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# LM3492: power factor issue

Part Number: LM3492
Other Parts Discussed in Thread: , TPS92561

Hi,

I am designing a luminaire with LEDs (VF=6.1V, IF=100mA) using LM3492 based on following:

- Vin= 24Vac to 32Vac

- Iout (300mA) , Vout (42.7V)

I am currently implementing the configure 7 series 3 parallel to LM3492AR111DEMO board. I can get good output but not a good power factor and effieciency.

Is there any suggestion to improve the power factor? Next, is ther any formula to calculate the value of ILIM and IREF?

Thank you.

Regards,

FY

• Hello,

The datasheet should have the equations for ILIM and IREF. But I am not sure this design was set up for PFC so how to improve it may be difficult. In any case our AC-DC expert is out of the office for a couple of days, but I will forward this to him and see if he has any ideas.

Regards,

Clint

• Hi Clint,

Thanks for the info on the equation. Besides, we are looking forward for flavorable solution on the power factor.

Regards,

FY

• Hello FY,

What are you looking for regarding PFC?  I just starting looking at the EVM.  It doesn't appear this is set-up for power factor correct though it's not going to be as bad as some converters.  Let me know what you are trying to do or what you are expecting this to do and I will see if it is possible.

Regards,

• Hi Irwan,

Our application are need three type of input voltage such as 12VAC, 24VAC and 32VAC. We are looking a solution can power up with three type of the input voltage and can get a better power factor and efficiency. Currently we are using LM3492 to power up 7 LED(6.1V) in series and 3 parallel with current 375 mA ( each line are 125mA) and 9 LED(6.1V) in series and 2 parallel with current 250mA ( each line are 125mA). As per our testing result with 24VAC , 32VAC we can't manage to get a good power factor on the LM3492AR111DEMO eval board. Can you manage to provides us with can drive above spec modification schematic on the LM3492AR111DEMO eval board or other better solution on ti product. Please help to advise us.

Thanks.

Regards,
Loon
• Hello Loon,

What is your input source?  Is it an electronic transformer or are you providing a low impedance sinusoidal input that doesn't require a minimum load?  The reason these designs aren't power factor corrected is because they are designed to be used with an electronic transformer (some including triac dimming).  Is this going to be triac dimmed?  An electronic transformer is difficult to interface with because the LED driver is a small load compared to what the electronic transformer wants to see.

The Vled is 7 X 6.1V = 42.7V and 9 X 6.1V = 54.9V?  Just want to make sure.

Adding an external circuit to adjust Iref should be able to improve the Power Factor Correction.  What level of Power Factor Correction are you trying to achieve?

If you are driving this from a low impedance AC source some components can be removed from this design.

Regards,

• Hi Irwin,

For the input source ,we are using the linear transformer to step down to different voltage such as 12VAC, 24VAC, and 32VAC. We are not using for triac dimmed .

The Vled is 7 X 6.1V = 42.7V and 9 X 6.1V = 54.9V? Yes

The better PFC we need to achieve is more than 0.9 and efficiency is more than 80%.

Thanks.

Regards,
Loon
• Hello,

It may be possible to use the Iref pin to adjust the current based on what the input voltage is doing.  There is a limit to how low the current can be set, 50 mA so it may not be enough for what you are trying to do.  There are other parts that could work for this as well.  The TPS92561 is one of them.  It has a connection to allow the rectified AC to create high power factor.  There are other parts that could accomplish this as well.  If you like the two channels with the dynamic headroom control I can look at what it will take to improve the power factor by adjusting the Iref pin.

Regards,

• Hi,

We did try to adjust the Iref to obtain desired ILed, but the power factor reduced to 0.7 when the input is 32VAC. What external circuit need to be added to improve the Power Factor Correction as mentioned earlier. Please advice. Thank you.

regards,

FY
• I would add a circuit to adjust Iref in relation to the input voltage. Unfortunately it's not as simple as injecting current into Iref from Vin. It is inverted meaning a circuit needs to source current into Iref when the input voltage is low and not inject current when the input voltage is high. It will take a few components and at least one transistor.