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Selection Criteria for NTC or Inrush Current limiting relay

sabu31

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Hi all,

I wanted to know typically, what is the power level beyond which Inrush current limiting relay is preferred as compared to that of NTC. Also What should typically allowed inrush current (any standard ) or references for its.

Thanks
 
Hi,

we don´t know whether you talk about AC, DC, what voltages and currents .. what application ...

And "what" to protect against overcurrent... Is it a semiconductor, the wiring, or want to prevent a mechanical switch contact to weld, or just want to prevent a fuse to trip...

Klaus
 
Hi,

we don´t know whether you talk about AC, DC, what voltages and currents .. what application ...

And "what" to protect against overcurrent... Is it a semiconductor, the wiring, or want to prevent a mechanical switch contact to weld, or just want to prevent a fuse to trip...

Klaus
Hi Klaust,

The NTC is to be used in Power factor correction (PFC) converter at 1.5kW level. The input is from 115V to 270V ac. Output Voltage is DC at 400V.

Will this information suffice for suggesting whether till what level NTC is to be used and what are other current limitng measures? The system efficiency and size is also important.

Thanks & Regards,
 
NTC are available for the intended current range, see e.g. https://product.tdk.com/system/file.../ntc-limiter/data_sheet/50/db/icl_16/s464.pdf
The cold resistance is however relative low and involves large inrush currents. You might consider semiconductor switches to bypass precharge resistors, e.g. SCR.
Thanks FvM for the reply.

What is the general limit of in-rush current.

I was using PFC converter with 28019 IC and at 1.6kW output load on instantaneous switching (the Bias was initially ON). The sense resistors (0.025Ohm ) and its capacitors got damaged. I was using NTC- Cl-30 which was rated for 8A. Even on slow start. The NTC gets heated very much , to the level of smoke coming.
 
CL-30 is rated for 8 A RMS, so it can work for 230 VAC 1.6 kW PFC, although there's not much margin.

There are several ways to damage the NTC in this circuit:
- have much higher Irms due to incorrectly operating PFC control
- restart the PFC without allowing the NTC to cool down
- use higher than rated load capacitance

Even under rated operation conditions, the NTC runs quite hot.
 

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