WEBINAR: GaN Performance Advantage in Totem Pole PFC and LLC Converters

The webinar compares GaN E-HEMT with Silicon and SiC MOSFETs in a Power Supply Unit (PSU) with Bridgeless Totem Pole PFC and LLC resonant converter topologies. The presentation concludes that GaN E-HEMT solutions provide higher efficiency than SiC and 40% higher power density than the conventional Si-based PSU design.

With Dr. Jimmy Liu, Technical Marketing Director at GaN Systems

In this webinar, you’ll learn:

  • The key transistor parameter advantages for GaN compared to Si and SiC
  • GaN-based Power Supply Unit (PSU) efficiency and power density benefits
  • System design considerations with GaN-based high switching frequency (CCM Totem Pole PFC at 200KHz and LLC at 500KHz)

Questions & Answers

For GaN, it can operate higher switching frequency than 500KHz for LLC DC/DC converter. However, we should also consider other components. A more careful selection of magnetic components is required, such as the resonant inductor and transformer. In these cases, we have customers switching at 600kHz, 800kHz and up to 2MHz, 6.78MHz in wireless power systems, and up to 40MHz in plasma laser drive systems.

GaN Systems has 300W PFC+LLC design. For 20W-40W LLC, we have designs with our GS-065-004-1-L and GS-065-008-1-L devices and would be happy to discuss with you.

For consumer electronics, there are several examples of GaN implementation including: AC adapters, audio products, wireless power products, and air-conditioners. For AC adapters, GaN allows for adapters to go higher in power and be 3-4X smaller in size. For audio products, the fast switching of GaN produces the highest quality sound and highest efficiency amplifier. For air conditioners, efficiency is critical, GaN helps meet the green energy requirements. And lastly, with GaN at high frequency, many advantages become available in wireless power including high power levels, charging at a distance, multiple device charging and many more. GaN is applicable from 20W to 250kW, from consumer to industrial to automotive.

Yes, 650V GaN can be suitable for 3-phase AC/DC, it can be used in a 6-pack topology or other 3-level topologies. The advantage of the single phase GaN based Totem Pole PFC can be applied to other 3phase AC/DC converter too.

For GaN, it can operate higher switching frequency than 500KHz for LLC DC/DC converter. However, we should also consider other components. A more careful selection of magnetic components is required, such as the resonant inductor and transformer. In these cases, we have customers switching at 600kHz, 800kHz and up to 2MHz, 6.78MHz in wireless power systems, and up to 40MHz in plasma laser drive systems.

We consistently see customers reduce system costs. Examples include: removing the cooling fan, removing heatsink or reducing its size by >50%, converting from water-cooled to air-cooled, power passives reduce in size and see 30-40% cost reduction, mechanicals reduce by 20-30%, cable/harness lengths are shorter. These are some of the ways our customers realize lower cost systems. There are also systems where saving energy results in significant ownership cost such as in Energy Storage Systems and Data Center.

In general, high switching frequency will lead to low cost or size for passive component (inductor and transformer). This can reduce systems costs, usually higher savings at higher power levels. Meanwhile, besides the BOM, we should consider OPEX or CAPEX too. For example, the high efficiency data center power supply, the OPEX is dramatically reduced with less electricity consuming and more space for data processing server on the rack.

The perception that GaN prices are significantly higher has become less true today and continues to even more true as time passes. The prices for GaN today, in most high power applications, result in system performance, efficiency and power density much better than silicon with system savings costs. As we are now in the growth phase for GaN, we will continue to see the difference between GaN and silicon to continually reduce. If you have a project, with high volume, you should request a quote from us. You will see that a strong business case can be built with the use of GaN.

At this power level, with high frequency, the PFC’s inductor and EMI filter can be reduced. So there is some cost reduction on the passive component to offset the GaN. Additionally, heatsink size is reduced as is other mechanicals and the case. More importantly, besides the BOM, in the case o a data center as an example, we should consider OPEX or CAPEX too. Here, the OPEX is dramatically reduced with less electricity consuming and more space for data processing server on the rack.

We are comparing the most latest Si SJ MOSFET and SiC MOSFET in the industry. They are all the best transistor in the relative semiconductor material catalog. And the typical Rdson is almost the same with GS66508B, but different other parameters, such as Qoss, Co(tr), Qg and Qrr. As is general practice in the industry, specifics are not publicly divulged.

The Rdson for the comparison is at 25C temperature. The GaN transistor has a little higher temperature coefficient compared to the SiC MOSFET. Our comparison is based on 100C temperature for real world implementation, So you can see the conduction loss for GaN is slightly higher than the SiC MOSFET. It is a fair comparison. Nevertheless, GaN still has much lower total loss than SiC when accounting for all types of loss (switching loss, gate drive loss, Qrr loss etc…)

In order to present all of these loss models we have to use simulation. However, the simulation model is proven and compared to a working system, where we validate that the model results for the same losses as our real test system. The losses are based on calculation from all components. We use very detailed and accurate simulation tools to compare the loss for transistors and other components.

Please take a look at this paper. Also, Please check slide Page 24 of this webinar, the minimum dead time equation for ZVS condition is very important which influence the design of efficiency and frequency for LLC. GaN transistors have the lowest vales on Co(tr) and Qoss, which can increase the switching frequency with ZVS condition and high efficiency.

