Using an oscilloscope to test SMPS controller IC signals
There are some options for you. You probably have a circuit like this:







simulate this circuit &ndash; Schematic created using CircuitLab

Your problem here is that the probe reference of the scope is tied to the mains ground. When you touch the probe reference in the circuit reference/ground, its ground is actually floating between the mains phase and neutral, depending on the time. So, you are connecting the mains phase/neutral to the mains ground and the fuse goes Poof.... It's a short circuit.

There are some scopes with isolated inputs or differential probes witch you could use, but they are expensive.  

The best to do would be to use an isolating transformer for your circuit board, between the mains and the board. This way the circuit is protected, you can touch it and not have a discharge, and so is the scope, since it's grounded. 

Some mistakes people usually do: They try to float (isolate) the scope. This is dangerous. You will be touching the tips, and the board (witch is not isolated) and you can easily be dealing with 110Vac on the board and touch the reference of another probe and zap!. 

Another one is that the two references are connected internally at the scope, so, do not put then in different potentials on the board, otherwise you will damage one (or two) probes. If you are not familiar with scopes, the best would be to remove the reference from one of the probes. You can have more noise on this channel, but you can rest in peace.