Can the cascode GaN Switch shown be called a "bi-directional GaN" and is it okay to run current in reverse for continuous operation, for example in the PFC Totem pole design with power flowing back to grid ? Is the body diode of the Silicon FET able to handle high continuous currents?
Thanks for the question. We reached out to an engineer who provided the following response - The term “bi-directional”, when applied to a semiconductor switch, normally means the device can block voltage in both directions as well as conduct current in both directions. So no, we would not call the cascode GaN switch “bi-directional” because it only blocks voltage in one direction (Vds>0). However, it can be used exactly as you describe. The current capability in the reverse direction is nominally the same as in the forward direction, limited ultimately by temperature rise. There is a maximum rating on the data sheet, which is call “source current”. From the GAN039-650NBB data sheet, for example: IS source current Tmb = 25 °C; VGS = 0 V 58.5 A As you probably understand, power dissipation during reverse conduction can be reduced by enhancing the MOSFET channel (applying voltage to the gate after an appropriate dead time), which is to say, using the FET as a synchronous rectifier. We hope this answers your question. Have a great day!
Hello! The body diode formed by the parasitic p-n junction in a Si MOSFET provides an alternative path for reverse, or freewheeling current, in cases where the channel is not enhanced. This is also the case for the Si MOSFET In the cascode GaN switch. If positive gate voltage is not applied, and the channel is therefore not enhanced, then current will flow in the body diode. Because this is a 30V MOSFET, the related recovery charge is quite low. (Note that the MOSFET, like any FET, would conduct reverse current even without the body diode. In most technologies, however, the forward voltage of the diode is less than the threshold voltage of the FET, and so reverse current doesn’t flow in the channel until the channel is intentionally enhanced with positive gate voltage.)
~Thanks for the reply, I just checked the message box and saw this, really appreciate it!I got what you mean:the diode acts like an alternative current path when the cascode device is used in reverse mode（which means a positive voltage at the source versus the drain), while the gate voltage is not positive enough to turn the Si MOSFET on. ~I am doing some research on GaN power conversion recently, and I am working on several textbooks about GaN and GaN HEMT.(CRC, Wiley textbook,etc). I can understand the physics of semidonductor&semiconductor devices part, but it's tough to understand the circuit and system part(so many circuit analysis experience and power conversion engneering background). ~For example:1. I don't know when the cascode is used conversely in what kind of circuit system ; 2.I have seen a variation of the cascode, they use a drive circuit to drive the gate of the GaN HEMT directly and the gate electrode of the Si device is called "enable" electrode. I don't see how the enable circuit outcome a traditional cascode...... ~My research is about how to improve HEMT performance through optimizing the isolation process in the device manufacturing process, so it's actually on manufacturing process. However, considering enhance mode GaN-on-Si HEMT mainly targets the power conversion market while the depletion mode device targets the RF market, I believe the research will be more logically if I have more profound understanding on it's application background. If I can find the relationship between the device characteristics and the power conversion system, I will know what process optimization should be made to improve the performance at a system level using some semiconductor physics knowledge. ~I guess I just lack some circuit analysis knowledge and power conversion background knowledge now......@@Nexperia
Hello! Thanks for your question. Depending on your application details, we can point out these 2 lists of 25/30 V devices which are often chosen for computing and/or switching. tinyurl.com/3tpd6kc7 tinyurl.com/3yhyy76a For more information or help please get in touch on nexperia.com
Can the cascode GaN Switch shown be called a "bi-directional GaN" and is it okay to run current in reverse for continuous operation, for example in the PFC Totem pole design with power flowing back to grid ?
Is the body diode of the Silicon FET able to handle high continuous currents?
Thanks for the question. We reached out to an engineer who provided the following response - The term “bi-directional”, when applied to a semiconductor switch, normally means the device can block voltage in both directions as well as conduct current in both directions. So no, we would not call the cascode GaN switch “bi-directional” because it only blocks voltage in one direction (Vds>0). However, it can be used exactly as you describe. The current capability in the reverse direction is nominally the same as in the forward direction, limited ultimately by temperature rise. There is a maximum rating on the data sheet, which is call “source current”. From the GAN039-650NBB data sheet, for example:
IS source current Tmb = 25 °C; VGS = 0 V 58.5 A
As you probably understand, power dissipation during reverse conduction can be reduced by enhancing the MOSFET channel (applying voltage to the gate after an appropriate dead time), which is to say, using the FET as a synchronous rectifier.
We hope this answers your question. Have a great day!
What's the usage for the diode which is parallel with the Si MOSFET in the internal circuit diagram for cascode GaN HEMT?
Hello! The body diode formed by the parasitic p-n junction in a Si MOSFET provides an alternative path for reverse, or freewheeling current, in cases where the channel is not enhanced. This is also the case for the Si MOSFET In the cascode GaN switch. If positive gate voltage is not applied, and the channel is therefore not enhanced, then current will flow in the body diode. Because this is a 30V MOSFET, the related recovery charge is quite low. (Note that the MOSFET, like any FET, would conduct reverse current even without the body diode. In most technologies, however, the forward voltage of the diode is less than the threshold voltage of the FET, and so reverse current doesn’t flow in the channel until the channel is intentionally enhanced with positive gate voltage.)
~Thanks for the reply, I just checked the message box and saw this, really appreciate it!I got what you mean:the diode acts like an alternative current path when the cascode device is used in reverse mode（which means a positive voltage at the source versus the drain), while the gate voltage is not positive enough to turn the Si MOSFET on.
~I am doing some research on GaN power conversion recently, and I am working on several textbooks about GaN and GaN HEMT.(CRC, Wiley textbook,etc). I can understand the physics of semidonductor&semiconductor devices part, but it's tough to understand the circuit and system part(so many circuit analysis experience and power conversion engneering background).
~For example:1. I don't know when the cascode is used conversely in what kind of circuit system ; 2.I have seen a variation of the cascode, they use a drive circuit to drive the gate of the GaN HEMT directly and the gate electrode of the Si device is called "enable" electrode. I don't see how the enable circuit outcome a traditional cascode......
~My research is about how to improve HEMT performance through optimizing the isolation process in the device manufacturing process, so it's actually on manufacturing process. However, considering enhance mode GaN-on-Si HEMT mainly targets the power conversion market while the depletion mode device targets the RF market, I believe the research will be more logically if I have more profound understanding on it's application background. If I can find the relationship between the device characteristics and the power conversion system, I will know what process optimization should be made to improve the performance at a system level using some semiconductor physics knowledge.
~I guess I just lack some circuit analysis knowledge and power conversion background knowledge now......@@Nexperia
What would you recommend for a +12V 2-phase Ćuk-Buck2 VRM for the latest AMD and Intel CPUs?
Hello! Thanks for your question. Depending on your application details, we can point out these 2 lists of 25/30 V devices which are often chosen for computing and/or switching.
tinyurl.com/3tpd6kc7
tinyurl.com/3yhyy76a
For more information or help please get in touch on nexperia.com