25W OFF-LINE AUTORANGING Battery CHARGER
by Domenico Arrigo
This note describes an off-line autoranging (85Vac-264Vac) Battery charger designed to operate in three differ-
ent functional modes:
– constant voltage
– constant current
– constant power
These requirements are typical of Battery chargers for notebook computers.
The resulting output characteristic of the system is shown in fig 1.
The A-B portion of the V-I characteristic is a constant voltage mode. This situation happen when the Battery pack is almost completely charged. Vo has to be set at the final value of the Battery pack. From point B to C the
charger operates in constant power mode, reducing the output voltage while increasing the output current.
The benefit of introducing the constant power mode is that the maximum output power is lower with respect to
the power output at point E. This reduces the power managed by the primary circuitry and in consequence re-
duces the total dissipation power. This is a useful improvement considering that the complete system is pack-
aged in a plastic case with a very limited capability of dissipating heat.
From point C to D the adapter works in constant current mode until the output voltage reaches 0V.
In practice it is difficult to have a constant current characteristic below an output voltage of 2-3V because the
primary auxiliary supply voltage drops at the controller turn-off threshold (internally fixed at 7.6V typ.) and the
V-I characteristic exhibit a foldback to zero.
If mandatory to operate in constant current mode till 0V, a secondary post-regular or other tricky solutions on
the windings, primary or secondary, are required.
Fig 2 shows the V-I characteristic obtained with the circuit of Fig 3.
Regulated output voltage
Maximum output power
Target efficiency @ full load
Fig 3 shows the electrical circuit.
Figure 1. V-I output characteristic
Figure 2. Practical V-I characteristics
85Vac to 264Vac
discontinuous current mode flyback