在线研讨会: 图腾柱PFC和LLC转换器中的GaN性能优势
该网络研讨会将GaN E-HEMT与具有无桥图腾柱PFC和LLC谐振转换器拓扑的电源单元（PSU）中的硅和SiC MOSFET进行了比较。 该演讲得出的结论是，GaN E-HEMT解决方案比SiC具有更高的效率，并且功率密度比传统的基于Si的PSU设计高40％。
在此网络研讨会中，您将学到
- 与Si和SiC相比，GaN的关键晶体管参数优势
- GaN基电源单元（PSU）的效率和功率密度优势
- 基于GaN的高开关频率（200KHz的CCM图腾柱PFC和500KHz的LLC）的系统设计注意事项
问题与解答
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Can you go higher than 500KHZ for DC-DC conversion for 3KW power supply?
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.
Do you have any example LLC converter designs for low power (20-40W)?
氮化镓系统 (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 applications, are there any other comments you have on GaN advantages?
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.
Can the GaN transistors do a 3-phase AC/DC converter topology?
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.
Have you tried to go higher that 500KHZ?
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.
Please give examples how GaN can save cost in a project, considering that GaN is usually more expensive than Si and SiC.
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.
How does the total BOM compares for typical 100W and 3kW power supplies?
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.
Prices are still significantly higher. When should we expect prices to drop comparable to SI?
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.
What are the BOM cost associated with the 1.5kW PFC analysis? Does GaN justify the higher cost?
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.
What are the models of SiC and Si you are comparing the GS66508B against?
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.
Why does the 50mOhm GaN device has a higher conduction loss than the 55mOhm SiC device?
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…)
Are efficiency and loss data based on actual test results or calculated?
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.
Can you explain ZVS in LLC again?
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.
Slide 32: What is the output voltage of this 2x 1.5 kW PSU?
The output voltage is 48V
What about the efficiency over the whole power range?
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.
What is the performance increase on LLC from 100 to 500khz with GaN?
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.
Did you do efficiency comparison for PFC at low line eg 100Vac?
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.
Do you see higher blocking voltage with GaN (1.2kV , 1.7kV etc..)?
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.
Do you already have 1200V version available in the market?
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.
EHV, Perception of Market Growth? especially china situation?
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.
How does Low Qrr translate to less EMI?
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 氮化镓系统 (GaN Systems) application note GN001
GaN does not have anti-parallel diode. How it can conduct reverse current during deadtime?
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.”
What is the challenges of gate driver for GaN? and how to take the heat out of the small package?
We have a detail application note and design tips for gate drive design of 氮化镓系统 (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.
What is PLEXIm software?
PLECS is a simulation tool. On the 氮化镓系统 (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.
You have PFC evaluation boards with GaN FETs that have high current capability; but still the output power of these evaluators is quite limited. What is the reason behind that?
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.
GaN device can work at high frequency and have high dV/dt. High dV/dt can lead to common mode current in gate-driver circuit that can circulate to control circuit. Does this common mode current cause noise or EMI issue in the gate driver circuit? Is there any way to reduce this common mode current?
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:
- Use high CMTI gate drive IC, 氮化镓系统 (GaN Systems) has a recommended gate drive list in the application note.
- For 氮化镓系统 (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.
- 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.
- The reverse recovery of Silicon and SiC MOSFETs often result in high frequency ringing with the highest dv/dt. With 氮化镓系统 (GaN Systems)’ GaN having zero Qrr, our EMI and noise impact is easier to manage”
