Electronic – Is a 9.6µs over-voltage transient long enough to fry the circuit (Details inside)

automotivecircuit-protectiontransient-suppressiontvs

I am designing a board for a 12V automotive environment, with a variety of input voltage transients including load dump and permanent 24V over-voltage. I am using a TVS at the input to clamp to around 60V followed by a TI LM5060, a power supply supervisor, which in case of an over-voltage above 18V (or overcurrent) will cut off the power to my circuit using an external 60V MOSFET. The LM5060 datasheet specifies a 9.6 microsecond delay from the onset of a over-voltage condition to the MOSFET gate turning off. On the other side of the LM5060 I have my bulk capacitor and number of linear regulators and other ICs which at their input can tolerate up to around 30V (or 45V depending on what parts I choose).

So my question is, during that 9.6µs it takes for LM5060 to kick in, how can I ensure that nothing on its down-side gets fried? Is a sufficiently large bulk capacitor (220uF?) enough to absorb that transient? Or would I be better off using a small zener? How would I choose a value for this zener/capacitor?

If anyone knows of a better/cheaper alternative to LM5060, please let me know. I only need it to provide over-voltage protection up to around 60V.

Best Answer

Just throwing this out there: Typical FET response is measured in nanoseconds.

schematic

simulate this circuit – Schematic created using CircuitLab

Vshutdown > VzZD1 + VgthFET1

If VzZD1 = 50V and VgthFET1 = 3V then Vshutdown > 53V

As shown, FET1 may (will try to) destroy itself to protect the load. Determine resistor sizes as follows:

R1 protects FET1  
(R2 > R1) ensures lower-impedance path through FET than load

Size R2 according to Ohm's law with Vout "grounded":

E = I * R
Edrop = Vin - Vout = 12V - 0V = 12V
R = Edrop / I
R = 12V / 1.4A = 8.57ohm

If Vzd1 = 12V and VgthFET1 = 3V, then Vshutoff = 15V:

Ir2 = E / R
Ir2 = 15V / 8.57ohm = 1.75A

FET1 will pass Vin - Vzd1 - VgthFET1 when active.
If R1 = 0 and Vin = 50V: 

VdsFET1 = 50V - 12V - 3V = 35V
I = E / R
I = 35V / 25mOhm = 1400A //Zap!

If R1 = 4 ohm ("less" than R2) and Vin = 50V:

VdsFET1 = 50V - 12V - 3V = 35V
I = E / R
I = 35V / (4 + 25mOhm) = 8.70A
EdropR1 = 8.70A * 4Ohm = 34.8V
EdropFET = 8.70A * 0.025Ohm = 0.2V
P = I * E
PFET = 8.70A * 0.2V = 1.74W
PR1 = 8.70A * 34.8V = 302.76W //Warm
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