LM5156: LM5156-flyback converter schmatic review

Hi Sekar,

Thanks for using the e2e forum.

I will review the design and get back to you within 1-2 days.

Thanks and best regards,
Niklas

Hi Moritz,

Thank your review feedback and kindly check below are the transformer datasheet also. 

750345364r00 Prelim Spec Sheet 03.22.23 10.14 AM.pdf

As per the above schematic the physical board output is not as per expected

Regards,

Sekar D

Hi Sekar,

What is the problem with the output?

Can you send waveforms?

Best regards,

Moritz

Dear Moritz,

Below are the input and output wave form

VIN: 28VDC

Iin: 110mA

Vout: 2V

Iout: No Load

Regards,

Sekar D

Hi Sekar,
Thanks for the Scopeshot.
On which output are you measuring? At Vout1? So you would expect 48V, not 2V, right?

Can you please send a more zoomed in Scopeshot of the startup?

And also please measure the SS-pin, the COMP pin and the current sense pin please.

Best regards,

Moritz

Dear Moritz,

Yes, your right the expected output voltage is 48VDC and below are waveform. 

CH1 = Output (Yellow color), CH2 = Input (Green), CH3 = SS (Bowen), CH4 = COMP (light violet )

On Time (R20 & C18 is Remove (DNI))

Off Time (R20 & C18 is Remove (DNI))

On Time (After mounting the R20 & C18)

Regards,

Sekar D

Hi Sekar,

Thanks for the scopeshots.

I seems like the softstart time is very short, and then, the controller wants to regulate to full output voltage when the input is only at around 5V.

This might cause overcurrent. Please try to increase C15 and C16 and place D6.

Furthermore try to adjust UVLO, so that the controller starts up at higher input voltage. For example R4=133k R16=23.7k.

Best regards,

Moritz

Dear Moritz,

Below are the wave form with same setting in scope

Changes:

Mounted the D6, C15 add 10nF (total 20nF), C16 Add 47uF (total 94uF), R4 changed to 130K and 22k for R16.

After Increasing the C15 110nF

Thanks,

Sekar D

Hi Sekar,

Thanks for the additional scopeshots.

Can you please include the waveform of the current sense pin in your measurement? (you can leave out comp voltage for that)

I would like to see if the device sees an overcurrent while trying to startup.

Best regards,

Moritz

Hi Moritz,

Below is the CS pin waveform with VIN(CH2), VOUT(CH1) and SS (CH3).

Regards,

Sekar D

Hi Sekar,

Thanks for the additional measurements. Can you please zoom in Signal at CS-pin where the peak of Vout is?  To around 4us/div

The next thing you could do is to decrease the current sense resistor R11 to 10mOhm.

Best regards,

Moritz

Dear Moritz

Same setup with CS pin zoom in wave form

 CS - pin.rar

After changing the R11 resistor to 11m ohm, the D1 diode is get damaged and  below the wave form. 

Regards,

Sekar D

Hi Moritz,

i got the output after pin swiping the transformer secondary side the pin 11 & 13, below is the waveform 

CH1 = Output (Yellow color), CH2 = Input (Green), CH3 = SS (Bowen), CH4 = CS (light violet )

R11= 22m ohm, D7= DFLS1100Q-7

Vin; 28V, Vout:46.5V (Open circuit)

Regards,

Sekar D

Dear Moritz,

The problem not solved, after load the 100mA load current the output voltage is going from 46.5V to 0V.

Connected up to 200mA load the output voltage is constant, after change the input voltage level from 28V to 12V.

Regards,

Sekar D

Hi Sekar,

Thank you for your reply, Could you please change the R11 to 11mohms and check if it helps?

Also, could you send some waveforms with 100mA load?

Thank you

BR,

Haroon

Hi Haroon,

Below is the 100mA load wave form and used 11mohms of R11.

CH1: Vout, Ch2: Anode pin of D1, CH3: COMP pin, CH4: CS

CH1: Vout, Ch2: Anode pin of D1, CH3: Vin pin, CH4: SS

Regards,

Sekar D

Hi Sekar,

Is it possible to measure the current? I want to see what the output current looks like alongside the other waveforms here, maybe you can replace the anode pin of the D1 with the output load(current)

Thank you

BR,

haroon

Dear Haroon,

Here is 100mA output load current waveform

CH1: Output current (Yellow), CH2: CS pin( Blue), CH3: Output voltage (Pink).

