Abstract:
A fine-turning circuit is used for adjusting the output voltage of a main transformer in an electrical power convert device and solving the problem of turns granularity of conventional transformers. The fine-turning circuit includes an auxiliary transformer having a primary coil and a secondary coil, and a voltage level clamper connected with the secondary coil of the auxiliary transformer via a diode. The primary coil of the auxiliary transformer is connected with the renovated winding in serial. When a current flows through the renovated winding, the secondary coil of the auxiliary transformer reacts to an amended voltage so as to amend the output voltage of the main transformer. The amended voltage is adjusted via changing the turn ratio of the primary coil to the secondary coil of the auxiliary transformer.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a fine-tuning circuit for the winding voltage of a transformer. In particular, this invention relates to a fine-tuning circuit that is used for adjusting the output voltage of a main transformer in a power switching and conversion device. 
   2. Description of the Related Art 
   Transformers comprise a primary coil and at least one secondary coil, and are used for converting electrical power. Generally, the primary coil of a transformer is connected with a primary circuit and the secondary coil is connected with a secondary circuit. Electrical power from the primary circuit is transmitted to the primary coil of the transformer. Then, the transformer converts the electrical power into magnetic force, and the magnetic force is transmitted to the primary side of the transformer via the iron core of the transformer and is converted into electrical power and outputted via the secondary coil of the transformer. The electrical power on the secondary coil, such as the current or voltage, is related to the number of turns of the coil. The desired voltage or current is outputted from the secondary coil by adjusting the number of turns of the primary coil and the secondary coil of the transformer. Therefore, a variety of output voltages are available by adjusting the number of turns of the primary coil and a plurality of secondary coils of the transformer. 
   Nowadays, electronic circuits usually require a high power rate and a low voltage output. The secondary coil of the transformer cannot have too many turns, because of the problem of turns granularity. An engineer usually adjusts the number of turns of the coil to change the output voltage. However, this method cannot fully provide the desired voltage. For example, when a circuit needs to be 8V and 5V, 5V can be outputted if the number of the turns of the secondary coil of the transformer is 2. Under this condition, 7.5V is outputted when the number of the turns of the secondary coil of the transformer is 3 or 10V is outputted when the number of the turns of the secondary coil of the transformer is 4. The output voltage does not equal 8V. Therefore, the engineer needs to use a secondary coil with 3.2 turns to output the desired 8V. It can be implemented in a transitional transformer and a special transformer is needed. However, the special transformer has drawbacks, such as it is large in size, expensive, and needs special electric-magnetic components that are not easily obtained, etc. 
   Reference is made to  FIG. 4 , which is a schematic diagram of U.S. Pat. No. 6,348,848 that discloses a structure of a transformer having fractional-turn winding. An alternative solution for this problem is a coil with an exact number of turns (meaning, for example, 2, 3 or 4 turns, as opposed to 2.1, 3.3 or 4.7 turns) adopted and a resistor element, such as a linear constant voltage regulator, or voltage-time eating means, such as a saturable reactor, adopted to lower the voltage. However, both the resistor element and the voltage-time eating means suffer heat problems and lose power in the process.  FIG. 5  is a circuit diagram of the power supply disclosed in U.S. Pat. No. 6,735,094. The power supply uses the saturable reactors Lsat 3  and Lsat 4  to lower the output voltages of secondary coils N 3  and N 4  of the transformers  1  and  2 . 
     FIG. 6A  is a circuit diagram of a linear constant voltage regulator that has the same effect as the resistor. 
     FIG. 6B  is a circuit diagram of a switching constant voltage regulator that has the same effect as the resistor. The switching constant voltage regulator amends the output voltage and reduces power loss. Because the switching constant voltage regulator is expensive, large and complex it is not extensively used in electronic circuits. 
