Design Con 2015

Product How-to: Drive LEDs with fluorescent ballasts (part 2)

Tom Stamm, Jianwen Shao, STMicroelectronics -December 08, 2013

In part 1 we discussed rectifying the ballast AC current to DC and we showed an open load protection circuit.


Direct replacement of fluorescent tubes with LED assemblies can lead to uneven lighting. It may be desirable to control the LED current to match the output of the less efficient tubes. The circuit in Figure 1 accomplishes this current regulation.



Figure 1 – AC/DC converter with active LED current control

 

The ballast can be regarded as a high-frequency current source. So long as the load current required is less than the ballast’s output, regulation can be achieved by short-circuiting the ballast for part of each half cycle.


The control signal will synchronize with output from ballast. It is important to synchronize the switching action of Q2 with the ballast output, otherwise the ballast might shut down. The control is very simple. When Q2 is off, the output current from the ballast will feed LED load. If LED current is higher than setting point, Q2 will be turned on.  So no power is transferred to LED. So the duration of “on time”, duty cycle, will determine the LED current. The controller will sense the LED current by Rsense, calculate the proper D, and send control signal to switch Q2. If the duty cycle is D, the LED current is:

The controller can be either analog controller or a microcontroller. The detail of the circuit operation can be found in Reference of the patent application info.


Open-load protection is necessary when a power current source feeds any electronics. Q1, an SCR, fires at about 82 volts if the load cannot sink all the current supplied by the ballast thru the transformer turns ratio.

 

The circuit has been tested with three different ballast types, and regulates the LED current within a fraction of a percent over a range of LED voltage.  The LED string length over which each ballast type works correctly is shown in Table 1.

Table 1 – LED string length for ballast type

Ballast Type

4 Foot T8 Ballast

Program start

4 Foot T5 Ballast

4 foot T8 Instant Start Ballast

Minimum String Length

9 LEDs at 700mA

9 LEDs at 700mA

9 LEDs at 700mA

Maximum String Length

16 LEDs at 700mA

19 LEDs at 700mA

22 LEDs at700mA

 

Actual LED current varied between 0.697A and 0.698A over the ranges above. Efficiencies from AC line through the ballast and regulator to the LED load varied from 75.2% to 77.2% with a load string of 16 LEDs.



Figure 2 – Typical Waveforms:

                                                                        Yellow: Input Voltage

Magenta: Input Current, 100mA/div

Blue: LED Voltage

Green: LED Current, 200mA/div, ref -3div

 

The input current harmonic content is quite high, sometimes leading to false zero crossing detection. The SMALL inductor (L1) at the input is used to reduce the harmonic content. Larger inductor values can resonate with the ballast’s output current limiting capacitor at the inverter frequency, defeating the ballast’s current limit mechanism.  Not advisable. But if the ballast operating frequency is known, a 5th harmonic parallel resonant trap can be inserted in place of L1, for an overall efficiency gain of 1-2%.


Reference:


“Power Converter for Interfacing a Fluorescent Lighting Ballast to a Light Emitting Diode Lamp”

US Provisional Patent Application No. 61/817,065 filed April 29, 2013

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