SIC Mosfet Gate negative ringing during ZVS switching in LLC converter. (Best way to reduce gate-Source negative spike and ringing in power MOSFET)

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dattsam
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During ZVS turn on in LLC, the body diode of Mosfet(IMW120R045M1) start conducting before gate pulse is being applied, during that instant a high negative Vgs gate ringing appears in Mosfet is observed which exceeds the dynamic limit of the Vgs recommended.

Few actions are taken to decrease the gate ringing

— Started tuning the drain to source snubber across the Mosfet- No improvement observed

— Added gate damper across the Dc link—No improvement is observed

— started tuning the external gate resistor to damp the oscillations in Vgs - small improvement observed

— started tuning the external gate resistor and external capacitor across the vgs - better improvement than above combine all.

But the negative peak voltage is exceeding the limit of the Vgs dynamic limit. Can any one help in the best way to reduce gate-Source negative spike and ringing in power MOSFET.

 

 

 

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1 Solution
vigneshkumar
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Hi Sampath,

A possible cause of the VGS ringing is stray inductance at the source.

How reliable, does the Infineon mosfets on gate oxide and internal mosfet characteristics in this situation on Long time reliability?

Transient voltage should not exceed the Max Vgs transient given in datasheet. We cannot guarantee the operation where Vgs transient exceeds the Max value given.  Nagative max transient for IMW120R045M1 is  -10V 

The equivalent capacitance across a TVS diode will have any impact on Mosfet gate switching?

Since it will be far less than Cgs, the corresponding capacitance across a TVS diode won't have any effect.

The current selection of TVS diode are done as per Breakdown voltage rating to guarantee the turn on before it crosses the dynamic limit values-Right/wrong?

A TVS or Zener diode addition will also help reduce the negative ringing.

There is no spike found above the Vgs max, during the negative ringing it will be in forward bias, peak current will be based on Rg and Vgs negavtive. You can choos the diode based on the peak current.

The gate ringing is happening during the inbuilt body of mosfet is conducting in reverse current direction ZVS condition-what making the gate terminal to see this type of ringing even though cds and cgd capacitors of mosfets are discharge

Stray inductance at the source may be the cause of the VGS ringing.

You can avoid this by making the source path shorter and wider. in the layout.

Already Rgs has been changed . you can try tuning Rg, this may increase the switching loos it may help you in damping the ringing.

A TVS or Zener diode addition will also help reduce the negative ringing.

View solution in original post

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vigneshkumar
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25 likes received 250 sign-ins 50 solutions authored

Hi 

Thanks for posted on Infineon Community,

Did you add resistor across gate and source ? 

is that possible to share schematic?

Regards

Vignesh kumar

 

 

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Hi vignesh,

I tried with keeping 2Kohm resistor across mosfet Gate to source and also 150pf to 1nf capacitor across the gate to source terminal.

This time i am planning to work with TVS diode across the Mosfet Vgs terminal. Any suggestion or any other improved solution looking.

How to select  while designing  the gate protection from ringing with TVS diode one can suggest.

dattsam_0-1678183049412.png

 

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dattsam
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I has few question ...

  1. In our testing, the gate ringing is going below the dynamic limit of datasheet in the span of less than 5 to 20us with 0.5 to 1volt margin . How reliable, does the Infineon mosfets on gate oxide and internal mosfet characteristics in this situation on Long time reliability?
  2. As discussed above post, if TVS diode are used to limit the peak negative voltage-The equivalent capacitance across a TVS diode will have any impact on Mosfet gate switching?
  3. The current selection of TVS diode are done as per Breakdown voltage rating to guarantee the turn on before it crosses the dynamic limit values-Right/wrong?
  4. The gate ringing is happening during the inbuilt body of mosfet is conducting in reverse current direction ZVS condition-what making the gate terminal to see this type of ringing even though cds and cgd capacitors of mosfets are discharged (please see the ringing highlighted in red box in below image)dattsam_1-1678353375055.png
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vigneshkumar
Moderator
Moderator
Moderator
25 likes received 250 sign-ins 50 solutions authored

Hi Sampath,

A possible cause of the VGS ringing is stray inductance at the source.

How reliable, does the Infineon mosfets on gate oxide and internal mosfet characteristics in this situation on Long time reliability?

Transient voltage should not exceed the Max Vgs transient given in datasheet. We cannot guarantee the operation where Vgs transient exceeds the Max value given.  Nagative max transient for IMW120R045M1 is  -10V 

The equivalent capacitance across a TVS diode will have any impact on Mosfet gate switching?

Since it will be far less than Cgs, the corresponding capacitance across a TVS diode won't have any effect.

The current selection of TVS diode are done as per Breakdown voltage rating to guarantee the turn on before it crosses the dynamic limit values-Right/wrong?

A TVS or Zener diode addition will also help reduce the negative ringing.

There is no spike found above the Vgs max, during the negative ringing it will be in forward bias, peak current will be based on Rg and Vgs negavtive. You can choos the diode based on the peak current.

The gate ringing is happening during the inbuilt body of mosfet is conducting in reverse current direction ZVS condition-what making the gate terminal to see this type of ringing even though cds and cgd capacitors of mosfets are discharge

Stray inductance at the source may be the cause of the VGS ringing.

You can avoid this by making the source path shorter and wider. in the layout.

Already Rgs has been changed . you can try tuning Rg, this may increase the switching loos it may help you in damping the ringing.

A TVS or Zener diode addition will also help reduce the negative ringing.

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