I have a device with a supply voltage range requirement 12-24AC. The B80S bridge rectifier I am using recommends a 3.5 Ohm series resistor to limit inrush to acceptable levels. Initially I decided to go for 2 x 2.7 Ohm in 0805 (metric) package. While testing the prototype it rarely happened that one of those resistors failed when using 24VAC.
I am somewhat unsure about the single pulse peak power rating that is required for the resistors and would like some feedback.
About 400uF of bulk capacitance (calculated with respect to my load requirements) are used to smooth out the AC before hitting a 5V switching regulator.
A quick estimation, ignoring parasitics like AC transformer resistance, inductance and capacitor ESR:
At 24VAC and an assumed no load factor of 1.3 for the transformer I expect a peak voltage of
U = 24V * SQRT(2) * 1.3 = 44V
With 2 x 2,7 = 5.4 Ohm and completely discharged capacitors this leads to an inrush current of about
I = 44V x 5.4 = 8.15A.
So the dissipated peak power on the resistor would be
P = 44V / 2 * 8.15 = 180W
Having 400uF load capacitance, the pulse duration would be about
t = 5 * R *C = 5 * 5.4 Ohm * 400uF = 10ms
So I would need a 2.7 Ohm resistor with a maximum pulse dissipation of 180W for 10ms.
Maybe this is a somewhat unrealistic estimation since the parasitics will work in my favor, so I performed a simulation:
While this looks "better", this would still lead to a pulse load requirement of about 90W for 3ms. I don't know if 500uH as transformer inductance are realistic though. Since the customers will used totally different supplies this will vary anyway.
Looking at some Pulse Proof Thick Film Chip Resistor data sheets:
I would have to select at least a 2010 resistor type.
Does all this sound/look reasonable or am I way off?