Demystifying High Pass Domain Controller Level 1 Power Supply Design: Power Supply Design and Calculation

The rapid development of the new energy vehicle industry has driven the explosive growth of each industry chain, automotive intelligence, autonomous driving has become the most important core competitiveness of new energy vehicles direction, to the highly integrated central brain and domain controller brings new challenges and opportunities, especially for the reliability of DC-DC switching power supply, high power density, switching power supply EMC, high efficiency, cost-effective to bring new opportunities and challenges .

Qualcomm as a supplier of intelligent cockpit domain controller, SA8155 and SA8295 occupies an important position, the central domain control SOC level 1 power supply (power supply converted from the battery input level 1) transient current, stable operating current, standby operating efficiency, cost, the contradiction between the switching power supply EMC design has become a huge challenge for BUCK power supply design. How to solve and balance these contradictions is the technical direction of switching power supply architecture, power supply chip, inductor, Mosfet, capacitor together.

This paper combines the large dynamic switching power supply current (100-300%) automotive central domain control level 1 power supply design, to explore the design of DC-DC switching power supply, including power supply scheme, inductor, capacitor selection and other design methods; taking into account the volume, cost, efficiency, performance challenges to explore and practical landing design.

This paper explores and implements the real-world design of a one-stage BUCK switching power supply using the Qualcomm SA8295 domain controller as an example.

This article series contains three series (will be updated continuously in the future):

01- Deciphering Qualcomm Automotive Domain Controller Level 1 Power Supply Design: Power Supply Design and Calculation (this chapter)

02- Demystifying Qualcomm's Automotive Domain Controller Level 1 Power Supply Design: Schematic Design and PCB Design

03- Deciphering Qualcomm Automotive Domain Controller Level 1 Power Supply Design: Performance Test Measurement Analysis

1- Design Objectives and Challenges

1.1 SA8295 Transient Current Requirements

Table 1: SA8295 Power Supply Design Requirements

1.2 SA8295 Standby Current Requirements

Qualcomm SOC 3.3V power supply standby power consumption within 4-7.5mA (including memory self-refresh power consumption), support standby wake-up.

Central brain (cockpit domain controller) the whole car overall current budget 7-10mA (13.5V), 4G/5G module alone consumption 4-5mA, Qualcomm SA8295 current 13.5V 3mA (40mW) within.

1.3 Three challenges

1.3.1 Qualcomm Domain Control SA8295 Switching Power Supply Current Output Challenge 1:

Large transient current, 3.3V, 18 Amp (0.1ms), 0.1ms for DC-DC switching power supply already belongs to the long period steady state output, need Buck power supply according to the 18 Amp stable output design.

1.3.2 Qualcomm domain-controlled SA8295 switching power supply high-current dynamic challenges 2:

SA8295 domain control steady state operating current in 5-9 amps, which will cause the switching power supply inductance (inductance and current size is inversely proportional to the size of the selection of the stable operating current of more than 300% of the difference between the volume, cost, frequency of a larger contradiction.

1.3.3 Qualcomm domain control SA8295 switching power supply micropower efficiency challenges 3:

Standby power consumption, the need for 13.5V 3mA consumption efficiency > 70%, which is the power controller architecture, inductor selection design is also a huge challenge.

This design is based on the design of the most challenging SA8295 one-level Buck power supply, to explore the core difficulties of switching power supply and DC-DC technology solutions.

2- Comparison of programme selection

2.1 Qualcomm SA8295 domain control power supply technical requirements

As shown in table 2:

Table 2: Qualcomm SA8295 power supply design specifications requirements

2.2 Programme design and technical information

MPQ2918, MPQ2930, LM25141-Q1, MAX20098, LTC7803, and LM25149-Q1 can meet the design requirements. In this design, LM25149-Q1 is chosen as the first-level power supply design scheme for this central brain domain controller.

2.2.1 LM25149-Q1 official address:

https://www.ti.com.cn/product/cn/LM25149-Q1?keyMatch=LM25149-Q1

Table 3: LM25149-Q1 Design References

2.2.2 LM25149-Q1 Datasheet:

LM25149-Q1 42-V Automotive, Synchronous, Buck, DC/DC Controller with Ultra-Low IQ and Integrated Active EMI Filter datasheet (Rev. B)

2.2.3 LM25149-Q1 development board:

LM25149-Q1 EVM User Guide (Rev. A) (ti.com.cn)

2.2.4 Active filter stability and performance:

How to Ensure Stability and Performance of Active EMI Filters (ti.com.cn)

2.2.5 LM5149-LM25149 Design Tools:

LM5149-LM25149DESIGN-CALC Calculation tool | TI.com

3- Synchronous BUCK power supply design and calculation

3.1 Main specifications and design parameters of LM25149

Table 4: Qualcomm SA8295 power supply design specifications requirements

Efficiency

Active EMI Filters

EMI Testing

Reference Design Schematic

Reference Design Solution Evaluation Board

3.2 LM25149 Synchronous BUCK Inductor Selection Calculation

3.2.1 Synchronous BUCK switching power supply calculation formula:

Table 5: Synchronous BUCK power supply design calculation equation

3.4 Minimum Inductance Calculation

(For formulae, see Table 5.)

