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LED Driver ICs Forum Discussions

Delmore
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At 9V, the PWM driver will see the LED flicker that can be recognized by human eyes. When testing the waveform with the oscilloscope, it is found that the power supply voltage is forcibly pulled down by a few tenths of a volt. When the voltage is adjusted to 10V or above, the flicker disappears, but the phenomenon of forcibly pulling down the power supply detected by the oscilloscope still exists.

This phenomenon is especially obvious when multiple devices supply power at the same time. The waveform of tail point is as follows. Please help analyze whether this phenomenon is normal and what causes it.

Delmore_1-1653308004770.png

 

 

Delmore_2-1653308004642.png

 

Delmore_3-1653308004755.png

 

The following attempts and changes have been made

1. The current limit of power supply is adjusted from 5A to 10A without improvement;

2. Cancel D2 (equivalent to removing the diode 0.7V voltage drop), 9V will not flicker with the naked eye, but the waveform still exists. The capacitance of vs pin is connected in parallel with a 1uF capacitor on the basis of 0.1uF, the phenomenon is not improved, and the power supply voltage is also pulled down;

3. Test the voltage drop of LED. The voltage drop of each LED is 2.2V, and the total voltage drop of LED is about 6.6v

It is equivalent to that after D2 is removed and replaced with 0 Ω resistor, the power supply voltage will still be forcibly pulled down, as shown in the following waveform.Please also help to confirm whether it is normal for the power supply voltage to be forcibly pulled down, and what is the reason for the flicker?

Delmore_0-1653307885878.png

 

 

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Kumarkanahai
Moderator
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Hello @Delmore 

Thanks for posting on the Infineon Developer Community.

Can you please let us know the following points 

1.Can you please share the complete modified schematics along with the point where you are measuring the waveform? Since ,I am not able to find the tail point you mentioned in the complete schematics.

2.We also analyzed the simulation with single TLD2141-3EP  But didn't get the voltage fluctuation at the Vs pin.

So, can you please share the tail waveform and the PWMI waveform together.

Regards

Kanahaiya

 

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Delmore
Level 2
10 replies posted 25 sign-ins 5 questions asked
Level 2

The complete circuit diagram of T1 is attached.

The voltage waveform of tail power supply is as follows:

Delmore_0-1653385018980.png

The waveform of pwmi is as follows:

Delmore_1-1653385037838.png

 

Is PWM working properly? Where should I confirm?

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Kumarkanahai
Moderator
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Hi @Delmore 

Thank you for sharing the schematics.

Can you please measure and share the current value at the tail point and the current waveform of the tail point.

Regards

Kanahaiya

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Delmore
Level 2
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Level 2
The waveform of tail point is as follows:
blue is voltage; yellow is current

Delmore_0-1653469108941.png

 

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Delmore
Level 2
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Level 2

Hi @Kumarkanahai 

The waveform of tail point is as follows:
blue is voltage; yellow is current
Delmore_0-1653537076020.png

 

 

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Delmore
Level 2
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Level 2

Hi @Kumarkanahai 

1. The waveform of tail point is as follows.
The waveform is as follows: blue is the voltage ,yellow is the current.
Delmore_0-1653537076020.png

 

2.Add a newly discovered phenomenon

It is found that the voltage (blue )waveform is related to the line length. The shorter the harness is, the less obvious the voltage waveform is. As shown in the figure, the voltage and current waveform is tested by adding the power directly to the PWR (the harness is very short). Only this board of the schematic diagram in the figure below is used for testing.

I hope it is helpful to analyze the problem.

Delmore_2-1653547268299.png

 

Delmore_3-1653547296307.png
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Kumarkanahai
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Hi @Delmore 

Thank you for sharing all those waveforms.

It is observed that the Resistor i.e. L2/L3 which is used at the power supply has the maximum power capability of 0.1 watts only which is too less than the requirement.

tld2141ques.jpg

So, It is recommended to short the resistor L2 & L3  and check whether the power supply voltage dip and the flickering issue are resolved or not.

In case of any further queries, feel free to reach us.

Regards
Kanahaiya

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Delmore
Level 2
10 replies posted 25 sign-ins 5 questions asked
Level 2

@Kumarkanahai 

1,Resistance l2/l3 is 0 Ω.

Short circuit resistor L2 and L3 as recommended,but the power supply voltage dip and the flickering issue are not resolved.

Can you share the normal current and voltage waveforms at at the tail point of your test for reference?

 

2.Add a newly discovered phenomenon

It is found that the voltage (blue )waveform is related to the line length. The shorter the harness is, the less obvious the voltage waveform is. As shown in the figure, the voltage and current waveform is tested by adding the power directly to the PWR (the harness is very short). Only this board of the schematic diagram in the figure below is used for testing.

Please use demo to test whether the extended harness length (part of PWR) will reproduce or have an impact?

