Loss reduction circuit for switching power converters

Details Loss reduction circuit for switching power converters Loss reduction circuit for switching power converters

2000-09-30

2002-01-22

Berhane, Adolf Deneke (Department: 2838)

Electric power conversion systems

Current conversion

Including automatic or integral protection means

C363S056090

Reexamination Certificate

active

06341076

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to DC-DC power converters, and, more specifically, to an active snubber circuit, method of operation thereof and power converter employing the same. The present invention is directed, in general, to pulse width modulated DC-DC power converters which process power from an input DC voltage source and deliver power to a load through an inductive energy storage component being alternatively connected to the input DC power source and to the load via electronic solid state switches.
BACKGROUND OF THE INVENTION
It is known from the art that increasing the operational frequency of DC-DC power converters results in reduction of total weight, size and cost as well as in increase of converted power density, i.e. number of watts per cubic inch.
However, the solid-state switches of the DC-DC power converters are subjected to high power losses as a result of the switch being changed from one state to another (i.e. when the switch turns on or off) while having simultaneously overlapped both a significant current through it and significant (exceeding the primary source level) voltage across it. This results in extensive heat dissipation within the switch during the switching transitions.
For “off-on” transition the switching losses may be defined as:
W
on
=0,5V
sm
I
sm
t
on
;  [1]
for “on-off” transition the switching losses may be defined as:
W
off
=0,5V
sm
I
sm
t
off
;  [2]
where: V
sm
is a voltage-across maximum value during the transition,
I
sm
is a current-through maximum value during the transition,
t
on
is a time-duration of the “off-on” transition,
t
off
is a time-duration of the “on-off” transition.
The switching transition losses evolve substantial constraints upon the potentially available performance rate of existing DC-DC power converters wherein the bipolar junction transistors (BJT), insulated gate bipolar transistors (IGBT) and metal-oxide-semiconductor field-effect transistors (MOSFET) are used as power switches.
Fast switching speeds, low power gate drive and state-on low resistance of MOSFETs have made them a wide practical choice. However, the MOSFETs exhibit large drain-to-source capacitance C
oss
. It reduces the dV/dt factor on turnoff and minimizes the power loss at this transition but increases the power loss at tum-on transition since the power stored in C
oss
is fully dissipated as heat within the MOSFET, which may be defined as:
P
on
=0,5C
oss
V
2
sm
f
op
,  [3]
where: f
op
is an operational frequency value.
To reduce the switching transition power losses within the DC-DC power converters, the prior art brought forward numerous passive, i.e. comprising the inductive and capacitive components only, and active, i.e. comprising solid-state semiconductor devices, snubber circuits optimally shaping the operating points trajectories of the switching devices, i.e. adjusting the shape-of-change of the voltage-across and of the current-through to minimize their simultaneous overlapping during the switching transition.
Passive snubbers are hardly attractive since the power absorbed within their passive components is dissipated as heat. Active snubbers are more efficient since the absorbed power may be re-circulated back to the primary source or forwarded to the load.
Shaping the operating points trajectories of the power switches becomes extremely important function with increasing the operational frequency, operational voltages and overall power conversion output.
As well as the power switches of the DC-DC power converters, the switching devices within prior art active snubbers are also subjected to power losses ascribed with [1] and [2].
Minimizing these “snubber” losses is no less important function both for high and low rates of power conversion since in the latter case the “snubber” losses may be in the row with power conversion output.
Therefore, a better method and apparatus for active shaping the operating points trajectories both of the power switches within the DC-DC power converters and of the switching devices within the apparatus itself is needed to be applicable for use in various DC-DC power converter topologies.
SUMMARY AND ADVANTAGES OF THE INVENTION
The benefits of the proposed invention may be better disclosed through prior appraisal of the state-of-the-art snubber circuits.
Although the present invention may be applicable equally to many existing DC-DC power converter topologies, the boost converter topology is chosen as an example to demonstrate the advantages of the present invention.
The output voltage of the boost converter is always higher than the peak value of the mains voltage, and would be typically between 300 and 400 volts. At these high voltage levels the switching transition losses are unavoidably great, and transient voltage and current spikes may well damage the solid-state semiconductor devices. For this reason a fast-recovery blocking rectifier is required. At a high operational frequency, a fast-recovery rectifier is subjected to substantial reverse-recovery current and, therefore, produces significant reverse-recovery loss when operated under a “hard switching” condition, i.e. when simultaneous overlapping of non-zero-voltage-across with nonzero-current-through during the switching transition.
Besides, as being galvanically non-isolated of the primary power source, the boost converters are quite sensitive to reverse-recovery sufficiency to prevent the internal components of electric shoot-through destruction. As a result, the “hard-switched” DC-DC power converters are operated at relatively low switching frequencies.
To reduce the switching transition losses while increasing the switching frequency and, therefore, to improve the efficiency of DC-DC power conversion, a number of “soft-switching” techniques have been proposed within the prior art.
“Soft-switching” condition occurs when no voltage appears across the switch and/or no current flows through the switch during the switching transition.
Turning the power switch into conducting state at zero voltage across it (ZVS=Zero Voltage Switching) results in elimination of two kinds of switching transition losses: the first, caused by blocking rectifier reverse-recovery loss as defined in [2] and, the second, caused by the power switch stray capacitance recharge as defined in [3].
Turning the power switch into nonconducting state at zero current through it (ZCS=Zero Current Switching) results in elimination of inductively stored power loss which may be defined as:
P
off
=0,5LI
sm
f
op
,  [4]
where: L is an inductance value of the power storage inductor.
FIG. 1
illustrates the circuit diagrams of DC-DC power converters comprising some of the prior art snubber circuitry for soft-switching conditions provision and for switching transition loss reduction, and indexed structures are as follows:
200
: prior art snubber;
209
: controllable commutator;
210
: controllable commutating switch within the controllable commutator
209
;
211
: rectifier within the first commutator
209
;
213
: commutating rectifier;
215
: damp rectifier;
216
: resonant inductor;
L
H
: auxiliary saturable inductor;
217
: first slope-shaping capacitor;
218
: second slope-shaping capacitor;
304
: main power switch;
305
: power storage inductor;
306
: blocking rectifier;
307
: output smoothing filter;
308
: primary power source;
309
: load.
These prior art techniques utilize an auxiliary active commutator
209
together with a few passive components like resonant inductor
216
and voltage slope-shaping capacitors
217
,
218
thus forming an active snubber to limit the rate-of-change of blocking rectifier
306
current (di
306
/dt) and to create the soft-switching conditions for the main power switch
304
. As a result the main power switch
304
is tuned-on into conducting state under zero-voltage across it. However, the auxiliary active commutator
209
shown in FIG.
1
(
a
) operates under hard-switchi

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