GB2438880A

GB2438880A - Adjusting the output voltage of a transformer

Info

Publication number: GB2438880A
Application number: GB0611133A
Authority: GB
Inventors: Chi Hung Cheung
Original assignee: Pi International Ltd
Current assignee: Pi International Ltd
Priority date: 2006-06-06
Filing date: 2006-06-06
Publication date: 2007-12-12
Anticipated expiration: 2026-06-06
Also published as: GB0611133D0; GB2438880B

Abstract

A main transformer 101 has a primary coil 102a of an auxiliary transformer 102 connected in series with its secondary winding 101b. The secondary coil 102b of the auxiliary transformer is connected via a diode 105 to a voltage level smoothing component such as capacitor 104. The output voltage of transformer 101 is thus reduced by the voltage of the primary coil 102a, which can be adjusted by its turns ratio. Thus the problem of turns granularity is solved by fine-tuning the transformer output voltage. The voltage level smoothing device may comprise a fixed voltage (fig 1C, 106) or a zener diode (fig 1D, 107) and may additionally be fed via a resistor (fig 3A, 301), whose resistance may be controlled to control the output voltage. The voltage level smoothing device may also comprise a linear voltage regulator (fig 3B, 302). When output current stops flowing in diode 103, diode 105 cuts off auxiliary transformer 102 from the loading circuit so that the voltage can fly back and the magnetic flux of the core is released, thus avoiding saturation. Transformer 101 may be the main transformer of a power switching device, or converter.

Description

CIRCUIT FOR ADJUSTING THE OUTPUT VOLTAGE OF A

TRANSFORMER

The present invention relates to a fine-tuning circuit for adjusting the induced output 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.

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 flux, and the magnetic flux is transmitted to the primary side of the transformer via the iron core of the transformer and is converted into electrical power and output 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 output 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 requires 8V and 5V, 5V can be output if the number of the turns of the secondary coil of the transformer is 2. Under this condition, 7.5V is output when the number of the turns of the secondary coil of the transformer is 3 or 1 OV is output 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 size, expense, and the need for special electro-magnetic components that are not easily obtained, etc. Reference is made to FIG.4, which is a schematic diagram from U.S. 6,348,848 that discloses a construction of a transformer having a 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) and a resistor element, such as a linear constant voltage regulator, or voltage-time integrating means, such as a saturable reactor, adapted to lower the voltage. However, both the resistor element and the voltage-time integrating means suffer heat problems and lose power in the process. FIG. 5 is a circuit diagram of the power supply disclosed in U.S. patent 6,735,094. The power supply uses the saturable reactors Lsat3 and Lsat4 to lower the output voltages of secondary coils N3 and N4 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 moilifies 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 T2 is an auxiliary transformer having a primary coil 23T and a secondary coil lOT. The primary coil 23T of the differential transformer T2 couples to the secondary coil lT of the main transformer Ti in parallel. The secondary coil 1 OT of the differential transformer T2 couples to another secondary coil iT of the main transformer Ti in series. The secondary coil lOT of the differential transformer T2 is used for amending the output voltage of another secondary coil IT of the main transformer Ti.

However, the differential transformer T2 still has a problem. Although the primary coil 23T of the differential transformer T2 does not carry the output current, the primary coil 23T can have more turns. However, the secondary coil lOT 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 ftilly 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 Ti makes the voltage-time product between the ends of the coil not equal to zero. Because the primary coil 23T of the differential transformer T2 couples to the secondary coil iT of the main transformer Ti 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 T2 must have a larger resistor and a magnetic core of the primary coil 23T so that the volume of the differential transformer T2 is large.

An object of the present invention is to provide a circuit to overcome or alleviate at least one of the above disadvantages.

In one embodiment the invention provides a fine-tuning circuit for reducing the induced output voltage of a secondary winding of a main transformer, the circuit comprising: an auxiliary transfcrmer having a primary coil and a secondary coil, wherein the primary coil of the auxiliary transformer is connected to the secondary winding of the main transformer in series; and a voltage level smoothing device connected to the secondary coil of the auxiliary transformer via a diode; wherein, when a current flows through the secondary winding of the main transformer, a voltage is induced in the secondary coil of the auxiliary transformer and the diode couples the voltage to the voltage level smoothing device.

This embodiment enables the output voltage of the main transformer in an electrical power conversion device to be adjusted. This embodiment alleviates the problem of turns granularity of the conventional transformer and provides a fine-tuning circuit that is simple and has high efficiency to dynamically adjust the output voltage of the main 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 idle, the magnetic flux of the auxiliary transformer is automatically reset.

