15
Types of Switched Capacitor Voltage Converters
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the above circuit capacitor C1 is charged to the input voltage during
first half of switching cycle. In the second half of switching cycle
its voltage is inverted and applied to capacitor C2 and load. The
output voltage is negative of the input voltage. The Duty cycle
defined as ratio of charging time of capacitor C1 to the entire
switching cycle time ? is usually 50% because that generally yields
the optimal charge transfer efficiency
Types of Switched Capacitor Voltage Converters
In
the above circuit capacitor C1 is charged to the input voltage during
first half of switching cycle. In the second half of switching cycle
its voltage is inverted and applied to capacitor C2 and load. The
output voltage is negative of the input voltage. The Duty cycle ?
defined as ratio of charging time of capacitor C1 to the entire
switching cycle time ? is usually 50% because that generally yields
the optimal charge transfer efficiency.
During
the time period of transient conditions start-up and steady state
condition the capacitor C1 has to supply only a small amount of
charge to the output capacitor on each switching cycle. The amount of
charge transferred depends upon the load current and the switching
frequency. Capacitor C1 is also known as charge pump capacitor.
During
the time the charge pump capacitor is being charged by the input
voltage, the output capacitor C2 must supply the load current. The
load current flowing out of C2 causes a droop in the output voltage
which corresponds to a component of output voltage ripple. Higher
switching frequency allow smaller capacitors for the same amount of
droop. Thus the switching frequency impacts the size of the external
capacitor required. Higher switching frequency allows the use of
smaller capacitor. The Switching frequencies are generally limited to
few hundred Khz.
Switched
capacitor inverters are low cost, compact and efficiency achieved are
greater than 90%. Typical switched capacitor inverters have maximum
output current of 150mA maximum.
The
Voltage inverters are used in applications where relatively low
current negative voltage is required in addition to the primary
positive voltage. This may occur in single power supply system where
only a few high performance parts require the negative voltage.
Voltage
Doubler
Voltage
Doubler works similarly to the Inverter, however, the pump capacitor
is placed in series with the input voltage during its discharge
cycle, thereby, accomplishing the voltage doubling function. In
voltage doubler, average input current is approximately twice the
average output current.
Voltage
doublers are used in low current applications where a voltage greater
than the primary supply voltage is required.
Regulated
Output Switched Capacitor Voltage Converters
Adding
regulation to the switched capacitor voltage converters increases its
usefulness in many applications. The most straightforward is to
follow the switched capacitor converter with a low dropout linear
regulator (LDO). The LDO provides the regulated output and also
reduces the ripple of the switched capacitor converter. This approach
however, adds the complexity and reduces the available output voltage
by the dropout voltage of the LDO.
Another
approach to regulation is to vary the duty cycle of the switch
control signal with the output of an error amplifier which compares
the output voltage with a reference. However , this approach is highly
non linear and requires long time constants in order to maintain good
regulation control.
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Tags: c1, c2, capacitor voltage, capacitors, charge transfer efficiency, cycle time, duty cycle, external capacitor, frequency capacitor, input voltage, inverters, khz, output capacitor, output voltage, second half, steady state condition, time period, transient conditions, voltage converters