Abstract:
A power converter including a first stage, a reservoir capacitor and a second stage. The first stage converts the voltage of a primary energy source, such as a battery, to a voltage on the reservoir capacitor, which stores a large amount of energy in the form of a voltage substantially larger than the voltage of the primary energy source. The second stage converts the voltage on the reservoir capacitor to a substantially constant voltage for a load device that demands current having the form of large, short, current pulses. The cascaded converter prevents the pulsating load currents of the load device, such as a GSM power amplifier, from causing a severe voltage loss at the battery. This increases the power available from the battery and reduces loses from the internal resistance of the battery.

Description:
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&#39;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 0.2Ω, 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 (μS) in burst transmission period (with a transition time of about 10 μS), and rests, in an idle period, for about 4060 μS. Thus, there are eight such transmission periods within the total cycle time of 4640 μS, 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&#39;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 ×0.2Ω=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 μF capacitor  35 , having an equivalent series resistance (ESR)  36  of about 50 mΩ, is connected in parallel with the lithium-ion battery  31 , which has a 0.2Ω 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 μS 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 mΩ 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 μF 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 the reservoir capacitor. The present invention uses relatively small reservoir capacitor to store energy at an elevated voltage. The boost converter stage essentially stretches the pulsating load current to almost a constant current (having a very small RMS to DC ratio), thus, greatly reducing the voltage droop and power loss across the source resistance of the battery. 
     The buck converter stage is capable of regulating the VCC voltage of the PA, by drawing power from the reservoir capacitor. Since the reservoir capacitor can store a relatively large amount of energy in the form of elevated voltage, the pulsating load current will only produce a large voltage droop on this reservoir capacitor, not on the battery. 
     One advantage of the present invention is that a much smaller energy-storage capacitor can be used. 
     Another advantage is that the load current waveform is changed from a high peak, low duty cycle waveform to a nearly constant waveform. 
     Yet other advantages are that (i) the power loss from the battery source resistance is reduced, thereby improving efficiency, (ii) the usable battery voltage is extended from 3.5 V to 2.8 V, which increases usable battery life, (iii) talk time is extended without over-driving the PA when the battery voltage is substantially higher than the 3.3V that is required by the PA, and (iv) increased power and thus communication range are available in an emergency situation, by setting PA&#39;s VCC to 4.0 Volts. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features, aspects and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where: 
     FIG. 1 shows a typical GSM power amplifier drawing a pulsating current from a lithium-ion battery; 
     FIG. 2A shows the pulsating current waveform of the GSM power amplifier of FIG. 1; 
     FIG. 2B shows the waveform of the battery output voltage under a pulsating load current; 
     FIG. 3 shows prior art in which a large capacitor is used to reduce the voltage droop caused by a pulsating load current; 
     FIG. 4A shows the current waveforms of the GSM PA and the battery output of the circuit as shown in FIG. 3; 
     FIG. 4B shows the waveform of the battery output voltage of the circuit as shown in FIG. 3; 
     FIG. 5 shows a preferred embodiment of the present invention using a boost converter input stage followed by a small reservoir capacitor and a buck converter output stage; 
     FIG.  6 A and FIG. 6B show key waveforms of the circuit as shown in FIG. 5; 
     FIG. 7 shows a detailed embodiment of the boost converter input stage of the FIG. 5 circuit; 
     FIG. 8A shows the duty cycle variation of the boost converter as a function of the output voltage; 
     FIG. 8B shows a boost converter inductor current waveform for different output voltage conditions; 
     FIG. 9A shows a load current waveform; and 
     FIG. 9B shows the waveforms when the boost converter re-charges the reservoir capacitor at different load current magnitude. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 5 shows one embodiment of a converter in accordance with the present invention. The converter includes an input stage  50 , a reservoir capacitor  58 , and an output stage  60 . The input stage includes an input inductor  53 , first and second switches  54 ,  56 , a current sense circuit  55 , and a first control circuit  59 . The output stage includes third and fourth switches  61 ,  62 , an output inductor  63 , an output capacitor  64 , and a second control circuit  66 . The input inductor  53  receives a battery voltage, such as 3.6 volts from a lithium-ion battery  51 , and the output capacitor connects to a power amplifier, such as PA  68  shown in FIG.  5 . 
