We know that the inrush current is refer to the maximum instantaneous input current drawn by an electrical device when first turned on. But, how the inrush current in power converter can be minimized effectively?
provide series resistor at the input, the value depends upon the input capacitor and required time for fully starting the power converter. the power rating of the resistor is also calculated w.r.t inrush current, once the power converter output voltage stabilizes short the input series resistor connecting a thyristor across it, by firing. this technique avoids any resistive steady state losses and only losses incurred is the forward conduction drop of the thyristor, this technique is most suitable for high power converters, otherwise the same old NTC circuit is enough.
Your question is very open, if you want a practical answer, I suggest that you specify more what type of converter, what voltages, what power...
However, here are some generalities :
- The power converter's power can range from less than 0.1W to over several MW
- Power converters can either be DC or AC powered
- The inrush current has, at least, two sources : the input impedance of the converter alone and the current drawn by the load of the converter
This makes the answer to your question difficult.
But if you want a quick one :
- a series impedance can be placed in the supply to reduce the inrush current, then a relay would shunt that impedance
- a pre-load circuit can be used when the input stage of the converter in very capacitive (similar to the solution above)
- a specific "soft start" control of the power switches in the converter can , in some cases, reduce the inrush current, (this solution can reduced the inrush current drawn by the load of the converter
- a current limiter can be places at the input of the power converter
- etc
Dear Dominique ,
Thank you for your suggestions.Your answer is pretty clear and informative.Actually, i would like to get a quick overview first but will look into that in depth.
- For detailed study(with depth) with mathematical modeling, for A.C inrush refer my paper," C.T performance for power transformer inrush(132KV and 220KV)........".
- Soft start (increase voltage applied, gradually and slowly) is applicable for both A.C and D.C power convertor.
- Time constant of circuit(T=L/R) plays important roll for A.C inrush, hence can be minimized by increasing time constant of circuit.
- Switch in instant, is also important for A.C inrush, hence can be minimized by switch in at 90 (degree electrical)instead at zero.
- Increase in Inductance (variation of flux with respect to current) can reduce inrush for A.C, can be achieved by putting reactor, during switch in and cut off after by controlled circuit.
- Putting current limiter during switch in can reduced in rush for both A.C and D.C power converters.
- Pre load circuit, of power converters in start, can reduced the in rush for both A.C and D.C....
The three classical systems to limit the inrush current caused by capacitor charge (or toroidal transformers) are:
1. For small currents, a fixed resistor.
2. Up to about 5 A, a NTC resistor. At first glance, the disadvantage of this approach seems that if the supply is switched off and again on fast, the NTC has no time to cool, but if the converter is inhibited when the supply is off, the capacitors remain charged for a long time and there is no problem. So, it is very popular method in switched power supplies up to say 1 kW.
3. A limiting resistor switched by a relay (or a triac).
I am enclosing an example of a quite popular circuit used with toroidal transformers, as it does not need an auxiliary power supply. The delay is caused by C2 being charged by the current limited by C1.
Another enclosed example is a starter circuit which measures the current through the charging resistor and switches the relay when this current is very low. So you may change the value of the charging resistor or the filter capacitor without worrying about the delay time. I designed this circuit for a range of AC motor drives.
Thank you very much Francesc for the enclosed example and many thanks everyone for supporting with answers.
Select the peak current you may allow. In any case, select the peak current well below the IFSM of the input rectifier, if any. Also consider the I2t of the input fuse. Then R >= Vpeak/I Peak.
Losses are only significant during the first very short charging time, if you use NTCs or a relay to short the resitor after the inrush current.
hi
all converter has a control circuit to generate switching pulses. you can use this control circuit for soft starting purpose but in some converter like boost converter you have to use additional component like NTCs or relay and resistor as Francesc Casanellas mentioned before.
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