MC33063A: Short circuit test failrue

James Zhan

Expert 4365 points

Part Number: MC33063A
Other Parts Discussed in Thread: TEST2

Hi,

My customer had come across a problem of short circuit test when using MC33063A for buck function.

Schematic sames as below except for two items:

Cin=200uF（electrolytic capacitor） parallel with 0.1uF（ceramic capacitor）
Add a magnetic bead between the diode and GND.

Test condition:

VIN=24VDC, Vout=5V(typical500mA)

Test1 :

Vout 5V short test, IC burn(pin 1)

Test2:

Change the Rsc resistor from 0.33ohm to 1ohm; Vout 5V short test for 5mins, no problem.

Test3:

Remove the magnetic bead, keep the Rsc=0.33ohm; Vout 5V short test for 5mins, no problem.

Question：

Why the current limit doesn't on the Test1?
Customer will keep the magnetic bead, which is their standard circuit. How to pass the S/C test under this condion? What could we do to achieve that?

If you need any more information, please let me know. Looking forward to your reply. Thanks.

over 5 years ago

0 Helen chen over 5 years ago

TI__Mastermind 40080 points

Hi Jams,

When use 0.33ohm resistor, the short circuit current into the pin1 is higher than the maximum current capability, so pin1 burn. Could you use higher resistor?

0 James Zhan over 5 years ago in reply to Helen chen

TI__Expert 4365 points

Hi Helen,

According to the design procedure, Ipk= 2 Ioutmax(0.5A), and Rsc=0.3/Ipk. When Ipk set to 1A, and Rsc=0.33ohm. Is this beyond the current limit?

0 Jasper Li over 5 years ago in reply to James Zhan

TI__Mastermind 46905 points

Hi James,

It seem the IC is damaged by voltage spike in pin 1 or longer switching time because of the bead. The layout is also critical here. the loop between the 0.1uF and the IC must be very small. could you share the image of the layout?

why customer need the bead?

0 James Zhan over 5 years ago in reply to Jasper Li

TI__Expert 4365 points

Hi Jasper,

I've told the customer to remove the bead, ant the short circuit test is good. But they want to learn about why this bead has such a great effect?

Could you please help to explain? or do you have some document for this ?

Thanks.

0 Jasper Li over 5 years ago in reply to James Zhan

TI__Mastermind 46905 points

Hi James,

I think it is caused by the voltage spike or the large current at pin. When the high side NPN try to shutdown, the bead in series with the diode prevent the current transferring from the NPN to the diode quickly. this cause high energy consumption in the pin 1 or high voltage spike in the pin 1.

we can measure the voltage spike at pin 1 with and without the bead to confirm this. However, this measurement is not easy, as the IC could be damaged.

0 James Zhan over 5 years ago in reply to Jasper Li

TI__Expert 4365 points

Hi Jasper,

Thanks for your information.

I've told my customer to remove the bead and they have pass the short circuit test.

Thanks again!

James

Power management

Power management forum

MC33063A: Short circuit test failrue