Ballast having a resonant feedback circuit for linear diode operation

A circuit, such as a ballast circuit, includes a resonant feedback signal from an inverter that provides substantially linear operation of rectifying diodes. In one embodiment, capacitors coupled in series with respective lamps resonate with a first winding of a feedback transformer so as to generate a feedback signal on a second winding of the transformer. The feedback signal is effective to produce substantially linear operation of the rectifying diodes.

CROSS REFERENCE TO RELATED APPLICATIONS 
Not Applicable. 
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
Not Applicable. 
BACKGROUND OF THE INVENTION 
The invention relates generally to rectifier circuits, and more 
particularly, to circuits having a feedback signal for producing 
substantially linear operation of rectifying diodes. 
There are many types of circuits for providing power to a load. Many such 
circuits include a rectifier circuit for receiving an alternating current 
(AC) signal and providing a direct current (DC) output signal. In one 
application, a ballast circuit for energizing a fluorescent lamp includes 
a rectifier circuit having an input coupled to a relatively low frequency 
AC power source and a DC output coupled to an inverter circuit. The 
inverter circuit applies a relatively high frequency AC signal to the lamp 
that is effective to cause a predetermined level of current to flow 
through the lamp and thereby produce visible light. 
Rectifier circuits generally contain one or more rectifying diodes coupled 
so as to form input (AC) and output (DC) terminals. In operation, each of 
the rectifying diodes is conductive for a part of the AC input signal. For 
example, a first rectifying diode may be conductive for a part of the 
positive portion of the AC input signal and a second rectifying diode may 
be conductive for a part of the negative portion. One problem associated 
with this arrangement is that the diodes which form the rectifier circuit 
do not operate in a linear manner. Typically, the rectifying diodes are 
only forward biased, i.e., conductive, when the AC input signal is at or 
near its peak value. The non-linear operation of the rectifying diodes has 
a negative impact on the efficiency of the circuit since only a limited 
amount of power from the AC power source is available to the circuit. 
Further, the total harmonic distortion (THD) and the power factor (PF) of 
the circuit are also adversely affected. 
It would, therefore, be desirable to provide a circuit including a 
rectifier having rectifying diodes that are operated in a substantially 
linear manner. 
SUMMARY OF THE INVENTION 
The present invention provides a circuit that generates a feedback signal 
for providing substantially linear operation of diodes that rectify an AC 
input signal. Although the invention is primarily shown and described with 
reference to ballast circuits, it is understood that many other 
applications are possible, such as electric motors and regulators. 
In one embodiment, a circuit, such as a ballast circuit, includes a full 
bridge rectifier for receiving a relatively low frequency input AC signal 
and providing positive and negative DC signals to an inverter. The 
inverter generates a relatively high frequency AC signal for energizing a 
plurality of loads, fluorescent lamps for example. Each load is connected 
in series with a respective one of a plurality of circuit elements, such 
as capacitors. A first winding of a feedback transformer is adapted for 
coupling to the lamps such that during operation of the circuit, the first 
winding of the feedback transformer resonates with the capacitors. A 
corresponding feedback signal is generated on a second winding of the 
feedback transformer, which is coupled to the rectifying diodes in the 
rectifier via feedback signal path. The feedback signal produces 
substantially linear operation of the rectifying diodes. 
In another embodiment, a circuit includes a voltage doubling type circuit 
for receiving an AC input signal and providing DC energy to an inverter 
that energizes one or more loads. The voltage doubling circuit includes 
first and second rectifying diodes coupled end-to-end across the DC output 
terminals of the circuit. The voltage doubling circuit energizes an 
inverter circuit for energizing a plurality of loads, such as lamps. Each 
of the lamps is coupled in series with a respective charge-storing circuit 
element, such as a capacitor. The lamp/capacitor series circuit paths are 
coupled to a first winding of a feedback transformer. A feedback signal 
path extending from a second winding of the feedback transformer is 
connected to a point between the rectifying diodes. A feedback signal 
generated on the feedback signal path by the feedback transformer second 
winding is effective to produce substantially linear operation of the 
rectifying diodes.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 shows a ballast 100 for providing substantially linear operation of 
diodes for rectifying an AC signal in accordance with the present 
invention. The ballast 100 includes a rectifier 102 for receiving an AC 
signal from an energy source 104 via first and second input terminal 
106a,b. The rectifier 102 provides DC signals to positive and negative 
voltage rails 108,110 of an inverter 112. The inverter 112 energizes a 
series of lamps L1-4, fluorescent bulbs for example, with an AC signal 
that is effective to regulate current through the lamps L1-4 such that 
they emit light. A relatively high frequency feedback signal that flows 
from the inverter 112 to the rectifier 102 via a feedback path 114 is 
effective to periodically bias the rectifying diodes to a conductive state 
such that they operate in a substantially linear manner. 
