Multiple output switching power supply.

BACKGROUND OF THE INVENTION

0001The present invention relates to a multiple output switching power supply, and more particularly to a current type multiple output switching power supply suitable to supply a power to an OA or FA equipment which requires a high precision of output voltage.

0002A current type switching power supply is disclosed in Japanese Unexamined Patent Publication No. 55-120369 which discloses that a recovery current of a flywheel diode inserted in a secondary circuit of a transformer does not flow into a switching device in a primary circuit of the transformer and flow of an excessive current in an input circuit is suppressed even if the transformer is saturated by a surge voltage from the input circuit. However, it does not recognize a problem encountered in the multiple output switching power supply.

0003Japanese Unexamined Patent Publication No. 55-144775 discloses a technique to operate a switching device to stabilize one of the outputs so that the multiple outputs are stabilized. In the disclosed technique, a choke coil inserted in a secondary circuit of a transformer has a plurality of magnetically coupled windings. In the disclosed technique, it is not possible to determine the output voltages by turn ratios of a plurality of secondary windings wound on a transformer but turn ratios of the windings wound on the choke coil should also be taken into consideration. As a result, the more the number of outputs is, the more difficult to design the power supply. Further, since one output is stabilized by the operation of the switching device, variations of other output voltages due to a variation of the one output voltage, that is, voltage variations due to so called cross-regulation increase. Causes of the cross-regulation are leakage inductances and winding resistances of the transformer. In the disclosed technique, the leakage inductances and the winding resistances which cause the cross-regulation during an on period of the switching device affect to the transformer as well as leakage inductances and resistances of the windings of the choke coil and make it difficult to precisely control all of the output voltages.

0004In a technique disclosed in Japanese Unexamined Patent Publication No. 56-19119, an inductor is connected to an A.C. input. As a result, a ripple of a current flowing through the inductor is large, a current capacity of a rectifying/filtering circuit in a secondary circuit of a transformer is large and a maximum current flowing through a leakage inductor and a resistor of the transformer is large. As a result, outputs are affected by voltage drops thereacross and precision of the output voltages is lowered.

SUMMARY OF THE INVENTION

0005It is an object of the present invention to provide a multiple output switching power supply which is easy to design, suppresses variations of the output voltages due to a cross-regulation and can supply the multiple output voltages to loads with a high precision.

0006In the present invention, it was noticed that the prior art multiple output switching power supply having a plurality of windings in a choke coil inserted in a secondary circuit of a transformer increases the cross-regulation by a combination of leakage inductances and winding resistances of both the transformer and the choke coil, and the choke coil is inserted in a primary circuit of the transformer in order to suppress an influence by the choke coil. In the present arrangement, therefore, the leakage inductances and the winding resistances of the transformer and the choke coil which cause the cross-regulation are not combined and hence the precise output voltages are produced. Since the choke coil is inserted in the primary circuit of the transformer, the voltages developed across the windings of the transformer during an on period of the switching device are substantially equal to the respective output voltages, and the voltages induced across the windings of the choke coil during an off period of the switching device are also substantially equal to the respective output voltages. Accordingly, the respective output voltages are determined by the turn ratios of the secondary windings of the transformer and the choke coil and the design of the multiple output switching power supply is simplified.

BRIEF DESCRIPTION OF THE DRAWINGS

0007<ul id="ul0001" list-style="none"><li>Fig. 1 shows one embodiment of the present invention.</li><li>Fig. 2 is an equivalent circuit of the present invention during an on period of a transistor Q.</li><li>Fig. 3 shows a circuit configuration of a prior art multiple output siwtching power supply having a plurality of windings on a choke coil.</li><li>Fig. 4 is an equivalent circuit of Fig. 3.</li><li>Fig. 5 shows energy discharge means of Fig. 1.</li><li>Fig. 6 is an equivalent circuit during an off period of the transistor Q, for illustrating an operation of the energy discharge circuit.</li><li>Figs. 7 to 9 show other embodiments of the present invention.</li></ul>

DESCRIPTION OF THE PREFERRED EMBODIMENTS

0008One embodiment of the present invention is explained with reference to Fig. 1. E denotes a D.C. input power supply. A current I_cflows, during an on period of a transistor Q which is a switching device, from a positive terminal of E through a reactor T₂ which stores a magnetic energy, a primary winding n_p of a transformer T₁ to a negative terminal of E. Thus, voltages are developed across the primary winding n_p and secondary windings n₁ and n₂ of the transformer T₁with doted ends being positive, and currents are supplied to loads from n₁ and n₂ through- diodes D1 and D₂, respectively. A capacitor C₁ smoothens the voltage supplied to the load connected to an output terminal 1 and it is charged with a voltage V_l. A capacitor C₂also functions in a similar manner to C₁ and it is charged with a voltage V₂ which is supplied to the load connected to an output terminal 2. Numeral 10 denotes means which discharges, during an off period of the transistor Q, the magnetic energy stored in the reactor T₂ during the on period of the transistor Q.

