BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to DC to DC converters, and more particularly to converter oscillators of the ringing choke type adapter for use in insect killing devices.

2. Description of the Prior Art

DC to DC conversion has had extensive applications in the past. Such converters can be found in gaseous lamps, automotive ignition systems of the capacitor discharge type, in laser pumping circuit and many other applications. Most frequently converters of the foregoing kind lacked the requisite efficiency for portable use. In particular devices like bug killers used by campers require portable battery power and thus the manner in which that power is used is of a paramount importance. The typical prior art converters normally experience substantial power losses during the switching transient of each oscillation. In the typical ringing choke configuration the collector winding develops current levels which are quickly above the saturated conduction levels of the base. At this point the base drive becomes insufficient to maintain the switching transistor in saturated conduction causing a slow turn-off with its attendant losses.

In the past various techniques were developed to improve this switching transient, both for optimization of efficiency and to improve frequency control. In each instance the improvements entail circuit complexity or the use of special purpose elements and their related expense.

SUMMARY OF THE INVENTION

Accordingly it is the general purpose and object of the present invention to provide a DC to DC converter which by way of a control transistor both improves switching efficiency and frequency control.

Other objects of the invention are to provide a DC to DC converter of the ring choke type having a control transistor in the base circuit of the switching transistor.

Yet additional objects of the invention are to provide a DC to DC converter which is reliable in use, entails few parts and is stable over a wide range of load and power inputs.

Briefly these and other objects are accomplished within the present invention by combining a base control transistor into the circuit of a ring choke oscillator, the base control transistor being switched in and out of conduction by the phase between the collector and base windings of the switching transistor forming the oscillator. To control the phase angle accurately a capacitor is provided across the base-emitter leg of the control transistor and it is the current ratio set by this base capacitor which control the conduction of the control transistor. By virtue of these additional elements the following advances are obtained: the main switching transistor is switched at a fast turn-off rate, resulting in lowered switching losses; the cycling rate of the control transistor is determined by an RC time constant rather than by transistor gain which varies from device to device, and supply voltage variations are compensated by faster turn-ons of the control transistor and low losses are further enhanced due to base diode which reduces reverse currents.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit schematic of a ring choke converter including a control transistor circuit constructed according to the invention herein; and

FIG. 2 is a variation of the circuit shown in FIG. 1 illustrating alternate circuit implementation for further reducing base loss including second transformer.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS

The following disclosure entails an improvement on a prior art device and for that reason alphabetic symbols designate the structure improved. Accordingly as shown in FIG. 1 a prior art ring choke oscillator includes a collector winding W_c tied between the collector of the switching transistor T_s and the collector source V_cc across which the collector current is applied. A base winding W_b is connected to the base T_s across circuit connection including a resistor R_b and a capacitor C_b . A further control or base bias is obtained by yet another resistor R_c deployed across the collector winding W_c and the base of transistor T_s. This configuration, without the added parts, comprises a typical ring choke converter which drives an output winding W_o. The current in winding W_o is rectified across a diode (optional) D_l to power the load R_l.

In this circuit as the transformer core T_l charges and discharges during each cycle the current through resistor R_c charges capacitor C_b and turns the transistor T_s on. Since the feedback through W_b is positive transistor T_s saturates and remains saturated until the base signal drops off across capacitor C_b. As the drop across capacitor C_b brings the base drive below the level necessary to maintain the transistor in saturation transistor T_s slowly begins turning off. When T_s turned off a forward voltage is developed across load R_l discharging the core. During this discharge period capacitor C_b is discharged through winding W_b.

In order to improve this slow transistion of the above circuit and the periodic reverse currents in windings W_b a control transistor 10 is connected between the base and emitter of transistor T_s. Transistor 10 is controlled by a resistor 16 in series with a variable resistor 11 disposed between the winding W_b and the base thereof and a capacitor 12 connected to ground. Resistor 11 together with resistor 16 and capacitor 12, control the base signal of transistor 10, thus controlling the on time of transistor T_s.

Accordingly with few additional parts substantial improvements and efficiency are obtained. More specifically as current flows through resistor R_c and resistor R_b to the base of transistor T_s, transistor T_s turns on. Concurrent with the increase of signal across resistor 11 the current in winding W_c increases. An increase in current through winding W_c is accompanied by an increase in winding W_b, increasing the charging rate of capacitor 12. When the charge on capacitor 12 reaches the conduction level of transistor 10 transistor T_s is turned off decreasing the current through winding W_c and concurrently the current in the winding W_b . As the signal across winding W_b decreases transistor 10 is turned off.

The additional features of this circuit provide for compensation of the collector voltage variations. More specifically as V_cc increases the voltage across winding W_b also increases. This is accompanied by an associated increase in the charging current to capacitor 12.

An alternative configuration for the above circuit is set out in FIG. 2. In this circuit a diode 14 in parallel with a capacitor 15 is substituted for capacitor C_b, reducing reverse current and cutting these losses substantially.

In addition, or in the alternative, a diode 17 across capacitor 12, limits peak reverse voltage on capacitor 12. This improves frequency control and improves regulation of circuit from power supply variations. Included further is a second transformer core T₂ having its primary W₅ connected in series with the primary W₁ of transformer T₁. The secondary W₆ of transformer T₂ extends from a tap of winding W₃ to one end of the load R_L, which in this case may be a gaseous tube. The other end of load R_L is connected to one end of the winding W₃ while the other end of the winding connects across a diode 18 to a capacitor 19. Capacitor 19 then connects to the common juncture of the winding W₆ and load R_L, there being two electrodes 20_A and 20_B disposed across the capacitor. It is between these electrodes that a potential is developed to kill flying insects.

Thus, when transistor T_s turns off the voltage across windings W₃ and W₆ is additive. The load, in the form of a gaseous tube, begins to conduct. The inductance of the winding W₆ then controls the discharge across the load. Accordingly the voltage across the gap between electrodes 20_A and 20_B is equal to the voltage across winding W₃ minus the voltage across the load. This voltage between the electrodes may be used to kill insects. When an insect flies into this gap the increased current must also flow through the load R_L, since the inductance of winding W₆ effectively limits the amount of current passed thereacross. This last feature assists in the maintenance of the power across the electrodes while the insect is electrocuted.

It is to be understood that any capacitance in the gap will reduce the gap voltage. Accordingly diode 18 is provided to charge up the gap capacitance thereby increasing output voltage with some input power while capacitor 19 provides the stored energy for the initial transient.

Obviously many modifications and variations to the above disclosure can be made without departing from the spirit of the invention. It is therefore intended that the scope of the invention be determined solely on the claims appended hereto.