LED Street Lighting Power supply

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

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

Hi Guys,

A lot is being said about topology choices and implementations. Enough to build something worthwile. Anyway, i like to share some interesting; the near future of power control.

AN1271 : Isolated Flyback converter to drive LEDS with active PFC function.

http://ww1.microchip.com/downloads/en/AppNotes/01271A.pdf

The scheme is not fully digital control as the microcontroller has analog peripherals for the control loop. This is a very nice way to make the next step. I have samples on my desk of this controller, now i have to find the time to play with the device.

Best regards, Enrico Migchels

Hi Enrico,

Great document! I think i am trying to build that circuit int the next days to see how it works. And i would be glad to hear about your experiments though.

Thanks for your sharing.

Firat

Hi Jorge,

I will set three type of output current beforehand which is 350mA, 700mA, 1050mA, that means my full load is about 36W and my light load is only 12W. The power factor become the big problem when light load and high voltage input.

Hi Peter,

Lighting certification and safety requirement is the main reason to design a Isolated driver, but the cost also take into account, maybe the Single-Stage Flyback is the best choice. What you say?

Jason

Hi Enrico,

The solution is wonderful, it is a singel stage topology but the controller is a MCU not the PFC controller. there is only primary detection, no secondary feedback. Use MCU can make the duty cycle accurately. But I have some questions:

1,How to protect the driver when the secondary occur Over Voltage, Over Current, or Over Load?

2,How to burn the code into the MCU, when the AC power is connect or not? If AC power is connect, how to isolated the ICSP Header and the AC input? If not, how to power the MCU?

Jason

Hi Firat,

What about you try,does it work well? I,m glad to hear about your experiment.

Jason

Hi Jason,

I hope you are doing well. I know this questions is for Enrico and I'm sure he'll chime in with his suggestions, but I'd like to take a crack at answering your questions.

1.  I think Over voltage, Over Current, and Over load should be defined first. An over load will generally be caused by an over voltage fault or an overcurrent fault so you don't have to think about the over load as a seperate fault condition, if you take care of overvoltage and overcurrent conditions than the over load condition is also taken care of.

Since your going to regulate current in the LEDs a short circuit condition will not be so bad. The flyback control loop will adjust the duty cycle in order to maintain a constant current, however you don't want the supply to stay in this condition for too long so once the condition is detected (say through a voltage divider across the output) then the MCU should shutdown the converter.

An open circuit condition is far more serious because the control loop is trying to regulate current in an open circuit current can't flow to the sensing element (resistor, current transformer, etc.) so the control loop will continually increase the duty cycle to try to increase the current over time the the stress on the transistor and diode will become excessive and these components will be destroyed. The best way to handle this situation is to have a switchable load on the output ( the simplest example is a power resistor which is switch in on the low side with an NPN BJT or N channel FET) Once the open circuit condition is sensed then the MCU can activate the load across the output at which point current flow and the circuit can maintain regulation.

2. Due to the low voltages and current needed by the MCU you'll generally have to make a secondary low power housekeeping supply and their are many ways to go about this. A few examples would be a bootstrap circuit , Auto oscillating buck converter, Royer converter, even a low power flyback would be suitable as a housekeeping supply.

Most MCUs save program data in non-volatile EEPROM memory so you don't have to worry about losing your code once the power is removed.

I hope this helps if you have any other questions please feel free to contact us.

Best Regards,

Jorge Garcia

Cadsoft Computer

Thanks Jorge,

It's very exciting that you replied.

I do not know well how dose the MCU work.If I use a PFC controller for Single-stage Flyback application, I well set my OPP in primary, OVP and OCP in secondary. Generally,OPP is caused by an over voltage fault or an overcurrent fault,  the circuit will continued work untill some components are broken. It is necessary to setting OPP in primary to prevent broken the circuit .

An open loop control is much difficult than closed-loop control. For open loop control, the contoller (no matter MCU or PFC IC) didn't know the situation of the output. But for closed-loop control, feedback signal will be detect by the controller.

By the way, use MCU as the contoller is much expensive and complex than use PFC controller, and MCU is difficult to me, I don't know how to write the code, how to burn the code into the MCU. Use MCU, that means 'mission impossibility'

So, use PFC controller to finish this project is my decision. I find Fairchild's FAN7530 is suitable to my project.

You can see my schematic circuit based on FAN7530 as follow.

Hi Jason,

I hope you are doing well.

Don't worry you're not obligated to use an MCU.

You're on the right track, the only problem is that as it stands you have only made a Flyback converter, the chip itself is designed for Critical Conductin mode PFC not to run a Flyback.

However from the brief look at the schematic you've given it seems like your trying to adapt it. If you've done this before than go with it, I'm just a little cautious because the chip isn't designed to run a Flyback but rather a boost PFC and I generally try to stick to manufacturer recommendations.

In one of my earlier post I mentioned TI's LED cookbook which already contains some reference solutions to the driver your trying to create, so I hope you can take a little time to look it through, I think it will be well worth your time.

hth,

Jorge Garcia

Cadsoft Computer