PUSH-PULL BASED VOLTAGE-CLAMPING ELECTRONIC BALLAST

Abstract

An electronic ballast that includes a DC power source and a transformer comprising first, second and third windings. The first, second and third windings being inductively coupled, and the third winding is connected in parallel with a load. The ballast also includes first and second circuit pathways connected in parallel. The first circuit pathway comprises a first switch connected in series with the first winding, and the second circuit pathway comprises the second winding connected in series with a second switch. The DC power source is connected in parallel with the first and second circuit paths to provide an input voltage source. A capacitor connects the point between the first switch and first winding of the first circuit pathway with the point between the second switch and the second winding of the second circuit pathway.

Description

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to electronic ballasts, and more particularly to electronic ballasts that clamp a voltage across a switch and recycle leakage energy of a transformer such that a power converter has high power conversion efficiency with no voltage spike in the switch.

[0003] 2. Description of the Related Art

[0004] Traditional ballasts for use in fluorescent lamps consisted of large and heavy magnetic coils. These have been replaced by electronic ballasts that are compact and light. A common device used as an electronic ballast is a self-oscillating, push-pull converter. One well-known push-pull converter is shown in FIG. 1. As shown in FIG. 1, the converter consists of a DC power source Vin, switches S1 and S2, and transformer T. The switches S1 and S2 are each a standard metal-oxide semiconductor, field-effect transistor (MOSFET). Ds1 and Cs1 represent body diode and internal capacitance, respectively, of switch S1, and Ds2 and Cs2 represent body diode and internal capacitance, respectively, of switch S2. Transformer T contains three windings, N1, N2 and N3, and is designed such that a current present in either winding N1 or N2 produces a current in winding N3 to drive the load Ro, typically a fluorescent lamp. Inductor Lr and capacitor Cr form a standard resonant tank to provide high frequency voltage to the load Ro. Three current paths flowing in the circuit are represented as having currents is1, is2 and iLr. Voltage V0 is shown across load Ro.

[0005] FIG. 2 is a graph of the waveforms associated with the conventional push-pull electronic ballast circuit of FIG. 1. As shown in FIG. 2 and also found in other like electronic ballasts is a phenomenon of overvoltage or high voltage spikes that occur across the switches when the device is switched off. For example, at t0 when S1 is switched off a spike is shown in FIG. 2 to occur in the voltage across S1, i.e. Vds1. The principle reason for the voltage spike is that the opening of switch S1 creates an abrupt interruption (discontinuity) in the current through N1. There is an release of stored energy due to a leakage inductance associated with winding N1 (represented as Lk1 in FIG. 1), thereby inducing a current that charges CS1 and consequently creating a high voltage spike across switch S1.

[0006] The same voltage spike characteristic occurs in the voltage across S2, i.e. Vds2, at time t2 when S2 is switched off The voltage spike is caused by transient or leakage inductance Lk2 associated with transformer winding N2. The circuit of FIG. 1 provides no avenue for the released energy of the winding to flow other than the internal capacitance of the adjacent switch, which results in the voltage spike. In the prior art the leakage inductances are always present to some extent and there is always a danger of inducing these high voltage transients when switching off. The faster the switching the greater the voltage spikes. An excessive voltage spike will result in permanent damage to the switching device such as a burn through of the semiconductor layers.

[0007] One method of reducing the voltage spikes is to include a “snubber” circuit comprised of an additional diode, capacitor and resistor, connected in parallel with the switch (such as S1 and S2). While the snubber circuit can limit the peak voltage of the spike, it slows down the effective switching speed of the circuit, and in doing so it absorbs energy that is dissipated as heat, thus reducing the overall power conversion efficiency of the ballast.

[0008] Another method of reducing the voltage spikes is to “clamp” the voltage across the switch (such as S1 and S2). This is accomplished by two conventional methods. A first method connects a zener diode across the switch. As the voltage spike occurs the Zener diode turns on allowing the current to flow through, thus reducing or eliminating the voltage spike across the switch. A second method connects a diode in series with a with a parallel capacitor and resistor network across the switch. The capacitor charges to a constant voltage through a current flow, thus absorbing the voltage spikes. The resistor dissipates the stray inductances while maintaining the voltage across the capacitor. As with any clamping circuit, stray circuit inductances exist, and voltage spikes will be produced. Also, a diode by its nature does not provide an instantaneous clamping action.

