V. 
     It is preferred that the first voltage V 1  is 3V to 6V, and the second voltage V 2  is 7V to 9V. Additionally, the drain-source voltage falling time and the drain-source voltage rising time of the switch transistor  32  at an output electrical power of 36 W are less than 200 ns and 100 ns, respectively. Specifically, the drain-source voltage falling time means a time when the drain-source voltage of the switch transistor  32  lowers from a highest drain-source voltage to a lowest drain-source voltage during a turn-on process, and the drain-source voltage rising time means a time when the drain-source voltage of the switch transistor  32  increases from the lowest drain-source voltage to the highest drain-source voltage during a turn-off process. The first rising period T 1 , the second rising period T 2 , the first falling period T 3  and the fourth falling period T 4  are dynamically adjusted and controlled by appropriately enhancing or reducing the driving capability of the PWM driving controller  31 . 
     In addition, the above lowest voltage VL and the highest voltage VH are also dynamically adjustable to match various MOS or transistors so as to reduce conduction loss and/or switching loss, thereby increasing power conversion efficiency. 
     To further clearly explain the actual performance of the present invention, an example for NMOS as the switch transistor  32  will be described in detail. 
     Specifically, the first voltage V 1  is configured to just turn on the switch transistor  32  such that the drain-source voltage Vds of the switch transistor  32  falls down. The first voltage V 1  is about Miller plateau of the switch transistor  32 , and Miller plateau is referred to the specific gate-source voltage Vgs of the switch transistor  32 , which is maintained as a constant during the switching transition from the turn-off state to the turn-on state or from the turn-on state to the turn-off state. When the drain current Id of the switch transistor  32  is zero, the first rising period T 1  is prolonged to reduce EMI issue because switching loss is not affected by the drain current Id of zero. In other words, the rising rate of the driving voltage of the PWM driving signal VD from 0V to the first voltage V 1  is kept as slow as possible within the allowable range of EMI. It is intended to adjust the second rising period T 2 , the first falling period T 3  and the fourth falling period T 4  as short as possible so as to reduce switching loss and conduction to a minimum value. The reason is that the drain current ID is not zero, and too slow rate for transition results in too much power consumption, leading to poor efficiency of power conversion. Thus, the first rising period T 1 , the second rising period T 2 , the first falling period T 3  and the fourth falling period T 4  are dynamically adjusted by enhancing or reducing the driving capability of the PWM diving controller  32 . 
     The effect of the above adjustment process will be described in detail. 
     First, the switching loss is not needed to consider but the EMI effect is taken in consideration when the initial turn-on current Ion is smaller at continuous conduction mode (CCM) like the very beginning of power conversion, or the initial turn-on current Ion is just zero at discontinuous conduction mode (DCM). That is, EMI is reduced as much as possible. This is achieved by properly prolonging the first rising period T 1 . 
     For the second rising period T 2  when the PWM driving signal VD is increased from the first voltage V 1  like 5V to the second voltage V 2  like 8V, the voltage and current of the switch transistor  32  are switched and completed, and the turn-on current Ion thus increases. To reduce the turn-on loss, it is needed to rise the PWM driving signal VD to exceed the second voltage V 2  like 8V so as to assure that the switch transistor  32  fast enter into the saturation state to minimize the turn-on resistance and the switching loss. 
     The first falling period T 3  for the PWM driving signal VD is substantially the time for the transition reversed to the second rising period T 2 . At this time, the voltage and current of the switch transistor  32  are not yet completed, so if the PWM driving signal VD is lowered too slow, the turn-on consumption is increased. Therefore, the first falling period T 3  is needed to shorten in order to fast reduce the turn-on current Ion. 
     Similarly, the second falling period T 4  is substantially the time for the transition reversed to the first rising period T 1 . At this time, the turn-on current Ion is larger and the efficiency has to be first considered. That is, the second falling period T 4  is needed to properly shorten to fast turn on the switch transistor  32 , thereby lowering the turn-on current Ion to zero or about zero. 
     Thus, the present invention performs the adjustment process based on the feedback signal to optimally adjust the PWM driving signal so as to change the driving capability of the switch transistor (the driving transistor or the driver). At the same time, both EMI effect and the turn-on loss are optimized to not only improve electrical performance but also greatly increase the overall efficiency of electrical conversion. 
     In addition, refer to  FIG. 6  illustrating the power converting apparatus with dynamical driving adjustment according to the second embodiment of the present invention. It should be noted that the second embodiment in  FIG. 6  is similar to the first embodiment in  FIG. 2 . The primary difference is that the second side coil of the transformer TR 1  in the power converting apparatus of the second embodiment has opposite polarity in comparison with the first embodiment, and the power conversion unit  30  of the second embodiment comprises an auxiliary diode DX and an output inductor Lo in addition to the transformer TR 1 , the PWM driving controller  31 , the switch transistor  32 , the output diode Do and the output capacitor Co. Thus, only the feature of the auxiliary diode DX and the output inductor Lo will be described in the following context. 
