Accuracy, Fast-Response, Cost-Efficient Solar Cell Regulator by ill20582


									   Accuracy, Fast-Response, Cost-Efficient Solar
      Cell Regulator Suitable for Low-Power
                  Jiann-Jong Chen, Fong-Cheng Yang, Hsin-Tung Li and, Yuh-Shyan Hwang

Abstract— An accuracy, fast-response, cost-efficient solar    as remote terminals. In space applications, this is the
cell regulator suitable for low-power applications is         only source of power, barring nuclear alternatives. The
presented. The proposed regulator is a current-to-voltage     need for small size and low weight has necessitated the
regulator and has been fabricated with TSMC 0.35µm            use of high-speed switching voltage regulators. The I-V
2P4M CMOS technology. The measurement results show            Curves of a Si-base solar cell under different radiances
the settling time which can achieve 400ns with 0.5% error
                                                              is shown in figure 1.
for full load-current. Furthermore, the line and load
regulations are 14µV/mA and 8ppm/mA, respectively. The           The characteristic of the solar cell is like a current
current efficiency can be up to 99.3% for 150mA output        source with an equivalent series resistor Rs and an
current. The active chip area is only 226µm x310µm.           equivalent parallel resistor Rp, its modeling can be
                                                              shown as figure 2. The resistor Rs is small and can be
Index Terms: current-to-voltage regulator, current-           considered as zero in most of the applications. The
mode voltage regulator, transient response.                   resistor Rp can be considered as infinity to simplify the
                                                              circuit analysis.
                 I. INTRODUCTION
  The current interest in commercial green power in the
developed world is about 25 years old, starting in the
mid-1970s after the first oil shock. Many a billion
dollars and many years later, the public is asking if
green power is for real. Electricity derived from any
renewable energy source is considered “green” because
of the negligible impact on greenhouse gas emissions.
In terms of commercial energy, this list currently
includes hydro, wind, biomass, geothermal, and solar.
Now there is a broader goal: to minimize the emission
of CO2 (the most common global warming gas) that
results from the burning of fossil fuels. It is possible to
generate electricity from the thermal part of the solar
spectrum. This is generally done by concentrating the
incoming sunlight and then trapping its heat, which can          Figure 1: I-V Curves of a Si-base solar cell under different
raise the temperature of a working fluid to a very high                                  radiances.
degree to produce steam and then generate electricity.
The concentration part of this explanation is very               As the solar cell is used in the application, which
important, as that is what allows the sunlight to produce     allowed the larger voltage variation, it is a good energy
enough heat to be used to generate electricity. Notice        resource. However, in most of the application, the load
that this process is different from that of a photovoltaic    can not be supplied a voltage source with larger
panel where the sunlight is directly converted into           variation. Therefore, we need a current-to-voltage
electricity without the intermediate heat collection.         regulator, as shown in figure 3, suitable for solar cell
                                                              applications. There are some regulators have been
  Photovoltaic conversion of solar energy is one of the
                                                              proposed in previous works [1-3]. However, they are
most promising ways of meeting the increasing energy
                                                              not cost-efficient in low-power application. On the
demand. The technology of photovoltaics has evolved
                                                              other hand, their transient responses are not fast enough
and is vying to become an economical alternative to
                                                              and their accuracy is not good enough. So, we proposed
other power sources in certain niche applications such
                                                              an accuracy, fast-response, cost efficient current-to-
voltage regulator suitable for low-power solar cell                              regulations. The first stage is the differential pair,
applications in this paper.                                                      detects the difference between the reference voltage
                                                                                 (Vref) and the feedback signal (Vfb), so as to provide
                                                                                 error signal to the power transistor for voltage
                                                                                 regulation. The power transistor is designed to operate
           Rs                                                                    in saturation region at dropout to reduce the transistor
                                                                RLoad            size and the parasitic gate capacitance [4].Consequently,
                                     Rp                                          the slew rate at the gate of the power transistor is
                                                                                 improved due to the smaller gate capacitance.
