GB1603387A - Switched mode power supply - Google Patents

Description

(54) SWITCHED MODE POWER SUPPLY (71) We, TDK ELECTRONICS CO., LTD., a Japanese company of 13-1, Nihonbashi 1-chome, Chuo-ku, Tokyo, Japan (formerly of 16-6, Uchikanda, 2chome, Chiyoda-ku, Tokyo, Japan) do hereby declare the invention, for which we pray that a patent may be granted to us and the method by which it is to be performed to be particularly described in and by the following statement :- The present invention relates to a switched mode power supply using a variable leakage transformer.

A transformer has, in general, a magnetic core defining a closed magnetic path and a primary and a secondary winding wound on said magnetic core. All the input power applied to the primary winding is available at the output of the secondary winding except for a small amount of loss in the transformer. The output voltage V2 across the secondary winding is given by :- V2= (n/n,) xV,, where V, is the voltage across the primary winding and n, and nl are the number of turns of the primary and secondary windings, respectively.

If it is required to control the output power of the transformer, a controllable switching device such as a SCR (Silicon Controlled Rectifier) or a transistor may be employed at the output of the transformer.

In such controllable switching device, the pulse width during each cycle is varied by controlling the conducting time of the device. However, a prior controllable switching device has the disadvantage that the circuit is very complicated and the cost of the device is rather high.

Another known arrangement for controlling an AC power source is a magnetic amplifier, in which a saturable reactor is inserted between the power source and the load, and by controlling the reactor, the power transferred from the source to the load is controlled. However, a magnetic amplifier has the disadvantage that the voltage across the load must be the same as that of the power source, and the saturable reactor does not function as a variable-voltage transformer.

The present invention seeks to overcome the disadvantages and limitations of the prior art by providing a new and improved switched mode power supply.

In accordance with the invention there is provided a switched mode power supply comprising a) a switching means connected to a direct current power supply to switch the same to provide an input rectangular waveform voltage, b) a variable leakage transformer supplied by said input rectangular waveform voltage of said switching means and operable to provide a controlled rectangular waveform output voltage, said variable leakage transformer comprising a core defining a closed main magnetic path and a closed sub-magnetic path, the main magnetic path having at least a portion in common with the sub-magnetic path, a primary winding having at least a portion which is wound on the common portion of the magnetic path, and connected so as to be supplied with said input rectangular waveform voltage, a secondary winding wound on the main magnetic path of the core and providing said controlled rectangular waveform output voltage, and a direct current control winding wound on the sub-magnetic path for controlling the magnetic flux in the submagnetic path, c) control means for providing a desired amplitude of said controlled rectangular wave form output voltage by adjusting the direct current in said control winding.

In order that the invention may be better understood, several embodiments thereof will now be described by way of example only and with reference to the accompanying drawings in which :- Figure I is the cross sectional view of a known variable leakage ; Figures 2 (A), 2 (B), 2 (C) and Figures 3 (A) and 3 (B) all show the operational waveforms of the transformer in Figure 1 ; Figure 4 is the cross sectional view of a variable leakage transformer suitable for use in the power supply of the present invention ; Figure 5 is the perspective view of another variable leakage transformer suitable for use in the power supply of the present ; Figure 6 is a plan view of the transformer of Figure 5; Figure 7 is a side view of the transformer of Figure ; Figure 8 shows the structure of the core for the use in the transformer of Figure 5; Figure 9 shows still another variable leakage transformer suitable for use in the power supply according to the-present invention ; Figures 10 and I I are modifications of the transformer shown in Figure 9; Figure 12 is the circuitry of a first embodiment of a switched mode power supply according to the present invention; Figure 13 is one embodiment of a control circuit for use in the power supply of Figure 13 ; Figures 14 and 15 show the operational waveforms of the circuit shown in Figure 12 ; Figure 16 is the circuit of another embodiment of a switched mode power supply according to the present invention; Figure 17 shows one embodiment of a control circuit for use with the power supply of Figure ; Figure 18 is the circuit of still another embodiment of a switched mode power supply according to the present invention.

