Improvement about low-temperature startup problems for high-power power supplies

1. Problem description

For larger power supplies (above 60W), if the lower limit of the required operating temperature range is -40°C, it is easy to cause the power supply to fail to start normally at a low voltage input (90Vac) in an environment of -40°C.

2. Analysis and improvement

Reason: The value of resistance for the thermistor of input is too high when in low temperature.

Generally, in order to ensure that the current value of INRUSH to meet the customer’s limit requirements when turn on the power supply, the thermistor needs to be connected in series at the input of the power supply, and the value range of resistance is from 1 ohm to 10 ohms. The resistance value of this thermistor will increase exponentially in a low temperature environment. For example, a 3-ohm thermistor will increase its resistance at -40°C to about 35 ohms. The power supply is turned on with full load, and the voltage drop of the input current on the thermistor reaches tens of volts, which makes the back-end voltage insufficient and the power supply cannot start normally.

Improvement measures:

Method 1: Adding relay

Using the relay to short-circuit the thermistors can effectively avoid the problem of increasing resistance value of the thermistor at low temperature, but the disadvantage is that the relay has large power consumption when at no load and higher device cost. Therefore, this method is suitable for occasions where there is not requirement on energy efficiency or cost, and is mostly used for power supply designs above 250W.

Method 2: Connecting thermistor and fixed-value wire-wound resistor in parallel

In order to take into account the INRUSH current and low-temperature starting performance, a single thermistor can be replaced with a thermistor in parallel with a fixed-value wire-wound resistor. Take advantage of the characteristic that resistance vale of the wire-wound resistor does not change with temperature to ensure that the impedance of the circuit in front of the rectifier bridge will not increase sharply in a low temperature, and guarantee the power supply to start smoothly.

In fact, except the thermistor, the low temperature startup problem is also related to the electrolytic capacitor of the power supply. This article only discusses the improvement and optimization of the thermistor.