VVR Design - comments and opinions welcome

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strelok
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Re: VVR Design - comments and opinions welcome

Post by strelok »

Correct. I don't think R9 at 1 watt will be too much of an issue. Typically most bias supplies have large dropping resistors that would effectively limit the current before r9 could exceed its rated value. Additonally the caps are usually so small that inrush current would also not be an issue. However we could probably up it to a 2 watt device to be on the safe side, since metal film/oxide resistors have gotten so small lately that a 2 watt device is now roughly the same size as the old 1 watts.
Ten Over
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Re: VVR Design - comments and opinions welcome

Post by Ten Over »

strelok wrote: Tue Apr 25, 2017 1:10 am Correct. I don't think R9 at 1 watt will be too much of an issue. Typically most bias supplies have large dropping resistors that would effectively limit the current before r9 could exceed its rated value. Additonally the caps are usually so small that inrush current would also not be an issue. However we could probably up it to a 2 watt device to be on the safe side, since metal film/oxide resistors have gotten so small lately that a 2 watt device is now roughly the same size as the old 1 watts.
It occurred to me that there is no reason to have a 10 Ohm resistor at R9 unless there is the possibility of enough current flowing through it to have a voltage drop sufficient to cause a 12V zener to break over. If you happen to be talented enough to design a circuit that could run 700mA through a 10 Ohm resistor, you better use at least a 5W one.
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roberto
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Re: VVR Design - comments and opinions welcome

Post by roberto »

Ten Over wrote: Mon Apr 24, 2017 7:21 pm R2 and R9 are parts of current limiting circuits. They set the maximum currents through the MOSFETs when the filter capacitors are initially charged or when there is a catastrophic failure of some sort.
Yes, but adding a BJT will permit to use way smaller resistors (the same 700 mA are obtained with a 1 Ω resistor, so 1/10th of the power will be dissipated).
strelok
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Joined: Wed Mar 26, 2014 5:46 pm

Re: VVR Design - comments and opinions welcome

Post by strelok »

Ten Over wrote: Tue Apr 25, 2017 4:12 am
strelok wrote: Tue Apr 25, 2017 1:10 am Correct. I don't think R9 at 1 watt will be too much of an issue. Typically most bias supplies have large dropping resistors that would effectively limit the current before r9 could exceed its rated value. Additonally the caps are usually so small that inrush current would also not be an issue. However we could probably up it to a 2 watt device to be on the safe side, since metal film/oxide resistors have gotten so small lately that a 2 watt device is now roughly the same size as the old 1 watts.
It occurred to me that there is no reason to have a 10 Ohm resistor at R9 unless there is the possibility of enough current flowing through it to have a voltage drop sufficient to cause a 12V zener to break over. If you happen to be talented enough to design a circuit that could run 700mA through a 10 Ohm resistor, you better use at least a 5W one.
That's a good point, I hadn't thought of that. Maybe we could go to a large value resistor or maybe better would be a lower value zener. That way it would actually have a chance at protecting the circuit.
roberto wrote: Tue Apr 25, 2017 8:15 am
Ten Over wrote: Mon Apr 24, 2017 7:21 pm R2 and R9 are parts of current limiting circuits. They set the maximum currents through the MOSFETs when the filter capacitors are initially charged or when there is a catastrophic failure of some sort.
Yes, but adding a BJT will permit to use way smaller resistors (the same 700 mA are obtained with a 1 Ω resistor, so 1/10th of the power will be dissipated).
Not a bad idea for sure. Though I think at this point we're just trying to keep things as simple as possible. Would be a good option should we decide we want to shrink the size of the board down a a bit.
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