Power conversion, isolation, overvoltage/overcurrent protection and accurate current measurement are requirements in virtually all power supply designs. These capabilities are necessary to help increase reliability and reduce downtime, but are also required for regulatory compliance to help ensure safe operation. From the range of components to choose from, what are the right magnetics, resistive, and protection solutions typically needed throughout a switched-mode power supply (SMPS) design?

Circuit Protection

Circuit protection at the power supply input typically is designed with an overcurrent device first so that it protects against failures of the overvoltage element as well as the rest of the power supply. Because lightning and other surge events will activate the overvoltage element, careful consideration must be made as the resulting surge current will be passed through the overcurrent device. It is advised to specify a minimum overcurrent trip value to allow this surge current to pass so that it doesn’t permanently take the power supply offline.

Figure 2: Shows the typical location for overcurrent and overvoltage circuit protection in a power supply

Fuses are the most common overcurrent technology, and multiple types of fuses are available that enable designers to tailor the fuse response. For instance, to eliminate nuisance fuse blowing, self-resetting polymer and ceramic PTC (positive temperature coefficient) devices are optimal solutions.

For overvoltage protection, Metal Oxide Varistors (MOVs) are popular solutions. However, MOVs are known to degrade over time from exposure to line voltage transients. To reduce the stress and extend the MOV’s lifespan, designers can place a Gas Discharge Tube (GDT) in series with the MOV.

Bourns has developed new hybrid protectors, such as its IsoMOV™ protectors, that integrate the GDT/MOV functionality into a single device to deliver an elevated level of surge protection performance, operational life and device reliability. These fully-integrated, compact hybrid designs give power supply designers the protection performance usually found in larger traditional MOV devices, allowing them to better tailor surge protection to their space requirements, and to upgrade their MOV overvoltage protection.

Offering protection from damage due to an overvoltage condition on a power supply’s AC input lines, Power TVS (PTVS) devices are a good option. For example, Bourns® Model PTVS20-015C-H bidirectional PTVS diodes feature low clamping voltage at peak pulse current and assist in meeting IEC 61000-4-5 8/20 μs current surge requirements.

Many designs can benefit from a multi-stage protection approach. Bourns offers one that combines its hybrid IsoMOV™ protector with its TBU® high speed protector and its TISP® thyristor. This solution effectively limits voltage let-through to the power supply, but also protects against sustained overvoltage events, often without interrupting the operation of the power supply.

The faults that trigger overcurrent and overvoltage protection can generate excess heat within the power supply. That’s why it is recommended to implement protection circuits to shut down the supply when an internal temperature rating is exceeded. Bimetals such as Bourns® mini-breaker thermal cutoff devices can be used both in the temperature monitoring and protection circuits with the ability to conduct power and then to cut the power at precise, pre-programmed temperature settings.

Magnetics Components

Depending upon the SMPS topology used, several magnetics component solutions are needed within the design. There are several forms of power conversion needed including DC-DC and AC-DC. The input filter stage prevents electromagnetic interference (EMC/EMI) generated by the SMPS from returning to the power line (i.e., the “grid”). Magnetics components typically used here are a common mode choke (CMC), a differential mode choke (DMC), or a combination of both plus accompanying capacitors to form a filter to block the unwanted noise signal.

Figure 3: Indicates where a resistor is placed for primary RC snubber circuit for flyback converter

Waveform rectification is often necessary to correct the ratio of real power and reactive power. This helps increase supply efficiency, and is known as Power Factor Correction (PFC). Boost inductors are typically used for both passive type and active type PFC in an SMPS where active type PFC is the most prevalent.

Specifying magnetics for an SMPS design requires to first define if it is either isolated or non-isolated. If it is isolated, then there must be electrical physical separation between the primary/input side of the SMPS and the secondary side. The bridging of primary to secondary is accomplished with a transformer to complete the circuit.

A non-isolated SMPS incorporates single-winding or multiple single-winding inductors for voltage and current conversion. With an isolated transformer, total energy is transferred from primary to secondary. In a non-isolated transformer (also called an isolated flyback transformer), energy is stored to complete the voltage/current conversion.

Depending on the SMPS topology used and the presence of conducted EMI, the output filter stage will likely be included. Here, a single-winding choke (inductor) or a combination of filter choke, DMC and CMC are necessary.

Current Measurement

Understanding the level of current flowing through a circuit and being delivered to a load can be very helpful in maximizing the operating performance of an SMPS. Current sense resistors are optimal, low-cost solutions that help OEMs create more efficient circuit design and work by detecting and converting current to voltage. To choose the right current sense resistor, it is important to look at the input common-mode voltage specification, which is the average voltage present at the input terminals of the amplifier.

Figure 4: A shunt resistor is placed in series with the electrical load whereby all the current to be measured will flow through it

The higher efficiency and stability needed in SMPS calls for tight tolerance of resistance values. The low resistance value of less than 10 – 20 milliohms in Bourns® CSS Series current sense resistors provides an optimal solution. Other benefits include a low TCR, low thermal EMF and long-term stability. Bourns® CSS and CSM Series offer excellent electrical characteristics and performance for SMPS designs.

More Technical Information

To learn more about which magnetics, circuit protection and resistive devices are the optimal component building blocks for an SMPS design, please see a related article in Power Systems Design. It provides additional technical and regulatory standards information that can help designers narrow and speed their component selection.