Abstract: A switching power supply contains a transformer. Connected on the secondary side of the transformer is a dummy load circuit. The dummy load circuit rectifies, voltage divides and low-pass filters the signal on the secondary side of the transformer. If the output of the low-pass filter is below a predetermined value, it is recognized that an insufficient load (which might be no load at all) is connected to the output of the switching power supply. In that case, the dummy load circuit activates a transistor which switches a dummy load resistor across the output of the power supply to assure at least a predetermined minimum electrical load on the output. If the electrical loads on the output of the switching power supply include a voltage source such as a battery, a relay is connected in series with the dummy load resistor within the switching power supply.

Abstract: Charging voltage and current control circuits for battery chargers provide for a constant output voltage above a first predetermined value of charging current and an increasing output voltage below the first predetermined value of charging current. The charger makes use of a pulse-width modulated control circuit utilizing a variable duty cycle proportional to charging current and couples a signal proportional to charging current via a diode-resistor network to the pulse-width modulator for values below the first predetermined value of charging current. A current limit control circuit provides a current limit function by driving the pulse-width modulator to a minimum duty cycle during overcurrent conditions.

Abstract: Charging voltage and current control circuits for battery chargers provide for a constant output voltage above a predetermined value of charging current and a step-wise increased output voltage below the predetermined value of charging current. The charger makes use of a pulse-width modulated control circuit utilizing a variable duty cycle to control charging current and couples a trim command signal responsive to a primary side signal representative of charging current to the pulse-width modulator for values below the predetermined value of charging current. A current limit control circuit provides a foldback current limit function by driving the pulse-width modulator to a reduced output current during overload conditions.