0009The operation of the present embodiment is explained with reference to Fig. 2 which shows an equivalent circuit during the on period of the transistor Q. L₂ denotes an inductance of the reactor T₂, Z₁ denotes an impedance including a leakage inductance, a winding resistance and a circuit resistance of the winding n_l, and Z₂ denotes an impednace including a leakage inductance, a winding resistance and a circuit resistance of the winding n₂. The current flowing through the inductance L₂ shunts to the output terminals 1 and 2 with currents I₁ and I₂, respectively. The currents I₁ and I₂ meet the following relationship.<maths id="math0001" num=""><img file="EP0176783A2_D0001.tif" /></maths> where V_Dl and V_D2 are voltage drops across the diodes D₁ and D₂, respectively. From the formula (1), the volage V₂ at the output terminal 2 is expressed by<maths id="math0002" num=""><img file="EP0176783A2_D0002.tif" /></maths>From the formula (2), it is seen that the voltage V₂ is represented by the voltage V₁ at the output terminal 1 and the circuit impedance.

0010Reffering to Fig. 1, the voltages induced across the windings of the transformer T₁ during the on period of the transistor Q is considered. The voltage induced across the winding n₁with the dotted end being positive is equal to V₁ + V_D1, as seen from Fig. 1. Because of the voltage drop I₁ Z₁due to the winding resistance and the circuit resistance of the transformer, the voltage V_p shown by the following formula must be developed across the primary winding np of T₁ with the dotted end being positive.<maths id="math0003" num=""><img file="EP0176783A2_D0003.tif" /></maths>

0011If Vp is given, a voltage V_n2 induced across the winding n₂ with the dotted end being positive is expressed by

0012<maths id="math0004" num=""><img file="EP0176783A2_D0004.tif" /></maths>From the formula (4), the voltage V₂ at the output terminal 2 is expressed by<maths id="math0005" num=""><img file="EP0176783A2_D0005.tif" /></maths>

0013The voltage drop I₁·Z₁, + V_D1 at the output terminal 1 is given by k₁ and the voltage drop I₂·Z₂ + V_D2 at the output terminal 2 is given by k₂. Thus, from the formula (5), the voltage V₂ is expressed by<maths id="math0006" num=""><img file="EP0176783A2_D0006.tif" /></maths>

0014From the above, it is seen that the voltage V₂ at the output terminal 2 is defined by the turn ratio of the secondary windings n₁ and n₂ of the transformer T_l, and the function of the voltage V₁ at the output terminal 1 and the voltage drops in the circuit. Since k₁ and k₂ are generally smaller than V₁, V₂ can be expressed as follows without a large error.<maths id="math0007" num=""><img file="EP0176783A2_D0007.tif" /></maths>

0015Thus, the turn ratios of the transformer T₁ can be substantially determined by the ratios of the output voltages and the circuit design is facilitated.

0016In the above embodiment, the multiple output switching power supply having two outputs was shown. When m outputs are included, the operation is similar and the output voltage V. (j = 2, ..., m) is expressed by<maths id="math0008" num=""><img file="EP0176783A2_D0008.tif" /></maths>

0017Accordingly, the design of the multiple output switching power supply is facilitated.

0018The embodiment of the present invention shown in Fig. 1 is compared with a prior art power supply having a plurality of magnetically coupled windings on a choke coil inserted in a secondary circuit of a transformer. Fig. 3 shows a configuration of the prior art power supply. T denotes a transformer, E denotes a D.C. input power supply, _L denotes a choke coil having magnetically coupled windings n_L1 and nL2, and V_I and V₂ denote voltages at output terminals 1 and 2, respectively. Voltages are induced, during an on period of the transistor Q, across the windings of the transformer T and the choke coils L, with dotted ends being positive.

0019From Fig. 3, it is seen that the following relationships must be met during the on period of the transistor Q.<maths id="math0009" num=""><img file="EP0176783A2_D0009.tif" /></maths><maths id="math0010" num=""><img file="EP0176783A2_D0010.tif" /></maths> where ^V_nLl and V_nL2 are voltages induced across the windings n_Ll and n_L2 of the choke coil L, respectively. In the formulas (9) and (10), the affects by the leakage inductances and winding resistances of the transformer T and the choke coil L, and diode voltage drops are omitted for the purpose of simplification.

