Next-generation high-frequency circuits with GaN


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The key to improving power density is to increase the switching frequency to reduce passive components, such as transformers, EMI filters, bulk capacitors, and output capacitors, while at the same time maintaining or improving efficiency. High-speed topologies such as active flyback (ACF) have been proposed by academics since 19961, but have been frustrated by the poor switching performance of silicon (QGD, Trr, COSS), not to mention the complexity and cost of the system. Gallium nitride (GaN) 3 is a “wide bandgap” material because it offers an electronic bandgap three times larger than silicon, meaning that GaN can handle 10 times stronger electric fields and deliver high power with dramatically smaller chips. With much smaller transistors and shorter current paths, extremely low impedance (RDS(on)) and capacitance (QGD, COSS, zero Trr) are achieved, enabling faster switching speeds of up to 100×. To deliver truly performance that fulfills the GaN promise, ICs4 harmoniously integrate GaN power (FET) and drive GaN power switches to control and protect the GaN power switch at high speeds. Three new topologies were introduced: a 50 W pulsed ACF, a 300 W CrCM PFC electrode totem, and a 1 kW half-bridge LLC. Pulsed ACF: Elimination of Bulk Capacitors by Electrolysis Reducing the bulk capacitor—or complete removal—has been an elusive topology for many years, with little or no success. The bulk capacitor (µF) rating is determined by the required output power, AC line voltage, and AC line frequency. Rating is the process of balancing the charging and discharging of the capacitor for each AC line cycle to provide the necessary output power, while maintaining the minimum DC voltage level (~400 V) needed to provide a stable DC output voltage. Increasing the switching frequency of the power shift stage itself has no effect on the size of the block capacitor, so it does not benefit from the same frequency reduction to size that we get with magnetism. Even if the switching frequency is increased enough that the magnetic elements shrink to the PCB-based “air cores”, the block capacitor voltage still has to be replenished by the AC line voltage at the very low AC line frequency (50/60 Hz) So the classification – and physical size – remain unchanged. However, if we change the output requirements of the transformer from, say, a tightly regulated DC voltage to a rectified AC voltage, we can change the rules of the game. Through the pulse output, we can get the rectified AC bulk capacitor voltage, which allows the value of the bulk capacitor capacitance to be greatly reduced and the DC bus voltage can directly follow the rectified AC line voltage. For fast smartphone chargers, pulse current is acceptable, especially if the phone battery charging algorithms are slightly modified to accept the pulse voltage waveform. To achieve the requirements of the new pulsed output voltage, the ACF topology can efficiently convert the rectified AC vector voltage into the DC pulsed output voltage. The traditional QR flyback system is a simple, low-cost but “hard toggle switch” during high-line conditions. Resonant LLC topologies provide full-load range ZVS operation but rely on a range-limited DC bus voltage. The ACF topology offers the best of both worlds by enabling ZVS operation across the entire line and a wide load and voltage range. Compared to the traditional QR flyback, the ACF topology includes an auxiliary high-side switch and a capacitor to lower the switching node voltage (VSW) to the opposite rail during the specified time and achieve ZVS. Megahertz ACF using GaN power ICs was demonstrated academically in 20165 and available to industry since the introduction of 2018 TI’s UCC2878x ACF PWM Controller. GaN enables high frequency ACF operation and results in a significant reduction in transformer size; For example, from the RM10 shuttle-based transformer of 22 mm diameter at 50 kHz to a thin 8 mm EI25 planar transformer at 500 kHz, as shown in Figure 1. Figure 1: How high frequency drives smaller passive components, 50-example of a fast charger : ~100 kHz conventional spool (22 mm high) (left) and ~500 kHz planar transformer (8 mm) (right) Reducing the volume by increasing the frequency and pulse operation (eliminating the block capacitor) introduced the Oppo ultra-thin 50W charger “Cookie” GaN-power-IC fast-charging in 2020. This was a perfect example of combining GaN with some new system partitioning to reduce transformer size and profile, ultimately creating a new device and unique user experience. High Frequency PFC, Without Bridge Conventional PFC topologies for medium power applications (100 to 500 watts) include an input bridge rectifier followed by a conventional boost converter. When the boost switch is turned on and off at a certain switching frequency, the on and off times are controlled so that the AC line input current follows the same shape and phase as the AC line voltage and the DC bus output voltage is kept at a constant level. During 90-VAC input and full load conditions, this circuit can reach an efficiency of about 96%. The boost converter itself can be made very efficient, but the AC input bridge losses are very high, causing extreme thermal conditions and poor overall efficiency. Enter the “bridgeless totem-pole” PFC topology. In conventional PFC circuits with a standard AC rectifier, at any given time, two input bridge diodes are always performing and generating >50% of the total PFC circuit losses. Several non-bridge PFC circuits have been investigated over the past few decades in attempts to phase out the input bridge rectifier and boost system efficiency, but few have succeeded in making their way out of the lab and into the mainstream market, mainly due to increased complexity and cost. These structures include the classic totem pole without bridges, semi-bridges, bi-directional, bridgeless and bridgeless. Each of these structures has its own set of pros and cons, but none of them is an optimal solution. While microcontroller-based designs were implemented for the multi-kilowatt SMPS data center, standby losses were too high to meet consumer market requirements such as DoE Level IV and Euro CoC Tier 2. Figure 2: 300 W CrCM totem pole PFC diagram and data efficiency With the advent of new controllers in 2021, a bridgeless CrCM high-frequency totem pole appears as a common topology due to lower electromagnetism, as well as simplified voltage and current sensing by the controller. The switching speeds can be increased up to 10×, from the fixed frequency 50 kHz CCM to 200-500 kHz to operate the CrCM totem, and the resulting low capacitance of GaN (COSS) produces an impressive and high efficiency result. High Frequency DC/DC: 6 x Power with Gallium For constant output voltage transformers in the power range from 100 to 3000 watts, the choice of a DC/DC converter is typically a limited liability resonant phase with a 400-VDC input. The 400-V bus can come from the upstream PFC stage inside a jacketed AC/DC SMPS or can be the mains rail in an HVDC installation. LLC’s topology has many advantages including ZVS operation, high efficiency, high power density and ZVS operation makes this converter an ideal platform to increase switching frequency and reduce the size of magnetic elements with high speed power train. At a quarter-brick industry-standard form factor (DOSA), best-in-class silicon-based designs reach 150 watts. By using GaN power integrated circuits and increasing the DC/DC switching frequency 3× from 275 kHz to 830 kHz, the power rating can be increased up to 6× to 1 kW. Written by Tom Rebarich, Director of Strategic Marketing at Navitas Semiconductor Please visit the e-book for the full article.


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