Abstract:
A self-oscillating switching power converter has a controllable reactance including an active device connected to a reactive element, wherein the effective reactance of the reactance and the active device is controlled such that the control waveform for the active device is binary digital and is not synchronized with the switching converter output frequency. The active device is turned completely on and off at a frequency that is substantially greater than the maximum frequency imposed on the output terminals of the active device. The effect is to vary the average resistance across the active device output terminals, and thus the effective output reactance, thereby providing converter output control, while maintaining the response speed of the converter.

Description:
FEDERAL RESEARCH STATEMENT 
     The U.S. Government may have certain rights in this invention pursuant to contract number DEFC2699FT40630 awarded by the U.S. Department of Energy. 
    
    
     BACKGROUND OF INVENTION 
     Self-oscillating resonant power converters, such as commonly used in compact fluorescent lamp ballasts, for example, typically operate by deriving a transistor switching waveform from one or more windings magnetically coupled to a resonant inductor. U.S. Pat. No. 5,965,985 of Nerone describes a circuit for such a ballast that allows control of the output to a load in order to provide lamp dimming capability. U.S. Pat. No. 5,965,985 describes the control of a self-oscillating ballast by effectively clamping the voltage excursion across an inductor. The effect is to control the reactance of the inductor clamp combination. A similar method of achieving such a result is to vary the effective reactance of a reactive element using a variable resistance coupled in series or parallel therewith. The variable resistance is typically implemented with an active element, e.g., a transistor, wherein the effective resistance across two terminals is a continuous function of the magnitude of the control signal. The applied control signal is also continuous and has a maximum frequency component that is substantially less than the switching frequency of the converter. 
     It is desirable to implement control circuitry, such as of a type described hereinabove, on an application specific integrated circuit (ASIC) in order to achieve low complexity and cost. It is furthermore desirable to implement as much of the control circuitry as possible in digital form. Unfortunately, the control method described hereinabove inherently requires an analog, continuous signal. Hence, a digital approach, when combined with the control method described hereinabove, requires a digital-to-analog converter to generate the control signal, adding to the complexity of the system. In addition, the analog approach may result in significant power dissipation in the control element, making it impractical to integrate on an ASIC chip. These latter drawbacks may be overcome using a switch control waveform synchronized to the converter power switching waveforms, as known in the art, but for a self-oscillating converter, this results in the requirement of a frequency tracking circuit, such as a zero-crossing detector or phase-locked loop. This requirement may substantially increase cost, complexity, and size of the system. 
     Accordingly, it is desirable to provide a control for a self-oscillating switching power converter using an active control device in a manner that does not require the control switch waveform to be synchronized with the converter switching frequency. It is furthermore desirable that such control device be operated in a digital manner, that is, with two operating states (on and of f and that the control input for the device also be digital. It is furthermore desirable that such a control avoid compromising the response speed of the converter, so that maximum performance may be obtained. 
     SUMMARY OF INVENTION 
     In accordance with exemplary embodiments of the present invention, a self-oscillating switching power converter has a controllable reactance comprising an active device connected in series or parallel with a reactive element, wherein the effective reactance of the controllable reactance and the active device is controlled such that the control waveform for the active device is binary digital and is not synchronized with the switching converter output frequency. Preferably, the active device is turned completely on and off at a frequency that is substantially greater than the maximum frequency imposed on the output terminals of the active device. The effect of such control is to vary the average resistance across the active device output terminals, and thus the effective output reactance, thereby providing converter output control, while maintaining the response speed of the converter. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 schematically illustrates a control for a switching power converter of a type described by U.S. Pat. No. 5,965,985; 
     FIG. 2 schematically illustrates a control for a switching power converter in accordance with an exemplary embodiment of the present invention; 
     FIG. 3 schematically illustrates circuitry and graphs useful for describing operation of the circuit of FIG. 2; 
     FIG. 4 schematically illustrates an exemplary application for a power converter and control of the present invention in a compact fluorescent lamp ballast; 
     FIG. 5 graphically illustrates exemplary start-up and steady-state waveforms for the ballast of FIG. 4; and 
     FIG. 6 graphically illustrates an exemplary transition from start-up to steady-state operation for the ballast of FIG.  4 . 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 illustrates a known implementation of a variable reactance control circuit  10  for a self-oscillating power converter. The control circuit comprises a dc control voltage  12  coupled to an active device  14 . A diode bridge network  16  enables the typically unipolar active device  14  to function as a bipolar resistive element. In the circuit of FIG. 1, the controlled reactive element comprises an inductor  18 . The effective resistance across terminals A and B of FIG. 1 is a continuous function of the magnitude of the control signal applied to device  14 . The applied control signal is also continuous and has a maximum frequency component substantially less than the switching frequency of the converter. The variation in resistance across terminals A and B results in a varied effective inductance, the switching converter output being controlled thereby. 
     Disadvantageously, the circuit of FIG. 1 is not practicable for ASIC applications, such as, for example, a compact fluorescent lamp ballast, due to the complexity of adding a required digital-to-analog converter and also the difficulty of integrating a control device capable of dissipating sufficient power for such application on an ASIC chip. Moreover, the circuit of FIG. 1 is not capable of an all-digital ASIC implementation. 
