Selection Criteria for GE CL-Products
1.
I
max
- GE’s 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.
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.
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.
2.
3.
Using the above criteria, the selection of the CL provides multiple solutions. One would opt for the smallest size CL to achieve
the required protection. The selection criteria is as follows:
1. I
max
>1.14 Amps
2. Max allowable Inrush current 100 Amps
3. Bulk Capacitance listed as 500 μfd
4. Choose smallest physical size that will allow protection for the device.
Criteria indicates that either the CL-150 or CL-160 would be suitable for the application. In the case of the CL-150 less heat is
dissipated allowing the operating resistance to drop but at a higher temperature. This increases efficiency of the system but
may lead to shorter component life.
www.ge-mcs.com
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names mentioned in this document may be trademarks or registered trademarks of their respective companies, which are not affiliated with GE.
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