GaN Transistor Gate Drive Optocouplers
This article features Broadcom's Gallium Nitride (GaN) Transistor Gate Drive Optocouplers and discusses its advantages, marketing, drive and protection.
Gallium Nitride (GaN) power semiconductors are rapidly emerging into the commercial market delivering several benefits over conventional Silicon-based power semiconductors. GaN can improve overall system efficiency and the higher switching capability can reduce the overall system size and costs. The technical benefits coupled with lower costs have increased the fast adoption of GaN power semiconductors in applications like industrial power supplies and renewable energy inverters.
Broadcom Limited (formerly Avago Technologies) gate drive optocouplers are used extensively in driving Silicon-based semiconductors like IGBT and Power MOSFETs. Optocouplers are used to provide reinforced galvanic insulation between the control circuits and the high voltages. The ability to reject high common-mode noise will prevent erroneous driving of the power semiconductors during high-frequency switching. This paper will discuss how the next generation of gate drive optocouplers can be used to protect and drive GaN devices.
Advantages of GaN
Gallium Nitride is a wide bandgap (3.4 eV) compound made up of Gallium and Nitrogen. The bandgap is a region formed at the junction of materials where no electron exists. Wide bandgap GaN has a high breakdown voltage and low conduction resistance characteristics. Unlike conventional Si transistor that requires bigger chip area to reduce ON-resistance, GaN device is smaller in size. This reduces the parasitic capacitance which allows high-speed switching and miniaturization with ease. The low conduction resistance is achieved because the on-resistance of the power semiconductor is inversely proportional to the cube of the electrical breakdown. In other words, it is expected that the GaN device will have an on-resistance approximately 3 digits lower than the limit of that of the Si device. In addition, GaN device has high electron saturation velocity that makes it suitable for high-speed applications.
Figure 1. Silicon vs. GaN transistor structure and size
Power semiconductor is the key device and works on a tremendous amount of power during electrical energy conversion. It is therefore important to optimize the efficiency of this device to minimize energy loss during their operation. GaN is the next generation power semiconductor able to minimize power loss with the following characteristics: miniaturization, high breakdown voltage, and high-speed switching.
Most of the GaN devices are however normally on which means the source and drain are conducting when no voltage is applied at the gate. To stop the conduction, a negative voltage must be used to reverse the conduction channel. A normally on transistor poses danger to the system if the gate is not controlled properly and silicon transistor, which is normally OFF, is more suitable for hazardous high voltage applications.
To speed up GaN adoption, Panasonic‘s X-GaNTM developed a normally-OFF structure by using the P-type GaN gate and diffuse AlGaN channel under the gate. At the same time, the P-type GaN adds holes near the drain which recombines with the electrons at high voltage. This method solves the current collapse problem whereby electrons trapped near the channel during high voltage increases the transistor on-resistance. If the increase of on-resistance is not controlled, the GaN device will overheat and destroy over time. Panasonic GaN transistors are capable of no current collapse for up to 850V.
Figure 2. Panasonic X-GaNTM transistor structure
Panasonic has done a concept demo of the world’s most compact 400W power supply. The power conversion stages, PFC and LLC operate at 100 kHz and 280 kHz respectively. The high frequencies reduce the cost and size of the power supply by more than 30%. The miniaturized power supply is measured 11.2cm x 4.95cm x 3.95cm and with an effective power density of 1.83W/cm3. It also achieved a high conversion efficiency of 94% with the low switching and conduction losses.
Figure 3. Panasonic world’s most compact 400W power supply with 94% conversion efficiency
GaN Market and Adoption
GaN technology is now widely recognized as a reliable alternative to silicon. Recent financial investments into GaN startups like GaN Systems and Transphorm and corporate partnership between Infineon and Panasonic indicate market confidence in GaN devices. GaN has a huge Total Accessible Market (TAM) like PF, EV/HEV, and PV inverter, which is one of the earliest adopters of GaN.
In 2014, Yaskawa Electric Corp launched the world’s first PV inverter using a GaN-based power semiconductor. The PV inverter has the ability to operate without cooling fans. It covers 60% of the volume of competing devices and with an overall peak efficiency above 98%.
