SLUAAP2 March   2023 LMG2610 , UCC28782

 

  1.   Abstract
  2.   Trademarks
  3. 1Introduction
    1. 1.1 Design Requirement 1: Managing Thermals Induced by Power Losses
    2. 1.2 Design Requirement 2: Reducing Energy Storage Requirement by Switching at High Frequency
  4. 2A Brief Introduction to GaN's Value
  5. 3The Active Clamp Flyback
    1. 3.1 Power Loss Saving 1: Zero-Clamp Loss
    2. 3.2 Power Loss Saving 2: Zero-Voltage-Switching
  6. 4The Value of GaN in Active Clamp Flyback
  7. 5Leveraging Integrated GaN to Simplify ACF Stage
  8. 6Physical Design Implementations Using LMG2610 Integrated Half-Bridge and UCC28782 ACF Controller
    1. 6.1 UCC28782EVM-030
    2. 6.2 PMP23146
  9. 7Leverage Design Tools for ACF
  10. 8Summary
  11. 9References

4 The Value of GaN in Active Clamp Flyback

The next discussion points highlight the value proposition of GaN within ACF. The required energy to realize ZVS can be explained through Equation 5.

Equation 5. E Z V S = 1 2 L M × I M - 2 ≥ 1 2 C p × V d s 2

Equation 5 makes it apparent that larger device capacitance of silicon devices require more energy for ZVS. As a result, this requires a longer on-time of Q2, which reduces the switching frequency and increases the primary peak current. The combination of higher peak current and longer on-time leads to an increase in the RMS currents that show up as conduction losses in Q1 and the transformer windings. In many cases, these incurred conduction losses can completely negate the other benefits of the ACF, such as zero-clamp-loss and ZVS. As a result, the reduced output capacitance of GaN, compared to Si, is valuable in the ACF topology, as it is a crucial factor in keeping the RMS currents low. An empirical comparison of the RMS currents from GaN and silicon based ACF stages is shown in Figure 4-1.

GUID-20230223-SS0I-DHPD-SGRP-VMLP9DVPFPSC-low.svg Figure 4-1 Current Waveforms of ACF Stage Operating with Silicon vs. GaN FETs

The higher output capacitance of the silicon FET requires more negative magnetizing current and produces higher peak currents, which leads to larger I2rmsR losses compared to GaN.

As a result, the ACF can switch at high frequency while providing valuable power loss savings, but only under the condition that the RMS currents are managed. In summary, ACF enables high efficiency and high frequency operation, while GaN successfully enables ACF.