SUMMARY OF THE INVENTION

[0009] It is therefore an aspect of the present invention to provide an electronic ballast that clamps the voltage across the main switch and eliminates voltage spikes due to the leakage inductance of the transformer.

[0010] It is another aspect of the present invention to provide an electronic ballast that recycles the leakage energy of the transformer to improve the power consumption efficiency of the circuit.

[0011] These and other aspects of the present invention are achieved by providing an electronic ballast that incorporates a clamping capacitor therein in such a manner that the overall circuit eliminates voltage spikes across the switches and recycles the transient leakage inductances of the transformer.

[0012] Thus, the invention comprises an electronic ballast that includes a DC power source and a transformer comprising first, second and third windings. The first, second and third windings being inductively coupled, and the third winding is connected in parallel with a load. The ballast also includes first and second circuit pathways connected in parallel. The first circuit pathway comprises a first switch connected in series with the first winding, and the second circuit pathway comprises the second winding connected in series with a second switch. The DC power source is connected in parallel with the first and second circuit paths to provide an input voltage source. A capacitor connects the point between the first switch and first winding of the first circuit pathway with the point between the second switch and the second winding of the second circuit pathway.

[0013] The first and second switches may be transistors, for example, metal-oxide semiconductor field-effect transistors. The first switch and the second switch are electrically connected to a digital controller or some other switching source that provides switching signals to the first and second switches. The first and second switches are switched off and on in a repeating cycle, the first switch switched on while the second switch is switched off and the first switch switched off while the second switch is switched on. When the first switch is switched off, a current in the first winding provides a charging of the capacitor. Likewise, when the second switch is switched off, a current in the second winding provides a charging of the capacitor. The voltage across the first and second switches is substantially constant over the interval that the respective switch is turned off.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The above and other aspects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which:

[0015] FIG. 1 is a circuit diagram of a conventional push-pull electronic ballast;

[0016] FIG. 2 is a graph of typical switching waveforms of the circuit of FIG. 1;

[0017] FIG. 3 is a circuit diagram of a push-pull based voltage clamping electronic ballast according to an embodiment of the present invention;

[0018] FIG. 4 is a graph of switching waveforms of the circuit of FIG. 3;

[0019] FIG. 5 is a circuit diagram representing the circuit of FIG. 3 when switch S1 is on; and

[0020] FIG. 6 is a circuit diagram representing the circuit of FIG. 3 when switch S2 is on.

DETAILED DESCRIPTION OF INVENTION

[0021] Preferred embodiments of the present invention will be described herein below with reference to the accompanying drawings. In the following description, well-known functions or constructions are not described in detail since they would obscure the invention in unnecessary detail.

[0022] FIG. 3 is a circuit diagram of a preferred embodiment of the present invention. A DC power source Vin supplies DC power to the circuit. Lk1 and Lk2 represent the leakage inductances of windings N1, N2 of transformer T. Ds1 represents body diode of switch S1. Ds2 represents body diode of switch S2. CS is a clamping capacitor that, as described farther below, acts to absorb and recycle leakage energy of transformer T. Transformer T has three coupled windings, primary windings N1 and N2 and secondary winding N3. In the preferred embodiment, the number of windings of N1 is equal to the number of windings of N2. The normal current paths through switches S1 and S2 and windings N1 and N2 are represented by is1, is2, in1 and in2x, respectively, having the sign conventions as shown. The voltages across switches S1 and S2 and capacitor Cs are represented by Vs1, Vs2 and VC, respectively, having the sign conventions as shown. Inductor Lr and capacitor Cr form a resonant tank and resonant tank current iLr induced in N3, provides a high frequency voltage V0 across the load R0, i.e. a fluorescent lamp. The operation of FIG. 3 will be described with reference to FIG. 4.