Figure I shows a known transformer. In this figure, a magnetic core 1, composed of the combination of either an E shaped magnetic core portion and I shaped magnetic core portions or a pair of E shaped magnetic core portions, is a three legged structure having 2 magnetic paths, a submagnetic path AA'B'B and a main magnetic path AA'C'C. The middle leg AA'and the side leg CC'of the magnetic core I are provided with a primary coil 2 and a secondary coil 3, respectively. The number of turns of the primary coil 2 is assumed to be N, and that of the secondary coil N2.

A load resistor RL is connected to the secondary coil 3 and an AC input voltage V, is applied to the primary coil 2. The magnetic flux generated thereby is divided into magnetic flux 0, flowing in the submagnetic path AA'B'B, and magnetic flux flowing in the main magnetic path AA'C'C. That is, the formula #=#1+#2 is satisfied.

Of these two fluxes, the one that supplied power to the load resistor R is the one that interlinks with the secondary coil 3, i. e. 0,.

The flux 0, does not contribute in the supply of power to the load resistor RL At this stage, if the magnetic reluctance is increased by application of a control magnetic field Hg to the sub-magnetic path AA'B'B from outside, the magnetic flux in path AA'B'B decreases and accordingly the magnetic flux in path ACC'A'increases.

Therefore, the output voltage VQ, oí the secondary coil 3, i. e. the power supplied to the load resistor RL, increases. It can thus be seen that power transferred from the primary coil to the secondary coil can be controlled by changing the strength of the control magnetic fie) d H. apptied to the submagnetic path AA'B'B from outside.

Suppose that a rectangular wave voltage is applied as input voltage Vh, such as shown in Figure 2 (A). If the sub-magnetic path AA'B'B is in a saturated condition due to the control magnetic field Hc then because this sub-magnetic path can be considered to no longer act as a magnetic circuit, the output wave form of the output voltage V thus obtained is identical with that of the input voltage Vs show in Figure 2 (B). The amplitude of VOU, is determined by the ratio between the number to turns N, on the primary coil 2 and the number of turns N, on the secondary coil 3. That is, V, =V (N/N,).

Suppose that the sub-magnetic path AA'B'B is put in a non-saturated condition by weakening the control magnetic Held Hé Some of the magnetic flux + generated by input voltage V, flows round the submagnetic path during the time t"i. e. from the time the input voltage V, is applied until the time the sub-magnetic path AA'B'B is saturated by the input voltage Vin, and the output voltage Vox'in effect becomes almost zero.

At the end of time period t ;, the submagnetic path AA'B'B becomes saturated.

This saturated condition is maintained until inversion of the input voltage V,,.

Therefore, as shown in Figure 2 (C) the output vo) tage V, (=V, N/N,) appears only during the time (T/2-t,). At this stage, the length of time period t, can be changed by changing the magnetized condition of the sub-magnetic path AA'B'B, or in other words by the magnitude of the control magnetic fie ! d H, applied to the submagnetic path AA'B'B.

Thus, by controlling the magnitude of the control magnetic Eleld Hc, the magnetic flux in the sub-magnetic path AA'B'B is controlled, and so the time (t,) during which the sub-magnetic path is not saturated is controlled. It should be noted that the output voltage does not appear during the time t"since all the magnetic flux generated by the input voltage goes around the submagnetic path but does not go around the main magnetic path. Therefore the conduction period or the pulse width of the output voltage VOU, can be controlled. Thus, the power consumption in the secondary winding is controlled by changing the pulse width of the output voltage. During the period t, in which the sub-magnetic path is not saturated, all the magnetic flux generated by the input voltage goes around the sub-magnetic path, but does not go around the main magnetic path. Therefore, the power consumption in the primary winding during the period t, is almost zero because the whole magnetic flux # flows in the sub-magnetic path and does not interlink with the secondary coil 3 on the main magnetic path. Durmg the period (T/2-t,) in which the sub-magnetic path is saturated, all the input power is transferred to the load through the secondary winding.

