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LM51551: Designing 12V 5A SEPIC DC-DC Converter - Current limitation debugging

Prit Varmora
Prodigy 30 points
Part Number: LM51551


Hello there, 

This is Prit, I am trying to design an DC-DC converter using LM51551 IC, I am attaching Schematic of my design. 

Design parameters are as below: 

Input voltage : 8V to 36V

Output Voltage : 12V Fix

Max Output Current: 5A

Topology used : SEPIC

When i am drawing not current from this designed supply its working fine, 11.89 V output I am observing, and its quite acceptable ripple with whole range of input voltage (8V - 12V).

But when i am drawing current of 1 Amp from it its getting tripped by Hiccup mode at input voltage of 17 V When i am using Mosfet IRFB3077, and i am being able to draw 3A at 17V input with MosFET IRFB3205

At this mentioned current of load i am not being able to go below 17V in input 

Calculator and references used for this designing :

Reference calculator excel sheet

Datasheet & AN-1484 Designing A SEPIC Converter

What are my questions ?

1. Even if IRFB3077 is having low Rds_on, Why its performance in circuit is lower then IRFB3205?

2. Even if all calculations seems to be right and done using Excel sheet provided by TI, I am not being able to get 12V @ 5A with 8.5 V of input supply ?

3. Why mosfets are heating up too much?

4. When i am observing waveform of voltage across sense resistor 0.005 Ohm (3W, also checked), i am getting Spike of 0.8V to 3 V which is equivalent to 100s of A running through it, And my supply is just capable of providing 10A  max

(I didn't observed supply getting tripped or CC mode because of these spikes)

5. Any other Circuit topology or Better IC to fulfil this requirements? 

What I have tried ? 

1. I have tried different Mosfets and replacing inductors with same kind of new one

2. Checked and verified calculations again and again 

3. attaching screenshots of DSO here (yellow : voltage across sense resistor, Pink : gate voltage) 

(Note: This DSO measurement has been taken using ground spring probe)

It would be very helpful for us if anybody can suggest some improvement or guidance

AL111_Rev1_Schematic.pdf

Thanks and Regards

Prit Varmora

  • 0
    TI__Mastermind 23550 points

    Hi Prit,

    Can you share your design calculator and layout?

    I can see the MOSFET you are using have very large Qg. It will increase switching loss.

    You may need to change the MOSFET or reduce the switching frequency.

    Best Regards,

    Feng Ji

  • 0 in reply to Feng Ji
    Prodigy 30 points

    Sure Sir,

    Thanks for your quick reply, Any suggetions for MosFET to be used ? 

    Here are both the things

    Calculator I have used with filled appropriate data in it : 

    LM5155_56_Excel_Quickstart_Calculator_for_SEPIC.xlsx

    Layout PCB Design file : (Can be viewed by this online tool here)

     LM51551_PCB.PcbDoc

    Regards, 

    Prit Varmora

  • 0 in reply to Prit Varmora
    TI__Mastermind 23550 points

    Hi Prit,

    The layout has many issues. You should follow LM5155EVM-SEPIC.

    You should select SMD diode and MOSFET for best performance.

    Best Regards,

    Feng Ji

  • 0 in reply to Feng Ji
    Prodigy 30 points

    Thanks Sir, I will improve on that,

    The Issue with that above design is I am not being able to find Coupled inductor as per required capacity like one used in your suggested design

    ------

    Can you help me find answers of the mentioned questions ? 

    It would help me to understand it better.

    Rewriting here: 

    1. Even if IRFB3077 is having low Rds_on, Why its performance in circuit is lower then IRFB3205?

    2. Even if all calculations seems to be right and done using Excel sheet provided by TI, I am not being able to get 12V @ 5A with 8.5 V of input supply ?

    3. Why mosfets are heating up too much?  -- May be Switching losses as you have mentioned ?

    4. When i am observing waveform of voltage across sense resistor 0.005 Ohm (3W, also checked), i am getting Spike of 0.8V to 3 V which is equivalent to 100s of A running through it, And my supply is just capable of providing 10A  max

    (I didn't observed supply getting tripped or CC mode because of these spikes)

    5. Any other Circuit topology or Better IC to fulfil this requirements? 

    Also Can you please Point out some Major issues in PCB Design that can Lead to this problem of degradation of Output Capacity ?

    Thanks in advanced

    Prit Varmora

  • 0 in reply to Prit Varmora
    TI__Mastermind 23550 points

    Hi Prit,

    Please follow the layout guidelines in the datasheet. It is difficult to say what exactly caused this issue.

    You can choose between SPEIC and buck-boost.

    Maybe it will be easier to start with TPS552892EVM.

    Best Regards,

    Feng Ji

  • 0 in reply to Feng Ji
    Prodigy 30 points

    Hello sir, 
    Can you help me understand this ? 

    After seeing equation in datasheet for calculating required maximum ESR for output capacitor of SEPIC DC- DC converter is as below :
    ESR <= (Vripple * 0.5)/(peak current from both inductors)

    which is giving me 6.5mOhm for my application but also in below document i found below sentence:

    "Since, for loop stability reasons, we should not use any output capacitor of less than 100 mΩ" 

    what is right ?
    Document : AN-1197 Selecting Inductors for Buck Converters 

    snva038b.pdf
    Regards

  • 0 in reply to Prit Varmora
    TI__Mastermind 23550 points

    Hi Prit,

    If you are using a SEPIC, follow SEPIC application note.

    Different topology/control method may have different requirement for output cap ESR.

    Best Regards,

    Feng Ji