U.S. patent application number 10/437622 was filed with the patent office on 2004-11-18 for soft-switching techniques for power inverter legs.
Invention is credited to Cao, Xiao Hong, Chung, Shu-Hung Henry, Hui, Shu-Yuen Ron.
Application Number | 20040228153 10/437622 |
Document ID | / |
Family ID | 33417419 |
Filed Date | 2004-11-18 |
United States Patent
Application |
20040228153 |
Kind Code |
A1 |
Cao, Xiao Hong ; et
al. |
November 18, 2004 |
Soft-switching techniques for power inverter legs
Abstract
This invention relates to new soft-switching techniques for
minimizing switching losses and stress in power electronic circuits
using inverter legs. By choosing the switching frequency with
specific relationships with the resonant frequency of the power
electronic circuits, the proposed switching technique enables the
power electronic circuits to achieve soft switching under full load
and short-circuit conditions at the defined frequencies for both
capacitive and inductive loads. This technique can be applied to an
electronic circuit with two switches connected in totem pole
configuration between two dc voltage rails or commonly known as a
power inverter leg or inverter arm. Examples of these circuits are
class-D power converter, half-bridge power converters and
full-bridge power converters or inverters. The proposed techniques
allow inverter circuits with resistive, capacitive and inductive
loads to achieve soft switching.
Inventors: |
Cao, Xiao Hong; (Hangzhou,
CN) ; Hui, Shu-Yuen Ron; (Hong Kong, HK) ;
Chung, Shu-Hung Henry; (Hong Kong, HK) |
Correspondence
Address: |
BARNES & THORNBURG
11 SOUTH MERIDIAN
INDIANAPOLIS
IN
46204
|
Family ID: |
33417419 |
Appl. No.: |
10/437622 |
Filed: |
May 14, 2003 |
Current U.S.
Class: |
363/71 |
Current CPC
Class: |
H02M 3/3376 20130101;
Y02B 70/10 20130101; H05B 41/2856 20130101; H02M 1/0058
20210501 |
Class at
Publication: |
363/071 |
International
Class: |
H02M 001/00 |
Claims
1. A method of operating a power electronics circuit comprising an
inverter and a load including a resonant tank, wherein said
inverter is switched at a frequency f.sub.s and said resonant tank
has a resonant frequency f.sub.r, wherein
K<f.sub.r/f.sub.s<K+1 where K is an even-numbered
integer.
2. A method of operating a power electronics circuit comprising an
inverter and a load, wherein said load as first and second
operating conditions associated with respective first (f.sub.r1)
and second (f.sub.r2) resonant frequencies, f.sub.r2 being greater
than f.sub.r1, and wherein said inverter is switched at first
(f.sub.s1) and second (f.sub.s2) switching frequencies
corresponding to said first and second operating conditions,
f.sub.s2 being greater than f.sub.s1, and wherein
K<f.sub.r2/f.sub.s1<K+1 where K is an even-numbered
integer.
3. A method as claimed in claim 2 wherein an auxiliary resonant
tank is provided between said inverter and said load and having a
resonant frequency (f.sub.a), and wherein K-f.sub.sf.sub.s1<K+1
where K is an even-numbered integer.
4. A method as claimed in claim 3 wherein f.sub.s2>f.sub.a.
5. A method as claimed in claim 2 wherein f.sub.s2>f.sub.r2.
6. A method as claimed in claim 2 wherein said power electronics
circuit is an electronic ballast for driving a high intensity
discharge lamp.
7. A method of operating a power electronics circuit comprising an
inverter and a load, wherein said load as first and second
operating conditions associated with respective first (f.sub.r1)
and second (f.sub.r2) resonant frequencies, f.sub.r2 being greater
than f.sub.r1, and wherein said inverter is switched at first
(f.sub.s1) and second (f.sub.s2) switching frequencies
corresponding to said first and second operating conditions,
f.sub.s2 being greater than f.sub.s1, wherein an auxiliary resonant
tank is provided between said inverter and said load and having a
resonant frequency (f.sub.a), and wherein
K<f.sub.a/f.sub.s1<K+1 where K is an even-numbered integer,
and wherein K<f.sub.r2/f.sub.s1<K+1, and f.sub.s2>f.sub.r2
and f.sub.s2>f.sub.a.
8. A method as claimed in claim 7 wherein f.sub.a is close to
f.sub.r2.
9. A method of operating a power electronics circuit comprising an
inverter and a load, wherein said load as first and second
operating conditions associated with respective first (f.sub.r1)
and second (f.sub.r2) resonant frequencies, f.sub.r2 being greater
than f.sub.r1, and wherein said inverter is switched at first
(f.sub.s1) and second (f.sub.s2) switching frequencies
corresponding to said first and second operating conditions,
f.sub.s2 being greater than f.sub.s1, wherein an auxiliary resonant
tank is provided between said inverter and said load and having a
resonant frequency (f.sub.a), wherein K<f.sub.r2/f.sub.s1<K+1
and f.sub.s1>f.sub.a.
10. A method of operating a power electronics circuit comprising an
inverter and a load including a resonant tank, wherein an auxiliary
resonant tank is provided between said inverter and said load
whereby in the event of the load acting as a short-circuit during
operation, current provided by said auxiliary resonant tank enables
soft-switching of said inverter.
11. A power electronics circuit comprising an inverter, a load
including a resonant tank, and an auxiliary resonant load provided
between said inverter and said load.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to methods for the
soft-switching of power inverter legs, for example, though by no
means exclusively, in electronic ballasts for high energy discharge
lamps.
BACKGROUND OF THE INVENTION AND PRIOR ART
[0002] Many power electronic circuits consist of inverter legs or
arms. An inverter leg is shown in FIG. 1. Each inverter leg
consists of two power switches (S1 and S2) connected across a dc
voltage rail. This switch arrangement across a do voltage rail is
also known as a totem-pole configuration. Each switch has an
anti-parallel diode (D1 or D2), which can be part of the switch
structure in a power mosfet or an externally connected diode. In
addition, there is also capacitance across the switch and diode.
