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Power Semiconductors

Critical Components Behind Switch-mode Power Supplies

19 May 2016

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The job of passive components in the input circuit of a power supply is to protect the electronic device from overcurrent and overvoltage and ensure electromagnetic compatibility—especially important considerations in today’s portable applications where long battery life is a necessity. The choice of the components used in switch-mode power supplies (SMPS) is extremely important, as their impact on performance is often not considered. These components—such as resistors, capacitors and inductors—are called upon to deal with heat, size, output power and device price tag.

Switch-mode power supplies switch on and off at high frequency, providing electric energy transfer from capacitors and inductors. SMPS are smaller and feature a lighter package and reduced manufacturing costs.

While there are many components involved in SMPS, a handful of critical devices include:

1. Capacitors

Capacitors perform energy storage, filtering, compensation and soft-start programming. The overriding loss for capacitors in SMPS applications is based on effective series resistance (ESR). ESR results from the physical construction of a capacitor, the resistance of internal interconnects, and even the behavior of the material used as an insulator between capacitor plates. Many capacitors are particularly sensitive to heat and a function of ESR is to dissipate heat in the presence of a current flow into or out of a capacitor. The ESR of a capacitor is typically the most important specification when selecting capacitors for SMPS applications—even more important than the component’s basic capacitance rating.

Capacitors represent the key components of the DC link. They smooth the rectifier ripple voltage and stabilize the DC link voltage. While all rectifiers produce an output voltage that has ripple superimposed on the DC, it varies in magnitude, waveform and frequency depending on the rectifiers. The downside of ripple includes possible reduced productivity or process equipment life, interference with process instrumentation, and electromagnetic coupling with nearby structures, resulting in heating of the structures and increased losses.

DC link capacitors expose the rectifier and power line to extremely high input currents at the moment of turn-on, potentially destroying the rectifier diode or causing the power fuse to blow. This can be avoided by using negative temperature coefficient (NTC) thermistors as inrush current limiters (ICL), for a gentle power up. When NTC ICLs are sufficiently heated by the current flow, their resistance drops to a negligible value. The B57*S series of EPCOS NTC thermistors offers a corresponding ICL for each required current strength.

EPCOS aluminum electrolytic capacitors are particularly suitable for this job based on their high CV value; they allow today’s compact power supplies to be easily implemented. The capacitors are distinguished by a very high ripple current capability, combined with low ESR and ESL values, leading to lower intrinsic heating. Depending on the required output and voltage of the link circuit, radial versions or snap-in types with two or four pins are used. The capacitors cover a voltage range from 10 V DC to 600 V DC and maximum temperatures of 85° C to 140° C.

In two EPCOS aluminum electrolytic capacitors with screw terminals (B43700* and B43720*), the permissible rated voltage increased from 550 V DC to 600 V DC. Covering a capacitance range from 680 µF to 6800 µF, the highly compact components are designed for temperatures up to 85° C. A new snap-in series (B43541*) for temperatures up to 85° C, offers a rated voltage of 600 V DC instead of the previous 550 V DC, and features capacitance values of between 47 µF and 270 µF.

Among the newcomers to the range are types with a higher-rated voltage for temperatures up to 105° C. Screw-terminal types are rated at 500 V DC instead of the previous 450 V DC, and the rating of the snap-in types increased from 500 V DC to 550 V DC. The benefit to developers is that they need to connect fewer series capacitors in the link circuit, which reduces costs and mounting space.

Alternatively, EPCOS MKP film capacitors can also be used to stabilize the DC link. The B3267* high-power series and the B3277* high-density series are designed for voltages from 450 V DC to 1300 V DC and cover a capacitance range from 0.47 µF to 110 µF. As with all EPCOS film capacitors, they are distinguished by a very long operating life and are self-healing. Dielectric breakdowns caused by over-voltages lead to local vaporization of the metallization, yet the capacitor remains fully functional.

EPCOS and TDK components in the input circuit of a power supply protect the electronic device against overcurrent and overvoltage, assuring EMC.EPCOS and TDK components in the input circuit of a power supply protect the electronic device against overcurrent and overvoltage, assuring EMC.

2. PTC Thermistors

In comparison to the NTC thermistors previously mentioned, positive temperature coefficient (PTC) thermistors provide protection from current surges, such as those that occur as a result of a short circuit. At excessive currents, these components heat up and acquire a high resistance. As with ICLs, they are inserted directly into the current path of the power input. The B5910*J series spans commonly used current levels and are available in leaded or packaged versions.

