Good Day,

I opened up a 220 Volts AC to 5 Volts DC SMPS power supply module. There was no DC output, the Power Schottky rectifier is shorted and damaged. Here are its details:

STMicroelectronics 45 V 40 A Diode Schottky 3-Pin TO-247 STPS4045CW

I wish to know if I can replace it with the following:

SBDF4045LCTB-JSCJ-45V 40A TO-220F Schottky Diodes ROHS

because the latter is less expensive and easily available.

Please advice if this can be done.

Thanks.

 

Electrically, both are dual common-cathode Schottky rectifiers with the same 45 V, 2 × 20 A rating and pin order:

1. Anode
2. Common cathode
3. Anode

However, it is not a true drop-in replacement. The original TO-247 device has better thermal performance and a 175 C maximum junction temperature, while the insulated TO-220F replacement has higher junction-to-case thermal resistance and a 150 C limit. It may overheat at the same load, despite having the same advertised current rating.

It could work if the supply operates well below 40 A and the replacement is mounted correctly to an adequate heatsink, but I would prefer another TO-247 part with equal or better voltage, current, forward-drop and thermal ratings. Also check the output capacitors and load for shorts before powering it again, as the failed diode may be a symptom rather than the only fault. Use appropriate isolation precautions because the primary side carries lethal mains voltage. You should check this SMPS power supply board: https://www.pcbway.com/project/shar...sign_for_Different_Applications_cfd0a641.html

 

However, it is not a true drop-in replacement. The original TO-247 device has better thermal performance and a 175 C maximum junction temperature, while the insulated TO-220F replacement has higher junction-to-case thermal resistance and a 150 C limit. It may overheat at the same load, despite having the same advertised current rating.

Ok then I will go with the original one. Thanks for the information. That is not mounted on a heat sink and there is no cooling fan as well. So it would overheat. The current rating of the module is 20 A and I would only use max 10 A so even though current is lesser I suppose the overheating problem is there.

 

Yes, the original TO-247 part is the safer choice. Even at 10 A, the diode can dissipate several watts because of its forward-voltage drop, and without a heatsink its junction temperature may still rise considerably.

Also check the output capacitors and load for shorts or excessive ripple current before fitting the replacement, the failed rectifier may be a symptom rather than the original fault.

 

Is it possible to add a small, black heatsink to that diode (?), as it appears that it likely will overheat even at 10A of current.

 

Is it possible to add a small, black heatsink to that diode (?), as it appears that it likely will overheat even at 10A of current.

Yes that could be the case. Thanks for pointing it out I will try to add heat sink and operate the module. Initially I thought it was of 20 A rating so that should be ok to draw 10 A current from it. That damaged the rectifier. Possible cause could be overheating.

 

I will try to add heat sink

The dissipation at 10A is likely about 4W, so you would want a black-anodized heat-sink that has a maximum thermal resistance of about 20°C/W in still air.

 

Years ago I had the lazyness to calculate the power a single resistor would dissipate and thought an educated guess would do. Tried a 0.25 W and it fried right away. From that day on, I calculate, even if in my mind. Thinking the circuit can handle is deceiving sometimes.

 

I thought I could give in my input after trying to repair that SMPS I tried replacing the rectifier with heat sink and replacing two other MOSFETs. Still something else is also damaged due to this overheating problem and no cooling fan is provided. Nevertheless I found repairing it and modifying it will be more time consuming and expensive than the SMPS itself.

I ordered a desktop computer CPU SMPS that matched my power requirements and it has three outputs 3.3V, 5V and 12V, very useful for my application, I only have to short a green and black wire to turn it ON. Regret not getting this idea first. Its working perfectly stable and fine.