This capacitance (C) arises from the inherent capacitance of the
switch and diode. In some applications, however, an additional
capacitor may be connected across the switch to increase the
capacitance if more capacitance is needed for achieving soft
switching.
[0003] Usually, the node between S1 and S2 is connected to the load
circuit (FIG. 1). The two switches are turned on and off in a
complimentary manner with a dead time in between. This means that
only one switch is turned on at any time. Between the change of
switching states, both switches are not turned on for a short
period of time that is known as the dead time. Usually, this dead
time is a small portion of the switching period Two inverter legs
can be used to form a single-phase full-bridge inverter (FIG. 2).
The inverter leg can be used to form a half-bridge inverter as
shown in FIG. 3(a) and FIG. 3(b). Capacitor Cb is simply a dc
voltage blocking capacitor (FIG. 3(b)). The function of the
inverter circuits are to generate an ac voltage from the do voltage
supply and apply this ac voltage across the load which may be an
energy-consuming component (ie a resistive load) or an
energy-storing component such as a capacitor and inductor forming a
resonant tank.
[0004] Examples of typical loads are shown in FIG. 4 and FIG. 5. In
FIG. 4, the overall load consists of a dc voltage blocking
capacitor, a resonant inductor, a resonant capacitor and an
equivalent resistive load. This is a commonly used circuit for an
electronic ballast for a lamp and the resistive load represents the
energy consuming lamp. The equivalent resistive load can also be a
transformer coupled circuit with the energy-consuming load
connected on the secondary transformer circuit via a rectifier
(such as the system for a switched mode power supply). FIG. 5 shows
multi-resonant circuit with an energy consuming load. This
multi-resonant circuit is a alternative electronic ballast circuit
for a high-intensity-discharge (HID) lamp, in which Lr2 and Cr form
a relatively low-frequency (e.g. 50 kHz) resonant tank to create a
high voltage to ignite the HID lamp and Lr1 and Cr form a
relatively high-frequency (e.g. 400 kHz) resonant tank for
operating the lamp under steady-state conditions, Here
Lr2>Lr1.
[0005] To understand the problems faced by existing technology,
existing soft-switching techniques for power electronic circuits
with inverter leg or legs will be described, using the half-bridge
circuit in FIG. 4 as an example. The directions of the load current
I.sub.input and load voltage V.sub.input as indicated in FIG. 4 are
assigned as positive for the following description.
[0006] In the example of FIG. 4, the de blocking capacitor Cb
eliminates the dc component of the ac voltage generated by the
inverter leg. The resonant tank consists of Lr and Cr and the
dominant resonant frequency is f.sub.r=1/2.pi.{square root}{square
root over (L.sub.rC.sub.r)}. If the switching frequency (fs) of the
AC rectangular voltage generated by the inverter circuit is higher
than fr, the overall load including the resonant tank and resistive
load is more inductive than capacitive. In this case (fs>fr),
the overall load is considered as inductive and the current Iz is
lagging behind the applied ac voltage Vz as shown in FIG. 6(a). On
the other hand, if fs<fr, the overall load is capacitive and the
current I.sub.input is leading the applied voltage V.sub.input as
shown in FIG. 6(b).
[0007] FIG. 7 shows three typical switching trajectories of a power
switch. The y-axis is the current through the switch and the x-axis
is the voltage across the switch. During the transition periods of
the turn-on or turn-off processes, a power switch will withstand
high transitional voltage (across the switch) and current (through
the switch). This is called hard switching. Hard switching not only
leads to switching loss and stress, but more importantly causes
switching transients or spikes that are major source of
electromagnetic interference (EMI). Such EMI problems may induce
noise in the gating signals of the power switches, causing
reliability problems. For example, if noise is induced in the gate
of a nominally-off power switch and triggers the switch to turn on,
the inverter leg may have a shoot-through or short-circuit
situation. As one solution to this problem it is known to connect a
snubber circuit consisting of resistor and capacitor to reduce the
high di/dt and dv/dt of the switch so as to reduce the switching
loss and stress. However, traditional snubber circuits are lossy
because part of the switching loss is transferred from the switch
to the snubber resistor. In order to achieve soft switching, it is
necessary to create a zero voltage and/or zero current condition
for the switch to turn on or off. If either the switch voltage or
switch current is zero, the instantaneous product of switch voltage
and current is zero. Thus, the switching loss becomes zero. In
practice, it may not be possible to achieve absolute zero switch
voltage and/or current. Instead, the switch voltage and/or current
can be clamped to near-zero value. Such near-zero voltage and/or
current zero-voltage and/or current switching may still be
considered to be zero voltage or zero current. The general term for
zero-voltage or zero-current switching is soft switching.
[0008] The following conditions have to be met in order to achieve
soft switching in circuits including an inverter leg.
[0009] (A) For Zero-Voltage `Turn Off` of Power Electronic Switches
S1 and S2
[0010] Condition (1)--Parallel capacitance is needed across the
power switches S1 and S2 in order to limit the dv/dt of the switch
so as to achieve zero-voltage turn off.
[0011] Parallel capacitance across the switch can come from the
power switches' device capacitance such as the drain-source
capacitance of the power mosfet. External capacitor can be added
across the switch if necessary. This is a well known technique for
zero-voltage turn off of power electronic devices.
[0012] (B) For zero-voltage `turn-on` of power electronic switches
S1 and S2
[0013] Condition (1*)--The tank current I.sub.input should be in
the correct direction as follows:
[0014] For the inverter circuit example (FIG. 4), soft switching
can be achieved if the overall load (including the resonant tank
and resistive load) is inductive. The normal understanding in the
prior art is that the frequency (fs) of the inverter's ac voltage
V.sub.input must be higher than the dominant resonant frequency
(fr) of the overall load so that the overall load is inductive. The
actual soft-switching condition is that the current I.sub.input is
positive just before S2 (bottom switch) is turned on and negative
just before S1 (top switch) is turned on (1*) (FIG. 6(a)). This is
a necessary condition for zero-voltage switching
[0015] When S1 is turned off, it is soft-switched off because the
parallel capacitor across S1 limits dv/dt of the switch voltage.
