High-Voltage Inrush Current Limiter

Purpose: 

To limit the surge current while first powering a power converter device with bulk capacitor as the load. 

Problem:

While providing input power to a 400V DC-DC converter with 6000 uF capacitance on the input side, the fuse of the driving power supply was blowing because of huge surge current drawn. Damage to the power supply was also a concern.

Figure 1: Experimental MOSFET based (in linear range) current limiter

Solution Idea and Experiment:

To achieve in-rush current limit, employed the following circuit, which is based on an Application Note by Motorola Semiconductor [1]. 

Result:

1) The prototype circuit worked reasonably well for bridge rectified Power Supply (Oscilloscope traces shown in Figure 4 below). 

2) However, the switching MOSFET had several initial failures when the Power Supply was a switched mode PFC. It is suspected that the excessive amount of EMI generated by the PFC was making the MOSFET fail. The high operating voltage (400V) might have also contributed to the failure of the MOSFET. This problem was mitigated by using a higher gate drive resistor and having a scrubber circuit between the load and the MOSFET Drain.

Conclusion:

• Advantage of this circuit is simplicity with very few components. 
• However, this circuit makes use of MOSFET's operating characteristics in the linear region. It is not the recommended practice of using a MOSFET.
• This inrush circuit is a simple and viable method if the DC supply output is clean output from a battery or a analog bridge rectified DC source.
• For PFC driven power supplies, additional EMI suppression circuit at the front-end and more careful layout of the PCB layout than what was tried in the prototype may possibly help.
• It is possible that this configuration was putting the MOSFET outside the maximum Safe Operating Area (SOA).

Circuit:

Figure 2: In-rush current limit circuit using Vishay IRFBC40 600V, 4A, Power MOSFET [2]

Note: The Drain scrubber circuit made up of L1, R5 and C3 is not included in the reference document [1]; this feature of circuit is added to improve the MOSFET reliability.

Calculations:

The detailed theory of operation of a MOSFET and the circuit are well explained in Reference [1]. However, the calculation of specific component values for the Figure 2 are shown below.

Figure 3: From Reference [1]

 

Oscilloscope Traces:

Channel #1 (Yellow): Drain Current ( Ids ): 5A/div => peak 12A)

Channel #2 (Green): Gate voltage (Vgs): 2V/div => Miller Plateau at ~3.7V; Final voltage = 10V

Channel #3: (Blue): Drain voltage (Vds): 100V/div => Initial voltage= 350V; final voltage = 0V

The oscillations (fluctuations) of the Ids is believed to be due to the bridge rectified DC power-supply. The measured frequency of the oscillations is 120Hz (twice the line frequency).

Figure 4: Oscilloscope trace for 350V supply voltage (Ids, Vgs and Vds).

Caution:

• Be extremely careful while working with high-voltage, high-current circuits. Wear necessary protective equipment such as safety glasses, shoes, etc. 
• While using the oscilloscope make sure that there is no ground loop formed between the Device Under Test (if it is powered directly by the mains) and the Oscilloscope probe. A ground current loop could destroy the oscilloscope or the probe.
• While experimenting place, install fast-acting fuses in the circuit so that in the worst case the fuse will blow, preventing a major burn-out.

References:

[1] "Active Inrush Current Limiting Using MOSFETs", Motorola Semiconductor Application Note, AN 1542, Prepared by C.S. Miller.

[2] Data sheet: IRFBC40, SiHFBC40, Vishay Siliconix.

Notice:

The information in this article is exclusively intended for technically qualified person. The author hereby disclaims any and all liabilities of any kind with respect to any and all information provided in this blog post.