SLYT857 August   2024 TPS1200-Q1 , TPS1211-Q1

 

  1.   1
  2. 1Introduction
  3. 2Output-voltage slew-rate control
  4. 3Parallel precharge path
  5. 4Automatic PWM-based capacitor charging
  6. 5Design considerations and test results
  7. 6Conclusion
  8. 7References
  9. 8Related Websites

2 Output-voltage slew-rate control

In this method, placing the capacitor (C) between gate-GND, the slew rate of the gate and the output voltage limits the inrush current. The circuit configuration with output voltage slew-rate control is shown in Figure 3.

Equation 1 and Equation 2 calculate the inrush current and power dissipation at startup as:

Equation 1. I I N R = C O U T   × d V O U T d t
Equation 2. P D ( V o u t = 0 ) = V I N   ×   I I N R

Because the MOSFET is operating in a saturation region, the inrush current should be low enough to keep the power dissipation within its safe operating area (SOA) during startup. MOSFETs can handle more energy (1/2 COUTVIN2) when their power dissipation is reduced and spread over longer durations. Thus, the inrush interval needs to stretch out over a longer period of time to support higher capacitive loads.

This method is suitable for slow charging requirements (for example, 5mF and 50ms), but the design must always include a trade-off between COUT, the FET SOA, the charging time and the operating temperature. For example, charging 5mF to 12V takes 40ms with an inrush current limit of 1.5A using TI’s high-side, switching controller, the TPS1211-Q1 as gate driver. Reference [1] iterates a procedure on how to check the FET SOA during startup using this method, while reference [2] is an online tool for estimating the SOA margin for a specific MOSFET.

Circuit for output voltage slew-rate control. Figure 3 Circuit for output voltage slew-rate control.