Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Patent
1981-06-16
1983-02-01
Shoop, William M.
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
363 97, 363124, H02M 322, H02P 1318
Patent
active
043719171
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a voltage conversion method to drive a switching voltage convertor yielding a high efficiency.
In the majority of DC-DC convertors, the voltage transformation is achieved by chopping a DC voltage, coming from a battery or a rectifier, by means of one or more switching transistors. The chopped DC voltage is then fed to the primary winding of a transformer which transforms the AC-component of said chopped DC voltage up or down, in such a way that one or more AC voltages with predetermined amplitudes are obtained at the secondary windings of the transformer. The AC voltages are then rectified and smoothed and then fed to the output connections of the DC-DC convertor.
The advantage of switched voltage conversion lies in the fact that, if a comparatively high switching frequency is applied, a much smaller transformer can be used than in the case when a 50 Hz AC voltage is transformed and rectified. Switched power convertors can therefore be made small and can have low weight.
In more advanced types of switched DC-DC convertors, the voltage on the secondary winding is sensed and the resulting feed-back signal regulates the pulse width of the switched DC-voltage being fed to the primary winding of the transformer.
In this way an output voltage is obtained which can be kept relatively constant even with a varying load. The feed-back signal can also be used to protect the convertor against short-circuiting and overloading. An example of such DC-DC convertors is the so called "fly-back" convertor.
In switched high voltage power supplies used to deliver high current, two basic convertors are applied. In one type the transformer has a ratio such that the secondary winding directly yields the desired high voltage after rectification. In another type the secondary winding yields only a fraction of the desired high voltage and the final high voltage generation is achieved through voltage multiplication.
The first mentioned type has the advantage of being uncomplicated and small in size. Its disadvantage lies first of all in the fact that a large secondary winding necessarily has a large inductance and large winding capacitance, so that the resonance frequency of the transformer is low. This means that at the moment when the DC voltage is switched, large current and voltage transients will occur at the switching transistors, which have to be dimensioned accordingly to withstand the power loss. The physical explanation why such current and voltage transients must occur is given below.
The transients are a serious problem and to a large extend have limited the applicability of the direct conversion method. Various ways of suppressing such transients have been tried. One of these methods is suggested in the U.S. Pat. No. 4,063,306 of December 1977. Here an actively switched damping circuit is described which limits the amplitude of the transients by activating a filter at the moment such transients occur.
Another method to decrease the power loss is based on a means of regulating the saturation of a transformer core, in order to limit the amplitude of the transients. This method is described in U.S. Pat. No. 4,061,957.
In both methods one has not tried to eliminate the transients, but only describe ways to eliminate, respectively decrease their effect.
The second type of convertor has a considerably high resonance frequency due to a smaller secondary winding. Transients occurring as a result of switching a DC voltage can damp sufficiently before the next switching event takes place. A disadvantage of this type of convertor is the need of a voltage multiplicator, which must contain several diodes and capacitors. This leads not only to a cost increase, but also to larger size and shorter MTBF (mean time between failure) as a result of the increased risk for component failure in the diodes and capacitors. Furthermore such convertors are generally more load sensitive than the first mentioned type.
SUMMARY OF THE INVENTION
The present invention solves the problems. The method is based on voltag
REFERENCES:
patent: 3618130 (1971-11-01), Garuts
patent: 4061957 (1977-12-01), Jan Vader
patent: 4063306 (1977-12-01), Perkins et al.
Andersson Hans
Shoop William M.
Wladimiroff Wladimir
Wong Peter S.
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