Lighting emergency gear – what waveform should we use?

November 22nd, 2019, Published in Articles: EE Publishers, Articles: Vector

This article explains the pitfalls in providing DC or a modified sinewave to drive LEDs in emergency mode.

Many LED luminaires have built-in drivers, so emergency gear must either simulate mains or attempt to drive the LED with DC power.

Lower-power LEDs with low power factors are easiest to drive due to their simple circuitry. However, efforts to increase power factors have increased the circuit complexity of newer generation LEDs. This can cause issues with some emergency gear. The best option is to provide a pure sinewave in emergency mode but generating a pure sine wave is relatively expensive compared to generating either DC or a modified sinewave.

Modified Sinewave

The whole idea behind the use of a simulated sine wave is to “fool” the lighting equipment into working as if it is fed by a pure sine wave (as it was originally designed to receive). By the way, the optimum timing for 230 Vrms, values of 5 ms high and 5 ms low and peak value of 325 V are required. This will optimally simulate a sinewave of 230 volts rms value.

However,


 

 

and
Vmean = Ton/Total x Vpeak

Typically, Vrms = 230 V and Vmean =162,5 V. In other words, Vrms and Vmean are incompatible. See the practical outcome in Fig. 1.

Fig. 1: Modified sinewave.

It is usually more desirable to get the mean voltage to be 230 V. In other words, for a peak of 325 V, we need 7 ms high and 3 ms low (the waveform in Fig. 1 shows 6,5 ms high; 2,5 ms low and peak of 320 V). This gives an rms value of 272 V which is dangerous for rms sensitive equipment. In practise, most lighting equipment has at least a full wave rectifier on the mains which, meaning it will not respond to the 272 Vrms but rather just to the peak value of 320 V.

Valley-fill power factor correction

One of the simplest forms of power factor correction used in many LED drivers is the valley-fill method shown in Fig. 2. This will typically raise the power factor to 0,7.

Fig. 2: Valley-fill circuit.

It is possible to drive this circuit with pure DC equal to the peak value which the internal rectifier will produce, i.e. 325 V. Driving it with a lower voltage (Vout in the valley fill circuit above) may result in the LED’s flashing or switching off thereby providing no emergency lighting.

Active power factor correction

Active power factor correction can raise the power factor to unity. This type of circuitry is becoming more popular to reduce power consumption. The simulated sinewave does not work well in all cases where the input of the product has more circuitry before the rectifier, usually power factor correction. Active power factor correction is required to get input current in-phase with supply voltage, i.e. a power factor of 1.

An example of a power system which will not work on a simulated sine wave or DC is active power factor correction shown in Fig. 3.

Fig. 3: Typical active power factor correction circuit.

Pure sinewave

In this case, the only option is to feed it with a pure sinewave (so that the product is unaffected by the difference between the drive from emergency gear and the original mains sinewave).

Fig. 4 shows the output of a Cosine pure sinewave emergency driver. The low distortion waveform can drive any light fitting without any compatibility issues.

Fig. 4: Measured pure sinewave output.

Fig. 5 shows a measured mains supply. The pure sinewave emergency provides a better waveform than the mains supply (granted, at the specific measurement location).

Fig. 5: Measured mains supply.

A pure sinewave provides a universal method of powering any light fitting in emergency mode. Also, the system can be used to power any load type without compatibility issues. An added advantage is a reduction of electromagnetic noise compared to a modified sinewave.

Contact Stirling Marais, Cosine Developments, Tel 031 579-2172, stirling@cosine.co.za

 

 

 

 

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