Solar array peak power tracker

Details Solar array peak power tracker Solar array peak power tracker

1996-05-29

2001-11-13

Wong, Peter S. (Department: 2838)

Electricity: power supply or regulation systems

Output level responsive

Using a three or more terminal semiconductive device as the...

C323S284000, C307S066000

Reexamination Certificate

active

06316925

ABSTRACT:

FIELD OF THE INVENTION
This invention relates generally to battery chargers and, in particular, to battery charger power regulators that operate from a solar array and which employ a peak power tracker (PPT).
BACKGROUND OF THE INVENTION:
Battery charger regulation control circuits that force a solar array to operate at a maximum power point of the solar array are generally known as peak power trackers (PPT). PPT circuits have been known for a considerable period of time. However, their general acceptance and widespread use has been hindered by their cost and complexity. One reason for employing a PPT circuit is that the operating characteristics of a conventional photovoltaic solar power array tend to degrade over time. This results in considerably different I-V characteristics at the beginning of the operating life than those exhibited at the end of the operating life.
One significant advantage to using a PPT is the ability to extract the maximum power available from a solar array. Another significant advantage is the ability to extend battery life in the presence of varying battery load or varying solar illumination. This second advantage is made possible by minimizing the depth of the discharge experienced by the battery. Both of these advantages are most beneficial when used in a satellite in a low earth orbit (LEO) that experiences periods of full illumination alternating with eclipse. Thus, despite the added cost and complexity of the PPT, relative to fixed operating point battery charging systems, the use of PPT can be very desirable.
One conventional approach to operating a PPT circuit is known as the disturb and observe approach. In this technique the operating point of the battery charging regulator is offset by some small amount, the power output of the solar array is then determined by multiplying the array output current by the array output voltage, and a search is then made for a maximum in the power output.
A recent example of the disturb and measure approach is described in an article entitled “Design and Analysis of a Microprocessor-Controlled Peak-Power-Tracking System”, by P. Huynh and B. H. Cho, Virginia Power Electronics Center, Bradley Electrical Engineering Department, Virginia Polytechnic Institute and State University (August 1992). The authors employ a microprocessor to compute the peak power point. Afterwards, the solar array is forced to operate at a voltage where the output power of the solar array is maximized. The system is said to operate essentially in two different modes: peak-power-tracking (PPT) mode and trickle-charge (TC) mode. As a spacecraft emerges from eclipse, the solar array is regulated at the peak-power voltage (PPT mode) to provide maximum power for the load, and the battery sources or sinks the additional power, depending on the load demand. When the battery is fully charged and the solar array output power exceeds the load power, the PPT system switches to the TC mode. When operating in this mode a small solar array current is used to charge the battery, to compensate for the battery leakage current, while adequate power from the solar array is provided for the load.
Other PPT techniques do not employ a microprocessor, but instead utilize complex analog signal processing which, while eliminating the requirement to provide a programmed microprocessor device, result in considerable complexity and require a significant amount of circuit board area to implement. As is well known, in any earth satellite application the conservation of weight and volume is an important goal.
An object of this invention is to provide a power supply system which provides the benefits of the prior art PPTs, while at the same time doing so at little or no additional cost and complexity, and with a minimized increase in weight and volume.
SUMMARY OF THE INVENTION
The foregoing and other problems are overcome by a method for operating a pulse width modulated switching power converter, and by a pulse width modulated switching power converter that is constructed to operate in accordance with the method. The power converter has an input coupled to an output of a solar array and an output providing an output current, the output current being coupled to a battery for applying a charging current I
CHARGE
to the battery while also supplying a current I
LOAD
to a load. The method includes the steps of: (a) incrementally increasing a duty cycle of the pulse width modulated switching power converter so as to incrementally increase a magnitude of the output current of the switching power converter; (b) sensing a first magnitude of both I
CHARGE
and I
LOAD
; (c) storing the sensed magnitude; (d) sensing a second magnitude of both I
CHARGE
and I
LOAD
; and (e) comparing the stored first magnitude to the sensed second magnitude. If the stored first magnitude is determined to be less than the sensed second magnitude, the method maintains the duty cycle at a current duty cycle increment. If instead the stored first magnitude is determined to be greater than the sensed second magnitude, the method decreases the duty cycle.
The step of incrementally increasing includes a step of periodically increasing the charge upon a capacitance that is coupled to a control node of the pulse width modulated switching power converter, and the step of decreasing the duty cycle includes a step of at least partially discharging the capacitance.
In this manner the circuit comes to oscillate about a point that corresponds to the peak power point of the solar array, and thus functions as a PPT. Due to the position of the current sensor at the output of the power converter, the circuit always tracks the maximum battery current operating point, even when this point changes due to external factors such as changes in I
LOAD
.
This invention also encompasses an earth satellite comprising at least one solar array, at least one switching power converter, at least one battery that is charged by the at least one switching power converter, and a load. The load may be comprised of a communications package that includes a transceiver coupled to an antenna.
The switching power converter includes a current control loop comprised of a pulse width modulator having an output providing a pulse width modulated signal to a control terminal of a switching device. The switching device is coupled between the at least one solar array and the at least one battery. The current control loop further includes a current sensor coupled to an output of the switching device, the current sensor having an output coupled to an input of the pulse width modulator for causing the pulse width modulator to vary the duty cycle of the pulse width modulated signal in accordance with the sensed current.
In accordance with an aspect of this invention the switching power converter further includes a peak power tracker loop operating in parallel with the current control loop. The peak power tracker loop includes a circuit arrangement for periodically increasing the duty cycle of the pulse width modulated signal so as to periodically increase a magnitude of the output current of the switching device. The peak power tracker loop also includes a sample and hold circuit, having an input coupled to an output of the current sensor, for periodically storing a sample of the sensed current; and further includes a comparator having a first input coupled to the output of the current sensor and a second input coupled to an output of the sample and hold circuit for comparing a value of a previous sample of the sensed current to a value of the presently sensed current. The comparator has an output coupled to an input of the circuit arrangement for periodically increasing the duty cycle of the pulse width modulated signal. The circuit arrangement is responsive to the output of the comparator for maintaining the duty cycle at a current duty cycle increment if the value of the previous sample of the sensed current is indicated to be less than the value of the presently sensed current, and is further responsive to the output of the comparator for decreasi

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