Use an Inrush Current Limiter

Figure 1

• A transformer draws inrush current that can exceed saturation current affecting the magnetic property of the core. This happens even if the transformer is on no load with its secondary open. The magnitude of the inrush current depends on the instant on the AC wave at which the transformer is switched on.
• If the instant happens to be such that the AC voltage wave is going through its peak value, then there will be no inrush current drawn by the transformer. The magnitude of the current in this case will be at normal no load value.

If the instant happens to be such that the AC wave is going through its zero value, then the current drawn will be very high that exceeds the saturation current (see Figure 1).

In this scenario, the transformer has to be protected from inrush current.

Protection of the Transformer

This application note provides a convenient solution (see Figure 2) to deal with the problem of inrush current exceeding saturation current in transformers. The solution uses NTC Thermistor in series with the primary. This NTC Thermistor offers high resistance at the beginning of switching and limits the inrush current. After a short time, the NTC Thermistor resistance decreases to a low value due to self heating and does not affect normal operation.

• Each transformer rating: 1000 VA, transformer step-down: 30 V
• Total transformer rating: 2000 VA
• Filter capacitors used: 30V, 2300 μF

NTC Selection Criteria: Energy

Energy required for the NTC: Inductive reactance of the transformer

Note:

• Peak Inrush Current occurs in one cycle = 564 A, as measured on the oscilloscope
• Input Voltage = 120 VAC
• Frequency = 60 Hz

NTC Selection Criteria: Steady State Current

Assume, Efficiency of transformer: 90%, Ambient Temperature: 85

Normally thermistors are rated up to 65 for their operating current, and then a derating factor must be taken in to account.

Decision criteria: choose an NTC Thermistor that can provide at least the steady state current as calculated above:

Using the de-rating curve at 80, use 90% of max rated steady state current,
= 0.90 X 36 A = 32.40 A

You can use any of the NTC Thermistors that are rated up to 36.0 A to meet your Steady State Current and Energy Requirements. See the charts for part numbers.