RP300X4.000-DAN - IMI Precision Engineering

Self-protecting charging resistors based on PTC ceramic are suitable for

smoothing capacitors in power supplies. In the event of a short circuit, they

limit the currents to safe levels.

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.

As shown in the table, the J20X family consists of the J201, J202 and J204 products.

Table: Key data for PTC charging resistors

* Not for new design

PTCs as charging resistors July 2007

Fail-safe charging

Type Ordering code Vmax [V] RR[Ohm]

In phenolic resin plastic case:

J201 * B59201J0140B010 550 20 ± 30%

J202 * B59202J0135B010 650 56 ± 30%

J204 * B59204J0130B010 800 100 ± 25%

J105 B59105J0130A020 360 22 ± 25%

J107 B59107J0130A020 620 56 ± 25%

J109 B59109J0130A020 800 100 ± 25%

Leaded disks:

B750 B59750B0120A070 360 25 ± 25%

B751 B59751B0120A070 360 50 ± 25%

B752 B59752B0120A070 360 80 ± 25%

B753 B59753B0120A070 620 120 ± 25%

B754 B59754B0120A070 620 150 ± 25%

B755 B59755B0115A070 800 500 ± 25%

Leaded disks, coated:

C1412 B59412C1130A070 620 120 ± 25%

C1451 B59451C1130A070 470 56 ± 25%

Applications & Cases

Typical applications for the J20X series are industrial power supplies, frequency converters and UPS systems in the

power range from 500 W to 50 kW. 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. Fig. 1 shows a

block diagram of a conventional rectifier or converter system.

The combination of an ohmic resistor and a relay as described above 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.

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.

Applications & Cases

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 malfunction, the temperature

and inherent resistance of the PTC ceramic increases in line with the increased ohmic losses (Fig. 2) and limits the

current to a safe level.

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 (Fig. 3).

A three-phase bridge rectifier connected to a power supply with a phase-conductor voltage of 400 VRMS is assumed.

The smoothing capacitor used has a capacitance of 940 µF. A parallel circuit comprising two charging resistors of type

B59204J0130B010 is used to limit the inrush current. Also known as a zero-potential resistor, it has a rated resistance

of 100 Ω at an ambient temperature of 25°C. The parallel connection of two components is needed in this case

because the energy that must be transferred to the capacitor during charging would cause a single B59204J0130B010

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.

FIGURE 3: RECTIFIER CIRCUIT

Typical configuration of a rectifier at the

three-phase power supply system with a

smoothing capacitor and self-protecting

charging resistors.

Applications & Cases

resistor to heat up to an undesirable extent and as a result become highly resistive. This should be avoided, as the link-

circuit capacitor would otherwise not be completely charged.

Applications & Cases

The number of required components from the J20X family may be calculated from the equation:

where:

The component B59204J0130B010, for example, has a heat capacity of approximately 2 J/K and a reference

temperature of 130°C. The two components may be connected in either parallel or series. Satisfaction of the above

equation ensures that the PTC ceramic does not exceed the reference temperature up to completion of charging and

thus remains in the low-resistance range.

When 95 percent of the maximum charging voltage of the capacitor is reached, the parallel-connected J20X

components are short-circuited and the load (represented by a 260 Ω fixed resistor) is connected in. Subsequently, the

behavior of this parallel circuit of two J204 components is compared with that of a 50 Ω fixed resistor. A current-time

diagram such as that shown in Fig. 4 represents fault-free charging.

The time curve of the charging currents is almost identical in the two cases. The slight divergences of the current

characteristic of the PTC ceramic from that of the fixed resister are due to:

n the number of required J20X elements

C the capacitance of the link-circuit capacitor in F

V the maximum charging voltage of the capacitor in V

Cth the heat capacity of a J20X charging resistor in J/K

TRef the reference temperature of the PTC ceramic in °C

TAmax the maximum ambient temperature at the insertion point of the charging resistor in °C

FIGURE 4: NORMAL CHARGE WITH A PTC AN OHMIC RESISTOR

The currents flowing during charging are

almost identical for the PTC ceramic (blue)

and the ohmic resistor (red).

Applications & Cases

the particular shape of the resistance-temperature characteristic of the PTC thermistor; and

the voltage dependence of the PTC ceramic that is particularly strong during the turn-on process. This voltage

dependence must be taken into account when dimensioning the peak inrush current.

After about 190 ms, charging is completed and the charging resistors are short-circuited. The energy absorption curves

and thus the degree of heating are also almost identical (Fig. 5). Their maximum corresponds to the energy in the

capacitor at the time of the short circuit.

The advantage of a PTC thermistor as a current-limiting element becomes evident in the event of malfunctions. If the

relay fails to close, the load current flows via the charging resistor and produces a high thermal stress that would

require corresponding dimensioning of the resistor. When a charging resistor based on PTC ceramic is used, its

resistance rises to several 10 kΩ due to the high initial power dissipation and accordingly limits the current flowing

during this malfunction (see Fig. 6). After about three seconds, the current flowing through the two resistors and thus

through the entire circuit has dropped to a few 10 mA. A comparison of the energies absorbed is shown in Fig. 7.

FIGURE 5: ABSORPTION DURING CHARGING

The energy absorption of the PTC ceramic

(blue) and the ohmic resistor (red) are

almost identical during normal charging.

FIGURE 6: CURRENT FLOW WITH A DEFECTIVE RELAY

With a defective relay, the PTC ceramic

heats up and the current drops to safe

levels after about three seconds (blue)

Applications & Cases

By going into the high-resistance state, the PTC ceramic limits its energy absorption to a non-critical value, whereas

the fixed ohmic resistor shows a linear rise of the absorbed energy. In the above example, the fixed resistor must have

a rated power of above 200 W in view of the temperature derating in order to avoid overheating and subsequent

destruction.

Malfunction – short-circuited capacitor at the start of charging

The high inrush current makes the two self-protecting charging resistors highly resistive after about 150 ms so that they

restrict the current. The current flowing through the fixed resistor is limited only by the very low power line impedance

and thus causes a very high power conversion in the fixed resistor (Fig. 8).

After a short time, both parallel-connected self-protecting charging resistors are in thermal equilibrium with their

surroundings and the energy absorbed rises only slightly due to the high resistance of the PTC ceramic. The resulting

energy absorption is similar to that shown in Fig. 7.

The above-mentioned malfunction, “short-circuited capacitor at the start of charging,” represents a very high load on

the charging resistor. Therefore, J201 charging resistor requires an additional fixed resistor to limit the short circuit

FIGURE 7: ENERGY ABSORPTION WITH A DEFECTIVE RELAY

After about four seconds, the energy

absorption of the PTC ceramic is constant,

whereas it increases at the ohmic resistor

(red).

FIGURE 8: CURRENT CURVE WITH SHORTED CAPACITOR

With a shorted capacitor, the current

flowing through the PTC ceramic drops

very quickly to non-critical values (blue).

With the ohmic resistor, however, the

current flow remains constant at high

values (red).

Applications & Cases

current. The charging resistors J202 and J204, however, can be used without any additional protection by a fixed

resistor.

Author: Dr. Stefan Benkhof, Product Marketing Manager PTC Thermistors

Applications & Cases