Charging voltage control and current limit for battery chargers
First Claim
1. In a battery charger of the type having an output charging voltage which remains substantially constant over a relatively wide output charging current range, the improvement in combination therewith comprising voltage trim means for step-wise increasing the output charging voltage when the output charging current decreases below a predetermined value.
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Accused Products
Abstract
Charging voltage and current control circuits for battery chargers provide for a constant output voltage above a predetermined value of charging current and a step-wise increased output voltage below the predetermined value of charging current. The charger makes use of a pulse-width modulated control circuit utilizing a variable duty cycle to control charging current and couples a trim command signal responsive to a primary side signal representative of charging current to the pulse-width modulator for values below the predetermined value of charging current. A current limit control circuit provides a foldback current limit function by driving the pulse-width modulator to a reduced output current during overload conditions.
45 Citations
22 Claims
- 1. In a battery charger of the type having an output charging voltage which remains substantially constant over a relatively wide output charging current range, the improvement in combination therewith comprising voltage trim means for step-wise increasing the output charging voltage when the output charging current decreases below a predetermined value.
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13. An improved method of operating a lead-acid-battery charger of the type having a pulse-width modulator producing an output charging current from a transformer coupled DC converter at a first level of charging voltage wherein the output charging current is responsive to a duty cycle of the pulse width modulator, the method comprising the steps of:
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a) time-average integrating a primary current to provide a signal proportional to and isolated from the output charging current; and b) increasing a charging voltage from the first level to a second level higher than the first level when then time-average integral of the primary current indicates the output charging current is in a range below a predetermined value. - View Dependent Claims (14, 15, 18, 19)
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- 16. The method of 13 wherein the predetermined value of output current is 2 amperes.
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20. An improved charging voltage trim circuit for use in a battery charger of the type having an isolation transformer coupled modulator providing charging voltage and current at an output terminal in response to a primary side variable duty cycle current control, the trim circuit comprising:
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a) a resistive voltage divider feedback network connected to the output terminal to provide a voltage proportional to the output charging voltage; b) optically-coupled switch means for switching between one of two ratios for the voltage divider feedback network; c) time-average integrating means for providing a running time-averaged integral value of a primary side current; and d) comparator means for comparing the time-averaged integral value with a predetermined value at the primary side of the isolation transformer and for driving the optically-coupled switch means at the secondary side of the isolation transformer to select; i) a first ratio for the voltage divider above the predetermined value, and ii) a second ratio for the voltage divider below the predetermined value such that the output charging voltage of the battery charger is controlled to a first value when the first ratio is selected and to a second value when the second ratio is selected.
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- 21. A method of operating a current limiting control loop to limit output charging current in a battery charger of the type having a variable duty cycle modulator providing the output charging current via an isolation transformer, the method comprising connecting the current limit loop reference to both a fixed command signal and an output of the duty cycle modulator such that the level at which current is limited is determined in part by the duty cycle of the modulator.
Specification