Abstract:
A method, which is for determining switching state of a transistor-based switching device that includes a set of transistors, includes the steps of: applying a bias voltage to a transistor having a fastest response so as to dispose the transistors in the set in a desired transistor state; detecting a voltage level at a transistor having a slowest response to the bias voltage; and comparing the detected voltage level with a predetermined threshold voltage level in order to determine the switching state of the switching device. A transistor-based switching device is also disclosed.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims priority of Taiwanese application no. 093135229, filed on Nov. 17, 2004.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The invention relates to a method for determining switching state of a transistor-based switching device, more particularly to a method for determining effectively the switching state of a transistor-based switching device.  
         [0004]     2. Description of the Related Art  
         [0005]      FIG. 1  illustrates a simplified transistor-based switching device  1  that includes first and second sets of field-effect transistors  9 ,  10 , a controller circuit  11  (e.g., a PWM or a PFM controller in a switching power supply), first and second logic circuits  12 ,  13 , and first and second driving circuits  14 ,  15 .  
         [0006]     The field-effect transistors in each of the first and second sets  9 ,  10  are connected in parallel.  
         [0007]     The first driving circuit  14  has input and output nodes connected electrically and respectively to the first logic circuit  12  and the gates of the field-effect transistors in the first set  9 , whereas the second driving circuit  15  has input and output nodes connected electrically and respectively to the second logic circuit and the gates of the field-effect transistors in the second set  10 .  
         [0008]     The first and second logic circuits  12 ,  13  are connected electrically to the controller circuit  11 . The first logic circuit  12  is further connected electrically across gate and source of the field-effect transistor in the second set  10  closest to the output node of the second driving circuit  15 . The second logic circuit  13  is further connected electrically a cross gate and source of the field-effect transistor in the first set  9  closest to the output node of the first driving circuit  14 .  
         [0009]     In operation, the first driving circuit  14  applies a voltage to the gates of the field-effect transistors in the first set  9  so as to dispose the transistors in the first set  9  in a non-conducting state. The second logic circuit  13  then detects a voltage level across the gates and sources of the field-effect transistors in the first set  9 , and compares the detected voltage level with a predetermined threshold voltage level. When the detected voltage level is found to be lower than the predetermined threshold voltage level, the second driving circuit  15  applies a voltage to the gates of the field-effect transistors in the second set  10  so as to dispose the transistors in the second set  10  in a conducting state. After a predetermined period, the second driving circuit  15  applies the voltage to the gates of the field-effect transistors in the second set  10  so as to dispose the field-effect transistors in the second set  10  in a non-conducting state. The first logic circuit  12  then detects a voltage level across the gates and sources of the field-effect transistors in the second set  10 , and compares the detected voltage level with the predetermined threshold voltage level. When the detected voltage level is found to be lower than the predetermined threshold voltage level, the first driving circuit  14  applies the voltage to the gates of the field-effect transistors in the first set  9  so as to dispose the field-effect transistors in the first set  9  in a conducting state. After the predetermined period, the whole operation is repeated.  
         [0010]     The problem with the simplified transistor-based switching device  1  is that, since the gates of the field-effect transistors are disposed at different distances from the output node of the corresponding driving circuit  14 ,  15 , the field-effect transistors in each of the first and second sets  9 ,  10  have different actual response times to the bias voltage. Since the voltage level is detected across the gate and source of the transistor in the first (second) set  9  ( 10 ) closest to the output node of the second (first) driving circuit  15  ( 14 ), the detected voltage level is not an accurate indication of the transistor state of all the field-effect transistors in each of the first (second) set  9  ( 10 ).  
         [0011]     In order to solve the above problem, with further reference to  FIG. 2 , a conventional transistor-based switching device  1 ′ further includes first and second delay circuits  16 ,  17 , and first and second detecting circuits  18 ,  19 .  
         [0012]     Each of the first and second delay circuits  16 ,  17  is connected electrically to a respective one of the first and second logic circuits  12 ,  13 .  
         [0013]     The first detecting circuit  18  is connected electrically to the first delay circuit  16  and across the gate and source of the field-effect transistor in the second set  10  closest to the output node of the second driving circuit  15 , whereas the second detecting circuit  19  is connected electrically to the second delay circuit  17  and across the gate and source of the field-effect transistor in the first set  9  closest to the output node of the first driving circuit  14 .  
