LED Street Lighting Power supply

Hi Jason,

The term ''valley PFC'' is probably the method to switch the PFC FET on when the voltage on the drain is the lowest. This is called ZVS (Zero Voltage Switching) and helps to reduce the turn on losses in the MOSFET. PFC techniques are often mandatory for a product (depends on power rating and field of use). Isolated or not has often to do with safety aspects. If you use a not isolated topology the chamber where the LEDS are (including heatsink) is connected to mains (live) and should have sufficient insulated towards any touchable parts. Flyback is an easy topology, but has less efficient than buck, the voltage on the MOSFET are higher (higher max Vds device) and the transformer is used less efficient (transformer = coupled inductor) but if you require an mains insulated application this would be an good choice.

Best regards, Enrico Migchels

Hi,Enrico

Thanks for your help.

I agree with what you said. There are so many method to light the LED when I want to design a LED street lighting, but it is difficult to choose. I want to know which is more popular, PFC+LLC+LED driver? Or PFC+Flyback? Or Valley fill+HV Buck? Or Single PFC? Too hard to choose.

By the way,''valley PFC''  I've said means a very simple and low cost PFC circuit. You can see the circuit in the following picture. This circuit will improve power factor when you not use a initiative power factor correction.

Hi Jason,

I know the Valley Fill circuit, it has disadvantages also. Factor factor is one thing, but the mains current distortion (MHR) is another. The MHR is not very good for the Valley Fill circuit, it draws nasty current spikes from the mains. The output voltage ripple is also high, but this is probably no issue for the second stage (feedback regulation is fast enough). I would probably go for the PFC (400Vdc boost converter) and LLC. The LLC can act as a current source for the LEDS.

Best regards, Enrico Migchels

Hi Jason,

Hope you are doing well.

I agree with Enrico in that you are better off using an active PFC based off a boost converter. The performance of the Valley Circuit is not going to be good enough for Street Lighting. However, I disagree with his recommendation of using an LLC resonant converter. The resonant converter topology is very common for Fluorescent Ballasts and the like however there's no need to go with such a sofisticated topology to drive LEDs.

An active PFC gives a 400V rail, a well designed buck topology could be used to power the LEDs. The simplified conversion function for a buck controller is Vo=D*Vin the current relationship similarly is In=D*Io, I bring this up because even though the PFC stage stepped up the voltage the current you'll be able to draw from it won't be very great depending the Power output you have designed for. The buck topology takes in very little current for a given output current depending on the conversion ratio so it meshes very well with the PFC stage.

I've designed drivers that make use of both the Buck and Boost topologies and they are simple to implement in an LED design. As Enrico mentioned these topologies are not isolated so if for safety reasons you must use an isolated supply then you may have to use one of the isolated topologies such as a Flyback. I mean no disrespect to Enrico his solution is valid, I just think it's more complicated then it needs to be.

hth,

Best Regards,

Jorge Garcia

Cadsoft Computer

Hi Jason,

Ok, probably the active PFC and flyback is better . I have found an interesting article:

http://powerelectronics.com/power_management/lighting_power_management/led-streetlight-high-brightness-0225/

Jorge, we use LLC all the time :-) We like the EMI behaviour and the lack of stress on the power components. :-) Okay, you are the expert! ;-)

Best regards, Enrico Migchels

According to Element-14 standings you're the expert.

Jason,

I'm sure Enrico will agree with me that we hope our responses have proven to be helpful in giving you some guidance as to how to proceed with your LED streetlight project.

Here to help,

Jorge Garcia

Cadsoft Computer

Hi Enrico, hi Jorge,

Thanks for your responses! It is very useful to my project.

I have designed a 350W LCD TV(52 inch)  power supply which use a active PFC and a LLC. I have to say that PFC+LLC is good choice for power supply >200W.  But this LED Street project only 36W, use active PFC+LLC will increase my cost. What about the Single-Stage Flyback topology which use PFC IC as its PWM controller? This solution can match both the PF requirement and safety requirement. The architecture as bellow:

Hi Jason,

By looking at your diagram, even if you are using a PFC chip to control the Flyback stage that doesn't mean you get built-in power factor correction. The only feedback loop your showing in the diagram is the LED current control loop. As you know in order to implement a PFC control algorithm you would have to monitor the input voltage waveform and inductor current.

If the biggest issue is price, then you may want to consider passive power factor correction. Granted I hope you don't have any size restrictions because the inductor you're going to need to implement passive power factor correction will be fairly large. This solution will work well as long as the number of LEDs you plan to drive is going to be constant, passive power factor correction tends to work best when you have a fixed predictable load.

Using a Passive PFC circuit will improve power factor to about .75-.85. If I remember correctly the latest energystar guidelines specify that devices consuming less than 75W must have a power factor of .7 or better. If I'm wrong I'm sure someone will correct me, but this could be an option since your driver is only for 36W.

I believe that your solution of a single stage-flyback+Active PFC should be lower in cost than LLC+Active PFC.

hth, and if I made a mistake in any of my statements please someone correct me.

Regards,

Jorge Garcia

Dear Jorge,

Brilliant analyse.

I'm sorry I don't show all the feedback loop that make you confused, there have other loop from the output of the bridge-rectifier and inductor current.

Generally, >75W power supply require PF>0.9, but for LED application, Commercial ≥ 0.90, no matter how many watt of the power supply,you can find the requirements by the following link:

ENERGY STAR Requirements for SSL Luminaires, Version 1.1 (page4 Residential ≥ 0.70 Commercial ≥ 0.90)

http://apps1.eere.energy.gov/buildings/publications/pdfs/ssl/energystar_sslcriteria.pdf

And, you can also find a Single Stage PFC solution of power supply in ON-Semi's website:

A Single Stage PFC+PWM Converter for 75-150 W Distributed Power

http://www.onsemi.com/site/pdf/ipecsa_paper.pdf

Maybe this solution is suitable for my application.

Do you have any idears?

Best Regards

Jason

Hi Jason,

It looks feasible, obviously you would have to modify the Feedback loop to regulate current instead of the output voltage (the tested design is set to output 12V).

The only point of concern is what input line voltages you plan to use your driver at. If you notice on the charts of page 5 the power factor falls below 0.9 at high line voltage and low output power. I looked at the standard and it doesn't specify at what conditions that power factor must be greater than 0.9. From this we can deduce that the power factor must be greater than 0.9 for all operating conditions, if that's the case then under some circumstances the App Note design will not meet the specifications.

If you plan to design the driver for a specific line voltage then you could optimize it for that specific line voltage, if not you will have to look at another solution.

Your designing a low power driver (36W), TI and National semiconductor have some good chips that you may be able to use. I know National doesn't make a PFC chip (other than their triac dimmable driver) but TI makes a couple. You may be able to then use one of National's specifically designed drivers, for the driver stage.

Check out TI's LEDCookbook.

http://focus.ti.com/lit/sg/slyt349a/slyt349a.pdf

You should find pages 10-18 very interesting

hth,

Best Regards

Jorge Garcia