UCC21520-Q1: UCC21520

user4186101

Prodigy 200 points

Part Number: UCC21520-Q1
Other Parts Discussed in Thread: UCC21520,

Hi, thanks for reading my question.

Recently I want to use bootstrap circuit to drive my LLC SiC Mosfet, and I choose UCC21520 as TI engineer recommand me.

I calculate the gate resistor, but I dont understand the equation below.

I think the circuit is :

Ciss+ ->Diode_Off->Roff->OUTA->Rol->VSSA-> Ciss-

So Rgeft_int is not in the circuit

and V_Ciss= VDD-VBDF-VGDF，i can understand this part, the voltage in Ciss is equal to VDD-VBDF-VGDF.

Am I right?

thank you.good luck.

over 7 years ago

0 Don Dapkus over 7 years ago

TI__Guru**** 189401 points

Hi,

Thanks for your interest in our UCC21520-Q1 device!

We have a TechNote that talks about gate resistors, this might provide you additional information. I suggest you review it:

www.ti.com/.../slla385.pdf

As for your specific question, we will get back to you later this week. Today is a holiday in the US.

0 user4186101 over 7 years ago in reply to Don Dapkus

Prodigy 200 points

Thank you for your anser and the technote you recommand, it has resolved my question very well.

0 user4186101 over 7 years ago in reply to Don Dapkus

Prodigy 200 points

Sorry to bother you again, when I use 21520 to design Mos driver circuit ,I met a new problem.

How do I choose the value of RGS(between Gate pin and Source Pin)? I choose 5K，but I dont know the theory.

Is there some TI technote about it?

Thank you again. Best wishes.

0 Don Dapkus over 7 years ago in reply to user4186101

TI__Guru**** 189401 points

Hi,

We don't have a TechNote that discusses that. That resistor is essentially a fail-safe element for two cases: before power is applied, and if something happens to the gate driver. It will keep the gate of the power transistor discharged so it won't conduct any current.

The lower value this resistor is, the better it will hold off the transistor. However, the trade-off is this resistor will steal some of the gate current away from the gate driver, slowing down switching speeds.

5-10 kohm is a pretty typical value, so I think you are ok.

0 user4186101 over 7 years ago in reply to Don Dapkus

Prodigy 200 points

Thank you for your answer. Your explaination is very clear.That is enough for me.

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UCC21520-Q1: UCC21520