         [0014]     In operation, the first driving circuit  14  applies a voltage to the gates of the field-effect transistors in the first set  9  so as to dispose the field-effect transistors in the first set  9  in a non-conducting state. The second detecting circuit  19  then detects a voltage level across the gates and sources of the field-effect transistors in the first set  9 , and compares the detected voltage level with the predetermined threshold voltage level. The second delay circuit  17  introduces a delay into the detected voltage level. The delay introduced by the second delay circuit  17  ensures that the field-effect transistors in the first set  9  are all in the non-conductive state. Thereafter, the second logic circuit  13  enables the second driving circuit  15  to apply a voltage to the gates of the field-effect transistors in the second set  10  so as to dispose the field-effect transistors in the second set  10  in a conducting state. After the predetermined period, the second driving circuit  15  applies a voltage to the gates of the field-effect transistors in the second set  10  so as to dispose the field-effect transistors in the second set  10  in a non-conducting state. The first detecting circuit  18  then detects a voltage level across the gates and sources of the field-effect transistors in the second set  10 , and compares the detected voltage level with the predetermined threshold voltage level. The first delay circuit  16  introduces a delay into the detected voltage level. Then, the first driving circuit  14  is enabled by the first logic circuit  12  to apply a voltage to the gates of the field-effect transistors in the first set  9  so as to dispose the field-effect transistors in the first set  9  in a conducting state. After the predetermined period, the whole operation is repeated.  
         [0015]     Although the aforesaid conventional transistor-based switching device  1 ′ achieves its intended purpose, it requires the first and second delay circuit  16 ,  17  and the first and second detecting circuits  18 ,  19 . This results in a larger size for the conventional transistor-based switching device  1 ′ and in higher fabrication costs. Furthermore, the predetermined delay time periods introduced by the first and second delay circuits  16 ,  17 , when not accurate, the field-effect transistors in the first and second sets  9 ,  10  may conduct simultaneously, which results in a poor operating efficiency for the conventional transistor-based switching device  1 ′.  
       SUMMARY OF THE INVENTION  
       [0016]     Therefore, the object of the present invention is to provide a method for determining switching state of a transistor-based switching device that is capable of overcoming the aforementioned drawbacks of the prior art.  
         [0017]     According to one aspect of the present invention, a method for determining switching state is implemented using a transistor-based switching device that includes a set of transistors. The transistors in the set are connected in parallel in such a manner that one of the transistors has a fastest response to a bias voltage in relation to the other transistors in the set, and that another of the transistors has a slowest response to the bias voltage in relation to the other transistors in the set. The method comprises the steps of:  
         [0018]     A) applying the bias voltage to a gate of the transistor having the fastest response so as to dispose the transistors in the set in a desired transistor state;  
         [0019]     B) detecting a voltage level across gate and source of the transistor having the slowest response to the bias voltage; and  
         [0020]     C) comparing the voltage level detected in step B) with a predetermined threshold voltage level in order to determine the switching state of the switching device.  
         [0021]     According to another aspect of the present invention, a method for determining switching state is implemented using a transistor-based switching device that includes first and second sets of transistors. The transistors in each of the first and second sets are connected in parallel in such a manner that one of the transistors in each of the first and second sets has a fastest response to a bias voltage in relation to the other transistors in the same one of the first and second sets, and that another of the transistors in each of the first and second sets has a slowest response to the bias voltage in relation to the other transistors in the same one of the first and second sets. The method comprises the steps of:  
         [0022]     A) applying a bias voltage to a gate of the transistor in the first set that has the fastest response so as to dispose the transistors in the first set in a first transistor state;  
         [0023]     B) detecting a voltage level across gate and source of the transistor in the first set that has the slowest response; and  
         [0024]     C) comparing the voltage level detected in step B) with a predetermined threshold voltage level.  
