Coordinated line protection for power supplies, relays and solenoids

Implementing coordinated circuit protection on a variety of new advanced equipment can help improve equipment reliability, reduce the number of components, and enable it to meet stringent safety regulations.

The coordinated approach helps protect power supplies, relays and solenoids from increased current due to line faults or overloads, as well as voltage spikes or exposure to steady-state overvoltage conditions.

SMPS Design Considerations

Switching mode power supplies (SMPS) are replacing linear regulators in many applications, including white goods, as they meet the size, weight, and energy conservation requirements of consumer electronics. However, because SMPS lack the inherent resistance of previous designs, they often require more robust line protection.

Polymer positive temperature coefficient (PPTC) overcurrent protection devices can help manufacturers meet the UL60950-1/LPS (limited power source) requirements for SMPS and improve the safety and reliability of equipment. Although the device cannot prevent the occurrence of faults, it can react quickly and limit the current to a safe level to prevent downstream components from being damaged. In addition, PPTC devices are small in size and can be easily used in space-constrained applications.

PPTC devices have a low impedance value under normal operating current, but will jump to a high resistance state when in an overcurrent state. The increase in resistance will reduce the current flowing through, changing from a fault state to a lower steady state, thereby helping to protect the device circuit. The device will remain locked until the fault is removed. When the circuit power returns to normal, the PPTC device will reset and restore the current, returning the current to normal operating conditions.

As shown in Figure 1, PolySwitch™ PPTC devices can be connected in series with the power input to help protect against damage caused by circuit shorts, line overloads, or user errors. In addition, placing a metal oxide varistor (MOV) across the input can also provide overvoltage protection.

Figure 1 Common circuit protection design for switching power supplies

PolySwitch can also be placed after the MOV. Many equipment manufacturers like to combine resettable PolySwitch devices with upstream fail-safe protection in the protection circuit. In this example, R1 is used as a ballast resistor in combination with the protection circuit.

Solenoid Design Considerations

A solenoid is an electromagnetic device that consists of a coil, a wire frame, an armature, and a support frame. The coil and the support frame are mechanically fixed on the wire frame, and the armature is inserted into the coil. When the coil is excited by current, a magnetic force is generated, which pulls the piston into the coil and fixes it to the support frame.

When the solenoid is energized, the end-of-travel position of the armature is detected by a sensor, which feeds the armature position information back to the system circuit to shut off the solenoid power. If the sensor fails and the armature is not pulled in, the intermittently operating solenoid will generate a lot of heat and may fail.

Combining PPTC devices with MOVs and installing them at the primary of the AC input can protect against damage caused by overcurrent and overvoltage failures. Unlike one-time fuses, resettable PPTC devices can also protect against damage caused by temperature rise due to failure with only slight current changes.

Certain overload conditions can also cause the MOV to remain clamped and continue to conduct electricity, which will eventually cause the device to overheat and fail. At this time, the PPTC device can be connected in series with the MOV device and placed close together so that their temperatures are close. As the MOV transfers heat to the PPTC device, the PPTC device parameter changes faster, limiting the current flowing through the MOV, so that the MOV can provide protection in a larger overload condition.

This technique allows designers to take advantage of the temperature response of the PPTC device and replace other thermal protection components in the circuit. Not only does the PPTC device perform two functions here, it is also a fully resettable solution.

The most common cause of solenoid failure is mechanical obstruction, either due to dirt contaminating the solenoid or debris getting stuck between the armature and the inner wall of the coil, preventing proper movement. Other problems include misalignment, a damaged spring, or opposing forces.

These conditions may cause the current flowing through the solenoid to remain unchanged while the coil temperature rises, eventually burning the coil insulation and wiring. As shown in Figures 2a and 2b, under normal conditions, the solenoid coil temperature rises every cycle, Figure 2c shows how an overtemperature fault causes the coil winding to burn, and Figure 2d shows a PolySwitch device inserted into the circuit that limits IIN when it rises to 120°C, causing the coil temperature to gradually decrease to protect the coil from damage.

Figure 2 PolySwitch devices prevent damage caused by overcurrent/overtemperature by limiting coil current

Relay Design Considerations

Relays are often used in consumer electronics to control high currents and high voltages with small signal levels, or to switch currents that must be isolated from the control circuit. The most basic parts of a relay are the coil, armature, and contacts. When the relay is connected to the circuit, the current will generate a magnetic field in the relay coil, which then acts on the armature to connect or disconnect the contacts connected to the relay output.

Excessive voltage or current may cause damage to the relay. A common problem is the voltage spike generated when the relay disconnects the current of the inductive load. If the voltage spike is large enough to exceed the rated voltage of the relay contact, the contact may be damaged (V = Ldi/dt). This damage may be sudden or slow, appearing after years of operation.

In addition, when the contacts are disconnected, if the current flowing through the contacts is too large, it will also cause damage. Excessive current and voltage will also damage the relay coil. If the relay coil is designed to have only a short excitation time under normal working conditions, then if the excitation time is occasionally extended under normal working current, the coil will also be damaged.

Figure 3 shows a common relay protection circuit. A PolySwitch device is placed in series with the relay coil to limit the current during a fault or accidental overload. The figure also shows the PolySwitch in series with the relay contacts.

Figure 3 Typical overcurrent/overvoltage relay protection circuit

When selecting a PPTC device, it is very important that its rated voltage is equal to or greater than the maximum possible voltage, and its holding current is equal to or greater than the maximum steady-state current under normal operating conditions. In addition, the maximum ambient temperature must also be considered, because an increase in ambient temperature will cause the holding current to decrease.

Figure 3 also shows an MOV or MLV in parallel with the relay contacts. The ratings are based on the voltage and maximum surge current. It is important that the device selected does not conduct large currents at the normal peak voltage. MOV parameters include the maximum allowable AC or DC voltage. Each MOV and MLV device also has a maximum rated surge current. A common standard practice for determining the rated surge current is to use an 8/20 microsecond waveform (8 microseconds rise time and 20 microseconds fall time to half the peak value). As the size of the varistor increases, the 8/20 microsecond surge current rating will also increase. The MOV or MLV can also be connected in parallel with the relay coil, as shown in Figure 3.

Summarize

Coordinated overvoltage/overcurrent line protection can help designers reduce the number of components, design safer and more reliable products, comply with regulatory requirements, and reduce warranty and repair costs. PPTC devices have recoverable functions and low line impedance characteristics. They are rated for 240VAC, allow a maximum voltage of 265VAC, and can be placed on the AC input line. MOV and MLV devices can help manufacturers meet a series of safety regulations and provide large current handling capabilities and energy absorption performance as well as fast response to transient overvoltages.