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HEMT (GaN) Forum Discussions

PowerStarter
Level 2
First like received 25 sign-ins 10 replies posted
Level 2

Hi guys,

I'm currently learning about the LLC converter. But a few questions popped up.

Q1: Uncertainties about the body diode Primary Side
Some sources state, that the LLC to be able to gurantee ZVS operation an intrinsic body diode is necessary.
Resonant inductor current must be high enough to discharge the voltage of effective capacitance (Coss) appearing in parallel with drain-sources of the power MOSFETs. Additionally the converter must work in the inductive region to achieve ZVS.
When the FET switches during body diode conduction (leading to zero), a ZVS transition happens.
Incomplete body diode reverse recovery can damage the power FET due to large current caused by the high reverse recovery charge. This happens mainly during startup. This can be prevented by using the FETs with fast body diode because its low reverse recovery charge as well as the short recovery time. There are multiple approaches to get rid of this problem. [1]

Some LLC converter designs use GaN HEMTs, which have low equivalent capacitance, which shortens the charging/discharging time Co(tr). But it they do not have an intrinsic body diode and therefore zero reverse recovery charge.

So does the body diode not affect the ZVS behavior and GaN can be used?

Q2: Uncertainties about the body diode Secondary Side
When GaN is used, it is mostly used only on the primary side, and not on the secondary side.
In one Abstract is written that "[...] reverse recovery and junction capacitance (C j ) of secondary-side diode critically affect the ZVS condition of primary-side switches." [2]

Is this maybe the main reason why mostly Si or SiC is used?

Can't one just use GaN HEMTs here as well?

Additionally on the secondary side are often multiple FETs used in parallel. Is this only to split the current?


[1] Primary Side MOSFET Selection for LLC Topology (infineon)
[2] Analysis of Diode Reverse Recovery Effect on ZVS Condition for GaN-Based LLC Resonant Converter

1 Solution
Abhilash_P
Moderator
Moderator 25 likes received 250 replies posted 250 sign-ins
Moderator

Hi,

 

    Thank you for posting on the Infineon Community. 

 

Q1: So does the body diode not affect the ZVS behavior and GaN can be used?

Ans: As per the shared application note, body diode does affect the ZVS behavior during start up. Please refer the Application note for more details. 
The shared application note by you does give the possible solutions for incomplete reverse recovery of the body diode. 

 

Q2a: Is this maybe the main reason why mostly Si or SiC is used? Can't one just use GaN HEMTs here as well?

Ans:  SR MOSFETs offer extra advantages such as the ability to operate in parallel to prevent overheating
in high-power applications. MOSFETs RDS(ON) have a positive temperature coefficient. When operating in
parallel, resistance increases with the temperature rise, shifting power to the cooler device. By contrast, diode
forward voltage drop has a negative temperature coefficient. Increasing temperature results in increased current flow in the hotter device, and increases the chance of overheating. It is worth noting that GaN have a positive temperature coefficient, however, the price is still too high in the market and most of them focus on high voltage (≥ 600 V) applications. 

Q2b: Additionally on the secondary side are often multiple FETs used in parallel. Is this only to split the current?

Ans: Yes, your understanding is correct. In higher power level applications, a number of MOSFETs can be paralleled to achieve lower on-state resistance and reduce conduction loss. Also, the RDS(ON) has a positive temperature coefficient so the FETs will automatically share current, facilitating optimal thermal distribution among the SR devices. This improves the thermal management.

But when MOSFETs are used in parallel, a current imbalance is caused during switching transitions by a mismatch between each device characteristics.
This current imbalance could increase power losses and lead to destruction of a MOSFET. More care should be taken when designing MOSFETs in parallel. For preventing the current imbalance please take care of the following,

Preventing this current imbalance.

  1. Make symmetrical layout.
  2. Assemble MOSFETs to one heatsink so that the thermal dissipation condition is the same for each MOSFET.
  3. Use an external gate resistor for each MOSFET.