FIG. 2 shows an exemplary embodiment of the ballast 100 of FIG. 1. The 
rectifier 102 includes an EMI input stage 116 for filtering the incoming 
AC signal from the energy source 104. The input stage 116 includes first 
and second inductors L1,L2 that are inductively coupled with each other in 
a common mode arrangement, which is indicated by conventional dot 
notation, and first and second capacitors C1,C2, which are coupled 
end-to-end across the first and second inductors L1 ,L2. The rectifier 102 
further includes four diodes D1-4 coupled in a full bridge configuration. 
More particularly, first and second diodes D1,D2 are coupled end-to-end 
across the output terminals of the rectifier, i.e., the positive and 
negative voltage rails 108,110 of the inverter. The third and fourth 
diodes D3,D4 are also coupled end-to-end across the rectifier output 
terminals. The rectifier 102 receives the relatively low frequency, e.g., 
60 Hz, AC signal from the energy source 104 and provides DC signals on the 
positive and negative rails 108,110 so as to energize the inverter 112. 
The inverter 112 receives the DC signals from the rectifier and provides a 
relatively high frequency AC signal to the lamps L1-4 that cause them to 
emit light. During normal operation, the applied AC signal causes a 
predetermined amount of current to flow through the lamps L1-4 at a 
characteristic voltage drop. It is understood that the inverter 112 can 
have a variety of configurations and can energize many different types of 
lamps. For example, the inverter can have a half bridge configuration, a 
full bridge arrangement, or other topologies, such as that disclosed in 
co-pending and commonly assigned U.S. patent application Ser. No. 
09/215,070, filed on Dec. 18, 1998, which is incorporated herein by 
reference. In addition, the number and type of lamps driven by the 
inverter can vary. In an exemplary application, the inverter 112 energizes 
from one to four fluorescent lamps, such as industry standard four foot T8 
lamps. 
In the exemplary embodiment shown in FIG. 2, the inverter 112 includes 
first and second switching elements Q1,Q2 coupled in a halfbridge 
configuration. It is understood that the switching elements Q1,Q2 can be 
selected from a wide variety of components known to one of ordinary skill 
in the art, such as transistors, e.g., BJTs and FETs. The first and second 
switching elements Q1,Q2, which are shown as bipolar junction transistors, 
are coupled in series across the positive and negative voltage rails 
108,110 of the inverter. The conduction state of the first switching 
element Q1 is controlled by a first control circuit 118 and the conduction 
state of the second switching element Q2 is controlled by a second control 
circuit 120. Circuits for controlling the conduction state of the inverter 
switching elements are well known to one of ordinary skill in the art. 
First and second bridge capacitors CB1,CB2 are connected in series across 
the positive and negative rails 108,110 of the inverter. 
An optional DC-blocking capacitor CDC has one end coupled to a point 
between the first and second switching elements Q1,Q2 and the other end 
coupled to a resonant inductive element LR. A resonant capacitor CR and a 
primary winding 120 of an isolation transformer TI are coupled in parallel 
between the resonant inductive element LR and a point between the bridge 
capacitors CB1,CB2. As known to one of ordinary skill in the art, the 
first and second switching elements Q1,Q2 are alternately conductive for 
resonant operation of the inverter. More particularly, the first switching 
element Q1 is conductive for a first halfcycle as current flows from the 
resonant inductive element LR to the isolation transformer T1, and the 
second switching element Q2 is conductive for the second half cycle as 
current flows in the opposite direction. The impedances of the circuit 
components, such as the resonant inductor LR and the resonant capacitor 
CR, determine the characteristic resonant frequency of the inverter. 
Resonant operation of the inverter is effective to provide an AC signal on 
the first winding 120 of the isolation transformer TI that ultimately 
energizes the lamps L1-4. 
The resultant signal on the second winding 122 of the isolation transformer 
TI is effective to cause a predetermined current to flow through the lamps 
L1-4. As known to one of ordinary skill in the art, the isolation 
transformer TI isolates the resonant current from the lamp terminals so as 
to limit the lamp terminal current to a predetermined level, as required 
by certain safety requirements. In an exemplary embodiment, each of the 
four lamps L1-4 is connected in series with a respective lamp capacitor 
CL1-4 between the second winding 122 of the isolation transformer TI and a 
first winding 124 of a feedback transformer TF. A second winding 126 of 
the feedback transformer TF is coupled between the first winding 120 of 
the isolation transformer TI and a point 128 between the first and second 
capacitors C1,C2. 
In operation, resonant operation of the inverter 112 produces an AC signal 
of desired frequency on the first winding 120 of the isolation transformer 
TI, which induces a corresponding signal on the second winding 122 of the 
transformer TI. This signal energizes the lamps L1-4 such that a 
predetermined current flows through the lamps at a characteristic voltage 
drop. 