The initial voltage across S1 is near zero during the turn-off
process of S1. Therefore, S1 is zero-voltage (soft) turned off. The
next important process is to ensure that S2 is soft-switched on. If
fs>fr, the overall load is inductive. The existing method is to
add a small dead time between the turn-off of S1 and turn-on of S2.
During this dead time, both gating signals for S1 and S2 are off.
However, this does not mean that the current I.sub.input is not
continuous. When S1 is turned off, the capacitor voltage across S1
will rise to the dc rail voltage whilst the capacitor voltage
across S2 will discharge to zero. Because the load is inductive,
I.sub.input must be continuous. So the anti-parallel diode across
S2 will be turned on so as to allow I.sub.input to flow
continuously during this dead time. This means that the voltage
across S2 will be clamped by its parallel diode's on-state voltage
which is typically 0.7V (this is a near-zero-voltage when compared
with the dc rail voltage of tens or hundreds of volts). Therefore,
a soft-switching condition is created for S2 to be turned on at
zero voltage condition.
[0016] Similar arguments apply to the soft-turn-off process of S2
and soft-turn-on process of S1. At the and of the on-time of S2,
I.sub.input is negative. S2 can be soft turned off because of its
parallel capacitor which limits the dv/dt of the voltage across S2.
So S2 can be zero voltage (soft) turned off S1 is not turned on
immediately after S2 is turned off because of the dead time. The
inductive load current I.sub.input has to flow into the
anti-parallel diode of S1 during this dead time, thus clamping the
voltage across S1 to zero, So S1 can be turned on under zero
voltage condition.
[0017] The main problem of the above soft-switching method for the
inverter circuit is that fs must be greater than fr so that the
overall load is inductive. If fs<fr, the overall load becomes
capacitive and the soft-switching condition that "the current
I.sub.input is positive just before S2 is turned on and negative
just before S1 is turned on" (1*) cannot be met (FIG. 6(b)). If
I.sub.input is negative just before S2 is turned on, the
anti-parallel diode of S2 is not conducting. Thus, the voltage
across S2 is not clamped to zero for S2 to turn on and
soft-switching condition is lost.
[0018] Condition (2*). Tank current I.sub.input must exceed a
minimum magnitude in order to fully discharge total equivalent
capacitance across the power switch for zero-voltage
switching--Equation (3).
[0019] It is necessary to find the current threshold for soft
switching in the operating frequency region. When tie current is
above the current threshold, soft switching can be achieved. The
current i.sub.input should be large enough to remove the charge on
(discharge) the total equivalent capacitance across the power
switch (such as the drain and source of the power mosfet). The
requirement can be expressed by below equation: 1 Q s = - t d / 2 t
d / 2 i input ( t ) t 2 C s V g ( 1 )
[0020] where Qs is the charge and Cs is the total equivalent
capacitance across the power switch (e.g. drain and source of the
power switch), Vg is the de inverter voltage and t.sub.d is the
dead time between the gating signals of S1 and S2.
[0021] If a resonant tank is used in the load circuit, the input
circuit can be approximated as a sinusoidal current because of the
filtering effect of the resonant tank.
i.sub.input(t)=I.sub.input sin(.omega..sub.st-.phi.) (2)
[0022] where I.sub.input is the peak magnitude of i.sub.input.sub.s
.omega..sub.s=2 .pi.fs is the angular frequency of the inverter, t
is the time variable and .PHI. is the phase angle between the
voltage generated by the inverter leg (V.sub.input) across the load
circuit.
[0023] Based on (1) and (2), the input current must obey the
following equation in order to create a zero-voltage condition for
the power switch to achieve soft switching: 2 I input C s V g s sin
( s t d 2 ) ( 3 )
[0024] Therefore, equation (3) must be met as a necessary condition
for soft switching. This equation provides a guideline to choose
the appropriate t.sub.d, Cr and fs.
SUMMARY OF THE INVENTION
[0025] The present invention provides new soft-switching techniques
for inverter bridges. According to the present invention there is
provided a method of operating a power electronics circuit
comprising an inverter and a load including a resonant tank,
wherein said inverter is switched at a frequency f.sub.s and said
resonant tank has a resonant frequency f.sub.r, wherein
K<f.sub.r/f.sub.s<K+1 where K is an even-numbered
integer.
[0026] In particular, a first preferred method enables soft
switching to be achieved in the inverter bridge with overall
capacitive load or for inverter operating at a frequency below the
dominant resonant frequency of the resonant tank(s). This may be
considered a "pseudo inductive soft-switching" method. Within the
nominal "capacitive" operating range (fs<fr), certain frequency
regions may be defined that can be considered to be
pseudo-inductive regions. Within the pseudo inductive regions, soft
switching can be achieved even though the frequency range is within
the capacitive region. A second preferred method includes the use
of an additional and unloaded resonant tank that provides a current
path to ensure soft-switching irrespective of the load condition.
This additional resonant tank lowers the minimum inverter frequency
at which soft switching can be achieved. Even if the inverter
operates in the nominally capacitive region of the original
resonant tank, the inductive effect of the additional resonant tank
makes soft switching possible at a lower inverter frequency.
