a flyback converter based on a switcher ic with pulse width modulation control of the switching transistor . the pulse width is controlled by changing the input current at the control input . under normal operating conditions the voltage at the control input is maintained at a fixed level , which voltage also serves as a supply voltage for the control unit . a charge pump provides extra charge for the smoothing capacitor at the control input when the load would cause the voltage across the smoothing capacitor to collapse below the fixed level , thereby preventing the control unit from entering the auto - restart mode of the switcher ic . in this way the flyback converter can be used as a constant power source for charging rechargeable batteries with a working voltage lower than the nominal operating output voltage of the converter .