The high and low input voltage limits within which a power supply or DC-DC converter meets its specifications.

 

Figure shows the typical voltage and current wave forms at the in- put to a SMPS. Current only flows to charge the capacitors when the rectified input voltage exceeds the voltage stored. Pulses of cur- rent are drawn from the supply near the peak point of the AC volt- age sine wave.

 

It can be seen that the input voltage is only slightly distorted by the very non-linear load presented by the capacitor input filter. The sinusoidal input is maintained because the line input resistance is very low. The input current however, is very distorted and discontinuous, but superficially would appear to be a part sine wave  in phase with the voltage. This leads to a common error: The product V_{in (rms)} x I_{in (rms)} is assumed to give input power. This is not so! The product is the input volt ampere product; it must be multiplied by the power factor (typically 0.6 for a capacitor input filter) to get true power.

The reason for the low power factor is that the non-sinusoidal cur- rent wave form contains a large odd harmonic content, and the phase and amplitude of all harmonics must be included in the measurement.

Apparent power = E(rms) X I(rms)

Active power = Apparent power X power factor

The ratio of total output power to active power, expressed in per- cent. This is normally specified at full load and nominal input volt- age.

 

 

The peak instantaneous input current drawn by the SMPS at switch ON.

For low-power applications, simple series resistor may be used. However, a compromise must be made, as a high value of resistance, which will give a low inrush current, will also be very dissipative under normal operating conditions. Consequently, a compromise selection must be made between acceptable inrush current and acceptable operating losses.

Negative temperature coefficient thermistors (NTC) are often used in the position of TH1 in low-power applications. The resistance of the NTCs is high when the supply is first switched ON, giving them an advantage over normal resistors. They may be selected to give a low inrush current on initial switch-on, and yet since the resistance will fall when the thermistor self-heats under normal operating conditions, excessive dissipation is avoided.