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Delving Into Different Types of Switched Capacitor Voltage Converters


Delving Into Different Types of Switched Capacitor Voltage Converters

In the first half of switching cycle, the circuit capacitor C1 is charged to the input voltage and in the second half of the switching cycle the voltage is then inverted and applied to a capacitor C2 and load. This leads to the input voltage to be negative of the output voltage and defined as the Duty cycle which is the ratio of charging time of a capacitor C1 to the entire switch cycle time. This is almost always because 50% is the yield of the optimal charge transfer efficiency. The capacitor C1 has to supply only a small amount of charge during the time period of transient conditions startup to the output capacitor on each switch cycle and on steady state conditions.

The capacitor C1 is also known as a charge pump capacitor with the charge transferred depending upon the current load and the frequency of the switches. The output capacitor C2 must supply the current load during the time the input voltage is charging the charge pump capacitor. This will cause a droop in the output voltage caused by the load current flowing out of the C2 which also happens to correspond the output voltage ripple of a component. The high frequency of the switching allows smaller capacitors to droop and switching the frequency will require a different size of external capacitor required though the frequencies will be limited to a few hundred Khz. While switched capacitors inverters are low cost in nature, the efficiency and compactness achieved are greater than 90% while the maximum output current of typical switched capacitor inverters are 150mA maximum.

Single power supply system only require a few high performance parts with negative voltage whereas relatively low current negative voltage is required in addition to positive primary voltage. Voltage doublers are primary used in low current applications and adding regulation to the switched capacitor voltage converters increases the effectiveness of its application. Finally LDO which is low dropout linear regulator is the most straightforward and provides the regulated output while also reducing the ripple of the switched capacitor converter. This approach reduces the available output voltage by the dropout voltage of the LDO but will also add to the complexity of the approach. Another highly non-linear approach requires long time constants to vary the duty cycle of the switch control signal in order to maintain good regulation control.

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