Resumen
The goal of this project is the analysis and design of a DC/DC converter for aircraft applications. This converter is part of the output of an isolated rectifier unit for the distribution of electric power inside the airplane.
This work is part of the research effort done by the Center of Industrial Electronics in Universidad Politécnica de Madrid (CEI-UPM), in the More Electric Aircraft (MEA) topic.
The system works as a rectifier of the current obtained at the turbines, the generator of the plane. A three-phased current flows to a device for eliminating the electronic interference (EMI). Then, a buck rectifier reduces the current, and finally a DC/DC converter adjusts its input voltage to the voltage at the load.
The objective of this project is designing an efficient and lightweight DC/DC converter in order to reduce fuel consumption on the plane. Its input voltage come from the rectifier. Output voltage is 28V as it is nowadays integrated in most planes. It is formed by active elements (switches and control) and by passive elements (transformer and bridge).
The topology chosen is full-bridge as isolation is needed and transformer is simpler than in half-bridges. Inside full-bridge, there are different possibilities: phase shifted, dual active bridge, resonant dual active bridge and triangular full bridge.
The Triangular Full-Bridge is the chosen topology because the soft switches on both bridges at the primary and the secondary, and it only needs one magnetizing component, which gives the leakage needed for obtaining the triangular wave form. Due to the high ripple on this current, high capacitance is needed at the output. This capacitance can cover a big volume, being a disadvantage for this topology.
It is mandatory to avoid Continuous Current Mode (CCM) and always operate with Discontinuous Current Mode (DCM), as behavior control of the converter changes and less hard-switchings are required. ZCS is achieved on most switches and less losses and noise on output are produced.
Anyway, there are some disadvantages that must be considered:
• High number of transistors: The main disadvantage of a full-bridge topology is that it needs minimum eight transistors. Because of this the total voltage drop in transistors duplicates of that in case of center-tap rectifiers, if both sides of the transformer are connected as rectifiers. Losses are increased and conversion efficiency is somewhat reduced.
• High RMS currents: The RMS current is the equivalent steady DC value which gives the same effect and therefore the same losses on transformer and transistors, as they have internal resistances.
Another disadvantage of high RMS current is high ripple, that leads to high capacitance on the output. With interleaving ripples can be compensated and capacitance can be smaller.
There are three different states per cycle on current waveform. The Dead state allows to have ZCS.
Different types of power semiconductor switches are designed for functioning in diverse conditions. Voltage, current and switching frequency of the application are the parameters that need to be considered for electing the right switch.
Increasing frequency allows the transformer to be smaller. As converter’s layout depends on the size of the transformer and specifications establish that it is needed a high power density. Therefore, a high frequency is obligatory.
MOSFET is the technology that best fits these exigencies. Some IGBTs are also good solutions. There are different kinds of MOSFETs depending on their material: Silicon MOSFETs are the traditionally used; Silicon Carbide MOSFETs are good for preforming with high voltages and high power, Gallium Nitride MOSFETs are excellent for high frequencies.
Interleaving consists in producing a multiphase current thanks to various converters connected in parallel. There are as many phases as converters. The main advantage is that every converter handles less power so current peaks are lower.
In this application, interleaving helps to take advantage of the leakage inductance of the transformer. With a single converter, for most turn ratios it is mandatory to add another external inductance to the circuit. This is a big disadvantage as it would force to operate with specifications that would not produce the less losses possible, or to place an external inductance that would drop power density.
In order to evaluate the efficiency of this converter, a proper calculation of losses in the devices as well as in the transformer is required. As it was explained before, the design and optimization of the transformer is out of the aim of this work so it will not be calculated. However, the optimized design is going to take into account some of the restrictions on the transformer design and the integration of the magnetizing components needed to achieve a good power density.
The parameters regarded for the design space are:
• Frequency
• Inductance
• Turns ratio of the transformer.
• Number of devices in parallel (Primary and secondary)
• Diodes or MOSFET in secondary
• Number of Parallel converters
• Stability
• Maximum current on devices
Primary bridge MOSFETs operate with 400V, therefore the best materials are Silicon and Silicon Carbide. It is possible to discriminate two types of MOSFET side:
- High side has switching, conduction and reverse recovery losses. Consequently, the number of MOSFET in parallel per side and the MOSFET employed are determined by the configuration that reaches the lower losses.
- Low side only has conduction losses. For that reason, the MOSFET chosen is the one with less RDS at temperature of operation. Also a high number of MOSFET per side is desired.
Secondary bridge performs with 28V, for that reason GaN MOSFETs and diodes can be taken into account as alternatives to Silicon and Silicon Carbide MOSFETs. Secondary side mainly has conduction losses. Therefore, the same configuration selection as in low MOSFETs in primary is applicable.
An important fact to take into account is that devices have to be able to endure the current that flows through them without breaking.
With low frequencies, the time demanded for reaching the current peak that delivers on output the correct power is bigger than in high frequencies. Consequently, slope is smaller and inductance bigger in low frequencies than in high frequencies.
But with more converters, different turn ratios produce different waveforms at the output, and consequently ripple changes as well. Ripple drops and capacitance thus is not forced to be that big. Its size changes with turn ratio as well. However, there are some turn ratios where adding more converters ripple increases.
Losses in transformer are reduced by ZVS. But, in order to achieve zero voltage, some magnetizing current is required. As a consequence, on dead state there is no zero current and ZCS on MOSFETs is not achieved. Therefore, switching losses increase in configurations where high magnetizing current is needed.
It is not possible to determine which is the best solution. Future research lines on the transformer will allow to choose the configuration with lower volume, weight and losses.