SLUP412 February   2022 LMG3522R030-Q1


  1. Introduction
  2. Comparing Different Technologies
  3. Advantages of Integrating the Driver With GaN FETs
  4. The GaN-Based 6.6-kW OBC Reference Design
  5. PFC Stage
  6. DC/DC Stage
  7. DC/DC Topology Selection
  8. Frequency Selection
  9. Core Loss
  10. 10Loss of ZVS
  11. 11Dead Time
  12. 12ISR Bandwidth
  13. 13Overall
  14. 14Resonant Tank Design
  15. 15Thermal Solution
  16. 16Layout Best Practices
  17. 17Control-Loop Considerations
  18. 18Conclusions
  19. 19References
  20. 20Important Notice

DC/DC Stage

The primary goals for the GaN-based OBC reference design are to minimize overall solution volume and maximize the electrical efficiency of the power conversion. To effectively optimize these parameters, the DC/DC stage must meet a few critical requirements. First and foremost, this converter is a battery charger. The wide range of output voltage values that the output needs to cover present significant challenges to many topologies; see the highlighted levels in Table 6-1. For example, a popular topology such as an inductor-inductor-capacitor (LLC) converter may have some difficulty meeting high efficiency over such a wide output range. In addition, the bidirectional nature of the converter might require a traditional LLC to have some additional tank components and facilitate the necessary gain.

Table 6-1 DC/DC converter requirements.
Description Minimum Typical Maximum Units
DC output voltage 250 400 450 V
DC output current 20 A
DC output power 6.6 kW

The final solution achieved an efficiency of just over 98%, as shown in Figure 6-1.

Figure 6-1 Measured efficiency of the capacitor-inductor-inductor-inductor-capacitor (CLLLC) converter.

For context, Figure 6-2 shows some essential waveforms during operation at full load.

GUID-20220218-SS0I-3PC1-QBLP-T9NDQMVLFWR0-low.png Figure 6-2 CLLLC essential waveforms at full load.

Topology selection, operating frequency, tank design, operating modes, thermal management, layout and control optimization are among some of the more significant design challenges that we solved in the course of the GaN-based OBC reference design. Now, let us explore these challenges more, along with the rationale behind our decision-making.