in the circuit below what is the maximum reverse voltage around the diodes?
but I dont know how to apply it ,
more over I dont know what does the question mean by vr1=vr2=150?
so the voltage around diodes will be 150 :| .
Real world answer: 100 V on each, but be sure to choose a diode rated for substantially higher than that, say 200 V.
Answer your professor wants: A few volts more on D1 than on D2, because part of the leakage voltage through D2 can't pass through D1...therefore it must go through R1, which increases the voltage across R1 (which is the same as the voltage across D1).
Not sure what the VR1/VR2 are supposed to mean. Let's ignore that bit. Maybe it's a mistake, maybe it's supposed to be a maximum rating or something of that ilk.
If you use a large negative voltage in your formula, the '1' dominates over the exponential term so the Is will just be the leakage current through each diode. Replace each diode with an appropriate fixed current source and analyze.