SLUSDD5A April   2019  – December 2020 UC1825B-SP

PRODUCTION DATA  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
  4. 4 Revision History
  5. 5 Pin Configuration and Functions
  6. 6 Specifications
    1. 6.1 Absolute Maximum Ratings
    2. 6.2 ESD Ratings
    3. 6.3 Recommended Operating Conditions
    4. 6.4 Thermal Information
    5. 6.5 Electrical Characteristics
    6. 6.6 Typical Characteristics
  7. 7 Detailed Description
    1. 7.1 Overview
    2. 7.2 Functional Block Diagram
    3. 7.3 Feature Description
      1. 7.3.1 Control Methods
      2. 7.3.2 Synchronization
      3. 7.3.3 High Current Outputs
      4. 7.3.4 Open Loop Test Circuit
    4. 7.4 Device Functional Modes
  8. 8 Application and Implementation
    1. 8.1 Application Information
    2. 8.2 Typical Application
      1. 8.2.1 System Design Theory
        1. 8.2.1.1 Switching Frequency
        2. 8.2.1.2 Transformer
        3. 8.2.1.3 RCD and Diode Clamp
        4. 8.2.1.4 Output Diode
        5. 8.2.1.5 Main Switching MOSFETs
        6. 8.2.1.6 Output Filter and Capacitance
        7. 8.2.1.7 Compensation
        8. 8.2.1.8 Sense Resistor
    3. 8.3 Application Curves
  9. 9 Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
      1. 10.1.1 Feedback Traces
      2. 10.1.2 Input/Output Capacitors
      3. 10.1.3 Compensation Components
      4. 10.1.4 Traces and Ground Planes
      5. 10.1.5 Ground Planes
    2. 10.2 Layout Example
  11. 11Device and Documentation Support
    1. 11.1 Documentation Support
      1. 11.1.1 Related Documentation
    2. 11.2 Receiving Notification of Documentation Updates
    3. 11.3 Support Resources
    4. 11.4 Trademarks
    5. 11.5 Electrostatic Discharge Caution
    6. 11.6 Glossary
  12. 12Mechanical, Packaging, and Orderable Information

Package Options

Mechanical Data (Package|Pins)
Thermal pad, mechanical data (Package|Pins)
Orderable Information

8.2.1.3 RCD and Diode Clamp

For the UC1825BEVM-CVAL a resistor and capacitor in combination with a diode was used to clamp the voltage of the switch node. The resistor and capacitor is generally a value that is found through testing, but starting values can be obtained. To figure out the resistor and capacitor needed for the RCD clamp, one must first decide how much the node is allowed to overshoot. The equation for finding the voltage of the clamp is Equation 30.

Equation 30. Vclamp=Kclamp×Nps×(Vout+VDiode)

Note that Kclamp is recommended to be 1.5 as this will allow for only around 50% overshoot. Knowing the parasitic inductance of the transformer and how much the RCD clamp voltage is allowed to change over the switching cycle, can allow one to figuring out starting values for the resistor and capacitor using Equation 31 and Equation 32.

Equation 31. Rclamp=Vclamp212×Lleakage×IPriPeak2×VclampVclamp-Nps×(Vout+VDiode)×fosc
Equation 32. Cclamp=VclampΔVclamp×Vclamp×Rclamp×fosc

A starting value of 10% is generally used for ΔVclamp.