0020The voltage _VnL2 of the formula (10) is expressed as follows.<maths id="math0011" num=""><img file="EP0176783A2_D0011.tif" /></maths>

0021From the formulas (9), (10) and (11),<maths id="math0012" num=""><img file="EP0176783A2_D0012.tif" /></maths>

0022From the formula (12), the voltage V₂ at the output terminal 2 is expressed by

0023<maths id="math0013" num=""><img file="EP0176783A2_D0013.tif" /></maths>Since the impednaces of the transformer T and the choke coil and the diode voltage drops are omitted in the formula (13) as described above, the formula (13) corresponds to the formula (7) which represents the voltage V₂ in the embodiment of Fig. 1. As seen from the comparison of the formulas (7) and (13), the output voltage V₂ of the embodiment of Fig. 1 is essentially determined by the turn ratio of the secondary windings n₁ and n₂ of the transformer T₁ while the voltage V₂ in the prior art of Fig. 3 cannot be defined without taking the voltage V_Ll induced across the winding of the choke coil and the turn ratio of the choke coil L into consideration, and hence proper design of the power supply is difficult to attain.

0024Referring to an equivalent circuit shown in Fig. 4, the operation of the prior art power supply of Fig. 3 is explained in detail. Fig. 4 shows the equivalent circuit during the on period of the transistor Q. E denotes the D.C. input power supply, I₁ and I₂ denote the currents supplied to loads connected to output terminals 1 and 2, respectively, Z₁ and Z₂ denote the impedances including the leakage inductances, winding resistances and circuit resistances of the transformer T, and Z_L1 and Z_L2 denote the impedances including the leakage inductances, winding resistances and winding resistances of the windings N_L1 and n_L2 of the choke coil L. As seen from Fig. 4, the currents I₁ and I₂ flowing through the output terminals 1 and 2 meet the following relationship.<maths id="math0014" num=""><img file="EP0176783A2_D0014.tif" /></maths>

0025From the formula (14), the voltage V₂ at the output terminal 2 is expressed by

0026<maths id="math0015" num=""><img file="EP0176783A2_D0015.tif" /></maths>From the formula (15), it is seen that when the output currents I₁ and I₂ change, voltage variations which are proportional to (Z ₁+ Z_Z1) and (Z₂+ ZL2), respectively, take place in ^V₂.

0027On the other hand, in the present embodiment, it is seen from the formula (2) that the variation of V₂ when _I1 and _I2 change is proportional to Z₁ and Z_2' As a result, the voltage variation due to the change of the load current is smaller in the embodiment of Fig. 1 and the precision of the output is higher.

0028As described above, the embodiment of Fig. 1 and the prior art of Fig. 3 are different from each other in their concepts and effects.

0029Fig. 5 shows a circuit configuration of the energy discharge means 10 shown in Fig. 1. The energy discharge means 10 comprises windings n_S1 and n_S2 magnetically coupled to the primary winding n_S of the reactor T₂ and diodes D₃ and D₄ connected to ends of the windings n_S1 and n_S2, respectively. The winding n_sl is connected through the diode D₃ in parallel to the secondary winding n₁ of the transformer T₁ and the winding n_s2 is connected through the diode D₄ in parallel to the winding n₂. During the on period of the transistor Q, voltages are induced across the windings of T₂ with the dotted ends being positive. Accordingly, the voltages induced across the windings n_S1 and n_S2 during the on period of the transistor Q are blocked by the diodes D₃ and D₄ and no current flows. The current flowing through the winding n_S is an exciting current of T₂ and a magnetic energy is stored by the exciting current. When the transistor Q is turned off, the current flowing through the winding n_S is cut off and voltages are developed across the windings of T₂ with the opposite ends to the dotted ends being positive, by the magnetic energy stored in T₂. As a result, currents flow out of the windings n_S1 and n_S2 into the loads connected to the output terminals 1 and 2, respectively.

0030The operation of the energy discharge means is explained in detail with reference to an equivalent circuit of Fig. 6 during the off period of the transistor Q. L₂ denote an inductance of the reactor T₂, and Z₁' and Z₂' denote impedances including the leakage inductances, winding resistances and circuit resistances of the windings n_sl and n_S2, respectively. As seen from the equivalent circuit, the voltage V₂ at the output terminal 2 is expressed by

0031<maths id="math0016" num=""><img file="EP0176783A2_D0016.tif" /></maths>The voltages induced across the windings of T₂ during the off period of the transistor Q is expressed by the formula (4) as seen from Fig. 5, and the voltage V_ns2 across the winding n_s2 is expressed by<maths id="math0017" num=""><img file="EP0176783A2_D0017.tif" /></maths>

0032From the equation (17), the voltage V₂ at the output terminal 2 is expressed by<maths id="math0018" num=""><img file="EP0176783A2_D0018.tif" /></maths>

0033In the formulas (16) - (18), V_D3 and V_D4 are voltage drops across the respective diodes D₃ and D_4'

0034The voltage drop I₁·Z₂' + V_D3 at the output terminal 1 is expressed by k_l, and the voltage drop I₂·Z₂' + V_D4 at the output terminal 2 is expressed by k₂. Thus, from the formula (18), V₂ is expressed by

0035<maths id="math0019" num=""><img file="EP0176783A2_D0019.tif" /></maths>It is seen from the above that the voltage V₂ at the output terminal 2 during the off period of the transistor Q is defined by a function similar to that shown in the formula (6) .

