Low-cost power factor correction circuit and method for electronic ballasts

• US 5,568,041 A
• Filed: 02/09/1995
• Issued: 10/22/1996
• Est. Priority Date: 02/09/1995
• Status: Expired due to Term

• Current Assignee: Universal Lighting Technologies Incorporated (Panasonic Holdings Corporation)
• Original Assignee: Magnetek Incorporated (Columbus Mckinnon Corporation)
• Inventors: Hesterman, Bryce L.
• Primary Examiner(s): Sterrett, Jeffrey L.
• Application Number:
  US08/385,906
• Time in Patent Office: 621 Days
• Field of Search: 323/205, 323/207, 323/222, 363/34, 363/37, 363/84, 363/89
• US Class Current: 323/207
• CPC Class Codes: H02M 1/0006 Arrangements for supplying an adequate voltage to the control circuit of converters
• CPC Class Codes: H02M 1/4225 using a non-isolated boost converter
• CPC Class Codes: H02M 1/425 using a single converter stage both for correction of AC input power factor and generation of a high frequency AC output voltage
• CPC Class Codes: H05B 41/28 using static converters
• CPC Class Codes: Y02B 70/10 Technologies improving the efficiency by using switched-mode power supplies [SMPS], i.e. efficient power electronics conversion e.g. power factor correction or reduction of losses in power supplies or efficient standby modes

Abstract

The present invention provides a low-cost power factor corrected electronic ballast circuit. A preferred embodiment includes a discontinuous conduction mode boost power factor correction circuit that is controlled with a simple pulse-width modulator (PWM) circuit comprising a few discrete components instead of an integrated circuit. The PWM circuit utilizes a reference waveform signal derived from the ballast inverter. The reference waveform is combined with a feedback signal to create a composite signal that is compared with a reference voltage to create a pulse-width-modulated signal. The feedback signal is used to regulate the bulk DC voltage. In an alternative embodiment, a feedforward signal that is proportional to the time-varying level of the rectified line voltage is added to the reference waveform and the feedback signal, and serves to modulate the pulse width of the boost circuit in a manner that reduces the harmonic distortion of the input current. In addition to the boost circuit, the power factor correction circuit can be realized with flyback and buck-boost topologies. The invention can also utilize several different types of inverter circuits.

Claims

1. A power factor corrected circuit comprising:

rectifier means for rectifying voltage from an AC source;

power converter means actuated by a control pulse signal for converting the rectified voltage into current pulses, the magnitude of the current pulses being dependent upon the duty cycle of the pulse signal;

capacitive bulk energy storage means for storing the output of the power converter means as a bulk DC voltage;

feedback signal means for developing a feedback signal related to the level of the bulk DC voltage;

oscillating inverter means for converting the bulk DC voltage into an AC signal for driving a load at a frequency that is substantially greater than the frequency of the AC source;

means for providing a reference waveform signal that is derived from the oscillations of the inverter;

pulse-width modulation means for providing as an output the control pulse signal, the pulse width modulation means including comparator means for comparing a combination of the reference waveform signal and the feedback signal with a threshold voltage, the pulse width modulation means further including an output terminal that is actuated when the combination of the reference waveform signal and the feedback signal exceeds the threshold voltage, such that a change in the level of the feedback signal causes an adjustment of the duty cycle of the control pulse signal so as to regulate the level of the bulk DC voltage.

2. A circuit according to claim 1, wherein the means for providing the reference waveform signal is inductively coupled to the inverter means.

3. A circuit according to claim 1, further including means for rectifying the reference voltage waveform signal to provide a power supply voltage for the pulse-width modulator means.

4. A circuit according to claim 3, wherein the feedback signal means includes a resistor and a reverse-biased zener diode connected in series between the power supply voltage for the pulse width modulation means and the input terminal of the comparator means.

5. A circuit according to claim 1, wherein the comparator means is a transistor.

6. A circuit according to claim 1, wherein the power converter is a boost converter.

7. A circuit according to claim 6, wherein the level of the rectified voltage from the AC source varies with time, and wherein the circuit further includes:

feedforward signal means for providing a signal proportional to the time-varying level of the rectified voltage form the AC source, the pulse width modulation means further including means for receiving the feedforward signal and combining it with the reference waveform signal and the feedback signal, thereby modulating the duty cycle of the pulse signal output of the pulse width modulation means so as to reduce harmonic currents drawn from the AC source.

8. A method for controlling the power factor of a ballast circuit, comprising the following steps:

(a) rectifying an AC supply voltage;

(b) converting the rectified AC supply voltage to current pulses using a switching power converter that is controlled by a pulse signal of variable duty cycle, the magnitude of the current pulses being dependent upon the duty cycle of the pulse signal;

(c) storing the output energy of the power converter as a bulk DC voltage across a capacitive energy storage means;

(d) converting the bulk DC voltage into an AC signal with an oscillating inverter that drives a load at a frequency substantially greater than the frequency of the AC supply voltage;

(e) deriving a reference waveform signal from the oscillations of the inverter;

(f) deriving a feedback signal that is related to the level of the bulk DC voltage;

(g) creating a composite control signal by combining the reference waveform signal with the feedback signal;

(h) generating the pulse signal used to control the power converter by comparing the composite control signal with a fixed reference voltage.

9. A method according to claim 8, wherein the amplitude of the rectified AC supply voltage varies with time and wherein the power converter is a boost converter, the method further including the following steps:

deriving a feedforward signal that is proportional to the time-varying amplitude of the rectified AC supply voltage;

combining the feedforward signal with the reference waveform signal and the feedback signal to create the composite control signal, so as to modulate the duty cycle of the pulse signal to reduce the harmonic currents drawn from the AC source.