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All told, isolated offline SMPS with sync rect outputs are often a waste of time?

Z

zenerbjt

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Hi,
Do you agree, Isolated offline SMPS of say 240W for eg Vout = 12V and iout = 20A , that switch off a PFC output should not bother using synchronous rectifiers, but instead just give a vout of say 60V and 4A…..then they don’t need synchronous rectifiers because output current is so low….and then the 12V , 20A (or whatever) can be cheaply provided by using eg a LM5116 sync buck controller?

Very few Two transistor forward or LLC controllers comprise sync rect drivers. Eg HiperTFS and HiperLCS don’t have sync rect drivers. Also, those sync rect controllers which sit on the secondary side and don’t communicate with the primary side controller are very dodgy under certain cirmustances, and should be avoided.

So do you agree that unless super small size is needed, then having a higher voltage output non-sync-rect, isolation-stage SMPS, followed by some cheap offtheshelf sync Buck controllers is the best way to supply your high current outputs?
 

dick_freebird

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Really more about the output voltage and your
efficiency requirement. Catch-diode simple buck
has a (diode_Vf/Vin_min) inefficiency term that
becomes unberarable in low voltage input buck
converters (and presumably other topologies).

0.7V / 28V = 2.5% loss term, so peak eff% 97.5% all \
other losses being nil. Not terrible.

0.7V / .5V (like POL buck) = 15.6% loss term, so 84%
peak eff% @ otherwise ideal, and roughly zero sales
on that one.
 
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Z

zenerbjt

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Thanks, thats right, and this is why ti.com have produced a range of low cost sync buck converter ICs up to 100V input voltage.

Anyway, i think if secondary side , independent sync rectifier controllers were worth anything, then power integrations would have made them by now, but they never have. The only sync rect controller in power integrations reportoire is the innoswitch, but that is a secondary side controlled chip, and the sec side controller directly controls the primary fet switching and the sync rect operation...which is how it needs to be with sync rects....ie one controller (on either pri or sec side), needs to directly control both primary fets and sec side sync rects....otherwise sync rects are liable to go badly wrong.
 

Easy peasy

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There are pleanty of good ASIC's out there for synch rect - just a matter of mating up to the primary side drive and providing the delay necessary so that the sec side can turn off just before ( ~ 200nS ) the pri side switches ...
 
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Z

zenerbjt

Guest
Thanks, yes that seems the best way, to actually be able to switch off before the pri fet switches, instead of reacting to it switching, which is the modus operandi of the SRK2000 sec side sync. rect. driver....

SRK2000

Page 15 of the SRK2000 datasheet warns that some mosfets will have too much bond wire inductance and will instigate faulty behaviour of the synchronous rectification....and the mosfet may need swapping out......such sensitivity seems very poor, and in fact, indicates that these kind of "reactive" type sec side sync rect drivers are dangerous.

Power integrations employ a vast army of SMPS engineers to make demo boards and eval boards, but it says it all that they have only used a “reactive” type sec side sync rect driver chip on one occasion (DER385). That was the SRK2000 discussed above. (whose datasheet tells of the perils of using it)
And in DER428, power integrations use “natural drive” (no chip) sync rectification for a 2TFC with 14A output….but even then only do sync rect for the power diode…not across the freewheel diode.
This says it all to me about “reactive” type sync rect drivers. They are plain dangerous. They promise vast reductions in sec side rectification losses, but are too risky.
 
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Z

zenerbjt

Guest
For a realistic offline 2 Tran forward converter with synch rects, the attached is the only realistic modus operandi to do it.
Would you agree?
The synch rects get switched off before the primary fets turn ON. There is no chance of reversing output inductor current in light load, as there is a load current monitor which disables the synch rects in cases of light load current.
This is backed up by a comparator which looks at the COMP pin of the primary side controller. Whenever this goes low in voltage, indicating light load, then again the synch rects are turned OFF. Thus there is no chance that the synch rect logic could end up keeping the "freewheel" synch rect ON permanently during light load.
Do you know why there are no offthe shelf chipsets which incorporate this circuitry?

LTspice sim and pdf schem attached
 

Attachments

  • 2TF_SRs _home brew LTC3723_1.zip
    4.8 KB · Views: 0
  • Schem_2TFC with Synch Rects.pdf
    146.5 KB · Views: 1
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Z

zenerbjt

Guest
...leading on from the above about synchronous rectifiers...the following sheds further light....

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
 
Z

zenerbjt

Guest
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. [Vin=390v, vout = 250w and 24v]

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 by contracted experts but thereafter 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?
 

Attachments

  • 2TFC LTC3723 LTC3901 SRs.pdf
    132.8 KB · Views: 0
  • 2TFC_LTC3723 LTC3901 _SRs.zip
    4 KB · Views: 0
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dick_freebird

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Figuring 0.8V/24V*((390-24)/390) is about a
3% inefficiency term, is the freewheel diode
really worth replacing with an active FET and
the expense of driving it? I guess it comes
down to requirements and competition.
 
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zenerbjt

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Thanks, good point, in this case it was really just so a smaller heatsink could be used.

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?
 
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