The Phase-Shift Full Bridge power topology has a lot of potential for its use in applications where
high reliability is a major design driver. Reliability requirements impose the use of specific
techniques, and these are easy to implement in this topology. However, relevant hi-rel markets
such as aerospace and defence have important component choice and component procurement
constraints related to part qualification, use of extended range components, radiation tolerance
and so on. This Application Note aims to highlight the merits of this topology for such uses, giving
practical examples of possible implementations.
CASE OF STUDY
The Phase Shift Full Bridge (PSFB) with an external inductor (Lr) and two clamping diodes (Figure1) is a very common topology for high power applications. The behavior of the topology changes depending on the operation mode. The key aspect is the location of the external inductor. When the lagging leg is connected to the external inductor, the load range of ZVS is wider. On the other hand, with this configuration, the losses for conduction in the clamping diodes are increased.
COMPARISON BETWEEN THE TWO OPERATION MODES
The following figures describes both possibilities of leading and lagging leg.
Operation mode B increases significantly conduction losses for in the diodes. It is possible that the diodes used in operation mode A, are not adequate in term of current for mode B. In order to choose the best operation mode in each application, a trade off between losses in clamping diodes and losses by commutation has to be achieved.
To demonstrate the analysis, a 270/48 V, 100 kHz, 600 W prototype of a PSFB have been built. In the Figure 4 is appreciated that Mode B presents better performance and low and medium power levels, with similar values at maximum power.
An analysis of two operation modes of the PSFB depending on the location of the external inductor is presented. It is determined the energy available for the commutations, especially for the lagging leg, which is the most complicated to achieve ZVS under medium load conditions. In the analysis is shown that the energy available for the commutations in the lagging leg in operation mode A is lower than the energy in operation mode B. In addition a design guideline is presented. The experimental results for the prototype built shown that in mode B the efficiency is better at medium load.