         [0025]     According to yet another aspect of the present invention, a transistor-based switching device comprises a set of transistors connected in parallel. Each of the transistors has a gate. The gate of one of the transistors has a fastest response to a bias voltage in relation to the other transistors in the set being provided with a drive end. The gate of another one of said transistors that has a slowest response to the bias voltage in relation to the other transistors in the set being provided with a detected end. The drive end is adapted to be applied with a bias voltage so as to dispose the transistors in the set in a desired transistor state. A voltage at the said detected end is detected for comparison with a predetermined threshold voltage level in order to determine switching state of the switching device. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]     Other features and advantages of the present invention will become apparent in the following detailed description of the preferred embodiments with reference to the accompanying drawings, of which:  
         [0027]      FIG. 1  is a schematic block diagram of a simplified transistor-based switching device;  
         [0028]      FIG. 2  is a schematic block diagram of a conventional transistor-based switching device;  
         [0029]      FIG. 3  is a schematic block diagram of the preferred embodiment of a transistor-based switching device according to the present invention;  
         [0030]      FIG. 4  is a flowchart of the preferred embodiment of a method for determining switching state of the transistor-based switching device shown in  FIG. 3 ; and  
         [0031]      FIG. 5  is a schematic block diagram of an alternative embodiment of a transistor-based switching device according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0032]     Referring to  FIG. 3 , the preferred embodiment of a transistor-base switching device according to this invention is shown to include first and second sets of transistors  26 ,  27 .  
         [0033]     In this embodiment, each of the transistors  260 ,  270  in each of the first and second sets  26 ,  27  may be one of an n-type metal-oxide-semiconductor field-effect transistor (MOSFET), an n-type metal-insulator-semiconductor field-effect transistor (MISFET), and an n-type junction field-effect transistor (JFET). In an alternative embodiment, each of the transistors  260 ,  270  in each of the first and second sets  26 ,  27  may be one of a p-type MOSFET, a p-type MISFET, and a p-type JFET.  
         [0034]     The transistors  260  in the first set  26  are connected in parallel in such a manner that one of the transistors  260  in the first set  26  has a fastest response to a bias voltage in relation to the other transistors  260  in the same first set  26 , and that another of the transistors  260  in the first set  26  has a slowest response to the bias voltage in relation to the other transistors  260  in the same first set  26 . In particular, the source (S) of each of the transistors  260  in the first set  26  is connected to those of the other ones of the transistors  260  in the first set  26 . The drain (D) of each of the transistors  260  in the first set  26  is connected to those of the other ones of the transistors  260  in the first set  26 . The gate (G) of each of the transistors  260  in the first set  26  is connected to those of the other ones of the transistors  260  in the first set  26 .  
         [0035]     Similarly, the transistors  270  in the second set  27  are connected in parallel in such a manner that one of the transistors  270  in the second set  27  has a fastest response to a bias voltage in relation to the other transistors  270  in the same second set  27 , and that another of the transistors  270  in the second set  27  has a slowest response to the bias voltage in relation to the other transistors  270  in the same second set  27 . In particular, the source (S) of each of the transistors  270  in the second set  27  is connected to those of the other ones of the transistors  270  in the second set  27 . The drain (D) of each of the transistors  270  in the second set  27  is connected to those of the other ones of the transistors  270  in the second set  27 . The gate (G) of each of the transistors  270  in the second set  27  is connected to those of the other ones of the transistors  270  in the second set  27 .  
         [0036]     The transistor-base switching device further includes a controller circuit  21 , first and second logic circuits  22 ,  23 , and first and second driving circuits  24 ,  25 .  
         [0037]     In this embodiment, the controller circuit  21  may be one of a pulse width modulator (PWM) or a pulse frequency modulator (PFM) controller commonly found in switching power supply applications.  
         [0038]     The first logic circuit  22  is connected electrically to and is controlled by the controller circuit  21 , and is connected electrically across the gate (G) and source (S) of the transistor  270  in the second set  27  that has the slowest response. On the other hand, the second logic circuit  23  is connected electrically to and is controlled by the controller circuit  21 , and is connected electrically across the gate (G) and the source (S) of the transistor  260  in the first set  26  that has the slowest response.  
         [0039]     The first driving circuit  24  is connected electrically to the first logic circuit  22 , and the gate (G) of the transistor  260  in the first set  26  that has the fastest response. On the other hand, the second driving circuit  25  is connected electrically to the second logic circuit  23 , and the gate (G) of the transistor  270  in the second set  27  that has the fastest response.  
