Central issue:
- Detailed explanation of the charger with positive temperature coefficient thermistor
- Failure analysis of a capacitor short circuit at the beginning of charging
Self-protecting charging resistors are based on PTC (Positive Temperature Coefficient) ceramics for smoothing capacitors in power supplies. When a short circuit occurs, they limit the current to a safe level.
Ordinary resistors are often used to limit current when charging a capacitor. However, this is often a technical risk. For example, when a capacitor is shorted, if the capacitor is shorted or the relay fails, the resistor will continue to be exposed to high power levels. This can cause damage to the resistor or the entire system. With a new J20X series of charging resistors based on PTC ceramics, EPCOS has developed a professional solution: in terms of self-protection, it also achieves a relatively compact size. As shown in the table below, the J20X series includes J201, J202 and J204 products.
Typical applications for the J20X series range from industrial power supplies, frequency converters and UPS (uninterruptible power supply) systems in the 500 W to 50 kW power range. In these applications, the link capacitor is used to flatten the generated DC voltage or as an energy storage device in the link.
When the capacitor is charged, it is usually necessary to connect a resistor in series to limit the charging current to avoid generating a strong current peak that exceeds the allowable range. This is usually done using a fixed normal resistor or a negative temperature coefficient (NTC) resistor. In most cases, a time or voltage controlled relay is used to short the current limiting component after charging. The regulation of the charging current is very important for the rectifier and converter system, because the peak value of the generated inrush current, if not limited, may trigger the fuse or subject the rectifier to a high current exceeding the allowable range. Figure 1 shows a block diagram of a conventional rectifier or converter system.
If there is no interference during operation, the combination of the above conventional resistors and relays is sufficient to limit the charging current. However, disturbances that occur during or after charging can cause these resistors to fail completely and cause a total failure of other components of the system.
To handle typical faults, such as capacitor shorts or short-circuit switches, it is recommended to use the J20X series of self-protecting charging resistors. In trouble-free charging, these components act like fixed-type resistors that limit the peak charge current. When a fault occurs, the temperature and internal resistance of the PTC ceramic will increase with increasing ohmic losses (see Figure 2) and limit the current to a safe level.
In contrast, if a fixed resistor is used as the charging current limiter, the above failure will result in a relatively high power loss of the resistor, which would require a certain size of the component, which is uneconomical. The following special example (see Figure 3) clearly illustrates this functional principle.
The following formula can be used to calculate the number of J20X series components required:
If the component B59204J0130B010 has a heat capacity of about 2 J/K and the reference temperature is 130 °C, then both components can be connected in series or in parallel. Satisfying the above equation ensures that the PTC ceramic does not exceed the reference temperature before charging is completed and remains within the low resistance range.
When the 95% limit charging voltage of the capacitor is reached, the parallel J20X components will be shorted and the load will be connected (represented by a 260 Ω fixed resistor). Therefore, the performance of a parallel circuit composed of two J204 components is equivalent to a fixed resistance of 50 Ω. For the case of trouble-free charging, see the current time diagram shown in Figure 4.
In both cases, the time profile of the charging current is almost the same. The nuances in the current characteristics of PTC ceramics and fixed resistors are due to:
* The resistance temperature characteristics of the PTC thermistor are special; in addition,
* PTC ceramics have a very high voltage dependence when turned on. When calculating the peak inrush current, it is necessary to consider the voltage dependence.
After about 190 ms, the charging resistor will be short-circuited after charging. The energy absorption curve and the degree of heating are also similar (see Figure 5). The highest point of both corresponds to the energy of the capacitor in the event of a short circuit.
The advantage of using a PTC thermistor as a current limiting component is obvious when a fault occurs. If the relay fails to turn on, the load current will flow through the charging resistor and generate strong thermal stress, which requires the resistor to have the appropriate size. With PTC ceramic-based charging resistors, the resistance rises to 10 kΩ due to the strong initial power loss, which limits the current during fault occurrence (see Figure 6). After about three seconds, the current flowing through the two resistors and then flowing through the overall circuit has dropped to a few 10 mA. See Figure 7 for a comparison of the absorbed energy.
After entering the high-resistance state, the PTC ceramic limits the energy absorption to a non-critical value, while the absorbed energy of the fixed ohmic resistor rises linearly. In this example, given the temperature derating, the fixed resistor must have a power rating of more than 200 W to prevent overheating and subsequent damage.
Fault - The capacitor is short-circuited at the beginning of charging. The powerful inrush current causes the two self-protecting charging resistors to generate high resistance after about 150 ms, thereby limiting the current. The current flowing through the fixed resistor is limited only by the very low power line resistance, so a very high power energy conversion occurs in the fixed resistor.
In a short time, the two self-protecting charging resistors connected in parallel achieve thermal equilibrium with the outside, and at the same time, due to the high resistance value of the PTC ceramic, the absorbed energy only slightly increases. The resulting energy absorption is similar to that shown in Figure 7.
The above fault - the capacitor is short-circuited at the beginning of charging - means that there is a very high load on the charging resistor. Therefore, the J201 charging resistor requires an additional fixed resistor to limit the short-circuit current. However, the application of the charging resistors J202 and J204 does not require any additional protection with a fixed resistor.
M8 distribution box is a kind of industrial connector. It is a device that converts a power supply or data into a shunt and outputs multiple current or data signals. General M8 distribution box refers to the line socket for M8 or M12 specifications of the distribution box. The difference between it and the ordinary socket is that the power output of the ordinary socket is only positive and negative two levels, or there is a ground wire, while the industrial distribution box, the power output is needle I/O port.
M8 Distribution Box,D-Sub Input Distribution Box,M8 distribution box 4 way, M8 Junction Box 8 port,M8 distribution box 12 way
Kunshan SVL Electric Co.,Ltd , https://www.svlelectric.com