Electricity: power supply or regulation systems – In shunt with source or load – Using choke and switch across source
Reexamination Certificate
2002-10-15
2004-09-28
Sterrett, Jeffrey (Department: 2838)
Electricity: power supply or regulation systems
In shunt with source or load
Using choke and switch across source
C323S266000, C323S271000, C323S282000
Reexamination Certificate
active
06798177
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
1. Field of the Invention
The present invention generally relates to power converters and more particularly a cascaded converter capable of storing energy in a reservoir capacitor and insulating pulsating load currents from a primary energy source, such as a battery.
2. Description of the Related Art
Many of today's wireless phones or cellular phones transmit signals in a time-division multiple access (TDMA) scheme such as GSM (Global System for Mobile Communications) or GPRS (General Packet Radio Signal). Up to eight phones share a frequency band and each phone transmits signals in a burst.
However, the operation of a TDMA phone causes a pulsating load current to be drawn from a battery source. Due to high source resistance in a battery, such as a lithium-ion battery, a pulsating load current can cause a severe voltage droop problem. In addition, pulsating load current aggravates the power loss caused by the source resistance.
FIG. 1
shows a power system for a typical cellular phone handset. A lithium-ion battery
11
supplies an output voltage VCC
13
directly to an RF Power Amplifier (PA)
14
. The battery output voltage VCC
13
also supports several linear regulators such as regulators
15
,
16
, for a digital signal processor and a flash memory, respectively. A typical lithium-ion battery for cellular phone has a substantial source resistance. A resistance of about 0.2&OHgr;, modeled by resistor Rs
12
, includes the source resistance and other resistances such as fuse and battery contacts.
FIG. 2A
shows the waveform of a pulsating load current drawn by a typical power-amplifier
14
for the above mentioned applications. The power-amp
14
draws about 2 Amperes (A) for about 580 microseconds (&mgr;S) in burst transmission period (with a transition time of about 10 &mgr;S), and rests, in an idle period, for about 4060 &mgr;S. Thus, there are eight such transmission periods within the total cycle time of 4640 &mgr;S, and the duty cycle of each burst is one eighth. For proper operation of the power amplifier PA
14
, it is important for the voltage on node
13
to have small ripple (typically less than 0.3 V). In addition, the RF PA
14
requires a minimum operating voltage of 3.3 V to assure sufficient transmission power and communication quality.
FIG. 2B
shows the waveform of the VCC node
13
, whose voltage drops from about 3.5 V to 3.1 V because of the 2 A peak current that is required during a transmission interval. The 0.4V ripple exceeds the ripple requirement of 0.3 V for the RF PA
14
. Furthermore, whenever the battery voltage falls below 3.7 V, VCC drops below 3.3 V during a 2 A load pulse. In other words, battery
11
cannot be used to support either the RF PA
14
or the LDO
16
if its voltage drops below 3.7 V, leaving as much as 40% of the battery's total energy unusable.
The power loss for a pulsating current, such as is shown in
FIG. 2A
, is the product of the source resistance Rs and the square of the Root-Mean Square (RMS) value of the current. In this case, the power dissipated by Rs is approximately ({square root over (2
2
/8)})
2
×0.2&OHgr;=0.1 W. Since the power consumed by the load is (2/8)A×3.3 V=0.825 W, the power loss from Rs
12
amounts to about 12% of the load power.
Another drawback for a power amplifier operating on an unregulated voltage is wasted power. When a lithium-ion battery is fully recharged, it has a nominal 4.2V output voltage. Driving the PA with 4.2 V consumes,
(
4.2
2
/
8
)
2
1.65
Ω
,
or about 1.34 W over an entire transmission cycle, i.e., from the beginning of T
0
to the beginning of T
2
in FIG.
2
A. Compared with the 0.825 W that is actually required during the transmission, about 63% of battery power is wasted in over-driving the power amplifier.