Regards,

Sekar D

Hi Sekar,

It seems like you are triggering the current limit as the CS pin is very noisy and it is triggering the over current, the current limit threshold is 100mV and it seems like you are crossing that, do you have filters for your CS pin? for your layout are those filters placed close to the IC?

BR,

Haroon

Dear Haroon,

Thanks for our support,

Yes, RC filter component are mounted and with closer to the IC below are the image and layout screen short.

Please let me know the LM5156 flyback convert EVM is availably (any input and output range), If yes, please share the related document.

Regards,

Sekar D

Hi Sekar,

I see but the values of the RC filter seems to be too small, could you change the R7 to 800 to 1k ohm?

As for EVMs we only have this LM5156 Boost https://www.ti.com/tool/LM5156EVM-BST  and also there is this LM5156 Flyback reference design https://www.ti.com/tool/PMP41012#description 

Hope this helps.

BR,

Haroon

Dear Haroon,

After changing the R7 to 1K resistor the CS pin voltage level reduced slightly. Observed the R11/Q1 source pin voltage pk-pk is 8V to 11V during load on time, below is the waveform.

CH1: I out, CH2:CS, CH3: V out, CH4:R11/Q1 Source pin 

Regards,

Sekar D

Hi Sekar,

Could you also add SW node waveform instead of the R11/Q1? when the load goes to 0 I think your device switches off and I think you are hitting the current limit, I would increase the R7 a bit more and see if the noise is reduced or not. Also, I feel like the layout might be the problem, feel free to attach it here for a review if you want.

BR,

Haroon

Dear Haroon,

Here is the layout file, please review and give your feedback. 

Design Files_48V DC-DC Flyback Converter - PCB_V0.01_230418_1540.rar

please let me know whare i have to probe and below screen for reference please mark and send with me.

Regards,

Sekar D 

Hi Sekar,

Please allow me some time to get back to you on this. I will write back to you at the start of next week with a layout review.

Thank you for your patience.

BR,

Haroon

Hi Sekar,

Could you please send the SW node waveform as suggested in my last message? 

BR,

Haroon

Dear Haroon

Below is the waveform 

CH1=VOUT, CH2=CS, CH3=Q1Drain, CH4=Q1Source - Without load

CH1=VOUT, CH2=CS, CH3=Q1Drain, CH4=Q1Source - With 100mA load

Regards,

Sekar D

Hi Sekar,

I will get back to you as soon as possible.

BR,

Haroon

Hi Sekar,

I meant the waveform of the SW node when the output is changing, when your Vout is dropping or CS is too high.

BR,

Haroon

Dear Haroon,

Yes, The Vout is get drop and same time CS also going to high when SW node is changes, after truing the value is the result I'm getting, any wrongs the value or circuit and Please guide me on what has to do with the circuit bring-up.

Regards,

Sekar D

HI Sekar,

I understand, but do you have a waveform of this? the SW node waveform when the Vout drops and CS hits the limit, could you share the waveform for this case?

BR,

Haroon

Dear Haroon,

Here is the wave form

CH1=VOUT, CH2=CS, CH3=Q1Gatepin, CH4=Q1Drain

Regards,

Sekar D

Hi Sekar,

thanks for the scopeshots. Please give me some time to look into this. i will come back to you until the end of the week.

Best regards,

Moritz 

Hello Sekar D,

We do not have an LM5156 flyback EVM, but you could use the LM5155 flyback EVM as reference. The devices are almost the same and can often be used on the same layout. Please find the LM5155 EVM here: LM5155EVM-FLY Evaluation board | TI.com

Best regards,
Brigitte

Hello Sekar D,

How do you have the transformer at the moment connected? Still pins 11 and 13 swapped? It is very important that the diode on the secondary side blocks when the transistor on the primary side is on. So the right orientation of the transformer is essential. Please check this and connect the transformer properly independent of the output voltage result. Debugging only makes sense if the transformer is connected correctly.

Best regards,
Brigitte

Dear Brigitte,

Yes the transformer mounted connected properly and still the pin is swapped stage. The secondary side Diode changed from V3P6LHM3/H to PDS4200HQ-13. Measured secondary wind of 12V pin 10 and 5V of pin 9 and below is wave form.

12V (Pin-10)

 5V (Pin-9)

Regards,

Sekar D

Hello Sekar D,

So it seems now you are getting a high overshoot on both outputs and they both regulate then to 20V, is this right? What is the regulated output doing? How much load do you have on the 12V and 5V outputs?

Best regards,
Brigitte