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Effect of brief surge currents?

Posted by: blackoakeng.com on

Hello,
we used PI Expert to design a dual output 12 V / 24 V TOPswitch-HX switcher. See attached graphic. Either one output or the other will be connected to the load. Max typical load current is 2.5 A.

Now, it turns out that there are occasional fault conditions in which the load surges to 16 A for 300 ms. This inrush current must be supplied to meet our spec. Large capacitors are not economically viable. After the surge the load settles to 200 mA.

1. How does the circuit respond to this transient event?
2. Could we put in a PTC that is balanced enough to pass the surge current for 300 ms while it heats up, then pass a 200 mA holding current thereafter? Would this keep the TOPwitch-HX within normal operating bounds?

Thanks.
manderson @ blackoakeng.com

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Comments

Submitted by PI-csbabu on 05/08/2020

Hi Manderson,

I would like to know few more details. What is the total output power requirement ( Both outputs together)? What is the duty of transient load ( 16A to 200mA)?
You are correct, large amount of capacitance required is to support 16A load for 300ms. You need to consider the max load current (peak power) instead of steady state current while designing the transformer. Since x-pin is shorted, it can support max switch current. Based on duty compute the average power and thermals will be based on average power.

I hope you are talking about using NTC not PTC. PTC will limit the current during transient due to positive temp coefficient. If steady state current is 2.5A, the losses during steady state very high.