Electricity: battery or capacitor charging or discharging – Serially connected batteries or cells
Reexamination Certificate
2002-07-16
2003-12-16
Tso, Edward H. (Department: 2838)
Electricity: battery or capacitor charging or discharging
Serially connected batteries or cells
Reexamination Certificate
active
06664762
ABSTRACT:
FIELD OF THE INVENTION
This invention pertains generally to the field of battery charging and particularly to the high voltage charging of multiple cells or batteries connected in series.
BACKGROUND OF THE INVENTION
High voltage batteries are a critical element of several important applications such as electric vehicle drives. As with any battery, charging high voltage batteries is a complex electrochemical process in which a charging system replenishes a discharged battery by supplying to it a controlled amount of energy from an electric network. Achieving wide market acceptance for high voltage battery applications demands an economically viable system for charging high voltage batteries. Addressing this demand requires developing a low cost, high power density charging system that can supply a controlled charging current at high output voltages. However, realizing such a system requires overcoming certain practical problems related to the high output voltage.
In principle, a battery charger is a power supply with controllable voltage and current limits. What differentiates a battery charger from a conventional power supply is the capability to satisfy the unique requirements of a battery. Typically, battery chargers have two tasks to accomplish. The first, and most important, is to restore capacity as quickly as possible and the second is to maintain capacity by compensating for self-discharge and ambient temperature variations. These tasks are normally accomplished by controlling the output voltage and current of the charger in a preset manner, namely, using a charging algorithm.
The two most common charging algorithms are constant-voltage charging and constant-current charging. In constant-voltage charging, the voltage across the battery terminals is held constant, with the state of the battery determining the charge current level. The charging process normally terminates after a certain time limit is reached. Constant-voltage charging is most popular in float mode applications.
By contrast, constant-current charging holds the charging current constant. This method is often used in cyclic applications as it recharges the battery in a relatively short time.
There are many variations of the two basic methods using a succession of constant-current charging and constant-voltage charging to optimize battery charge acceptance. These variations, however, require a controlled charger with both voltage and current regulation capability.
Chargers are commonly divided into uncontrolled and controlled chargers. Uncontrolled chargers are the oldest, simplest, and cheapest chargers available. They are typically less efficient and have slow dynamic response. The simplest uncontrolled charger consists of a low frequency power transformer along with an uncontrolled bridge rectifier. Such a charger is suited for constant-voltage charging, where the battery's state of charge sets the charging current. The advantages of such chargers include simple structure and low cost. However, with these chargers, the output voltage depends on the input voltage and has considerable voltage ripple. In addition, this type of charger could cause damage to batteries because it lacks control of the charging current.
Alternatively, controlled chargers can overcome these limitations. Controlled chargers offer the ability to control the charging current as well as to implement both constant-voltage and constant-current charging methods. The simplest form of controlled chargers are SCR chargers, consisting of a low frequency transformer, an SCR bridge rectifier, and a DC choke. SCR chargers offer a simple and low cost solution to implement a fully controllable charging system. They are still in use in many low to high power industrial applications. However, SCR chargers are bulky and have relatively low efficiencies and slow dynamic response.
Transistor controlled chargers comprise another class of controlled chargers. They consist of a low frequency transformer, an uncontrolled bridge rectifier, and a series pass transistor. These chargers can implement both constant-voltage and constant-current charging methods and have fast dynamic response. However, they have low efficiencies and are generally bulky due to the low frequency transformer.
Switch mode power supply (SMPS) based chargers offer improved performance compared to the SCR and the transistor controlled chargers. These chargers offer high efficiency power conversion due to high frequency operation. The high frequency power conversion stage results in significant size reduction for the energy storage elements (transformers, inductors, and capacitors). In addition, these chargers have fast dynamic response. The basic components of an SMPS charger include an input filter stage, an input rectification stage, a power factor correction stage (if required), a high frequency power conversion power stage, a high frequency isolation transformer, and an output rectification and filtering stage. A central analog/digital controller is normally employed to regulate the charger voltage/current and to implement the desired charging algorithm. Considering that a well designed switch mode power supply is inherently current limited, the combination of constant-current and constant-voltage charge is available.
In order to implement both constant-voltage and constant-current charging methods, a SMPS charger would employ an output filtering stage that allows for output current limiting. This is typically achieved by using an inductive output filtering stage (DC choke), which smoothes the output charging current and limits it through the charger control circuitry. The inductor serves as the main energy storage device. Consequently, the charging current is smoothed out and is prevented from changing instantaneously. This allows the SMPS charger to implement accurate current limiting as well as protect against any short circuit conditions that may arise across the output terminals. The inductor current is normally sensed and regulated by the control circuitry to achieve the desired level of output current and to implement the constant-current intervals of the charging algorithm. An output capacitor is normally used to filter out any remaining current ripple in the filter inductor and thus supply a pure DC current to the battery. The voltage across the capacitor, which is the same as the battery voltage, is normally sensed and regulated by the control circuitry to achieve the desired level of output voltage and implement the constant-voltage intervals of the charging algorithm.
For low power battery charging needs (<1 kW), the single switch and the two switch forward converters are the simplest isolated SMPS battery charger topologies with an inductive output filtering stage. A battery charger using a single switch forward converter power stage may employ a half wave rectifier on the secondary side.
For high power charging needs (>1 kW), the full-bridge converter of the type shown in
FIG. 1
(H-bridges of transistors Q
1
-Q
4
) with an inductive output filter L
0
is the most suitable power converter topology. A full wave rectifier composed of diodes D
1
-D
4
is employed on the secondary side to rectify the primary voltage and current waveforms. Typically, a center-tapped (push-pull) or a full-bridge rectifier is used in association with a full-bridge converter topology. Typical voltage and current waveforms for the full-bridge SMPS charger are shown in FIG.
2
. With an inductive output filtering stage, the secondary rectifiers are normally subjected to high voltage transients (ringing) during switching transitions. This is due to the reverse recovery of the output diodes where the transformer leakage inductance resonates with the secondary diodes' junction capacitance causing a two per-unit voltage stress across them. With an input DC bus voltage of VDC and a transformer turns ratio of 1:a, the diode voltage stress is approximately twice the transformer secondary voltage, namely 2·a·VDC. Consequently, the secondary diodes' voltage rating should be higher
Foley & Lardner
Power Designers, LLC
Tso Edward H.
LandOfFree
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