FIG. 3
shows a prior-art circuit that employs a large storage capacitor to reduce the effects of pulsating load current on a battery source with a substantial source resistance. A 4700 &mgr;F capacitor
35
, having an equivalent series resistance (ESR)
36
of about 50 m&OHgr;, is connected in parallel with the lithium-ion battery
31
, which has a 0.2&OHgr; internal resistance
32
. The energy stored in capacitor
35
provides a low impedance energy source for the pulsating current of the RF power amplifier
34
and helps to reduce the voltage droop of the battery output voltage
33
.
FIG. 4A
shows the waveforms of the current drawn by the PA
34
in FIG.
3
and the current supplied by the battery
31
. In particular, waveform
41
shows the PA
34
drawing
2
A during the 580 &mgr;S transmission interval and no current outside of the interval. Waveform
42
shows the current supplied by the battery
31
. The difference between the two waveforms is the current supplied by the capacitor
35
. As is clear from the figure, the addition of capacitor
35
reduces the ripple and RMS value of the battery output current.
FIG. 4B
shows the waveform
43
of the battery output voltage
33
with the additional capacitor
35
. The battery output voltage ripple is reduced to about 0.2 V. Starting at time T
0
, a 2 A current flows causing about 0.08 V to be dropped across the 50 m&OHgr; ESR
36
of capacitor
35
. Between T
0
and T
1
, capacitor
35
provides the most of the load current to PA
34
. Battery current increases gradually from about 0.4 A at T
0
to about 1.05 A at T
1
, and VCC
33
drops further from 3.42 V to about 3.3 V at T
1
.
At T
1
, the load current of PA
34
drops to zero. Voltage VCC
33
jumps back to 3.38 V (due to the ESR effect). Battery output current now gradually recharges capacitor
35
back to 3.5 V at T
2
to prepare for another current pulse at T
2
.
It is clear that adding a large capacitor in parallel to a battery reduces the ripple voltage to less than 0.2 V, and extends the usable battery voltage range to about 3.5 V from the previous 3.7V. However, a 4700 &mgr;F capacitor adds significantly to the cost of the system. Such a capacitor is bulky and requires a large amount of PC board space. Furthermore, adding a large capacitor will not reduce wasted power when the battery has a high voltage (greater than 3.8V).
Thus, there is a need for a method and apparatus that uses a much smaller capacitor to reduce the ripple voltage of a RF power amplifier in a cellular phone handset, that regulates the supply voltage to the PA at 3.3V, and that avoids over-driving the PA when the battery voltage is substantially higher than 3.3V.
BRIEF SUMMARY OF THE INVENTION
The present invention is directed to these needs. A method in accordance with the present invention includes providing a voltage on a reservoir capacitor by converting a voltage and current provided by a primary power source, and maintaining an average constant current supplied to the reservoir capacitor, where the capacitor voltage is greater than the power source voltage. The method further includes, while maintaining average constant current to the reservoir capacitor, converting the capacitor voltage to a predetermined output voltage for the load device, and maintaining the predetermined output voltage substantially constant, while providing a current pulse to the load device.
An apparatus in accordance with the present invention includes a reservoir capacitor, a first converter stage and a second converter stage. The reservoir capacitor is used to store energy obtained from a primary power source. The first converter stage is configured to convert the primary power source voltage to a voltage on the reservoir capacitor while maintaining a substantially constant average current to the reservoir capacitor. The second converter stage is configured to convert the capacitor voltage to an output voltage for a load device, while maintaining the output voltage substantially constant and providing a current pulse to the load device. According to a version of the invention, the first converter stage is a boost converter and the second converter stage is a buck converter. Between the two converters is
Hsieh George
Liu Kwang H.
Negru Sorin L.
Shih Fu-Yuan
Arques Technology, Inc.
Dechert LLP
Diependbrock, III Anthony B.
Sterrett Jeffrey
LandOfFree
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