SNVA940 November   2021 LM5157 , LM5157-Q1 , LM51571-Q1 , LM5158 , LM5158-Q1 , LM51581 , LM51581-Q1

 

  1.   Trademarks
  2. 1LM5157 Boost Design Example
  3. 2Calculations and Component Selection
    1. 2.1  Switching Frequency
    2. 2.2  Inductor Calculation
    3. 2.3  Slope Compensation Check
    4. 2.4  Inductor Selection
    5. 2.5  Diode Selection
    6. 2.6  Output Capacitor Selection
    7. 2.7  Input Capacitor Selection
    8. 2.8  UVLO Resistor Selection
    9. 2.9  Soft-Start Capacitor Selection
    10. 2.10 Feedback Resistor Selection
    11. 2.11 Control Loop Compensation
      1. 2.11.1 Crossover Frequency (fcross) Selection
      2. 2.11.2 RCOMP Selection
      3. 2.11.3 CCOMP Selection
      4. 2.11.4 CHF Selection
    12. 2.12 Power Loss and Efficiency Estimation
  4. 3Implementation Results
  5. 4Small Signal Frequency Analysis
    1. 4.1 Boost Regulator Modulator Modeling
    2. 4.2 Compensation Modeling
    3. 4.3 Open Loop Modeling

2.12 Power Loss and Efficiency Estimation

The total loss of the boost converter (PTOTAL) can be expressed as the sum of the losses in the device (PIC, excludes the power MOSFET loss), MOSFET power losses (PQ), diode power losses (PD), inductor power losses (PL), and the loss in the sense resistor (PRS, see explanation below). The sum of (PIC) and (PQ) is the power dissipation in the converter device and should be designed within the reasonable range to prevent excessive temperature rise of the IC.

Equation 25. GUID-23E791B3-71F1-446B-972D-1B7667A0B1A9-low.gif

PIC can be separated into gate driving loss (PG) and the losses caused by quiescent current (PIQ).

Equation 26. GUID-CFC5E57C-7A75-4C4E-AFC3-C83244935307-low.gif

Each power loss is approximately calculated as follows:

Equation 27. GUID-0A353963-CBCE-4DFE-B134-4EFD427858E4-low.gif
Equation 28. GUID-BA406820-65CF-49CE-81D5-E35DB3847474-low.gif

IBIAS values in each mode can be found in the LM5157x and LM5158x data sheet.

PQ can be separated into switching loss (PQ(SW)) and conduction loss (PQ(COND)).

Equation 29. GUID-263BE9AC-BDC9-4229-8E61-0C49740A4378-low.gif

Each power loss is approximately calculated as follows:

Equation 30. GUID-C932777C-4D52-4C82-948C-4A9A5CD17426-low.gif

tR and tF are the rise and fall times of the integrated power MOSFET. ISUPPLY is the input supply current of the boost converter.

Equation 31. GUID-444B11A3-D344-4E99-B299-353B6F2E2C98-low.gif

RDS(ON) is the on-resistance of the MOSFET given in the LM5157x and LM5158x data sheet. Consider the RDS(ON) increase due to self-heating.

PD can be separated into diode conduction loss (PVF) and reverse recovery loss (PRR).

Equation 32. GUID-72FAB8F9-5EFC-4CC3-BCE4-FAE3D912709F-low.gif

Each power loss is approximately calculated as follows:

Equation 33. GUID-C790B063-9478-42DF-948F-A352D83649DE-low.gif
Equation 34. GUID-990765FB-66B6-403A-9264-74AE210E2DBC-low.gif

QRR is the reverse recovery charge of the diode and is specified in the diode data sheet. Reverse recovery characteristics of the diode strongly affect efficiency, especially when the load voltage is high.

PL is the sum of DCR loss (PDCR) and AC core loss (PAC). DCR is the DC resistance of inductor which is mentioned in the inductor data sheet.

Equation 35. GUID-EE8FA34E-8E5B-437D-BDD7-9E317477B01A-low.gif

Each power loss is approximately calculated as follows:

Equation 36. GUID-A89DB8D9-5C4E-41C4-BEF2-92765014F4F0-low.gif
Equation 37. GUID-7F971EC1-D61E-418F-9330-4B008657DB35-low.gif
Equation 38. GUID-C5967F64-156D-4433-9778-F14C493AD705-low.gif

∆I is the peak-to-peak inductor current ripple. K, α, and β are core dependent factors which can be provided by the inductor manufacturer.

Due to the current sensing technique implemented in LM5157x and LM5158x, the sensing resistance and the power loss PRS is negligible.

Efficiency of the power converter can be estimated as follows:

Equation 39. GUID-FB154740-0CE3-436B-8FB6-85A9C7941B8C-low.gif