     FIG. 7  is a circuit diagram of a converter having multiple outputs. A circuit of the converter having multiple outputs was disclosed on Feb. 1, 2001 in EDN by Robert Bell. Robert Bell utilized a differential transformer to solve the problem of turns granularity of conventional transformers. The differential transformer T 2  is an auxiliary transformer having a primary coil  23 T and a secondary coil  10 T. The primary coil  23 T of the differential transformer T 2  couples to the secondary coil  1 T of the main transformer T 1  in parallel. The secondary coil  10 T of the differential transformer T 2  couples to another secondary coil  1 T of the main transformer T 1  in serial. The secondary coil  10 T of the differential transformer T 2  is used for amending the output voltage of another secondary coil  1 T of the main transformer T 1 . 
   However, the differential transformer T 2  still has a problem. Although the primary coil  23 T of the differential transformer T 2  does not carry the output current, the primary coil  23 T can have more turns. However, the secondary coil  10 T cannot have more turns because the output current is already loaded on it. Therefore, the amended voltage is limited and the problem of turns granularity cannot be fully solved. 
   The differential transformer has another problem. The drop in voltage caused by a DC current flowing through the secondary coil  1 T of the main transformer T 1  makes the voltage-time product between the ends of the coil not equal zero. Because the primary coil  23 T of the differential transformer T 2  couples to the secondary coil  1 T of the main transformer T 1  in parallel, the non-zero voltage-time product nearly saturates the magnetic core of the differential transformer. In order to prevent saturation from occurring, the differential transformer T 2  must have a larger resistor and a magnetic core of the primary coil  23 T so that the volume of the differential transformer T 2  is large. 
   SUMMARY OF THE INVENTION 
   One particular aspect of the present invention is to provide a method for adjusting the output voltage of a main transformer in an electrical power convert device. The present invention improves upon the problem of turns granularity of the conventional transformer and provides a fine-tuning circuit that is simple and has high efficiency to dynamically amend the output voltage of the main transformer. 
   The present invention includes an auxiliary transformer. When the auxiliary transformer operates during the power transmitting cycle, its action is similar to the differential transformer. However, this auxiliary transformer does not suffer the problem of saturation. Because the auxiliary transformer is removed from the main transformer when the main transformer is operating in idle, the magnetic flux of the auxiliary transformer is automatically reset. 
   In another embodiment, the present invention uses a voltage clamper connected with the secondary coil of the auxiliary transformer to adjust the output voltage of the main transformer. 
   In one further embodiment, the voltage clamper is replaced by a linear constant voltage regulator. The circuit of the linear constant voltage regulator is simple and is composed of commonly available components. The power needed for this circuit is low because the surplus voltage of the main transformer winding can be converted into voltage of a proper quantity and forward transmitted to the output circuit. 
   For further understanding of the invention, reference is made to the following detailed description illustrating the embodiments and examples of the invention. The description is only for illustrating the invention and is not intended to be considered limiting of the scope of the claim. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The drawings included herein provide a further understanding of the invention. A brief introduction of the drawings is as follows: 
       FIG. 1A  is a circuit diagram of the first embodiment of the present invention; 
       FIG. 1B  is a circuit diagram of the second embodiment of the present invention; 
       FIG. 1C  is a circuit diagram of the third embodiment of the present invention; 
       FIG. 1D  is a circuit diagram of the fourth embodiment of the present invention; 
       FIG. 2A  is the same as  FIG. 1B ; 
       FIG. 2B  is a circuit diagram of the fifth embodiment of the present invention; 
       FIG. 2C  is a circuit diagram of the sixth embodiment of the present invention; 
       FIG. 2D  is a circuit diagram of the seventh embodiment of the present invention; 
       FIG. 3A  is a circuit diagram of the eighth embodiment of the present invention; 
       FIG. 3B  is a circuit diagram of the ninth embodiment of the present invention; 
       FIG. 3C  is a circuit diagram of the tenth embodiment of the present invention; 
       FIG. 3D  is a circuit diagram of the eleventh embodiment of the present invention; 
       FIG. 4  a schematic diagram of a structure of a transformer having the fractional-turn winding disclosed in U.S. Pat. No. 6,348,848; 
       FIG. 5  a circuit diagram of a power supply disclosed in U.S. Pat. No. 6,735,094; 
       FIG. 6A  is a circuit diagram of a linear constant voltage regulator; 
