Detailed explanation of DC/DC in electric vehicles
Electric vehicles (EVs) are equipped with large high-voltage lithium-ion batteries and low-voltage lead-acid batteries used in engine vehicles. Both batteries need to be charged, lithium-ion batteries need to be charged at charging stations, and lead-acid batteries need to be charged from lithium-ion batteries. Currently, the device that converts a certain DC power into a different DC power is a DC/DC converter. We will explain the functions and system configuration of DC/DC converters.
What is a DC/DC converter?
Overview of DC/DC converters
All EV devices rely on electricity to operate. The driving motor, etc. relies on the power of large/high-voltage lithium-ion batteries to operate, and many other onboard devices (ECU, cameras, lights, etc.) rely on the power of low-voltage lead-acid batteries to operate. If the lead-acid battery is not charged, it will run out of power, so it needs to be charged from the lithium-ion battery. Among them, the device that converts high-voltage DC power into low-voltage DC power is a DC/DC converter. By converting to a low voltage, various on-board devices of EVs can operate at an appropriate voltage.
Different uses of high voltage and low voltage
- Uses of high voltage
To operate the driving motor, high voltage is required. Since the motor requires high power, if it is attempted to operate at low voltage, a large current will be generated, resulting in increased losses in the circuit and poor conversion efficiency. To improve efficiency, high voltage (400V or more) is required to suppress the current.
- Uses of low voltage
Low voltage is used for equipment other than those related to the driving motor (various equipment in the car and headlights, etc.). In addition, even for high-voltage equipment, the internal control circuit operates at low voltage. Therefore, a DC/DC converter that converts from high voltage to low voltage is essential.
About the circuit structure of the DC/DC converter
Overall structure
- Voltage detection (input side): Measures the input voltage from the lithium-ion battery
- Noise filter (input side): Removes noise using capacitors and coils
- Voltage conversion circuit: Converts voltage by switching through insulation transformers and FETs
- Noise filter (output side): Removes noise using capacitors and coils
- Voltage detection (output side): Measures output voltage
- Control circuit: Controls conversion circuit, etc.
- DC/DC converter: Supplies power to the control circuit
- Communication circuit: Communication circuit with the outside
On EVs, most of the onboard devices other than the motor operate at a voltage much lower than the vehicle's main power supply voltage. Among them, the device that converts the high-voltage DC output of the lithium-ion battery into low-voltage DC is the DC/DC converter. In the future, as the number of EVs increases, the number of DC/DC converters installed will also increase.
In addition, as various devices become more electrified and more powerful, it is also necessary to pursue higher output of DC/DC converters. In the future, people will put forward higher performance requirements for automotive electronic components, such as high voltage (high withstand voltage), large current, low loss, high heat resistance, and miniaturization.
