SLYT865 March   2025 LMG3422R030 , LMV797 , SN74LV1T125 , TLV713P-Q1 , TMS320F280033

 

  1.   1
  2. 1Introduction
  3. 2Inrush current vs. re-rush current
  4. 3Proposed re-rush current control method
  5. 4Test result
  6. 5Conclusion
  7. 6References

2 Inrush current vs. re-rush current

Power factor correction (PFC) is required for front-end power supplies (PSUs) greater than 75W. PFC forces the input current to follow the input voltage so that the electronics load appears as a resistor. PFC usually has a large output capacitor. Before startup, the PFC output capacitor is fully discharged. Because the PFC structure provides a current path when VAC > VOUT, applying the AC voltage will generate a huge current, since the input voltage is essentially applied to the PFC output capacitor directly. This current is called the inrush current.

Placing a thermistor (RT) with a positive temperature coefficient and a mechanical relay at the PFC input side will limit the inrush current, as shown in Figure 1. During PFC power up, the relay is off. The inrush current is limited by RT to a low value, and the PFC output bulk capacitor (CBULK) charges gradually. Once the output voltage (VOUT) charges to equal the peak value of the AC voltage (VAC), the inrush current drops to 0. Then the relay turns on, with RT bypassed to reduce power losses during normal operation.

Using RT and a relay to limit the PFC inrush current. Figure 1 Using RT and a relay to limit the PFC inrush current.

Re-rush current is different; it occurs during normal PFC operation. As shown in Figure 2, the AC input voltage suddenly drops out when PFC is operating normally. Since the load is still applied, the PFC VOUT could drop to a lower value. Then when the AC voltage returns, if the AC input voltage is higher than VOUT, there will be an inrush current again. This current is called the re-rush current.

M-CRPS re-rush current limit and timing. Figure 2 M-CRPS re-rush current limit and timing.

Previously, it is solely relies on the power stage components’ ability to handle the re-rush current. Test results show that the re-rush current can jump to more than 10 times higher than the PFC-rated maximum input current. Such a high re-rush current can either damage the power supply or reduce its lifetime, which is why the M-CRPS specification limits the amount of re-rush current after the AC voltage returns. The root-mean-square value of the re-rush current should be less than five times the maximum PSU current rating (5 × Irated,RMS) over a half cycle of input frequency, and less than 3.5 × Irated,RMS over one cycle of input frequency. In addition, the input current should settle to a value ≤2 × Irated,RMS within two cycles of the input frequency after applying the AC input.

It gets more complicated when considering PFC pulse-width modulation (PWM) operation during this period. If the PFC is not well controlled, an inappropriate PWM duty cycle may occur when the AC voltage resumes, resulting in another large input current spike that may also exceed the M-CRPS specification.

On the other hand, when the AC voltage resumes, PFC needs to supply enough current to boost the PFC output voltage to its regulation level as soon as possible; otherwise, VOUT will keep dropping because of the heavy load and eventually trip the input undervoltage lockout level of the DC/DC converter. Charging the PFC output capacitor once the AC voltage resumes will require a large input current, either from re-rush when VIN > VOUT or from the PFC control loop when VIN < VOUT.

This paper provides a solution to handle this re-rush current so that when the AC voltage comes back from dropout, both the re-rush current (when VIN > VOUT) and the non-re-rush current (when VIN < VOUT) are well controlled and high enough to rapidly boost VOUT but not exceed the M-CRPS limit specification.