       FIG. 6B  is a circuit diagram of a switching constant voltage regulator; and 
       FIG. 7  is a circuit diagram of a differential transformer. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Reference is made to  FIG. 1A , which shows a circuit diagram of the first embodiment of the present invention. The main transformer  101  has a primary coil  101   a , a secondary coil  101   b , and another secondary coil  101   c . The secondary coil  101   b  is a subject secondary winding and has the problem of turns granularity. An auxiliary transformer  102  has a primary coil  102   a , and a secondary coil  102   b . The primary coil  102   a  of the auxiliary transformer  102  is connected with the renovated winding  101   b  of the main transformer  101  in series. The primary coil  102   a  of the auxiliary transformer  102  is connected with the secondary coil  102   b  in serial to form a secondary coil of a differential transformer straddling over the subject secondary winding  101   b . An amended voltage that is equivalent to the end voltage of the renovated winding  101   b  reacts to the two ends of the primary coil  102   a . A diode  103  is connected with the subject secondary winding  101   b  via the primary coil  102   a . The output voltage of the subject secondary winding  101   b  is adjusted via the amended voltage of the two ends of the primary coil  102   a , and is transmitted to an output filtering capacitor  104  via the diode  103 . The surplus voltage on the subject secondary winding  101   b  caused by the turns granularity is cancelled by the amended voltage of the two ends of the primary coil  102   a . The problem of turns granularity is solved. 
   Reference is made to  FIG. 1B , which shows a circuit diagram of the second embodiment of the present invention. The main transformer  101  and the auxiliary transformer  102  are the same as those in  FIG. 1A . The primary coil  102   a  of the auxiliary transformer  102  is not connected with the secondary coil  102   b  in serial. The secondary coil  102   b  of the auxiliary transformer  102  is connected in serial with a level-clamping diode  105 . The secondary coil  102   b  straddles over the output filtering capacitor  104  via the level-clamping diode  105 . The output voltage of the subject secondary winding  101   b  is transmitted to the output filtering capacitor  104  via the primary coil  102   a  and the diode  103 . When output current flows through the primary coil  102   a  of the auxiliary transformer  102 , a voltage is reacted on the secondary coil  102   b  of the auxiliary transformer  102  so as to output a secondary current. The secondary current flows into the output filtering capacitor  104  via the level-clamping diode  105 . The cathode of the diode  103  is connected with the cathode of the level-clamping diode  105  and both have a forward bias. Therefore, the voltage on the anode of both diodes is also similar. 
   The operation of this embodiment is the same as that of  FIG. 1A , in which the anodes are connected together. The amended voltage at the two ends of the primary coil  102   a  can be adjusted by changing the number of turns of the coil of the auxiliary transformer  102 , as with the differential transformer. However, the difference between this embodiment and the differential transformer is that when the output current flowing through diode  103  stops, the level-clamping diode  105  also cuts off. At this time, the auxiliary transformer  102  escapes from the loading circuit. When the auxiliary transformer  102  escapes from the loading circuit, the voltage on the coil flies back freely and the magnetic flux of the iron core is also released. 
   Reference is made to  FIG. 1C , which shows a circuit diagram of the third embodiment of the present invention.  FIG. 1C  is similar to  FIG. 1B . The difference is that the secondary coil  102   b  of the auxiliary transformer  102  is connected with a fixed voltage  6 , not an output filtering capacitor  104 . When output current flows through the primary coil  102   a  of the auxiliary transformer  102 , a voltage reacts on the secondary coil  102   b  of the auxiliary transformer  102 . The voltage is limited by the fixed voltage  6  via the level-clamping diode  105 . The voltage value is the sum of the drop in voltage of the level-clamping diode  105  and the fixed voltage  6 . The voltage reacts to the primary coil  102   a  of the auxiliary transformer  102  according to the turn ratio of the coils of the primary coil  102   a  of the auxiliary transformer  102 . Via this connection relationship, the output voltage of the subject secondary winding  101   b  is adjusted by changing the voltage on the primary coil  102   a  of the auxiliary transformer  102 . The voltage on the primary coil  102   a  of the auxiliary transformer  102  is determined by the drop in voltage of the level-clamping diode  105 , the voltage value of the fixed voltage  6 , and the turn ratio of the coils of the auxiliary transformer  102 . Thereby, the present invention changes the turns ratio of the coils of the auxiliary transformer  102  to amend the output voltage on the subject secondary winding  101   b  so as to provide the desired voltage. Alternatively, the present invention also can dynamically change the voltage value of the fixed voltage  6  to achieve the same effect. 