Thus, input power is not consumed in the transformer in spite of the power control.

Therefore, operation is always highly efficient.

Performance of the transformer of Figure I shows that the pulse width of the output voltage VO4t can be controlled by increasing or decreasmg the magnitude of the control magnetic field H It should be appreciated of course that an input voltage of sinusoidal waveform is also possible and provides the same effect as a rectangular waveform, although the drawings shows a rectangular waveform for simplicity of explanation.

As explained above, the leakage of the magnetic flux from the main magnetic path to the sub-magnetic path can be controlled by varying the magnetic flux in the submagnetic path, and thus, the coupling between the primary and secondary windings and the conduction period in each cycle of the input voltage are controlled.

And it should be noted that the control of the conduction period provides the control of the power transmission from the primary winding to the secondary winding.

In the transformer of Figure 1, the primary coil 2 was wound around the middle leg of the magnetic core I and the secondary coil 3 was wound around one of the side legs. As a result of this arrangement, the magnetic coupling between the primary coil 2 and the secondary coil 3 has a tendency to be insufficient. If the coupling is insufficient, even if the rectangular wave input voltage V, as given in Figure 3 (A) is applied, the waveform of the output voltage V,,, becomes deformed as shown in Figure 3 (B).

This results from insufficient coupling between the primary coil 2 and the secondary coil 3.

Figure 4 depicts a transformer suitable for use in the power supply of this invention and which provides a configuration with improvements on the above mentioned drawback. In this configuration, the primary coil 2 surrounds the middle leg AA' of the magnetic core 1. The secondary coil 3 is wound over the side leg CC'. In an alternative arrangement (not shown) the primary coil 2 is divided into a seriesconnected first primary coil surrounding the middle leg AA'of core I and a second primary coil surrounding the side leg CC', the secondary coil 3 being wound over the second primary coil. With this arrangement, the coupling between the primary and the secondary coils becomes greater and the waveform of the output voltage VOliX and that of the input voltage V can be made almost identical.

The other side leg BB'is provided with a control magnetic core 5 surrounded by a control coil 4. The control magnetic core 5 is an E shaped magnetic core portion with its middle leg D designed to be surrounded by the control coil 4.

In the arrangement shown in Figure 4, passage of a control current If through the control coil 4, causes the generation of a control magnetic field by the control coil 4.

This in turn generates: (I) a control magnetic flux 5c that passes through the middle leg D of the control magnetic core 5, the side leg BB'of the magnetic core I and to the side leg F of the control magnetic core 5 ; and (2) a control magnetic flux that passes through the middle leg D, side leg BB'and to the side leg G. At this stage, if the control current Ic is increased or decreased, the control magnetic flux 0. is also increased or decreased proportionally.

Therefore, if the control current I is increased, the magnetic reluctance of the sub-magnetic path AA'B'B increases and the resultant condition is susceptible to magnetic saturation. Therefore, if rectangular waves such as shown in Figure 2 (A) are applied as input voltage V,,, part of the sub-magnetic path AA'B'B is saturated in a short time with less magnetic flux p1.

Consequently, the time period t, indicated in Figure 2 (C) becomes short, while the pulse width of output voltage VOlJt increases.

Conversely, if control current I is decreased, the magnetic reluctance of the sub-magnetic path AA'B'B falls and the possibility of magnetic saturation becomes remote. Under these conditions, it will take time to saturate part of the sub-magnetic path. Consequently, the time period t, shown in Figure 2 (C) becomes long and the pulse width of output voltage Vs falls.

The embodiment shown in Figure 4 is capable of controlling the pulse width of the output voltage VOlUt very efElciently through increase or decrease of control current I.

