Selection Criteria for Thermometrics CL-Products
1. I max - Thermometrics CLs are rated for maximum steady state current. The maximum steady current is mainly determined
by the acceptable life of the final products for which the thermistor becomes a component. The differential equation Pdt
= HdT + δ(T – T A )dt reduces to Power = I 2 R = δ(T – T A ). An example in the case of a 100 watt power supply with an efficiency
rating of 80%, 100% load is calculated to be 125 watts. The maximum input current is calculated from the minimum
supply voltage. For a standard 120V supply, this could be rated as low at 110V. Therefore, input current would be
calculated by 125 watts/110 V = 1.14 Amps. Selection of the CL should have an I max rating of at least 1.14 Amps.
2. The second step of selection of the CL is to understand the desired maximum inrush current allowable. This is generally
specificed by the components in line of the CL, such as the diode bridge. In the case of the diode bridge rated at 200 Amps,
one would should select a CL that would limit max surge current to 50% of the rating, therefore limit surge to a maximum
of 100 Amps. The listed maximum current flow is rated at 25°C, so derating is required if the ambient temperture is greater
than 25°C.
3. The next selection of criteria for the CL is to understand the bulk Capacitance of the device to be protected. On power, the
bulk capacitance of the device appears as a short to the system. The designer needs to understand the bulk capacitance
at the RMS voltage rating of the system. Assuming the input capacitance is approximately 500 μFds, the selection of the
CL needs to be able to absorb input energy.