Broadcom gate drive optocouplers have been used extensively in driving Silicon-based semiconductors like IGBT.
GaN Transistor and Gate Drive Optocouplers
Broadcom has been working closely with GaN market leader Panasonic, to determine suitable gate driver for GaN operation. We have evaluated gate drive optocoupler ACPL-352J with Panasonic GaN transistor, PGA26E19BA using a 100-150V, 5A chopper board at 100 kHz.
The ACPL-352J is a smart gate drive optocoupler with a high output current of 5A. The high peak output current, together with wide operating voltage makes it ideal for driving GaN transistor directly. The device features fast propagation delay of 100 ns with excellent timing skew performance and has very high common-mode transient immunity (CMTI) of more than 50kV/μs. It can provide GaN with overcurrent protection and fail-safe functional safety reporting. This full-featured gate drive optocoupler comes in a compact, surface-mountable SO-16 package. It provides the reinforced insulation certified by safety regulatory IEC/EN/DIN, UL, and CSA.
The PGA26E19BA is a 600V, 10A GaN enhancement-mode transistor. It uses Panasonic’s proprietary Gate Injection Transistor (GIT) technology to achieve normally OFF operation with single GaN device. This extremely high switching speed GaN is capable of no current collapse for up to 850V and has zero recovery loss characteristics.
Figure 4. 100-150V, 5A chopper board with ACPL-352J and PGA26E19BV
Driving GaN Transistor
Figure 5. ACPL-352J driving circuit for GaN transistor
Figure 5 shows the ACPL-352J gate drive outputs, VOUTP/MClamp, and VOUTN and external resistors and a capacitor for switching the GaN transistor. The full chopper board schematic can be found in figure 11.
Figure 6. GaN transistor gate current and voltage switching waveform
The initial in-rush charging current to turn ON the GaN quickly is provided by ACPL-352J VOUTP and the peak current limited by R9. The C16 is used to turn ON the GaN faster by increasing the charging current momentarily. The required IG_CHARGE can be calculated by the GaN’s Qgd and turn ON time Δt, for example, 10ns.
IG_CHARGE = Qgd / Δt = 4.5nC / 10ns = 450mA (1)
The value of R9 can then be calculated by the gate drive supply, VCC, GaN gate plateau voltage, Vplateau and IG_CHARGE :
R9 = (VCC – Vplateau) / IG_CHARGE = (24V- 2.9V) / 450mA = 46Ω (2)
51Ω is selected for R9.
Figure 7. GaN transistor VGS vs. Qg characteristic
The “speed-up” capacitor, C16 can be calculated using the Qg characteristic graph which shows the gate charge needed to turn on the GaN is 4.5nC.
C16 > Qg / (VCC-VGS-∆V(neg)) = 4.5nC / (24V – 3.6V–5V) = 292pF (3)
A higher C16, 1nF is chosen to ensure more accumulation charge for faster turn ON.
The GIT GaN transistor would require 4.75mA ON-state current to continuously bias the VGS diode at 3.6V to maintain the transistor in ON-state. This is provided VOUTP and the value of R15 can be calculated:
R15 = (VCC – VGSF) / IG_ONSTATE = (24V- 3.6V) / 4.75mA = 4.3kΩ (4)
4.3kΩ is selected for R15.
Switching OFF or discharging the gate of the GaN is done by ACPL-352J’s VOUTN and R10. The ACPL-352J is connected to a bi-directional power supply and the gate is discharged through VOUTN to -9V. At the same time, the active Miller clamp (Mclamp) will turn ON when the gate discharge to -7V. GaN transistor has very low typical gate threshold voltage of 1.2V. The negative gate voltage and active Miller clamp help to hold the transistor in OFF-state and shunt parasitic Miller current to prevent false turn ON. The peak discharging gate current can be calculated:
IG_DISCHARGE = (VGSF – VEE2) / R10 = (3.6V- (- 9V)) / 27Ω = 0.467A (5)
Assuming R10 of 27Ω is used.
