Why use gate drive transformer instead of bootstrap high side drive for SMPS?

[treez]

Hello,

This concerns the use of high side fet drives in SMPS….

On page 25 of the LTC3722 datasheet (bottom RHS), it says that bootstrap high side drive ic’s such as LTC4440 can be used when input voltage is less than 80V….it implies that if above 80v, gate drive transformers are needed instead. Why do they say this?, bootstrap high side drive ic’s are used with 400V (post PFC) input voltages and there’s no problem.

LTC3722 datasheet
http://cds.linear.com/docs/en/datasheet/372212fb.pdf

 

[skylarks]

High side driver ics has a working limits, With in these limits these operate perfactly , beyond these limits these ics fail burns ,can not withstand higher voltages

 

[FvM]

Why do they say this?

Because Linear doesn't own a HV IC process and has no highside drivers for higher supply voltages...

 

[treez]

I agree with FvM, I mean, there are loads of bootstrap high side driver ic's, eg ncp5181..
https://www.onsemi.com/pub_link/Collateral/NCP5181-D.PDF

..not only that, but virtually every single switch mode fluorescent ballast driver in the world uses a bootstrap high side drive with ballasts switching off of 400V post PFC bus

 

[mtwieg]

The datasheet doesn't show a gdt being used. The transformer is for signal isolation, and they then use a gate drive IC with a bootstrapped supply. So it has the exact same function as a bootstrapping gate driver IC. Asto why they show this approach, it's likely because LT does not make and high side high voltage gate drivers.

 

[dick_freebird]

A transformer has a nice property of shorting gate to source
absent stimulus. Nice for fault and transitional operation,
well behaved, hard to break, and cheap. Could get chubby
at low frequency though.