   Reference is made to  FIG. 1D , which shows a circuit diagram of the fourth embodiment of the present invention.  FIG. 1D  is similar to  FIG. 1C . The difference is that a Zener diode  107  replaces the fixed voltage  6 . The operating method and principle of this embodiment is the same as  FIG. 1C . 
   Reference is made to  FIG. 2A , which is the same as  FIG. 1B . The voltage on the output filtering capacitor  104  is supplied by the output voltage of the subject secondary winding  101   b , and the output filtering capacitor  104  is used as a voltage clamper. The secondary coil  102   b  of the auxiliary transformer  102  is connected with the level-clamping diode  105  in serial, and both the secondary coil  102   b  and the level-clamping diode  105  are straddled over and connected with the output filtering capacitor  104 . The secondary coil  102   b  of the auxiliary transformer  102  obtains the output voltage of the output filtering capacitor  104  via the level-clamping diode  105  and reacts with the obtained voltage to the primary coil  102   a  of the auxiliary transformer  102 . The voltage on the primary coil  102   a  of the auxiliary transformer  102  is used to reduce the surplus output voltage on the subject secondary winding  101   b.    
   Reference is made to  FIG. 2B , which shows a circuit diagram of the fifth embodiment of the present invention. One end of the secondary coil  102   b  of the auxiliary transformer  102  is connected with the level-clamping diode  105  in serial, and is then connected to the output filtering capacitor  104 . Another end of the secondary coil  102   b  of the auxiliary transformer  102  is connected with another secondary coil  101   c  of the main transformer  101 . Therefore, the secondary coil  102   b  of the auxiliary transformer  102  is clamped to a voltage. The voltage is the voltage value of the output filtering capacitor  10  subtracted from the voltage value on another secondary coil  101   c  (the drop in voltage on the level-clamping diode  105  can usually be ignored). This embodiment solves the problem of turns granularity of the main transformer  101  and transfers the demanded loading current to improve the voltage cross-regulation when another secondary coil  101   c  of the main transformer  101  requires additional loading to balance the loads between the outputs. 
   Reference is made to  FIG. 2C , which shows a circuit diagram of the sixth embodiment of the present invention. A rectifier rectifies the AC voltage on another secondary coil  101   c  of the main transformer  101  into DC voltage. The DC voltage is outputted to a secondary circuit. The secondary circuit is connected with a second output filtering capacitor  202 , and the second output filtering capacitor  202  is used as a voltage clamper. The secondary coil  102   b  of the auxiliary transformer  102  is connected with the level-clamping diode  105  in serial, and the secondary coil  102   b  and the level-clamping diode  105  are straddled over the second output filtering capacitor  202 . The voltage on the secondary coil  102   b  of the auxiliary transformer  102  is obtained from the voltage of the second output filtering capacitor  202  via the level-clamping diode  105 . Therefore, this embodiment solves the problem of turns granularity of the main transformer  101  and transfers the demanded loading current to improve the voltage cross-regulation when the secondary circuit requires additional electrical power to balance the loads between the outputs. 
   Reference is made to  FIG. 2D , which shows a circuit diagram of the seventh embodiment of the present invention. The secondary coil  102   b  of the auxiliary transformer  102  is connected with the level-clamping diode  105  in serial, and the secondary coil  102   b  and the level-clamping diode  105  are straddled over the subject secondary winding  101   b  of the main transformer  101  and the second output filtering capacitor  202 . Therefore, the secondary coil  102   b  of the auxiliary transformer  102  is clamped to a voltage. The voltage is the voltage value of the value subtracted from the voltage value on the subject secondary winding  101   b  of the main transformer  101  (the drop in voltage on the level-clamping diode  105  can usually be ignored). Therefore, this embodiment solves the problem of turns granularity of the main transformer  101  and transfers the demanded loading current to improve the voltage cross-regulation when the renovated winding  101   b  of the main transformer  101  requires additional electrical power to balance the loads between the outputs. 