     The input stage  50  operates to boost the voltage on the reservoir capacitor  58  to a level greater than the battery voltage. The charging current delivered to the reservoir capacitor  58  is about 50% of the current drawn from the battery by the input stage  50 . The output stage  60  operates to down convert the boosted voltage on the reservoir capacitor  58  to an output voltage, for example, 3.4V, needed by PA  68 , which the stage regulates. During the transmission period, the reservoir capacitor  58  supplies, via the output stage, most of the energy to PA  68 . The input stage supplies only a small portion of the energy (η 1 ×η 2 ×0.3A×3.6V×580 μs)=565 μJ (microJoules)) delivered during the transmission period, where η 1  is the efficiency of the input stage, typically about 95% and μ 2  is the efficiency of the output stage, also about 95%. 
     Waveform  71  in FIG. 6A shows the current needed by the power amplifier  68 , and waveform  72  shows the battery output current. Waveform  73  in FIG. 6B shows the voltage across the reservoir capacitor  58  and waveform  74  shows VCC voltage  65  at the PA  68 . 
     When the PA begins a transmission, voltage  57 , in FIG. 5, falls gradually from 8.0V (V 1 ) at T 0  to about 6.15 V (V 2 ) by T 1 . The remaining voltage V 2  on the reservoir capacitor  58  is determined by the following equation: 
     
       
         η 2 ·0.5· C   RES ·( V   1   2   −V   2   2 )+η 1 ·η 2   ·P   BATT   ·T   ON   =P   PA   ·T   ON   
       
     
     where η 2  is the efficiency of the output stage, η 1  is the reservoir capacitor  58  voltage when it is fully charged (8.0 V), V 2  is the remaining reservoir capacitor voltage at T 1 , T ON  is the transmission interval, η 1  is the efficiency of the input stage, P BATT  is the power delivered by the battery during the interval, and P PA  is the power consumed by the power amplifier during the transmission interval. The above equation states that the net energy delivered by the reservoir capacitor equals the energy consumed in the power amplifier. 
     The energy consumed by the load is (V PA I PA T ON ). Assuming I PA =2 A, V PA =3.4 Volts at the power amp and T ON =580 μs, the energy consumed by the power amp is 3,944 μJ. 
     The net energy supplied to the load by the battery is η 1 η 2 V BATT I BATT T ON . If the battery voltage V BATT =3.6 volts, and I BATT =0.3 Amps, and the input stage efficiency η 1  is 95%, the energy supplied by the battery is 565 μJ. 
     This means that the reservoir capacitor need only supply about 3,379 μJ. If charged to 8.0 volts (V 1 ), a reservoir capacitor with a value of C RES =270 μF has an energy storage of about 8,208 μJ. This is a relatively large amount of energy, much more than is needed by the load. Thus, the reservoir capacitor  58  discharges to a remaining voltage of about 6.15 volts, after the transmission interval T 0 -T 1 . 
     After T 1 , the power amplifier PA  68  is idle for about 4.06 msec. The input stage continues to provide 150 mA of average output current to recharge capacitor  58 . At T 2 , about 3.35 msec after T 1 , the capacitor  58  is recharged to 8.0V, at which point the input stage stops further charging. At T 3 , the power amplifier PA  68  starts its next transmission period. 
