Dual Board Net Systems in Automobiles

Modern electric vehicles are increasingly using dual board net systems. These contain both a 12 VDC bus and a 48 VDC bus. One of the key building blocks in the architecture of these vehicles is the high-power, bidirectional 48 VDC to 12 VDC converter. Energy flows in either direction between the two batteries—48 V and 12 V. This helps to optimize the overall efficiency of the vehicle. The direction of the energy flow depends on the demands the vehicle’s electrical system places on the batteries and their state of health.

Vishay offers a complete 3 kW 48 V / 12 V buck-boost type DC/DC converter for electrical vehicles. The design has a standard FR-4 controller board mounted on an IMS or Insulated Metal Substrate that sports a heat sink for the power stage. As these converter designs do not operate at maximum efficiency over a wide power range, Vishay has designed them as six modular power stages operating at 500 W each.

It is possible to switch the protection MOSFETs on/off in each stage. This allows the system to activate or deactivate each power stage individually. Vishay uses this topology for maximizing efficiency under various operating conditions. Moreover, this also provides built-in redundancy, preventing a total breakdown in the event of any failure in an individual power stage.

The converter design from Vishay has another important detail—the half-bridge design uses different MOSFETs. As the high-side MOSFET operates at one-fourth the output current, its on-resistance is not essential. Instead, the gate-drain charge and the gate-source charge of the MOSFETs are more significant. Rather than use the regular low-power thick film resistors, Vishay uses thin-film MELF resistors for driving the gates.

Thin-film MELF resistors can handle large pulses, while not drifting over time and temperature. This prevents an increase in switching losses at frequencies of 100-150 kHz. Switching losses are the dominating power-loss factors at these frequencies. To minimize the drain-to-source resistance in the low-side MOSFET, Vishay connects two of them in parallel, as this resistance is the largest factor dominating the power loss.

The DC/DC converter has a primary storage inductor. This inductor must support both the DC output current and the ripple current. The inductance value and the switching frequency determine the ripple current amplitude. Although increasing the inductance value or the switching frequency helps in reducing the ripple current, it is necessary to consider a tradeoff in performance and size. The designer must ensure the inductor rating is adequate for the output current it must handle, without saturation and high self-heating.

Vishay uses IHDM inductors for primary storage. These have a good combination of low core loss (AC), low DC loss, and very good saturation performance. The IHDM series of inductors from Vishay cover a wide range of inductor values, ranging from 0.1 µH to 200  µH. Their current handling capacity ranges from a few amperes to several hundred amperes. The inductor series also comes in several materials, allowing efficient operation when the converter is operating between 100 kHz and 5 MHz.