As current flows through the lamps L1-4, the lamp capacitors CL1-4 resonate 
in series with the first winding 124 of the feedback transformer TF. This 
resonance generates a corresponding signal on the second winding 126 of 
the feedback transformer TF that is fed back to the point 128 between the 
first and second capacitors C1,C2 via the feedback path 114. The feedback 
signal has a voltage that is sufficient to forward bias the rectifying 
diodes D1-4 so as to provide linear operation of the diodes D1-4. 
FIG. 3, in combination with FIG. 2, shows an exemplary relationship between 
the conduction state of the rectifying diodes D1-4, which corresponds to 
the relatively high frequency of the feedback signal, and the relatively 
low frequency input AC signal 150. The relatively high frequency of the AC 
signal generated by the inverter 112 is effective to cause the pairs 
diodes (D1,D2, and D3,D4) to become conductive many times during each half 
cycle of the AC input signal, e.g., 60 Hz, 110 volt signal. In an 
exemplary embodiment, the first and second diodes D1,D2 are periodically 
conductive during the negative halfcycle 152 of the AC input signal and 
the third and fourth diodes D3,D4 are periodically conductive during the 
positive half cycle 154. This results in substantially linear operation of 
the rectifying diodes D1-4. 
FIG. 4 shows a more detailed circuit diagram 160 of the circuit of FIG. 2, 
including a startup circuit 162, a lamp terminal wiring diagram 164, and 
transformer windings T4Q1,T4Q2 coupled to the switching elements Q1,Q2. 
The circuit 160 further includes exemplary component values, which can be 
readily modified by one of ordinary skill in the art. The startup circuit 
162 includes capacitor C2 and diac D10, which combine to initiate resonant 
circuit operation. More particularly, and as well known to one of ordinary 
skill in the art, the start up capacitor C2 initially charges until the 
voltage triggers the diac D10 to bias the second switching element Q2 to 
the conductive state and thereby start operation of the inverter. The lamp 
wiring diagram 164 includes industry convention to indicate respective 
yellow, blue, and red terminals for a four T8 lamp configuration, as 
shown. And the transformer windings T4Q1,T4Q2, which are inductively 
coupled to resonant inductor T4 and connected to respective switching 
elements Q1,Q2, are effective to isolate the load from the line and to 
drive the inverter. 
FIG. 5 shows an exemplary ballast 200 having a so-called voltage doubler 
arrangement providing substantially linear diode operation. The ballast 
200 includes a rectifier 202 having an input stage with first and second 
common mode inductors L1,L2, first and second differential mode inductors 
L3,L4, and a capacitor CIS. The rectifier 202 includes first and second 
rectifying diodes D1,D2 coupled across the DC output terminals 204a,b of 
the rectifier. First and second capacitors C1,C2 are also coupled 
end-to-end across the output terminals 204 of the rectifier. As known to 
one of ordinary skill in the art, the voltage doubler arrangement is 
effective to roughly double the voltage of the incoming AC signal, as 
compared to the DC signals provided to the inverter. 
The rectifier 202 provides DC energy to a resonant inverter 112 that 
generates an AC signal for causing the lamps L1-4 to emit light. The 
inverter 112 is substantially similar to the inverter shown in FIG. 2, 
wherein like reference numbers indicate like elements. The feedback signal 
path 114' extends from the second winding 126 of the feedback transformer 
TF to a point 206 between the first and second rectifying diodes D1,D2. 
In operation, the feedback signal generated on the feedback signal path 
114' is effective to periodically bias the first and second diodes D1,D2 
to a conductive state. In an exemplary embodiment, the first diode D1 is 
periodically conductive during the positive portion of the AC input signal 
and the second diode D2 is conductive during the negative portion of the 
signal. The relatively high frequency associated with the lamp current 
causes a respective one of the rectifying diodes D1,D2 to become 
conductive many times during each halfcycle of the relatively low 
frequency AC input signal. It is understood that the duty cycle of the 
diodes D1,D2 corresponds to the operating frequency of the inverter. 
The feedback transformer TF in combination with the feedback signal provide 
certain advantageous operating characteristics in addition to 
substantially linear diode operation. For example, the reactance of the 
lamp capacitors CL is substantially reduced due to the resonance with the 
feedback transformer. Thus, the load appears to be substantially resistive 
thereby increasing circuit efficiency. And by providing generally linear 
operation of the rectifying diodes D1-4, the efficiency of the circuit is 
further increased since more energy will flow directly from the energy 
source and less energy is transferred from the various circuit components. 
Further, in the case where the lamps are removed from the circuit, the 
feedback signal is substantially eliminated. This obviates apotential 
hazard associated with some known ballasts in that rail voltages can be 
driven to excessively high levels when lamps are removed from the ballast. 
One skilled in the art will appreciate further features and advantages of 
the invention based on the above-described embodiments. Accordingly, the 
invention is not to be limited by what has been particularly shown and 
described, except as indicated by the appended claims. All publications 
and references cited herein are expressly incorporated herein by reference 
in their entirety.