Consequently, the power transferred to the lead resistor RL from the input can be electrically controlled. Furthermore, it will be seen that the direction of the magnetic flux, flowing in the sub-magnetic path AA'B'B is inverted every half cycle, following the input voltage V i,. However, since the control magnetic flux Xc is constantly flowing in both directions, either one of the magnetic fluxes 0. and the magnetic flux, are always available to combine to cause magnetic saturation.

Therefore, control current le can be a direct current and is readily controllable.

Figures 5,6 and 7 illustrate a second construction of transformer suitable for use in the power supply of the invention. In these figures, the magnetic core IA forming the main magnetic path is made up of the combination of a U shaped core portion and an I shaped core portion, and the magnetic core 1B forming the sub-magnetic path is made up of a combination of an E shaped core portion and an I shaped core portion.

The primary coil 2 is wound in common around one of the legs of the magnetic cores I A and the middle leg of the magnetic core I B while the secondary coil 3 is wound around the other leg of the magnetic core IA. Both the side legs of the magnetic core 1B are provided with respective control coils 4A and 4B. The control coils 4A and 4B are connected in series so that the voltage induced in these coils will cancel each other when the input voltage V, is applied to the primary coil 2. As with the embodiment of Figure 4, the secondary coil may be split into two series-connected parts, with the first part wound in common about one of the legs of the magnetic cores I A and the middle leg of the magnetic core 1B, and the second part wound around the other leg of the magnetic core IA, with the secondary coil 3 wound over the second part of the primary coil.

In the above arrangement, the magnetized condition of the magnetic core IB forming the sub-magnetic path can be changed by the control current I. fed to the control coils 4A and 4B. For example, when the rectangular wave such as shown in Figure 2 (A) is applied as input voltage V, the magnetization strength of the su- magnetic path becomes large if control current le is large, while the time period t, in Figure 2 (C) becomes short. On the other hand, if the control current lc is small, the strength of magnetization of the sub magnetic path falls, and the time period t, becomes long. As a result, the pulse width of output voltage VOut can be controlled.

The magnetic core is made up of the E-1 magnetic core portions and the U-1 magnetic core portions combined.

However, it should be appreciated that a magnetic core having the same effect may be formed by a combination of a fourlegged magnetic core portion IC and a T shaped magnetic core portion ID as depicted in Figure 9.

Although Figure 5 illustrates the use of combined U-I core portions and E-1 core portions, it should be appreciated that many modifications of this particular arrangement, for instance using U-U core portions and E-E core portions, are possible.

Figure 9,10 and 11 show a still further construction of transformer suitable for use in the power supply of the present invention. In these figures, the main magnetic path is provided by a U shaped magnetic core portion 10a and an I shaped magnetic core portion 10b, and a pair of sub-magnetic paths are provided by a pair of U-1 core portions I IA (a) and I IA (b) and I I B (a) and I I B (b). The primary winding 2 is wound commonly on the first legs of each core portion (10 (a), IlA (a), IlB (a)), while the secondary coil 3 is wound on the other leg of the core portion 10a. On the other legs of the core portions ! ! A (a) and l ! B (a), control coils 4A and 4B are provided. The U-1 shaped core portions 10a, 10b, llA (a), IlA (b), IlB (a) and IlB (b) can be rotatably fixed around the leg around which the primary coil 2 is wound, so that the angles ( IA 02, 03) between each core portion can be adjusted at will.

The transformer shown in Figures 9 to 11 performs the same operation as that shown in Figure 5. Accordingly, when an input voltage V, n is applied to the primary winding 2, the output voltage V, U, across the secondary coil 3 can be controlled by a control current 1. flowing in the control coils 4A and 4B. It will be appreciated that a flat transformer can be obtained by arranging the angles between each core according to the shape of the available mounting space.

As explained above, the transformer shown in Figures 9 to 11 can take any shape according to the mounting position by changing the angles between each core, thus making effective use of the available mounting space. Further, as the shape of all the cores is the same, manufacture is simple. Although the construction in Figures 9 to H uses U-1 shaped core portions, any core shape which can provide a closed magnetic path, including a U-U shaped core, can be utilized.