[0027] According to conventional resonant circuit theory, a series
resonant tank works in the "capacitive" region when the inverter
operating frequency f.sub.s is below its resonant frequency
f.sub.r, namely 1/2 .pi.{square root}{square root over
(L.sub.rC.sub.r)} of the resonant tank. However, in embodiments of
this invention that, in the nomina capacitive region (fs<fr),
when the frequency ratio N (=f.sub.r/f.sub.s) is larger than an
even number and smaller than the nearest odd number, soft switching
can still be achieved as if the operation is in inductive region as
described in the background section, Because the soft switching
conditions required can be met even though fs<fr, we call these
soft-switching regions within the capacitive region `pseudo
inductive` regions. The corresponding soft-switching technique
proposed in this invention is called `pseudo inductive`
soft-switching technique,
[0028] The invention also provides a method of ensuring that there
is a threshold current for enabling soft-switching in the event,
for example, of the load acting as a short-circuit using an
auxiliary resonant load. In particular the invention also extend to
a method of operating a power electronics circuit comprising an
inverter and a load including a resonant tank, wherein an auxiliary
resonant tank is provided between said inverter and said load
whereby in the event of the load acting as a short-circuit during
operation, current provided by said auxiliary resonant tank enables
soft-switching of said inverter.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] Some embodiments of the invention will now be described by
way of example and with reference to the accompanying figures in
which:
[0030] FIG. 1 illustrates a typical inverter leg,
[0031] FIG. 2 illustrates a single-phase full-bridge inverter,
[0032] FIGS. 3 (a) and (b) show alternative forms of single-phase
half-bridge inverters,
[0033] FIGS. 4 and 5 illustrate half-bridge inverters different
loads,
[0034] FIGS. 6 (a) and (b) show respectively typical voltage and
current waveforms for an inverter bridge in (a) the inductive
region and (b) the capacitive region,
[0035] FIG. 7 illustrates typical switching trajectories of a power
switch,
[0036] FIG. 8 shows a half-bridge inverter loaded by a series
resonant tank,
[0037] FIGS. 9 (a) and (b) show (a) classification of the inductive
and capacitive regions according to frequency ratio and (b)
required current direction as a function of frequency ratio to
achieve soft-switching.
[0038] FIG. 10 illustrates a half-bridge inverter used in
experimental verification of embodiments of the present
invention,
[0039] FIGS. 11(a), (b) and (c) show (a) simulated and (b) measured
tank voltage and current respectively, and (c) measured gate
signals in a first test,
[0040] FIGS. 12 (a), (b) and (c) show (a) simulated and (b)
measured tank voltage and current respectively, and (c) measured
gate signals in a second test,
[0041] FIGS. 13 (a), (b) and (c) show (a) simulated and (b)
measured tank voltage and current respectively, and (c) measured
gate signals in a third test,
[0042] FIGS. 14 (a), (b) and (c) show (a) simulated and (b)
measured tank voltage and current respectively, and (c) measured
gate signals in a fourth test,
[0043] FIG. 15 shows a half-bridge inverter circuit for use in a
method according to a second embodiment of the invention,
[0044] FIG. 16 shows simulated (left) and measured (right) voltage
and current waveforms in a test of the second embodiment of the
invention,
[0045] FIG. 17 shows simulated (left) and measured (right) voltage
and current waveforms in a test of the second embodiment of the
invention,
[0046] FIG. 18 shows simulated (left) and measured (right) voltage
and current waveforms in a test of the second embodiment of the
invention,
[0047] FIG. 19 shows simulated (left) and measured (right) voltage
and current waveforms in a test of the second embodiment of the
invention,
[0048] FIG. 20 shows a half-bridge inverter circuit similar to FIG.
15 but in an alternate embodiment,
[0049] FIG. 21 illustrates schematically the effect of the
auxiliary resonant tank on the inductive region,
[0050] FIG. 22 plots calculated auxiliary inductance upper limit
against switching frequency,
[0051] FIG. 23 plots calculated maximum auxiliary inductor current
against switching frequency,
[0052] FIG. 24 shows simulated (left) and measured (right) voltage
and current waveforms in a test of an alternative form of the
second embodiment of the invention,
[0053] FIG. 25 shows simulated (left) and measured (right) voltage
and current waveforms in a test of an alternative form of the
second embodiment of the invention,
[0054] FIG. 26 shows simulated (left) and measured (right) voltage
and current waveforms in a test of an alternative form of the
second embodiment of the invention, and
[0055] FIG. 27 shows simulated (left) and measured (right) voltage
and current waveforms in a test of an alternative form of the
second embodiment of the invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0056] In preferred embodiments of this invention a novel
pseudo-inductive soft-switching technique is provided that can be
applied to the circuits described in FIGS. 1-5. The overall load Z
can consist of different combination of resonant tank(s) and is not
restricted to the forms shown in FIG. 4 and FIG. 5.
[0057] A first embodiment of the present invention (which may be
termed a "pseudo-inductive soft-switching" method) will now be
described firstly by reference to theory, and then by experimental
verification of the theory.
[0058] The half bridge inverter loaded by series resonant tank
shown in FIG. 8 is considered as an example to illustrate the
soft-switching technique in the capacitive region based on the
pseudo-inductive region concept. Based on the Fourier analysis
approach, the input voltage V.sub.input and input current
I.sub.input to the resonant tank can be expressed by following
equations:
[0059] The rectangular ac voltage applied to the resonant tank is:
3 V input = V g 2 + 2 V g n = 2 k - 1 .infin. 1 n sin ( n t T s / 2
) , k = 1 , 2 , 3 , , .infin. ( 4 )
[0060] Normally a DC-blocking capacitor is used to remove the Dc
component V.sub.g/2. The AC current entering the resonant tank is:
4 i input = 2 V g n = 2 k - 1 .infin. 1 n ( j n s L r + 1 j n s C r
) sin ( n t T s / 2 ) = 2 V g n = 2 k - 1 .infin. 1 ( n 2 s L r - 1
s C r ) cos ( n t T s / 2 ) ( 5 )
[0061] V.sub.input and i.sub.input have only odd harmonic
components, where V.sub.2 is the DC link voltage and T.sub.s is the
switching cycle; .omega..sub.s=2 .pi.f.sub.s=2 .pi./T.sub.s. The
resonant frequency is f.sub.r=1/2 .pi.{square root}{square root
over (L.sub.rC.sub.r)} and the characteristic impedance
Z.sub.r={square root}{square root over (L.sub.r/C.sub.r)}.
