LM5025A: LM5025A circuit issue

Ting YI

Prodigy 140 points

Part Number: LM5025A

Tool/software:

HI,

I would like to understand how the CS1 and CS2 circuits of the LM5025A are designed ?

4 months ago

0 Hong Huang 4 months ago

TI__Guru 64115 points

Hi,

CS1 is for cycle by cycle current limit, and CS2 is for hiccup, so you need to decide at which current you want to make cycle by cycle current limit, and at which hiccup. Then adjust the resistor values to get reach threshold.

0 Ting YI 4 months ago in reply to Hong Huang

Prodigy 140 points

Hi,

So, the turns ratio of the current sensing transformer is set to 100:1 to prevent CS1 and CS2 from exceeding the rated 0.5V?

0 Hong Huang 4 months ago in reply to Ting YI

TI__Guru 64115 points

Hi,

100:1 is to help convert the current to a proper range so to help make resistor design to get voltage compatible to the thresholds to each CS1 and CS2.

0 Ting YI 4 months ago in reply to Hong Huang

Prodigy 140 points

Hi,

Then I'd like to know how the resistor values for R3 to R7 were calculated?

0 Hong Huang 4 months ago in reply to Ting YI

TI__Guru 64115 points

Hi,

First decide at which current you want to trigger CS1 and which current you want to trigger CS2. Then divide that current by 100, then times the current after division with a resistor value to get the CS1 and CS2 threshold.

0 Ting YI 4 months ago in reply to Hong Huang

Prodigy 140 points

Hi,

Then may I ask if you know what the trigger current settings are for CS1 and CS2 on the LM5025AEVAL?

0 Hong Huang 4 months ago in reply to Ting YI

TI__Guru 64115 points

Hi,

The user's guide has the answer. Its CS2 is setup about corresponding to 33A load current. Then you can make calculation to find CS1 setup based on the EVM circuit.

0 Ting YI 4 months ago in reply to Hong Huang

Prodigy 140 points

Hi,

May I ask which resistor you were referring to when you multiplied the resistor value earlier?

0 Hong Huang 4 months ago in reply to Ting YI

TI__Guru 64115 points

Hi,

R3 and R4 for the EVM for CS1. CS2 has divider of R6 and R7. You need to converter 33A to the primary side first then do the calculation of CS1 and CS2.

You can use the below LM5025A design calculator for your design.

https://dr-download.ti.com/design-tools-simulation/calculation-tool/MD-dQGlmGWwXo/01.00.00.00/sluc655.zip

0 Ting YI 4 months ago in reply to Hong Huang

Prodigy 140 points

Sorry, the CS2 voltage I calculated is still very incorrect. Could you tell me where I made a mistake? When the secondary side is at 33A, the primary side is 3.03A.

(3.03A/100)*R2*(R7/R6+R7)=39.6V

0 Hong Huang 4 months ago in reply to Ting YI

TI__Guru 64115 points

Hi,

R3//R4 does show in your calculation, why R2.

33A is average you need to use peak current.

But the big error should be R3//R4 not in your calculation.

0 Ting YI 4 months ago in reply to Hong Huang

Prodigy 140 points

Hi,

Is this the correct calculation method?

(4.28A/100)*(R2//R3//R4//(R6+R7))*(R6/R6+R7)=0.046V

0 Hong Huang 4 months ago in reply to Ting YI

TI__Guru 64115 points

Hi,

(4.28A/100) * (R2//R3//R4) * R6 / (R6+R7)

0 Ting YI 4 months ago in reply to Hong Huang

Prodigy 140 points

Hi,

The calculated result is still 0.046V,But the value seems to be incorrect.

+1 Hong Huang 4 months ago in reply to Ting YI

TI__Guru 64115 points

Hi,

The resistor values are from the bench tune up due to leakage spikes and capacitor charges. Also at high line of input voltage to deliver same average 33A current peak current will be much higher than low line due to different duty cycle. So these will make difference between calculation and bench reality. The calculation process is correct. You can follow that but you will need to tune up.