Fuse & TVS Diode For Protection Design

[themaccabee]

HI ,,
I want to design an overvoltage + transient protection to a circuit using a fuse + unidirectional TVS diode combination.
**broken link removed**

Uploaded with ImageShack.us
The input power supply is 3V,Max current required is 2.7A

The problem is the ideal selection of the fuse & TVS diode so that the TVS diode will survivie in an unwanted input transient condition & only the fuse will blow.

I was reading the basic considerations for selecting the fuse & TVS diodes.




The fuse parameters are
1)current rating
2)volatage rating
3)I2t rating

TVS Diode parameters
1) Reverse Standoff Voltage
2) Break Down Voltage
3) Peak Pulse Current rating

The current ,voltage of the fuse could be designed easily
like wise the reverse stand off & Break down voltage of TVS can be selected easily.

When comes to the I2t rating & Peak Pulse Current rating, i dunno what to do,

Ive got the basic idea that when the transient condition occurs the TVS will conduct current largely after the break down.The maximum allowable current fllowing at this time must be limited & the fuse must blow before the TVS could be damaged.
The fuse parameter determining melting is I2t ,,how could could i determine that fuse will blow with sufficient margin protecting theTVS and rest of the circuit..

Can somebody help with this design,The datasheets of fuse and the TVS i m considering are attached.

Thanks & Regards

 

[FvM]

The question can't be answered without specifying:
- the expectable maximum input voltage and current
- the allowed overvoltage of the protected circuit
If you possibly think of a virtually unlimited input source, the design can't work without being able to assume a maximum input current respectively introducing a current limiting means. If necessary, the fuse resistance can act as such, but then the input voltage must be limited.

 

[themaccabee]

The actual transient current cannot be defined., or atleast i dunno how to define.
for the identification of the source of the threat ;(that is, whether lightning, switching, ESD or NEMP. )

I d explain the scenario where the unit is expected to serve.
the Unit is getting powered from a Linear Power Supply of 3V,3A;The unit may be plugged to the power rails alive. There is a hotswap module kept to take care of any over current event after the fuse and the TVS.But it doesnt have any over voltage shutdown facility.

So i suppose the expected transient condition is when the unit is hot plugged into the Power plane.Or any worst case event of the Linear Power Supply getting damaged/accidental mistake by giving a high voltage input.

The intended Purpose of Fuse+TVS is to limit the over voltage due to any transients to 4V (Break Down of TVS) and to Blow up the fuse if the overvoltage condition persists without damaging the TVS.

can somebody help me now.??

 

[FvM]

The actual transient current cannot be defined.

Of course it can. Every serious EMC regulation does.

Regarding your specification. Assuming 4V would be the maximum tolerable protected circuit voltage. The diode reaches already 4.1V at 1 mA current and e.g. a maximum of 10.3V at 200 A. The said high currents would be achieved with standard surge or ESD pulses, the fuse won't react on them anyway.

Looking at slower scenarios. A defective voltage regular supplies e.g. 12 V to the circuit. Two interesting variants

- the input current is above 10A, a 3 or 4A fuse will blow below 1 sec. The TVS is safe, but the voltage will reach 6.5 to 7V.

- the input current is below 6 A. The fuse will blow after minutes, the TVS is likely to be damaged. The circuit has to withstand at least 6V during this period.

I fear, your circuit won't feel comfortable with any of the sketched situations.

For sensitive circuits, a more exact voltage limiting (most likely by active electronic circuits, e.g. a good old thyristor crow bar) is obviously required.

 

[themaccabee]

Well then, now im thinking of active electronic crowbar with out any relay..can somebody tell me to find a circuit diagram to start with??

 

[blapcb]

First of all why don't you have a diode in series to your input to protect against reverse polarity.....
Anyway, check out the Transil series from ST or a resettable fuse such as Polyswitch

 

[FvM]

now im thinking of active electronic crowbar

A thyristor (SCR), a zener diode between anode and gate, a gate-cathode resistor to set a defined trigger current if the thyristor hasn't it built-in already.

A fuse is needed in addition if the input source is able to exceed the thyristor's i²t or continuous imax rating.

 

[themaccabee]

hmm

1)the diode to protect against reverse polarity will take up atleast 0.3V drop across it rite?
Input being 3V i cant afford 0.3V drop.

2)The active crow bar circuitry ..i was thinking about implementation using transistor or a mosfet..Im not sure but it seems in some of the posts i ve read about mentioned that simple crowbar with SCR and fuse will have a problem that it wont reset automatically...i mean recover from an over voltage condition occuring for a short period of time...Is that rite??

 

[FvM]

simple crowbar with SCR and fuse will have a problem that it wont reset automatically

Yes. I fear however, that only a SCR crowbar will be able to provide protection for your circuit at high input currents. But it clearly depends on the expectable overvoltage specification.