By: Michael Larsson, Field Applications Engineer (Power and Analog), Future Electronics
LED lighting is extremely valuable as an energy-saving technology compared to traditional incandescent lamps. But today, it is not as efficient as it might be. One source of the problem? The switch mode power supply (SMPS) housed in the LED lamp or its power adapter, which converts AC mains to the low voltage DC required by LEDs.
Uncorrected, this converter circuit has a low power factor and causes high harmonic distortion. This degrades grid power quality and causes power losses in the transfer of energy from the generator, via the grid, to the consumer.
Of course, the power factor problem is well understood by the industry, and for relatively high power LED lighting systems (>25W), European authorities have laid down regulations requiring the implementation of active power factor
At a maximum load <25W, however, there is today no requirement for PFC. But demand for domestic and commercial low power LED lighting is forecast to explode in the next three years. Without PFC, these millions of new LED lights will in aggregate cause a vast, and avoidable, waste of generated electricity. And there is clearly on the horizon the potential for today’s recommendations, which require lamps <25W to implement some PFC, to become mandatory; it would be prudent for LED lighting manufacturers to begin to take account of them now.
Several power IC manufacturers have recently released active PFC controllers that can easily and cheaply be adopted in LED lamps of <25W. This article looks at the choices available to designers of power systems for <25W LED lighting equipment, and explains the different approaches each takes to curing the power factor problem.
The Perfect Load
Power factor is the ratio of active power (measured in W) to apparent power (measured in VA). This figure shows how effectively energy is transferred to the load – in other words, how much extra power has to be delivered to the load in addition to the actual power consumed by it. The phenomenon of apparent power leads to resistive losses in the grid that increase in proportion with the square of the current as given by the formula P=I² x R.
Purely resistive loads, such as an incandescent bulb, have a power factor of 1: the current and voltage waveforms follow each other in shape and phase. By contrast, the AC/DC power supply in an LED lamp, if it has no PFC, operates as a non-linear load with a typical power factor of around 0.5.
In the AC/DC converter, energy is taken in a very short period of the alternating current cycle when the incoming voltage from the rectifier is higher than the voltage at the rectifier’s input capacitor. Because the time for energy transfer is so short, the current must be very high. This means that the current’s waveform has a sharp flank which generates multiple strong harmonics. Those conflict with the grid’s frequency.
These harmonics together with the low power factor serve to degrade the power network, affecting the performance of other electrical equipment on the circuit, while giving rise to power losses in the distribution network.
For instance, when the power factor in an LED light is 0.5, the apparent (VA) power will be twice the active (W) power. And in a three-phase electrical system, the powerful third harmonic will create an imbalance in the system that causes current to flow in the neutral wire. If this wire were not sized to carry reactive current, there could be a safety risk.
Of course, in previous years the total installed base of LED lights was small, so the safety risk and the potential harm to power grids attributable to this type of lighting equipment were both negligible (fluorescent lamps, on the other hand, have presented these problems for many years). Now, however, the situation is changing: production volumes of LED lights in Europe are estimated to be around 30 million units today, and are forecast to rise by the end of the decade to 200 to 300 million. Anyone concerned with grid power quality and energy efficiency will therefore be interested to see that many low power LED lights on the market today have a low power factor.
Measurement of LED Lights Available Today
Table 1 shows the results of measurements of a representative range of <25W LED lights on sale in Europe in October 2013. The measurements were performed with a PM100 single-phase power analyzer from Voltech.
It shows that one LED lamp features an impressive power factor of 0.96, providing effective
suppression of harmonic distortion. But the figures also show that in the majority of cases, the power factor is either totally uncorrected or inadequately corrected.
|Incandescent light bulb 60W||65.08||65.08||0.90||1.00||2.03|
|10W, 600lm LED lamp||10.46||10.93||3.06||0.96||11.57|
|8,1W, 400lm LED lamp||8.79||15.33||12.56||0.57||135.71|
|7,5W, 400lm LED lamp||7.45||9.86||6.47||0.75||73.42|
|7,5W, 400lm LED lamp||6.90||9.47||6.46||0.74||75.77|
|5,0W LED lamp||5.30||5.82||2.39||0.91||44.80|
|4,0W, 200lm LED lamp||4.25||7.33||5.98||0.57||127.00|
|3,5W, 200lm LED lamp||3.81||6.37||5.11||0.60||118.87|
|2,3W, 90lm LED lamp||2.55||4.96||4.26||0.51||36.00|
|2,3W, 80lm LED lamp||2.48||4.71||4.01||0.53||36.52|
|2,3W, 90lm LED lamp||2.45||4.62||3.94||0.53||36.20|
|Exterior LED lamp||12.72||22.80||18.88||0.56||117.80|
|8W, 310lm LED spotlight||8.64||10.01||5.07||0.86||53.10|
|4W LED lamp||4.78||8.55||7.04||0.56||119.80|
W is active power VA is the apparent power VAr is the reactive power flowing through the reactive load PF is the power factor %ITHD is the total harmonic distortion, the ratio between the sum of all harmonic current and the fundamental current
Table 1: power factor measurements of various sub-10W LED lamps
It is possible, however, that such uncorrected power supplies will in the future have to be withdrawn from sale. The question of power factor (otherwise known as ‘displacement factor’) has already been addressed in the European Commission’s regulation 1194/2012 of 12 December 2012, which implements Directive 2009/125/EC of the European Parliament and of the Council with regard to ‘eco-design requirements for directional lamps, LED lamps and related equipment.’