Delmore_0-1653639885500.png

 

 

Delmore_1-1653639885271.png

 

 

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Kumarkanahai
Moderator
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Hello, @Delmore

The time period and the peak value of the waveform are not clear in the image, you have shared. So, can you please measure and share the marked value of T & ΔI?
tld.jpg

 

Thanks & Regards
Kanahaiya

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Delmore
Level 2
10 replies posted 25 sign-ins 5 questions asked
Level 2

Hi @Kumarkanahai 

1, T ≈1.5ms, ΔI≈280mA

Delmore_0-1653879357304.jpeg

2,After the length of power harness is shortened, the change of Vs is reduced.

Delmore_1-1653879783958.jpeg

 

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Kumarkanahai
Moderator
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Hi @Delmore 
1. It is recommended in the datasheet to use a 4.7 or 10 nF capacitor near the Vs pin and place it close to the supply pin. So, it is suggested to change the capacitor C41 from 100nF to 4.7/10 nF.

1.PNG

2. Refer to this article for ISO pulse circuit.https://community.infineon.com/t5/Knowledge-Base-Articles/Do-we-need-an-external-Reverse-Polarity-Pr... 

3. The nominal waveform of the tail point is as follows

Kumarkanahai_1-1653914190433.png

4.. The value of the capacitor C42 can be estimated  as follows
I=C(dv/dt)
where i is the current i.e. 280 mA
              dv is the dip in voltage allowed.
              dt is the time period i.e. 1.5ms.
if dv is 200 mV then C=2.1 mF.So, it is suggested to select the capacitor value more than the calculated one.

Regards
Kanahaiya

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Delmore
Level 2
10 replies posted 25 sign-ins 5 questions asked
Level 2

@Kumarkanahai Thank you for your support!

We will modify and debug according to your suggestions.

Is there any explanation about the selection of capacitor C42?

 

 

Delmore_0-1653916791219.png

 

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Kumarkanahai
Moderator
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Hello @Delmore 

I explained in the last comment. Please check point number 4 of the last reply.

Regards
Kanahaiya

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Delmore
Level 2
10 replies posted 25 sign-ins 5 questions asked
Level 2

@Kumarkanahai 

The product is composed of four boards:MB, STOP, UP and DOWN. See the circuit diagram for details. MB is the power supply input module, and the three boards (STOP, UP and DOWN) are connected in parallel.See attachment for details.

Delmore_0-1653980760738.png

 

Changed:

Shorten the power harness,and Vs bypass capacitors C32 and C33 are 4.7nF.

Phenomenon:

The three boards (stop, up and down) are connected in parallel.

When the voltage is 9V, the naked eye cannot see the flicker;However, the voltage is 7V, and flickering can be seen with the naked eye.

The three boards (stop, up and down) are disconnected in parallel,  MB is independently linked, so there is no flicker.

 

The three test points are as follows:

MB board:

Delmore_1-1653981097236.png

STOP  board:

Delmore_2-1653981105689.png

 

1,The three boards (stop, up and down) are connected in parallel.The voltage is as follows. 

test point 1

Delmore_7-1653983375417.png

test point 2

Delmore_8-1653983429834.png

 

test point 3

Delmore_9-1653983463666.png

 

2,The waveform current of a single board is the current on the power line. The three boards (stop, up and down) are disconnected in parallel.The voltage is as follows. 

test point 1

Delmore_4-1653983089650.png

test point 2

Delmore_5-1653983151090.png

test point 3

Delmore_6-1653983236824.png

Please help with the analysis.

At the same time, please share your actual test waveform on the development board. Thank you!

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Kumarkanahai
Moderator
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Hello @Delmore 
1. The device is not in proper current regulation at Vs = 7 V, which causes the flickering. Please ensure the following points mentioned on the page number 14 of the Datasheet

Kumarkanahai_0-1654072582943.png

2.To minimize the  flickering during startup, it is recommended to replace the resistors on the EN/DEN pin.
To do so, set the DEN threshold to be more than Vout+VPS(cc) ->Vden > 7.6V. This should eliminate the flickering.

3.Have you changed the capacitor value C42 as recommended?

4.Can you please share the ERRN measurement and the output voltage waveform?

Regards
Kanahaiya

 

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Delmore
Level 2
10 replies posted 25 sign-ins 5 questions asked
Level 2

According to the suggestion, the board is made again, and the voltage drop still exists (the single board test vs drops by about 1V). In addition, when all the light boards of the whole vehicle are connected to the power supply, the LED light can still be found to flicker slightly.

This voltage drop cannot be explained clearly whether it is normal, and then whether this slight flicker is caused by this voltage drop.

The test waveform is as follows:

Purple is the input power supply voltage of the product;

Yellow is the ERRN pin voltage;

Blue is the power supply current in the case of three tld2141-3ep cascades

Delmore_0-1656988301208.png

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Kumarkanahai
Moderator
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Hello,

The waveforms in the last image appear to be normal. There is no open load condition indicated by the ERRN pin. So, Could you please send us the ERRN and PWM waveforms from one of the boards when flickering is noticed? In the last waveform, It is observed that the frequency of the pulses is 200 Hz. The components selected for pwmi -pin is also producing a 200 HZ signal. Whereas, earlier the frequency was 100 Hz. So, can you let us know the reason behind the change? Can you please also confirm that the 10nF capacitor is mounted in parallel to the each LED or not?

Regards
Kanahaiya

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