It has been a while since I shared LED driver experiments. I have done a lot with different LED driver topologies and drivers lately yet had no time to sit down and do a proper write-up. So, here’s a little post on linear LED drivers. They are also called driver on board (DOB) LED light engines!
This is a project that is a long-range remote controlled barn light. I focused on playing with various AC230V DOB LED Plates. I settled with cheap 9W types because of my changing evaluations, and the great support provided by my nearest electronics store. There will be further progress of the project within a few weeks. Stay tuned!
In the following section I will provide a set of random lab notes which are aimed at shedding some light on today’s high-voltage linear LED drivers. I hope you will find this post useful, and please take the time to provide any feedback.
Linear LED Drivers
Linear LED drivers are generating considerable interest in the maker community because of their simple architecture, low noise operation, miniature circuit board size and low parts count. A linear LED driver uses linear, non-switching techniques to regulate the power to an LED or strings of LEDs. The linear LED driver owes its name to the linear regulator because a linear regulator operates by controlling the voltage drop across a pass element biased in the linear region.
A typical linear LED driver includes a full wave bridge rectifier, a ripple filter and a linear regulator. The ac power is rectified into dc power and is smoothed out by a low pass filter to achieve a steady output voltage with low ripple current. The refined dc voltage is fed to a linear regulator which consists of an internal reference voltage, an error amplifier, a feedback circuitry and a series pass transistor. The error amplifier continuously compares the difference between the reference voltage and the feedback signal provided by the feedback circuitry. The series pass transistor, operates in the linear region, adjusts the input voltage to the desired output using the error amplifier. Rapid advances in linear LED driver technology gave birth to DOB LED Light Engines – the so called driverless AC LED Plates!
By contrast, a typical DOB mechanism uses high-voltage LEDs which can be directly operated by regular AC voltage only by employing a compact high-voltage linear driver circuit. Therefore, it has the advantages of small size, high luminescence efficiency, and cheerfully high power factor (PF).
Power Factor (PF)?
What is PF and why is it significant? Simply, PF is an expression of energy efficiency, usually expressed as a percentage – the higher the percentage, the highly efficient power usage is. In other words, PF is a measure of how effectively incoming power is used in your electrical system (energy efficiency) and is defined as the ratio of real (true) power to apparent (total) power.
→ Power Factor (PF) = Real Power / Apparent Power
The ideal PF is unity (1) which means that all the energy supplied by the power source is consumed by the load. You can look at the simple beer mug analogy in order to easily understand these terms.
It seems that a linear LED driver chip tailored for driving cascaded loads often incorporates multiple current regulators which are set with different voltage and current steps, so that the voltage and current steps are made large for the first regulator and small for the last regulator. This makes the load roughly sinusoidal in phase with the power line voltage and thus results in a high PF and low THD (total harmonic distortion).
There are some proven methods for power factor correction (PFC) to restore the power factor to as close to unity as is economically viable. This Toshiba application note discusses the basic topologies of the PFC circuits and their operations in detail https://toshiba.semicon-storage.com/info/docget.jsp?did=68570
AC230V LED Plate
This session will walk you through the lazy circuit analysis of a randomly picked AC230V/9W LED Plate. I proposed this plate for the barn light project.
The AC230V/9W white LED Plate I picked is a lovely piece of engineering on a 40mm diameter metal core printed circuit board (MC PCB). It holds a total of 20 cool white SMD LEDs. There’s also an 8-pin driver IC, a bridge rectifier, and a fixed resistor. That’s all!
Since I didn’t get the schematic of this particular module from anywhere, I simply prepared a rough schematic to understand its design. Below you can see that reverse-engineered circuit diagram, but the accuracy is not guaranteed!
The PCB has blank solder pads for one capacitor and two resistors. I think the first resistor (R1) pad (across the power supply input) was actually intended to add a metal oxide varistor (MOV). But nothing is known about the second resistor (R2) pad, and why the capacitor C1 was omitted.
As with most cheap chips, the YZ1000AT IC’s datasheet does not seem to be easily available on the web (at least in my search). I’m sure its basic functionality is similar to the SM2082D linear LED driver still used in some DOB LED modules. Below you will find an excerpt from its datasheet (https://pdf.dzsc.com/99999/2016111142730107.pdf).
Here is the datasheet of another linear driver (GVN3501) you might be interested in https://content.web-repository.com/s/90881806155485785/uploads/Images/GVN3501_Preliminary_Data_Brief_r1_2-0308443.pdf
In the quick live test, the LED plate worked as expected. More interesting is its PF value. My trusty digital micro power monitor showed that the PF is unity (1). Yes, this clever design has a much better power factor than the classic design.
Caution: Do not operate the DOB LED plate for more than a few seconds without a proper heat sink, as it may kill the entire electronics assembly!
Disassembling electronic devices can be very instructive. Often, taking it apart will uncover an unusual circuit or a clever design trick. Although I know about linear LED drivers, this high voltage linear DOB LED driver idea really surprised and inspired me. I also want to do something following this great idea.
We might be able to add a dimmer with this design. The first plan is to experiment with a MOSFET, driven by an external PWM signal, as illustrated in the image below. Since they are available everywhere and are inexpensive, I am willing to sacrifice one or more DOB LED modules for this endeavor. Let’s do it!