MC33063A: The transistor stops switching

domenyko

Part Number: MC33063A

Dear all,

one of my AA is designing a Buck following the application info present in the datasheet.

Vin 24V, Vout 12V, Iout 300mA

The Buck has been tested connecting an active load and increasing the load current. As soon as the load current approaches 250mA the transistor stops switching and the device is damaged. After the damage the designer has measure 12ohm of resistance between CE pin of the output transistor.

Below the schematic.

Do you have any hints ? My idea is that the transistor does not have enought time to discharge itself.

regards,

Domenico

over 6 years ago

0 Jasper Li over 6 years ago

TI__Mastermind 46905 points

Hi Domenico,

Could you share me the layout of the circuit? i'm afraid the damage is caused by the voltage spike because of bad layout.

0 domenyko over 6 years ago in reply to Jasper Li

TI__Genius 9655 points

Hi Jasper,

Thanks for the hint.

below a screenshot of the layout and a screenshot of the scope when the device is still working (around 200mA output current).

Is the discontinous mode fine in this condition ?

regards,

Domenico

0 Jasper Li over 6 years ago in reply to domenyko

TI__Mastermind 46905 points

Hi Domenico,

Could you place an 1uF ceramic capacitor between the PIN 1 and Pin 4. the loop between the capacitor and the IC should be as small as possible.
this capacitor is to reduce the voltage spike of the internal NPN.

0 Jasper Li over 6 years ago in reply to Jasper Li

TI__Mastermind 46905 points

Hi Domenico,

I haven’t heard back from you, I’m assuming you were able to resolve your issue. If not, just post a reply below

0 domenyko over 6 years ago in reply to Jasper Li

TI__Genius 9655 points

Hi Jasper,

just yesterday the customer implemeted your suggestion with a manual rework on the board.

It solved the issue.

Is it possible to add this note in the DS too ?

thanks,

Domenico

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MC33063A: The transistor stops switching