         [0040]     It is noted that the transistor  260  in the first set  26  that has the fastest response is at a location closest to an output node of the first driving circuit  24 , and the transistor  260  in the first set  26  that has the slowest response is at a location farthest from the output node of the first driving circuit  24 . Moreover, the transistor  270  in the second set  27  that has the fastest response is at a location closest to an output node of the second driving circuit  25 , and the transistor  270  in the second set  27  that has the slowest response is at a location farthest to the second driving circuit  25 .  
         [0041]     The preferred embodiment of a method for determining switching state of a transistor-based switching device according to this invention includes the steps shown in  FIG. 4 .  
         [0042]     In step  41 , the first driving circuit  24  applies a voltage to the gate (G) of the transistor  26  in the first set  260  that has the fastest response so as to dispose the transistors  260  in the first set  26  in a non-conducting state.  
         [0043]     In step  42 , the second logic circuit  23  detects a voltage level across the gate (G) and source (S) of the transistor  260  in the first set  26  that has the slowest response.  
         [0044]     In step  43 , the second logic circuit  23  compares the voltage level detected in step  42  with a predetermined threshold voltage level.  
         [0045]     In step  44 , when the voltage level detected in step  42  is found in step  43  to be lower than the predetermined threshold voltage level, the flow proceeds to step  45 . Otherwise, the flow goes back to step  42 .  
         [0046]     In step  45 , the second driving circuit  25  is enabled to apply a voltage to the gate (G) of the transistor  270  in the second set  27  that has the fastest response so as to dispose the transistors  270  in the second set  27  in a conducting state.  
         [0047]     After a predetermined time period, in step  46 , the second driving circuit  25  is enabled to apply the voltage to the gate (G) of the transistor  270  in the second set  27  that has the fastest response so as to dispose the transistors  270  in the second set  27  in the non-conducting state.  
         [0048]     In step  47 , the first logic circuit  22  detects a voltage level across the gate (G) and source (S) of the transistor  270  in the second set  27  that has the slowest response.  
         [0049]     In step  48 , the first logic circuit  22  compares the voltage level detected in step  47  with the predetermined threshold voltage level.  
         [0050]     In step  49 , when the voltage level detected in step  47  is found in step  48  to be lower than the predetermined threshold voltage level, the flow proceeds to step  50 . Otherwise, the flow goes back to step  47 .  
         [0051]     In step  50 , the first driving circuit  24  is enabled to apply the voltage to the gate (G) of the transistor  260  in the first set  26  that has the fastest response so as to dispose the transistors  260  in the first set  26  in the conducting transistor state. After the predetermined time period, the flow goes back to step  41 .  
         [0052]     From the above description, since the first (second) logic circuit  22  ( 23 ) detects the voltage level at the transistor  270  ( 260 ) in the second (first) set  27  ( 26 ) that has the slowest response to the bias voltage, the detected voltage level is an accurate indication of the transistor state of all the transistors  270  ( 260 ) in the second (first) set  27  ( 26 ).  
         [0053]     In an alternative embodiment, as illustrated in  FIG. 5 , each of the transistors  260 ,  270  in the first and second sets  26 ,  27  may be one of an n-type and a p-type double gate MOSFET.  
         [0054]     The first driving circuit  24  is connected electrically to a first gate (G 1 ) of the transistor  260  in the first set  26  that has the fastest response, whereas the second driving circuit  24  is connected electrically to a first gate (G 1 ) of the transistor  270  in the second set  27  that has the fastest response.  
         [0055]     The first logic circuit  22  is connected electrically across second gate (G 2 ) and a source of the transistor  270  in the second set  27  that has the slowest response, whereas the second logic circuit  23  is connected electrically across a second gate (G 2 ) and a source of the transistor  260  in the first set  26  that has the slowest response.  
         [0056]     Since the operation of the transistor-based switching device of this embodiment is similar to that described hereinabove, a detailed description of the same will be dispensed with herein for the sake brevity.  
         [0057]     While the present invention has been described in connection with what is considered the most practical and preferred embodiments, it is understood that this invention is not limited to the disclosed embodiments but is intended to cover various arrangements included within the spirit and scope of the broadest interpretation so as to encompass all such modifications and equivalent arrangements.