GRAFIS 08/04/PDF

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Power Inductors 7 Design Tips

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SMD power inductor types

Switching regulators are becoming increasingly important thanks to their high efficiencies. The trend is towards regulators with output voltages lower than 1V, load currents up to 60A and switching frequencies up to 2 MHz. At the same time, users demand the smallest possible types.

Switching regulator design is supported by specialised software, for example from Texas Instruments (SWIFT/TPS40K Designer) or Linear Technology (Switcher CAD III).

The relevant SMD power inductor design kits from Würth Elektronik offer quick access to a range of components for the construction of in-house prototypes or for optimisation. But what has to be taken into account when using power inductors?

A practical guide for the selection of power inductors for DC/DC converters Switching frequency

Inductor current ratings

The switching frequency of typical converter ICs on the market is in the range 100 kHz to 2 MHz. First generation regulators operated in the range 30 kHz to 55 kHz. This leads to the following recommendations:

The current load for power inductors can be calculated very accurately in terms of DC current load and ripple current load (core losses) using the manufacturers' simulation software. The following approach can be chosen as a rough calculation:

Design Tip 1 Switching frequency < 100 kHz: suitable core materials: iron powder, ferrite Switching frequency > 100 kHz: suitable core material: ferrite

Step-down regulator: Nominal current of the inductor: Maximum coil current:

IN = Iout Imax = 1.5 x IN

Step-up regulator: Nominal current of the inductor: Maximum coil current:

IN = (Uout / Uin) Iout Imax = 2 x IN

Inductance value If there is no application note or software available, inductance can be calculated using the following rule-of-thumb formula:

Step-down regulator:

L=

Step-up regulator:

L=

(U in - U out) U out U in · 0.3 · I out · f (U out - U in) U 2in 2 · 0.2 · I out · U 2out · f

with the ripple current factors 0.2 to 0.4 (selected as 0.2 and 0.3 in these example). I out is the operating current of the circuit to be supplied, Uout the output voltage and U in the input voltage. f is the switching frequency of the regulator IC. Standard values for inductance L can be selected on the basis of the calculated value. If, for example, the value 37.36 µH is obtained as the result - you would select the standard values 33 µH, 39 µH and possibly also 47 µH for testing.

Design Tip 2 Inductance value => higher inductance - smaller ripple current => lower inductance - higher ripple current The ripple current is essential in determining the core losses. Besides the switching frequency, it is therefore an important parameter for minimising the power loss of the power inductor.

Design Tip 3 Please observe the definitions for the data sheet specifications. The nominal current for power inductors is usually linked to the specified self-heating with DC current – here self-heating of 40°C is common at the nominal current. According to semiconductor manufacturers' recommendations, the saturation current is the point at which the inductance value has fallen by 10%. Unfortunately, this is not a standard value for power inductor data sheet specifications and often leads to misinterpretation among users.

DC resistance Once the required values for inductance L and inductor currents are calculated, you select a power choke with the minimum possible DC resistance. Here the demands are often counteractive: Small size, high energy storage density and low DC resistance. Using suitable winding methods and new series, such as the Würth Elektronik WE-HC flat-wire inductors, this ideal case is very close to realisation. The data sheet definition must also be observed here: Is the DC resistance specified as a typical value or as the max. value required for calculating the circuit under worst case conditions?

Design Tip 4 DC resistance with the same size => higher inductance - higher DC resistance => lower inductance - lower DC resistance => same inductance for a shielded inductor - lower DC resistance The DC resistance is essential in determining the wire heating losses; this is another important parameter for minimising the power loss of the power inductor.

Type and EMC

Output L-C filter

Magnetic shielded power inductors like WE-PD, WE-TPC, WE-DD or WE-HC are recommended for EMC-critical applications. The externally fitted shielding prevents uncontrolled magnetic coupling of the windings with neighbouring conductor tracks or components.

An L-C filter at the DC converter output is recommended if a very clean output voltage is required. The components can be selected as follows [2]:

from DC converter

Design Tip 5 Use a magnetically shielded power inductor if at all possible. Do not route any conductor tracks under the component and do not place any circuit boards directly above the component, as this could give rise to coupling via the air gap remaining.

Unshielded power inductors like WE-PD4 can be used for uncritical applications or for low power circuits. Many packaging series can even be changed from shielded to unshielded versions while maintaining solder pad compatibility.

Design Tip 6

Output L-C filter

Design Tip 7 => Select cut-off frequency at 1/10 of the switching regulator frequency => Select output capacitor (e.g. 22 µF) => Calculate inductance

L= Advantage of magnetically shielded inductors of the same type: => higher AL value, therefore lower DC resistances for the same inductance = lower wire losses

1 (2 · π · f ) 2 · C

WE-TPC

WE-PD4 XL

WE-DD

WE-PD XL WE-HC

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Efficiency

Summary

Finally, a comparative measurement between unshielded and shielded power inductors. The Texas Instruments Evaluation Board for the TPS54610 regulator

The power inductor selection steps described are based on the design tips given in this article and are linked to the data sheet specifications. The relevant design software from the semiconductor manufacturers serves to reduce development times. Power inductors from Würth Elektronik are also listed in the leading semiconductor manufacturers' software solutions and hence they are immediately available for inclusion in the simulations. Correspondingly assembled sample kits help optimise prototypes. Magnetically shielded power inductors should be deployed for EMC-critical applications.

Evaluation Board TEXAS Instruments with TPS54610

was used with the following parameters [3]:

References: [1] Gerfer, A.; Rall, B.; Zenkner, H.: Trilogy of Inductors, 2nd extended edition 2002, Swiridoff Verlag, ISBN 3-934350-73-9 [2] Schramm, C.; DC-Wandler: Ausgangsspannung "säubern" [DC converters: „clean up“ output voltage]; ELEKTRONIK, Issue 23/2001. pg. 88ff [3] Texas Instruments: Evaluation kit for switching regulator family TPS546xx [4] SMD power choke data sheets, Würth Elektronik eiSos GmbH & Co. KG, www.we-online.com [5] Linear Technology Switcher CAD III, download at: www.linear-tech.com/software/ [6] Texas Instruments, SWIFT Designer, download at: www.ti.com/sc/power-w

DC converter with TPS54610 Switching frequency f = 550 kHz; Uin = 5 V; Uout = 3.3V; Iout = 5.65A The unshielded power choke WE-PD4 XL from Würth Elektronik, ref. no. 74457047 has the following characteristic data: L = 4.7 µH, RDC = 8.8 mΩ typ., IN = 8.5 A and Isat = 15 A; Size = 22 x 15 x 7 mm3. The magnetically shielded inductor WE-PD XL from Würth Elektronik, ref. no. 74477004 has the following characteristic data: L = 4.7 µH, RDC = 12 mΩ typ., IN = 6.8 A and Isat = 12 A; Size = 12 x 12 x 8 mm3. Despite the smaller size of the WE-PD XL, the circuit had the same high efficiency of 90.7%; the self-heating of the component was 46°C. The example impressively demonstrates that analysis of different power inductors in the design phase pays off: The shielded version achieves the same favourable data and requires less board space; it is therefore the better component from the EMC perspective.

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GRAFIS 08/04/PDF

EMC & Inductive Solutions