     FIG. 7 shows an embodiment of the input stage in accordance with the present invention. The input stage includes an input inductor  83 , a first switch  84 , a second switch  86 , a current sense circuit  85 , and a control circuit  50 . In one embodiment, the first switch  84  is a n-channel transistor and the second switch is a p-channel transistor. The control circuit includes an operational amplifier (OP-AMP)  91 , a compensation capacitor  92 , a first comparator  93 , a level shifter  94 , a SR flip-flop  95 , a Clock &amp; Ramp generator  97 , a second comparator  96 , and a Valley-voltage detect &amp; Current-setting circuit  89 . 
     The inductor  83  and the second switch  86  are connected in series between the battery  81  and the reservoir capacitor  88 . The first switch  84  is connected between ground and the junction of the inductor  83  and the second switch  86 . The control circuit connects to the first switch  84  and the second switch  86  in four places, the control input of the first switch  84 , the control input of the second switch  86 , the side of first switch  84  that connects to ground, and the output  87  of switch  86 . The control circuit thereby controls the state of the switches  84 ,  86  and senses the current in the first switch  84  and the voltage at the output of the second switch  86 . 
     In the control circuit  50 , the control inputs of the switches are driven by a SR flip-flop  95 . The control input of the first switch  84  connects, in one embodiment, directly to the Q output of the SR flip-flop  95  and the control input of the second switch  86  connects, via the level shifter  94 , to the Q output. In an embodiment in which first and second switches  84 ,  86  are implemented as n-channel and p-channel transistors, respectively, first switch  84  is on when second switch  86  is off and visa-versa. The S input of the flip-flop  95  is connected to an ‘AND’ gate whose inputs receive the clock output of the Clock &amp; Ramp circuit  57  and the output of the second comparator  96 . The second comparator  96  has a positive input connected to a reference voltage, e.g., 8.0 volts, and a negative input connected to the output  87  of switch  86 . Normally, the output of the second comparator  96  is high, enabling the AND gate, and allowing the S input of the flip-flop to receive the clock output of the Clock &amp; Ramp circuit  97 . Setting the flip-flop  95 , causes its Q output to have a logic high state, which, in turn, causes first switch  84  to be on and switch  86  to be off. When switch  84  is on, the inductor  83  is charged by the battery. When switch  84  turns off and switch  86  turns on, the charged inductor delivers its energy to the reservoir capacitor  88 , by charging the reservoir capacitor  88 . 
     The reset input of the flip-flop  95  is connected to the output of the first comparator  93 , which has a positive input connected to the ramp output of the Clock &amp; Ramp circuit  97 , and a negative input connected to the output of OP-AMP  91 . OP-AMP  91  has a positive input connected to the output of the Valley Detect &amp; Current Setting circuit  89 , and a negative input connected to a current sensing circuit output  85 . A compensation capacitor  92  is connected from the output of OP-AMP  91  to its negative input, and the current sensing circuit  85  is connected to the side of the first switch  84  that connects to ground. The Valley Detect &amp; Current Setting circuit  89  has an input connected to the output  87  of the reservoir capacitor  88 . 
     The current sense circuit  85  detects the average current flowing through switch  84 . OP-AMP  91  and the close loop control of the input stage maintain the average current in first switch  84  to about 150 mA. In particular, the OP-AMP  91  provides a determination of whether the current in the first switch  84  is at the predetermined current level provided by the Valley Detect &amp; Current Setting circuit  89 . If the current in the first switch  84  is less than the predetermined current level, then the output voltage of OP-AMP  91  moves higher and causes the first comparator  91  to produce a reset pulse to the flip-flop that is further delayed from the time the flip-flop is set. This keeps switch  84  on longer causing an increase in the current flowing in the first switch  84 . If the current in the first switch  84  is greater than the predetermined current level, then the first comparator produces a reset pulse sooner after the flip-flop  95  is set, causing first switch  84  to shut off sooner thereby maintaining or decreasing the current in the first switch  84 . Additionally, the voltage at the output of the input stage is sensed, and if greater than a predetermined voltage level, the second comparator  96  in conjunction with a logic AND gate, prevents the flip-flop  95 , from being set. 
