Electricity: power supply or regulation systems – Output level responsive – Using a three or more terminal semiconductive device as the...
Reexamination Certificate
2001-05-18
2002-07-16
Sterrett, Jeffrey (Department: 2838)
Electricity: power supply or regulation systems
Output level responsive
Using a three or more terminal semiconductive device as the...
Reexamination Certificate
active
06420861
ABSTRACT:
FIELD OF THE INVENTION
The present invention pertains to a drive signal supply circuit for a switching regulator. More specifically, the present invention pertains to a drive signal supply circuit for a switching regulator characterized by the fact that it produces a stable output voltage even for deviations of the power source voltage.
BACKGROUND OF THE INVENTION
Usually, a regulator is used as a device for supplying a stable DC voltage to a load.
An example of a conventional switching regulator is the switching regulator
101
shown in FIG.
13
. Switching regulator
101
comprises switching transistor
102
, rectifying/smoothing circuit
180
, and controller
127
to be explained later.
Switching transistor
102
is an n-channel MOS transistor (hereinafter referred to as nMOSFET). Its drain terminal is connected to a power source voltage supply line that supplies power source voltage Vcc. Its gate terminal is connected to controller
127
to be explained later, and with this constitution switching transistor
102
can be turned ON/OFF corresponding to the output signal of controller
127
.
Rectifying/smoothing circuit
180
comprises rectifying MOSFET
103
, choke coil
105
, and smoothing capacitor
106
. The source terminal of switching transistor
102
is connected to one end of choke coil
105
. The other end of choke coil
105
is connected to one end of smoothing capacitor
106
, and, at the same time, this end is connected through output terminal
107
to one end of load
108
. The other end of the load is grounded. The other end of smoothing capacitor
106
is grounded.
Rectifying MOSFET
103
is formed by an nMOSFET. Its drain terminal is connected to the source terminal of switching transistor
102
, its source terminal is grounded, and its gate terminal is connected to controller
127
. Its constitution is dependent on the output signal of controller
127
, such that it is OFF when switching transistor
102
is ON, and it is ON when switching transistor
102
is OFF. Also, in the figure,
104
represents the internal parasitic diode of rectifying MOSFET
103
.
In said switching regulator
101
, when rectifying MOSFET
103
is OFF, switching transistor
102
is turned from OFF to ON. As a result, choke coil
105
is connected through switching transistor
102
to the power source voltage supply line, current flows in choke coil
105
, and this current also flows through output terminal
107
into load
108
.
In this state, if switching transistor
102
is turned OFF and rectifying MOSFET
103
is turned ON, an electromotive force is generated between the two terminals of choke coil
105
. Due to this electromotive force, a negative voltage is asserted on the drain terminal of rectifying MOSFET
103
. As a result, parasitic diode
104
inside rectifying MOSFET
103
is forward-biased, and the energy stored in choke coil
105
is supplied to load
108
.
The ON/OFF state of said switching transistor
102
is controlled by the voltage output from driver
125
of controller
127
to be explained later, and switching transistor
102
is turned ON/OFF repeatedly. Here, the potential of output terminal
107
varies correspondingly. However, because smoothing capacitor
106
is connected in parallel to load
108
, in company with the ON/OFF switching, there is repeated charging/discharging of said smoothing capacitor
106
, so that the potential at output terminal
107
is smoothed. The smoothed voltage is output from output terminal
107
as an output voltage across load
108
. While this output voltage is asserted on load
108
, it is also input to controller
127
.
Controller
127
comprises voltage dividing circuit
122
, reference voltage generating source
119
, error amplifier
111
, comparator
112
, sawtooth wave generating circuit
113
, driver
125
, and negative feedback circuit
128
. Voltage dividing circuit
122
is composed of two resistors
121
1
and
121
2
which are connected in series between output terminal
107
and ground. The output voltage is input to voltage dividing circuit
122
, and the output voltage is divided according to the resistance ratio of resistors
121
1
and
121
2
to generate a sampling voltage which is input to the inverting input terminal of error amplifier
111
. Reference voltage generating source
119
is connected to the non-inverting input terminal of error amplifier
111
, so that reference voltage Vref is input from reference voltage generating source
119
. Between the output terminal of error amplifier
111
and the inverting input terminal, there is negative feedback circuit
128
formed by a series circuit of a resistor and a capacitor. From error amplifier
111
, the voltage of the error difference between reference voltage Vref and the sampling voltage is amplified by a prescribed gain determined by the impedance of negative feedback circuit
128
, and it is then output.
The output voltage of error amplifier
111
is input to the non-inverting input terminal of comparator
112
. A sawtooth wave is input from sawtooth wave generating circuit
113
to the inverting output terminal of comparator
112
. Comparator
112
compares the output voltage of error amplifier
111
and the sawtooth wave, and it outputs a pulse signal which defines the ON period of switching transistor
102
.
Said driver
125
turns switching transistor
102
ON/OFF corresponding to the pulse signal. When the output voltage rises higher than a prescribed voltage value, the output is lowered. On the other hand, when the output voltage falls lower than a prescribed voltage value, the output voltage is raised by means of driver operation. By means of this operation, it is possible to keep the output voltage at a prescribed level.
Because the response speed of error amplifier
111
in said conventional switching regulator
101
is slow, when there are rapid fluctuations in power source voltage Vcc, it becomes impossible for error amplifier
111
to respond to these fluctuations, and the output voltage becomes unstable.
In
FIG. 14
, curves (J)-(M) show the operation waveforms of the various circuits that form conventional switching regulator
101
when power source voltage Vcc is constant.
Curve (J) represents the waveform of the sawtooth wave output from sawtooth wave generating circuit
113
; curve (K) represents the output waveform of error amplifier
111
; curve (L) represents the output waveform of comparator
112
; curve (M) represents the waveform of the source terminal of switching transistor
102
.
FIG. 15
is a diagram illustrating the operations of the various circuits when the power source voltage Vcc falls rapidly. In
FIG. 15
, curve (N) represents the waveform of power source voltage Vcc that falls rapidly; curve (O) represents the waveform of the output voltage from comparator
112
; curve (P) represents the waveform of the source terminal potential of switching transistor
102
; curve (Q) represents the waveform of the sawtooth wave; curve (R) represents the output waveform of error amplifier
111
; and curve (S) represents the waveform of the output voltage.
As can be seen, when the power source voltage Vcc falls rapidly according to curve (N), error amplifier
111
cannot respond to such a rapid change. Consequently, the output of error amplifier
111
becomes unstable, so that the output voltage also becomes unstable as shown in curve (S). It becomes stable again after the transition period T
1
shown in FIG.
15
.
FIG. 16
illustrates the operations of the various circuits when power source voltage Vcc rises rapidly. In
FIG. 16
, curve (T) represents the waveform of power source voltage Vcc that rises rapidly, and curve (U) represents the waveform of the output voltage of comparator
112
. Curve (V) represents the waveform of the source terminal potential of switching transistor
102
; curve (W) represents the waveform of the sawtooth wave; curve (X) represents the waveform of the output of error amplifier
111
; and curve (Y) represents the waveform of the output voltage.
As can be seen, when power source voltage Vcc rises rap
Miyazaki Takahiro
Ochi Taizo
Brady W. James
Kempler William B.
Sterrett Jeffrey
Telecky , Jr. Frederick J.
Texas Instruments Incorporated
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