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Why is this excellent FET drive method for SMPS never mentioned?

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Flyback

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Hello,
The following article on Mosfet gate drive methods is amongst the world’s finest. Why then, does Balogh not mention the method of using an external Gate drive supply SMPS? As you know, this method means having isolated outputs which simply feed a gate drive IC next to the high side FET. A digital isolator couples the gate drive signal to the high side gate driver.

Mosfet Gate drive article:
http://www.radio-sensors.se/download/gate-driver2.pdf

attached ltspice simulation and pdf shows this excellent gate drive method in action
 

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  • Full Bridge SMPS.pdf
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  • Full Bridge SMPS.ASC
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It seems like that method is significantly more complicated than using a bootstrap driver.
 
When I started driving high side transistors and MOSFETs the opto isolators were too slow.
The digital isolators I have used are fast but cost too much, but the price is much better now.
 
Thanks,
The problem with pulse transformer gate drive is the following…..

1..You always need to drive the lower FETs through pulse transformers too, in order to get the same propagation delay, so you need more coils in the former.
2A….The source of the gate drive current is on the primary side of the pulse transformer, so that high current pulse has to traverse a potentially long bit of PCB to get to the FET gate. (with the method of post #1, there can be a gate driver right next to the FET itself, so the high pulse current can be in a tiny loop)
2B….Two gates can be driven from the same former, however, this means your gate drive has to be able to provide all that extra current to actually adequately drive those two FETs from the one driver. With the method of post #1, each FET can have its own dedicated drive chip.
3….Pulse transformer gate drives virtually always need a turn-off PNP to be included, so there’s extra parts.
4….Pulse transformers for FET gate drives need to be precision engineered, so that they have absolute minimum leakage inductance. If your winder has a bad day when winding them, then your SMPS batch may suffer too high switching losses and fail.
Its noteable how even tiny amounts of leakage inductance in a pulse transformer gate drive can really slow up the switching transient of the FET.
5….Pulse transformers for gate drives are rarely if ever available to your particular spec off-the-shelf, you virtually always have to get them custom wound.
6…Pulse transformer gate drives always need series resistance to be included in order to prevent ringing with the leakage inductance of the pulse transformer. There needs to be a resistor in both primary and secondary sides of the pulse transformer. This resistance always ends up being bigger than you would like, and means that your FET’s switching transients are always slower than you would like, meaning you get more switching losses.
7….If the method of post #1 is used, then you can use an 1EDI20N fet driver right next to the fet, and get absolute optimal turn off and turn on of the FET. (the 1EDI20N FET driver has separate pins for turn off and turn on.)

1EDI20N gate driver:
https://www.infineon.com/dgdl/Infineo...4790299add6112

.....after all this, I am amazed that nobody is using the method of post #1 with SMPS.
Attached simulation shows a ltspice sim of Pulse transformer solution
 

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  • Full Bridge _Pulse transformer FET drive.asc
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  • Full Bridge _Pulse transformer FET drive.pdf
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Transformers do not like wide duty cycle changes. They will not do 90% or 10%.
I have used them many times where the duty is around 50%. (example phase shift modulations) (or 60/40% in TV horizontal)

The old opto isolators had trouble above 10khz and I needed 100khz and up.
Many isolators do not like fast voltage slopes. They will false trigger. They work on resonant supplies where the slope is slow. If you FETS can switch 400 to 800 volts in 30nS many isolators will trigger on that noise.
 
My thoughts are that it isn't mentioned because its fairly old tech.

For a good example I know commercial welders and plasma cutter power supplies have been using that type of drive system for large SMPS type systems for several decades now and that's just two out of dozens more high capacity power supplies.
 
Many isolators do not like fast voltage slopes. They will false trigger. They work on resonant supplies where the slope is slow. If you FETS can switch 400 to 800 volts in 30nS many isolators will trigger on that noise.
Thanks, may i ask are you referring to "digital" isolators here?
Do you think it would be better to couple the gate drive signal up to the high side with eg a high side bootstrap high side drive ic...Eg...NCP5181
https://www.onsemi.com/pub_link/Collateral/NCP5181-D.PDF
 
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Here is what I call a digital isolator. (not LED+transistor) Output is 0/5V not linear. It is a term you used in post #1.
Note it can not take 1000 volts/uS slope, input to out put.
 

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  • TLP2200_datasheet_en_20140901.pdf
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Thanks, actually the digital isolators that I speak of have much higher transient immunity....around 50v/ns.
I presume the problem that you spoke of would not happen with any of the following digital isolators...?

The Si823x digital isolator FET drivers have transient immunity to 25 to 45 Volts per nanosecond.
Si823x digital isolator datasheet:
https://www.silabs.com/Support Documents/TechnicalDocs/Si823x.pdf
The si8610 digital isolators have 35 to 50V/ns of transient immunity. (it is strange that the plain digital isolators have more transient immunity than the digital isolators which are meant to be used as isolated fet frivers.)
Si8610 digital isolator datasheet:
https://www.silabs.com/Support Documents/TechnicalDocs/Si861x-2x.pdf

The NCP5181 (Bootstrap high side fet driver ic) has 50V/ns of transient immunity
NCP5181 bootstrap fet driver datasheet:
https://www.onsemi.com/pub_link/Collateral/NCP5181-D.PDF
King of transient immunity is the infineon 1EDI60N isolated fet driver IC. 1EDI60N has 100v/ns of transient immunity..
1EDI60N isolated fet driver datasheet
https://www.infineon.com/dgdl/Infin...N.pdf?fileId=db3a3043427ac3e201428e5da08f372a
 
I love the NCP51xx. Love SI823x. Have not used the 1EDI60.
Why is this excellent FET drive method for SMPS never mentioned?
I think it is mentioned by many people.

Some of us have been driving "high side switches" before you could get power MOSFETs worth using.
Us old guys were using isolated drivers long before the SI823x were patented. Before you were born.
You talked about all the bad ways to doing this. Some of us only had those ways. We made our drives out of discreet parts. There was no IC that could do this job. We used transformers because that is what we had. I have used every thing in that PDF in post #1.

We also made our PWMs out of op-amps and comparitors and transistors. Because that is what we had to work with.
You should thank the gods you have the NCP51xx.

"Grand dad, why did you ride a horse to school when cars are faster?"
"I rode a horse because it is faster than walking."
 
Thanks, I see your point, I must admit though, I believe the high side FET drive method of post #1 is so much better than all the other ways, that I am surprised that Balogh did not mention it in his paper which was written recently. I think the point you kindly make about high dv/dt is the only problem it could have. However, I believe now the modern parts mitigate this, as you describe.
 
https://www.infineon.com/dgdl/Infineo...4790299add6112

.....after all this, I am amazed that nobody is using the method of post #1 with SMPS.
Attached simulation shows a ltspice sim of Pulse transformer solution
I came here after searching for high side gate driving techniques. Sorry for resurrecting the dead post. I didn't want to create a separate thread for this issue.

I'm working on a 2-sw forward converter. The problem I've faced with is, as you might guess, is driving the high side MOSFET (I couldn't succeed on driving /w 1:1:1 transformer, btw). The part and datasheet that you provided has caught my attention. Although typical application shows a half-bridge configuration, I want to use it in my application. Is this possible?

Thanks.
 
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