SLVAFT6 September   2024 TPS23521 , TPS23523 , TPS23525

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
  5. 2Existing Design and Challenges
  6. 3Negative Hot-Swap Controller – TPS2352x
  7. 4Output Voltage Clamping with TPS2352x
  8. 5Design Procedure and Implementation
    1. 5.1 Configuring the Current Limit Switch-Over Threshold for TPS2352x
    2. 5.2 Feedback and Control Loop Response
    3. 5.3 Powering the Feedback Amplifier
    4. 5.4 Noise Immunity
  9. 6Test Results
    1. 6.1 Startup
    2. 6.2 NEBS Transient Response
  10. 7Summary
  11. 8References

2 Existing Design and Challenges

A typical design requirement for a Remote Radio Unit (RRU) is shown in Table 2-1. Traditional hot-swap designs in Figure 2-1 can handle such requirements with the robust protection schemes:

  • Inrush current management
  • Under-voltage and over-voltage protection
  • Reverse current blocking
  • Reverse polarity protection
  • Fast recovery during line transients
  • Over-current and short-circuit protection

However, a challenge comes from a voltage transient event, during which the input voltage rises to 75V (+20%/-0%) for 10ms (+20%/-0%) requiring the system to remain the normal operation without any damage from the over-voltage.

Traditional Negative Hot-Swap Protection CircuitFigure 2-1 Traditional Negative Hot-Swap Protection Circuit
Table 2-1 Typical system specifications
DESIGN PARAMETERVALUE
Input Voltage Range-36V to -60V
Nominal Voltage-50V
Maximum Load Power700W
Maximum Load Current700W / 36V = 20A
Target Current Limit (12 x maximum load current)24A
Voltage Transient Event75V for 10ms over-voltage as per NEBS/ATIS-0600315.218
Maximum Output Voltage62.5V ± 5%
Level of IEC61000-4-5 to pass± 2kV Line to Line with 2Ω series impedance
MOSFET RƟJA (Function of layout)20°C/W
Maximum Ambient Temperature85°C

The existing hot-swap design utilizes the over-voltage protection (OVP) functionality at the input side. Figure 2-2 shows conceptual waveforms during an over-voltage event in the existing hot-swap design where the OVP functionality is utilized at the input side. Once the voltage rises over 62.5V±5%, the voltage turns off the hot-swap FET Q1, and the hold-up capacitor (COUT) powers the load for 10ms. For a 700W load, 4.7mF of the large hold-up capacitor is required to prevent the under-voltage lockout (UVLO) of the downstream system as calculated in Equation 1. In addition, hot-swap FET Q1 can withstand significant power stress during recovery from over-voltage events. The huge voltage gap between the input node and the output node brings a high inrush current hitting the current limit. Due to such limitations, the traditional hot-swap design can be not a viable design for high-power telecom systems, especially Remote Radio Unit (RRU) and Active Antenna System (AAS) which typically requires >500W.

Equation 1. C o u t > 2 × P l o a d × T h o l d v _ n o m 2 - v _ m i n 2 = 2 × ( 700 W ) × ( 10 m s ) ( 65 V ) 2 - ( 36 V ) 2 ≈ 4.7   m F

Power stress on the FET Q1 during recovery from the OVP event is defined in Equation 2.

Equation 2. PFETW=12Vfinal-VinitialILIMT=0.5×62.5-36×24=350
Conceptual Waveforms During 75V/10ms Over-Voltage EventFigure 2-2 Conceptual Waveforms During 75V/10ms Over-Voltage Event