[0023] FIG. 4 is a graph of the various waveforms associated with the switching of the circuit of FIG. 3 during a steady state switching. Waveforms i and ii give the relative switching cycles of S1 and S2, respectively. Just prior to t0, switch S2 is conducting and carries its maximum current (see waveform vi) at t0, which is equal to in2-in1. Since switch S2 has been on for the duration of a switching cycle at t0, the current in2 through N2 due to Vin has increased to a maximum (see waveform viii), while capacitor CS is discharging a voltage (described further below), thus creating a negative current in1(see waveform vii). By the sign convention of in1 shown in FIG. 3, both of these currents add to is2.

[0024] During the time period t0 to t2, switch S1 is conducting or on and switch S2 is off During t2 to t4 switch S2 is on and switch S1 is off (see waveforms i and ii of FIG. 4). It is noted that a switch is also considered “on” where the associated body diode is conducting. Thus, for example, waveform v at t0 shows that is1 has a negative value even though waveform i shows that switch S1 remains off for a brief period after t0. The negative current flow in this interval is through Ds1.

[0025] FIG. 5 is the equivalent circuit of FIG. 3 during time period t0 to t2, when S1 is on and S2 is off. (For ease of description, the leakage inductances Lk1, Lk2 associated with windings N1 and N2 are omitted from the description of the basic operation of FIG. 5 (and FIG. 6 below); the impact of the leakage inductance at switch off will be considered in further detail below.) As shown in FIG. 5, the input source voltage Vin is applied to winding N1. Since winding N1 is coupled to winding N2 the increasing current in1 created by Vin induces a voltage and a resulting current in2 in N2. Since the number of windings of N1 and N2 are equal (have a turns ratio of 1:1), the induced voltage across N2 is equal to (N2/N1)*Vin, and thus is equal to Vin. By the dot convention of N1 and N2, the voltage drop is across N2 in the upward direction in FIG. 5 (i.e., in the direction of point Z to X). Because point X in the circuit has a voltage level Vin, the voltage at point Z must be 2Vin. Because the voltage at point Y also has voltage level Vin, the voltage drop from Z to Y is equal to Vin. Thus, capacitor voltage Vcs=Vin.

[0026] Because of the coupling between N2 and N1, current in2 is equivalent to −in1, with a slight DC bias voltage offset due to energy absorbed from Vin as shown in waveforms vii and viii of FIG. 4. The current ics, of capacitor CS must equal the current in2 flowing through N2, as shown in waveforms viii and ix. Thus, from time t0 to t1 the current ics acts to charge capacitor Cs, whereas from time t1 to t2 the current ics acts to discharge Cs.

[0027] As noted, the voltage at point Z of the circuit of FIG. 5 is equal to 2Vin. This is also the voltage drop across the open switch S2 in the time interval t0 to t2, as shown in waveform iv of FIG. 5. Focusing again on t0 of FIG. 4, it is seen from waveforms ii and iv that switch S2 does not have the voltage spike when switched from on to off, as in the prior art devices. Instead, waveform iv shows that the voltage across S2 rises directly to a value of 2Vin and is maintained at that level for the duration t0 to t2. When S2 is turned off at t0, the current in2 flowing in N2 is immediately shunted to the circuit loop that includes capacitor Cs, thus immediately charging Cs with the current in2. Thus, any current contributed to in2 by a release of magnetic energy stored in leakage inductor Lk2 associated with N2 at t0 acts to charge Cs from t0 to t1. Thus, there is no mechanism in the circuit that creates a voltage spike across S2.

[0028] It is again observed that the discharge of capacitor Cs from t1 to t2 creates a negative current in cs (which is equivalent to in2) as shown in FIG. 5 by the sign convention. The current thus flows counter-clockwise around the loop including Cs, N2 and S1, thus contributing (along with the current in1 through N1 created by Vin) to the relatively large positive current is1 through switch S1 from t1 to t2.

[0029] By symmetry, analogous description applies during the portion of the switching cycle when S1 is off and S2 is on during t2 to t4, as shown in FIG. 6. In short, supply voltage Vin is applied across N2 (including associated leakage inductor Lk2). Thus, current in2, which now flows through closed switch S2, begins to build from a negative value at t2 to a positive value at t4. By the coupling of N2 and N1, voltage Vin is induced across N1 in the upward direction (by the dot convention). This implies that the voltage at point Z′ is −Vin, VC=Vin and Vs1=Vin−(−Vin)=2Vin, as shown in waveform iii of FIG. 4.

