In my last blog, Power Electronics: Re-Invented, I spoke about the coming re-invention of power electronics. The trifecta of resonant topologies, fast GaN power devices and mixed-signal GaN power IC integration is making high-frequency and high efficiency not only a technical possibility, but also simple and cost-effective for high-volume commercial designs. Let’s take a closer look at how these exciting developments are occurring.
Resonant Topologies & GaN Power ICs: the LLC converter
An LLC topology is an excellent example of a resonant topology that is gaining popularity in the 100-500W range. When designed correctly with state-of-the-art GaN devices, soft-switching can be maintained over line and load. This topology has the benefit of a front-end power factor correction circuit in this power range, so a stable DC input is provided. The key to achieving and maintaining soft-switching is timing the turn-off when enough current is flowing in the resonant tank to transfer the charge in the switch node capacitance without loss to the load. While the corresponding losses of the output cap are eliminated, an increase in recirculating currents does increase the I2R losses throughout the system. This is the reason that very low and more linear COSS of GaN devices is critical to make LLC systems high efficiency, even at increasing switching frequencies.
While soft-switching topologies, like LLC, automatically achieve zero switching losses at turn-on, this is not easily achieved at turn-off. The turn-off losses can be appreciable if the design experiences any inductance in the gate loop. That inductance can cause ringing on the gate and lead to unintended FET turn-on and excess loss. To mitigate this possibility, damping resistors can be added but then switching is slowed down and switching losses increase. With GaN power ICs, where the driver is fully integrated in to the GaN device, the source inductance is effectively zero and turn-on losses can be virtually eliminated without any additional circuity.
In addition, GaN power ICs can integrate the bootstrap and level-shifter circuits very cost-effectively while reducing the power loss of those circuits at high frequency by 5-10x. Even with the very efficient LLC converter, as frequencies rise, the efficiency with discrete Si or discrete GaN can drop. However, with AllGaNTM integrated drivers, level shifters, bootstrap circuits and even voltage regulators, those frequency dependent losses are reduced to near zero and the sky’s the limit for high-frequency and high efficiency designs.
Resonant Topologies & GaN power ICs: the ACF converter
All of these same opportunities exist with the emergence of the active clamp flyback (ACF) to bring soft-switching and high-frequency, high-efficiency operation to lower power applications where PFC is not required. A single-switch flyback, or quasi-resonant flyback, is well proven as a low-cost, medium-efficiency design workhorse in the industry. The transition to a soft-switching ACF design can enable very high frequencies, with even higher efficiencies, delivering a power density improvement, but what about EMI and the added cost and complexity of an additional high-side switch? That additional switch can be expensive and limit high-frequency efficiencies given the extra circuity needed for level shifting, driving, bootstrap capacitor charging, and possible voltage regulation.
Here again, with GaN power ICs, all those circuits are integrated directly in to the GaN power devices, lowering the cost, size and complexity of those circuits while reducing the power loss vs frequency by up to 10x. While EMI is a natural concern with increased frequencies on a traditional QR flyback, with soft-switching ACF designs, the well-controlled soft edges of the waveforms, with no overshoot and no ringing, translate to higher-frequency EMI performance that is substantially better than a low-frequency QR design. There are now proven filtering techniques that easily & effectively mitigate lower frequency EMI and keep radiated noise will below the EMI limits, even when pushing ACF frequencies into the 300kHz to 1MHz range.
Taking all these advances together, we see a broad range of ACF and LLC designs that exploit new GaN power ICs to improve efficiencies to 95% or higher, reduce EMI filters while easily passing EMI certification limits, and enable power densities in the range of 15-25 W per in3, which is 2-3x greater than the benchmarks today.
Look for my next blog where we explore advances in magnetics that are contributing to these efficiency and density gains using high frequency GaN power ICs. Up, up and away!!