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Calculation Error

GHM Messtechnik
Prodigy 10 points
Part Number: LM5181DESIGN-CALC
Other Parts Discussed in Thread: LM5181, LM5180, LM5180-Q1

Tool/software:

Hello Team,

I wan't to start a design for LM5181 with the following data:

Uin = 20...28V 

Uout1 15V / 0.1A 

Uout2  8V 0.5A

These a voltages are a bit different from my requests but I want to take a ready wound transformer from würth.

So I take the design tool and change the turn ratio of the transformer

If I select 1:1 or 2:1 ratio I get a result with bad efficiency.

If I select 1.2:1 or 1.5:1 it calculates the duty cycle to 100%

So is this a calculation error ?

Thanks for your answer !0511.LM(2)518x PSR flyback converter quickstart tool revB.xlsxWürth_für_LM5181.pdf

  • 0
    TI__Genius 15566 points

    Hi GHM,

    I will reply back to you soon today.

  • 0 in reply to Manuel Alva Hernandez
    TI__Genius 15566 points

    Hi GHM,

    To deliver the amount of power required with LM5181 with your VIN specs, you would need to increase the magnetizing inductance Lm and turn ratio Nps significantly but it also means higher duty cycle and higher peak currents at the secondary side which leads to higher conduction losses. 

    I recommend using LM5180 which has a higher current rate integrated MOSFET (1.5A). LM5180 will allow you to deliver the power required with Lm=25uH and Nps=1:1 and without compromising Dmax at Vin_min. Please, see attached the updated Calculator Design with LM5180-Q1.

    Thank you.

    LM5180_CalculatorTool_GHM_02262025.xlsx

  • 0 in reply to Manuel Alva Hernandez
    Prodigy 10 points

    Hello Manuel,

    thank you for your answer in relation of the magnetizing current.

    So my first requirements are met.

    But one problem still remain

    If I select a turn ratio between 1:1 and 2:1 there is still a calculation error.

    Do you have an idea if there is a possible way to fix this problem in Excel. Which parameter are influenced by the selection box.

    This is because the  requirements in my question are only the start point of the calculation.

    I just took the values from the manufacturer of the transformer.

    (The real requirements are 12V and 5V // DCRs of the transformer turns could be higher  // effiency as high as possible including the voltage drop of appended linear regulators should be as low possible)

    So the calculator tool is ideal for an effective calculation !

    Another question:

    What does the curve Flyback Diode Loss in the diagram power rating mean ?

    Thanks in advance for your answer.

    Best regards

    Stefan

  • 0 in reply to GHM Messtechnik
    TI__Genius 15566 points

    HI GHM,

    I don't understand the first question. As mentioned in my previous answer, when using LM5181, you either will have an Max Pout at Vin_min lower than Pout needed (red cell shown) or if using high Lmag and Nps to be able to meet Pout needed, Duty cycle at Vin_min will be higher than 75% (red cell shown) and secondary peak current will be high.

    As recommended above, LM5180 is a better option. You should not have these red cells (or errors if you want to call them like that) showing any longer as the internal FET has twice current rate.

    Flyback diode loss are the power losses at the secondary power diode of Flyback. It comes mainly from conduction losses when the diode is conducting Ploss=I_sec_DC^2*R_diode_on. Losses are estimated in mW in the plot.

    Thank you

  • 0 in reply to Manuel Alva Hernandez
    Prodigy 10 points

    Hello Manuel,

    thanks again for your answers.

    But the problem with the turn ratio is still pending.

    The duty cycle at Vin(min) is very low (38,1%). And it becomes even lower at Vin(max).

    That's the reason while the magnetizing current is so high.

    So, if I increase the turn ratio i.e. to 1,5:1 the duty cycle rises and the magnetizing  current will be lower

    And then perhaps we can take the LM5181, with lower costs.

    The tool works for  the turn ratios 1:1 and 2:1.

    You can also select turn ratios as 1,2:1 and 1,5:1 and 1,8:1.

    But for these ratios the tool calculates nonsense

    Do you know any mail address where I can report this error so that it reaches a department at TI which is responsible for the tool?

    That is because all other skills of the tool a very good !

    Thanks

    Stefan

  • 0 in reply to GHM Messtechnik
    TI__Genius 15566 points

    Hi GHM, let me chime in here

    "The duty cycle at Vin(min) is very low (38,1%). And it becomes even lower at Vin(max).That's the reason while the magnetizing current is so high."

    [MA]: Low values of Duty cycle D such 38.1% are okay. The lower the D, the lower the peak current of magnetizing inductance Lmag and switching current because Ipeak is directly proportional to D (Ipeak=Lmag*D/(Vin*fsw)). D is set by the controller when the internal PWM comparator reaches the current sense level of the primary switching current (Current Mode Control).

    "So, if I increase the turn ratio i.e. to 1,5:1 the duty cycle rises and the magnetizing  current will be lower"

    [MA]: If Nps turns ratio is increased and keep the same Vout and Lmag, first your power level will increase because you are increasing the secondary peak current but at the same time, you need higher D and Lmag at Vin_min will need to increase to be able to deliver that power (see equation above).

    "The tool works for  the turn ratios 1:1 and 2:1.You can also select turn ratios as 1,2:1 and 1,5:1 and 1,8:1"

    [MA]: I have checked the calculator tool with both LM5181 and LM5180 and both are correct. When I increase Nps, Pout_Vinmin increased, Lmag_min increased and D_Vinmin increases. I am responsible for the Calculator Tool. I hope this helps.

    Thank you