SLUSDD4B April   2019  – December 2020 UC1843B-SP


  1. Features
  2. Applications
  3. Description
  4. Revision History
  5. Pin Configuration and 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 Diagram
    3. 7.3 Feature Description
      1. 7.3.1 UVLO
      2. 7.3.2 Reference
      3. 7.3.3 Totem-Pole Output
    4. 7.4 Device Functional Modes
  8. Application Information Disclaimer
    1. 8.1 Application Information
    2. 8.2 Typical Application
      1. 8.2.1 Design Requirements
      2. 8.2.2 Detailed Design Procedure
        1. Switching Frequency
        2. Transformer
        3. RCD Diode Clamp
        4. Output Diode
        5. Output Filter and Capacitor
        6. Compensation
        7. Sense Resistor and Slope Compensation
      3. 8.2.3 Application Curves
  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
    2. 10.2 Layout Example
  11. 11Device and Documentation Support
    1. 11.1 Receiving Notification of Documentation Updates
    2. 11.2 Support Resources
    3. 11.3 Trademarks
    4. 11.4 Electrostatic Discharge Caution
    5. 11.5 Glossary
  12. 12Mechanical, Packaging, and Orderable Information

Package Options

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


The transformer of the design consists of two major values, turns ratio and primary side inductance. There is no minimum limit to the turns ratio of the transformer, just a maximum limit. The equation below will give the turns ratio as a function of duty cycle, which if you put in the maximum duty cycle of the converter will give you a maximum turns ratio. The UC1843B-SP design targeted a duty cycle of 50%, which is somewhat low for this controller. The suggested value would be around 70% duty cycle to take advantage of the fact the UC1843B-SP has full duty cycle range. The equation of the turns ratio of the transformer is Equation 3.

Equation 3. NpsMAX=VinMIN×Dlim(Vout+VDiode)×(1-Dlim)
Equation 4. NpsMAX=20 V×0.5(5 V+0.7 V)×(1-0.5)=3.5

Often the turns ratio will slightly change in design due to how the transformer is manufactured. For the UC1843B-SP design a turns ratio of 3.33 was used. Another turns ratio that is important is the turns ratio of the auxiliary winding. The auxiliary winding is found by figuring out what positive voltage is needed from the auxiliary winding. Picking what voltage the auxiliary winding should have lets one pick the turns ratio from the secondary to the auxiliary winding, which in turn allows for the turns ratio from primary to auxiliary to be found. The equation for the turns ratio for the auxiliary winding is Equation 5.

Equation 5. Npa=Nps×(Vout+VDiode)Vaux
Equation 6. Npa=3.33×(5 V+0.7 V)13 V=1.46

An auxiliary winding of 1.43 was used for the UC1843B-SP design due to manufacturing constraints. The primary inductance of the transformer is found from picking an appropriate ripple current. A higher inductance will often mean reduced current ripple, thus lower EMI and noise, but a higher inductance will also increase physical size and limit the bandwidth of the design. A lower inductance will do the opposite, increasing current ripple, lowering EMI, lowering noise, decreasing physical size, and increasing the limited bandwidth of the design. The percent ripple current can be anywhere from 20% to 80% depending on the design. The equation for finding the primary inductance from the percentage ripple current is Equation 7.

Equation 7. LPRI=VinMAX2×DMIN2Vout×Iout×fosc×%Ripple
Equation 8. (40 V )2×0.2525 V×10 A×200 kHz×0.4=25 μH

There are quite a few physical limitations when making transformers, so often this inductance will change slightly. For the UC1843B-SP design a primary inductance of 21 µH. This corresponds to a percent ripple of around 0.475. The peak and primary currents of the transformer are also generally useful for figuring out the physical structure of the transformer. See the following equations for proper calculations.

Equation 9. IRipple=Vout×Iout×%RippleVinMAX×DMIN
Equation 10. IRipple=5 V×10 A×0.47540 V×0.25=2.375 A
Equation 11. IPriPeak=Vout×IoutVinMIN×DMAX×η+IRipple2
Equation 12. IPriPeak=5 V×10 A20 V×0.5×0.8+2.3752=7.44 A
Equation 13. IPriRMS=D×(Vout×IoutVin×D)2+IRipple23
Equation 14. IPriRMS=0.5×(5 V×10 A20 V×0.5)2+(2.375 A)23=3.79 A
Equation 15. ISecRMS=(1-D)×Iout2+(IRipple×Nps)23
Equation 16. ISecRMS=0.5×(10 A)2+(2.375 A×3.33)23=8.42 A