JAJU873 August   2020

 

  1.   概要
  2.   リソース
  3.   特長
  4.   アプリケーション
  5.   5
  6. 1 System Description
    1. 1.1 Medical Respiratory Systems
    2. 1.2 Respirator System Components
    3. 1.3 Key System Specifications
  7. 2System Overview
    1. 2.1 Block Diagram
    2. 2.2 Design Considerations
      1. 2.2.1 Brushless DC Motor (BLDC)
        1. 2.2.1.1 DRV8323RS BLDC Motor Driver Design Calculations
        2. 2.2.1.2 BLDC Motor Driver Circuit
      2. 2.2.2 Solenoid Valve Drivers
        1. 2.2.2.1 DRV8847 Solenoid Driver Design Calculations
        2. 2.2.2.2 Solenoid Driver Circuit
      3. 2.2.3 Power Tree Architecture
        1. 2.2.3.1 Input protection - overvoltage and reverse voltage
        2. 2.2.3.2 LM5122 Boost Design Calculations
        3. 2.2.3.3 LMR33630 Buck Design Calculations
        4. 2.2.3.4 Secondary Power Stage – TPS62840 3.3V Buck
        5. 2.2.3.5 Secondary Power Stage – TPS7A02 3.3V LDO
        6. 2.2.3.6 Power Tree Circuit
    3. 2.3 Highlighted Products
  8. 3Hardware, Software, Testing Requirements, and Test Results
    1. 3.1 Hardware and Software Requirements
    2. 3.2 Test Setup
      1. 3.2.1 Hardware Configuration
      2. 3.2.2 Software Configuration
    3. 3.3 Test Results
      1. 3.3.1 Motor Test Result
      2. 3.3.2 Valve Test Result
      3. 3.3.3 Power Tree Test Result
      4. 3.3.4 Key Test Summary
  9. 4Design and Documentation Support
    1. 4.1 Design Files
      1. 4.1.1 Schematics
      2. 4.1.2 BOM
    2. 4.2 Documentation Support
    3. 4.3 サポート・リソース
    4. 4.4 Trademarks
  10. 5About the Author

2.2.3.1 Input protection - overvoltage and reverse voltage

This system input voltage range is 6-28 V. A general rule-of-thumb for selecting a transient-voltage-suppression (TVS) device is by selecting the standoff voltage rating 25% above the maximum input voltage (i.e. 35 V). This will allow some margin for device process and temperature variations to minimize unnecessary TVS activation during normal operation. In this design, TVS3301 was selected to provide the input overvoltage protection (OVP). The device has a standoff rating of 33 V and IEC 61000-4-5 clamp rating of 40 V.

The target reverse voltage protection (RVP) should include the maximum input voltage 28 V plus at least a 25% margin to account for transient behaviors and device tolerances (i.e. 35 V). For this design, a 60 V reverse voltage protection target was chosen.

The RVP circuit included an ideal diode controller (LM74700) and an NMOS (CSD18532Q5B). The MOSFET selection for the reverse voltage protection circuit is guided by the typical application conduction current and the VDS voltage rating. There is a tradeoff between reverse voltage detection and conduction loss. As recommended by the LM74700 data sheet, the RDS(ON) should be (20 mV / ILoad(Nominal)) ≤ RDS(ON) ≤ ( 50 mV / ILoad(Nominal)). This guideline balances reverse current detection sensitivity and conduction loss. In this design, ILoad(Nominal) is 5 A. Thus, 4 mΩ ≤ RDS(ON) ≤ 10 mΩ.

CSD18532Q5B was selected to provide a -60V RVP and it is rated at:

  • 60-V VDS(MAX) and ±20-V VGS(MAX)

  • RDS(ON) 3.3-mΩ typical and 4.3-mΩ maximum rated at 4.5-V VGS

  • MOSFET Vth: 2.2-V maximum

Table 2-3 Input protection design summary
PARAMETER SPECIFICATION COMMENTS
Clamping voltage 40 V at 27-A (IEC 61000-4-5) Limited by TVS3301
Maximum reverse voltage -60 V Limited by CSD18532Q5B
Reverse block time <0.75 µs Limited by LM74700

Input Inrush Protection

Although not implemented in this design, it is recommended to add inrush current protection (for example, soft-start) to the design. Due to the inherent path from input to output, a large inrush current can flow when the input voltage rises quickly and charges the output capacitor. The slew rate of input voltage rising should be controlled by a hot-swap controller or by starting the input power supply softly for the inrush current not to damage the inductor, sense resistor or high-side N-channel MOSFET switch. An alternative low-cost option is to use a NTC or PTC thermistor for soft-start.