Electricity: power supply or regulation systems – Output level responsive – Using a three or more terminal semiconductive device as the...
Reexamination Certificate
2002-03-22
2003-11-11
Riley, Shawn (Department: 2838)
Electricity: power supply or regulation systems
Output level responsive
Using a three or more terminal semiconductive device as the...
C323S284000
Reexamination Certificate
active
06646426
ABSTRACT:
BACKGROUN OF THE INVENTION
1. Field of the Invention
The present invention relates to a current mode DC/DC converter provided with a current mode control circuit, serving as a feedback circuit for stabilizing an output voltage, which detects a current flowing through a choke coil and controls switching operation of a switching element according to a result of comparison between a detection signal due to the current and a reference signal.
2. Description of the Related Art
Generally, a DC/DC converter, specifically a current mode DC/DC converter with a current mode control circuit is one where a current flowing through a choke coil at an output side is detected and a DC output voltage supplied to a load is stabilized by controlling switching operation of a switching element according to a result of comparison between a detection signal due to the current and a reference signal.
FIG. 3
shows a circuit diagram displaying one example of such a current mode DC/DC converter. In the figure, reference symbol E denotes a DC source supplying an input voltage Vi and a series circuit of a switching element Q
1
and a free wheeling diode D
1
is coupled across the DC source E, while a series circuit of a choke coil L
1
and a smoothing capacitor C
1
is coupled across the free wheeling diode D
1
and a DC output voltage Vo generated in the smoothing capacitor C
1
according to switching operation of the switching element Q
1
is supplied to a load resister RL, that is a load, coupled between output terminals +V and −V. A current mode control circuit
51
, serving as a feedback circuit for stabilizing an output voltage Vo, is coupled, which is provided with; voltage divider resisters R
1
, R
2
, for detecting the output voltage, coupled between the terminals +V and −V, an error amplifier A
1
for amplifying an error between an output voltage detection signal output from a junction point of the voltage divider resisters R
1
, R
2
and a reference voltage Vref, a current detector
11
for detecting the current I
2
flowing through the choke coil L
1
, a current/voltage converter
21
for converting a detection current from the current detector
11
to a voltage, a comparator for outputting a reset pulse to turn off said switching element Q
1
when a voltage V
1
of a coil current detection signal supplied from the current/voltage converter
21
exceeds a voltage V
3
of an error signal serving as a reference signal output from an error amplifier A
1
, and a RS flip-flop circuit
26
for turning on the switching element Q
1
by a set pulse with cycle T output from an oscillator
25
and for turning off the switching element Q
1
by a reset pulse from the comparator A
2
.
In the circuit in
FIG. 3
abovementioned, when the switching element Q
1
turns on by the set pulse from the oscillator
25
, the free wheeling diode D
1
turns off so that an input voltage Vi is applied to the series circuit of the choke coil L
1
and the smoothing capacitor C
1
and so a coil current I
2
increases linearly with the lapse of time. As a result, when the coil current I
2
becomes greater than a current consumed by the load resister RL, that is, a load current Io, the smoothing capacitor C
1
is charged so that an output voltage Vo across the smoothing capacitor C
1
, in turn, across the load resister RL increases. On the other hand, in the current mode control circuit
51
, a voltage detection signal yielded by dividing the output voltage Vo with the voltage divider resisters R
1
, R
2
is compared with the reference voltage Vref by the error amplifier A
1
so that an error signal obtained by amplifying the magnitude of the error is fed to one side of input terminals of the comparator A
2
. Apart from this, the coil current I
2
flowing through the choke coil L
1
is detected by the current detector
11
so that the coil current detection signal corresponding to the coil current I
2
is fed from the current/voltage converter
21
to the other input terminal of the comparator A
2
. Then, the comparator A
2
compares the voltage V
1
of the coil current detection signal with the voltage V
3
of the error signal and when the voltage V
1
of the coil current detection signal exceeds the voltage V
3
of the error signal, a reset pulse is output from the comparator A
2
so that a voltage level of the output terminal is changed from a H (High) level to a L (Low) level to turn off the switching element Q
1
.
When the switching element Q
1
turns off, the free wheeling diode D
1
turns on so that energy already stored in the choke coil L
1
is discharged. As a result, the coil current I
2
of the choke coil L
1
decreases linearly with the lapse of time and when the coil current I
2
becomes less than the load current Io, electric charges are supplied from the capacitor C
1
to the load resister RL so that the output voltage Vo decreases. After the lapse of one cycle, the set pulse is generated from the oscillator
25
so that the switching element Q
1
turns on again and the coil current I
2
and the output voltage Vo begin to increase also again.
Thus, the output voltage Vo changes with ripples by switching the switching element Q
1
. However, an amplitude of the variation is almost negligible in comparison with the magnitude of the output voltage Vo and therefore the output voltage Vo can be regarded as substantially stable at a certain value. Further, at the time of ON state of the switching element Q
1
, when the coil current I
2
of the choke coil L
1
increases, one fourth of a cycle later, the output voltage Vo also begins to increase and at the time of OFF state of the switching element Q
1
, when the coil current I
2
of the choke coil L
1
decreases, one fourth of a cycle later, the output voltage Vo also begins to decrease. In other word, the coil current I
2
and the error signal from the output terminal of the error amplifier A
1
are mutually in proportional relationship.
FIG. 4
shows each of waveforms of, the load current Io at the time of steady state, the charge and discharge current I
1
of the capacitor C
1
and the coil current I
2
, in the circuit of abovementioned FIG.
3
. As described above, at the time of ON state of the switching element Q
1
, the coil current I
2
increases linearly and when the coil current I
2
becomes greater than the load current Io, a flowing direction of the charge and discharge current I
1
of the capacitor
1
is converted from that for discharge to that for charge. On the other hand, when the switching element Q
1
turns off, the coil current I
2
decreases linearly and when the coil current I
2
becomes less than the load current Io, the flowing direction of the charge and discharge current I
1
of the capacitor C
1
is converted from that for charge to that for discharge. At steady state, according to switching operation of the switching element Q
1
, the charge and discharge current I
1
and the coil current I
2
change with ripples. (see &Dgr;I
1
, &Dgr;I
2
, in FIG.
4
).
Now, in the DC/DC converter with abovementioned current mode control circuit
51
, due to delay of response or the like at the time of an abrupt change of a load current in control systems comprising the error amplifier A
1
and the comparator A
2
, there occurs a problem that stability of the output voltage Vo is lost so that the output voltage Vo changes in a great degree. Specifically, as shown in
FIG. 5
, if the output current Io increases abruptly, for example, at the time to, in the first place, electric charges corresponding to the abrupt increase are supplied to the load resister RL from the smoothing capacitor C
1
and next, the coil current I
2
, in turn, also the voltage V
1
of the coil current detection signal increases gradually. However, the current mode control circuit
51
, due to delay of response within the circuit, cannot immediately feeds a pulse drive signal, that is required to keep the output voltage Vo constant, to the switching element Q
1
so that the output voltage Vo decreases in a great degree immediately after an abrupt change in the lo
Densei-Lambda Kabushiki Kaisha
Riley Shawn
Senterfitt Akerman
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