At the moment a high-power system is switched on, high loads can result in serious damage—even when the extra load is only for short time. Inrush current limiters (ICLs) can help prevent these issues. In this article, TDK Electronics’ Matt Reynolds examines ICLs based on NTC and PTC thermistors, discussing the underlying technology and the device options.
When high-power devices and systems such as power supplies, frequency converters or on-board chargers are switched on, loads that are often many times the rated current can cause significant stress or damage. Although this extra load is only for a short period of time, it can damage the system, trip fuses or cause other issues with how the device operates. In order to protect the devices and circuitry, ceramic inrush current limiters (ICLs) may be used that are based on NTC and PTC thermistors (Figure 1).
With NTC (negative temperature coefficient) thermistors, the resistance decreases with increasing temperature. In PTC (positive temperature coefficient) thermistors, resistance increases as temperature increases. When a specific temperature is exceeded, PTC thermistors show a sharp rise in resistance.
High inrush currents come in two different types. First, inductive loads that occur in transformers and motors require very high currents to create the magnetic fields needed to operate properly. Second, high-capacitance capacitors in DC links cause high charging currents and cause significant stress to the capacitors and especially to the rectifiers at the moment of connection (Figure 2).
The most traditional way to limit inrush currents is by using low-ohmic power resistor to reduce the current. However, once the inrush is over, the resistor continues to cause a power loss that affects the entire system, which is a significant drawback to this method.
Another, more effective method involves the use of thermistors as ICLs. Both NTC and PTC thermistors have thermal characteristics that can be used—although they differ in resistance characteristics. As a result, they may be used in different applications that require their different resistance characteristics. In some cases, they can be used in combination with each other to provide the desired resistance needed for the application. We will explore the characteristics and applications of both NTC and PTC thermistors below.
NTC thermistors limit high input-side inrush currents. NTC thermistors are temperature-dependent resistors whose resistance drops as the temperature rises, and are typically made from ceramic materials. The resistance of NTC thermistors depends on the ambient temperature at the time it begins to receive power. When the ambient temperature is low, the NTC thermistor’s resistance is relatively high resulting in longer charging times due to lower charging currents. However, higher temperatures cause the NTC ICL to be in a low ohmic state which limits its ability to suppress inrush currents. In other words, as more current flows through the component, it heats up and provides less resistance. In addition, losses of the rated current are also low.
When selecting an NTC thermistor, it’s important to know the initial resistance and maximum current that will flow through the thermistor. The initial resistance must be high enough that when it is connected in series with the load, the current is limited and does not cause the fuse to trip or cause other damage to components including rectifiers. The maximum current is determined by the power rating of the load and as a result, the NTC thermistor must be de-rated.
Cooling time of approximately 90 seconds depending on type should be ensured when using the ICLs. However, this can be problematic when loads are frequently switched at short intervals. The reason why is that a warmed-up NTC thermistor offers little current limiting and is very low ohmic. To overcome this, an NTC thermistor can be bypassed using a thyristor or relay just a short time after switching on and loads are at the rated current. When this occurs, the NTC thermistor does not experience ohmic-reducing warming.
A Zener diode and time constants determine the response time of the bypass circuit depending on the tolerances of the components (Figure 3). Due to the charging current the relay responds to the current requirement. If loads have high rated currents, the power demand of the circuit is less than the losses caused by the NTC thermistor’s continuous current flow.
NTC thermistors have a high resistance at room temperature and when they are energized, they generate heat by themselves and the resistance falls as their temperature rises. Due to these attributes, NTC thermistors can be used as current protection devices for electrical and electronic devices that easily and effectively limit abnormal currents including an inrush current at the time of powering on. When used as current protection devices, NTC thermistors are also called power thermistors.
