**DC-DC Converter (1): Enhancing Voltage Conversion Efficiency**
TDK has introduced its DC-DC converters for hybrid and electric vehicles. These vehicles operate with high-voltage battery systems, often reaching hundreds of volts. The DC-DC converter plays a key role by stepping down the battery voltage to 14V, which is then used to power the lead-acid battery. This battery supplies energy to auxiliary systems such as the engine's starter, windshield wipers, and headlights.
The first mass-produced hybrid vehicle has been on the road for over a decade. As technology advances, the power density of DC-DC converters—measured per unit volume—has continuously improved. This trend is expected to continue in the future, driven by the demand for more compact and efficient components.
TDK’s DC-DC converter was first implemented in hybrid vehicles as early as 1997. It is now used in Honda’s current Civic Hybrid and the new Insight model (see Figure 1). The technology has also been adopted by several overseas manufacturers, highlighting its reliability and performance.
One of the main reasons Honda chose TDK’s DC-DC converter is its compact and lightweight design. By reducing the size of the PCU (Power Control Unit), including the DC-DC converter and inverter, along with the Ni-MH battery, Honda managed to create more space in the vehicle. In the Civic Hybrid, these components were placed behind the rear seat, while in the Insight, they are located under the luggage compartment. This not only increases cargo space but also lowers manufacturing costs.
The latest DC-DC converters used in the Insight are 45% lighter and 5% smaller than those in the Civic Hybrid (see Figure 2). Their weight is less than 1kg, and they maintain a conversion efficiency above 90%.
By eliminating the need for an alternator in hybrid and electric vehicles, the DC-DC converter can replace it. Traditional alternators use engine rotation to generate electricity for the lead-acid battery (as shown in the image).
Electric vehicles rely on large-capacity rechargeable batteries. With the DC-DC converter, the lead-acid battery can be charged directly from the main battery, removing the need for an alternator (see image).
The Insight does not have an alternator (see image), which helps reduce weight and complexity. With a rechargeable battery and DC-DC converter, the lead battery can be charged regardless of engine speed. In traditional gasoline cars, low engine speeds may cause battery drain when multiple accessories like air conditioning, stereo, and lights are used simultaneously. However, with a DC-DC converter, power is available at any engine speed.
**DC-DC Converter (2): Retaining Lead Storage Battery**
Hybrid and electric vehicles still retain lead storage batteries, even though they can technically save costs. There are two primary reasons for this. First, keeping the lead battery reduces the overall cost of the vehicle. Second, it ensures redundancy in the power supply system.
Lead batteries can deliver high currents quickly, which is essential for powering accessories like air conditioning, wipers, and lights. Replacing them with a rechargeable battery would require a larger and more expensive DC-DC converter, negating any cost benefits. Lead batteries are inexpensive, making them a practical choice.
Additionally, the lead battery acts as a backup power source. If the DC-DC converter fails, the lead battery can keep critical systems running, preventing a complete loss of power. For example, if the wipers or lights stop during rain or at night, having a lead battery allows the driver to reach a safe location.
A future development for DC-DC converters is bidirectional operation. Current models only convert voltage in one direction, but there is growing demand for two-way functionality. This would allow the lead battery to supply power to the main battery in emergencies. Bidirectional DC-DC converters are being explored to enhance system reliability.
TDK has developed a basic circuit platform for DC-DC converters, which has evolved through generations such as GEN3 (launched in 2001), GEN4 (2005), and GEN4.5 (2008). The next generation, GEN5, is currently in development. Each version improves efficiency and meets specific requirements set by automakers.
The design of each DC-DC converter includes factors like cooling type (air or water), terminal placement, and housing shape, tailored to the vehicle model. The basic structure is optimized for reliable performance in harsh environments.
Efficiency has steadily increased from the second to the fifth generation. At 10A, the efficiency rose from 84% (GEN2) to 89% (GEN4.5). At 70A, it went from 86% (GEN2) to 88% (GEN4.5). The upcoming GEN5 is expected to exceed 90%. (Continued, special contributor: Kondo Hiroshi, Minister of EV Power Division, TDK Power Systems Business Group)
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