TIDUF57 November   2023

 

  1.   1
  2.   Description
  3.   Resources
  4.   Features
  5.   Applications
  6.   6
  7. 1System Description
    1. 1.1 Key System Specifications
  8. 2System Overview
    1. 2.1 Block Diagram
    2. 2.2 Design Considerations {Required Topic}
    3. 2.3 Highlighted Products
      1. 2.3.1 LMG3624
  9. 3System Design Theory
    1. 3.1 Quasi-Resonant Operation
    2. 3.2 Transformer Design
    3. 3.3 GaN FET Switching Device
    4. 3.4 Current Sense Emulation Resistor
  10. 4Hardware, Testing Requirements, and Test Results
    1. 4.1 Required Hardware
      1. 4.1.1 Hardware
      2. 4.1.2 Testing Equipment
    2. 4.2 Test Setup
    3. 4.3 Test Results
      1. 4.3.1 Efficiency Results
      2. 4.3.2 Thermal Results
      3. 4.3.3 Switching Waveforms
      4. 4.3.4 Switching Transients
  11. 5Design and Documentation Support
    1. 5.1 Design Files
      1. 5.1.1 Schematics
      2. 5.1.2 BOM
    2. 5.2 Tools and Software
    3. 5.3 Documentation Support {Required Topic}
    4. 5.4 Support Resources
    5. 5.5 Trademarks
  12. 6About the Author

3.1 Quasi-Resonant Operation

The quasi-resonant converter is chosen because this converter provides the best balance between cost, power loss, and simplicity.

GUID-20231115-SS0I-DRFP-6SZF-8WQ2RRJ52KBL-low.svg Figure 3-1 Quasi-Resonant Converter Abstract Schematic

The converter consists of a diode rectifier stage, switching FET, transformer with an inherent magnetizing inductance, passive RC clamp, synchronous secondary rectifier, and output capacitor.

The diode rectifier stage consists of the diode bridge and bulk capacitor. The diode bridge rectifies the ac line voltage and stores energy in the bulk capacitor, which maintains a relatively constant positive voltage.

The controller switches the FET on and off to deliver energy to the secondary side. While the FET is in the on state, current (energy) is built up in the magnetizing inductance of the transformer.

This is described by Equation 1:

Equation 1. E S T O R E D = 1 2 L M A G × I P K 2

The power delivered is calculated with Equation 2:

Equation 2. P = E S T O R E D × f S W

While the FET is in the off state, the rectifier on the secondary side of the transformer turns on and delivers the current to the load.

To maximize efficiency, the converter operates under quasi-resonant operation by turning on the FET at the resonant voltage valley formed by the resonance of the magnetizing inductance,LMAG, and the total switch-node capacitance,CD.

GUID-20231115-SS0I-9KGB-BGN0-RWGW4T9WD4HB-low.svg Figure 3-2 Drain-Source Waveform Switching at Resonant Valley

At this voltage valley, the FET suffers from the lowest possible turn-on switching power loss as described using Equation 3:

Equation 3. P L O S S , T U R N - O N - E N E R G Y = E C O S S × f S W = 1 2 C D × V V A L L E Y 2 × f S W

where

  • CD is the lumped capacitance at the switching node, consisting of FET output capacitance, transformer interwinding capacitance, and parasitic board capacitance
  • VValley is the drain-source voltage when the FET turns on
  • fsw is the switching frequency

The minimum valley at which the converter can switch is given with Equation 4:

Equation 4. V V A L L E Y , M I N = V I N , D C - N P S × V O U T = 2 × V A C   - N P S × V O U T

Equation 4 illustrates that the VVALLEY,MIN reaches zero volts when the rectified input voltage, VIN,DC, is equal to or less than the reflected voltage, NPS × VOUT. Hence, the quasi-resonant converter operates with zero-voltage switching under conditions when the input voltage is low and the reflected voltage is high, meaning that device losses are only conduction-dominated.