Actual Tech Question -MP24943DN switching regulator output shorts

When you say that pin 8 "shorts to gnd" do you mean that part breaks ? Lets its smoke out ?

If not, that switch node will usually go to zero volts when the input transistor inside the chip turns off when there is current in the inductor. Actually, the SW node voltage will go slightly negative, to the forward drop voltage of the diode drop.

It looks like a buck converter.

Yes, resistance essentially reaches zero ohms Pin 8 to common/GND and we lifted the leg (P8) to confirm short.

I think you meant a f..ked converter!

John :-#)#

IF for some reason you needed to adjust the loop gain, you could play with R4, There are fixed Feedback components inside around the Error amp.

I've seen these things butchered from the origanl design notes in those 12v car ligher plug chargers. Like a high school project gone wrong. They work better when wired up properly.

Cheers

What's your maximum load? The chip says it can do 3A, are you using it all the away at 3A, no safety margin? The catch diode will be on most of the time. At 3A it drops about 0.55 volts, or 1.6 watts. If it doesn't have a small clip-on heat sink, its junction may be running pretty hot, and eventually fail, as a short. Shortly after it shorts, it'll take out the switcher.

Hi Win,

Thanks, that may well be it. There is no heat sink on that diode.

I will monitor the temperature of the catch/Schottky diode as see how high it gets in normal use as well as monitoring the load current.This circuit is used in a commercial pinball game for the networked driver boards

Much appreciated!

John :-#)#

Networked driver boards, what's that?

If there's enough space next to the TO-220 diode, the upgrade will be very easy, just slide on a heatsink.

My go-to heatsink is the Wakefield 233-60AB. It takes about 0.5-inch square out from the body of the TO-220, and at 1.6 watts, the heatsink temp rises only 45 deg. The MBR760 diode junction is another 8 deg = 103 C, if the box ambient is say 50C. It should last forever.

They are using their own twisted pair network in the game - much like automobile CAN (Controller Area Network) - to communicate with all the switch, lamp (LED), and solenoid driver boards scattered about.

Yes, this should be easy to install.

Mouser has those under $0.40USD each, much appreciated!

John :-#)#

Not sure about that. The thermal for his package should be similar to this one, which they list as 62.5oC/W RsubJ-A:

Your heat sink buys less than 20oC/W margin, which should put him at about 8x increase in MTBF compared to without it.

heatsink is 30oC/W? Makes that 32x increase MTBF. Maybe forever depends on what he has now.

Currently forever is around one to five years...

John ;-#)#

Okay, heat sink will make it forever. The original design almost got there, they must have underestimated operating ambient air temp.

The pin short and the diode short shouldn't be found in the same unit. A dry solder joint in one might explain the other.

Sync buck converters may ( repeat: 'may') have undocumented behavior at shut-down that produces an abnormally long conduction interval in the internal shunt mosfet (ie pin8 to ground). If the system load has a lot of capacitance, the abnormal conduction period can cause reverse current in the output filter choke. When the switch turns off, you get a load dump effect, with voltages limited by the node Z under these conditions. The current can be insanely large w/r to normal operating conditions.

The amount of energy absorbed may not be 'much', though it scales to LxIp^2, but neither diode nor chip are designed to do that. Your schematic has a TVS on input that may be too high a clamping voltage. An external diode in parallel with the upper switch might do something.

You could spend the rset of your life trying to capture this event on a scope, if a particular unit will oblige - temperature and conditions of use may dominate both behavior and results, so keeping things cool can't hurt.

RL

The slow start capacitor size has also been known to influence chip behavior during shutdown, as well as start-up.

RL

The diode is a through hole device, so not much chance of dry solder joint there...

The typical load for this 6V3A supply are a string of colour LEDs. I don't think they have any back EMF potential. However your suggestion is interesting. I do recall linear supplies with a back diode from the output to the input, when there were large capacitors on the load side, to avoid this problem.

Good suggestions for further research on the shut-down side.

Thanks!

John :-#)#

Testing two of these TO-220 diodes after running all day in the game they run at least 50C with ambient temp of 24C. The derating curve doesn't cut in until the device reaches about 105C. I will put a thermocouple on the tab and let it run for a day and see how high it gets when I do not have the cabinet open to do a temp test.

John :-#)#