Reason for multiple components in Buck converter

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Kutty simba

Joined May 6, 2020
7
Attached is a snip from buck converter circuit. I have three questions 1.What is the purpose of using multiple diodes instead of using on higher rated? 2. Reason for tho inductor in series instead of one with higher value. 3. Reason for multiple resistors in parallel?
Please help me to understand
 

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jpanhalt

Joined Jan 18, 2008
9,418
I don't see any reason single components of comparable specifications to the array could not be used. Of course, for diodes in series, one may do that to get a greater voltage drop. Similarly, for sense resistors in parallel, you need to consider the current rating.
 

Wolframore

Joined Jan 21, 2019
1,748
parallel diodes are a bad idea. They have different transfer characteristics and their internal resistance goes down as they heat up. If you must do this each of them must be rated for the full current.
 

MisterBill2

Joined Jan 23, 2018
6,069
I don't see any reason single components of comparable specifications to the array could not be used. Of course, for diodes in series, one may do that to get a greater voltage drop. Similarly, for sense resistors in parallel, you need to consider the current rating.
First, it would be better to show us the whole circuit so that we can see what is happening in context.
But, as for mltiple diodes in parallel, and all of those little resistors in parallel, consider that the supply is probably a small package where size matters a lot, and consider that there may not be height available for a more highly rated diode. And with diodes in parallel redundancy brings reliability. And more likely, a group of smaller diodes is able to spread the heat better. Two other things are response times and charge storage, where it takes a bit of time for the diode to switch off. Smaller diodes can switch faster. And it may also be that the re were a few million of those diodes i stock, available immediately, while the single diode production line was down.
So there are many possible real world reasons why that part of the circuit uses parallel parts. The rest of the circuit may also have some interesting choices.
 

Papabravo

Joined Feb 24, 2006
13,725
It is hard to throw stones at the original designer if you have no insight into his motivations. I certainly don't see anything egregious in this design.
 

ronsimpson

Joined Oct 7, 2019
680
I can not see enough to know what is happening. but
C14 2.2nF capacitor on the MOSFET. Not a common thing to do.
Gate drive circuit. I question if it can pull up enough to turn on the MOSFET.
 

MisterBill2

Joined Jan 23, 2018
6,069
I can not see enough to know what is happening. but
C14 2.2nF capacitor on the MOSFET. Not a common thing to do.
Gate drive circuit. I question if it can pull up enough to turn on the MOSFET.
The gate drive may be adequate, we can't know because it is not clear what the supply voltage is, given that part of the circuit is not shown. AND, C14 is directly across the group of 4 diodes, which are the ones that are supposed to allow the energy stored in the inductor flow into the load.
 

ScottWang

Joined Aug 23, 2012
6,898
3. Reason for multiple resistors in parallel?
I'm not sure the reason for the circuit, but I'm thinking of two reasons why we will use multiple resistors in parallel in the circuit of power supply, and the one is to reduce the power dissipation for each resistor and another one is to get a more precisely (or desired) values of current.
 

MisterBill2

Joined Jan 23, 2018
6,069
I'm not sure the reason for the circuit, but I'm thinking of two reasons why we will use multiple resistors in parallel in the circuit of power supply, and the one is to reduce the power dissipation for each resistor and another one is to get a more precisely (or desired) values of current.
Very true! and it spreads the heat over a larger area. The other advantage is that if one resistor fails the rest are still thgere.
 

Wolframore

Joined Jan 21, 2019
1,748
Yeah no problems with parallel resistors. Easy way to get a precision current shunt. It’s the parallel diodes that I’m not liking. Maybe there’s a reason for it thats not apparent from what we can see.
 

MisterBill2

Joined Jan 23, 2018
6,069
Yeah no problems with parallel resistors. Easy way to get a precision current shunt. It’s the parallel diodes that I’m not liking. Maybe there’s a reason for it thats not apparent from what we can see.
Like I stated before, possibly to get some specific performance characteristic, maybe switching time or charge storage capacitance or some less commonly considered parameter. There is a lot to diodes in a switching supply. AND, there also, it may have been just for the power rating and the redundancy.
 

Wolframore

Joined Jan 21, 2019
1,748
As a rule of thumb I avoid this due to there is always one diode that will have a lower forward voltage. Due to the kT/q the resistance on the one that starts first goes down and can lead to thermal runaway. Redundancy doesn’t make sense, power diodes usually fail short. I would like to know, I’m just trying to make sense of it.

I found this... it makes me cringe a little but as long as it’s under control its possible.
https://www.st.com/resource/en/application_note/dm00098381-current-sharing-in-parallel-diodes-stmicroelectronics.pdf
 
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tindel

Joined Sep 16, 2012
716
Attached is a snip from buck converter circuit. I have three questions 1.What is the purpose of using multiple diodes instead of using on higher rated? 2. Reason for tho inductor in series instead of one with higher value. 3. Reason for multiple resistors in parallel?
Please help me to understand
1) As others have pointed out, the diodes in parallel, aren't the greatest - but can work - just not great engineering.
2) Assuming the D17 is ~15V and R39 is 22ohms, then the gate drive maximum current is about 0.5A if Q5 saturates. This is a pretty weak drive current in a reasonably high power supply (~200W). meaning the switch frequency is likely pretty low, 10-50kHz I'm guessing. 100uH with a 9.5A saturation (?) current makes for a pretty large inductor in a slow switching power supply. The engineer may have had to split them up to meet space or cost requirements. Could have also been the inductor used in a lower power design of similar topology and is being reused.
3) The resistors are likely used for current sensing somewhere. By using several of them, power is spread out a bit more and the part is probably cheaper. At 10A, power is about 750mW. A 1W resistor is 75 cents at quantity and power rating doesn't have much margin. Conversely, a 68mohm, with 0.5% accuracy and 0.25W rating in an 0805 rating is about 4.3 cents.
4) C14 is really strange. I don't get that one either. Maybe slowing the switch edges to meet radiated emissions? It's a power hit, and may be a sign of a bigger issue.

Do you have a pcb?
 

tindel

Joined Sep 16, 2012
716
Another thought on C14 - could be a path for the switching transistor charge current to return to the 28V supply without having to go back all the way to output capacitor. Minimizing the loop area.
 
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