     FIG. 8A shows the duty cycle variation at different voltage levels of capacitor  88 , where duty cycle is defined as the ratio of the on-time to the cycle time (i.e., the sum of the on-time and off-time) of switch  84 . Assuming battery output voltage is 3.6 V, and an early stage of recharging, the voltage  87  of capacitor  88  is at 6.2 V, the duty cycle is about 42%, As voltage  87  increases, both the Vo/Vin ratio and the duty cycle increase. At 7.2V, the duty cycle is 50% and at 8.0V, the duty cycle is 55%. 
     FIG. 8B shows the current waveform of inductor  83  at these different duty cycles. Waveform  101  occurs when V BATT =3.6V, V RES =6.15 V, and duty cycle is 42%. Waveform  102  is when V BATT =3.6V, V RES =7.2 V, and duty cycle is 50%. Waveform  103  is when V BATT =3.6V, V RES =8.0 V, and duty cycle is 55%. It is clear that as the duty cycle of switch  84  increases, the amount of ripple in the current waveform increases. 
     FIG. 9A shows a current waveform for a GSM power amplifier. The power amplifier transmits for about 580 μsec, and then idles for 4.06 msec. 
     FIG. 9B shows the voltage waveform for capacitor  88  in FIG. 7, when recharged by a 300 mA constant current converter stage. If the V PA  is set to 3.4 V and draws an I PA  of 2 A in a transmission, the voltage  87  drops to 6.15 V at the end of 580 μsec transmission interval. It takes the input stage about 3.35 msec to recharge capacitor  88  back to 8.0V. This is shown as waveform  113  in FIG.  9 B. As soon as voltage  87  reaches 8.0V, comparator  96  in FIG. 7 outputs a logic low state, inhibiting flip-flop  95  from turning on switch  84 , and the input stage stops delivering charging current to reservoir capacitor  88 . 
     If the PA draws less energy during the transmission, by setting V PA  to 3.0V is used during a transmission interval. Voltage  87  drops to 6.67 V. In this case, it takes only 2.39 msec to recharge capacitor  88  from 6.67 V to 8.0 V. This is shown as waveform  114  in FIG.  9 B. 
     During transmission periods, valley detect circuit  89  monitors the level of voltage  87  at the end of a 580 μsec transmission interval. During receiving intervals, the PA is turned off and consumes no power and the Valley Detect circuit  89  voltage  87  remains at 8.0 V. It sets the reference current level of 0 mA to OP-AMP  91 , thereby turning off the input stage. 
     On the other hand, if the PA draws more power than normal, such as in the case of setting V PA  to 4.0V for emergency high-power call, voltage  87  may drop to about 5.08 V. Valley detect circuit  89  then sets a higher current, for example, 400 mA. This higher current level charges capacitor  88  to 8.0V in 3.94 msec, slightly within the 4.06 msec time limit. 
     The present invention requires a much smaller energy-storage capacitor, 490 μF (270 μF and 220 μF) in comparison with the 4700 μF of the prior art. Yet, it achieves better ripple performance for the RF power amplifier. 
     In conclusion, the present invention transforms a pulsating load current into a nearly constant DC load current equal to about 15% the prior art current drawn from the battery. The result is reduced source resistance loss and improved efficiency. Furthermore, the present invention extends the usable battery voltage range to 2.8 V from 3.6 V, thus increasing talk time by 40% or more. The present invention also provides an efficiently regulated PA V PA  which further extends talk time without over-driving the PA when the battery voltage is substantially higher than what is actually required by the PA. The present invention can also provide an extended communication range (by setting V PA  voltage to as high as 4.5V) in an emergency situation. 
     Although the present invention has been described in considerable detail with reference to certain preferred versions thereof, other versions are possible. Therefore, the spirit and scope of the appended claims should not be limited to the description of the preferred versions contained herein.