SLUS157Q December   1999  – October 2019 UCC1895 , UCC2895 , UCC3895


  1. Features
  2. Applications
  3. Description
    1.     Device Images
      1.      Simplified Application Diagram
  4. Revision History
  5. Pin Configuration and Functions
    1.     Pin Functions
  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. Detailed Description
    1. 7.1 Overview
    2. 7.2 Functional Block Diagrams
    3. 7.3 Feature Description
      1. 7.3.1  ADS (Adaptive Delay Set)
      2. 7.3.2  CS (Current Sense)
      3. 7.3.3  CT (Oscillator Timing Capacitor)
      4. 7.3.4  DELAB and DELCD (Delay Programming Between Complementary Outputs)
      5. 7.3.5  EAOUT, EAP, and EAN (Error Amplifier)
      6. 7.3.6  OUTA, OUTB, OUTC, and OUTD (Output MOSFET Drivers)
      7. 7.3.7  PGND (Power Ground)
      8. 7.3.8  RAMP (Inverting Input of the PWM Comparator)
      9. 7.3.9  REF (Voltage Reference)
      10. 7.3.10 RT (Oscillator Timing Resistor)
      11. 7.3.11 GND (Analog Ground)
      12. 7.3.12 SS/DISB (Soft Start/Disable)
      13. 7.3.13 SYNC (Oscillator Synchronization)
      14. 7.3.14 VDD (Chip Supply)
    4. 7.4 Device Functional Modes
    5. 7.5 Programming
      1. 7.5.1 Programming DELAB, DELCD and the Adaptive Delay Set
  8. Application and Implementation
    1. 8.1 Application Information
    2. 8.2 Typical Application
      1. 8.2.1 Design Requirements
      2. 8.2.2 Detailed Design Procedure
        1.  Power Loss Budget
        2.  Preliminary Transformer Calculations (T1)
        3.  QA, QB, QC, QD FET Selection
        4.  Selecting LS
        5.  Selecting Diodes DB and DC
        6.  Output Inductor Selection (LOUT)
        7.  Output Capacitance (COUT)
        8.  Select Rectifier Diodes
        9.  Input Capacitance (CIN)
        10. Current Sense Network (CT, RCS, RR, DA)
          1. Output Voltage Setpoint
          2. Voltage Loop Compensation
          3. Setting the Switching Frequency
          4. Soft Start
          5. Setting the Switching Delays
          6. Setting the Slope Compensation
      3. 8.2.3 Application Curves
  9. Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
    2. 10.2 Layout Example
  11. 11Device and Documentation Support
    1. 11.1 Documentation Support
      1. 11.1.1 Related Documentation
      2. 11.1.2 Related Links
    2. 11.2 Receiving Notification of Documentation Updates
    3. 11.3 Community Resource
    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

Preliminary Transformer Calculations (T1)

Transformer turns ratio (a1):

Equation 8. UCC1895 UCC2895 UCC3895 qu2_lua560.gif

The voltage drop across the RDS(on) of the primary side FETs is negligible. We assume a 0.5-V forward voltage drop in the output rectifiers.

Equation 9. UCC1895 UCC2895 UCC3895 eq_9_slus157.gif

Select transformer turns based on 70% duty cycle (DMAX) at minimum specified input voltage. This will give some room for dropout if a PFC front end is used.

Equation 10. UCC1895 UCC2895 UCC3895 qu2_lua560.gif
Equation 11. UCC1895 UCC2895 UCC3895 eq_11_slus157.gif

Turns ratio rounded to the nearest whole turn.

Equation 12. UCC1895 UCC2895 UCC3895 qu6_lua560.gif

Calculated typical duty cycle (DTYP) based on average input voltage.

Equation 13. UCC1895 UCC2895 UCC3895 eq_13_slus157.gif

Output inductor peak-to-peak ripple current is set to 20% of the output current.

Equation 14. UCC1895 UCC2895 UCC3895 qu8_lua560.gif

Care must be taken in selecting the correct amount of magnetizing inductance (LMAG). The following equations calculate the minimum magnetizing inductance of the primary of the transformer (T1) to ensure the converter operates in current-mode control. As LMAG reduces, the increasing magnetizing current becomes an increasing proportion of the signal at the CS pin. If the magnetizing current increases enough it can swamp out the current sense signal across RCS and the converter will operate increasingly as if it were in voltage mode control rather than current mode.

Equation 15. UCC1895 UCC2895 UCC3895 qu9_lua560.gif

Figure 19 shows the transformer primary and secondary currents during normal operation.

UCC1895 UCC2895 UCC3895 fig2_lua560.gifFigure 19. T1 Primary and Secondary Currents

Calculate T1 secondary RMS current (ISRMS):

Equation 16. UCC1895 UCC2895 UCC3895 qu10_lua560.gif
Equation 17. UCC1895 UCC2895 UCC3895 qu11_lua560.gif
Equation 18. UCC1895 UCC2895 UCC3895 qu12_lua560.gif

Secondary RMS current (ISRMS1) when energy is being delivered to the secondary: (OUTA = OUTD = HI or OUTB = OUTC = HI).

Equation 19. UCC1895 UCC2895 UCC3895 qu13_lua560.gif

Secondary RMS current (ISRMS2) during freewheeling period: (OUTA = OUTC = HI or OUTB = OUTD = HI).

Equation 20. UCC1895 UCC2895 UCC3895 qu14_lua560.gif

Secondary RMS current (ISRMS3) caused by the negative current in the opposing winding during freewheeling period, please refer to Figure 19.

Equation 21. UCC1895 UCC2895 UCC3895 qu15_lua560.gif

Total secondary RMS current (ISRMS):

Equation 22. UCC1895 UCC2895 UCC3895 qu16_lua560.gif

Calculate T1 Primary RMS Current (IPRMS):

Equation 23. UCC1895 UCC2895 UCC3895 qu17_lua560.gif
Equation 24. UCC1895 UCC2895 UCC3895 qu18_lua560.gif
Equation 25. UCC1895 UCC2895 UCC3895 qu101_lusa16.gif
Equation 26. UCC1895 UCC2895 UCC3895 qu19_lua560.gif
Equation 27. UCC1895 UCC2895 UCC3895 qu21_lua560.gif

T1 Primary RMS (IPRMS1) current when energy is being delivered to the secondary.

Equation 28. UCC1895 UCC2895 UCC3895 qu19_lua560.gif

T1 Primary RMS (IPRMS2) current when the converter is free wheeling.

Equation 29. UCC1895 UCC2895 UCC3895 qu22_lua560.gif

Total T1 primary RMS current (IPRMS):

Equation 30. UCC1895 UCC2895 UCC3895 qu23_lua560.gif

We select a transformer with the following specifications:

Equation 31. UCC1895 UCC2895 UCC3895 qu24_lua560.gif
Equation 32. UCC1895 UCC2895 UCC3895 qu25_lua560.gif

Transformer Primary DC resistance:

Equation 33. UCC1895 UCC2895 UCC3895 qu27_lua560.gif

Transformer Secondary DC resistance:

Equation 34. UCC1895 UCC2895 UCC3895 qu28_lua560.gif

Estimated transformer core losses (PT1) are twice the copper loss.


This is just an estimate and the total losses may vary based on magnetic design.

Equation 35. UCC1895 UCC2895 UCC3895 qu29_lua560.gif

Calculate remaining power budget:

Equation 36. UCC1895 UCC2895 UCC3895 eq_36_slus157.gif