A power supply circuit for a device that runs off the 12V battery of a car, is shown below.
LM7805 is used to generate a 5V supply for the circuit - max load current is 200mA. A forward diode and a TVS diode are used for negative/positive spike protection (load dump). And a 10R, 2W resistor is used to limit the inrush current due to the 470uF cap at the input of LM7805. To prevent tripping during load dump or inrush current events, I am using a 3A slow-blow fuse (1.27 A2s).
One normal current overload situation is inrush current due to the 470uF cap (inrush current = 1.2A but might be 4x that if a huge positive spike occurs on start up). How do I calculate the inrush current pulse duration?
Another normal current overload situation is load dump wherein, as per ISO-7637-2 pulse #5, the pulse duration can be as much as ~500ms (Vp ~ 100V). Vc for the TVS diode I am using is 30V so the circuit could potentially see a current pulse of (100-30)/10 = 7A for 500ms due to load dump. Is my calculation correct?
3.Considering the above, I am using a 3A slow-blow fuse but this might actually be useless considering that LM7805 should prevent more than 1.5A being drawn through the circuit and if LM7805 input node gets shorted, then a maximum of 12V/10R = 1.2A (nominal) would flow through the circuit. This 1.2A would be sufficient to burn out resistor & is therefore a concern. However, if I use a lower current rating for the fuse (say 1 or 1.5A), it might blow up during either of the 2 normal current overload situations above. Plus if I take derating factor of 0.5x due to thermal cycling and temperature, a 1 or 1.5A fuse might blow that much more easily couple of months down the line. Also, I don't think I can increase the resistor value much - the max voltage drop I can afford across it is ~3V (min LM7805 i/p voltage = 7V and there will be voltage drop across the FWD diode and fuse in addition to R) which for a 200mA peak load current, means 15 ohm max.What would be the optimal tradeoff here with respect to selecting the fuse?
- What sort of situations could actually result in a short to GND at any of the nodes connected to 7805 i/p? i.e., assuming the device is mechanically robust, should I still be concerned about a potential short to GND there?