LM3445: Will LM3445 dim the LED brightness when input AC is dimmed by phase cut?

Hello,

Both the LM3445 and LM3450 are intended to be used with phase cut dimmers. They will reduce the LED current as the dimmer is turned down.

Universal input phase dimming is difficult to achieve and optimize for.

The LM3445 demo board is set up for a higher voltage, lower current and lower output power. The transformer is not set up for low voltage higher current and, due to the increased power, would have to be designed for the application

The LM3450A eval board could, more easily, be changed to lower the output voltage and raise the current. The flyback is already set up for 30 watts the LM3409 buck would have to have components changed and verify the freewheel diode can handle the extra current and duty cycle.

Stepping down to 12 volts at 2 amps will be less efficient than a higher stack voltage with lower current.

The LM3445 and LM3450 can be used without the LM3409 but loses some of the two stage benefits such as low LED current ripple and line rejection.

The LM3445 and LM3450 have decoding circuitry that can be used to reduce the LED current as the dimmer is turned down.

Other parts, do you need isolated? Does it have to be a flyback?

Thanks,

Thanks for quick response.
I need isolated. it doesn't have to be flyback. Efficiency is not critical. Low LED current ripple is more concerned if it translates to brightness ripple noticeable by a human eye.

Hello,
LM3445 is a bit easier to design with, the LM3450 has a lot of features to interface with a triac dimmer but was originally intended to be used as a two stage solution. Another part to look at is the LM3447.
Thanks,

I'm leaning to LM3445, since it's simpler than LM3450, and supports reverse phase cut, while LM3447 doesn't.
I'm thinking about using LM3445 eval board as ref to re-design for 12VDC/2A for LED. However, LM3445 eval document does not have calculation detail. LM3445 DS has only detail for buck. where can I get design help?

Hello,

LM3447 supports leading and trailing edge dimmers.

The LM3445 was originally design to be a buck converter running from a valley fill circuit.  Later on evaluation boards were made to run isolated.  There is a description of the magnetic in the evaluation board documentation that could be modified to work for different output configurations.  Your application is higher power so it probably will require a larger transformer.  I'm not sure if you plan on designing your own flyback transformer or having a company like Wurth create it.

Thanks,

Hi,

I can find only section 12. Transformer Design, which shows only spec for the eval board, not how to design a new design.

I bought 2 eval boards (120VFLBK-NOPB) from Mouser. They are both non-functional, putting out 0V, with or without load. Input is 120VAC. The transformers were getting real hot.

I plan to have a third party to make the transformer (either Wurth or other). First, I need to know how to design the entire circuit for dimmable 12VDC out, 2A max.

Does TI have any design tool for LM3445 (or a similar device)?

I clicked "Open Design" on  WEBENCH® Designer LM3445 link, it does not do anything.

 

Hello,

LM3445 was originally designed to be a buck converter running from a valley fill.  Other designs have been developed from it but as reference designs.  PMP7753 is a valley fill that is closer to your output but there won't be a dedicated design tool for flyback PFC since that came out later.

There are other ICs that show designing a flyback transfomer, the LM3447 is one of them though that is a fixed frequency design.  Wurth or other may be a better place to start if you are not familiar with transformer design and safety requirements.

The evaluation boards should work.  What are you connecting to them?  Did you change the board in any way?

Thanks,

The eval boards were DOA. I didn't change the board in any way, only plugged in 120VAC and measured DC out with a volt meter. There was no load attached. I also attached load. The problem was still exist.

I found only a design spreadsheet for LM3450. This is the minimum tool I need to design a complete circuit.

Hello,

These boards are current sources, they do not regulate voltage and they are a flyback.  This means the output voltage will rise until it finds a path to conduct current.  This design does not have an OVP circuit or voltage clamp so either D5 has to take the full current or the flyback clamp, D1, does depending on which conducts first.  Neither of these parts can handle the output power, both are TVSs rated at one to two watts and will probably fail shorted depending on the power dissipation.  You can measure the output and see if it is shorted, if it is most likely it is D5, if not check D1.

Regards

When I applied load, the load is a 150 Ohm power resistor. So current could be only ~100mA max. I checked that day, one of the diodes was shorted.

Hello,

With 120 VAC input the output current will be 365 mA typical. If the load resistor only takes a portion of it the rest of the current must go elsewhere possibly damaging components such as D5. The load should be LEDs or diodes in series since this is a current regulator. Also the load must be able to handle the power delivered by the EVM.

Regards,

The load is a resistor. So it will take any current that the EVM pushes thru. The EVM was already non-functional without load, before I connected the load resistor. So I believe the problem here is not the load. It is that the EVM will burn up if there is no load. If that is the case, then this design is not what I'm looking for. In really life, it is likely that the AC is on while there is no load.
Load is not the big concern at this time. The concern is how I get design detail and tool to design a circuit to my requirements.

Hello,

A few references

AN-2150

LM3447 has equations for designing a flyback transformer.

Wurth will design one as well.  Just have to give them the output power, output voltage/current.  Input voltage and range, switching frequency.  If you plan on using the LM3445 it will be a discontinuous mode power factor corrected flyback.

Designing a flyback power supply isn't trivial, there are a lot of considerations to take into account.

Also attached is a design calculator for the LM3447.

Also note that an LED driver does not need to regulate voltage, many designs the LEDs are part of the board or fixture that never gets disconnected, if they do it's a failure.

Regards,3223.LM3447_Design_Tool_sluc365.xlsx