0036The same is applicable to the multiple output switching power supply having m outputs. The output voltage ^V. (j = ^2, 3, ..., m) is expressed by

0037<maths id="math0020" num=""><img file="EP0176783A2_D0020.tif" /></maths>Accordingly, the output voltage V. is essentially defined by the turn ratio of the secondary windings n₁ and n_jof the reactor T₂as is done for the transformer T₁ and the design of the power supply is facilitated. Further, in the present embodiment, since the magnetic energy stored in the reactor T₂ is supplied to the loads by the energy discharge means 10, the efficiency of the multiple output switching power supply is improved.

0038Fig. 7 shows another embodiment. In the present embodiment, a voltage V₁ at an output terminal 2 is a sum of a voltage V₁developed across a winding n₁ and a voltage developed across a winding n₂. Windings n_S1 and n_s2 are similarly configured to develop a voltage V₂ at an output terminal 2.

0039In the embodiment of Fig. 1, the turn ratio of the windings n₁ and n₂ is essentially same as the ratio of the output voltages V₁ and V₂, as shown in the formula (7). The same is true for the turn ratio of the windings n_S1 and n_s2. On the other hand, it is seen from the formulas (5) and (8) that the smaller the ratios n₂/n_l and n_s2/n_sl are, more the variation of the voltage V₂ due to the change of the output current I₁is suppressed. Thus, in the present embodiment, the numbers of turns of the windings n₂ and n_s2 are reduced to suppress the variation of the voltage V₂ and improve the output precision.

0040In the circuit configuration of Fig. 7, the numbers of turns of the windings n₂ and n_s2 are selected such that the voltage V₂ at the output terminal 2 is a sum of the voltage V₁ at the output terminal 1 and the voltages developed across the windings n₂ and n_s2. Thus, the turn ratios of the windings n₂ and n_s2 are selected as follows.<maths id="math0021" num=""><img file="EP0176783A2_D0021.tif" /></maths><maths id="math0022" num=""><img file="EP0176783A2_D0022.tif" /></maths>

0041Accordingly, the turn ratios n₂/n_l and nS₂/n_S1 are smaller than those of the embodiment of Fig. 1, and hence the variation of the voltage V₂ at the output terminal 2 is suppressed and the precision of the output is improved.

0042Fig. 8 shows another embodiment. In the present embodiment, a control circuit 11 is provided to control the switching of the transistor Q to keep the voltage V₁ at the output terminal 1 constant. It is seen from the formulas (6) and (19) that if the voltage V₁ at the output terminal 1 is constant, the voltage at the output terminal 2 can be stabilized within a variation of a voltage drop across an impedance in the circuit.

0043In the present embodiment, since the voltage V₁at the output terminal 1 is kept constant, the voltage V₂ at the output terminal 2 need not take the change of V_l into consideration as shown in the formulas (6) and (19), and the output precision of the voltage V₂ is further improved. Alternatively, the control circuit 11 of the present embodiment may be provided in the embodiment of Fig. 1 to control the switching of the transistor Q in order to keep the voltage V₁ of the output terminal 1 constant.

0044Fig. 9 shows another embodiment. In the present embodiment, m outputs are supplied to the loads in the embodiment of Fig. 8. By controlling the switching of the transistor Q by the control circuit 11 to keep the voltage V₁ at the output terminal 1 constant, m stable voltages can be supplied to the loads.

0045Alternatively, the control circuit 11 may be provided in the embodiment of Fig. 1 to control the switching of the transistor Q in order to keep the voltage V₁at the output terminal 1 constant, so that stable outputs are supplied from m secondary windings wound on the transfermer T₁ and the reactor T_2'

0046The control circuit 11 shown in Figs. 8 and 9 may be Hitachi switching power supply control IC HA17524 or Texas Instruments TL 494. The configuration and operation of those IC's are well known and they are not explained here.

0047In accordance with the present invention, the output voltages of the multiple output switching power supply can be defined by the turn ratios of the secondary windings of the transformer and the windings of the energy discharge circuit of the reactor, and the design of the power supply is facilitated. When the control circuit is used to control the switching device such that one of the output voltages is kept constant, the voltage variations of other outputs can be less than ±5%. Thus, the multiple output switching power supply used for the OA and FA equipments are simplified and the size and cost thereof can be reduced.