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Designing GaNFast Mobile Chargers

In my last blog, “Fast Chargers … Going GaN Fast!!”, I spoke about the opportunity to create fast mobile chargers using GaN power IC technologies. With a 3-5x increase in power density, chargers can now charge your mobile devices 3-5x faster and can miniaturize those bulky chargers to match the thin, sleek and lightweight design of the mobile devices they are charging… let’s talk about the steps to kick the brick!

Selecting your Architecture

GaN power ICs are fast, very fast, and high switching frequencies means more power and faster charging in smaller & lighter weight footprints and outlines. But which architecture is best suited to take advantage of such a fast power device? The answer isn’t “hard”! Soft-switching circuits, as compared to hard-switching ones, create minimal switching losses and less electrical noise or EMI, even at higher frequencies. When you combine soft-switching architectures with GaN power ICs, the sky is the limit for high-frequency and high-efficiency, low-noise power supplies.

For fast mobile chargers in the range of 65W and below which is common for laptops, tablets and smartphones, an excellent choice is the Active Clamp Flyback (ACF). Compared to a traditional flyback, the ACF architecture uses a second FET in place of the flyback’s clamping diode, to create an active clamp circuit. This minimizes voltage stresses and recycles leakage energy, while allowing control of the voltage across FETs so that the devices turn on after the drain voltage reaches zero. In doing so, zero-voltage switching (ZVS) is achieved which is a very effective form of soft-switching.

Selecting your Control ICs

Traditional ACF controllers are slow and cannot meet efficiency requirements, especially during light-load conditions. While a general-purpose microcontroller or DSP might give you the flexibility to implement the required control algorithms needed for an ACF charger, the cost and power consumption of such controllers can be prohibitive. Luckily, IC suppliers are developing application-specific control ICs this year which are suitable for GaN-enabled, high-frequency ACF mobile chargers with very attractive cost points and low power modes to meet regulatory standby power requirements such as DOE Level VI and CoC Tier 2. Look for product announcements from control IC suppliers soon.

Selecting and Designing Magnetics

While it’s important to combine GaN power ICs with soft-switching architectures and high-frequency control ICs, a GaNFastTM charger will only be possible with careful selection and design of the magnetics. Both the transformer and the EMI filters are very big opportunities for size and cost reduction. Existing transformers optimized for low-frequency (100 kHz) operation are large, expensive and inefficient. With a 3x-10x increase in switching frequency, these transformers can be reduced in size, weight and cost by 50% or more. Multiple magnetics suppliers such as Hitachi Metals, TDK EPCOS and Ferroxcube offer new magnetic core materials that can achieve very high-efficiency and very small core size when operating in the 300 kHz to 1 MHz range.

Once the core material is selected, the design of the transformer windings and thermal-mechanical design is also critical to achieve the desired efficiency, density and thermal targets. Navitas has significant experience in designing such high-frequency, high-density transformers to assist our mobile charger customers. EMI filters are the other key magnetic components requiring careful attention. New EMI cancellation techniques and filters are needed to minimize noise and meet EMI regulations. Navitas has extensive experience and proven techniques to advise customers in implementing such methods. With these methods, EMI can actually be reduced using EMI filters that are 50% smaller than existing ones and passing EMI requirements is a fast and easy process!

Putting it all together

In a typical mobile charger design, the magnetic components can represent 60% or more of the total volume. A 50%+ reduction in size of these magnetics represents a 30%+ reduction in overall size of the design. In addition, a typical charger design today dissipates significant power, due to inefficient operation in the range of 85-90% at full load and worst-case 90 VAC input. This dissipated power requires a significant portion of the size and cost to be dedicated to thermal dissipation (i.e., heatsinks, thermal glue, potting materials, etc.). With the high efficiency enabled by GaN power ICs, typically approaching 94% or greater, thermal solutions are minimized or even eliminated, saving another 10-20% of the volume of the charger design.

Finally, the high level of integration enabled by these GaN power ICs combining up to 10 components in to one miniaturized surface-mount package, saves another 5-10% of the volume of the design. Taken together, these chargers can shrink by 50-80% for a given power level or, conversely, can increase their power output by 2-5x within the same charger case or outline. With this size reduction, also comes cost reductions as the transformer, EMI filter, PCB and housing all come down in cost as their size is reduced.

For help in designing your first GaNFastTM mobile chargers, please don’t hesitate to reach out to me at gene.sheridan@navitassemi.com or our support team at info@navitassemi.com. Let’s get going GaNFastTM !

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