      Is                                                                             Figure 5 shows the detail circuit of the proposed
                                                                                 regulator. The transistors Ma10-Ma14 form a single-stage
                                                                                 differential pair and the high-swing second stage is
                                                                                 composed of Ma21-Ma23. In addition, a transconductor
                                                                                 boosting circuitry [4] is formed by Mb0-Mb2 which is
    Modeling of solar cell
                                                                                 used to increase gm2 to enhance the accuracy of the
  Figure 2: The modeling of solar cell connected with loading
                                                                                 output voltage. The current-mode regulator mainly
                          resistor.                                              display as output voltage and time that the voltage
                                                                                 return stable, when the load current change in the
                                                                                 twinkling of an eye. Those can affect the transient
                                                                                 response including output capacitor, output capacitor
   Solar               Current-to-Voltage                                        equivalent series resistance (ESR), and maximum load
    Cell                     Regulator                                           current, etc.
                                                                                     In the following, when the loading changes, we will
                                                                                 analyze the output voltage in several parts. When the
                                                                                 loading draw the current from regulator suddenly, the
   Figure 3: The proposed regulator is applied with solar cell.                  regulator does not offer a flood of current, due to
                                                                                 smaller the unit gain frequency. This action makes the
  In this paper, a description of proposed regulator will                        output voltage to product a voltage dip, and the output
be included in section II, and the small-signal analysis                         capacitor offered a large amount of current that load
is shown in section III. The experimental results of                             needs temporarily, flowed into Vout by Cout in this
proposed regulator are shown in section IV. Finally, the                         moment. However, there are some tradeoffs among the
conclusion is made in section V.                                                 stability of conventional linear regulators and other
                    II. CIRCUIT DESCRIPTIONS                                     performances, such as the transient response, maximum
                                                                                 load current, current efficiency, and accuracy of the
                                                                                 output voltage, etc. Hence, this paper provides
                  EA                            Feedback resistors
                                   Iin                               Vout        current-mode methods to improve the load transient
                                                                                 response and maintain the stability simultaneously. It
   Vref                                                                          has a better load transient response, less output noise
           1st       2nd           POWER               Rf1                       and ripples, and few off-chip components. The voltage
          stage     stage          Transistor          Vfb     Resr         RL   dips of the output voltage can be improved obviously
                                                       Rf2                       by the use of the low power bounce current mode
       Error            Cm    Rm                                                 circuit, as the output load current is switched from 0 to
                                                                                 150mA. Finally, we use the bandwidth extension skill
                                                                                 to improve the output transient response for the output
           Figure 4: The architecture of solar cell regulator                    load current variation. By the bandwidth-extension skill,
                                                                                 the value of the Miller compensation capacitor is
   The architecture of the proposed regulator is shown                           reduced. In addition, the voltage dips, and settling time
in figure 4. The solar cell is considered as a current                           of the output voltage for the load transient response are
source Iin. From figure 4, a moderate-gain stage is                              improved simultaneously by the use of the bandwidth
inserted between the error amplifier and the power                               extension skill [5].
transistor to reduce the value of the Miller capacitance
and enhance the loop gain to improve line and load
                                                        Figure 5: The proposed solar cell regulator

