A Full H-Bridge
Buck-Boost Converter with high
Following on from the half H-bridge SMPS buck-boost
converter is a circuit using a full
H-bridge architecture, which is potentially more efficient at higher
one described here uses P-type MOSFETs for
the high side switches, although N-type
MOSFETs can be used. The circuit was intended for use as a battery
charger, hence the two
current sense resistors at the output, one for battery current and the
other for load current. More details of this circuit are
given in the description of the battery
charger for which it was developed. The circuit can be used also
for investigating maximum power point tracking
algorithms for sources such as solar panels and wind generators.
The following circuit was developed to investigate the algorithms needed to drive the H-bridge buck-boost converter. The load and battery current is measured as well as the battery and source voltages.
In this circuit the Atmel ATTiny461 was used as it provides three direct and inverting PWM output pairs with dead-time controls, the latter being necessary for the proper working of a synchronous SMPS. Only two of the PWM outputs are needed. The device also provides differential ADC inputs with several programmable gains. This would allow some of the operational amplifiers to be eliminated, however the sample and hold delay is twice that of the single-ended ADC inputs. This is too slow for the circuit needs. As with other circuits that have been tested here, the Atmel devices are not well suited to SMPS work because of the low A/D conversion speeds.
The ATTiny461 analogue reference voltage was chosen to be an internal 2.56V rather than the 5V used in other circuits. This was originally intended to allow the differential inputs to be used, and was retained after they were discounted as being unusable. The voltage dividers drop the input and output voltages by 0.145, while the currents are measured at 1V per ampere. In practice these had to be changed empirically by quite a significant amount. The reason for this was not investigated; it may have been due to inaccuracies in the analogue reference voltage.
Control AlgorithmsThe control algorithm used is the same as that used in the half H-bridge circuit.