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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
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