3. Varistors

Varistors (variable resistors), commonly referred to as Metal Oxide Varistors (MOVs), are ceramic-based components used to protect susceptible circuitry against damaging overvoltage conditions. Varistors are voltage-dependent resistors whose resistance decreases with increasing voltage. Connected in parallel with the electronic device, or circuit that is to be protected, a varistor forms a low-resistance shunt when voltage increases. Common overvoltage sources include lightning, inductive load switching and electrostatic discharge (ESD).

EPCOS offers an extensive range of varistors including leaded and SMD equivalents, ThermoFuse types and Multilayer SMDs.

  • Leaded and SMD equivalents span voltages ranging from 11 to 1000 Vac, surge current ratings from 100A thru 100kA, energy ratings from .3 thru 6000 joules, as well as new125° high-temperature types
  • ThermoFuse varistors range from 130 to 1000 Vac and surge current ratings to 10,000A . A varistor disc in the series with a thermally coupled fuse is packaged in a plastic housing. If the varistor overheats, the thermal fuse activates, disconnecting the varistor from the circuit.
  • Multilayer SMD types in case sizes 0201 thru 2220 feature voltages ranging from 4 thru 60 Vac with surge current ratings up to 1200A

4. EMC and Transient Protection

The ability of a system to function in an electromagnetic environment without introducing electromagnetic disturbances that may interfere with other electrical products in the same environment is called electromagnetic compatibility (EMC). EMC and transient protection is delivered via a broad range of EPCOS X- and Y-capacitors and common-mode chokes. For example, the electromagnetic interference (EMI) capacitors of the B3292*, B3202*, B3291* and B81123 series are designed for voltages of 250 V AC to 350 V AC.

In addition, EPCOS also offers a broad range of double, triple and quadruple common-mode chokes with the B827* series, with voltages that extend from 250 V AC to 690 V AC and permissible current-handling capacities from 0.25A to 200A.

5. High-performance and Compact Inductors

Inductors are magnetic energy-storage elements that consist of a wire coil wound around a ferromagnetic core. Current that flows through the inductor induces a magnetic field in the core. This field is the mechanism for energy storage. Current in an inductor cannot change instantaneously; when a voltage is applied across an inductor, the current will ramp.

When the switch closes, the full voltage appears across the inductor. When the switch is open, the current in the inductor will ramp down at the same rate and a large voltage is generated as the magnetic field collapses. The magnetic field is the energy-storage mechanism.

Two main core materials are used in inductors for SMPS—powdered iron and solid ferrite. A powdered iron core has air gaps within the material, giving a soft saturation curve. These are for applications that need large, instantaneous current. In comparison, ferrite core inductors saturate faster but are lower cost and have lower core losses.

To reach various voltage levels at the SMPS output, MOSFET-switched power transformers of the EPCOS B78* series are required. They are offered in the most varied core shapes, as well as in SMD or plated through hole (PTH) versions. Despite their compact dimensions, outputs of up to 200 W can be realized per transformer. Versions with several outputs are available. The new chokes and transformers of the EPCOS e-mobility platform represent a special highlight. They were developed specifically for on-board chargers and are characterized, apart from their compact dimensions, especially by low losses.

Power inductors are used in SMPS applications with high requirements for temperature range and reliability.Power inductors are used in SMPS applications with high requirements for temperature range and reliability.

To smooth the output voltages after rectification, EPCOS SMT inductors must be present in the current path; the EPCOS series B8246*, B8247* or B8255* are suitable for this purpose. The range covers types with current-handling capabilities from 0.11 A to 71 A in shielded and unshielded versions. Thanks to the flat-wire helical windings of the high-current versions, this produces a very high copper fill factor so that these components have a low space requirement despite their high performance. TDK inductors are ideal for this purpose. Single-ended aluminum electrolytic capacitors of the B41* series or film capacitors are recommended for smoothing the voltage path.

EPCOS current sensors are indispensable for exact regulation of SMPS. The B82801* series specifically targets this purpose. The sensors are available with transformation ratios of 1:20 to 1:125 and designed for currents from 7 to 40 A.

SMPS can be used in computers, instrumentation, laser and medical applications. They are ideal for use across a wide range of power classes from a few watts when charging a phone, to kilowatts in high-performance industrial power supplies. Design engineers must carefully select such critical components in order to deliver higher performance and greater reliability to the end design.

For more specific product information regarding EPCOS components, please contact Matt Reynolds, field applications engineer, EPCOS Inc., A TDK Group Company at matt.reynolds@epcos.com or 732-906-4382.



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