In an embodiment, a voltage damper connected to the secondary coil of the auxiliary transformer is used to adjust the output voltage of the main transformer.

In one further embodiment, the voltage damper is replaced or augmented 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 traisformer winding can be converted into the desired voltage and applied to the output circuit.

Preferred embodiments of the invention are described below by way of example only with reference to Figures. 1A to 7 of the accompanying drawings wherein: FIG. IA is a circuit diagram of the first embodiment of the present invention; FIG. I B is a circuit diagram of the second embodiment of the present invention; FIG. 1 C 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.3 1D 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. patent 6,348,848; FIG. 5 a circuit diagram of a power supply disclosed in U.S. patent 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.

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 lOla, a secondary coil 1011, and another secondary coil lOic. The secondary coil 101 b is a regenerating winding and has the problem of turns granularity. An auxiliary transformer 102 has a primary coil 102a, and a secondary coil 1 02b. The primary coil I 02a of the auxiliary transformer 102 is connected with the secondary winding lOib of the main transformer 101 in series. The primary coil 102a of the auxiliary transformer 102 is connected with the secondary coil 1 02b in series to form a secondary coil of a differential transformer straddling over the secondary winding 101 b. A modified voltage that is equivalent to the end voltage of the secondary winding lOib is induced between the two ends of the primary coil 1 02a. A diode 103 is connected to the secondary winding lOib via the primary coil 102a. The output voltage of the secondary winding 101 b is adjusted via the modified voltage of the two ends of the primary coil 1 02a, and is transmitted to an output filtering capacitor 104 via the diode 103. The surplus voltage on the secondary winding lOib caused by the turns granularity is cancelled by the modified voltage between the two ends of the primary coil 102a. 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 I 02a of the auxiliary transformer 102 is not connected with the secondary coil 102b in series. The secondary coil 102b of the auxiliary transformer 102 is connected in series with a level-clamping diode 105. The secondary coil 102b straddles over the output filtering capacitor 104 via the level-clamping diode 105. The output voltage of the renovated winding 10 lb is transmitted to the output filtering capacitor 104 via the primary coil 1 02a and the diode 103.

When output current flows through the primary coil 1 02a of the auxiliary transformer 102, a voltage is induced in the secondary coil 102b 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 to 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. 1 A, in which the anodes are connected together. The modified voltage at the two ends of the primary coil I 02a 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 is cut off from the loading circuit. When the auxiliary transformer 102 is cut off 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 lC is similar to FIG. lB. The difference is that the secondary coil 102b of the auxiliary transformer 102 is connected with a fixed voltage 106, not an output filtering capacitor 104. When output current flows through the primary coil 102a of the auxiliary transformer 102, a voltage is induced in the secondary coil 102b of the auxiliary transformer 102. The voltage is limited by the fixed voltage 106 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 106. The voltage reacts to the primary coil I 02a of the ixiliary transformer 102 according to the turn ratio of the coils of the primary coil 102a of the auxiliary transformer 102. Via this connection relationship, the output voltage of the secondary winding lOib is adjusted by changing the voltage on the primary coil 102a of the auxiliary transformer 102. The voltage on the primary coil 102a 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 106, and the turns ratio of the coils of the auxiliary transformer 102. Thereby, the present embodiment changes the turns ratio of the coils of the auxiliary transformer 102 to modify the output voltage on the secondary winding lOlb so as to provide the desired voltage. Alternatively, a variant of the present embodiment also can dynamically change the voltage value of the fixed voltage 106 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. IC. The difference is that a Zener diode 107 replaces the fixed voltage 106. 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 lB. The voltage on the output filtering capacitor 104 is supplied by the output voltage of the secondary winding bib, and the output filtering capacitor 104 is used as a voltage damper (smoothing device). The secondary coil 102b of the auxiliary transformer 102 is connected with the level- clamping diode 105 in series, and both the secondary coil 1 02b and the level-clamping diode 105 are straddled over and connected with the output filtering capacitor 104. The secondary coil 1 02b 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 102a of the auxiliary transformer 102. The voltage on the primary coil I 02a of the auxiliary transformer 102 is used to reduce the surplus output voltage on the secondary winding 10 lb. 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 I 02b of the auxiliary transformer 102 is connected with the level-clamping diode 105 in series, and is then connected to the output filtering capacitor 104. Another end of the secondary coil 102b of the auxiliary transformer 102 is connected with another secondary coil 101 c of the main transformer 101. Therefore, the secondary coil 102b of the auxiliary transformer 102 is clamped to a voltage.