                                                          1         2 Cm C g 
             Lg o (1 + sCout Resr )1 + sC m  Rm −                  −s              
                                                    g m 2 g mn Ro 2      g m 2 g mn                           Lgo (1 + sCout Resr )                          (2)
  Lg ( s ) ≈                                                                           ≈
                       s                           Cout        2 C p 2 Cout              s                             Cout        2 C p 2 Cout 
               1 +
                           1 + s Cout Resr +                  +s                    1 +         1 + s Cout Resr +                 +s            
                     p−3db 
                                                g m 2 g mn Ro 2      g m 2 g mn       
                                                                                              p−3db   
                                                                                                                         g m 2 g mn Ro 2    g m 2 g mn 

                                                                                     (1) Cout >>(Cm,Cp2) >> Cp(1,2) ,
                                                                                     (2) gmn >> gm1 and gm2 ,
                                                                                     (3) Ro1gm2Ro2gmnRout>>Rm,
                                                                                     (4) The parasitic zeros are high enough to be ignored.
                                                                                     The transfer function is calculated in equation (2) and
                                                                                     the loop gain at DC can be described as

                                                                                                 Rf1 
                                                                                         Lg 0 =          g g g R R R                                          (3)
                                                                                                 R + R  m1 m 2 mn o1 o 2 out
                                                                                                 f1   f2 

    Figure 6: The small-signal modeling of proposed solar cell                       ,and P-3dB is the dominant pole and can be obtained as
   Figure 6 shows the equivalent small-signal model of                                    p−3dB =                                                               (4)
the proposed regulator with the single Miller                                                            g m 2 g mn Ro1 Ro 2 Rout C m
capacitance (SMC), where Ro(1,2), gm(1,2,n), and Cp(1,2) are                         After comparing the second-order function in (2) with a
the output resistance, trans-conductance, and lumped                                 standard second-order function, we can get the value of
output parasitic capacitance of the gain stages,                                     the nature frequency pc and the damping factor ς as
respectively. Cm is the compensation capacitor, and Cout
is the loading capacitance. Resr is the equivalent series                            shown in equation (5-6).
resistance of Cout, and Rout can be written as                                                          g m 2 g mn
                                                                                             pC ≈                                                              (5)
   Rout = RL // Ropass // (R f 1 + R f 2 )                              (1)                             C p 2Cout

, where RL is loading resistance, Ropass is the output
resistance of the power transistor and Rf1 and Rf2 are                                         1               1         g m 2 g mn Cout
                                                                                            ς ≈  Resr +
                                                                                                                                                             (6)
the feedback resistances.                                                                      2        g m 2 g mn Ro 2 
                                                                                                                               C p2
  To solve the small-signal model, we assume
following assumptions:
                                                                                                       IV. EXPERIMENTAL RESULT
   The proposed regulator has been implemented in
0.35µm 2P4M CMOS technology. The photograph of
the regulator R is shown in figure 7, and the active chip                                                                            Line Regulation                Iout=150mA
area is 226µm x310µm. The maximum output current is
150mA with a dropout current of 1.0741mA. The
proposed regulator with 1.8V output voltage is capable

                                                                                                    Vout [V]
of operating from 152mA to 420mA, and consumes
880µA quiescent current. Figures 8 and 9 show the line
regulations at Iout =0mA (light load) and Iout =150mA
(heavy load), and the measured line regulations are                                                            1.7930

47µV/mA and 14µV/mA, respectively. The measured                                                                1.7910
                                                                                                                          160       200     250       300    350   400     420
load regulation is 8ppm/mA (Iin=200mA), 40ppm/mA
(Iin=300mA) as shown in figures 10-11. The                                                                                                        Iin [mA]

measurement results of the load transient responses of                                      Figure 9: The Measured line regulation at Iout=150mA.
the proposed regulator are measured under two
conditions: light load to heavy load, shown in figure 12,
and heavy to light, shown in figure 13. It is noted that
the load transient responses are measured at 1-µF
(Resr=8Ω) off-chip capacitors. The spectrum of outout                                                                          Load Regulation                 Iin=200mA

voltage is shown in figure 14 with 150mA loading                                        1.8010
current. Finally, we summarize the specifications of the
proposed regulator in Table I.                                                          1.8005

                                                                             Vout [V]



                                                                                                                0   20    40    60    80    100 120 140 150 160 180
                                                                                                                                          Iout [mA]

                                                                                         Figure 10: The Measured load regulation at Iin=200mA.

 Figure 7: The photograph of the proposed solar cell regulator

                                Line Regulation            Iout=0mA
                                                                                                                               Load Regulation
             1.8060                                                                                1.8050
             1.8040                                                                                1.8040
  Vout [V]

                                                                                        Vout [V]

             1.7980                                                                                1.8020
             1.7920                                                                                1.8000

             1.7900                                                                                1.7990
                      1   10   50   100    150       200   250   300   330
                                          Iin [mA]                                                                  0    20    40    60    80 100 120 140 150 160 180
                                                                                                                                            Iout [mA]
              Figure 8: The Measured line regulation at Iout=0mA.
                                                                              Figure 11: The Measured load regulation at Iin=300 mA.
                                                                                                    Table I
                                                                                     Summary of Measured Performance
                                                                                      Technology             TSMC 0.35µm 2P4M
                                      150 mA                                         Supply Current            152mA~420mA
                Iout                                                                 Output Voltage                1.8V

