KSGER T12 2.1s soldering station - small explosion, help needed to repair

[tszaboo]

I remember when I said that the lack of protection on these is disturbing.

Thanks for the schematic. I cannot believe I even considered buying this. Not even a fuse on the output or on the input. if you micro locks up, the iron becomes hotter than the sun. or if ESD kills Q2 gate. Not to mention all the routing and assembly problems. What a piece of junk.

And I got wall of text messages telling me that I expect too much and its actually fine the way it is. 

[kind3r]

My first attempt after replacing R1, U2, Q3 and the 22uF cap went ... not so well: https://youtu.be/-eFd1yWiqpU 
The result was blown R1 and onboard fuse.

So I started looking around and found that D6 and R6 were also blown, replaced them and we're back in business !

Now just need to find some way to replace the stupid THT fuse.

[DavidAlfa]

Always use a 50-100W bulb in series!! 

Pu the battery far as you can from the heat, the life span on these willshorten a lot.
Glueing it on top of the transformer is not the smartest thing!

[kind3r]

Indeed, moved the battery, prepared the case for grounding and also chopped the corner of the radiator that was resting over the PCB trace. Just waiting for the fuse and 1M resistors 

[Ian.M]

The sequence of events when this topology of SMPS blows is:

• For whatever reason the switching MOSFET gets overstressed and blows. As it does so it shorts drain to source and gate.
• The current sense resistor between the MOSFET source and primary side common (negative side of reservoir capacitor) fails open circuit.
• The controller chip gets massive overvoltage (>300V) on its gate drive and current sense pins via the gate and current sense coupling resistors.
• All sorts of parts round the controller chip and the controller chip itself object violently to the order of magnitude or more overvoltage releasing magic smoke!
• Eventually, some milliseconds later, the fuse or other primary side overcurrent protection blows.

After an event like that, *ALL* components round the SMPS controller IC are suspect, even if they don't appear to be damaged.  Also you still have to find and fix the original fault that caused the MOSFET to be overstressed.   That could be anything from a cracked component in the snubber network to a hard short on the transformer secondary + a bunch of other possibilities.  Some faults cause a rapid or immediate blowup, others will come back and bite you hours or even days later.

If you haven't done much SMPS repair work, https://www.repairfaq.org/sam/smpsfaq.htm may help.

The optocoupler is a possible casualty - if its blown open you can get massive overvoltage on the output, so after you've removed the carnage,  worth back-feeding the secondary side output from a bench PSU (with no primary side mains supply) and cranking it up, with a DMM on diode test across the optocoupler's phototransistor.  It should go from open to a few tens to hundreds of mV as the TL431 turns on at the output voltage setpoint.  If it doesn't do that close to the nominal output voltage, or it draws significant current from the bench PSU, you need to take a close look at the secondary side.

You can get push-on axial wire ended caps for standard 20mm fuses, to allow them to be directly soldered as through-hole components.  Check if the blown fuse has these caps - if you are lucky they can be carefully pried off and fitted to a new fuse, otherwise you'll need to buy a pair from a distributor or even Ebay.