U.S. patent application number 09/730619 was filed with the patent office on 2002-06-06 for push-pull based voltage-clamping electronic ballast.
Invention is credited to Qian, Jinrong, Weng, DaFeng.
Application Number | 20020067140 09/730619 |
Document ID | / |
Family ID | 24936069 |
Filed Date | 2002-06-06 |
United States Patent
Application |
20020067140 |
Kind Code |
A1 |
Qian, Jinrong ; et
al. |
June 6, 2002 |
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.
Inventors: |
Qian, Jinrong;
(Croton-On-Hudson, NY) ; Weng, DaFeng; (Yorktown
Heights, NY) |
Correspondence
Address: |
Corporate Patent Counsel
U.S. Philips Corporation
580 White Plains Road
Tarrytown
NY
10591
US
|
Family ID: |
24936069 |
Appl. No.: |
09/730619 |
Filed: |
December 6, 2000 |
Current U.S.
Class: |
315/224 |
Current CPC
Class: |
Y02B 20/183 20130101;
H05B 41/282 20130101; Y02B 20/00 20130101 |
Class at
Publication: |
315/224 |
International
Class: |
H05B 037/02 |
Claims
What is claimed is:
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
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
V.sub.in, switches S.sub.1 and S.sub.2, and transformer T. The
switches S.sub.1 and S.sub.2 are each a standard metal-oxide
semiconductor, field-effect transistor (MOSFET). D.sub.s1 and
C.sub.s1 represent body diode and internal capacitance,
respectively, of switch S.sub.1, and D.sub.s2 and C.sub.s2
represent body diode and internal capacitance, respectively, of
switch S.sub.2. 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 L.sub.r and capacitor
C.sub.r 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 i.sub.s1, i.sub.s2 and i.sub.Lr.
Voltage V.sub.0 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 t.sub.0 when S.sub.1 is switched off a spike is shown in FIG. 2
to occur in the voltage across S.sub.1, i.e. V.sub.ds1. The
principle reason for the voltage spike is that the opening of
switch S.sub.1 creates an abrupt interruption (discontinuity) in
the current through N.sub.1. There is an release of stored energy
due to a leakage inductance associated with winding N.sub.1
(represented as L.sub.k1 in FIG. 1), thereby inducing a current
that charges C.sub.S1 and consequently creating a high voltage
spike across switch S.sub.1.
[0006] The same voltage spike characteristic occurs in the voltage
across S.sub.2, i.e. V.sub.ds2, at time t.sub.2 when S.sub.2 is
switched off The voltage spike is caused by transient or leakage
inductance L.sub.k2 associated with transformer winding N.sub.2.
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 S.sub.1 is on; and
[0020] FIG. 6 is a circuit diagram representing the circuit of FIG.
3 when switch S.sub.2 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 V.sub.in supplies DC power to
the circuit. L.sub.k1 and L.sub.k2 represent the leakage
inductances of windings N.sub.1, N.sub.2 of transformer T. D.sub.s1
represents body diode of switch S.sub.1. D.sub.s2 represents body
diode of switch S.sub.2. C.sub.S 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 N.sub.1 and N.sub.2 and secondary winding N.sub.3. In the
preferred embodiment, the number of windings of N.sub.1 is equal to
the number of windings of N.sub.2. The normal current paths through
switches S.sub.1 and S.sub.2 and windings N.sub.1 and N.sub.2 are
represented by i.sub.s1, i.sub.s2, i.sub.n1 and i.sub.n2x,
respectively, having the sign conventions as shown. The voltages
across switches S.sub.1 and S.sub.2 and capacitor C.sub.s are
represented by V.sub.s1, V.sub.s2 and V.sub.C, respectively, having
the sign conventions as shown. Inductor L.sub.r and capacitor
C.sub.r form a resonant tank and resonant tank current i.sub.Lr
induced in N.sub.3, provides a high frequency voltage V.sub.0
across the load R.sub.0, 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
S.sub.1 and S.sub.2, respectively. Just prior to t.sub.0, switch
S.sub.2 is conducting and carries its maximum current (see waveform
vi) at t.sub.0, which is equal to i.sub.n2-i.sub.n1. Since switch
S.sub.2 has been on for the duration of a switching cycle at
t.sub.0, the current i.sub.n2 through N.sub.2 due to V.sub.in has
increased to a maximum (see waveform viii), while capacitor C.sub.S
is discharging a voltage (described further below), thus creating a
negative current i.sub.n1(see waveform vii). By the sign convention
of i.sub.n1 shown in FIG. 3, both of these currents add to
i.sub.s2.
[0024] During the time period t.sub.0 to t.sub.2, switch S.sub.1 is
conducting or on and switch S.sub.2 is off During t.sub.2 to
t.sub.4 switch S.sub.2 is on and switch S.sub.1 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 t.sub.0 shows that i.sub.s1 has a
negative value even though waveform i shows that switch S.sub.1
remains off for a brief period after t.sub.0. The negative current
flow in this interval is through D.sub.s1.
[0025] FIG. 5 is the equivalent circuit of FIG. 3 during time
period t.sub.0 to t.sub.2, when S.sub.1 is on and S.sub.2 is off.