Figure 12 depicts a first embodiment of a switched mode power supply for use with the transformer described above. In the drawing, the primary winding N, of a pulse width control transformer 21 is a so-called bifiler coil with a center tap. To this primary winding N, is connected a self-exiting push- pull converter 23 incorporating transistors Qt and Q, and an oscillation transformer 22.

A direct current power source 24 is connected between the center tap of the primary winding N, and the emitter of the transistors Q, and Q2 so that dc power is fed to the converter 23.

The secondary winding N, of the transformer 21 is also provided with a center tap. A full-wave rectification circuit 25 composed of rectifier diodes D,, D2, and choke L and capacitor C,, is connected to the secondary winding N. The DC output voltage V of full-wave rectification circuit 25 is fed to a load 26. The transformer 21 is equipped with an auxiliary supply winding N, and a control winding N,. A d. c. control voltage V, produced by rectification by diode D3 and smoothing by capacitor C2, is applied, together with the AC output of the auxiliary supply winding N,, to a control circuit 27. The control circuit 27 detects changes in the output voltage VOU, and controls the value of the control current I ; to be fed to the control winding N,.

It should be appreciated in Figure 12 that the transformer 21 is a variable leakage transformer of the type described above.

The pus-pull converter 23 provides the input voltage of the rectangular waveform to the primary winding N, of the transformer 21.

As illustrated in Figure 13, the control circuit 27 comprises a series circuit consisting of a transistor Q,, which feeds control current I. to the control winding N4, upon receiving the control voltage Vf, and a resistor R,. The control circuit also comprises a zener diode ZD that detects the output voltage VOU, of the rectification circuit 25, and a transistor Q, controlled by zener diode ZD.

The transistor Q, is switched on upon break-down of the zener diode ZD when the output voltage V. exceeds the zener voltage of the zener diode ZD, and thus reduces the emitter current of transistor Q,, i. e. control current lc.

In the circuit of the Figures 12 and 13, if the sub-magnetic path of the transformer 21, is sufficiently saturated by having the prescribed control ! current I, passing through the control winding N4, the submagnetic path effectively becomes non existent. The P-P'voltage of the selfexciting pus-pull type converter 23 assumes a rectangular wave form as indicated in Figure 14 (a). The collector current I, of the transistor Q, and the collector current 1, of the transistor Q2 assume wave forms as depicted in Figures 14 (B) and 14 (C) respectively. Therefore, the voltage induced in the secondary winding N2, as in the case of an ordinary transformer, becomes a rectangular wave form similar to that induced in the primary winding as shown in Figure 14 (D).

In this situation, transfer of power from the primary to the secondary is maximised and the output DC voltage VO"1 also reaches its maximum value.

When the terminal voltage of the DC power source 24 rises, or the power required by the load 26 decreases resulting in an increase in the output DC voltage Vox,, current flows through the zener diode.

ZD in the control circuit 27 and the base bias current of the transistor Q3 is divided by the transistor Q,. As a result, emitter current of the transistor Q3, i. e. control current If decreases and the magnetic reluctance of the sub-magnetic path of the transformer 21, drops. Under these conditions, most of the magnetic flux generated by the primary winding N, flows through the sub-magnetic path and reduces the coupling with the secondary winding N2.

Therefore, even if the P-P'voltage of the self exciting push-pul ! type converter 23 assumes the rectangular wave form indicated in Figure 15 (A), the collector current 1, of the transistor Q, and the collector current I2 of the Q2 assume wave forms as shown in Figures 15 (B) and 15 (C) respectively. That is, until the end of the time period t, when the submagnetic path has been magnetically saturated by the magnetic flux established by the primary winding N"only very little current corresponding to the exciting current for exciting the magnetic core of the transformer 21, flows as collector current of each transistor. At the end of time period t,, the sub-magnetic path becomes saturated and the magnetic flux of the primary winding N, couples with the secondary winding N,. Therefore, the voltage appearing in the secondary winding N2 becomes a bipolar type pulse such as shown in Figure 15 (D). That is, compared with Figure 14 (D) the pulse width becomes shorter by as much as the time duration t, and therefore, in response thereto, the output DC voltage v drops.