Although they provide fixed resistance, an NTC thermistor always causes a power loss and a decrease in performance. Therefore, an NTC thermistor limits an inrush current with its high initial resistance, and then its temperature rises because of energization and its resistance falls to a few percent of its level at room temperature. In that way it achieves a power loss that is lower than when a fixed resistor is used.
In other words, the effect of limiting inrush currents obtained by using an NTC thermistor is greater than that obtained by using a fixed resistor with comparable initial power losses. As a result of these characteristics, NTC thermistors can be used as inrush current limiting devices for switching power supplies, AC-DC power modules, DC-DC converters, industrial inverters and more.
In DC link circuits, high-capacitance capacitor banks and capacitors may short circuit when switched on. PTC thermistors should be used instead of fixed resistors to have reliable current limitation. PTC thermistors offer more consistent and reliable protection against inrush current surges and short circuits, while providing accurate temperature control and measurement. When current flows through a PTC thermistor, it heats up and its resistance increases, making it self-protecting and providing a significant advantage to other forms of inrush current limiting.
This behavior is the opposite of NTC thermistors, making PTC thermistors intrinsically safe and limits the current to values that are harmless to the system in the case of a short circuit, something that fixed resistors cannot do. This is an ideal characteristic for frequency converters. In banks of capacitors, engineers should make sure not to exceed the maximum thermal capacitance and maximum permissible temperature of the PTC thermistors. The necessary thermal capacitance can be achieved by connecting the PTC ICLs in parallel.
The PTC ICL must be bypassed after charging the DC link capacitors in normal operations to eliminate or reduce power losses. There must be no bypass, however, if there is a short circuit in the DC link—caused perhaps by damaged capacitors. The most significant parameter for a bypass circuit, therefore, is the DC link voltage (Figure 4). If it reaches the setpoint after charging it will not fault. However, it will short circuit if it remains at a very low value for a longer period of time. This enables a comparator circuit to be implemented with little effort, which bypasses the PTC thermistor only after charging of the DC link capacitor.
The inverting input of the comparator may be controlled by a Zener diode. When specified voltages are applied the comparator at the output trip to positive potential and switches the relay, causing the PTC thermistor to be bypassed. In this way, the varistor and the Zener diode serve to protect the non-inverting input of the comparator against overvoltages.
Because PTC thermistors are based on special semiconductor ceramics with a high positive temperature coefficient, they are temperature-dependent. They exhibit relatively low resistance values at room temperature. When a current flows through a PTC thermistor the heat generated raises its temperature and once a pre-defined temperature—called a Curie temperature—is exceeded, the resistance of a PTC thermistor rises significantly.
This attribute can be used to protect circuits or devices from overcurrents. In this case, the overcurrent brings the PTC thermistor to a high temperature and the resulting high resistance then limits the overcurrent and eliminates this cause of possible malfunction. When the cause for possible failure is eliminated the PTC thermistor will cool down, and act as a resettable fuse and can trip. As a result, PTC thermistors can act as a robust overcurrent protection device for on-board chargers, industrial inverters, on-board DC motors, solenoids and more.
NTC AND PTC TOGETHER
It is possible to combine the advantages of NTC and PTC inrush current limiters and leverage both of their functions in the case of high-power loads that have DC link capacitances. The main applications for this are industrial power supplies and converters. When this is done, a voltage-controlled ON time should be employed to bypass the NTC thermistor on the power input side. To accomplish this, a relay with two changeover contacts is needed in which the NTC and PTC and switched at the same time.
Using a combined solution to address inrush current limitations results in better protection of components, reliably limiting current to prevent DC link short circuits and preventing the tripping of internal or supply-side fuses. Regardless of whether or not an NTC, PTC or both thermistors are employed, the entire system’s high-power loads cause less damage and stress, thereby increasing the life of the design.
For detailed article references and additional resources go to:
TDK Electronics | www.tdk-electronics.tdk.com
PUBLISHED IN CIRCUIT CELLAR MAGAZINE • JULY 2019 #348 – Get a PDF of the Issue