Electricity: power supply or regulation systems – Output level responsive – Using a three or more terminal semiconductive device as the...
Reexamination Certificate
2002-08-20
2004-06-08
Berhane, Adolf (Department: 2838)
Electricity: power supply or regulation systems
Output level responsive
Using a three or more terminal semiconductive device as the...
Reexamination Certificate
active
06747441
ABSTRACT:
TECHNICAL FIELD
The present invention relates to electrical circuits and more particularly to direct current (DC) to direct current (DC) power conversion and regulation.
BACKGROUND OF INVENTION
There is an ever increasing demand for power conversion and regulation circuitry to operate with increased efficiency and reduced power to accommodate the continuous reduction in size of electronic portable devices. Many times these devices are battery powered, and it is desirable to utilize as little power as possible to operate these devices, so that the battery life is extended. Therefore, the prior 5-volt industry standard has decreased to a 3.3 volt industry standard, which may soon be replaced by an even lower standard. Voltage regulators have been implemented as an efficient mechanism for providing a regulated output in power supplies. One such type of regulator is known as a switching regulator or switching power supply, which controls the flow of power to a load by controlling the on and off duty-cycle of one or more power switches coupled to the load. Many different classes of switching regulators exist today. One class of switching regulators is known as non-synchronous buck switching regulators. Non-synchronous buck switching regulators are subject to operating in a discontinuous mode under light or no load conditions. This undesirable result occurs when the current through the inductor is reduced to zero or near zero and then tends to stay at zero or near zero.
FIG. 1
illustrates a conventional switching regulator
10
(e.g., a non-synchronous buck converter). The switching regulator
10
includes a control circuit
12
that is operative to control the duty cycle of pulses provided to a power switch
14
through a driver
16
. The control circuit
12
provides a pulse wave signal that is inverted by the driver
16
. In the illustration of
FIG. 1
, the power switch
14
is an N-type MOSFET device. In order to turn on the N-type MOSFET device, the gate must be pulled higher than the source. A capacitor
18
, referred to as a bootstrap capacitor or a boot cap, is coupled to the source of the power switch
14
and a diode
20
. The diode
20
is also coupled to V
IN
. The common node of capacitor
18
and the diode
20
is labeled BOOT
1
and is also coupled to the supply input of driver
16
and to a resistor
22
. The resistor
22
is representative of the load placed on the boot capacitor
18
by one or more level shifters (not shown) of the driver
16
. The other end of resistor
22
is coupled to a node labeled PH
1
, the driver
16
, and the source of power switch
14
. The node PH
1
is also coupled to the capacitor
18
, an inductor
24
and a diode
26
.
In order to turn on the power switch
14
, an N-type MOSFET device, the gate must be pulled higher than the source. When V
PH1
is pulled to V
IN
through power switch
14
, V
BOOT1
will be pulled to approximately 2*V
IN
. If V
BOOT1
is approximately 2*V
IN
, then the supply input to driver
16
will be at 2*V
IN
allowing the output from the driver
16
and the gate of power switch
14
to be pulled higher than the source. When the input to the gate of power switch
14
is high, the source to drain input impedance will be low and the voltage at node PHI (V
PH1
) will be approximately equal to V
IN
. When V
PH1
, is approximately equal to V
IN
, the inductor current I
L1
through inductor
24
will begin to increase. I
L1
continues to increase until V
PH1
, changes.
When the output of the control circuit
12
goes high, the output of driver
16
goes low and the power switch
14
turns off. Since the current I
L1
through inductor
24
tends to remain unchanged, V
PH1
will be pulled below ground so that current I
L1
can be supplied through diode
26
. At low loads, I
L1
decreases until it reaches approximately zero. When I
L1
, reaches approximately zero, V
PH1
will approximately equal to V
OUT1
, the voltage across a capacitor
28
and a load resistor
30
. The inductor
24
tries to maintain I
L1
equal to zero. With I
L1
, equal to zero and with no source driving node PH
1
, V
PH1
and V
OUT1
will ring (fluctuate up and down) until the next switching cycle when the power switch
14
is again turned on.
When I
L1
is equal to zero and the ringing described above occurs, the circuit is said to be operating in a discontinuous mode. The current through the inductor
24
is in the form of a triangle wave, increasing when power switch
14
is on and decreasing when power switch
14
is off. This triangle waveform is known as the ripple current. The decreasing portion of the triangle waveform is known as the reverse current. When an adequate minimum load exists, the current through the inductor will not reach zero because the triangle waveform (the ripple current) resides on top of a nominal load current level. However, under light or no load conditions, the inductor current I
L1
can reach zero when the negative ramp portion of the triangle reduces to zero. When this occurs, the circuit is said to be operating in the discontinuous mode and the fluctuating voltage (ringing) problems described above will occur.
FIG. 2
is a plot
40
of the voltage at node PHI (V
PH1
) versus time and a plot
42
of node BOOT
1
(V
BOOT1
) versus time. These plots are merely representative of the type of ringing and voltage fluctuations that can appear on these nodes and are not scale drawings with respect to frequency or amplitude. The scale has been altered to illustrate the problems herein discussed. Looking first to the plot
42
, it can be seen that at T1 the voltage V
PH1
is approximately equal to V
IN
when power switch
14
is turned on. At T2, power switch
14
turns off. Since I
L1
tends to remain unchanged, V
PH1
is initially pulled below ground as diode
26
supplies I
L1
. I
L1
decreases until reaching zero and after initially being pulled below ground, V
PH1
, begins to ring and fluctuate above and below V
IN
until T3, when power switch
14
is again turned on and pulls V
PH1
, to V
IN
.
At the same time, V
BOOT1
exhibits similar behavior. At T2, when power switch
14
is turned off, V
BOOT1
rings and fluctuates in voltage along with V
PH1
. At T3, when power switch
14
is turned on, V
BOOT1
stops ringing and the voltage at V
BOOT1
is initially equal to 2*V
IN
, but gradually decreases. With each successive cycle, the initial voltage of V
BOOT1
is slightly lower than the previous cycle and continues to decline during the period in which power switch
14
is on. V
BOOT1
continues to reduce each cycle until the boot cap
18
is eventually discharged and proper operation ceases.
FIG. 3
is a corresponding plot
44
of the output voltage V
OUT1
during this same time. V
OUT1
fails to regulate properly and continues to float higher until it eventually climbs to a value equal to V
IN
, about twice the desired regulated output voltage. However, if the load current is greater than a minimum value, then I
L1
will not reduce to zero and the problems discussed above are avoided. For this reason, non-synchronous regulators are often limited to uses where there is a guaranteed minimum load. Synchronous regulators can be used in light load applications, however, synchronous regulators are often more expensive to produce.
SUMMARY OF INVENTION
The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the invention. It is intended to neither identify key or critical elements of the invention nor delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later.
The present invention relates to circuits and a method for providing a regulated output voltage at light or low loads from an unregulated input voltage, V
IN
. A switching power supply or switching regulator (e.g., non-synchronous buck converter) is provided with a control circuit (e.g., a pulse width modulator, FM mo
Fowler Thomas L.
Johnson Alan Michael
Berhane Adolf
Brady W. James
Swayze, Jr. W. Daniel
Telecky , Jr. Frederick J.
Texas Instruments Incorporated
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