Thus, in the circuit of Figure 13, the output DC voltage VO% lt can be constantly maintained at a given level, if the control current Ie and the characteristics of the zener diode ZD, are appropriately set in dependence upon the output DC voltage Vo, desired. In this situation, the collector currents of the transistors Q, and Q2 of the aforementioned converter become only exciting current at the time t,, and the output end of the converter 23 reaches an open circuit or no-load condition.

Therefore, the control does not cause a power loss, while the conversion efficiency of the converter 23 hardly decreases. In the conventional pulse control system, in order to control the operation of a switching element a feed back circuit from the secondary to the primary of element. If multiple regulated outputs are desired, the control transformers may be connected in parallel to a single converter.

There has been described a high performance regulated power supply device of a simple configuration incorporating a control transformer.

Claims (7)

1. The present application is divided out of application 20778/78, serial no. 1603386, which describes and claims a variable leakage transformer comprising a core defining a closed main magnetic path and a closed sub-magnetic path, the main magnetic path having a part in common with the sub-magnetic path, a primary winding, a secondary winding wound on the main magnetic path of the core, and a control winding for providing a control magnetic flux in said sub-magnetic path so that magnetic flux generated by said primary winding is shared between the main magnetic path and the sub-magnetic path according to the strength of said control magnetic flux, said primary winding comprising a first primary winding portion and a second primary winding portion connected in series with each other, the first primary winding portion being wound on the common magnetic path part and the second primary winding portion being wound on part of the main magnetic path other than the common magnetic path part, and wherein the second primary winding portion is closely coupled magnetically with the secondary winding.

   WHAT WE CLAIM IS : 1. A switched mode power supply comprising: a) a switching means connected to a direct current power supply to switch the same to provide an input rectangular waveform voltage, b) a variable leakage transformer supplied by said input rectangular waveform voltage of said switching means and operable to provide a controlled rectangular waveform output voltage, said variable leakage transformer comprising a core defining a closed main magnetic path and a closed sub-magnetic path, the main magnetic path having at least a portion in common with the sub-magnetic path, a primary winding having at least a portion which is wound on the common portion of the magnetic path, and connected so as to be supplied with said input rectangular waveform voltage, a secondary winding wound on the main magnetic path of the core and providing said controlled rectangular waveform output voltage, and a direct current control winding wound on the sub-magnetic path for controlling the magnetic flux in the sub-magnetic path,

   c) control means for providing a desired amplitude of said controlled rectangular wave form output voltage by adjusting the direct current in said control winding.
2. 2. A switched mode power supply according to claim 1, further comprising rectifier means connected to the output of said secondary winding.
3. 3. A switched mode power supply according to any one of claims I to 3, wherein said switching means is associated with an intermediate transformer with a plurality of secondary windings to supply the input rectangular waveform voltage to a plurality of said variable leakage transformers, the arrangement being such that the current in each control winding to each variable leakage transformer is independently adjustable.
4. 4. A switched mode power supply according to any one of the preceding claims wherein said variable leakage transformer further comprises an auxiliary winding connected to supply current to said control means through a further rectifier means.
5. 5. A switched mode power supply according to any one of the preceding claims, wherein said primary winding has a first primary winding portion and a second primary winding portion connected in series with each other, the first primary winding portion being wound on the common magnetic path, the second primary winding portion being closely coupled magnetically with the secondary winding.
6. 6. A switched mode power supply according to any one of claims I to 4 wherein the core of the variable leakage transformer comprises at least three closed magnetic core portions having a common leg on which the primary winding is wound, and wherein the secondary winding and the control winding are provided on other legs of said magnetic core portions, each of said magnetic core portions being rotatably mounted around the axis of the common leg.
7. 7. A switched mode power supply substantially as hereinbefore described with reference to the accompanying drawings.