Fast Battery Charger using
Buck-Boost Full H-Bridge Circuit
full H-Bridge circuit described elsewhere is
adapted to provide a battery charger for a number of
different battery types, by adding an SLA battery charging
algorithm to the
SMPS algorithm. The full H-Bridge buck-boost converter provides
somewhat better efficiency at higher currents than the half H-bridge,
but it adds its own challenges, particularly when used as a battery
was tested with 1.4AH and 7.5AH, 12V, SLA batteries by Diamec.
is intended for use with sensitive radio
equipment to provide direct battery power for periods up to 8 hours,
with automatic switch over to recharge when the equipment is turned
simple load current detection provides the switching.
with this circuit is to prevent the battery
discharging back through the MOSFETs and coil during the startup phase.
To avoid this, the charger is kept turned off after power on or reset
analogue circuitry has settled and a steady battery terminal voltage
can be read. Then the charger is activated, with the upper boost
MOSFET turned off, until a measurable current starts to flow into the
the output voltage climbs above that of the originally measured battery
This ensures that the battery cannot discharge back through the
circuit. The Schottky diode (1A capacity) that bypasses the upper boost
MOSFET can provide the boost function
adequately with the
low currents that occur during this stage. Once charging has begun, the
upper boost MOSFET is allowed to operate. This will reduce the power
of the circuit (in which the full output current is flowing).
battery current is the critical parameter to be monitored for the
battery charger control. The battery voltage tends to be clamped
heavily by the battery, but increasing the terminal voltage above the
battery's natural terminal voltage can cause the current to increase
strongly. As such the current is measured every program cycle and is
unfiltered to allow rapid response of the algorithm.
parameters to be measured are the input voltage, the battery voltage
and the load current. Each of these is measured every four cycles and
is filtered with an exponential
filter to reduce the variance. The SMPS control
algorithm is computed every program cycle while the remaining
algorithms are computed every four cycles. As an A/D conversion takes
13 microseconds, the program cycle will take on average about 26
microseconds; the computation time of the algorithms is designed to be
very short by making use of integer arithmetic and avoiding
for the program is as follows:
A suitable SMPS control algorithm is described for the half H-bridge SMPS battery charger.