The voltage is the voltage value of the output filtering capacitor 104 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 lOic of the main transformer 101 requires additional 1oaling 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 lOic of the main transformer 101 into DC voltage. The DC voltage is output 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 damper. The secondary coil 1 02b of the auxiliary transformer 102 is connected with the level-clamping diode 105 in series, and the secondary coil 1 02b and the level-clamping diode 105 are straddled over the second output filtering capacitor 202. The voltage on the secondary coil 102b 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 102b of the auxiliary transformer 102 is connected with the level-clamping diode 105 in series, and the secondary coil 102b and the level-clamping diode 105 are straddled over the secondary winding lOib of the main transformer 101 and the second output filtering cajiacitor 202. Therefore, the secondary coil 102b of the auxiliary transformer 102 is clamped to a voltage. The voltage is the voltage value of the second output filtering capacitor 202 subtracted from the voltage value on the secondary winding lOib 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 secondary winding lOib 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 301 is connected with the secondary coil 1 02b of the auxiliary transformer 102 and the level-clamping diode 105, and the resistor 301, the secondary coil 102b, 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 lO2b "id reacts to the primary coil 102a. The voltage on the primary coil 102a cancels the reaction voltage on the secondary winding lOib of the main transformer 101 so as to lower the voltage on the output filtering capacitor 104. Therefore, the present embodiment changes the resistance of the resistor 301 to adjust the output voltage.

The resistor 301 can be a variable resistor. The resistance 301 can be changed by the user according to the magnitude of the DC voltage on the output filtering capacitor 104. When the DC voltage becomes higher, the resistance 301 can be increased. When the DC voltage becomes lower, the resistance 301 can be decreased. Alternatively, the resistance 301 can be a control circuit that is included in a control system. The control circuit changes the value of the resistance 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 1 02b of the, auxiliary transformer 102 and the level-clamping diode 105 in series. 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 dashed 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 output from the secondary coil 102b 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 102a and the secondary coil 102b of the auxiliary transformer 102 also decreases. Thereby, the effect on the induced voltage of the secondary winding lOib 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 102b 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 1 02a and the secondary coil 1 02b of the auxiliary transformer 102 is then increased. Thereby, the effect on the induced voltage of th 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.

V102a=V1 -V2 (1) In formula (1), Vl is the reacting voltage on the secondary winding lOib of the primary transformer 101; V 102a is the amended voltage on the primary coil 102a of the auxiliary transformer 102; V2 is the desired voltage renovated with the amended voltage via the secondary winding 10 lb. V102b = K * V 102a = K * (V1-V2) (2) In formula (2), K: the turn ratio of the auxiliary transformer 102; V102b is the voltage on the secondary coil 102b of the auxiliary transformer 102.

1102b=IlO2a/K (3) in formula (3), I 102a and I 102b are the current flowing through the primary coil 1 02a and the secondary coil 1 02b of the auxiliary transformer 102.

The power consumed by the linear constant voltage circuit 302 is obtained by formula (4).

P302 = V302 * 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_lO2b flowing through the secondary coil 102b 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 l02bV2)* I_102b (5) Combining with formulas (1), (2), and (3), the formula (5) is changed to formula (6).

P302 (K*(V1V2)V2)*(11O2a/K) =(VlV2)* I 102a -V2*IlO2aJK (6) In formula (6), (VlV2)* I_102a is a power loss on a conventional linear constant voltage circuit. V2*I lO2aIK 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 P302 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 102b of the transformer 102 is connected with the level-'!amping diode 105 and the resistor 301 in series, and is then connected with the output filtering capacitor 104. Another end of the secondary coil 102b 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.

Claims

CLAIMS: I. A fine-tuning circuit for reducing the induced output

voltage of a secondary winding of a main transformer, the circuit comprising: an auxiliary transformer having a primary coil and a secondary coil, wherein the primary coil of the auxiliary transformer is connected to the secondary winding of the main transformer in series; and a voltage level smoothing device connected to the secondary coil of the auxiliary transformer via a diode; wherein, when a current flows through the secondary winding of the main transformer, a voltage is induced in the secondary coil of the auxiliary transformer and the diode couples the voltage to the voltage level smoothing device.

2. A fine-tuning circuit for reducing the induced output voltage of a secondary winding of a main transformer, the circuit, comprising: an auxiliary transformer having a primary coil and a secondary coil, wherein the primary coil of the auxiliary transformer is connected with the secondary winding of the main transformer in series; and a voltage level smoothing device connected with the secondary coil of the auxiliary transformer via a resistance and a diode; wherein, when a current flows through the secondary winding of the main transformer, a voltage is induced in the secondary coil of the auxiliary transformer and the diode couples the voltage to the voltage level smoothing device via the resistance.