(For ease of description, the leakage inductances L.sub.k1,
L.sub.k2 associated with windings N.sub.1 and N.sub.2 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 V.sub.in is applied to winding N.sub.1. Since
winding N.sub.1 is coupled to winding N.sub.2 the increasing
current i.sub.n1 created by V.sub.in induces a voltage and a
resulting current i.sub.n2 in N.sub.2. Since the number of windings
of N.sub.1 and N.sub.2 are equal (have a turns ratio of 1:1), the
induced voltage across N.sub.2 is equal to
(N.sub.2/N.sub.1)*V.sub.in, and thus is equal to V.sub.in. By the
dot convention of N.sub.1 and N.sub.2, the voltage drop is across
N.sub.2 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 V.sub.in, the voltage at point Z must be 2V.sub.in. Because
the voltage at point Y also has voltage level V.sub.in, the voltage
drop from Z to Y is equal to V.sub.in. Thus, capacitor voltage
V.sub.cs=V.sub.in.
[0026] Because of the coupling between N.sub.2 and N.sub.1, current
i.sub.n2 is equivalent to -i.sub.n1, with a slight DC bias voltage
offset due to energy absorbed from V.sub.in as shown in waveforms
vii and viii of FIG. 4. The current i.sub.cs, of capacitor C.sub.S
must equal the current i.sub.n2 flowing through N.sub.2, as shown
in waveforms viii and ix. Thus, from time t.sub.0 to t.sub.1 the
current i.sub.cs acts to charge capacitor C.sub.s, whereas from
time t.sub.1 to t.sub.2 the current i.sub.cs acts to discharge
C.sub.s.
[0027] As noted, the voltage at point Z of the circuit of FIG. 5 is
equal to 2V.sub.in. This is also the voltage drop across the open
switch S.sub.2 in the time interval t.sub.0 to t.sub.2, as shown in
waveform iv of FIG. 5. Focusing again on t.sub.0 of FIG. 4, it is
seen from waveforms ii and iv that switch S.sub.2 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 S.sub.2
rises directly to a value of 2V.sub.in and is maintained at that
level for the duration t.sub.0 to t.sub.2. When S.sub.2 is turned
off at t.sub.0, the current i.sub.n2 flowing in N.sub.2 is
immediately shunted to the circuit loop that includes capacitor
C.sub.s, thus immediately charging C.sub.s with the current
i.sub.n2. Thus, any current contributed to i.sub.n2 by a release of
magnetic energy stored in leakage inductor L.sub.k2 associated with
N.sub.2 at t.sub.0 acts to charge C.sub.s from t.sub.0 to t.sub.1.
Thus, there is no mechanism in the circuit that creates a voltage
spike across S.sub.2.
[0028] It is again observed that the discharge of capacitor C.sub.s
from t.sub.1 to t.sub.2 creates a negative current in .sub.cs
(which is equivalent to i.sub.n2) as shown in FIG. 5 by the sign
convention. The current thus flows counter-clockwise around the
loop including C.sub.s, N.sub.2 and S.sub.1, thus contributing
(along with the current i.sub.n1 through N.sub.1 created by
V.sub.in) to the relatively large positive current i.sub.s1 through
switch S.sub.1 from t.sub.1 to t.sub.2.
[0029] By symmetry, analogous description applies during the
portion of the switching cycle when S.sub.1 is off and S.sub.2 is
on during t.sub.2 to t.sub.4, as shown in FIG. 6. In short, supply
voltage V.sub.in is applied across N.sub.2 (including associated
leakage inductor L.sub.k2). Thus, current i.sub.n2, which now flows
through closed switch S.sub.2, begins to build from a negative
value at t.sub.2 to a positive value at t.sub.4. By the coupling of
N.sub.2 and N.sub.1, voltage V.sub.in is induced across N.sub.1 in
the upward direction (by the dot convention). This implies that the
voltage at point Z' is -V.sub.in, V.sub.C=V.sub.in and
V.sub.s1=V.sub.in-(-V.sub.in)=2V.sub.in, as shown in waveform iii
of FIG. 4.
[0030] Input voltage V.sub.in applied to N.sub.2 acts to reverse
current i.sub.n2, as shown in waveform viii of FIG. 4. A closed
circuit is also formed by the loop including C.sub.S, N.sub.1 and
S.sub.2; thus i.sub.n1 and i.sub.cs are equal from t.sub.2 to
t.sub.4, as shown in waveforms vii and ix of FIG. 4. Currents
i.sub.n1 and i.sub.cs are positive from t.sub.2 to t.sub.3, thus
charging capacitor C.sub.S as the current in N.sub.1 is reduced. As
described above, this shunting of the current to C.sub.s avoids a
voltage spike across S.sub.1 at t.sub.2. (See waveform iii at
t.sub.2). The combination of current i.sub.n1 (or equivalently,
i.sub.cs) and current i.sub.n2 through switch S.sub.2 from t.sub.2
to t.sub.4 results in a relatively large current through S.sub.2 at
the end of the cycle t.sub.4. (See waveforms vii, ix and vi at
t.sub.2 to t.sub.4.)
[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 i.sub.n1 and i.sub.n2 through N.sub.1
and N.sub.2 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 L.sub.r and
resonant capacitor C.sub.r. 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 N.sub.1 and N.sub.2 is always continuous. The leakage
energy is thus recycled.
[0033] Given again that the number of windings of N.sub.1 equals
those of N.sub.2, the maximum voltage across switches S.sub.1 and
S.sub.2 is equal to twice the input voltage V.sub.in. Compared with
the conventional push-pull resonant inverter, in the present
invention the windings N.sub.1 and N.sub.2 carry continuous current
and work together at all times to produce the resonant tank current
i.sub.Lr, and the voltages across the switches S.sub.1 and S.sub.2
are always equal to twice the input voltage V.sub.in. 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.
* * * * *