[0062] Define a variable to represent the ratio between f.sub.s and
f.sub.r: 5 N = f r f s ( 6 )
[0063] When 0<N<1, the resonant tank works at the inductive
region as explained in the background section and the input current
to the resonant tank lags the input voltage pulse. In other words,
i.sub.input is positive when S1 is turned off and just before S2 is
turned on and negative when S2 is turned off and just before S1 is
turned on. This is the essential condition for soft switching,
[0064] Referring now to FIG. 9(a) the nominal capacitive region
when N>1 can be considered. Substitute in the equation of
i.sub.input, 6 i input = 2 V g n = 2 k - 1 .infin. 1 nZ r ( n N - N
n ) cos ( n N r t ) ( 7 )
[0065] Note the n is an odd number in the equation.
[0066] From this equation, it can be found that when N equals to
one of the odd numbers, such as 1,3,5 . . . , the factor 7 ( n N -
N n )
[0067] will be equal to zero when n equals to N, and thus the n-th
harmonic component will make i.sub.input infinite, and the
frequency 8 f r N ( N = 1 , 3 , 5 , )
[0068] acts as a resonant frequency, which is called a
sub-resonant-frequency. When the switching frequency of the
inverter f.sub.s comes near such sub-resonant-frequencies, the
local dominant resonant frequency will be predominated by the
sub-resonant-frequency accordingly. In summary, the whole
capacitive region can be subdivided into many small regions by the
sub-resonant-frequencies as shown in FIG. 9(a).
[0069] When the inverter switching frequency f.sub.s is higher than
the dominant resonant frequency f.sub.r of the resonant tank, i.e.
0<N<1, this operating region is said to be inductive because
the soft-switching condition (1*) that: the current I.sub.input is
positive when S1 (top switch) is turned off and just before S2
(bottom switch) is turned on, and the current I.sub.input is
negative when S2 is turned off and just before S1 is turned on is
complied with.
[0070] The region of N>1 (i.e. f.sub.s<f.sub.r) is usually
considered as capacitive and previously considered as unsuitable
for soft switching. However, under certain conditions soft
switching can actually be achieved in this nominal capacitive
region. In particular, the capacitive region can be divided into
two types, namely capacitive regions and pseudo-inductive
regions.
[0071] The capacitive regions (in which soft switching cannot be
achieved) meet the following two conditions; 9 ( I ) f s < f r (
II ) f r K + 1 < f s < f r K or K < f r f s < K + 1 or
K < N < K + 1 , where K = 1 , 3 , 5 , ( 8 )
[0072] Under these conditions, i.sub.input is negative when S1 is
turned off and before S2 is turned on, and i.sub.input is positive
when S2 is turned off and just before S1 is turned on. The
anti-parallel diode of the incoming (to be turned on) power switch
is not conducting and will not clamp the voltage of the incoming
switch to zero, resulting in hard switching.
[0073] However, soft switching can be achieved within the pseudo
inductive regions in the nominal capacitive region. By choosing an
appropriate value of N (ratio of fr and fs), soft switching can be
achieved in the capacitive region. In the capacitive region of
N>1, when 10 f r K < f s < f r K - 1 or K < f r f s
< K + 1 or K < N < K + 1 , where K = 2 , 4 , 6 , ( 9 )
[0074] i.sub.input is positive when S1 is turned off and negative
when S2 is turned off. This is the condition required by soft
switching and is similar to that in the inductive region. These
equivalent inductive regions may be called pseudo-inductive
regions.
[0075] In summary, for N>1 (i.e. f.sub.s<f.sub.r), the
nominal capacitive region is further divided into two types:
[0076] (1) The Capacitive Regions:
[0077] Odd integers<N<Even integers.fwdarw.capacitive
characteristics, such as:
1<N<2, 3<N<4, 5<N<6, . . .
[0078] (2) The Pseudo-Inductive Regions:
[0079] Even integers<N<Odd integers.fwdarw.pseudo-inductive
characteristics, such as:
2<N<3, 4<N<5, 6<N<7, . . .
[0080] Soft switching achieved in the pseudo-inductive regions can
be explained in an intuitive way. Consider N=fr/fs again. If
N>1, there are more than one resonant period within the inverter
switching period. If N is chosen to satisfy equation (9), the
resonant current is in the positive half cycle when the top switch
S1 is turned off, and it is in the negative half cycle when the
bottom switch is turned off. Therefore, if equation (9) is
satisfied, the soft switching conditions (1*) can be met. The
required direction of i.sub.input for the inverter to achieve
soft-switching condition is shown in FIG. 9(b).
[0081] Experimental Verification:
[0082] The pseudo-inductive soft-switching technique is illustrated
with a half-bridge power inverter circuit example (FIG. 10) that is
suitable for electronic ballast of high-intensity-discharge (HID)
lamp. There are two resonant frequencies in this system. Inductance
Ls is much larger than inductance Lr. The operating procedure is as
follows:
[0083] (1) By operating the inverter frequency fs=fs.sub.L close to
a low resonant frequency fr=fr.sub.L (about 56 kHz in this
example), which is due to the resonant tank that consists of Cr and
Ls+Lr (Ls>>Lr), a high voltage can be generated across the
resonant inductor Is to ignite the discharge lamp. Before ignition,
the lamp behaves like an open circuit.
[0084] (2) in the lamp's glow-to-arc transitional period, the lamp
is close to a short-circuit situation (shorting the large inductor
Ls).
[0085] (3) Once the lamp arc is established, the lamp is like a
resistor. The steady-state inverter frequency is then increased to
a high value (fs=fs.sub.H) higher than the high resonant frequency
fr=fr.sub.H (about 307 kHz in this example), which is due to the
resonant tank consisting of Cr and Lr.
