In pure electric cars, in addition to the high-voltage power motor, people often use air conditioners, radios, speakers, car light systems, electric windows, windshield wipers, power steering systems, hydraulic brakes, pneumatic brakes, etc. Air-conditioning heaters are collectively referred to as auxiliary subsystems, and they are mostly 14V or 28V, and higher voltages are also useful. When the traditional gasoline engine is at low speed, if the air conditioner, stereo and car lights are used at the same time, even if the engine is still running, there will be insufficient power under some conditions. After using the power battery and DC/DC, you can charge the lead-acid battery without considering the engine speed. In traditional fuel cars, only the starting battery is used for starting, and generally only a 12V or 24V battery is used to supply power to the auxiliary subsystem.
(1) The necessity of retaining the auxiliary battery
Pure electric cars use power batteries as power sources and can be repaired using DC/DC to charge auxiliary batteries. Therefore, after a pure electric car is equipped with DC/DC, the alternator of the original car can be omitted.
Pure electric cars still retain the auxiliary battery. There are two main reasons for this: one is to retain the auxiliary battery, which can reduce the cost of the entire vehicle, and the other is to ensure the redundancy of the power supply. Water heaters and car lights are amplified currents. If the auxiliary battery is omitted. The use of electric car battery power for air conditioners and wipers through DC/DC will increase the size of the DC/DC and increase the overall cost. In addition, the auxiliary batteries are mostly lead-acid batteries, which are cheap. Therefore, there is currently no cost advantage in replacing lead-acid batteries with power batteries (lithium batteries). When the CD/CD fails and stops supplying power, if there is no auxiliary battery, the low-voltage electrical appliances will immediately stop running, and the lights at night will not turn on, and the wiper will stop running in rain, etc., which will affect driving. If there is an auxiliary battery, the car can be driven to the nearest home or repair shop.
(2) Low-voltage system
The electrical appliances in the fuel car (gasoline car) usually use 12V, so the power supply is reduced by DC/DC and the output is 14V. For the fuel gas (diesel car) of the 24V electrical system, the pressure should be reduced to 28V.
(3) Sub-high pressure system
In order to save energy, high-voltage power supply is required for those devices with high power, such as motor controllers, power steering systems, hydraulic or pneumatic braking systems, and air conditioning defrosters (heaters). Therefore, there are several DC/DCs. In addition to stepping down and outputting the conventional 14V and 28V, they also use 48V, 120V or even higher sub-high voltages. This makes the auxiliary battery system of electric cars more complicated than the original car system of fuel cars.
The energy consumption of the auxiliary subsystem of pure electric cars is much larger than that of fuel cars. The power consumption of various auxiliary subsystems is shown in Table 1. From Table 1, it can be seen that the air conditioning system is the subsystem with the largest power consumption among the auxiliary subsystems of pure electric cars, and its power consumption accounts for all auxiliary subsystems. 60%~75% of power consumption. In order to reduce the loss of the air-conditioning system, the voltage level of 120V is usually used for power supply. In addition, in order to avoid exhaustion of the auxiliary battery power in a short time, high-power subsystems, such as air conditioning systems, power steering systems, hydraulic brakes (or pneumatic brakes) and defrosting devices, should only be closed when the contactor is closed. It can only work when it is time, so that the required power can be obtained directly from the main power source.
|Auxiliary subsystem||Working status||Power consumption/W|
|Energy Management System||continuous||150|
|Headlights and taillights||continuous||120|
|Stop lights, turn signals and interior lights||Intermittent||50|
|Power steering system||continuous||400|
|Hydraulic brake or pneumatic brake||continuous||1500|
|Electric car window||Intermittent||80|
|Car window defroster||continuous||250|
Generally, pure electric passenger cars have only one DC/DC, such as stepping down 400V DC high voltage to 14V or 28V DC voltage. Such an air conditioning system directly uses the DC voltage of the power battery to supply power, and the AC high voltage supplies power to the air compressor motor and the steering oil pump motor. High-end electric cars can have several DC/DCs to generate different DC voltages. At the same time, there are also different DC/AC converters that generate different AC voltages to drive motors of different systems. Of course, low-voltage direct current, such as 200V, can be raised to high-voltage direct current 600V, but this kind of boost is regarded as the use of power motors to drive cars.
Figure 1 shows the DC/DC for electric cars exhibited by Nichicon of Japan at “CEATEC JAPAN 2008”. The converter is equipped in Fuji Heavy Industries’ electric cars “Rle” and “Plugin STELLA Concept”. DC/DC is used to reduce hundreds of volts of car battery voltage to 14V or 28V DC voltage that can be used in the car, and the output current is 60~120A.
In order to support the home power supply voltage of countries all over the world, the input voltage of DC/DC and charger is 100~265V. The maximum output voltage is 2.6kV, and the maximum conversion efficiency is 88%.
The main factor that determines DC/DC performance is the transformer, including the size, shape, and supported switching frequency of the transformer. By increasing the switching frequency, the size of the transformer and the rectifier circuit can be reduced, because increasing the frequency can increase the number of switching of the power semiconductor per unit time. However, in order to prevent access to the frequency of radio AM broadcasting, the 70kHz frequency band has been used in the past. Recently, due to the advancement of noise suppression technology, the 110kHz color band has been adopted.
The core material of the transformer adopts the ferrite material “PC95”. The raw materials of PC95 are Fe (iron), Mn (manganese), and Zn (zinc). The mixing ratio of Fe is different, mainly to reduce the increase in iron loss and the decrease in efficiency that occur at certain temperatures. The latest iron core can reduce iron loss in a wide temperature range. The core loss is dominated by hysteresis loss and also includes eddy current loss.
One of the directions for DC/DC function improvement in the future is bidirectional. The DC/DC used now only changes the voltage in one direction. There is also a need to change the voltage in both directions. When the power of the power battery is insufficient, the power of the auxiliary battery can be input to the power battery for emergency needs. This is also a method to ensure redundancy. Some models also add converter components such as DC/AC output terminals and boost converters.
DC/DC continues to be miniaturized, lightweight, and efficiency continues to improve. The type of DC/DC and the basic structure of the converter circuit are different due to design differences and technical requirements in different periods. Water cooling/air cooling, terminal position, shell shape, etc. are designed according to the car type used, and the basic structure is designed on the premise of air cooling in severe environments.