Lossless switching converter with DC transformer

Details Lossless switching converter with DC transformer Lossless switching converter with DC transformer

2001-03-22

2002-05-14

Riley, Shawn (Department: 2838)

Electric power conversion systems

Current conversion

Including d.c.-a.c.-d.c. converter

Reexamination Certificate

active

06388896

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to the field of switching DC-to-DC power conversion and in particular to the new class of switching converters employing two novel methods: lossless switching method and method for novel magnetics structure. Lossless switching improves substantially the conversion efficiency, while new DC Transformer structure either minimizes or entirely eliminates the DC energy storage in magnetics core structures hence resulting in a very compact size of magnetics and efficiency improvements. Additional performance benefits are in increased DC overload current capability and reduced EMI noise with improved reliability.
BACKGROUND OF THE INVENTION
Definition and Classifications
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 by now has acquired wider meaning and refers generically to circuits with DC quantities;
2. AC—Shorthand notation historically referring to Alternating Current but by now has acquired wider meaning and refers to all Alternating electrical quantities (current and voltage);
3. i
1
, v
2
—The instantaneous time domain quantities are marked with lower case letters, such as i
1
and v
2
for current and voltage;
4. I
1
, V
2
—The DC components of the instantaneous periodic time domain quantities are designated with corresponding capital letters, such as I
1
and V
2
;
5. &Dgr;i
1
—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
;
6. i
CC
—The composite current equal to sum of currents through the input switch S
1
and complementary input switch S′
1
, that is i
CC
=i
S1
+i
S′1
The following common defining relationships and notations related to magnetic circuit descriptions is used consistently throughout:
1. Flux linkage &lgr; is the total flux linking all N turns and is defined as &lgr;=N&PHgr; and &PHgr; is the total flux in the magnetic core;
2. Inductance L is defined as the slope of the &lgr;-i characteristic, i.e., L=&lgr;/i;
3. Flux density B is the flux per unit area defined by B=&PHgr;/S where S is a magnetic core cross-section area;
Present invention imposes also a need to introduce completely new terminology for the two major novelties, neither of which is present in prior-art switching converter terminology:
1. New magnetic devices with substantially reduced size and increased efficiency made possible by a special switching converter structure and corresponding magnetic circuit structure;
2. Novel methods of switching, which make possible the complete elimination of switching losses (except for gate-drive losses) and thus result in the highest possible efficiency improvement.
The new magnetic devices come in two basic variants named as follows:
1. DC Transformer is a special magnetic structures with multiplicity of inductor windings on a common magnetic core, in which DC current flow of each winding and AC voltage polarity of each winding as imposed by a present invention non-isolated switching converter are such to result in the reduction of the total DC ampere-turns of all winding and hence in reduced DC flux in the common magnetic core and in some instances even substantially zero total DC Ampere-turns and substantially zero DC flux in the common core.
2. Isolated DC Transformer is a special magnetic structures having inductors and isolation transformer windings with the same performance features as DC Transformer but having in addition a galvanic isolation between the source and load.
Lossless Switching Methods require new definition of the switches, switching intervals and transition intervals they create as well as the respective duty ratio D as follows:
1. S
1
, S
2
, S′
1
, S′
2
—Switch designations respectively for input switch, output switch, complementary input switch, and complementary output switch and, at the same time, designate the switching states of the respective active, controllable switches as follows: high level indicates that active switch is turned-ON, low (zero) level that active switch is turned-OFF;
2. D—The duty ratio is defined as D=t
ON
/T
S
where t
ON
is the ON time interval during which the input switch is closed (turned ON) and T
S
is the switching period defined as T
S
=1/f
S
where f
S
is a switching frequency;
3. D′—The complementary duty ratio D′ is defined as D′=t
OFF
/T
S
where t
OFF
is the OFF time interval during which the input switch S
1
is open (turned OFF);
4. State-1 Interval—The time interval during which input switch S
1
and output switch S
2
are turned-ON (closed), while complementary input switch S′
1
and complementary output switch S′
2
are both turned OFF (open);
State-2 Interval—The time interval during which input switch S
1
and output switch S
2
are both turned OFF (open), while complementary input switch S′
1
and complementary output switch S′
2
are both turned ON (closed);
6. (1-2) transition interval—The time interval between State-1 and State-2 interval during which, in precisely defined sequence and timing, input switch S
1
and output switch S
2
reverse their state from ON to OFF while complementary input switch S′
1
and complementary output switch S′
2
reverse their state from OFF to ON;
7. (2-1) transition interval—The time interval between State-2 and State-1 interval during which, in precisely defined sequence and timing, input switch S
1
and output switch S
2
reverse their state from OFF to ON while complementary input switch S′
1
and complementary output switch S′
2
reverse their state from ON to OFF;
8. CR
2
, CR′
2
—Designation for the output switch and complementary output switch implemented as a current rectifier (CR) diodes and their corresponding switching time diagram. Since diode is a two-terminal passive switch, switching time diagram represents also the state of diode switch as follows: high level indicates that the diode is ON and low level that diode is OFF;
9. I—Designates one quadrant switch operating in the first quadrant. The Roman number (I through IV) within a rectangular box around ideal switch signifies limitation to particular one-quadrant operation;
10. CBS—Together with the rectangular box around the ideal switch and this symbol designates the Current Bi-directional Switch (CBS) as a three-terminal, controllable semiconductor switching device, which can conduct the current in either direction when turned ON, but blocks the voltage of only one polarity when turned OFF.
Switching Converter Categorizations
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 problems, 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 an

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