A Full H-Bridge Buck-Boost Converter with high side P-type MOSFETs

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 powers. The 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.

Experiment

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 Algorithms

The control algorithm used is the same as that used in the half H-bridge circuit.


First created 4 December 2010

Last Modified 26 December 2010
© Ken Sarkies 2010