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[SOLVED] Purpose of adittional resistor in a voltage clamp

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CataM

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Hello everyone,

I was looking at the schematic (page 26) of a PSFB implemented by TI using the UCC28950 and I have seen two unusual things (unusual for me :roll:) which I do not understand their purpose.
Both of them are located at the output stage, after the SRs.

1st unusual thing: Why is there R8//R9 connected between the clamped voltage and the output voltage ?
2nd unusual thing: I do not see the purpose of C13 connected between the output stage and the input stage since there is no current flowing thorough it (it has no return path for the current).

Any comment is much appreciated !

Thank you for your time !
 

Hello CataM,
(1) R8 and R9 are connected to give an overall resistance of 500 ohms. Perhaps it is needed for accuracy and/or current sharing.
(2) C13 is there for further isolation between the hot and cold side of the PSU and should be rated to at least 2kV.
Though others would be able to explain this in further detail. But that is what I can tell about those components.
Regards,
Relayer
 
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    CataM

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Discharging the capacitor to Vout instead of ground recovers part of the stored energy.

C13 is the usual Y-capacitor found in any isolated power converter. It's draining common mode noise current generated by capacitive coupling of the transformer windings.
 
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    CataM

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It's draining common mode noise current generated by capacitive coupling of the transformer windings.
Then I guess it should be higher than the "parasitic" capacitor between the primary and secondary windings of the transformer in order to provide a low impedance path for the common mode noise, isn't it ?
 

It forms a capacitive voltage divider, depending on the acceptable common mode noise level, it must be much higher than the interwinding capacitance.
 
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    CataM

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I have just noticed another unusual thing that went unnoticed the first time I watched that schematic.

It is the series connection of R4, R14 and R17 (each 1 MΩ) connected in parallel with the input capacitor.
The first thing that came to my mind was that their purpose is to give a discharge path for the energy of the input caps and other elements that store energy in the circuit when the circuit is powered off, but that would take too long... (330 uF in parallel with 3 MΩ takes about 990 seconds to discharge...) , so I discard that option.

Do anyone know what could be their purpose ?
 
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We have to consider that the schematic is incomplete, e.g. not showing the dc power input and control supply. But as shown, capacitor discharge is the only plausible purpose of the 3x1M resistors. Even if the time constant is rather large, it serves a purpose.

It's generally required that a warning notice about charged capacitors is printed in the operator manual and also on the enclosure. The time until the capacitor voltage falls below 60 V should be mentioned. Without a defined discharge path, the time constant is infinite.

A discharge circuit with a preferred low time constant would either have a huge power dissipation or require an active switch.
 
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    CataM

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Discharging the capacitor to Vout instead of ground recovers part of the stored energy.
R6 can be removed then, right ? Or is it there for some other reason I do not see ?
 

I mean, R6 must be used in general RCD clamp, but if one employs R8//R9 connected to the output, one can manage without R6 since it would only add additional losses and the voltage would still be clamped.

I am not saying this regarding this specific application, I am asking in general, since I do not see any advantage of having both R6 and other resistor connected to the output. The resistor connected to the output implicitly does the job of the former resistor (resistor used in the usual RCD clamp).
 

I've used a very similar energy-recovery snubber in one of my designs. But in my case, I used it to generate an additional lower current bias voltage.
Here, they are just dumping the snubber energy into the output, which may increase efficiency a little bit.

One has to remember that application circuits are generic platforms only, which have not been fully optimized.
It is expected, as part of the engineer's learning curve, that he/she will play around with the circuit and make modifications, and eventually tailor the circuit to his/her particular application and expected materials cost.

One thing I've found for instance, is that Mosfets in eval boards are not optimized for cost. So one of the very first things I do, is to replace the Mosfets with others which I've chosen, and compare the performance results.

So I would encourage you to remove R6 and see what happens.
 
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