PTCs as charging resistors for fail-safe charging

Ohmic resistors are generally employed to limit the current when charging capacitors. This, however, is accompanied by technical risks. If, for example, the capacitor short circuits or the relay has a malfunction when short-circuiting the capacitor, the resistors will be subjected to high power levels continually. This can lead to the destruction of the resistor or the system. With the new J20X series of charging resistors based on PTC ceramic, Epcos has developed a professional solution that is self-protecting despite its relatively compact size. The J20X family consists of the J201, J202 and J204 products. Typical applications for the J20X series are industrial power supplies, frequency converters and UPS systems in the power range from 500W to 50kW. In these applications a link-circuit capacitor is generally used to smooth the generated DC voltage or as an energy storage device in the link circuit.

In order to avoid impermissibly high current peaks when charging capacitors, as a rule it is necessary to limit the charging current by a resistor connected in series. This function is often implemented by fixed ohmic or NTC resistors. In most cases, the current limiting element is short-circuited after charging via a time or voltage-controlled relay. This limitation of the charging current is especially important for rectifier and converter systems, as the resulting inrush current peaks may otherwise trigger the fuses or expose the rectifier to impermissibly high currents. Figure 1 shows a block diagram of a conventional rectifier or converter system.

Figure 1: Block diagram of a rectifier unit with a smoothing capacitor

The ohmic resistor limits the charging current. As soon as the capacitor is charged, the resistor is shorted via a relay in order to avoid operating losses. The combination of an ohmic resistor and a relay as described is sufficient to limit the charging current under undisturbed operating conditions. However, disturbances occurring during or after charging can lead to total failure of these resistors and consequently to that of other system components. The use of self-protecting charging resistors of the J20X series is recommended in order to handle typical malfunctions such as short-circuiting the capacitor or failure of the short-circuit switch. In fault-free charging, these components act like a fixed ohmic resistor and limit the peak value of the charging current. In the case of a mal-function, the temperature and inherent resistance of the PTC ceramic increases in line with the increased ohmic losses (Figure 2) and limits the current to a safe level.

Figure 2: Resistance of a charging resistor based on PTC

Figure 2: Resistance of a charging resistor based on PTC ceramic The high short-circuit current heats up the PTC ceramic and makes it highly resistive. In contrast, when a fixed resistor is used as a charging current limiter, these malfunctions would produce a very high power dissipation at the resistor, thus requiring an uneconomic overdimensioning of the component. This functional principle is clearly illustrated by a specific example (Figure 3). A typical configuration of a rectifier at the three-phase power supply system with a smoothing capacitor and self-protecting charging resistors.

Figure 3: Rectifier circuit