In this demo, we will show the advantages of the FL7740, Constant-Voltage Primary-Side-Regulation PWM Controller for Power Factor Correction and the FL7760, Analog/PWM Dimmable 60 V input Buck Controller. We will demonstrate wide range of dimming capabilities using the demo. This particular demo features a high power factor constant voltage front end, coupled to a buck regulator for the second side.
Check out the demo video (transcript available below video).
Hi. My name is Jim Young. I’m an applications engineer at ON Semiconductor and today I would like to talk to you about some solutions we have come up with for Led lighting.
This particular demo features a high power factor constant voltage front end, coupled to a buck regulator for the second side.
The CV controller is an FL7740 that has a high voltage startup to maximize the capabilities of start while maintaining low standby power.
This converter will maintain a power factor of greater than 0.9 at highline with half load. This is due to a dynamic optimizer that’s inside that automatically compensates for the EMI filter phase shift.
It maintains about 3 percent regulation under static conditions on the output voltage, and under dynamic loading it’s about 10 percent.
This is far better than a lot of single stage converters which have a very slow compensation loop, and so it takes them longer to adjust to any dynamic change in either the input or in the output.
The converter is operating in DCM, discontinuous mode, at 65 kilohertz. As the load is reduced, the frequency is cut back. This allows for higher efficiency operation at lighter load.
Ultimately in standby mode the frequency can drop down to as low as 800 hertz. With this configuration you can achieve less than 300 milliwatts of standby power even at highline.
It has cycle by cycle peak current limit, it has short circuit protection, and if the auto shutdown timer is activated, it’s about a 3 second delay and then it goes through an auto restart.
The operation of this is not limited to just LED applications. It would also be suitable for other CV outputs where you needed high power factor on the input.
We followed this with a buck regulator. The buck regulator is able to maintain tight control of the LED current, even with 2 separate strings.
Perhaps you have one at one color temperature and the second string at another color temperature, and you’re able to balance the 2.
The FL7760 is a continuous conduction buck regulator. It directly drives the output FET with a high current driver, and it can operate up to about 2 megahertz operation.
Maximum about 60 volts input for the input line, and the LED current is directly monitored with a high side current sensor. This provides very very tight control over the LED current that’s flowing through the output.
It has a dimming input pin. This dimming pin accepts a combination of analog and digital PWM type controls.
This allows you very very wide range operation because while you’re in a PWM mode you can also adjust the amplitude to lower the output current and get a very very wide dynamic range.
The configuration of this demo board, we have a selector switch on these 2 independent buck regulators. We can select either analog input, where we’re using for in this case a 1 to 10 volt input, or a PWM input.
And I’ll be demonstrating later independent control where we can have one channel running an analog and one channel running in PWM.
Here is the FL7760 which is in TSOP-6 package. And this is the switching FET which is driven by the FL7760.
Here we have the selector switch that decides whether the input is being driven from the analog dimmer or the PWM dimming and they are available for each channel independently.
Those outputs go over here to two independent strings of LEDs. We are providing the AC power from this AC source. We have a meter on each of the LED stings of the output.
And this power supply is running a PWM oscillator. It provides us with the PWM control.
This is a conventional 1 to 10 volt analog dimmer that can also control the buck regulator.
And here we have on the screen, this is showing the PWM signal that has been provided by this oscillator.
So let’s start off by turning on the AC power.
We are running about 230 volts and as you can see, our standby power is about 120 milliwatts.
We will turn on the analog dimming. Right now we have set the both LED strings that are being controlled by the analog dimming.
And we can run the setting up to its maximum, for this instance we have about 560 milliamps to each channel. We run the dimmer down, we get down to for this dimmer minimum of 17 to 19 milliamps for the 2 independent strings.
We have the capability to change the input. We are going to change one of the PWM dimming channels from analog to PWM.
Now we can see on the screen as I change the PWM duty factor. We can see the brightness of the LEDs changing.
And we see current through that string changing. So I am going to turn off the analog control string and I will focus mainly on this digital string.
We can run the duty factor down to very narrow level here. And right now we are running about half a milliamps which is 0.1 % of our full current.
At that point, we can see, I uncover the LEDs, there is just a very thin glow. We have the option now to actually reduce the amplitude of the PWM signal to achieve even further down the dimming result.
So now we are down to less than 0.2 milliamps of current going through these LEDs. So it gives you very very wide range operations.
Now we can increase this back up. And let’s turn the analog string back on. We see we have independent control of the analog and the PWM dimming for the two strings.
So this is the FL7740 and FL7760 buck regulator, and thank you very much.
