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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