DC-DC converters are not a "dead short across the input for part of the cycle" in normal operation - rather the voltage is across the inductor. If the switch stays on too long and the inductor reaches its saturation current, or one of the many other (cascading) failure modes, then can you end up with effectively a short across the input. This can happen to many kinds of electronics (eg a simple tantalum decoupling cap, or an IC's SCR latchup), but designing the power topology is a good place to think about these failure modes.
(Although going 5V->120V with USB as the power source, I can understand how "dead short" was a decent intuition)
> DC-DC converters are not a "dead short across the input for part of the cycle" in normal operation - rather the voltage is across the inductor. If the switch stays on too long and the inductor reaches its saturation current, or one of the many other (cascading) failure modes, then can you end up with effectively a short across the input.
Right. Which is why under-designed AC-line powered power supplies can catch fire.
The failure mode of MOSFETS is usually to the "on" state, so the switch staying on is quite possible.
Sure, I don't see how FETs having a common failure mode of passively conducting is exceptional though? You've got to design (and test) for likely failure modes. Like even if you use a linear regular, you should be thinking about what happens when there is a short downstream.
The problem with fires isn't the type of circuit per se, but rather that anything connected to the mains can unleash a significant amount of power. And that designing/building for safety increases cost, for additional components or even things like larger PCB area for creepage. We basically take a lot of the work that goes into electrical safety for granted.
(Although going 5V->120V with USB as the power source, I can understand how "dead short" was a decent intuition)