3. A fine-tuning circuit for reducing the induced output voltage of a main transformer as claimed in claim 2, wherein the voltage level smoothing device is an output filtering capacitor, and the output filtering capacitor stores current flowing through the secondary winding of the main transformer to generate a DC voltage.

4. A fine-tuning circuit for reducing the induced output voltage of a main transformer as claimed in claim 3, wherein the resistance is a variable resistor, and the value of the resistance can be changed according to the magnitude of the DC voltage, wherein, when the DC voltage becomes higher, the value of the resistance can be increased, and when the DC voltage becomes lower, the value of the resistance can be decreased.

5. A fine-tuning circuit for reducing the induced output the winding voltage of a main transformer as claimed in claim 3, wherein the resistance is constituted by a control circuit, and the control circuit controls the current flowing from the secondary coil of the auxiliary transformer to the voltage level smoothing device according to whether the voltage on the voltage level damper is lower or higher than a target voltage.

6. A circuit for adjusting the output voltage of a secondary winding of a main transformer, comprising: an auxiliary transformer having a primary winding connected in series with the secondary winding of the main transformer wherein saturation of the auxiliary transformer is avoided; and a rectification arrangement connected to an output of the auxiliary transformer.

7. A power supply circuit substantially as described hereinabove with reference to Fig. IA or Fig. lB or Fig. 1C or Fig. 1D or Fig. 2B or Fig. 2C or Fig. 2D or Fig 3A or Fig 3B, or Fig. 3C or Fig. 3D of the accompanying drawings.

Amendments to the claims have been filed as follows CLAIMS: I. A fine-tuning circuit for reducing the induced output voltage of a secondary winding of a main transformer, the circuit comprising: an auxiliary transformer having a primary coil and a secondary coil, wherein the primary coil of the auxiliary transformer is connected to the secondary winding of the main transformer in series; and the secondary coil of the auxiliary transformer being connected via a diode to a filtering capacitor which receives current flowing through one winding of the main transformer to develop a DC voltage; wherein, when a current flows through the secondary winding of the main transformer, a voltage is induced across the secondary coil of the auxiliary transformer, and the diode couples the voltage to the filtering capacitor, and when the current flowing through the secondary winding stops, the secondary coil is decoupled from the filtering capacitor allowing the secondary coil to reset automatically, whereby saturation of the auxiliary transformer is avoided. * .

*:::
2. A fine-tuning circuit for reducing the induced output voltage of a * SS * : secondary winding of a main transformer, the circuit, comprising: S. Si *:. 20 an auxiliary transformer having a primary coil and a secondary coil, * : wherein the primary coil of the auxiliary transformer is connected with the * . secondary winding of the main transformer in series; and the secondary coil of the auxiliary transformer being connected via a resistance and a diode to a filtering capacitor which receives current flowing

I

through one winding of the main transformer to develop a DC voltage; wherein, when a current flows through the secondary winding of the main transformer, a voltage is induced across the secondary coil of the auxiliary transformer, and the diode couples the voltage to the filtering capacitor, and when the current flowing through the secondary winding stops, the secondary coil is decoupled from the filtering capacitor allowing the secondary coil to reset automatically whereby saturation of the auxiliary transformer is avoided.

3. A fine-tuning circuit for reducing the induced output voltage of a main transformer as claimed in claim 2, wherein the resistance is a variable resistor, and the value of the resistance can be changed according to the magnitude of the DC voltage, wherein, when the DC voltage increases, the value of the resistance is increased, and when the DC voltage decreases, the value of the resistance is decreased.

4. A fine-tuning circuit for reducing the induced output the winding :. voltage of a main transformer as claimed in claim 2, wherein the resistance is *::: constituted by a control circuit, and the control circuit controls the current * a..

*: flowing from the secondary coil of the auxiliary transformer to the filtering see.

*:. 20 capacitor such that when the DC voltage increases, the current is decreased, * and when the DC voltage decreases, the current is increased.

6. A power supply circuit substantially as described hereinabove with reference to Fig. 1A or Fig. lB or Fig. 1C or Fig. 1D or Fig. 2B or Fig. 2C or auxiliary transformer is avoided; and a rectification arrangement connected to an output of the auxiliary transformer.

7. A power supply circuit substantially as described hereinabove with reference to Fig. 1A or Fig. lB or Fig. 1C or Fig. 1D or Fig. 2B or Fig. 2C or Fig. 2D or Fig 3A or Fig 3B, or Fig. 3C or Fig. 3D of the accompanying drawings.