Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2000-03-24
2002-06-04
Wong, Peter S. (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S097000, C323S266000
Reexamination Certificate
active
06400579
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the field of DC-to-DC power conversion and in particular to the class of converters distinguished by ultra high efficiency, high overload capability, small size and weight, high power density at a moderate switching frequency.
BACKGROUND OF THE INVENTION
The following notation is consistently used throughout this text in order to facilitate easier delineation between various quantities:
1. DC—shorthand notation historically referring to Direct Current but now has wider meaning and refers to all Direct electrical quantities (current and voltage);
2. AC—shorthand notation historically referring to Alternating Current but now has wider meaning and refers to all Alternating electrical quantities (current and voltage)
3. The instantaneous time domain quantities are marked with lower case letters, such as i
1
and v
2
for current and voltage. Often these instantaneous quantities contain a DC component, which is designated with corresponding capital letters, such as I
1
and V
2
.
4. The difference between instantaneous and DC components is designated with &Dgr;, hence &Dgr;i
1
designates the ripple component or AC component of current i
1
.
Over the last two decades a large number of switching DC-to-DC converters had been invented with the main objective to improve conversion efficiency and reduce the converter size. The past attempts to meet both of these objectives simultaneously have been hampered by the two main obstacles, which up to now seemed to be inherent to all switching DC-to-DC converters:
1. The large DC current bias present in the filtering inductors at either input or output of the converters (as well as the DC-bias current present in the isolation transformer of some of the isolated converters) resulted in a big size of the magnetic components, since an air-gap proportional to the DC current bias must be inserted in the AC flux path in order to prevent magnetic core saturation. This also resulted in a very inefficient use of the magnetic material, which was largely wasted. Even a relatively small air-gap, in the order of 1 mm (40 mils), drastically reduces the total inductance. This loss of inductance was compensated by either an inordinately large increase of the switching frequency (hence increase of losses) or by increasing the size of the magnetic cores, or both.
2. An implementation of soft switching methods to reduce significant switching losses at increased switching frequencies was DC load current dependent and required for its operation an unwanted large output inductor AC current ripple (larger than twice the magnitude of the maximum DC load current) thereby diminishing most of the recovered energy due to increased conduction losses caused by this large AC ripple current.
Magnetic Saturation with DC Current Bias
First, the problem associated with the DC-bias of magnetic components (inductors and transformers) can be best understood with reference to the classical buck converter shown in prior art
FIG. 1
a
and the accompanied output inductor current waveform of
FIG. 1
b
. Since the converter output supplies DC power to the load, the inductor in the buck converter must pass the DC component of the load current, which is I
DC
. Hence, it clearly cannot be designed as an ordinary inductor used in alternating current (AC) applications such as the inductor in
FIG. 2
a.
Several quantities which are used throughout the text are now described with their defining relationship:
Flux linkage &lgr; is the total flux linking all N turns and is &lgr;=N&PHgr; where &PHgr; is the flux in the magnetic core;
Flux density B is the flux per unit area defined by B=&PHgr;/S where S is a magnetic core cross-section area.
Inductance L is defined as the slope of &lgr;−i characteristic, i.e., L=&lgr;/i;
Duty ratio D of the switch is defined as D=t
ON
/T
S
where t
ON
is ON time of the switch, and T
S
is the switching period defined as T
S
=1/f
S
where f
S
is a constant switching frequency;
Complementary duty ratio D′ of the switch is defined as D′=1−D.
An AC inductor is wound on magnetic core material in order to dramatically increase its inductance value. For example, typical ferrite core material has at room temperature a relative permeability on the order of &mgr;
r
.3,000. Hence the inductance of the coil is magnified by a factor of 3,000 simply by inserting the magnetic core material without any air gap as in
FIG. 2
a
. The corresponding flux linkage “&lgr;” versus current “i” characteristic is as in
FIG. 2
b
with a high slope illustrating the high inductance value L (maximum attainable with that core material). The flux linkage excursions (caused by the AC current) are symmetrical around the center of the magnetic core operating characteristic. Even if a very small DC current I
DC
shown in
FIG. 2
b
were to pass through this coil, the magnetic core material would saturate and instead of the desirable large inductive impedance, the inductor would look like a short circuit. Thus, to avoid core saturation, all present switching converters “solve” this DC-bias problem in a “brute-force” way by inserting an air-gap in the magnetic flux path as illustrated in
FIG. 3
a
. This clearly reduces the inductance value proportionally to the inserted air-gap size (the larger the DC current, the bigger air-gap is needed, hence the smaller is the resulting inductance value), as seen by the flux linkage characteristic of
FIG. 3
b
for an un-gapped and gapped core and their corresponding inductances L and L
g
. Clearly three very detrimental factors did occur:
1. By insertion of the air-gap, the inductance value is drastically reduced. It is not uncommon to see the original un-gapped inductance L reduced by a factor of 100 to 1000 to the inductance L
g
with the air-gap included. In order to compensate for this loss of inductance, the switching frequency is radically increased or a much bigger core size is used, or both.
2. The already small AC flux linkage excursions due to the finite and relatively low saturation flux density B
SAT
of 0.3 T (tesla) for ferrite material, is further significantly reduced due to the presence of the DC-bias in the core. For example, in typical applications, the DC-bias might correspond to a flux density of 0.25 T thus leaving only 0.05 T for the superimposed AC flux excursions. This in turn results in either larger core size requirements or increased switching frequency, or both.
3. The waste of ferromagnetic material is even larger, since the negative part of the saturation characteristic is not utilized at all, and thus another &Dgr;B=B
SAT
=0.3 T is also wasted.
The DC-bias problem is not only limited to all inductors used up to now in DC-to-DC converters but is also present in many isolation transformers, such as for example in the popular flyback converter shown in
FIG. 4
a
. This transformer does provide galvanic isolation and the ability to step-up or step-down the voltage through the transformer turns ratio, but contrary to the ordinary AC line transformer, it has a large DC-bias and requires a correspondingly big air-gap as shown in
FIG. 4
b
. Hence the magnetic core is biased in one direction thus limiting the superimposed AC flux excursions as seen in FIG.
5
.
Up to now, the detrimental effect of the large DC-bias and hence the large air-gap was introduced qualitatively. Let us now also quantify these effects on an output inductor design example for a 5V, 100 W buck-like converter, having a DC load current I
2
=20 A, and number of winding turns N=6, which is implemented on a ferrite core having a saturation flux density B
SAT
=0.3 T (tesla) out of which B
DC
=0.2 T is available for the DC-bias and the remaining 0.1 T is allocated for the superimposed AC flux excursions. To support NI=120 ampere-turns the required air-gap is given by formula l
g
=&mgr;
0
NI/B
DC
=30 mils=0.75 mm where &mgr;
0
=4&pgr;10
−7
H/m is the permeability of free
Fernandez A. M.
Laxton Gary L.
Wong Peter S.
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