Driven by the urgency of energy conservation and environmental protection, along with the latest global energy efficiency regulations, enhancing energy efficiency has become a universal goal across industries. Among various switching technologies, such as flyback, forward, two-switch flyback, two-switch forward, and full bridge, soft-switching technologies like LLC (dual inductor plus single capacitor), active clamp flyback, active clamp forward, and symmetrical half bridges (AHB) offer significantly higher energy efficiency. Consequently, in energy-conscious applications, soft-switching technology is gaining increasing popularity among designers.
On the other hand, the half-bridge configuration is ideal for medium and low-power applications requiring high energy efficiency and power density. This topology involves two primary types of MOSFET drivers: high-side drivers and low-side drivers. The high-side driver allows the MOSFET source to float between ground and the high-voltage input, while the low-side driver keeps the source always grounded, as illustrated in Figure 1. When the high-side switch turns off, the MOSFET source voltage increases from ground to the high-voltage input, necessitating an isolated or floating gate drive circuit. Conversely, the source of the low-side MOSFET remains grounded, allowing the gate drive voltage to also be grounded, simplifying the driving of the low-side MOSFET.
Table 1 provides a cross-reference for ON Semiconductor's high-side MOSFET drivers.
The project comparison suggests that we evaluate two solutions for a 24V@10A LLC half-bridge circuit: one using a transformer drive scheme and the other employing a silicon driver IC. Both solutions utilize the LLC controller NCP1395 with dual DRV outputs, but the former uses a transformer to drive the LLC converter’s MOSFETs, whereas the latter employs the NCP5181 driver IC. While the waveforms of the two solutions appear similar, the driver IC shuts off the high-side MOSFET faster—by 70ns—reducing switching losses. Additionally, when the high-side MOSFET turns on, the driver IC ensures a safe and adequate dead time between the high-side and low-side MOSFETs, outperforming the transformer drive scheme. In terms of energy efficiency, both solutions exhibit comparable performance at the same input power (for more details, refer to Reference 1).
Which solution should you choose? Careful design makes both viable. As a leading provider of high-performance, energy-efficient silicon solutions for green electronics, ON Semiconductor recommends opting for the silicon driver solution. This approach simplifies wiring and design, eliminating the need for manual transformer insertion. Furthermore, it circumvents issues associated with transformer schemes, such as broken isolation, broken magnetic flux, and unexpected ringing post-shutdown. Additionally, for slim designs, the height of the transformer in flat power supplies poses a challenge, which the silicon chip driver scheme avoids entirely.
To summarize: For applications focused on energy efficiency, half-bridge topologies utilizing soft-switching technology are becoming increasingly popular among designers. However, driving high-side MOSFETs in a half-bridge topology requires choosing between transformer-based or silicon chip drivers. This paper explores the design considerations, related challenges, and solutions for these different drive schemes, comparing the two approaches from multiple angles. Although both drive solutions perform well when designed properly, ON Semiconductor advocates for the silicon driver solution, such as the NCP5181, to streamline wiring and design while mitigating many of the drawbacks of transformer drive schemes. This reduces the design cycle and expedites product launches.
Reference Materials:
1. Half-bridge driver: Transformer or full-silicon driver, ON Semiconductor training course, [Link to PDF].
2. NCP5181 data sheet, [Link to datasheet].
In conclusion, soft-switching technologies in half-bridge configurations are increasingly favored in energy-efficient applications. When selecting a driver for high-side MOSFETs, designers must weigh the pros and cons of transformer-based versus silicon chip solutions. This article evaluates both options, highlighting the benefits of silicon driver solutions like the NCP5181, which simplify design and reduce potential complications inherent in transformer schemes.
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