Electronics > Power/Renewable Energy/EV's

Using Logic level FETs unecessarily in synchronous Buck?

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Faringdon:
Hi, do you know why the LM5143 (Synch Buck controller) was used for this Synch Buck spec (PMP31210 ref design)
of 20-58vin and 13.5Vout and 50A out?...

PMP31210 Ref Design:
https://www.ti.com/tool/PMP31210

Surely they should have just used the same_priced LM5119 instead?

The LM5143 has a max gate drive voltage of just 4.5V, meaning they are restricted to logic level FETs and their Rds(on)
is much higher at the 80V rating needed.
The LM5119 has a gate drive voltage up to 15V...meaning far lower Rds(on) FETs are available , and so FETs would be much cheaper.

Unfortunately we dont have an official company address, and so cant ask on the ti.com forum.
You would think they would advertise products in their suitable useage case, rather than in a totally unsuitable case?

I mean, 58Vin Synch Buck, with 700W output, and they are using a gate drive of 4.5V ??? When they could have gone with 12V gate drive for
same price solution.???

Maybe the SMPS is operated in sub zero temperature buildings and the extra heat dissipation is useful? -only plausible explanation.

PMP31210 Ref Design:
https://www.ti.com/tool/PMP31210

LM5143:
https://www.ti.com/product/LM5143

LM5119:
https://www.ti.com/product/LM5119

*********************************************
As you know, the only real reason to use logic level FETs in any SMPS is that your Vin is below say 7V, and you dont want to bother with a bias booster so you just put up with logic level FETs. As you know, people are not using logic level FETs because of any advantage of them. They have no advantages over "normal" threshold FETs. Especially in a synchronous buck, logic level FETs are a poor choice, because with their low gate threshold voltage, they may get spuriously turned ON by the other FET turning ON and pulsing a current spike through the drain gate capacitance. Hot logic level FETs have a ridiculously low VGS(th)......as you know, they are a very poor choice, and one that you "put up with".

Not least is the fact that if you choose a logic level gate driver, then you are restricted to only logic level FETs...so you likely end up paying more for your FETs, and being more susceptible to FET nil-stocking.

Also, how does a logic level hi side gate driver supply (bootstrapped) in a synch buck chip ever do "UVLO" of the driver?...as you know...it cant really do it......when VGSth is that low you cant really do it....if you have a 12V gate driver, then you can have it "UVLO out" at say 7V.

Logic-level devices, also, have higher gate charge than standard-level parts of similar V(BR)DSS and RDS(on)

David Aurora:

--- Quote from: Faringdon on April 10, 2024, 05:39:40 am ---Hi, do you know why the LM5143 (Synch Buck controller) was used for this Synch Buck spec (PMP31210 ref design)
of 20-58vin and 13.5Vout and 50A out?...
...

--- End quote ---

Yes but we can't tell you, it's a secret.

jc101:

--- Quote from: Faringdon on April 10, 2024, 05:39:40 am ---Unfortunately we dont have an official company address, and so cant ask on the ti.com forum.
--- End quote ---

What? Absolute rubbish.  If you are a company you have a registered address, it is a legal requirement.  Use that.

Anyway, registering simply requires a postcode. Assuming you aren't a company just use, for example, WC2B 6NH, which is a bookable We Work space—perfectly justifiable to use. If they want the full address, it's Aviation House, 125 Kingsway London, WC2B 6NH.

Just go and ask TI, please.

tggzzz:

--- Quote from: jc101 on April 10, 2024, 04:26:09 pm ---
--- Quote from: Faringdon on April 10, 2024, 05:39:40 am ---Unfortunately we dont have an official company address, and so cant ask on the ti.com forum.
--- End quote ---

What? Absolute rubbish.  If you are a company you have a registered address, it is a legal requirement.  Use that.

--- End quote ---

Might it be that the only company "they" have is himself? Sad possibility.

mtwieg:

--- Quote from: Faringdon on April 10, 2024, 05:39:40 am ---Hi, do you know why the LM5143 (Synch Buck controller) was used for this Synch Buck spec (PMP31210 ref design)
of 20-58vin and 13.5Vout and 50A out?...

PMP31210 Ref Design:
https://www.ti.com/tool/PMP31210

Surely they should have just used the same_priced LM5119 instead?

The LM5143 has a max gate drive voltage of just 4.5V, meaning they are restricted to logic level FETs and their Rds(on)
is much higher at the 80V rating needed.
The LM5119 has a gate drive voltage up to 15V...meaning far lower Rds(on) FETs are available , and so FETs would be much cheaper.

--- End quote ---
You misunderstand the purpose of these reference designs. It doesn't mean that TI thinks this is the best chip for the application. It just means the reference design is a good way for engineers to evaluate the chip for their application. They're trying to sell all their chips, and only putting their latest and greatest stuff in reference designs doesn't help that.

Also the datasheet says the gate  5V, but can be up to 6.5V if you apply an external voltage with the VCCX pin. Still restricted in FET selection, but a big difference from 4.5V...

--- Quote from: jc101 on April 10, 2024, 04:26:09 pm ---
What? Absolute rubbish.  If you are a company you have a registered address, it is a legal requirement.  Use that.

Anyway, registering simply requires a postcode. Assuming you aren't a company just use, for example, WC2B 6NH, which is a bookable We Work space—perfectly justifiable to use. If they want the full address, it's Aviation House, 125 Kingsway London, WC2B 6NH.

Just go and ask TI, please.

--- End quote ---
I believe he was referring to an email address/domain, not a street address. A few years ago TI decided to cull their own userbase by only allowing people with "company" email addresses create new threads.

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