Electrical Considerations of LED Bulbs

Governments all over the world regulated the future use of the inefficient incandescent lamps. For example, the member states of the European Union agreed to a phasing out of incandescent lamps by 2012. The initial European-wide ban applies the first step to nondirectional light bulbs. The first types of bulbs to be banned are frosted (non-clear) bulbs and clear bulbs over 100 W, which will be phased out completely by September 2009. The power limit will be moved down to lower wattages, and the efficiency levels raised by the end of 2012.
The replacement of 3.5 billion incandescent lamps installed throughout Europe with more-efficient lighting technologies, such as LEDs will lead to a relevant reduction in the power consumed by lighting systems.

The efficiency values and the light quality of LED bulbs are the focal point nowadays. Nevertheless, the behavior of the LED bulb technology in respect to the electrical parameters is of interest as well. LED professional has tested seven LED bulbs from four manufactures to get an inside view and reports the surprising results in this article.

Testing Conditions
Most recently produced LED bulbs from ATG Electronics, Line Lite, Exceed and Lemnis Lighting were analyzed (see Table 1). The first test stage covered the electrical parameter measurements of the mains such as input power, input current, power factor, current and voltage distortions. In the second test stage the LED bulbs were disassembled to extract the electrical circuit diagrams, and to study the mechanical designs. All tests were performed under room temperature and the devices were powered through a 500 VA electronic power amplifier simulating stable mains conditions.

See Figure 1

Measurement Results: Power Analysis
The power factors of the measured LED Bulbs showed poor values in general, ranging from 0.32 to 0.48 (see Figure 2). The low power factor values are the reasons why only 32-48% of the apparent mains power is transmitted to the LEDs as active power (see Figure 3 - LpR magazine). Nowadays, international standards claim for power factor correction circuits only for power values above 25 W. Nevertheless the reactive power has to be transferred from the power plants to the loads, ending up in unnecessary power demands and increased transmission losses.

Load Reduction Potential of Power Plants in the European Community:
Today, 3.5 billion incandescent lamps are installed in the European Community [1]. Two billion lamps are replaced each year with an intermediate lifetime of about 1,000 hours [2]. These figures imply that in average two billion lamps must be switched on for three hours every day. Due to the European time zones, it is possible that these lamps can be switched on at the same time. With an average lamp power level of 40 W (400 lm/bulb), a peak power demand calculation shows a result of 80 GW. The overall energy consumption per year can be calculated by multiplying 40 W by the intermediate lifetime by the number of replaced lamps per year, 40x1,000x2,000,000,000 = 80 TWh/a.

The replacement of the installed incandescent lamps with 8 W LED bulbs (50 lm/W, warm-white) and similar lumens output values as the incandescent lamps, would cut down the energy consumption per year to 16 TWh/a. In other words, an energy saving of 64 TWh/a could be reached, an amount that needs over 50 large 1,200 MW light water reactors. But what does this mean for the peak power load of the power plants?

The measurements showed that most of the recent LED bulbs have a relatively poor power factor resulting in a high apparent power. Hence an apparent power of 20 VA has to be calculated, in the worst case even 25 VA. As a result the peak power load is between 40 GW and 50 GW. This means a reduction between 40 - 50%, setting free a capacity of up to 25 of the named light-water reactors for other tasks. At first glance, that does not look so bad, but this is just 50% of what one would expect regarding the energy saving calculations. The reason is that only 35% of the apparent power is active power.

A replacement by LED bulbs with high-power factor values between 0.9 and 1.0 would reduce the peak power demand by an additional 50 -65%, to 16-18 GW, hence set free peak power capacity of up to 64 GW compared to incandescent lamps. Improving the power factor to 0.9-1.0 finally is the key factor to freeing up the capacities at existing power plants. It has to be recognized that most compact fluorescent lamps suffer from the same problem - at least as much as LED bulbs do. On the other hand, it has to be mentioned that there are also products available that offer power factor values of 0.85 or even more.

See Table 2 (see LpR magazine)

Measurement Results: Mains Current Distortions
The effects of the poor power factors can also be obtained at the mains input currents. Figure 4 and Figure 5 show very typical signals of the mains input currents for LED bulbs without power factor correction means. Near the input mains voltage maximum high input currents flow to charge up the internal capacitors of the LED bulbs and to supply the LEDs. The phase angle of the current flow is small and reaches only values of 10-15o. In an electrical installation, the high currents will occur at nearly the same time and will lead to a distortion of the mains voltage, generating additional wire losses. The di/dt current values of such LED bulbs vary in a wide range between 1.0 mA/μs to 253 mA/μs.

See Table 3 (see LpR magazine)

See Figure 4 (see LpR magazine)

See Figure 5 (see LpR magazine)

Figure 6 shows the mains current signal of a tested product. The phase angle of the mains current is much higher due to the usage of a passive valley fill circuitry. The switching frequency (some kHz) of the internal circuit can be measured almost unfiltered at the LED bulb mains terminals.

See Figure 6 (see LpR magazine)

Circuit Diagrams
Typical circuit diagrams of LED bulbs are shown in Figure 7 and Figure 8. The circuit in Figure 7 uses a passive valley fill circuit in the mains input part built up with the components C1, C2, and D1-D3. This topology connects the C1 and C2 capacitor in series for the charging phase while the capacitors C1 and C2 are connected in parallel through the diodes D1 and D3 during the discharging phase. This enlarges the phase angle of the mains current flow and improves the power factor. This circuit corresponds to the signals showing in Figure 6. The high-frequency switching of the LEDs through the MOSFET Q1 leads to a high distortion of the input current and voltage. The LED strings are connected in parallel to C3. There is no galvanic isolation of the LED strings from the mains input voltage. The actual product uses only the coating material of the metal core substrate to isolate the outer parts(metal housing) from the mains voltage.

See Figure 7 (see LpR magazine)

See Figure 8 (see LpR magazine)

Figure 8 shows an improved circuit with a galvanic isolation between LED output and mains input circuit parts. Since there is no power factor correction means, the power factor is still poor and the phase angle of the current flow is low. An optocoupler (OC1) is used to close the feedback loop to guarantee the galvanic isolation.

Conclusions
Incandescent lamps are banned. The market is ready for new technologies such as LEDs. LED professional has tested seven actual LED bulb products regarding the mains parameters and the electrical circuitries to check the quality of their electrical designs.

Firstly, the security issue seems to be most important. One product uses the metal core substrate coating material to isolate the housing from the mains. Regardless of the coating specification, this seems to be i