     FIG. 2 illustrates a variable reactance control circuit  20  useful in a self-oscillating switching converter in accordance with exemplary embodiments of the present invention. Control circuit  20  comprises a bi-directional active device  21  having a pulse modulator  24  with a control input  23  thereto. A diode network  26  enables bi-directional operation to be achieved with a typically uni-directional active device  22 . A resistor  28  (R) is coupled between switch  22  and the diode network  26 . The reactance  30  to be controlled is illustrated in FIG. 2 as comprising an inductor  31 . 
     In operation, the control frequency F C  for device  22  is substantially greater than the maximum switching frequency F S  imposed on terminals A and B. Typical values of F S  might lie in the range of 10 kHz to 200 kHz, and a typical value for F C  could be 1 MHz. In one embodiment, pulse modulator  24  provides a pulse width modulated (PWM) waveform with a duty cycle D. FIG. 3 illustrates PWM control and the effective resistance between terminals A and B, as represented by Vtest/Itest. The effect of the PWM waveform is to vary the average resistance in parallel with the inductance L between terminals A and B, wherein the average equivalent resistance  32  (Req) is given by Req =R/D, assuming that the value of resistance R is substantially greater than the on-resistance of switch  22 . As a result, the effective resistance between terminals A and B is varied to provide the desirable control. 
     Advantageously, because the control frequency of switch  22  is substantially greater than the converter output frequency, the intrinsic bandwidth of the converter is not compromised. In particular, the control switch can respond to a change in input several times during each switching cycle, whereas the response of the switching converter is limited by the switching frequency and the even slower response of the reactive elements that form part of most switching converters. Thus, the control device is faster than the switching converter; hence, the bandwidth of the total system is limited by the switching converter. In addition, because no synchronization is required, circuit complexity is reduced. Another advantage is that more of the control ASIC is implementable in digital form, while reducing the analog portion. As a result, the converter is more robust, costs less, and has fewer ASIC support components. Still further, since the value R is substantially greater than the on-resistance of switch  22 , most of the power dissipation occurs in R. The component R is preferably not on the ASIC, and the reduced dissipation in switch  22  enables integration of switch  22  on the ASIC. As yet another advantage, the effective resistance is substantially independent of active device parameters such that the effect is more consistent and predictable even with relatively large active device parameter variations. 
     An exemplary application for a variable reactance control in accordance with preferred embodiments of the present invention is in a dimmable compact fluorescent lamp (CFL) ballast. FIG. 4 schematically represents an exemplary CFL ballast  40  and lamp  42  system employing control circuit  20  (FIG.  2 ). in FIG. 4, block  44  represents a ballast and lamp system such as of a type described in U.S. Pat. No. 5,965,985, cited hereinabove. In the ballast, a converter comprises switches  120  and  122  that cooperate to provide ac current from a common node  124  to a resonant inductor  126 . A resonant load circuit  125  includes resonant inductor  126  and resonant capacitor(s)  128  for setting the frequency of resonant operation. The gates of switches  120  and  122  are connected at a control node  134 . Gate drive circuitry  136  is connected between the control node and the common node for implementing regenerative control of switches  120  and  122 . A gate drive inductor  127  is mutually coupled to resonant inductor  126  in order to induce in inductor  127  a voltage proportional to the instantaneous rate of change of current in load circuit  125 . A control inductance, comprising coupled windings  30  and  31 , has inductance L controlled by control circuit  20  (FIG.  2 ). In particular, winding  30  is connected in series with gate drive inductor  127  between the control node and the common node. A bidirectional voltage clamp  140  connected between nodes  124  and  134 , such as the illustrated back-to-back Zener diodes, cooperates with inductor  30  in such manner that the phase angle between the fundamental frequency component of voltage across resonant load circuit  125  and the ac current in resonant inductor  126  approaches zero during lamp ignition. A capacitor  146  may be connected in series with inductors  30  and  126 , as shown. The lamp current is regulated by sensing the lamp current using current sensing circuitry  147  and comparing to a reference signal  150  via error amplifier circuitry  149 . The output of the error amplifier is used to control the ballast in the manner described herein. In the exemplary dimmable ballast application, the reference signal  150  to the error amplifier  149  is provided, for example, via a dc power supply  152  and resistors  152  and  154  and may be adjusted in order to adjust the lamp current, which in turn adjusts the lumen output. 
     FIG. 5 graphically illustrates start-up and steady-state waveforms for the ballast of FIG.  4 : Waveform  50  represents the duty cycle D; waveform  52  represents the input to the control circuit at point  53  in the circuit of FIG. 4; waveform  54  represents the lamp power; and waveform  56  represents the lamp current. As illustrated, after an initial transient  55 , the control loop regulates the lamp current. Without the control loop, the ballast would be unstable, and the lamp arc would extinguish. 
     FIG. 6 graphically illustrates operation of the ballast of FIG. 4 when the in control loop begins to regulate the current. Waveform  60  represents the PWM signal to switch  22 . Waveform  62  represents the duty cycle D. Waveform  64  represents the control inductor (winding  30 ) voltage, and waveform  66  represents the control inductor (winding  30 ) current. The pulsed current in the control inductor occurs when switch  22  is on. While the peak current is high, the average current is such that the equivalent average resistance is the same as the resistance produced by the original circuit of FIG.  1 . The duty cycle changes as the control loop brings the lamp current into regulation. 
     While the preferred embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions will occur to those of skill in the art without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.