 

 

Regards,
Abhilash P

 

View solution in original post

3 Replies
Abhilash_P
Moderator
Moderator 25 likes received 250 replies posted 250 sign-ins
Moderator

Hi,

 

    Thank you for posting on the Infineon Community. 

 

Q1: So does the body diode not affect the ZVS behavior and GaN can be used?

Ans: As per the shared application note, body diode does affect the ZVS behavior during start up. Please refer the Application note for more details. 
The shared application note by you does give the possible solutions for incomplete reverse recovery of the body diode. 

 

Q2a: Is this maybe the main reason why mostly Si or SiC is used? Can't one just use GaN HEMTs here as well?

Ans:  SR MOSFETs offer extra advantages such as the ability to operate in parallel to prevent overheating
in high-power applications. MOSFETs RDS(ON) have a positive temperature coefficient. When operating in
parallel, resistance increases with the temperature rise, shifting power to the cooler device. By contrast, diode
forward voltage drop has a negative temperature coefficient. Increasing temperature results in increased current flow in the hotter device, and increases the chance of overheating. It is worth noting that GaN have a positive temperature coefficient, however, the price is still too high in the market and most of them focus on high voltage (≥ 600 V) applications. 

Q2b: Additionally on the secondary side are often multiple FETs used in parallel. Is this only to split the current?

Ans: Yes, your understanding is correct. In higher power level applications, a number of MOSFETs can be paralleled to achieve lower on-state resistance and reduce conduction loss. Also, the RDS(ON) has a positive temperature coefficient so the FETs will automatically share current, facilitating optimal thermal distribution among the SR devices. This improves the thermal management.

But when MOSFETs are used in parallel, a current imbalance is caused during switching transitions by a mismatch between each device characteristics.
This current imbalance could increase power losses and lead to destruction of a MOSFET. More care should be taken when designing MOSFETs in parallel. For preventing the current imbalance please take care of the following,

Preventing this current imbalance.

  1. Make symmetrical layout.
  2. Assemble MOSFETs to one heatsink so that the thermal dissipation condition is the same for each MOSFET.
  3. Use an external gate resistor for each MOSFET.

 

 

Regards,
Abhilash P

 

PowerStarter
Level 2
First like received 25 sign-ins 10 replies posted
Level 2

@Abhilash_P wrote:

Q1: So does the body diode not affect the ZVS behavior and GaN can be used?

Ans: As per the shared application note, body diode does affect the ZVS behavior during start up. Please refer the Application note for more details. 
The shared application note by you does give the possible solutions for incomplete reverse recovery of the body diode. 


Thanks for the response. But I am still confused about the body diode and the usage of GaN HEMTs.  Due to the absence of the intrinsic body diode and the reverse recovery current.

For my understanding the body diode is crucial for the LLC converter to work in ZVS.
As soon as the Coss is charged/discharged (causing a resonant transition and drain-source voltage is Vin - enough magnetizing current is needed) the body diode conducts. Due to the discharging of one e.g. Coss1 to almost zero and Coss2 to Vin one requirement for ZVS for Q1 is fulfilled. Therefore a low equivalent capacitance, which shortens the charging/discharging time Co(tr), is desired. To further bring down the voltage between the drain and source the body diode is needed. The conducting body diode is responsible for bringing the voltage down to zero, so the other switch can get turned on with ZVS.

GaN HEMTs have relatively higher switching-on loss compared with switching-off loss. Therefore it would be practical to use ZVS for turn on.
They don't have a body diode effect. In the reverse conduction mode, the drain of GaN HEMT will behave as the source and the source will act as the drain.

So I do not fully get why GaN HEMTs can here being used while keeping the ZVS.

Best regards

0 Likes
Abhilash_P
Moderator
Moderator 25 likes received 250 replies posted 250 sign-ins
Moderator

Hi,

 

   Yes, clearly as explained by you, during reverse conduction the drain becomes source and source becomes drain. Hence no additional power loss during reverse conduction. 

Also please go through the following IEEE papers for better understanding, 

Paper 1

Paper 2

Paper 3

 

Regards,
Abhilash P