[0086] This circuit is good example to illustrate the usefulness of
the invention. Before the lamp is ignited in stage (1) at a lower
starting frequency fs.sub.L, the lamp is like an open circuit. The
dominant resonant frequency of the resonant tank fr.sub.L. The
starting inverter frequency fs.sub.L should be slightly higher than
fr.sub.L, in order that the resonant voltage across Ls is large
enough for lamp ignition and the resonant tank operated in the
inductive region for achieving soft switching. However, when the
lamp starts to ignite and gets into the glow-to-arc transition, it
behaves like a short circuit (shorting the large inductor Ls). In
this case, the effect of Ls suddenly disappears and the resonant
tank consists of Cr and Lr only. This means that the dominant
resonant frequency is suddenly changed from fr.sub.L to fr.sub.H
during the glow-to-arc transition. Since the initial inverter
frequency is slightly higher than fr.sub.L, the initial inverter
switching frequency is in the `capacitive` region of the
high-frequency resonant tank circuit of Cr and Lr. If the starting
inverter switching frequency fs.sub.L is not chosen to be in the
pseudo-inductive region of the in the high-frequency resonant tank
according to equation (9), then hard-switching will occur and the
inverter could be damaged by the high switching loss and stress in
the power switches. The HID lamp load is a good example of a
changing load even under steady-state high-frequency f.sub.rH
operation. The lamp arc behaves like a resistive load under normal
state operation and could change into an open circuit if the lamp
arc is broken due to acoustic vibration. Therefore, soft switching
has to be achieved under different conditions:
[0087] In summary, the HID lamp ballast example has the following
operating modes:
[0088] a) The lamp behaves like an open circuit before ignition,
when a relatively low inverter starting frequency fs.sub.L is used
and a dominant resonant frequency is fr.sub.L,
[0089] b) The lamp behaves like a short circuit in the glow-to-arc
transition during the ignition process, with the inverter operating
at fs.sub.L and a dominant resonant frequency suddenly changed to
fr.sub.H and the inverter frequency remains fs.sub.L,
[0090] c) Immediately after the ignition process is completed, the
lamp behaves like a resistive load at an inverter frequency of
fs.sub.L.
[0091] d) The inverter frequency is then increased to a relatively
high values fs.sub.H steady-state lamp operation. The lamp behaves
like a resistive load.
[0092] e) The lamp behaves like an open circuit when the lamp arc
is broken due to acoustic resonance, when the inverter frequency is
a relatively high fs.sub.H.
[0093] Among these operating modes, modes (b) and (e) have the
potential danger of hard switching. In mode (b), the sudden change
of dominant resonant frequency to fr.sub.H while the inverter
switching frequency remains at fs.sub.L. Consequently, the inverter
circuit could be operated in the nominally capacitive region with
hard switching and Condition (1*) cannot be met. In mode (e), if
the load becomes an open circuit, the input current can become
smaller than the minimum level as required in equation (3) because
the inverter frequency is now high (fs=fs.sub.H). Thus, Condition
(2*) may not be met.
[0094] In this experimental system, the DC link is set at 310V.
HIE-E27 150 W metal halide is selected for testing. The starting
inverter frequency fs.sub.L should be higher than the low resonant
frequency fr.sub.L (56 kHz) and the steady-state inverter switching
frequency fs.sub.H should be higher than the high resonant
frequency fr.sub.H (307 kHz). In this system, it is thus safe to
choose a steady-state inverter frequency fs.sub.H (after the lamp
is fully turned on) to be 400 kHz because the resonant tank would
be in the inductive region for achieving soft switching. The key
question is how to choose the starting inverter frequency fs.sub.L
appropriately so that it is in the pseudo-inductive region of the
high-frequency resonant tank of Cr and Lr during the starting
period.
[0095] Test1: Confirmation of the Inductive Region
(fs.sub.H>fr.sub.H)
[0096] The large inductor Ls in FIG. 10 is shorted so that the
equivalent resonant tank consists of Lr and Cr only. This high
resonant frequency fr=fr.sub.H is about 307 kHz. The steady-state
inverter switching frequency fs.sub.H is set at 400 kHz so that the
frequency ratio N<1 (N=fr.sub.H/fs.sub.H=0.77) and the resonant
tank operation should be in the inductive region.
[0097] FIG. 11(a) and FIG. 11(b) show the simulated and measured
tank voltage V.sub.input and current I.sub.input, respectively,
when the steady-state inverter switching frequency fs=fs.sub.H is
set at 400 kHz (i.e. fs.sub.H>fr.sub.H). The simulations and
measurements confirm that the tank current I.sub.input is lagging
behind the tank voltage V.sub.input. This inductive feature meets
the soft switching requirement (1*). FIG. 11(c) shows the measured
gating signals of the power mosfets S1 and S2. It can be seen that
these gate signals are relatively `clean` without any induced
voltage spikes that commonly arise from hard switching.
[0098] Test 2: Confirmation of Capacitive Region with the
Steady-State Inverter Frequency fs.sub.H<fr.sub.H and N Failing
to Satisfy the Pseudo-Inductive Requirement in Equation (9)
[0099] Similarly to Test 1, the large inductor Ls in FIG. 10 is
shorted so that the equivalent resonant tank consists of Lr and Cr
only. When the steady-state inverter frequency fs=fs.sub.H is
changed to 200 kHz, N>1 (N=fr/fs=1.54). As fs.sub.H<fr.sub.H,
this operating region is capacitive. Note that N is higher than an
odd integer and less than an even integer in this case. This means
that N fails to meet the pseudo-inductive requirement in equation
(9).
[0100] FIGS. 12(a) and (b) show the calculated and measured
V.sub.input and I.sub.input, respectively, under this operation. It
can be seen that I.sub.input is leading V.sub.input, confirming the
capacitive characteristic of the circuit. Soft switching condition
1* is not met in this case. The switching noise arising from hard
switching can be observed from the high induced voltage spikes in
the measured gate signals of S1 and S2 as shown in FIG. 12(c). This
switching noise could be a serious reliability problem because it
can inadvertently turn on a power switch and result in a
short-circuit situation in the inverter bridge.
[0101] Test 3: Confirmation of Capacitive Region with Starting
Inverter Frequency fs.sub.L<fr.sub.H and N failing to Satisfy
the Pseudo-Inductive Requirement in Equation (9)
[0102] When the HID lamp is in the glow-to-arc transition the
starting inverter switching frequency fs.sub.L is slightly higher
than 56 Hz. But the lamp behaves like a short circuit, shorting Ls.