Table 6: Calculated graph of minimum inductance (∆I=0.3)

Table 7: Minimum inductance calculation

3.4.1 Summary of inductance calculation data:

① If the design covers the range of 6-20A (AI=0.3 calculation), 16V input, 6A output, inductance ≥ 0.69μH.

② Theoretical calculation of switching power supply inductance Lmin: ≥ 0.69μH (theoretical);

③ Considering the actual design selection and inductance error ± 20%, select 0.82μH and 1.0μH as the best design (inductance value increases, inductance volume increases, cost increases, SRF decreases).

3.5 Inductor Current Calculations

(Formula: refer to tables 1 and 2 of table 5)

Table 8: 0.82μH Inductor Current Calculation

Table 9: 1.0μH Inductor Current Calculation

3.5.1 Theoretically calculated inductor saturation current ≥ 20.76A, rounded to 21A: 

Table 10: Inductance Indicators

4- Switching power supply inductor selection

Table 11: Inductor Selection

4.1 LM25149 Switching Power Supply Inductor Current Sampling Resistance Calculation

Table 12: Theoretical calculation of inductor current sampling resistance

Table 13: Inductive Sampling Resistor Selection

4.2 Synchronous BUCK switching power supply output capacitance calculation

(Calculation of output capacitance: refer to the formula in Table 5)

Table 14: Synchronous BUCK switching power supply output capacitance calculation

For synchronous BUCK switching power supply design, input and output filter capacitor performance, volume, cost there is a contradiction, capacitance specification index test is completed under specific conditions, the test process instrumentation differences, the same indicators, there may be 10-50% difference, the final design performance needs to be verified in the debugging process of the scientific practice and testing (no optimal solution to the design, only the selection of suitable scenarios) (There is no optimal solution for design, only choose the one suitable for the scenario).

Switching capacitors need to meet: capacity ≥ 320uF (Overshoot requirements), ceramic capacitor capacity greater than 2.435uF (not the core conditions, to meet can)

Table 15: Recommended Model Selection for Switching Power Supply Output Filter Capacitors

Table 16: Switching power supply output filter capacitor design

4.3 LM25149 Power Supply Input Capacitance Calculation

4.3.1 Input Capacitance Calculations

Table 17: Switching Power Supply Input Filter Capacitance Calculations

Table 18: Switching Power Supply Output Filter Selection

4.4 LM25149 Mosfet Selection Calculation

4.4.1 Mosfet Calculations

LM25149 datasheet does not have too many calculations and selection calculations, QG calculations and selection based on empirical estimates backwards, the calculation results select 4.5-5.0V Vgs, ≤ 22nC, the calculation process refer to the following table, select the Miller plateau for 2-3V (close to 3V is also acceptable), Rdson select ≤ 8mΩ.

Table 19: Mosfet Selection and Calculations

4.5 Mosfet Selection Recommendations

Table 20: Mosfet Selection Models

4.6 LM25149 FB and Compensation Calculations

Table 21: FB and compensation calculations

4.7 LM25149 EMC design calculations

Without over-analysing, refer to the specification.

5- Design Summary

5.1 LM25149BUCK power supply design selection summary

Table 22: Design and Selection

5.2 Programme Summary

Synchronous switching power supply performance and efficiency is affected by many factors, performance and indicators need to take into account the actual factors, this chapter is used for theoretical calculations, theoretical guidance on the actual design, the design of the performance and indicators are closely related to the performance of the components, the use of conditions, layout, etc., need to be rigorous testing and verification.

Synchronous buck power supply design for high-pass domain controller is a difficult technical field of controller design technology, need to balance the performance, volume, cost, Kodak Ka focus on inductor independent research and development and design, CSEB0660-1R0M is suitable for the development and application of the high-pass platform, has a high cost-effective, strong resistance to saturation current, small heat and other technical advantages, with an industry-leading power to volume ratio; Kodak Ka focus on Technology R&D, technological innovation, research and development of excellent products for the inductor industry, to help the development and application of electronic products.