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

1.2 Design Requirement 2: Reducing Energy Storage Requirement by Switching at High Frequency

Another aspect of power-dense designs that must be discussed is the switching frequency and the relation to the energy storage requirements of the system. This is what is commonly stated as increasing the switching frequency reduces the size of the power converter. Analyzed at a deeper level, the DC power delivered to a load is processed by a power converter which fundamentally works on the principle of storing energy from the input for some time and then releasing it to the load. The power that is delivered to the load is mandated by two factors: the amount of energy that can be stored then released every cycle, and the frequency at which the process happens.

As an analogy, suppose a large shipment of goods has to be made from one port to another across a body of water within a set amount of time. On one hand, consider a) a cargo ship that can deliver the goods in just a few trips. On the extreme contrary, consider b) a speed boat that can make multiple quick trips with less goods per trip. In this analogy, the goods on the vessel represent the energy stored, while the number of trips represent the switching frequency. In the end, both the cargo ship and the speed boat deliver the same amount of goods (energy) in the same amount of time, maintaining constant power. Although the net result of the delivery is the same, there are two differences in the two scenarios: a) transfers more stored energy at a slower frequency, and b) one transfers less stored energy at a higher frequency, or Equation 1.

Equation 1. P=constant=Estored×fsw

In a power converter, this energy storage mechanism is carried out by the largest components, primarily the magnetics such as inductors and transformers. For magnetics, size is directly proportional to energy storage, while energy storage is directly proportional to inductance, or Equation 2.

Equation 2. Size∝Estored∝L
Note: While the larger bulk capacitors form an integral part of AC/DC systems, the capacitors are not considered here because there is little that can be done to reduce the size as capacitors are dependent on the slow 50/60-Hz line frequency rather than the much faster switching frequency. In small form-factor AC/DC designs, most attention is given to the magnetics, where energy storage is directly mandated by the switching frequency.

Because of this relationship, small-form-factor designs must switch at high frequency to reduce the energy storage requirement, and as a result, reduce the overall size. As shown in Figure 1-1 the inductance (energy storage) requirement for a given power level is inversely proportional to the switching frequency. However, it is important to keep in mind that switching losses are incurred by increasing the switching frequency, and can become a thermal concern. Techniques on how to mitigate these losses are explained in the following sections.

GUID-20230223-SS0I-2WGM-WRT3-VXT1MFTLGD20-low.svg Figure 1-1 Resulting Switching Frequency vs. Magnetizing Inductance of Transformer in a 65 W Transition-Mode Flyback Converter