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Conventionally, a centralized power supply is applied in an automotive powertrain through a multichannel transformer that converts 12-V dc power from the battery to supply six, isolated gate-drive circuits (Fig. 1).
Designers face many challenges in a centralized transformer model, including layout complexity and electromagnetic interference (EMI). There are also challenges associated with larger board space and higher printed-circuit-board (PCB) cost, as more layers are needed to route isolated signal/power lines.
A distributed power supply can be easily built using a few discrete components and a small high-efficiency transformer placed next to the IC integrating an automotive-grade smart gate drive with integrated flyback controller (Fig. 2). This reduces the overall footprint and minimizes EMI and noise coupling between insulated-gate bipolar transistor (IGBT) channels.
With a distributed-power-supply architecture, designers have more flexibility in planning the circuit layout; the low-voltage plane can be distinguished and isolated easily from the high-voltage plane. In addition, overall PCB routing becomes more manageable and straightforward.
Figures 3 and 4, respectively, compare a six-channel IGBT gate-driver board based on a centralized power supply versus one based a distributed power supply. It’s obvious that a distributed-power-supply architecture offers a simplified PCB layout and more efficient routing. There are no PCB traces or power planes crossing between low- and high-voltage circuits, enhancing the signal integrity and avoiding unfavorable noise disturbance to the signal lines.
The transformer in a distributed power supply is typically 14 times smaller in volume versus a centralized transformer. Figure 5 shows an individual transformer placed next to a centralized transformer. The table shows actual dimensions of a centralized transformer and an individual transformer.
A low-profile single transformer for each driver also improves reliability and robustness, compared to heavier, higher-profile transformers that are more vulnerable to mechanical vibration. While the power-supply capacitors used in a centralized-power-supply architecture tend to be larger and in a radial CAN package with a high profile, designers choose a smaller SMD package capacitor for a distributed power supply. The voltage ratings required for these capacitors is at least 10 to 20 V lower than those required for a conventional centralized-power-supply circuit.
On top of design simplicity and robustness, another benefit of choosing a distributed power supply is cost savings through the minimization of overall board size and PCB layers. A distributed system of drivers and a single transformer allows these components to be placed closer together, saving critical board space.
Figure 6 shows an example of a well-designed half-bridge gate-driver circuit (top and bottom channel), placed within a 39.7 mm (width) by 52 mm (length) area, that’s less than 50% of a standard card’s size. In this instance, a six-channel gate-driver circuit may only require a PCB area of one and a half that of a standard card’s size.
A distributed system also helps reduce the number of PCB layers because there’s no crossing of low-voltage traces/planes between the high-voltage traces/planes. This ensures that no extra layers are needed for passing the crossing signals. Figure 7 shows an example of a compact six-channel gate-driver board designed for the Fuji M651 IGB that uses only four layers of PCB.
Better EMI Performance
A large six-channel transformer in a centralized power system typically emits a lot more EMI noise than individual small transformers. In a distributed power supply, each smart gate-drive optocoupler drives an individual transformer with an integrated dc-dc controller to provide power to the secondary side for driving the IGBT arm.
Measurements show significantly higher EMI noise from a centralized six-channel transformer (Fig. 8) as compared to small individual transformers (Fig. 9).
A distributed power supply further simplifies automotive multichannel IGBT gate-drive design versus that of a gate-drive board using a centralized power supply. Furthermore, it improves robustness, EMI performance, and module cost when compared to a conventional centralized-power-supply architecture.
“AV02-4412EN Design of Isolated Flyback Converter for IGBT Gate Driver,” Application Note, Avago Technologies, December 2015
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