Protecting GaN Transistor
Figure 8. ACPL-352J overcurrent protection circuit for GaN transistor
The drain-source voltage of the GaN is monitored by ACPL-352J’s OC pin through high voltage blocking diode D2. The chopper is designed to operate at 5A and overcurrent threshold is set at 7A. When overcurrent occurs, the VDS of the GaN increases to about 0.8V. The ACPL-352J has an internal over-current threshold voltage, VOC of 9V. The threshold of the overcurrent detection can then be set by Zener diode, Z2.
Z2 = VOC – VD2 – VDS_OVERCURRENT = 9 – 0.7 – 0.8 = 7.5V (6)
During over current, if the GaN is shut down abruptly, high overshoot voltage induced by the load or any parasitic inductance can develop across the drain and source of the GaN. The overshoot will damage the GaN if it exceeds the breakdown voltage. To minimize such damaging overshoot voltage, ACPL-352J’s pin 13, SS does a soft shutdown when overcurrent is detected. GaN gate voltage is slowly reduced to low-level OFF-state. The rate of soft shutdown can be adjusted through external transistor Q1 and resistor R8 to reduce the overshoot voltage.
The entire overcurrent protection is completed by reporting the /Fault through the isolated feedback path to the controller. Beside overcurrent fault, the ACPL-352J also reports under high side under voltage lockout fault (/UVLO) and GaN gate status fault (/Gfault).
Figure 9. ACPL-352J functional safety fault reporting
Chopper Board Switching Performance
The chopper board is designed to switch the GaN transistor at 100 kHz with DC bus voltage from 100-150V. The GaN nominal working drain current is 5A and overcurrent threshold is set at 7A. Figure 10 shows the switching waveforms of the GaN VGS, VDS, and IDS. As the chopper board is not connected to any load to dissipate the energy, IDS increases on the very switching pulse and eventually triggers the ACPL-352J’s VOC, overcurrent detection threshold. The waveform on the right zooms into the soft shutdown process once overcurrent is detected.
Figure 10. Chopper board switching performance, overcurrent detection and soft shutdown
Other Design Considerations
Figure 11. Chopper board schematic
The ACPL-352J is powered by a RECOM Econoline DC/DC converter REC3.5-0512DRW. It is a 3.5W regulated converter and provides up to 10kVDC of reinforced isolation. The 24V dual output is split by a 15V Zener diode Z1 to provide bi-directional gate voltage of +15 for turning on and -9V for turning OFF.
Active clamping is provided by TVS diode TVS2, D5 and R14 to clamp the VDS of the GaN from exceeding 300V. The 15V Bi-directional TVS diode TVS1 is used to protect the gate of the GaN transistor. Schottky diode D3 is used to clamp negative transient at ACPL-352J’s VOC to prevent any false fault triggering.
About the Authors
Robinson Law works as the Applications Engineer at Broadcom Limited. He has been responsible for the application support for Broadcom's Optocoupler products in industrial design-in activities since he joined the company in 2001. He also takes care of the product marketing for Hall-effect current sensors. He graduated from University of Malaya in 1984 holding a Bachelor’s degree in Electrical Engineering.
Tee Chun Keong started his career with Avago Technologies in 2006 as an application engineer in the Isolation Products Division. He is now the Broadcom Inc. worldwide product manager for gate drive optocouplers and is involved in the new product development and market expansion. He holds a bachelor's degree in electrical engineering from Singapore's Nanyang Technological University.
References
- “GaN Power Devices Overview,” Panasonic Semiconductor http://www.semicon.panasonic.co.jp/en/products/powerics/ganpower/
- “All-In-One Power Supply,” Panasonic Semiconductor
- Dr. Hong Lin, Dr. Pierric Gueguen, “GaN & SiC Devices. GaN & SiC for power electronics applications report,” YOLE Développement, July, 2015.
- “PGA26E19BA Datasheet,” Panasonic Semiconductor
- “ ACPL-352J 5.0 Amp Output Current IGBT and SiC/GaN MOSFET Gate Drive Optocoupler with Integrated Over Current Sensing, FAULT, GATE, and UVLO Status Feedback,” Avago Technologies, pub-005603.
This article originally appeared in the Bodo’s Power Systems magazine.
