MC34063A: Technical Issue Report – MC34063A Overheating and Failure Under Load

Hello Devaraj Kannan,

I assume, you are using a circuit similar to this one: https://www.ti.com/document-viewer/MC34063A/datasheet#application_and_implementation/SLLS6364793

Is this right?

If yes, please measure the the voltage from pin 2 to pin 4 and zoom into the low level. I am interested in the forward voltage of the used diode while the device slowly increases in temperature. Please use one of the devices that heats up, but does not get damaged.

In addition, please share the lot number. This is the number on the paper box behind the word LOT which included the reels. This is a 10 digit number which indicates the lot.
I do not know what you mean with batch number.

Best regards,
Brigitte

Thank you for your response and guidance.

Yes, we are using a circuit similar to the one provided in the TI datasheet

1. Output Voltage on Defective Units

In the units exhibiting thermal failure, we observed the output voltage approximately 4.6V, 

2. Lot Numbers: RL91081L02, We understand now that what we previously referred to as the batch number is the marking found on the IC body itself, refer to the image
3. 
4. Schematic and Layout

We have attached the circuit schematic we are using for your reference. Please let us know.

Hello Devaraj Kannan,

The top side marking of the device you showed in the picture is the second and third line, so it is: 47MADX9 
The circle on the bottom right is the pin 1 marking.
You are showing just one picture, but mentioned 3 batches above. Do you have pictures of the other 2 batches as well?

Please measure the the voltage from pin 2 to pin 4 and zoom into the low level. I am interested in the forward voltage of the used diode while the device slowly increases in temperature. Please use one of the devices that heats up, but does not get damaged.

Best regards,
Brigitte

Dear Brigitte,

Please find below the waveform measurement between the PIN 2 & 4. We don't have the picture of the other 2 batches. Please advice .

Hello Devaraj,

Can you please measure the way that we can really see the switch waveform? 

Please measure the the voltage from pin 2 to pin 4 and zoom into the low level. I am interested in the forward voltage of the used diode while the device slowly increases in temperature. Please use one of the devices that heats up, but does not get damaged.

I cannot see the low level of this pin with such a wide time/div.

Best regards,
Brigitte

Hello Brigitte,

As requested, we have captured the switch waveform between Pin 2 and Pin 4 with a zoomed-in time/div setting to clearly show the low-level details.

Please find attached the oscilloscope screenshot from our measurement:

• The low-level region corresponds to the diode forward voltage.

• Frequency observed is approximately 28.35 kHz.

• The low-level voltage is approximately 180 mV (as shown in the measurement cursor), and the switching amplitude is around 17.4V, with a peak-to-peak value of 25.6V.

This measurement was taken from a device that heats up but remains operational  in line with your guidance.

Please review the attached waveform and let us know if any further details or additional measurements are required.

Thank you for your continued support.

Hello Devaraj,

Thank you very much for the measurement. The result is sadly not conclusive.

Do you think you could measure the current that is flowing in the inductor and the current that is flowing in the diode? Best while the MC34063A is already relatively hot but does not get damaged.

Another test would be using a Schottky diode that has a lower forward voltage and check if the behavior of the heating device changes with the reduced forward voltage.

Please check as well the Timing capacitor pin voltage on the failing units.

Best regards,
Brigitte

Dear Brigitte,

Thank you for your feedback.

We have carried out the requested current measurements on a unit that heats up under load but has not yet failed.

Measured (device hot but operational):

• Inductor current (average): ≈ 180 mA
• Diode current (average): ≈ 138 mA

Note: during this particular test, the device failed after approximately 25 seconds of operation and became completely non-functional (burnt).

Regarding the Schottky diode test:
We will not be able to perform this test, as a Schottky diode is not suitable for our application requirements.

Timing capacitor pin voltage on failing units:
We measured approximately 653 mV on the timing capacitor pin for the failing devices.

Hello Devaraj,

The diode you show in your schematic is a Schottky diode. Why do you mention that a Schottky diode is not suitable for your application?

Can you please share pictures of the labels on the reel package you mention for failing units?

Best regards,
Brigitte

Hello,

Brigitte said:

The diode you show in your schematic is a Schottky diode. Why do you mention that a Schottky diode is not suitable for your application?

What we meant earlier is that we cannot switch to a different Schottky diode for this test, as the one currently used is specifically selected to meet our application’s electrical and environmental requirements. Using a different diode type or specification would not be suitable for our design constraints.

The Schottky diode currently used is Vishay SS34-E3/57T. This part has been in our design since 2010, and the same design has been in continuous use without issues until the recent failures. This is not a new design.

We tried to get the reel photos of the issue batch, but those are currently not available. But SO# T05260015 is the SO number in which these IC's are brought. Earlier we have share the item marking of the IC (47MADX9).

Hi Devaraj,

Can you please check the current through R140 when the IC is hot? Or, you can also check the voltage across R140.

Best Regards,
Feng

Hello,

As requested, we have measured the current/voltage across R140 when the IC is hot:

• Voltage across R140: 22 mV

• Calculated Current through R140: ~35.5 mA

Please let us know if you would like us to take any further measurements under different conditions.

Hi Devaraj,

Sorry, I mean measuring the waveform with an oscilloscope.

Best Regards,
Feng

Dear Feng,

As requested, please find below the waveform,

Hi Devaraj,

This waveform is not helping. Please use a horizontal scale of around 10us/div.

Best Regards,
Feng

Dear Feng,

Find attached the waveform as requested, Hope this waveform is as per your request.

Hi Devaraj,

I cannot understand this waveform.

Best Regards,
Feng

Dear Feng,

Thank you for your feedback. We would like to ensure that we capture the waveform exactly as you require. Could you please clarify the following so that we can repeat the measurement accordingly:

1. Which specific pin-to-pin measurement you would like us to capture , we have captured the waveform between resistor(R140) output pin and ground.
2. The voltage scale you recommend in addition to the horizontal scale of 10 µs/div.
3. Whether you would like to see multiple switching cycles or a single cycle zoomed in.

We will remeasure and share the corrected waveform once we have your guidance.

Hi Devaraj,

1. Please measure the voltage across R140. That is the two terminals of R140. Connect the probe GND to "+34VF".

3. Please start from a few cycles.

Best Regards,
Feng