Components for power supplies
Reliable protection and stabilization
TDK offers developers of power supplies one-stop shopping for passive components: The extremely broad range, especially of EPCOS components, covers all types of powerful DC link capacitors. They are complemented by an extensive range of transformers, chokes, protection devices and EMC components, and more.
Very few electrical devices are still powered by AC voltage from the power line. Whether in communications and information technology, consumer electronics, industrial electronics or lighting, power supplies are omnipresent in a large variety of topologies.
Power supply technology encompasses converters for photovoltaic and wind power plants and frequency converters for industrial drives as well as switch-mode power supplies in a very wide range of power classes. The latter extends from a few watts for mobile phone chargers up to the kilowatt range for high-performance industrial power supplies. Figure 1 shows a typical layout of the input circuit of switch-mode power supplies (SMPS) with all passive components upstream of the rectifier.
|Figure 1: Input circuit layout of a switch-mode power supply|
Safe from current surges
DC link capacitors expose the rectifier and power line to extremely high input currents at the moment of turn-on. In extreme cases, this can lead to the destruction of the rectifier diode or cause the power fuse to blow.
This can be avoided by the use of NTC thermistors as inrush current limiters (ICL), which ensure a gentle power-up. These thermistors have a relatively high resistance at the moment of turn-on and thus limit the inrush current. Only when they have been sufficiently heated by the current flow does their resistance drop to a negligible value. The B57*S series of EPCOS NTC thermistors offers a corresponding ICL for every required current strength.
During operation, PTC thermistors assure protection from current surges, such as during a short circuit. At excessive currents, these components heat up and acquire a high resistance. Just like the ICLs, they are inserted directly into the current path of the power input. The B5910*J series covers the commonly used current levels. These components are available in leaded or packaged versions. Figure 2 shows NTC and PTC thermistors for overcurrent protection and a varistor of the ETFV series, a component that combines multiple protection functions.
|Figure 2: Ceramic protection devices|
Decentralized power generation, especially via wind power and solar plants, increases the risk of voltage fluctuations and dangerous overvoltages. Safe operation of power supplies can be assured by using the EPCOS varistors which make TDK the world market leader in overvoltage protection devices. These ceramic components are continuously being further developed and improved. Thus, types that can handle multiple high-voltage pulses are now available. The varistors of the ETFV series feature a special highlight (Figure 2 right). They are equipped with an integrated thermal fuse that disconnects the component in the event of overload. These components also offer a monitor output via which a malfunction can be signaled. Thanks to their heat resistance and flame-retardant design, these varistors satisfy UL 94 V-0 specifications. Moreover, they are qualified to UL 1449 3rd edition, UL 60691 and IEC 6 0950-1 Annex Q. The ETFV varistors can be connected via circuit board clamps, which significantly simplifies maintenance. In addition to many variants of monolithic varistors with diverse connection configurations and power ratings, SMD multilayer varistors are also very well suited to protect the control electronics of power supplies.
EMC – an absolute must
For EMC and transient protection, EPCOS offers one of the broadest ranges of X- and Y-capacitors and common-mode chokes worldwide. The EMI capacitors of the B3292*, B3202*, B3291* and B81123 series are designed for voltages of 250 V AC to 330 V AC. These components have the corresponding approvals to UL1414, UL1283, CSA C22.2 and EN 60384-14.
EPCOS offers a wide range of double, triple and quad common-mode chokes with the B827* series. Their voltages extend from 250 V AC to 690 V AC, and their permissible current handling capacities from 0.25 A to 200 A.
To achieve maximum efficiency with a simultaneously low power-line load via a large conduction angle, a PFC boost stage is connected immediately after the rectifier. EPCOS inductors are ideally suited for this purpose. They offer a wide range of current capabilities and are available as both leaded and SMD components.
|Figure 3: EPCOS EMC components for power supplies|
|Figure 4: Circuit diagram of an SMPS with DC link circuit, converter and output filtering|
Capacitors for the DC link
The key components of the DC link are the capacitors. They smooth the rectifier ripple voltage and thus stabilize the DC link voltage. EPCOS aluminum electrolytic capacitors are suitable for this purpose. With their high CV values, they allow highly compact power supplies to be implemented. EPCOS aluminum electrolytic capacitors are distinguished by a very high ripple current capability and 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 2 or 4 pins are used here. The capacitors cover a voltage range from 10 V DC to 600 V DC and are designed for maximum temperatures of 85 °C to 140 °C.
In two newly developed series of EPCOS aluminum electrolytic capacitors with screw terminals (B43700* and B43720*), the permissible rated voltage has now been increased from 550 V DC to 600 V DC (Figure 5). These capacitors cover a capacitance range from 1200 µF to 6800 µF. They are designed for temperatures of up to 85 °C and are also highly compact. A new snap-in series (B43541*) for temperatures of up to 85 °C also offers a rated voltage of 600 V DC instead of the previous 550 V DC, and it has capacitance values of between 47 µF and 330 µF. Among the newcomers to the range are types with a higher rated voltage for temperatures of up to 105 °C. The screw terminal types are now rated at 500 V DC instead of the previous 450 V DC, and the rating of the snap-in types has been increased from 500 V DC to 550 V DC. Thanks to these significant voltage increases, developers need to connect fewer series capacitors in the link circuit, which reduces costs and requires less mounting space.
Alternatively, EPCOS MKP film capacitors can also be used to stabilize the DC link. The B3267* (High Power) or B3277* (High Density) series are suitable for this purpose. They are designed for voltages from 450 V DC to 1300 V DC and cover a capacitance range from 0.47 µF to 110 µF. Like all EPCOS film capacitors, they are distinguished by a very long operating life. They are also self-healing, meaning that dielectric breakdowns caused by over-voltages lead to local vaporization of the metalization, so that the capacitor nevertheless remains fully functional.
|Figure 5: EPCOS aluminum electrolytic capacitor for 600 V|
High-performance and compact inductors
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 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.
To smooth the output voltages derived in this way 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 the SMPS. The B82801* series was developed for this purpose. These sensors are available with transformation ratios of 1:20 to 1:125 and are designed for currents from 7 A to 40 A.
|Table: EPCOS and TDK components for power supplies|