Electric power conversion systems – Current conversion – With condition responsive means to control the output...
Reexamination Certificate
2001-10-17
2002-08-13
Riley, Shawn (Department: 2838)
Electric power conversion systems
Current conversion
With condition responsive means to control the output...
C323S222000, C361S091700
Reexamination Certificate
active
06434029
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to DC—DC converters and particularly, to an improved circuit topology for boost converters.
BACKGROUND OF THE INVENTION
A boost converter is a type of flyback converter where a smaller input DC voltage is increased to a desired level. A prior art typical boost converter
10
is shown in FIG.
1
. As shown in
FIG. 1
, the typical boost converter includes an inductor
15
, to which the input voltage Vin is coupled, that is in series with a boost diode
16
connected to an output capacitor
22
across which the load
19
is connected. A transistor switch
14
is connected to a node
12
between the inductor
15
and boost diode
16
and ground to provide regulation of the output voltage. The control circuit
20
for the transistor switch
14
typically includes a comparator (not shown) for sensing and comparing the output voltage of the converter to a voltage reference to generate an error voltage. This error voltage is then coupled to a duty cycle constant frequency pulse width modulator circuit (PWM). The PWM converts the error voltage into a control signal. A gate resistor
18
couples this control signal to the transistor switch
14
control input for controlling the timing of the on and off transition of the transistor switch
14
. When the transistor switch
14
is on, the inductor current increases, storing energy in its magnetic field. When the transistor switch
14
is off, energy is transferred via the diode
16
to the output energy storage capacitor
22
and the load
19
. The transistor switch
14
is operated at a high frequency relative to the resonance of the inductor capacitor network.
Drawbacks of such conventional boost converter circuits include the creation of switch voltage and current stresses resulting in low efficiency power conversion. Another drawback of switched power circuits is the electromagnetic interference (EMI) arising from the large change in current (di/dt) and voltage (dV/dt) that occurs when the switch changes state. More specifically, one drawback of the conventional boost converter circuit in
FIG. 1
is the recovery current of the boost diode
16
added to the power loss due to the discharge of the switch output capacitance, Coss, of the switch
14
(1/2 Coss V
2
f) at turn on. Increased EMI noise is also generated due to the snap off of the boost diode
16
after it stops conducting. Another drawback of the boost converter
10
is the losses at turn off of transistor switch
14
. The Coss of the transistor switch
14
is so low that the turn off loss is significant. Increasing this capacitor does not overcome this because the losses would only be transferred at turn on. To overcome these drawback, boost converters have been proposed that provide soft switching, i.e., switching at low voltage and current stress across the transistor switch. A prior art boost topology
30
to overcome the drawback at turn on of the main switch
14
is shown in FIG.
2
.
As shown in
FIG. 2
, an input voltage V
IN
is converted into output power (V
OUT
) using a resonant network in addition to the conventional components of a boost converter. The resonant network comprises a snubber inductor
32
, coupled in series with resonant diode
34
and a
36
. Auxiliary switch
38
and resonant diode
36
are in series and are connected in parallel with main switch
14
. The snubber inductor
32
, with a value significantly smaller than the boost inductor
15
, in conjunction with the auxiliary switch
38
is added to control the recovery current of the boost diode
16
at its turn off. This topology allows a zero voltage switching (ZVS) on the main switch
14
and a zero current switching (ZCS) on the auxiliary switch
38
. In operation, a ZVS detection circuit (included in control circuit
44
, details not shown) monitors the voltage across the main switch
14
to turn it on at zero volts. Snubber inductor
32
limits the current at turn on of the auxiliary switch
38
to achieve the ZCS.
A drawback exhibited by the boost topology of
FIG. 2
is that the energy in the parasitic capacitor of resonant diode
34
is transferred to the snubber inductor
32
at the turn off of the main switch
14
. This transfer results in a current in resonant diode
34
that turns resonant diode
36
on. When the auxiliary switch
38
is turned on, the recovery current of resonant diode
36
will generate a current spike that causes losses in the auxiliary switch
38
and increased EMI noise. The power lost through the auxiliary switch
38
reduces the efficiency of the boost converter. A boost topology
40
to overcome the drawback associated with turn off of the main switch
14
and auxiliary switch
38
is shown in FIG.
3
.
The boost topology
40
in
FIG. 3
adds a snubber capacitor
42
and resonant diode
44
to the topology shown in FIG.
2
. In operation, at turn off of the main switch
14
, the snubber capacitor
42
is already charged to the output voltage. As a result, the current circulates into snubber capacitor
42
and the resonant diode
44
to smooth the dv/dt across the main switch
14
. The snubber capacitor
42
will discharge to zero in order to turn the boost diode
16
on. At turn off of the auxiliary switch
38
, the series combination of the discharged snubber capacitor, resonant diode
36
and main switch
14
are in parallel with the auxiliary switch
38
and smooth the dv/dt. The snubber capacitor
42
will again be charged to the output voltage. For this operation, energy is only exchanged between the Coss of each switch via the snubber capacitor
42
, thus there is no additional energy dissipation. The topology of
FIG. 3
addresses the dv/dt at turn off the switches, however, a drawback exhibited by this topology is associated with losses due to the recovery current of resonant diodes
36
and
44
at turn ON of the auxiliary switch. A boost topology
50
to overcome this drawback is shown in FIG.
4
.
As shown in
FIG. 4
, a boost topology
50
adds an inductor bead
52
and a clamping circuit formed by diodes
54
and
56
to the topology of
FIG. 3. A
commonly assigned U.S. Pat. No. 6,236,191 ZERO VOLTAGE SWITCHING BOOST TOPOLOGY which is incorporated by reference herein. U.S. Pat. No. 6,236,191 discloses a topology similar to boost topology
50
without the clamping circuit. This topology adds the inductor bead
52
in conjunction with the slower resonant diodes
36
and
44
, to overcome the drawback of recovery current of those diodes at turn on of the auxiliary switch
38
. For this topology, resonant diode
34
is a fast recovery type diode, such that it stops conducting (and recovers) before the remaining resonant diodes. The remaining charges in the slower resonant diodes
36
and
44
begin to charge the parasitic capacitor of resonant diode
34
. In operation, resonant diodes
36
and
44
must be slow enough to ensure that resonant diode
34
recovers first, but fast enough to be recovered before the parasitic capacitor of resonance diode
34
is charged. If resonant diodes
36
and
44
are not recovered when the parasitic capacitor of resonant diode
34
is fully charged, a current spike will occur upon turn ON of the auxiliary switch
38
. This charge up of the parasitic capacitor of resonant diode
34
is completed by the parasitic capacitor, Coss, of the auxiliary switch
38
. However, because resonant diodes
36
and
44
are slower than resonant diode
34
, the parasitic capacitor, Coss, of the auxiliary switch
38
will not discharge as much as if the diodes were the same. This reduces the resonance between the snubber inductor
32
and Coss of the auxiliary switch which reduces the current in resonant diodes
36
and
44
.
As the first resonant diode
34
is a fast recovery type diode, it recovers the stored charge that is dissipated by the snubber inductor
32
and stops conducting the corresponding current before the second and third resonant diodes
36
,
44
recover their stored charges. In this fashion, the current flowing through the first resonant diode
34
a
Cyr Jean-Marc
Verreau Richard
Astec International Limited
Coudert Brothers LLP
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