Week 4 Session 5

answer:

DC/DC converters for electric vehicles

The different configurations of EV power supply show that at least one DC/DC converter is necessary to interface the FC, the Battery or the Supercapacitors module to the DC-link.

In electric engineering, a DC to DC converter is a category of power converters and it is an electric circuit which converts a source of direct current (DC) from one voltage level to another, by storing the input energy temporarily and then releasing that energy to the output at a different voltage. The storage may be in either magnetic field storage components (inductors, transformers) or electric field storage components (capacitors).

DC/DC converters can be designed to transfer power in only one direction, from the input to the output. However, almost all DC/DC converter topologies can be made bi-directional. A bi-directional converter can move power in either direction, which is useful in applications requiring regenerative braking.

The amount of power flow between the input and the output can be controlled by adjusting the duty cycle (ratio of on/off time of the switch). Usually, this is done to control the output voltage, the input current, the output current, or to maintain a constant power. Transformer-based converters may provide isolation between the input and the output. The main drawbacks of switching converters include complexity,

Non-isolated converters

The non-isolated converters type is generally used where the voltage needs to be stepped up or down by a relatively small ratio (less than 4:1). And when there is no problem with the output and input having no dielectric isolation. There are five main types of converter in this non-isolated group, usually called the buck, boost, buck-boost, Cuk and charge-pump converters. The buck converter is used for voltage step-down, while the boost converter is used for voltage step-up. The buck-boost and Cuk converters can be used for either step-down or step-up. The charge-pump converter is used for either voltage step-up or voltage inversion, but only in relatively low power applications.

Isolated converters

Usually, in this type of converters a high frequency transformer is used. In the applications where the output needs to be completely isolated from the input, an isolated converter is necessary. There are many types of converters in this group such as Half-Bridge, Full-Bridge, Fly-back, Forward and Push-Pull DC/DC converters

Electric vehicle converters requirements

In case of interfacing the Fuel Cell, the DC/DC converter is used to boost the Fuel Cell voltage and to regulate the DC-link voltage. However, a reversible DC/DC converter is needed to interface the SCs module. A wide variety of DC-DC converters topologies, including structures with direct energy conversion, structures with intermediate storage components

some design considerations are essential for automotive applications:

• Light weight,

• High efficiency,

• Small volume,

• Low electromagnetic interference,

• Low current ripple drawn from the Fuel Cell or the battery,

• The step up function of the converter,

• Control of the DC/DC converter power flow subject to the wide voltage variation on the converter input.

Each converter topology has its advantages and its drawbacks. For example, The DC/DC boost converter does not meet the criteria of electrical isolation. Moreover, the large variance in magnitude between the input and output imposes severe stresses on the switch and this topology suffers from high current and voltage ripples and also big volume and weight. A basic interleaved multichannel DC/DC converter topology permits to reduce the input and output current and voltage ripples, to reduce the volume and weight of the inductors and to increase the efficiency. These structures, however, can not work efficiently when a high voltage step-up ratio is required since the duty cycle is limited by circuit impedance leading to a maximum step-up ratio of approximately 4. Hence, two series connected step-up converters would be required to achieve the specific voltage gain of the application specification. A full-bridge DC/DC converter is the most frequently implemented circuit configuration for fuel-cell power conditioning when electrical isolation is required. The full bridge DC/DC converter is suitable for high-power transmission because switch voltage and current are not high. It has small input and output current and voltage ripples.

The full-bridge topology is a favorite for zero voltage switching (ZVS) pulse width modulation (PWM) techniques.

Electromagnetic compatibility regulation

Fast semiconductor devices make it possible to have high speed and high frequency switching in power electronics converters. High speed switching helps to reduce weight and volume of equipment; however, it causes some undesirable effects such as radio frequency interference (RFI) emission. It is believed that high dv/dt or di/dt due to modern power device switching is mainly responsible for the EMI emissions. Application of electrical equipment especially static power electronic converters in different equipment is increasing more and more. Power electronics converters are considered as an important source of electromagnetic interference and have undesirable effects on the electric networks. Some residential, commercial and especially medical consumers are very sensitive to power system disturbances including voltage and frequency variations. Also, for Electric vehicle, there is limitation of the EMI. The best solution to reduce the interference and improve the power quality is complying national or international EMC regulations. CISPR, IEC, FCC and VDE are among the best known organizations from Europe, USA and Germany who are responsible for determining and publishing the most important EMC regulations. Compliance of regulations is evaluated by comparison of measured or calculated conducted interference level in the mentioned frequency range with the stated requirements in regulations. In European community compliance of regulation is mandatory and products must have certified label to show covering of requirements.

Depending on the type of function performed, power electronic converters are categorized into following types.

• AC to DC = Rectifier: It converts AC to unipolar (DC) current
• DC to AC = Inverter: It converts DC to AC of desired frequency and voltage
• DC to DC = Chopper: It converts constant to variable DC or variable DC to constant DC
• AC to AC = Cycloconverter, Matrix converter: It converts AC of desired f

  Here the fixed DC voltage is obtained from batteries or by DC link in most power electronic converter. The output of the inverter can be variable/ fixed AC voltage with variable/fixed frequency.

  This conversion from DC to AC along with variable supply is produced by varying the triggering angle to the thyristors. Most of the thyristors used in inverters are employed with forced commutation technique.

  These can be single phase or three phase inverter depending on the supply voltage. These converters are mainly divided into two groups. One is PWM based inverters and other multilevel inverters.

  Further, these are classified voltage source inverter and current source inverter. Each type is subdivided into different types such as PWM, SVPWM, etc. Multilevel inverters are more popular in industrial applications.

  requency and/or desired voltage magnitude from a line AC supply.