   Reference is made to  FIG. 3A , which shows a circuit diagram of the eighth embodiment of the present invention. A resistor  310  is connected with the secondary coil  102   b  of the auxiliary transformer  102  and the level-clamping diode  105 , and the resistor  310 , the secondary coil  102   b , and the level-clamping diode  105  are straddled over the output filtering capacitor  104 . When current flows through the resistor  301 , there is a voltage difference on the resistor  301 . The voltage difference on the resistor  301  increases the voltage on the secondary coil  102   b  and reacts to the primary coil  102   a . The voltage on the primary coil  102   a  cancels the reaction voltage on the subject secondary winding  101   b  of the main transformer  101  so as to lower the voltage on the output filtering capacitor  104 . Therefore, the present invention changes the resistance of the resistor  301  to adjust the output voltage. 
   The resistor  301  can be a variable resistor. The resistance of the resistor  301  is changed according to the magnitude of the DC voltage on the output filtering capacitor  104 . When the DC voltage becomes higher, the resistance of the resistor  301  also becomes higher. When the DC voltage becomes lower, the resistance of the resistor  301  also becomes lower. Alternatively, the resistor  301  can be a control circuit that is included in a control system. The control circuit changes the resistance of the resistor  301  according to the magnitude of the output voltage so as to control the voltage clamping level of the secondary coil of the auxiliary transformer. Therefore, the output voltage is automatically adjusted. 
   Reference is made to  FIG. 3B , which shows a circuit diagram of the ninth embodiment of the present invention. A linear constant voltage circuit  302  is used to replace the resistor  301  in the eighth embodiment (as shown in  FIG. 3A ). The linear constant voltage circuit  302  includes an output terminal, an input terminal, and a grounding terminal. The input terminal of the linear constant voltage circuit  302  is connected with the secondary coil  102   b  of the auxiliary transformer  102  and the level-clamping diode  105  in serial. A filtering capacitor  303  is straddled over and connected with the input terminal. The filtering capacitor  303  is not a necessary component (represented by a dash line). It usually cooperates with a general linear constant voltage regulator IC, such as 78xx IC series. The output terminal of the linear constant voltage circuit  302  is connected with the output filtering capacitor  104 . The grounding terminal of the linear constant voltage circuit  302  is connected with a reference voltage. Therefore, the linear constant voltage circuit  302  can detect the output voltage. When the output voltage is lower than a target voltage, the linear constant voltage circuit  302  conducts the current outputted from the secondary coil  102   a  to the output filtering capacitor  104 . At this moment, the operating principle of the linear constant voltage circuit  302  is the same as the resistor  301  operating under a low resistance. Then the voltage on the primary coil  102   a  and the secondary coil  102   b  of the auxiliary transformer  102  also decreases. Thereby, the renovation acting on the reacting voltage of the subject secondary winding  101   b  of the primary transformer  101  becomes less and the voltage on the output filtering voltage  104  increases. 
   Alternatively, when the output voltage detected by the linear constant voltage circuit  302  is higher than the target voltage, the linear constant voltage circuit  302  stops outputting the current from the secondary coil  102   b  to output filtering capacitor  104 . At this moment, the operating principle of the linear constant voltage circuit  302  is the same as the resistor  301  operating under a high resistance. The voltage on the primary coil  102   a  and the secondary coil  102   b  of the auxiliary transformer  102  is then increased. Thereby, the renovation acting on the reacting voltage of the subject secondary winding  101   b  of the primary transformer  101  becomes stronger and the voltage on the output filtering voltage  104  decreases. The output voltage on the output filtering capacitor  104  can be adjusted by setting the target voltage in the linear constant voltage circuit  302 . 
   In this embodiment, the linear constant voltage circuit  302  merely adjusts the input/output voltage difference and part of the output current. Therefore, the power loss of the linear constant voltage circuit  302  is lower than the conventional linear constant voltage regulator, as shown in  FIG. 6A . 
   Reference is made to  FIG. 3B . The merits of the present invention adopting the linear constant voltage circuit  302  is illustrated by the following formulas.
 