     As shown in  FIG. 6 , a positive end of the output diode Do is connected to an end of the second side coil, a positive end of the auxiliary diode DX is connected to another end of the second side coil, and a negative end of the output diode Do and a negative end of the auxiliary diode DX are connected to an end of the output inductor Lo. Another end of the output inductor Lo is connected to an end of the output capacitor Co, and another end of the output capacitor Co is connected to the other end of the second side coil. Accordingly, the output capacitor Co is connected to the external RL in parallel, and generates the output power. 
     Furthermore, the specific design of the structure shown in  FIG. 6  is a forward structure, and the design shown in  FIG. 2  is a flyback structure. These two structures are commonly used in the current power conversion design. 
     The adjustment process of the present invention is applicable to other power conversion structures like full bridge or half bridge, even boost structure or buck structure for controlling the recharging operation of rechargeable batteries. The following context will describe these structures to further explain the aspects of the present invention.  FIGS. 7, 8 and 9  show the third, fourth and fifth embodiments of the present, respective, for the power conversion unit  30  implemented by the full bridge structure, and  FIGS. 11, 12 and 13  show the sixth, seventh and eighth embodiments of the present, respective, for the power conversion unit  30  implemented by the half bridge structure. 
     As shown in  FIG. 7 , except the structure of the PWM driving controller  31 , other components of the power conversion unit  30  are similar to the second embodiment, and thus the description about the similar components are omitted. The PWM driving controller  31  is a full bridge structure and comprises a transformer TR 2 , four switch transistors Q 1 , Q 2 , Q 3  and Q 4 , an auxiliary diode DX, an output inductor Lo, an output diode o and an output capacitor Co. The transformer TR 2  comprises the first side coil and the second side coil with a center tap end. One end of the second side coil is connected to a positive end of the output diode Do, another end of the second side coil is connected to a positive end of the auxiliary coil, a negative end of the output diode Do is connected to a negative end of the auxiliary coil and one end of the output inductor Lo, another end of the output inductor Lo is connected to one end of the output capacitor Co, and another end of the output capacitor Co is connected to the center tap end of the second coil. Further, the output capacitor Co is connected to the external RL in parallel and generates the output power. 
     Moreover, the PWM driving controller  31  performs the adjustment process based on the feedback signal FB from the feedback unit  40  so as to generate four PWM driving signals for controlling the turn on and off operation of the switch transistors Q 1 , Q 2 , Q 3  and Q 4 , respectively. Particularly, the corresponding PWM driving signal for each of the switch transistors Q 1 , Q 2 , Q 3  and Q 4  has a specific waveform. This skill has been well known in the prior arts, and the description for the operation is thus omitted. However,  FIGS. 3 to 5  show the features of the PWM driving signals for clearly specifying the adjustment process of the PWM driving controller  31 . 
     The switch transistors Q 1 , Q 2 , Q 3  and Q 4  shown in  FIG. 7  are implemented by NPN bipolar transistors, and each of the switch transistors Q 1 , Q 2 , Q 3  and Q 4  shown in  FIG. 8  is NMOS. Further, the fifth embodiment of the present invention in  FIG. 9  uses two PMOS transistors and two NMOS transistors, which operate as the above transistors, and the related electrical operation is thus omitted. 
       FIG. 10  typically shows the operation waveform for the PWM driving signal VD′ of the PMOS switch transistor Q 1  or Q 3  of the fifth embodiment in  FIG. 9 . The waveform also indicates the first rising period T 1 ′, the second rising period T 2 ′, the first falling period T 3 ′ and the second falling period T 4 ′ for the PMOS Q 1  or Q 3 . It should be noted that the waveform of the switch transistor of PMOS is basically opposite in phase to the waveform of NOMS, and the operation of rising and falling is reversed so as to properly turn on and off the four switch transistors. 
     As shown in  FIG. 11 , the PWM driving controller  31  of the sixth embodiment is a half bridge structure and substantially comprises a transformer TR 2 , a PWM driving controller  31 , two switch transistors Q 1  and Q 2 , an auxiliary diode DX, an output inductor Lo, an output diode Do and an output capacitor Co. Specifically, both the switch transistors Q 1  and Q 2  are NPN bipolar transistors. The auxiliary diode DX, the output inductor Lo, the output diode Do and the output capacitor Co of the sixth embodiment are connected similar to the third embodiment shown in  FIG. 7 , and the related description of electrical operation is omitted. In addition, the first side coil of the transformer TR 2  is connected to the two switch transistor Q 1  and Q 2  as the prior arts. It should also be noted that the adjustment process performed by the PWM driving controller  31  is accordingly similar to the above embodiment. 
     Further refer to  FIGS. 12 and 13  for the seventh and eighth embodiments, respectively. Except different types of switch transistors Q 1  and Q 2  are used, the other components are the same as the above embodiment. The switch transistors Q 1  and Q 2  in  FIG. 12  are NMOS. In  FIG. 13 , one switch transistor is NMOS, and the other switch transistor is PMOS. The seventh and eighth embodiments exhibit the electrical operation similar to the sixth embodiment. 
