Electronic – TVS diode dissipation help

The TVSs on the inputs makes sense and your layout looks reasonable. The question is how far do you want to go to get what level of protection? It's a probability game. Just the TVSs should take care of most ESD events.

If you want to go a little farther, put a resistor in series with each input before the TVS, not between the TVS and the CPLD. This gives the TVS some minimum guaranteed impedance to work against. Going even farther, add a little capacitance accross the TVS. That will slow down the edges of really fast spikes so that the TVS can catch them effectively. It goes on and on, with each level of higher complexity decreasing the probability a damaging event will be encountered even further.

Only you know what environment these boards will be in, the cost of the extra board space, and the cost of failure.

Both solutions are not fully correct. First of all, look at the datasheet of 1.5KE series. It claims that 1.5KE39 has nominal breakdown voltage \$V_{br}\$ of 39V, but don't forget about accuracy and possible breakdown voltage in 37V to 42V range (7% accuracy). In practice breakdown voltage of TVS never equals to its nominal, I've checked that many times.

First diode (Z2033) has nominal \$V_{br}\$ of 33V (29.7 to 36.3V), so less than Your original one. The best choice from VRD series is Z2039 with 39V nominal breakdown voltage.

If you combine two 1.5KE18 (nominal \$V_{br}=18V\$) in series, theoretically you will have 36V breakdown voltage. But due to accuracy of every diode, the breakdown voltage will be somewhere between 34V and 38V - it's still too little. You should combine two 1.5KE20, then range of the breakdown voltage will be 39V to 42V.