Con...fused?

[Jalca]

I am trying to better understand fusing in a linear PSU. I understand the need for an appropriately sized fuse on the primary. This fuse seems universally accepted as a requirement and makes sense. However, I have found that there are then differing views on fuses on the secondary side and what the role of a fuse is.

Most views seem to be that there should be fusing on the DC side but differ on where in the DC rails they should be. Some advocate for just after the first smoothing cap and others at the output.

Yet I have spoken with those that say that a primary side fuse is all that is needed – any short on the secondary will cause the primary fuse to blow. Additionally, they say the electronics after the PSU (e.g. transistors in an amp) are going to fail before any fuse blows. Given how wide the tolerance on fuses are it make sense that the trying to match fuses to blow before damaging circuits is likely asking for headaches.

I’ve also heard different reasons as to what the job of a fuse actually is:

-   fuses only protect upstream (e.g. the house wiring)
-   fuses only protect downstream (e.g. the circuit in front of the fuse, where the fault is)
-   fuses protect upstream and downstream

Logic tells me that I would want a fuse to protect the house from burning down due the house wiring overheating (upstream protection) as well as protecting the house from burning down due to the PSU or its attached electronics overheating / bursting into flames.

So my questions are:

1.   Must there be fuses on the DC rails?
2.   If so, where on the rails?
3.   If only a fuse on the primary is used, what happens (worst case) if there is a short on the secondary in:
     a.   The PSU itself; or
     b.   An attached device such as power amplifier?
4.   What exactly are fuses meant to protect?

Many thanks for any help!

[bdunham7]

1.   Must there be fuses on the DC rails?
2.   If so, where on the rails?
3.   If only a fuse on the primary is used, what happens (worst case) if there is a short on the secondary in:
     a.   The PSU itself; or
     b.   An attached device such as power amplifier?
4.   What exactly are fuses meant to protect?

For 1,2 and 4, it depends on the configuration of your linear PSU and what it is used for.  For example, a lab supply will have different answers than the PSU section of an integrated audio amplifier--perhaps the lab supply needs to be protected against reverse polarity or backfeeding power so they need to be right at the outputs, but the audio amplifier is designed to protect the loudspeakers if the power amp section goes rogue so it will have fuses between the filter caps and the power amp section.

For 3, assuming everything is designed reasonably well and the correct fuse is installed, the answer is that the primary fuse should be sized to prevent any catastrophic damage or fires.  For many designs, just a primary fuse is adequate.  Of course, installing the wrong fuse or bypassing it may be problematic, so in some consumer goods they may add internal protection that shuts the device down more or less permanently. 

[Doctorandus_P]

The main use of fuses is to prevent fires during overload, short circuit and malfunctions.
They are usually far to slow to protect electronics.

With the old fashioned chunk of iron transformers using only a primary fuse works quite well. If there is an overload on the secondary side, it pulls the iron core into saturation, which causes the primary to draw too much current and the fuse blows.

[nightfire]

The definition of a fuse is to protect equipment against overcurrent and the following damages, which usually can result in fire and nasty smoke from burning insulators like rubber or PVC.

There are several types of fuses tailored towards the equipment, that has to be protected.
The most common types of fuses we see in our daily life are fuses (and breakers) that are intended to protect the cabling/wiring.
In the distribution box coming from the main distribution/street transformer are fuses or breakers that are suited for a certain kind of two kinds of overcurrent situation:
- slight overload, like too many devices pulling energy at the same time on the same outlet
- short circuit somewhere, so a sharp spike of current has to be dealt with

The energy needed to trip the breaker/fuse is coordinated with the cable (diameter), so the electrician does usually not need to calculate complicated stuff.
In germany, where VDE standards take place, usually it is something like: 1.5 mm² will be able to carry 16A, so use a fuse 16A gG, or a breaker 16A characteristic B- if there are lots of cables in the same duct, or in an isolated wand, some reduction factors apply.

Usually fuses  should be "coordinated"- this means, that when they should act, only the fuse that is next to the problem should act.
Fuses (and breakers) also have a maximum breaking capacity, so when a short-circuit situation arises and a small fuse is not able to really switch off in time, the next upstream fuse will also blow.

This said, a fuse on the primary of a PSU usually will protect against slight overload. In a short-circuit situation I would suspect that the fuse/breaker of the wall socket/circuit also has a good chance to act, depending on the short-circuit-current produced. (So called "backup protection")

For protection of sensitive (and expensive) semiconductors, there are specialized fuses- fast acting and able to take high short circuit currents- in germany there is the aR/gR line of fuses for mains distribution, and for parts within appliances usually fuses class "FF" would come to mind, that are able to break DC currents without problems.
In this case, you would have to look for your sensitive portions of the device, have a look at the short circuit currents which could occur at the problematic points and have a look how fast those current would overheat your part.
From this value (current and time) you calculate the integral value I²t in A²s, and look into the datasheet of the fuse in question- and the voltage drop of the fuse...
Then you look if this would help your circuit or if protection can be achieved in other regards.