The perfect buck or booost converter?
The perfect buck or booost converter?
(OP)
Hello,
I am doing SMPS's for a single product which sometimes runs off a battery, and sometimes off the secondary rail of an offline converter.
So:
I am doing a Boost converter as follows:-
V(in) = 2.1V
V(out) = 27V
P(out) = 2W
Switching frequency = 100KHz
(I have a 5V power rail available to act as a bias supply)
Also, I will be doing a Buck converter as follows:-
V(in) = 30V
V(out) = 2.4V
P(out) = 0.4W
Switching frequency = 100KHz
Given the above specification, is the following circuit the perfect way to do both of these circuits (as a synchronous buck or boost converter)..................
Schematic of synchronous buck or boost converter
http://i31.tinypic.com/eflwty.jpg
.......taken from page 8 of .............
http:// www.nxp.co m/document s/applicat ion_note/A N10661.pdf
It looks cheap, simple and highly efficient.
It seems just too perfect to be true
Is it really as good as it seems?
Can you see any bugs?
I am doing SMPS's for a single product which sometimes runs off a battery, and sometimes off the secondary rail of an offline converter.
So:
I am doing a Boost converter as follows:-
V(in) = 2.1V
V(out) = 27V
P(out) = 2W
Switching frequency = 100KHz
(I have a 5V power rail available to act as a bias supply)
Also, I will be doing a Buck converter as follows:-
V(in) = 30V
V(out) = 2.4V
P(out) = 0.4W
Switching frequency = 100KHz
Given the above specification, is the following circuit the perfect way to do both of these circuits (as a synchronous buck or boost converter)..................
Schematic of synchronous buck or boost converter
http://i31.tinypic.com/eflwty.jpg
.......taken from page 8 of .............
http://
It looks cheap, simple and highly efficient.
It seems just too perfect to be true
Is it really as good as it seems?
Can you see any bugs?





RE: The perfect buck or booost converter?
Looks like a simple driver chip. Not really what you stated.
RE: The perfect buck or booost converter?
RE: The perfect buck or booost converter?
TTFN
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RE: The perfect buck or booost converter?
Synchronous Boost Converter:
http://i29.tinypic.com/2vdi23t.jpg
Synchronous Buck Converter:
http://i32.tinypic.com/263ww7b.jpg
we don't like the specific actual "buck-boost" circuit since the output voltage is inverted, and this affects us in many ways.
RE: The perfect buck or booost converter?
Additionally, even if you could make it work in the correct direction, you'd never get closer than Vth to Vboost, and again, a diode would be better.
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
RE: The perfect buck or booost converter?
Synchronous Boost:
http://i29.tinypic.com/2vdi23t.jpg
-Surely the high-side NFET is properly turned on by the NPN of the "Qa" complimentary pair.
(though it does look like the high-side NFET would need a parallel Schottky so as to keep its intrinsic diode from starting to conduct when the high-side FET is turned OFF)
RE: The perfect buck or booost converter?
http://en.wikipedia.org/wiki/Sepic
RE: The perfect buck or booost converter?
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
RE: The perfect buck or booost converter?
..i believe this is what you are saying?, though you are using more exact terminology
(-i will add a Schottky in parallel with M1 drain-source so that its intrinsic diode does not conduct)
We thought of the SEPIC , but really it would be more inefficient here, than a synchronous boost or buck
RE: The perfect buck or booost converter?
VE1BLL:
"Having now followed the other link, I believe that you're off track by a country mile."
-Judging by the number of blown FETs we're getting, i guess you are right.
My guess is that you looked at the fact that this circuit was designed to drive a motor winding at a few KHz, and that such a circuit would be very unlikely to be able to drive SMPS FETs at 100KHz?
The thing is , the complimentary BJT drivers, "Qa" and "Qb" above are PMD3001D from NXP............
PMD3001D DATASHEET:
ht
.....Now page 8 and page 10 of the PMD3001D datasheet reveal hfe values of around 200 for Ic of around a few 100 mA's..........
PMD3001D hfe values at high Ic current:
http://i29.tinypic.com/2d92xcg.jpg
....this high hfe value should be well enough to ensure we get decent FET gate drive current for the FETs "M1" and "M2" above.
-though i would confess that the PMD3001D datasheet does not provide a "fT" value for the BJT's.....and this is a nuisance because it must be around 150MHz for a 100KHz MOSFET driver....and the PMD3001D datasheet gives no indication of "fT" value.
(source for this is page 4 of AN18 from Zetex...........
http:
So, my question is, how can you tell, from the PMD3001D datasheet that these BJT complimentary pairs cannot drive FETs at 100KHz?
RE: The perfect buck or booost converter?
TTFN
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RE: The perfect buck or booost converter?
RE: The perfect buck or booost converter?
You're reading way too much into my two brief responses to your original post. Your original post pointed to links that were not as stated. That was as far as I went. And in the 12 minutes between my first and second responses, I certainly didn't string together the missing information you've subsequently provided.
Good luck with your design.
RE: The perfect buck or booost converter?
TTFN
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RE: The perfect buck or booost converter?
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
RE: The perfect buck or booost converter?
RE: The perfect buck or booost converter?
Synchronous Boost:
http://i29.tinypic.com/2vdi23t.jpg
i.e., also, i am interested in knowing how the PMD3001D datasheet can be used to assess its ability as a MOSFET driver in a 100KHz SMPS.
PMD3001D DATASHEET:
ht
.....For me there is no way of assessing, from the PMD3001D datasheet, whether or not this part has MOSFET driver potentail at 100KHz switching frequency.
-in particular, because the "transition frequency" (fT) is missing from the datasheet
am i correct?
RE: The perfect buck or booost converter?
Can the PMD3001D provide enough current to discharge the gate (i.e. turn off the FET)?
gate charge = I * t
I had a simulation that showed I could turn a FET off in 15 nS. Luckily I dug into it and saw that I could as long as I pulled 9.8A out of the gate! My real circuit is a bit slower than that (147 mA @ 1 uS).
John D