Voltage Reference burns out on TNY267 design.
Hello, we have a problem. About 3% of power supplies get voltage reference D8 burnt out.
I'm attaching schematics of it. We added Capacitor C16 to improve Conducted Emissions. May be problem is there. One leg of C16 is very close to D8.
Second possibility is C21 being too large of a value, may need to be decreased to 0.1uF.
In any case problem appeared once we introduced C16 into the circuit. Removing C16, makes too much noise in conducted emissions. I'm taeting smaller value of C16 but the oproblem is we need a lot of units run for too long to be able to tell if there is any difference in performance. Anyone seen something like this?
Thanks
Comments
PI-Worf.
Sorry, I didn't know image got resized, I attached PDF.
The capacitor of the diode in the snubber circuit was suggested by PTCRB (cellmodem), supposedly it is easier to pass certification for EMI, I argued with them but left it there. They made us put capacitor across each diode in power supply. Didn't seem like it mattered, may be it does?
Other questions from above you can see on schematic.
Thanks for help.
Every time U3 turns ON, It has to conduct the current that comes from the extra capacitance you are placing. So I think you are having a huge current spike into U3 at On time. I t may not the cause D8 is getting damaged but I think you need to check your DRAIN current at U3 to make sure you are not putting extra stress on it. About D8. I think you need to reduce C21 to about 100nf.
Measurements need to be done at the cathode of D8 when you start up the power supply to see if the transient voltage is exceeding its rated voltage
Thanks PI. I will try to decrease C21. How would you recommend measuring current on U3? Resistor in series?
Thanks
depends on your resources to take measurements. If you have acurrent probe then you can use it. If not, just add a sensing resistor in series with the reference
I am sorry I can not see your schematic. Please post a larger file with good resolution.
In the main time I will assume that you mean D8 as the TL431...am I right?
What is the value of the resistor to you are connecting from the VOUT to the cathode of the TL431?
What is the value of your output voltage?
I can not see what capacitor is C16
I see you have a capacitor in parallel with the blocking diode in the DRAIN snuber circuit. What is this for? When you turn the controller ON, this capacitor will be a short circuit and a very large current will flow to the controller.
Anyway, Please post a better file to be able to see your schematic