The effective resonant frequency will suddenly change from
fr=fr.sub.L (due to Cr and Ls+Lr) to fr=fr.sub.H (due to Cr and
Lr). In this test, the starting frequency is chosen so that the
frequency ratio N does not meet the pseudo-inductive requirement in
equation (9). In this case, fr=fr.sub.H307 kHz. The starting
frequency is set at fs.sub.L=86 kHz so that N=3.56 which is higher
than an odd integer instead of an even integer.
[0103] FIG. 13(a) and FIG. 13(b) show the calculated and measured
V.sub.input and I.sub.input, respectively. As predicted, the
soft-switching condition 1* is not met, FIG. 13(c) shows the gating
signals of the two switches S1 and S2. The induced voltage spikes
arising from hard switching can clearly be observed.
[0104] Test 4: Confirmation of Pseudo-Inductive Soft-Switching
Technique in the Nominally Capacitive Region (fs.sub.L<fr.sub.H)
with N greater than an even integer and smaller than the last odd
integer.
[0105] Test 3 shows that if the starting inverter frequency
fs.sub.L does not meet the pseudo-inductive requirement in equation
(9), hard switching will occur and the induced switching noise
(FIG. 13(c)) could be quite significant, As assumed in tests 1 to
3, the lamp is in the short-circuit condition so that the effective
resonant frequency is fr=fr.sub.H (due to Cr and Lr only). The
starting inverter frequency fs=fs.sub.L is now set at 64 kHz, which
is higher than fr.sub.L=56 kHz. The effective frequency ratio
N=fr/fs=fr.sub.H/fs.sub.L=4.8, which is higher than an even
integer. This means that this is in the pseudo-inductive
region.
[0106] FIG. 14(a) and FIG. 14(b) show the calculated and measured
V.sub.input and I.sub.input, respectively. Despite the fact the
fs<fr (equivalent capacitive load). Input is positive when S1 is
turned off. The soft switching condition described in condition 1*
is met. FIG. 14(c) shows the gating signals for the S1 and S2. It
can be seen that no switching noise is induced in them.
[0107] A second embodiment of the present invention uses an
additional or auxiliary resonant tank. This embodiment will now be
described again with regard to theory first of all, and then with
experimental justification. There are two different versions of
this embodiment: one in which the additional resonant tank has a
relatively high resonant frequency, and another in which the
additional resonant tank has a relatively low resonant frequency.
When the additional resonant tank has a relatively high resonant
frequency there are more than one resonant cycles in the resonant
tank within one cycle of the inverter switching frequency and this
may be called the resonant mode of operation. On the other hand,
when the additional resonant tank is operated at a relatively low
resonant frequency, the resonant tank is charged and discharged
once within each inverter cycle. This may be termed a linear
mode.
[0108] The basic concept of the use of an additional resonant tank
is illustrated with the use of FIG. 15 and FIG. 20. FIG. 15 shows a
resonant circuit similar to the circuit example in FIG. 10, except
that an additional unloaded resonant tank capacitor Ca and inductor
La is added. An alternative implementation is to use the dc
blocking capacitor Cb and La to form an additional unloaded
resonant tank as shown in FIG. 16
[0109] (A) Additional or Auxiliary Parallel Resonant Tank (Resonant
Mode) (FIG. 15)
[0110] Through proper selection of the components' parameters of
the auxiliary capacitor and auxiliary inductor, the auxiliary
resonant tank can operate at an `inductive` state in the high
frequency range (namely the operating frequency fs.sub.H of the
load such as an HID lamp), and at a `pseudo-inductive` state in the
low frequency range (namely the starting inverter frequency
fs.sub.L). Thus lagging current can be generated independently,
regardless of state of the load (lamp) branch. As has been
discussed above, that operating mode (e) could have an I.sub.input
too small that soft-switching condition 2* may not be met. But with
the existence of the extra current through the auxiliary resonant
tank, enough inductive current flows through the two MOSFETs, thus
satisfying condition 2* for soft switching.
[0111] The use of the auxiliary resonant tank for soft switching
makes it easy to meet conditions 1* and 2*. For a specific
selection of the auxiliary tank components' parameters, when the
switching ratio is given, the maximum current through the auxiliary
tank is determined accordingly. When this current is above the
current threshold, soft switching can be achieved. Of course,
superfluous inductive current undoubtedly ensures soft switching,
but it gives rise to larger conduction loss in the power switches
and higher switch's current ratings requirement. So components'
parameters and switching frequency ratio should be carefully chosen
in the consideration of soft switching current threshold and
switch's conduction loss and current ratings.
[0112] In the prototype circuit, the parameters are selected like
this, C.sub.r=2.35 nF , L.sub.r=114 .mu.H, L.sub.s3.5 mH
L.sub.a=160 .mu.H C.sub.a=2.2 nF . Thus in the original LCL
resonant tank, the higher resonant frequency f.sub.rH is 308 kHz,
the lower resonant frequency f.sub.rL is 55 kHz, one
pseudo-inductive region (f.sub.rh/5, f.sub.rh/4) of f.sub.rH is
within (62 kHz, 77 kHz). In this embodiment of the present
invention, the parameters of the auxiliary resonant tank are chosen
in such a way that some of its pseudo-inductive regions at least
partially overlap with those of the original resonant circuits, and
the starting inverter frequency can be chosen to be within the
pseudo-inductive region of the auxiliary tank. In this case, the
resonant frequency of the auxiliary can be relatively high in a
sense that it is close to the high resonant frequency of the
original resonant circuit. For the auxiliary resonant tank in this
example, the resonant frequency f.sub.a is 280 kHz, and one
pseudo-inductive region (f.sub.a/5, f.sub.a/4) is within (56 kHz,
70 kHz). Thus the starting frequency region can be selected as
being somewhere between 57 kHz and 69 kHz, which is within the
pseudo-inductive frequency region of the auxiliary resonant tank
and overlaps with the pseudo-inductive region of the original
resonant circuit.
[0113] Once the lamp is turned on, the inverter switching frequency
can be increased from fs=fs.sub.L to fs=fs.sub.H (a relatively
higher switching frequency higher than the resonant frequencies of
the original resonant tank fr.sub.H and the auxiliary resonant tank
fa, so that both resonant branches are in the inductive state.