                         Settling time:400ns(0.5%error)                  Line                Iout= 0mA                47µV/mA
                         Voltage:30mV                                  Regulation
                                                                                             Iout= 150mA              14µV/mA
       0 mA
                                                                                Load Regulation                      8ppm /mA
                                                                                Quiescent Current                      880µA
                                                                              Operation Temperature                 -55℃~130℃
                                                                     Settling Time       Iout= 0mA to 150mA             400ns
                                                                     (0.5% error)
                                                                                         Iout= 150mA to 0mA             600ns
                                                                                       Iout= 0mA
                                                                       Voltage dip      to 150mA
Figure 12: The load transient response at Iin=200mA, Cout=1µF                           Iout=150mA to 0mA               70mV
Light load to heavy load, voltage dip= 30mV. ( horizontal scale :                     Active Area                  226µm x 310µm
200ns/div; vertical scale: 50mV/div and 50mA/div,from top to
                                                                                      V. CONCLUSIONS
                                                                        An accuracy, fast-response, cost-efficient solar cell
                                                                    regulator suitable for low-power applications has been
         150 mA                                                     designed with TSMC 0.35um 2P4M CMOS processes.
                         Settling time:600ns(0.5%error)
              Iout       Voltage dip:70mV
                                                                    The experimental results confirmed with the theoretical
                                                                    analysis are presented in this paper. The proposed
                                                                    regulator is suitable for current-type electrical source in
                                         0 mA                       low-power applications.
                                       Vout                            The authors would like to thank National Science
                                                                    Council   for    project   supporting     and    Chip
                                                                    Implementation Center for chip fabrication. This work
                                                                    was sponsored by NSC94-2213-E-027-052.
Figure 13: The load transient response at Iin=200mA, Cout=1µF,
heavy load to light load, voltage dip= 70mV. (horizontal            [1] Kimiyoshi Kobayashi, Hirofumi Matsuo, Yutaka Sekine,” Novel
scale :200ns/div; vertical scale: 50mV/div and 50mA/div,from top        solar-cell power supply system using a multiple-input DC–DC
to bottom)                                                              converter,” IEEE Trans. on Industrial Electronics, Vol. 53, No. 1,
                                                                        pp. 281-286, Feb. 2006.
                                                                    [2] Kimiyoshi Kobayashi, Hirofumi Matsuo, Yutaka Sekine,” An
                                                                        excellent operating point tracker of the solar-cell power supply
                                                                        system,” IEEE Trans. on Industrial Electronics, Vol. 53, No. 2,
                                                                        pp. 495-499, Apr. 2006.
                                                                    [3] Il-Song Kim, Myung-Bok Kim, Myung-Joong Youn,” New
                                                                        maximum power point tracker using sliding-mode observer for
                                                                        estimation of solar array current in the grid-connected
                                                                        photovoltaic system,” IEEE Trans. on Industrial Electronics, Vol.
                                                                        53, No. 4, pp. 1027-1035 ,Aug. 2006.
                                                                    [4] K. N. Leung and P. K. T. Mok, “A Capacitor-Free CMOS
                                                                        low-dropout regulator with damping-factor-control frequency
                                                                        compensation,” IEEE Journal of Solid-State Circuits, Vol. 38, No.
                                                                        10, pp. 1691-1702, Oct. 2003.
                                                                    [5] C. K. Chava and J. Silva-Martinez, “A frequency compensation
                                                                        scheme for LDO voltage regulators,” IEEE Transactions on
                                                                        Circuits and Systems I, Vol. 51, No.6, pp. 1041-1050, Jun. 2004.
Figure 14: The spectrum of the output voltage in proposed           [6] X. Fan, C. Misgra and E. Sanchez-Sinencio,“ Single Miller
regulator.                                                              capacitor frequency compensation techniquie for low-power
                                                                        multistage amplifiers,” IEEE Journal of Solid-State Circuits, Vol.
                                                                        40, No. 3, Mar. 2005.

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