|1.||A power unit which includes a selfoscillating series resonance converter and which is intended to supply a load, for instance an electric bulb, a battery charger and/or electronic equipment, wherein the series resonance convert¬ er includes two mutually connected transistors (TRl, TR2), each of which is activated by a respective control trans¬ former (Tl), and further includes an inductor (LI) and a capacitor (C4) which are connected in series, wherein each transistor (TRl, TR2) and an associated seriesconnected capacitive voltage divider (C6, C7) and/or a further transistor (TR3, TR4) are connected in parallel with said inductor (LI), capacitor (C4) and control transformer (Tl), and wherein the load is connected across the capacitor (C4), characterized in that the magnetic flux through the control transformer can be influenced by means of a magnetic flux applied externally via a magnet or corre¬ sponding means to change the oscillation frequency of the transistors (TRl, TR2) and therewith the voltage output of the power unit.|
|2.||A unit according to Claim 1, characterized in that an electromagnet (T3) is connected to influence externally the magnetic flux through the control transformer (Tl).|
|3.||A unit according to Claim 2, characterized in that the current through the winding circuit (Wl) of the electromag¬ net (T3) can be controlled by means of the voltage output of the power unit.|
|4.||A unit according to Claim 3, characterized in that the winding circuit (Wl) includes a zener diode (Zl) for detecting the output voltage.|
|5.||A unit according to any one of Claims 24, character¬ ized in that the control transformer (Tl) includes a ferrite ringcore (K) .|
|6.||A unit according to Claim 5, characterized in that in addition to the primary winding (P), the control transform¬ er (Tl) also includes two secondary windings (SI, S2) with connection to the base of respective transistors (TRl, TR2).|
|7.||A unit according to Claim 6, characterized in that the yoke (O) of the electromagnet (T3), said yoke preferably being comprised of transformer plate, is connected across the ring core (K) in the vicinity of the centre point of the primary winding (P) and the space between the secondary windings (SI, S2).|
The present invention relates to a power unit which includes a self-oscillating series-resonance converter and which functions to supply a load, such as an electric bulb, a battery charger and/or electronic equipment for instance, wherein the series-resonance converter includes two mutually connected transistors, each of which is intended to be activated by a control transformer, and a series- connected inductor and capacitor, and wherein each transis¬ tor and a corresponding series-connected capacitive voltage divider belonging to said transistor and/or a further transistor is connected in parallel with said inductor, capacitor and control transformer, and wherein the load is connected across the capacitor.
A power unit of the aforedescribed kind is known to the art and is found described, for instance, in Swedish Patent Specification No. 7512267-1. This power unit operates in accordance with the series-resonance principle and provides many advantages over other known power units. These advantages include low switch losses at high operating frequencies, while problems relating to radio interference or disturbance are negligible.
One of the drawbacks with a power unit that operates in accordance with the series-resonance principle isr that it requires the presence of complicated drive electronics. This results in a high total cost for the manufacture of such a power unit. Since there is a high demand for power units which can be used in low-price equipment, it is desirable to make the drive circuits of the power unit as cheap as possible. This desideratum is realized with a power unit of the kind defined in the introduction which has the characteristic features set forth in the character¬ izing clause of Claim 1.
Advantageous embodiments of the inventive construction are made apparent in the dependent Claims.
The invention will now be described in more detail with reference to the accompanying drawings.
Fig. 1 is a circuit diagram for a power unit operating in accordance with the series-resonance principle.
Fig. 2 is a diagram which illustrates a voltage/current relationship in the power unit.
Fig. 3 is a circuit diagram for the power unit according to Fig. 1, said circuit enabling the magnetic flux through the control transformer belonging to the power unit to be regulated or adjusted.
Fig. 4 is a view from above of the arrangement of a power unit control transformer having an electromagnet connected externally thereto in accordance with the principles of the invention.
Fig. 5 is a circuit diagram which relates to a modified version of the power unit illustrated in Fig. 3.
The manner of operation of the power unit illustrated in Fig. 1 will now be described.
