Power Electronics

Planar Magnetic Technology: Reshaping Power Supplies

Compact and high-efficient power conversion is critical to the miniaturization of telecommunication, computer, vehicle and other industrial and military systems. The creation of any high-efficiency switchmode power supply requires a proper selection of magnetic materials for the transformer and inductor.

The answer for miniaturization and material performance is planar magnetic technology. It's benefits include:

  • Low Profile — Planar magnetics have intrinsically much lower profile than conventional wire wound devices, which aids miniaturization. Typically, the height of a planar magnetic device will be 25% to 50% less than its wire wound counterpart.
  • High Power Densities — Planar transformers tend to have a higher surface-to-volume ratio than conventional magnetic designs. As a result, they are more effective at conducting heat and tend to have low thermal sensitivities compared with conventional wire wound parts. Therefore, it's possible to achieve very high power densities, particularly at frequencies ranging from 100 kHz up to 2.5 MHz.
  • Highly Constant Parasitics — the leakage inductance and capacitance of conventional wire wound magnetics are highly dependent on wire placement. As a result, a significant lot-to-lot variation in these electrical parameters is typical. Inconsistent parasitics effect power supply operation and can cause failure due to voltage and current spiking. With planar magnetics, the windings are highly regular and variations in the parasitics from lot-to-lot are extremely low.

A new set of flat ferrite cores employs innovative planar magnetic technology suitable for high-frequency operations and thin, flex, and pre-tooled circuits (Photo 1 and the Table). As a result, these transformers for quarter and half-brick SMPS have the smallest dimensions for their output power range.

High Power Applications

Planar transformers are the ideal solution for welding and industrial heating applications that generate extremely high output current. In most cases the associated transformers have relatively few secondary turns. A flat planar transformer winding, which is the best for high currents, has advantages in welding applications because of its ability to operate efficiently. For example, a welding SMPS using planar magnetics can dramatically reduce transformer size, weight, and cost.

The planar transformer is also the preferred option for induction heating applications. It's now possible to imagine a 20 kW induction heating transformer for a 200 kHz resonant SMPS with dimensions as L×W×H=142×90×40 mm.

Pre-Magnetized Ferrite Inductors

A newly patented pre-magnetized ferrite technology introduced a new approach in designing dc planar chokes, providing double inductance with the same current, or double current with the same inductance; and up to four times decrease of the power losses, or 30% to 40% smaller footprint with the same dc current and inductance.

Although the idea of a pre-magnetized ferrite inductor is very well known, so far it hasn't been widely used due to its high price, high-frequency limitations, and performance degradation due to magnet demagnetization.

The new chokes overcome these shortcomings. They use ferrite-based magnets, which are inexpensive. The parts were tested at operating frequencies of up to 1 MHz with no magnet degradation observed. There was no degradation of the choke's inductance due to the demagnetization of the magnet, when it was tested at field levels up to 10 times higher than the operational field. Photo 2 compares the pre-magnetized with a conventional choke.

Resonant Chokes — Hybrid Planars

For high-frequency resonant chokes, there's a newly-developed family of hybrid planar inductors: a special “six-legs” planar ferrite combined with a litz wire winding. This combination enables users to obtain a high quality factor (Q) at high frequencies. For example, a 40 µH/3Arms resonant choke has a Q of 500 at 1 MHz operating frequency.

Planar common-mode chokes provide extremely high noise attenuation. We can reduce their ratio of leakage inductance to self-inductance down to 0.005%. Due to the high self-capacitance, a planar common-mode choke can incorporate the input and output capacitors in the choke itself. Thus, a planar common-mode choke can operate as a common-mode filter. It's now also possible to design a planar common-mode choke with low losses that will allow operation at currents up to 200A.

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