Voltage Doubler - Switched Capacitor Circuits

Switched Capacitor Circuits

It is possible to use the simple diode-capacitor circuits described above to double the voltage of a DC source by preceding the voltage doubler with a chopper circuit. In effect, this converts the DC to AC before application to the voltage doubler. More efficient circuits can be built by driving the switching devices from an external clock so that both functions, the chopping and multiplying, are achieved simultaneously. Such circuits are known as switched capacitor circuits. This approach is especially useful in low-voltage battery-powered applications where integrated circuits require a voltage supply greater than the battery can deliver. Frequently, a clock signal is readily available on board the integrated circuit and little or no additional circuitry is needed to generate it.

Conceptually, perhaps the simplest switched capacitor configuration is that shown schematically in figure 5. Here two capacitors are simultaneously charged to the same voltage in parallel. The supply is then switched off and the capacitors are switched into series. The output is taken from across the two capacitors in series resulting in an output double the supply voltage. There are many different switching devices that could be used in such a circuit, but in integrated circuits MOSFET devices are frequently employed.

Another basic concept is the charge pump, a version of which is shown schematically in figure 6. The charge pump capacitor, C_P, is first charged to the input voltage. It is then switched to charging the output capacitor, C_O, in series with the input voltage resulting in C_O eventually being charged to twice the input voltage. It may take several cycles before the charge pump succeeds in fully charging C_O but after steady state has been reached it is only necessary for C_P to pump a small amount of charge equivalent to that being supplied to the load from C_O. While C_O is disconnected from the charge pump it partially discharges into the load resulting in ripple on the output voltage. This ripple is smaller for higher clock frequencies since the discharge time is shorter, and is also easier to filter. Alternatively, the capacitors can be made smaller for a given ripple specification. The practical maximum clock frequency in integrated circuits is typically in the hundreds of kHz.