[0114] Experimental Verification
[0115] Test 5:
[0116] Simulated and experimental waveforms of V.sub.input and
i.sub.input were obtained from th circuit example under the
following conditions are included.
[0117] (i) Starting Inverter frequency fs.sub.L=64 kHz, with an
open circuit load (representing a lamp before ignition).--FIG.
16.
[0118] (ii) Starting Inverter frequency fs.sub.L=64 kHz, with a
short circuit load (representing a lamp in the glow-to-arc
transition).--FIG. 17.
[0119] (iii) Steady-state inverter frequency fs=fs.sub.H=370 kHz
(higher than fr.sub.H=308 kHz and fa=280 kHz), with an open circuit
load (representing a lamp arc extinction).--FIG. 18.
[0120] (iv) Steady-state inverter frequency fs=fs.sub.H=370 kHz
(higher than fr.sub.H=308 kHz and fa=280 kHz), with an short
circuit load (representing short-circuit load condition).--FIG.
19.
[0121] All simulated and measured results confirm that soft
switching can be achieved at the relatively low starting inverter
frequency operation and the high inverter frequency operation under
both open and short circuit conditions. These results verify the
high reliability offered by the proposed auxiliary resonant branch
and pseudo-inductive soft-switching method.
[0122] (B) Alternative Implementation of Auxiliary Parallel
Resonant Tank (Linear Charging and Discharging Mode) (FIG. 20)
[0123] An alternative way to implement the auxiliary resonant
branch is shown in FIG. 20. In this case, the dc blocking capacitor
Cb, that is commonly used in high-frequency inverter to remove the
dc voltage component, is employed as part of the auxiliary resonant
tank. Because the size of Cb is relatively large (typically in the
order of micro-Farad), the resonant frequency fa of Cb and La is
relatively low.
[0124] Consider the nature of the resonant tanks of the original
resonant tank and the auxiliary one in FIG. 21. If the resonant
frequency fa of the auxiliary resonant tank is chosen to be lower
than the resonant frequency fr of the original resonant circuit,
the use of the auxiliary resonant tank can widen the inductive
frequency range of the overall circuit for achieving soft
switching
[0125] Equations (1)-(3) can be rewritten for FIG. 20 as
follows:
[0126] The current through the auxiliary inductor can be formulated
by the following equation: 11 i input = - V g 2 L a ( 1 2 f s - T d
) + V g L a t ( 0 t 1 2 f s - T d ) ( 10 )
[0127] where Vg is the dc voltage of the inverter, Td is the dead
time between S1 and S2. The charge to be removed from the power
switch's total equivalent capacitance is: 12 Q s = - t d / 2 t d /
2 ( V g 2 L a ( 1 2 f s - T d ) ) t 2 C s V g ( 11 )
[0128] where Qs is the charge and Cs is the total equivalent
capacitance across the drain and source of the power switch. This
equation can be simplified as: 13 T d 4 C s ( 1 2 f s - T d ) L a (
12 )
[0129] This means that the inductance of the auxiliary inductor
cannot exceed a certain limit as shown in (12) in order to achieve
soft switching. By using this equation, the needed auxiliary
inductance can be determined. When the dead time T.sub.d and the
parallel capacitance of MOSFET C.sub.s are given, the upper limit
of the auxiliary inductance can be determined by switching
frequency. FIG. 22 shows the relationship between switching
frequency and the upper limit of the inductance in the example.
[0130] From this graph, it is clear that the auxiliary inductance
should be set below 180 uH in this example if the ballast works up
to about 500 kHz. When the value of the auxiliary inductor is set
at 180 kHz.mu.H, then the maximum current through the auxiliary
branch can be determined by equation (10) and the relationship
between this maximum current and the switching frequency can be
shown by FIG. 23.
[0131] In the prototype circuit (FIG. 20), the selected parameters
are: C.sub.r=1.1 nF, L.sub.r=170 .mu.H, L.sub.s=1.5 mH, L.sub.a=120
.mu.H, C.sub.b=1 .mu.F. Thus in the LCL resonant tank, the higher
resonant frequency f.sub.rH is 368 kHz, the lower resonant
frequency f.sub.rL is 123 kHz, one pseudo-inductive region
(f.sub.rH/3, f.sub.rH/2) is within (124 kHz, 185 kHz). The starting
frequency region can be selected as between 130 kHz and 140 kHz.
Note that the resonant frequency of the auxiliary resonant tank
consisting of La and Cb is f.sub.a=1/(2 .pi.{square root}{square
root over (L.sub.aC)})=14.53 kHz, which is much lower than the
minimum resonant frequency (123 kHz) of the original multi-resonant
tank circuit. If necessary this resonant frequency can be set at a
higher value by using smaller auxiliary inductor L.sub.a.
[0132] Test 6:
[0133] Simulated and experimental waveforms of V.sub.input and
i.sub.input were obtained from the circuit example (FIG. 20) under
the following conditions are included.
[0134] (v) Starting Inverter frequency fs.sub.L=140 kHz (higher
than fa=14.5 kHz and f.sub.rL=123 kHz), with an open circuit load
(representing a HID lamp before ignition).--FIG. 24.
[0135] (vi) Starting Inverter frequency fs.sub.L=140 kHz (higher
than fa=14.5 kHz and f.sub.rL=123 kHz), with a short circuit load
(representing a HID lamp in the glow-to-arc transition).--FIG.
25.
[0136] (vii) Steady-state inverter frequency fs=fs.sub.H=400 kHz
(higher than fr.sub.H=368 kHz), with an open circuit load
(representing a lamp arc extinction).--FIG. 26.
[0137] (viii) Steady-state inverter frequency fs=fs.sub.H=400 kHz
(higher than fr.sub.H=368 kHz), with an short circuit load
(representing short-circuit load condition).--FIG. 27.
[0138] The simulating and experimental waveforms of V.sub.input and
i.sub.input at various operating conditions using the circuit
example of FIG. 20 are given below:
* * * * *