Electricity: power supply or regulation systems – Output level responsive – Using a transformer or inductor as the final control device
Patent
1997-12-29
1999-05-25
Wong, Peter S.
Electricity: power supply or regulation systems
Output level responsive
Using a transformer or inductor as the final control device
323259, 323346, G05F 114, G05F 124, G05B 2402
Patent
active
059072369
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to voltage converters. More particularly, although not exclusively, the present invention relates to resonance tapped transformer circuits in which the effective turns ratio of the transformer is dependent on the frequency of the input signal.
BACKGROUND OF THE INVENTION
DC--DC converter techniques which are known in the art include pulse width modulation (PWM) DC--DC converters and resonant series and parallel converters. Such methods are known to be susceptible to switching losses, particularly in the former case, and conduction losses.
In a conventional PWM DC--DC converter using switching transistors, the switches have an inherent capacitance which results in a residual voltage across the switch when the switches are alternately open and closed. Accordingly, it is desirable to delay switching to allow discharge of the capacitance by placing an inductor in series with the load resistance. This will resist the change in the current flow due to the residual field in the inductor. With no inductance, the power factor is close to unity. If an inductor is added in series with the load the power factor lags and soft switching is provided.
Ideally the current and voltage in a circuit element will be in phase. However, if there is a slight phase difference there will be instants where the current is positive and the voltage negative (or vice versa). In this situation the instantaneous power will be negative and the power transmitted will be effectively reduced. Thus the power transmission is less for the same time averaged values of voltage and current.
To obtain the same or a desired level of power transmission, the voltage must be increased but this increases the current correspondingly which leads to increased I.sup.2 R losses (or conduction losses). The traditional method of reducing output voltage in a PWM converter is to reduce the duty cycle. But as the duty cycle is reduced, the peak current is increased if the same average current is to be maintained. However, as the peak current is increased the I.sup.2 R losses become disproportionately high.
In presently known resonant circuits, it is possible to reduce the voltage by putting a reactance in series with the load. However, this may significantly impair the power factor. The present invention avoids the need for such a reactance and further provides an inductance to effect a lagging power factor (or soft switching). The present invention further provides for a converter where the turns ratio may be varied (and therefore the conversion ratio adjusted) while retaining the use of all of the primary windings.
It is an object of the present invention to overcome or at least mitigate some of the disadvantages inherent in the prior art or to at least provide the public with a useful choice.
DISCLOSURE OF THE INVENTION
In one aspect this invention provides a transformer circuit adapted to selectively connect a plurality of primary coils of a transformer in series and/or in parallel, wherein the connection type is dependent upon the line frequency, thereby changing the transformation ratio of the transformer.
Preferably the transformer circuit comprises: function of the line frequency and are connected in such a manner that at a first frequency the plurality of primary transformer coils are connected in parallel and at a second line frequency the plurality of primary transformer coils are connected in series.
In a further embodiment the present invention provides for a transformer circuit comprising: elements, connected in series with the second circuit element, is connected in parallel with a first primary transformer coil and the other first circuit element, connected in series with the second circuit element, is connected in parallel with a second primary transformer coil.
Preferably the first and second circuit elements comprise impedance elements having first and second resonant frequencies respectively.
Preferably the circuit elements can include inductive and/or capacitive components.
Preferably the ci
REFERENCES:
patent: 1639695 (1927-08-01), Heegner
patent: 3845437 (1974-10-01), Goodman
patent: 5625543 (1997-04-01), Barrett
patent: 5668707 (1997-09-01), Barrett
Patel Rajnikant B.
Swichtec Power Systems Limited
Wong Peter S.
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