The output voltage is 48V

We show peak load efficiency on the summary page but GaN is efficient over the entire curve. Because GaN has very low switching losses, at light load and half load, the efficiency of GaN is much higher than other MOSFET technology. The key values for GaN are shown in the presentations with low switching loss.

GaN at 100KHz has better efficiency than 500KHz GaN based, due to lower switching and gate drive loss. However, GaN is the best technology for trade-off on efficiency and power density. So GaN has more value at high frequency LLC converter other than conventional 100KHz frequency LLC.

The comparison shown for PFC is based on 220Vac input. However, in other cases such as in adapters and lower power AC/DC power supplies, we’ve shown GaN efficiency is superior at low line as well.

A variety of voltage levels are on the roadmap. We have demonstrated 1200V capability in R&D. Rollout of 1200V and other voltage levels as well as other current levels and package options all depend on many market variables. When appropriate, announcements will be made.

A variety of voltage levels are on the roadmap. We have demonstrated 1200V capability in R&D. Rollout of 1200V and other voltage levels as well as other current levels and package options all depend on many market variables. When appropriate, announcements will be made.

The eMobility trends are creating innovation in all vehicle types including Mild Hybrid, HEV and BEV. The need for more range, lightweighting, battery-shrinking, etc demand the use of new designs and topologies. We’re seeing the implementation of GaN into this market growing every year and look forward to production rollouts starting in 2020 and growing significantly over the next few years.

The MOSFETs, either Si or SiC, have an intrinsic PiN body diode, which will result in minority carrier recombination on the drift region during body diode turn-off. In hard-switching applications such as BTP-PFC, the reverse recovery effect of the body diode greatly increases the switching loss on the diode, as well as in the complementary transistor, thus there is a short period shoot-through between the body diode and the complementary transistor, and this shoot-through will result in a large high di/dt on the bridge topology, which will bring high frequency noise. You can refer to this paper: S. Walder; X. Yuan; N. Oswald, “EMI generation characteristics of SiC diodes: Influence of reverse recovery characteristics”, 7th IET International Conference on Power Electronics, Machines and Drives (PEMD 2014). On the other hand, the absence of reverse-recovery behavior for the GaN transistor is expected to reduce EMI generation caused by high switching di/dt, especially for radiated noise, refer to GaN Systems application note GN001

GaN has no intrinsic body diode. The GaN E-HEMT is turned on when the electrical potential is formed from source to drain and gate voltage exceeds the threshold voltage. As the presentation mentioned, during the dead time, the GaN will operate at 3rd quadrant operation with current conducting from source to drain with electrical potential formed from source to drain. Please review the page 6 for GaN structure. You may also refer to our paper, titled, “Common Misconceptions About the MOSFET Body Diode.

We have a detail application note and design tips for gate drive design of GaN Systems’ GaN transistor. Meanwhile, there is a webinar to discuss the gate drive design and PCB layout. For the package thermal, there are several ways, for examples, Insulated Metal Substrate (IMS) is a proven method. You can also visit our website to understand the variety of demo boards we have to offer which show the different methods.

PLECS is a simulation tool. On the GaN Systems website, you can use the online free simulation for most of popular topologies, such as Totem pole PFC and LLC. Please visit our Circuit Simulation Tools.

We had 3KW and 1.5KW reference design for Totem Pole PFC in our website, if you need higher current capability PFC, Please contact with our sales representative. You can provide more detail requirement, then we can further discuss your request. Meanwhile, in our website, we have Bridgeless Totem Pole PFC simulation tools for loss and thermal, you can use it to select your transistor based on your power.

Common mode current issue are happening all fast switching transistor, this is not only GaN device, but also Si or SiC MOSFET. There are several ways to avoid these issues:

  1. Use high CMTI gate drive IC, GaN Systems has a recommended gate drive list in the application note.
  2. For GaN Systems GaN transistor, we have separated the gate source and power source with Kelvin connections, so it can minimize ringing or spike on the gate due to the power source parasitic inductance with high di/dt.
  3. Minimize the gate drive loop layout, and use negative voltage for turn-off, this can avoid parasitic re-turn on due to the high dv/dt. For more detail, please visit our website to get the application note on the gate drive design for GaN transistor.
  4. The reverse recovery of Silicon and SiC MOSFETs often result in high frequency ringing with the highest dv/dt. With GaN Systems’ GaN having zero Qrr, our EMI and noise impact is easier to manage”
GaN Webinar Playback - GaN Performance Advantage in Totem Pole PFC and LLC Converters

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Speaker: Dr. Jimmy Liu

Technical Marketing Director at GaN Systems

He has more than 20 years of experience on the technique and system application of power semiconductors, especially for GaN and SiC devices. He holds a PhD. in power electronics.

Moderator: Paul Wiener

VP Strategic Marketing at GaN Systems

Paul Wiener is GaN Systems’ Vice President of Strategic Marketing. Prior to joining GaN Systems, Paul led the power magnetics business unit at Eaton. Paul brings more than 25 years’ experience in operations, sales and marketing, and business development. His experience includes vice president of sales at Fultec Semiconductor Inc. and several management roles at Genoa, BroadLogic, and Raychem.

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