 V   — 102 a=V 1− V 2  (1)
 
   In formula (1), V1 is the reacting voltage on the renovated winding  101   b  of the primary transformer  101 ; V_ 102   a  is the amended voltage on the primary coil  102   a  of the auxiliary transformer  102 ; V2 is the desired voltage renovated with the amended voltage via the subject secondary winding  101   b. 
 
 V   — 102 b=K*V   — 102 a=K* ( V 1− V 2)  (2)
 
   In formula (2), K is the turn ratio of the auxiliary transformer  102 ; V_ 102   b  is the voltage on the secondary coil  102   b  of the auxiliary transformer  102 .
 
 I   — 102 b=I   — 102 a/K   (3)
 
   In formula (3), I_ 102   a  and I_ 102   b  are the current flowing through the primary coil  102   a  and the secondary coil  102   b  of the auxiliary transformer  102 . 
   The power consumed by the linear constant voltage circuit  302  is obtained by formula (4).
 
 P   — 302= V   — 302* I   — 302  (4)
 
   In formula (4), V_ 302  is a voltage difference between the input terminal and the output terminal of the linear constant voltage circuit  302 ; I_ 302  is a current flowing from the input terminal of the linear constant voltage circuit  302  to the output terminal, the magnitude of the current is the same as the current I_ 102   b  flowing through the secondary coil  102   b  of the auxiliary transformer  102 . 
   The drop in voltage on the level-clamping diode is ignored and the formula (4) can be modified to formula (5).
 
 P   — 302=( V   — 102 b−V 2)* I   — 102 b   (5)
 
   Combining with formulas (1), (2), and (3), the formula (5) is changed to formula (6).
 
 P   — 302=( K *( V 1− V 2)− V 2)*( I   — 102 a/K )=( V 1− V 2)* I   — 102 a−V 2* I   — 102 a/K   (6)
 
   In formula (6), (V1−V2)*I_ 102   a  is a power loss on a conventional linear constant voltage circuit. V2*I_ 102   a /K means less power is needed in this embodiment than the conventional linear constant voltage circuit. 
   If the turn ratio K of the auxiliary transformer  102  is set to a threshold value, such as 1/(V1/V2−1), the power loss P_ 302  on the linear constant voltage circuit  302  is zero. This is a condition where no drop in voltage occurs in the linear constant voltage circuit  302 . Because there is no drop in voltage in the linear constant voltage circuit  302 , the output voltage cannot be dynamically adjusted. Therefore, K has to be higher than the threshold value. The power loss of the linear constant voltage circuit  302  is still lower than the conventional linear constant voltage circuit. 
   Reference is made to  FIGS. 3C and 3D , which show circuit diagrams of the tenth and eleventh embodiments of the present invention. In the tenth embodiment, one end of the secondary coil  102   b  of the transformer  102  is connected with the level-clamping diode  105  and the resistor  301  in serial, and is then connected with the output filtering capacitor  104 . Another end of the secondary coil  102   b  of the transformer  102  is connected with another secondary coil  101   c  of the main transformer  101 . In the eleventh embodiment, the linear constant voltage circuit  302  is used to replace the resistor  301 . When the secondary coil  101   c  of the main transformer  101  needs additional loading, these embodiments can transfer the desired loading current from another output set to improve the cross-regulation of the voltage. 
   The description above only illustrates specific embodiments and examples of the invention. The invention should therefore cover various modifications and variations made to the herein-described structure and operations of the invention, provided they fall within the scope of the invention as defined in the following appended claims.