UC3854B: Enhance gate freq & FET Vds problem

benhsiao

Prodigy 130 points

Part Number: UC3854B
Other Parts Discussed in Thread: UC3854, , UCC28180

Hi TI engineer .

For the research , I want to enhance freq to 680k Hz .

But I refer UC3854 Technical documents design the Ct = 174pF,Ccp = 10pF,Ccz = 62p.

I check the Ct cap frequency is 400k Hz . Help me to enhance freq .

I find some times signal Vds raising slowly , as figure (yellow line : Vgs Blue line :Vds)

General signal as figure :

over 5 years ago

0 John Griffin1 over 5 years ago

TI__Mastermind 34465 points

Hello benhsiao,

It is not possible to use the UC3854B with a 680kHz oscillator frequency.

This part is characterised at 100kHz and operation at a frequency greater than 100kHz will result in degradation of performance as you have discovered.

The UCC28180 is characterized at 250kHz so it may be more suitable for your application.

Regards

John

0 benhsiao over 5 years ago in reply to John Griffin1

Prodigy 130 points

Hello John ,

thanks your answer .

I am curious about problem 2 , that's same simulation, catch different timing .

Can you express what happen cause this different .

0 John Griffin1 over 5 years ago in reply to benhsiao

TI__Mastermind 34465 points

Hello benhsiao,

You are seeing a slow turn off of the FET because the gate drive output of the UC3854B is not able to perform a fast turn off of the FET at this high switching speed. Are you driving the FET directly from the ic ?

Regards

John

0 benhsiao over 5 years ago in reply to John Griffin1

Prodigy 130 points

Hello John ,

The upper chart show the FET slow turn off , But second chart FET trun off fast.

There are same simulation , but different catch timing .

The driver circuit show in figure .

And circuit annex

UC3854_Circuit.docx

0 John Griffin1 over 5 years ago in reply to benhsiao

TI__Mastermind 34465 points

Hello benhsiao,

A 2 Ohm turn off resistor on the gate will attempt to sink way too much current into the GDA or GDB drivers.

I think the simulation model is not accurate in reference to the gate drive capability of the ic.

Regards

John

Power management

Power management forum

UC3854B: Enhance gate freq & FET Vds problem