Fuse before DC/DC converter

[Nikos A.]

Hi everyone,

I design a PCB and I am going to use a dc/dc converter with output voltage of 5V and max output current of 1000mA (5W output power).
I need to protect the circuit by adding a fuse before the converter. The converter will supplied with 5vdc

I calculate that the input current is 5W/12V = 416.7mA and intend to use a slow blow fuse of 500mA.

Is it correct my approach?

Thanks in advance

[Nusa]

Probably a bit on the small size, especially if ambient is high and the enclosure is even higher. For a rule of thumb, I'd multiply by 1.25 and then go to the next highest value available. So 417*1.25 = 521 --> 600mA. Remember that fuse ratings are at about 25C, and electronics enclosures usually run hotter than ambient. Temperature affects the actual fuse rating. Google "electronics fuse sizing" if you want more factors to consider. Plenty of good information out there.

Note that protecting the circuit is not the primary function of a fuse. A fuse is to prevent a larger disaster, such an overloaded circuit turning into an actual fire or literal meltdown.

[Zero999]

Hi everyone,

I design a PCB and I am going to use a dc/dc converter with output voltage of 5V and max output current of 1000mA (5W output power).
I need to protect the circuit by adding a fuse before the converter. The converter will supplied with 5vdc

I calculate that the input current is 5W/12V = 416.7mA and intend to use a slow blow fuse of 500mA.

Is it correct my approach?

Thanks in advance

Don't you mean the converter will be supplied with 12V, not 5V?

Anyway, the fuse will not protect the transistors inside the DC:DC converter, but the wires/PCB traces on the primary side. The fuse can easily be rated to double the nominal input current. I would consider a 1A fuse, especially if the 12V could be lower than that.

The transistors in the DC:DC converter need to be protected by the controller reducing the duty cycle.

You should also consider some over-voltage protection on the secondary side, if there's a risk the controller or output transistor could fail in a way which would increase the output voltage, for example the switching transistor failing short circuit in a buck converter.

[Nikos A.]

Thank you for your replies!!

First of all I want the fuse to protect traces and avoid "catastrophic" results in case of overload.

Don't you mean the converter will be supplied with 12V, not 5V?

Yes, the converter will by supplied with 12V, sorry for the mistyping.

You should also consider some over-voltage protection on the secondary side

Do you have to suggest any over-voltage protection controller?

Furthermore, I should use fast or slow blow concerning the fuse?

[nctnico]

IMHO a TVS should do the trick to deal with an overload situation.
When it comes to fuses: a fuse is a device to prevent fire. It is not intended to safe electronic components. All in all the fuse rating should be less than the maximum current the PCB traces can handle.

[Zero999]

A crowbar is probably a good idea to protect against the output transistor failing short circuit. It's just an SCR and zener diode. When the zener conducts, the SCR is triggered, which short circuits the power supply. The capacitor filters out any spikes which could cause nuisance tripping.

[Nikos A.]

Thank you guys!!!

[aju11]

You must consider the efficiency of your DC-DC converter in the calculation of current.

For example if the  efficiency = 85%
Then the 5W Load would draw  = 5W/(12 x 0.85) = 0.49A

Also note that there would be inrush current due to bulk capacitors (if any).