SN6501: SN6505A: About disorder of switching issue

Hi Anjana,

Have you been able to troubleshoot the SN6505 system with replacement diodes?


Thank you for your time,
Manuel Chavez

Hello Manuel,

I tried, it dint help.

Regards,

Anjana 

Hi Anjana,

Manuel is on  work travel today and tomorrow, so I am writing here to gather more details.

Did you specifically try the RB168MM diodes on both paths of the secondary side? If not, could you please the part number you used , so that we can look up its reverse leakage characteristics ?

The original symptom you described where the current was increasing - this is usually consistent with the breakdown mechanism in the diodes, current leakage increases (exponentially) with temp, and this can cause the transformer to saturate, which leads to significantly increased current consumption. 

To rule out other causes, is it possible to share a block diagram of the power supply path you are using? Primary points of information we are looking for are:

• Input voltage value
• Transformer Turns ratio, part number   [This will help us assess the reverse bias on the diodes on the secondary side, since leakage depends on this also]
• Existing diodes being used (part number)
• (nominal) load current on the output 
• Is an LDO being used ?

To reconfirm: this issue is not there at room temp or cold but only shows up hot temps, right ? [ > 85C approx] 

Thanks,

Abhi

Hi Anjana,

Are you able to follow up on the information Abhi asked for above? These questions are all helpful and important to fixing this issue.

I will await your response.


Thank you,
Manuel Chavez

Hello Manuel,

Sorry for the delay in my response.

I came to a conclusion that the issue is because the transformer is saturating. The transformer I used has a volt sec product of 11Vus while the required is 20Vus.

The characteristic of inductance at higher temperature is also contributing to the failure condition.

Please confirm if my understanding is correct. The requested details is as below:

Input voltage - 5.5V

Turns ratio - 1:1.3 ; Current rating - 150mA

Nominal load current : 70mA

Regards,

Anjana G

Hi Anjana,

No worries about the delay; a transformer saturating could cause this behavior, especially because at 5.5V operation, the SN6505A requires a transformer with at least 22Vμs V-t product. On this note, the SN6505B requires a transformer with V-t product of 8Vμs or greater in the same conditions due to its faster oscillating frequency, so a potential fix is to replace the SN6505A with an SN6505B, which is pin-to-pin compatible.

The load current specifications seem acceptable for SN6505 systems, but if you'd like to continue using the SN6505A and change the transformer instead, please share the part numbers of the transformer you are using and replacements you are considering for insight on compatibility.


Thank you for your time,
Manuel Chavez

PSA: Original post edited to include SN6505B

Hello Manuel,

I am still confused how did the circuit work fine at nominal temperature?

I expect it to fail at nominal operating conditions.

Regards,

Anjana G