1. Problem description

For larger power supplies (above 60W), if the lower limit of the required working temperature range is -40°C, it is prone to the phenomenon that the power supply cannot start normally under load with low voltage input (90Vac) at -40°C.

2. Reason analysis and improvement countermeasures

Reason: The low-temperature resistance of the thermistor at the input end is too large

Generally, in order to ensure that the INRUSH current value of the power supply meets the customer's limit value requirements, a thermistor must be connected in series at the input end of the power supply, and the resistance value varies from 1 ohm to 10 ohm. The resistance of the thermistor increases exponentially in a low temperature environment. For example, a 3 ohm thermistor will increase its resistance to about 35 ohms at -40°C. When the power supply is turned on with full load, 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 be started normally.

Improvement measures:

Method 1: Add a relay

Using the relay to short-circuit the thermistor can effectively avoid the problem of the resistance increase of the thermistor at low temperature, but the disadvantage is that the no-load power consumption of the relay is large, and the device cost is high. Therefore, this method is suitable for occasions where there is little pressure on energy efficiency or cost, and is mostly used in power supply designs above 250W.

Method 2: Parallel connection of the thermistor with a fixed-value wire-wound resistor

In order to take into account the INRUSH current and low-temperature start-up performance, a single thermistor can be replaced by a thermistor connected in parallel with a fixed-value wire-wound resistor, and the resistance value of the wire-wound resistor does not change with temperature to ensure that the impedance of the circuit before the rectifier bridge stack will not change. In the low temperature environment, it increases sharply, and the smooth start of the power supply has never been guaranteed.

In fact, the low-temperature start-up problem is not only related to the thermistor, but also related to the electrolytic capacitor of the power supply. This article only discusses the improvement and optimization of the thermistor.