Electric lamp and discharge devices: systems – Periodic switch in the supply circuit – Silicon controlled rectifier ignition
Patent
1994-02-28
1996-10-01
Gonzalez, Frank
Electric lamp and discharge devices: systems
Periodic switch in the supply circuit
Silicon controlled rectifier ignition
315219, 315222, 315223, 315206, H05B 3700
Patent
active
055613497
DESCRIPTION:
BRIEF SUMMARY
The invention concerns a frequency modulated converter with a series-parallel resonance, particularly for driving any ohmic or inductive load, including gas discharge tubes, wherein a commutating voltage switch in the form of a transistor is provided and in connected in series between the negative electrode of a direct current voltage source and a first terminal of an inductor, wherein a pulse generator circuit is provided between the voltage source and a control electrode of the transistor and wherein a second terminal of the inductor is connected to the primary winding of a transformer.
The invention further comprises a first capacitor and a rectifier diode provided in a first and second parallel branch respectively between the charge emitting and the charge receiving electrode of the transistor, and a second capacitor is provided across the electrodes of the voltage source and additionally provides a smoothing capacitance for the voltage source, the second capacitor being connected in series with the inductor via the diode.
The later years have seen a dramatic reduction in the physical dimensions of power converters, something which has been achieved by increasing the operating frequencies. Common quasi-square pulse converters now approach an operating range whose upper limit lies about 0.5 MHz. This allows a significant reduction in the size of the most important passive power components such as the magnetic components and the capacitors, compared to for instance with switches for 20 kHz. However, these converters, being for the greater part pulse width modulated converters, have high switching losses in the power semiconductors, something which leads to reduced efficency and hence a need for more cooling, which decreases the possibilities of a reduction in the physical dimensions of the converters.
In order to increase the capability of power converters to handle overloads or large load varations, for instance when used as power supplies for gas discharge tubes, there has been proposed to monitor or avoid saturation of the switching transistors by special circuit arrangements, as will be evident from for instance PCT application NO WO 90/01248 and GB-PS-No. 1 378 465.
A more effective way of converting power at increasingly higher frequencies is based on the so-called "zero current switching" wherein a sinusoidal voltage which may be generated by a LC-resonant tank connected either in parallel or series, is used. Such converters are called "resonant converters". The advantage of using a sine voltage is that losses in the power semiconductors are dramatically reduced, as the switching generally takes place at the zero crossing. The disadvantage of resonant converters is that at a given power level the peak current is many times greater than that of a pulse width modulated converter. By using semiconductors with lower resistance in conduction it is however possible to increase the operating frequencies to above 1 MHz. Thus power densities well beyond 1 W/cm.sup.3 may be attained.
For use in such converters there is now known a frequency modulated control in the form of an integrated circuit which may be used in the range beyond 1 MHz, with the designation LD 405, obtainable from Gennum Corporation, Burlington, Ontario, Canada. The use of this control circuit in a frequency modulated converter is described in a LD 405 application note from Gennum Corporation with the title "Using LD 405 in a 125 W resonant mode power supply". To this end the same corporation has introduced a resonant circuit of which the embodiment in principle is shown in the appended FIG. 1. The circuit comprises an inductance L, a capacitance C, a resistance R and a load R.sub.L. Before the inductance L a commutating switch S, for instance in the form of a transistor, has been provided its purpose is to supply direct current from a source V to a series resonant tank LC. The resistance of the load R.sub.L drains current from the tank. As soon as the resonant process has been terminated, the switch S opens and the power conversion from
REFERENCES:
patent: 4613796 (1986-09-01), Bay
Gonzalez Frank
Ratliff Reginald A.
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