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AS321R030 Datasheet(PDF) 1 Page - AMETHERM Circuit Protection Thermistors |
AS321R030 Datasheet(HTML) 1 Page - AMETHERM Circuit Protection Thermistors |
1 / 3 page AS Series INRUSH CURRENT LIMITERS When a battery is connected to a load with capacitive input, there is an Inrush current surge as the capacitance is being charged to the battery voltage. The Input current depends on the input capacitance; the larger the batteries and the more powerful the load, the larger the input capacitance. A large Inrush current (in the precharge circuit, without protection) can cause the following: • Damage to input filter capacitors • Blowing of the main fuse if asked to carry the inrush current without protection • Contact failure (as well as reduction in current carrying capacity) due to arcing and pitting that results from high inrush current • Damage to the battery cell, which is not rated for inrush current A typical precharge circuitry for battery operation is below with the timing diagram, showing how the circuit operates. (Courtesy of Lithium -ION BMS) Thermistor Protection for Precharge Circuit on Lithium Ion Batteries + – R1 K1 10 BATTERY CONTROLLER PRECHARGE HV OUT K2 B= K3 B- K1+ K1+ K1– K1+ K1+ K1– + – The minimum resistance of the thermistor is determined by the following: 1. Ambient temperature 2. Input capacitance value (of the precharge circuit) 3. Battery voltage The precharge surge current reaches 63.2% (1/e) of its initial value after a time τ = RC. In the selection of the thermistor, we consider a time value of “five time-constant” when the capacitances are fully charged and the surge current reaches the normal operating current. For the purpose of our design, let us assume the following quantitative values: Precharge time: 20 millisecond Ambient operating temperature: Varies between 10°C to 50°C. Battery voltage: 100 volt Capacitor bank : 50,000 µF 5τ = RC R = 5τ / C = 5 (0.02 sec) / 0.05F = 2.0 . Now, look at the at R-T curves for Ametherm thermistor at ambient of 50°C. The material “C”exhibits R @ 50°C/ R @ 25°C = 0.412 @ R @ 10°C / R @ 25°C = 1.70 Therefore, minimum resistance @ 25°C = 2.0 / 0.454 = 4.40 , so our standard part has 5.0 ohm nominal resistance At 10°C, the standard part will have a resistance of 5.0 x 1.70 = 8.50 , which will meet our minimum resistance. Determine the energy the thermistor needs to handle with out self-destruction, E = ½ C V2 = ½ (0.05) (100) 2 = 250 Joules. The steady state current is not calculated because in most precharge circuits the steady state current goes through the contactor. The part, which would meet your specification, is AS32 5R020. In its most basic form, the Precharge circuit operates as follows: • OFF: When the system is OFF all relays / contactors are off. • Precharge: When the system is first turned on, K1 and K3 are turned on to Precharge the load, until the Inrush current has subsided. R1 shows the location of Thermistor in the Precharge circuit. • ON: After Precharge, contactorK2 is turned on (relay K1, must be off to save coil power) V Precharge Surge Normal Operating Current Load Voltage t A Battery Current t Off On Pre charge K1 K1 K1 t Off SELECTION OF THE THERMISTOR For this application note, let us limit our discussion to the selection of the Thermistor |
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