How do they sell SMPS synchronous rectifier controllers that give serious product warnings in the datasheet?

[opampsmoker]

Hi,
I have just come across a fantastic new current mode control chip for offline flybacks!

..Its like the UCC28C43 but way, way better. Only thing is you need to add a “loop” in the PCB tracking near the chip, in order to create some stray inductance (4nH, or maybe more), which cancels out stray inductance involving the MOSFET bond wires etc………if you don’t add this stray inductance, then you will suffer malfunctional operation……..The amount of stray inductance that you need to add can vary depending on layout and mosfet package tolerance in its bonding wires, and indeed, varies with how long the TO220 leads are cut off at, etc etc.

…….OK, if you read the above, you will no doubt be thinking that that controller is in fact extremely poor, and you’d wonder at the sanity of the semico that produced the chip.

But here is such an SMPS control chip, which suffers this exact problem….its called the NCP4303, and is a synchronous rectifier FET driver….
Pages 15 and 16 of the datasheet reveal the dire situation as decribed above.

Does anyone know why people use these type of synchronous rectifier drivers that rely on the chip monitoring the secondary side switching node in order to control the switching of the synchronous FETs? I mean, they all have this kind of cut-throat, skull and cross-bones warning in their datasheets.

Why are people not using chips like the ICE2HSO1G LLC controller? …which controls the synchronous FETs without any of this grief…and controls the synchronous FETs from the primary side, in coordination with the primary side fet switching. It even costs no more money.

NCP4303 Synchronous rectifier driver

https://www.onsemi.com/pub/Collateral/NCP4303-D.PDF

 

[JimB]

if you read the above, you will no doubt be thinking that that controller is in fact extremely poor, and you’d wonder at the sanity of the semico that produced the chip.

Err, no actually...

I was wondering what this whining little multi-aliased person is dripping on about now.

It is just an example of one of many solutions to a problem, as postulated by member Visitor in a recent thread.

JimB

 

[unclejed613]

OK, if you read the above, you will no doubt be thinking that that controller is in fact extremely poor, and you’d wonder at the sanity of the semico that produced the chip.

actually, since no semiconductor is perfect, it makes sense to put in design notes that help the end designer in building a power supply that doesn't "let out the magic smoke" as soon as the power is applied... one thing that cannot be included in an IC design is internal inductors larger than a few hundred picohenries. so it makes sense that any inductors in the drive leads to MOSFETS are going to be external to the chip. if i were designing an airplane, i would certainly publish in the tech manual for the plane what the max climb/dive rates, max airspeed, and minimum stall speed were for the aircraft. and since exceeding these parameters can damage the aircraft, i would put them in in the form of attention-getting warnings.

 

[gophert]

Why are people not using chips like the ICE2HSO1G LLC controller?

umm, if you have to ask, you either don't have a need or you don't understand the part. I features (ultimate adjustability, fast switching times and automatic conduction compensation - all in the name of maximzing efficiency. I suggest you pick the plug-n-play parts.

 

[opampsmoker]

Thanks...the thing is the following is a really solid, really simple way to do synch rects......

The attached is a Two Transistor forward , done with a Full bridge controller which has a synch rect driver output which can be used to drive the “freewheel” synch fet via a small Pulse transformer as in the attached LTspice sim and pdf schem. A “freewheel” synch fet is all that’s needed as the duty cycle can be made low such that the “power” synch fet can be avoided and so just a diode is OK here.

I am sure (?) many would agree that this is a perfectly satisfactory way to reduce secondary rectifier losses. Also, it avoids the dreadful problem of shoot through which can happen with those secondary side synch rect control chips which are afflicted by noise as they “look” at the noisy switching node. Also, the shown method here means you can slap in TO220 synch rects in parallel and not worry about lead or bonding wire stray inductance effects. No messing about trying to heatsink those “low stray inductance” SMD FET packages…..they are lousy as they need heatsinking through the FR4 PCB. (albeit with thermal vias but thermal vias are pretty lousy compared to a good solid metal TO220 tab screwed to a metal heatsink with just a little 100um thick insulating spacer)

The LTC3723 assures that the synch rect drive is delayed and “clipped” such that there is no shoot through. This means less field failures and so the extra cost of the LTC3723 is worth while.
This is also in the name of "design for maintenance" where SMPS's are designed and have to be maintained by engineers who do not have a decade of experience of SMPS design.

(By the way, the other reason to use a full bridge controller is that it has the “spare” output which could be used for pri side bootstrap high-side drive capacitor refresh, ..good in light load.)

I am wondering why no semi-co’s are making chips like this? The LTC3723 can be “hacked” to do it as shown here but its expensive at approx $4.5 per 1000 pces. I am sure this could be done cheaper than the LTC3723 chip?

 

[gophert]

Ok, then use that.

 

[opampsmoker]

The LTC3723 in this use-age has been "hacked" to do the job.
LTC3723 is expensive but i am not using its full functionality here. I am surprised there is not a chip available that does what i am doing here?
All it needs is a standard pwm controller, with an output which is the "delayed and clipped" inverse of the main gate drive. (the "delay and clip" to get the dead times)....i cant find anything off the shelf like this. Any body else?

 

[schmitt trigger]

Bringing into market a new, complex IC is a lengthy and expensive proposition.
And the question is always: What is the return on the investment? Does the proposed IC has significant improvements over existing devices that would entice customers, provide a good sales volume and profit margins?

These are tough questions to answer.
And in such a crowded market as power conversion, unless you have compelling arguments, the proposal will go nowhere.