[0030] Input voltage Vin applied to N2 acts to reverse current in2, as shown in waveform viii of FIG. 4. A closed circuit is also formed by the loop including CS, N1 and S2; thus in1 and ics are equal from t2 to t4, as shown in waveforms vii and ix of FIG. 4. Currents in1 and ics are positive from t2 to t3, thus charging capacitor CS as the current in N1 is reduced. As described above, this shunting of the current to Cs avoids a voltage spike across S1 at t2. (See waveform iii at t2). The combination of current in1 (or equivalently, ics) and current in2 through switch S2 from t2 to t4 results in a relatively large current through S2 at the end of the cycle t4. (See waveforms vii, ix and vi at t2 to t4.)

[0031] Subsequent switching cycles are analogously described. Thus, the inventive circuit avoids the voltage spikes across the switches when a switching off occurs.

[0032] The winding currents in1 and in2 through N1 and N2 are always continuous sinusoidal waves as long as the switching frequency is chosen close to the resonant frequency, which is determined by the values of resonant inductor Lr and resonant capacitor Cr. In any operation mode, the winding currents are always continuous and the switch voltages are always clamped up to two times the input voltage. Actually, the leakage inductances of transformer T can be used as part of the resonant inductance. In any operation mode, there are two sub circuits operating in parallel. There is no source within the circuit that generates the high voltage spike since the current flowing through windings N1 and N2 is always continuous. The leakage energy is thus recycled.

[0033] Given again that the number of windings of N1 equals those of N2, the maximum voltage across switches S1 and S2 is equal to twice the input voltage Vin. Compared with the conventional push-pull resonant inverter, in the present invention the windings N1 and N2 carry continuous current and work together at all times to produce the resonant tank current iLr, and the voltages across the switches S1 and S2 are always equal to twice the input voltage Vin. As a result, the present invention has a high efficiency and low voltage stress. And due to the fact that the present invention recycles the leakage inductances, there is no requirement for the leakage inductances of the transformer to be taken into effect during design.

[0034] It will be understood that the invention is not limited to a particular type of switching device, and that other switching devices such as bipolar-junction transistors (BJTs) or junction field-effect transistors (JFETs) can be used.

[0035] While the invention has been shown and described with reference to certain preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.

Claims

1. An electronic ballast, comprising:

a DC power source;

a transformer comprising first, second and third windings, said first, second and third windings being inductively coupled, the third winding being connected in parallel with a load;

first and second circuit pathways connected in parallel, the first circuit pathway comprising a first switch connected in series with the first winding, and the second circuit pathway comprising the second winding connected in series with a second switch, the DC power source also connected in parallel with the first and second circuit paths to provide an input voltage source;

a capacitor connecting the point between the first switch and first winding of the first circuit pathway with the point between the second switch and the second winding of the second circuit pathway.

2. The electronic ballast of claim 1, further comprising:

a first diode connected in parallel with said first switch; and

a second diode connected in parallel with said second switch.

3. The electronic ballast of claim 1, wherein said load is a resonant circuit comprising a fluorescent lamp.

4. The electronic ballast of claim 3, wherein said resonant circuit is comprised of an inductor in series with a capacitor and a fluorescent lamp, said capacitor and said fluorescent lamp being in parallel with each other.

5. The electronic ballast of claim 1, wherein said first and second switches are transistors.

6. The electronic ballast of claim 5, wherein said transistors is a metal-oxide semiconductor field-effect transistors.

7. The electronic ballast of claim 1, wherein the first switch and the second switch are electrically connected to a controller, the controller providing switching signals to the first and second switches.

8. The electronic ballast of claim 7, wherein the switching signals provided by the controller switches the first and second switches off and on in a repeating cycle, the first switch switched on while the second switch is switched off and the first switch switched off while the second switch is switched on.

9. The electronic ballast of claim 8, wherein the voltage across the first and second switches is substantially constant.

10. The electronic ballast of claim 9, wherein a current in the first winding provides a charging of the capacitor when the first switch is switched off and a current in the second winding provides a charging of the capacitor over the interval that the respective switch is turned off