Various switchmode power supply topologies use inductors for energy storage. The photos, below, show examples of an unshielded (left) and shielded (right) power inductor. The necessary inductance characteristics for a step-down converter depend on the keying ratio that defines the relationship between the input and output voltages:
VOUT = Output of the step-down converter
VIN = Input to the step-down converter
t1 = First timing interval, shown in the step-down converter schematic in Fig. 1, on page 46.
t2 = Second timing interval, also shown in Fig. 1.
The transistor conducts during a time t1. This charges L and C, then transfers energy to the load, R. This blocks the transistor during period t2, during which time the diode conducts. The energy stored in L during t1 then discharges to C during t2 and transfers to load R. The current flowing through the transistor has a triangular to trapezoidal wave characteristic, whereas the voltage is square-wave.
A rough approximation for the storage inductance's value in a step-down converter is:
ΔI = Maximum ripple current
f = Clock frequency
In a specific example:
Input = 12V
Output = 5V at 1A
Maximum ripple current = 0.5A
Clock frequency =100 kHz
This results in an inductance of approximately 100 µH and a maximum inductor current, Imax, of 1.25A (Imax = Imean + ΔI/2).
A step-up converter requires a different configuration. Fig. 2 shows the switching principle of a step-up converter whose keying ratio is:
The transistor conducts during t1 and is blocked during t2. The inductance, L, maintains the current with respect to VIN and supplies an increased output voltage (VOUT) via the diode.
A rough approximation for the inductance of a step-up converter is:
For example, if:
Input = 3V
Output = 6V at 1A
Ripple current = 0.5A
Clock frequency = 100 kHz
This results in an L of approximately 30 µH and maximum coil current, Imax, of 2.25A (Imax = Imean + ΔI/2, where Imean = IOUT × [VOUT/(VOUT — VIN)].
DC/DC Converter Inductors
Regardless of the exact configuration, inductors must satisfy the following requirements:
- High rated currents: saturation current must be large enough for the inductor to operate in the linear range.
- Low-ohmic resistance: important for high efficiency of the dc-dc converter.
- Low losses at high frequencies: important for high efficiency and low heating of the dc-dc converter.
- Surface-mount design: important for low-cost placement in volume production.
For dc-dc converters with low power, the SIMID series 1812-T and SIMID 2220-H, part numbers beginning with B82432-T and B82442-H, respectively, offer very good current-handling capability for dc-dc converters at lower power. The SIMID 1812-T can handle rated currents up to 1.3A, and the SIMID 2220-H handles up to 2.5A.
Due to the inductors' construction (laser welding, lead-free contacts) and materials (LCP encapsulation, temperature-resistant copper enamel wire), they can be used at ambient temperatures up to 125°C and, in special cases, up to 155°C.
The SIMID series is characterized by its high operational reliability — even at high ambient temperatures. Figs. 3 to 5, on page 48, show the drift characteristic at 150°C storage, at the rated current IR, and at 125°C for up to 10,000 hr in each case, with 1,000 temperature reversal cycles of -55/+125°C. In all three tests, the changes are well below the limits required by standards with a scatter width of 6σ.
For dc-dc converters with high power, the rated currents of the SIMID series may not be sufficient. Thus, SMD power inductors were developed, initially with conventional soldered connections (the wire is soldered to the terminals). A wide range of unshielded SMD power inductors satisfy the general requirements at ambient temperatures up to 85°C. They have cylindrical ferrite cores wound with copper wire.
The smallest version of the unshielded SMD power inductors has a footprint of 5.8 mm × 5.2 mm, the largest 18.6 mm × 15.3 mm. Inductance values range from 1 µH to 1,000 µH at rated currents in the range from 0.3A to more than 8A and with low-ohmic resistance. The rated current is referred to an ambient temperature of 85°C. Supplied on tape, these inductors offer all the benefits of surface mounting. They are suitable for re-flow soldering and can be used, depending on type, in a temperature range from -40/-30°C to 85/125°C.
Rated currents of the SMD power inductors are shown in Fig. 6 as a function of inductance, compared with the SIMID series 1812-T and 2220-H. Rated current ranges of the SMD power inductors seamlessly complement those of the SIMID series.
Some applications require shielded SMD power inductors. A series with a footprint of 7 mm × 7 mm (type B82472) is currently available for standard requirements at ambient temperatures up to 85°C. Inductances range from 1 µH to 100 µH, and the associated rated currents from 0.45A to 2.9A.
For tougher requirements and high reliability, the series with conventional soldered connections cannot cope with the higher temperatures required in some applications. Therefore, a new series with footprints of 6 mm × 6 mm (type B82462) and 10 mm × 10 mm (type B82464) have been developed in two different heights in shielded and unshielded versions. They are available with inductance values from 1 µH to 1000 µH and rated currents from 0.1A to 7.5A. The temperature range was widened for this new series and extends from -55°C to +125°C. The manufacturing processes used for the power inductors were largely adopted from the proven SIMID technologies.
Fig. 7 shows the rated currents as a function of inductance for unshielded inductors with footprints of 6 mm × 6 mm and 10 mm × 10 mm in both standard and low-profile versions. The low-profile version with a footprint of 6 mm × 6 mm can handle a current up to 2.5A (unshielded version) or 2.6A (shielded version) with a maximum height of only 2.5 mm.
The version with a footprint of 10 mm × 10 mm and low height can handle a rated current of 4A (unshielded) or 3.8A (shielded) at 3.3 µH. The maximum height is only 3 mm.
Infineon offers the TLE 6363 controller for typical automotive applications (14V system), such as supplying power to airbags. It contains a step-up converter with an output adjustable from 8V to 35V and a step-down converter with a fixed output of 4.75V to 5.25V. The outputs can operate independently of each other.
Two coils are used (Fig. 8, on page 55). One of them, part number B82442-A1104-K or B82464-A4104-K, steps up the voltage. The first is rated at 100 µH, 0.25A and 1.28W, and the second is an SMD power inductor rated at 100 mH, 1.15A and 0.29W. The second one steps the voltage down; this is the part number B82422-H1224-K or B82464-A4224-K, type 2220, rated at 220 µH, 0.24A and 2.72W or SMD power inductor rated at 220 mH, 0.8A and 0.62W (Fig. 8).
The Infineon TLE6389G50 buck controller (for 14V and 42V systems) has an output up to 15W (Fig. 9, on page 55). It uses an SMD power inductor and can step down voltages in the range between 5V and 60V to a range between 5V and 15V. This buck controller uses the 10 mm × 10 mm version, (series B82464), rated at 47 µH, 1.6A and 0.145W so that the buck controller can be operated as efficiently and reliably as possible — even at high power. It is unshielded and satisfies the elevated requirements of the automobile industry.
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