Electricity: power supply or regulation systems – Output level responsive – Zero switching
Reexamination Certificate
2001-10-15
2003-01-14
Nguyen, Matthew (Department: 2838)
Electricity: power supply or regulation systems
Output level responsive
Zero switching
C323S315000
Reexamination Certificate
active
06507175
ABSTRACT:
BACKGROUND OF THE INVENTION
1) Field of the Invention
This application relates generally to electronic circuits and more particularly to an electronic circuit for detecting a zero current condition, where such a circuit can be used in voltage regulators and switching power converters (“SPC”), including multiphase power converters.
2) Background
Power regulators are often used in electronic equipment to supply power at a predetermined voltage to a system. For example, a typical desktop computer may contain a power supply that converts alternating current (“AC”) from a wall socket, to direct current (“DC”) with a voltage that is usable by the various components of the computer system. With continued reference to computer systems, a hard disk drive may require a 12 volt (“V”) power input, while various integrated circuit components may require, for example, power at 5.0 V, 3.3 V, or 1.5 V. A power supply must thus contain power regulators to generate the required voltage levels.
Buck power regulators are often used to generate power outputs for microelectronic devices because they are relatively efficient and provide high current stewing (di/dt) capability. When providing a microprocessor with a regulated input voltage, di/dt and response time are very important considerations. The output inductor value of the regulator determines the di/dt capability of the regulator and also the boundary between continuous conduction mode (“CCM”) (when the inductor current is continuous) and discontinuous conduction mode (“DCM”) (when the inductor current is not continuous, but drops to zero until the transistor is turned ON; DCM typically occurs when a low load resistance is coupled to the buck power regulator).
With reference to
FIG. 1
, an exemplary buck (step-down) power regulator
100
, which converts a DC voltage to a lower voltage, is presented. A supply voltage, V
s
, is input into transistor
102
, which is coupled to a diode
104
that, in turn, is coupled to ground. Coupled to the junction of transistor
102
and diode
104
is an LC circuit comprising an inductor
106
and a capacitor
108
. A load
110
thus receives power at the required voltage, where the voltage is determined by the duty cycle of transistor
102
(i.e., the percentage of time when transistor
102
is turned on).
When transistor
102
is on, inductor
106
is being charged and the supply voltage supplies the output current. When transistor
102
is turned off, inductor
106
“freewheels” through diode
104
and supplies the energy to load
110
. The purpose of the diode is not to rectify, but to re-direct current flow in the circuit and to ensure that there is a path for the current from the inductor to flow. Capacitor
108
serves to reduce the ripple content in the voltage, while inductor
106
smoothes the current passing through it.
A problem of the buck power regulator is that, as low voltage outputs are required, the voltage drop of diode
104
leads to various consequences. For example, the circuit becomes less efficient because of the voltage drop of approximately 0.7 volt across the diode. Such inefficiencies become less tolerable when devices run on battery power as opposed to AC power.
In response to the above deficiencies, buck power regulator
200
, detailed in
FIG. 2
, was developed. As can be seen, buck power regulator
200
is similar to buck power regulator
100
, with a transistor
204
replacing diode
104
. Transistor
204
may be configured to have a low on resistance. Transistor
102
is usually termed the high-side switch and transistor
204
is the low-side switch. In addition drivers
222
and
224
control the operation of transistors
102
and
104
, respectively. By controlling the on and off cycles of transistors
102
and
204
, drivers
222
and
224
are able to more efficiently control the output voltage, V
out
, that is present at load
110
, and supply the desired amount of current.
In normal operation of a power converter, there is a ripple in the output current, due to the charging and discharging of inductor
106
. One method of reducing the ripple of the output current is the use of a multiphase power supply. Instead of having, for example, a single source supplying a 20 amp output, there may be four phases, each of which supply 5 amps. An exemplary multiphase buck power converter is shown in FIG.
12
.
In multiphase power converter
1200
, it is desired to convert an input voltage
1202
to an output voltage
1204
across a load
1206
. In a manner similar to that described above with respect to
FIG. 2
, transistors
1212
and
1214
are each coupled to the input voltage
1202
. Coupled to the junction
1211
of transistors
1212
and
1214
is inductor
1216
. Similarly, transistors
1222
and
1224
are each coupled to the input voltage
1202
. Coupled to the junction
1221
of transistors
1222
and
1224
is inductor
1226
. Similarly, transistors
1232
and
1234
are each coupled to the input voltage
1202
. Coupled to the junction
1231
of transistors
1232
and
1234
is inductor
1236
. Similarly, transistors
1242
and
1244
are each coupled to the input voltage
1202
. Coupled to the junction
1241
of transistors
1242
and
1244
is inductor
1246
. Each of the transistor pairs is coupled to capacitor
1208
to provide the output needed at output
1204
. Because of the presence of four power converters, each converter is only responsible for one-fourth of the total current needed, resulting in smaller transistors and inductors and a corresponding reduction in cost. In addition, the ripple in the output current is reduced because each of the converters is only responsible for a portion of the output current. The phases are slightly offset from each other such that the peak current of each individual phase do not coincide with each other. This is shown in
FIG. 15
, which shows the individual output currents for each phase as well as the total output current. As can be readily seen, the ripple in the output current is substantially reduced from the ripple in the current of each individual phase, and the period of the ripple is approximately one-fourth of the ripple of each individual phase.
FIG. 3
presents a plot of the inductor current of an exemplary buck power regulator. Axis
302
represents the passage of time, while axis
304
details the current flowing through inductor
106
. The current flowing through inductor
106
rises for the time period T
on
when transistor
102
is on and the current falls during time period T
off
, when transistor
102
is off. The period, T, is T
on
plus T
off
. The output voltage would be the input voltage times T
on
.
Problems may arise, however, when buck power regulator
200
is required to produce a voltage through a smaller load. An exemplary resulting current plot is shown in FIG.
4
. It can be seen that the current through inductor
106
becomes negative during a portion of the cycle, i.e., the current through inductor
106
reverses direction and flows into the ground. This behavior is undesirable because of the various inefficiencies that occur because the inductor is basically wasting power that would ideally remain in the system. Such a problem may not be present in buck power regulator
100
of
FIG. 1
, as diode
104
automatically “turns off” when the polarity of the inductor current changes.
It is desirable to develop a method and apparatus for converting voltage that alleviate the above and other problems that may be present in the prior art.
SUMMARY OF THE INVENTION
The present invention uses a Zero Current Detection (“ZCD”) circuit to determine the direction of current flow in various circuits, such as a switch of a switching power converter (“SPC”). In such a manner, once zero current is detected, the operation of the circuit can be changed such that inefficiencies are reduced.
In one embodiment, the ZCD circuit may comprise a pair of current mirrors supplying current to a matched pair of transistors. One of the transistors is coupled to ground while the other transistor is coupled to the node of in
Goodfellow Ryan
Susak David
Nguyen Matthew
Primarion, Inc.
Snell & Wilmer
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