     In addition,  FIG. 14  illustrates the power converting apparatus according to the ninth embodiment of the present invention providing boost conversion and dynamical driving adjustment. Specifically, the power converting apparatus according to the ninth embodiment comprises the PWM driving controller  31 , the first switch transistor  32 A, the second switch transistor  32 B, the boost resistor  60 , the buck resistor  70 , the inductor L 1  and the capacitor C 1 . The PWM driving controller  31  performs the adjustment process to convert the input power with a lower voltage from the first external power unit  80  into the output power with a higher voltage so as to supply the first external loading device  90 . Thus, the boost function is achieved. Since the operation of power conversion for the boost structure is well known in the prior arts, the following description is only focused on the electrical operation of the PWM driving controller  31  for controlling the first switch transistor  32 A and the second switch transistor  32 B to implement dynamic adjustment and reduce both switching loss and conduction loss. 
     As shown in  FIG. 14 , the first external loading device  90  is connected the grounded level through the first switch transistor  32 A and the second switch transistor  32 B in series. The first switch transistor  32 A is NMOS or PMOS, and the second switch transistor  32 B is NMOS, PMOS or diode. The gates of the first switch transistor  32 A and the second switch transistor  32 B are controlled by the PWM driving controller  31 . The first external power unit  80  is connected to the connection point P of the first switch transistor  32 A and the second switch transistor  32 B through the inductor L 1 , and the PWM driving controller  31  senses the voltage of the connection point P for controlling the first switch transistor  32 A and the second switch transistor  32 B. Further, the PWM driving controller  31  is connected to the first external power unit  80  through the boost resistor  60 , and connected to the first external loading device  90  through the buck resistor  70 . Additionally, one end of the capacitor C 1  is connected to the first external loading device  90 , and the other end of the capacitor C 1  is grounded. 
     The adjustment process of the PWM driving controller  31  is similar to the above embodiment shown in  FIG. 2 , and not described in the following context. It should be noted that the primary differences between the ninth embodiment in  FIG. 14  and the first embodiment in  FIG. 2  is that the PWM driving controller  31  in the power converting apparatus of the first embodiment controls only one switch transistor  32 , but the PWM driving controller  31  of the ninth embodiment can control the first switch transistor  32 A and the second switch transistor  32 B at the same time. Thus, when the first switch transistor  32 A is NMOS, the PWM driving controller  31  of the present embodiment operates according to the waveforms shown in  FIGS. 3, 4 and 5  to control the first switch transistor  32 A and the second switch transistor  32 B, and when the first switch transistor  32 A is PMOS, the PWM driving controller  31  operates according to the waveform shown in  FIG. 10 , besides the waveforms shown in  FIGS. 3, 4 and 5 . 
     Furthermore, to prevent the first switch transistor  32 A and the second switch transistor  32 B from damage due to over-current, it is crucial to keep the first switch transistor  32 A and the second switch transistor  32 B not turned on at the same time with reference to  FIGS. 3, 4 and 5 . That is, only when the first switch transistor  32 A is turned off, the second switch transistor  32 B can be turned on. Accordingly, only when the second switch transistor  32 B is turned off, the first switch transistor  32 A can be turned on. Alternatively, both the first switch transistor  32 A and the second switch transistor  32 B are turned off. 
       FIG. 15  shows the power converting apparatus with dynamical driving adjustment according to the tenth embodiment of the present invention, which is similar to the ninth embodiment shown in  FIG. 14 . The difference is that the connection for the first external power unit  80  and the first external loading device  90  of the tenth embodiment is opposite in comparison with the ninth embodiment. Therefore, the power conversion apparatus of the tenth embodiment can convert the power with higher voltage from the first external power unit  80  into the power with lower voltage supplied to the first external loading device  90 . The buck function of power conversion is achieved and the dynamic adjustment for driving capability is implemented so as to effectively reduce EMI and switching loss. 
     Accordingly, the PWM driving controllers  31  of the ninth and tenth embodiments are optionally implemented by an electrical circuit formed of discrete electronic elements, or alternatively by a central processing unit or a microcontroller executing a software program or firmware program. Preferably, the PWM driving controllers  31  performs digital operation by use of CPU or MCU, that is, a single integrated circuit (IC). 
     From the above mentioned, one aspect of the present invention is to employ the adjustment process performed by the PWM driving controller to slow down the turn-on rate of the switch transistor as much as possible when the initial conduction current is zero in DCM, thereby lowering the switching slope of the voltage, increasing EMI margin and reducing EMI. In addition, when the switch transistor operates in DCM and the initial conduction current is not zero, the turn-on rate of the switch transistor is configured as fast as possible to reduce switching loss and increase power conversion efficiency. Also, the electrical property is assured. In particular, the present invention provides the adjustment process for driving capability which is applicable to various circuit structures like forward, full bridge, half bridge, boost structure or buck structure, and thus exhibits much industrial applicability for wide application fields. 
     Furthermore, the present invention may use different MOS or different transistor as the switch transistor to be driven, and specifically implements the effect of reducing conduction loss and switching loss so as to increase the overall power conversion efficiency. 
     Although the present invention has been described with reference to the preferred embodiments thereof, it is apparent to those skilled in the art that a variety of modifications and changes may be made without departing from the scope of the present invention which is intended to be defined by the appended claims.