When the power unit is connected to the mains voltage, which in the present case reaches an alternating voltage of 220 volts, the voltage is rectified with the aid of four bridge-connected rectifier diodes D1-D4. The full-wave rectified mains voltage is filtered downstream of the diode bridge with the aid of a first capacitor Cl. The direct voltage across the capacitor Cl will reach about 310 volts. A current which charges a capacitor C2 now passes through a resistor Rl. When the voltage across this latter capaci-
tor C2 has risen to above the threshold voltage of a trigger diode D5, the trigger diode is ignited or fired and conducts current to the base of a first transistor TRl. The purpose of the illustrated diode D6 is to prevent trigger- ing of the trigger diode D5 when the circuit self-oscil¬ lates. The transistor TRl is now conductive and a capacitor C3 is discharged via a further resistor. The voltage across the transistor TRl will reach about 310 volts before the transistor becomes is turned on. When the transistor TRl is turned on, current passes through the transformer Tl, the inductor LI and the capacitor C4. The current passing through the primary winding P of the transformer Tl gives rise to magnetic flux in the core K of the transformer Tl. In turn, this flux induces a voltage in a secondary winding SI of the transformer Tl, which causes the first transistor TRl to become conductive. The current passing through the transformer Tl will increase while the first transistor TRl is operative. The core K of the transformer Tl becomes saturated after a given length of time, as illustrated in the Fig. 2 diagram. This causes the first transistor TRl to cease to conduct. Because of the back electromotive force generated by the inductor LI, the voltage in the common contact point A of the first transistor TRl and of a second transistor TR2 will be converted from low potential to high potential. The current passing through the transformer Tl thereafter changes direction, causing the earlier described sequence to be repeated. In turn, this causes the transis¬ tors TRl, TR2 to conduct alternately. When the components included are dimensioned correctly, the switching frequency of the circuit concerned will normally be higher than the resonance frequency of the inductor LI and the capacitor C4. The voltage across the capacitor C4 is determined by how close the resonance frequency of the inductor LI and the capacitor C4 lies to the switching frequency. As illustrated in the drawings, a load can be connected across the capacitor C4. The diodes D9-D10 are antiparallel to the transistors TRl and TR2.
The illustrated arrangement thus provides an extremely simple construction. However, this construction has a serious drawback, namely that it is not possible to adjust or control the output voltage in a simple manner. The invention provides an arrangement which with the aid of an external magnetic field enables the core K of the trans¬ former Tl to be saturated rapidly when the external magnetic field increases, and to be saturated more slowly when the external magnetic field decreases. This is achieved with the power unit illustrated in Fig. 3, which is a development of the arrangement illustrated in Fig. 1 and which operates in accordance with this principle. The voltage across the capacitor C4 is dependent on the oscillation frequency of the transistors TRl, TR2. Connect- ed across the capacitor C4 is a transformer T2 which transforms the capacitor voltage to a suitable level. The voltage is rectified by the diodes D7, D8. The rectified voltage is then filtered by means of the inductor L2 and the capacitor C5. When the output voltage increases, so that the zener diode Zl placed in the winding circuit of the electromagnetic T3 begins to conduct, current will pass through the zener diode and also through the winding Wl of the illustrated electromagnet T3. The current passing through the winding Wl gives rise to a magnetic flux which hastens saturation of the core K of the transformer Tl, which in turn causes the oscillation frequency to increase and distance itself from the resonance frequency of the inductor LI and the capacitor C4. As a result, the voltage across the capacitor C4 becomes lower, as does also the power unit output voltage. There is thus obtained a power unit having a controlled or adjusted output voltage.
Fig. 4 illustrates a suitable embodiment of the transformer and of the transformer magnetizing arrangement. The transformer Tl operates as a typical ferrite transformer, in which the primary winding P induces an alternating flux in the ring core K. In turn, the flux induced in the ring
core K induces a voltage across the secondary windings SI, S2. The oscillation frequency is now governed by the speed at which the ring core K of the transformer Tl becomes saturated; see also Fig. 2. Application of an externally arriving flux in addition to the flux arriving from the primary winding P of the transformer T will cause the ring core K of the transformer Tl to be saturated more rapidly. Thus, the oscillation frequency increases. The magnetic flux induced externally via the electromagnetic T3 in the ring core K is added to the flux from the primary winding P of the transformer Tl. The ring core K of the transformer Tl can thus be brought to saturation more or less quickly, by changing the current in the winding Wl of the electro¬ magnet T3.
The electromagnet T3 used in the illustrated arrangement includes an iron yoke O manufactured from transformer plate in a known manner. As before mentioned, magnetization of the yoke O through the medium of the current in the winding P of the electromagnet T3 will also change the flux in the ring core K of the transformer Tl. The iron yoke O is mounted externally of the ring core K with a connection transversely across the core, between the centre point of the primary winding P and the space between the secondary windings SI, S2.
The arrangement illustrated in Fig. 2 relates to a so- called half bridge, wherein σapacitive voltage dividers C6, C7 are connected in parallel with respective transistors TRl, TR2. Naturally, the voltage dividers C6, C7 can be replaced with transistors TR3, TR4, so as to obtain a so- called full bridge according to Fig. 5.
The winding circuit of the electromagnet T3 is connected to the power unit output in both the arrangement illustrated in Fig. 3 and the arrangement illustrated in Fig. 5.