How to get through the problem related to MOSFET Switching?

[SACHIN C]

can SOA restrict the voltage & current depend upon switching frequency?




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[FvM]

can SOA restrict the voltage & current depend upon switching frequency?

Only indirectly by calculating average power and thermal derating.

 

[SACHIN C]

But what about time period mention in SOA plot for MOSFET?
Limited by Rdson means?


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[SACHIN C]

If for any MOSFET in SOA plot manufacturer has given Time period for switching from 10ms to 100us,then is that means we can't operate that particular MOSFET below 100us?


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[FvM]

that means we can't operate that particular MOSFET below 100us?

It surely doesn't mean this. Try a different explanation.

 

[SACHIN C]

Dear All,
As i changed the MOSFET for 1200VDC ,now using SCT30N120
But,in datasheet Rise & Fall Time not mention.So how to calculate?
Only Charge at required Vgs known,but rise & fall time Vs gate resistor not mention in datasheet ?
Vgs=12VDC,

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[SACHIN C]

Dear All,
As i changed the MOSFET for 1200VDC ,presently using SCT30N120
But,in datasheet Rise & Fall Time not mention.So how to calculate?
Only Gate Charge at required Vgs known,but rise & fall time Vs gate resistor not mention in datasheet ?
Vgs=12VDC,Fsw=10KHz

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[FvM]

Parameters not written in a datashet are unspecified.
I don't believe that you can expect a reasonable answer at Edaboard. Particularly because SCT30N120 is new SiC technology. Did you actually get hold of some samples? Product status is still flagged as "preview".

I presume, you noticed the differences to Si MOSFET, e.g. high suggested gate drive voltage. Some informations can be probably derived from a related technical article.

 

[SACHIN C]

FvM,
Currently i am using SCT30N120 MOSFET in Half Bridge,but i didn't found any Rise & Fall Time Vs Gate Resistor Plot in datasheet?
Gate Driving Parameters are as follows,
Vgs =12VDC,
Rg =16ohm,(Gate Resistor)
Rgs =4.7K(Gate to Source Resistor)

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[FvM]

A brief view at the SCT30N120 output characteristic suggests that 12 V Vgs isn't a good choice.

I agree that a switching speed versus gate resistance data would be fine. But firstly you can't expect it for brand new devices with preliminary datasheets. Secondly these data don't say much about achievable speed in your application. At the end of the day, you have to try it in a real hardware setup.

 

[SACHIN C]

FvM,
As i am using Driver ic which suggest voltage for gate to source as 12V,
So do i need to change the MOSFET?

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[FvM]

Why? Most popular gate drivers gate work with 18 or 20V.

 

[SACHIN C]

FvM,
i am using si8234AB Driver IC?



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[FvM]

i am using si8234AB Driver IC

O.K., it has 6.5 to 24 V supply voltage range.

 

[SACHIN C]

FvM,
But they are saying use 12V for Gate Supply,when Label on IC with Suffix indicate A (which represents UVLO at 5V),
So i am using 12V,guide me whether it is fine to use 12V or we can change the Voltage ?

Also i am currently observing with Vgs =12V ,Rg =16ohm,Rgs =4.7K that one of the gate waveform for half bridge starts shifting to left as we go on increasing the Drain to Source Voltage beyound 100V because of which at voltage say around 350VDC (Vds) the cross conduction occurs.
I am not able to understand the issue of signal shifting ?


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[FvM]

You should be able to determine if dead time variation happens internal to Si8234, e.g. due to crosstalk to DT node, or only in the gate voltage. At the end of the day, you'll adjust the dead time for correct operation with your final bus voltage.

I have one additional comment regarding gate driver supply voltage. If you set it to e.g. 18 or 20 V for SiC MOSFETs, you would prefer "C" devices with 10V UVLO threshold.

 

[SACHIN C]

FvM,
Thanks for your Technical Artical of SCT30N120 Link in comment 68,i gone through it.I have a querry,in artical they applied Gate Voltage 20V mentioning it is at high current.
So can we apply lower voltage to test up to 25A & also Gate Threshold Voltage is 1.8V?
In datasheet they have provided the plot for Vgs Vs Gate Charge.
So accordingly 12V corresponds to 65nC

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[FvM]

AT 20 A, 12 V gate voltage is just enough to switch the transistor fully on. And the datasheet doesn't specify type variations, you should definitely provide some margin.

 

[SACHIN C]

FvM,
I am still not able to find the rise time for SCT30N120 as not mention in datasheet.
Which is important to calculate gate current & to select Power Supply Regulator of that current capacity.
What will be rise time for Qg =65nC ,Vgs =12V (So that Rg can be selected )?


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[FvM]

I don't understand the problem.

Average gate driver current will be determined by MOSFET capacitances respectively gate cjharge and switching frequency.
Gate resistors only change the peak current and how the dissipated power is shared between driver IC and resistor.

The peak current will be sourced by bypass capacitors rather than a voltage regulator. Providing sufficient gate driver bypass capacitors allows for any gate resistor value without affecting voltage stability.