At the moment, the directive only provides a ‘recommendation’ on the minimum displacement factor to be required of applicable lamps. But such a recommendation is normally a precursor to full regulation. LED lamp manufacturers would, therefore, be well advised to prepare their designs for compliance now, to avoid the need for hasty changes at a later date.
The standard to which manufacturers will need to comply will be IEC 62612 Edition 1. It states that ‘no negative effects on the power grid are to be expected from self-ballasted LED lamps’ when complying with the following recommended specifications:
|Lamp maximum power consumption||≤2W||2W to 5W||5W to 25W||>25W|
|Displacement factor||No limit||≥0.4||≥0.7||≥0.9|
Many of the tested lamps shown in Table 1 fail to meet this recommendation for 5W to 25W lamps.
In North America, the US Department of Energy’s Energy Star program mandates a minimum power factor of 0.7 for the domestic market, and 0.9 for commercial LED lights.
The most immediate concern for manufacturers of LED lamps, then, is the prospect of stricter European regulation of power factor. There is also a debate among power generators and distributors about the possibility of charging consumers for apparent power rather than active power (a practice which already applies to large industrial customers). This would also turn the economics in favor of LED lights with a high power factor.
For example, a single 5W LED light with no PFC consumes around 125kWh over a lifetime of 25,000 hours. This requires apparent power of 245kVAh. On average domestic electricity costs around Europe, the additional cost of implementing active PFC in this LED light is just 1% of the cost of the extra 120kVAh of apparent power.
If consumers were made to pay for the apparent power, integrating active PFC into low power LED lamps would save consumers money as well as being good for the environment.
PFC and Power Regulation in a Single Device
The latest power converter ICs for LED power supplies implement PFC and power regulation in a single, low cost package. An excellent selection of PFC ICs, which generally cost just $1 or less, is shown in Table 2.
The most obvious approach to the power supply design is to use a single-stage converter operating directly from the rectified mains supply. This normally entails the lowest cost, since only one FET switches the power supply, and few components are required overall. Single-stage converter ICs may be isolated or non-isolated. In addition, unlike other topologies, a single-stage converter implementing primary-side regulation requires no 400V electrolytic input capacitor and no optocoupler. Since both of these device types are prone to early failure, their absence helps the designer achieve a long specified operating life.
A potential drawback of this topology is its output voltage ripple, which will have a component at the rectified line frequency (100Hz or 120Hz), together with a short hold-up time because of the lack of an input capacitor. While voltage ripple can be disastrous in sensitive analog systems such as image sensors and RF receivers, however, in LED applications it can be tolerated, although designers should be aware of its effect on efficacy, and of the risk that it will generate unacceptable flickering.
As Table 2 shows, single-stage conversion with PFC is implemented with different techniques by different IC suppliers. Almost all use primary-side regulation, in which the feedback loop uses current measurements taken at an extra winding on the same transformer that supplies the controller with power (the NCL30000 from ON Semiconductor senses on the secondary side, and sends feedback through an optocoupler.)
This winding also provides information about the current waveform, so that the device can achieve near-zero Volt switching in Quasi-Resonant (QR) mode, producing soft switching that supports high efficiency and generates little EMI.
|Manufacturer||Part Number||PF (typ.)||THD||Isolated||Input Range AC||Power Level||Total Efficiency||Dimmable||PFC Technique||Control Topology||Internal FET||Package|
|Diodes Inc.||AP1694||>0,90||<30%||Yes/No||80V to 300V||<15W||≤85%||With extra components||BCM||Flyback or buck converter with PSR||No||SOIC-8|
|Diodes Inc.||AP1690||>0,90||<20%||Yes/No||80V to 300V||<30W||≤82%||With extra components||DCM||Flyback or buck-boost converter with PSR||No||SOIC-8|
|Fairchild||FL7701||>0,94||<25%||No||80V to 308V||≤85%||0V to 10V||CCM or DCM||ZCD buck converter||No||8-pin SOIC|
|Fairchild||FL7730||>0,98||<12%||Yes||80V to 308V||≤86%||0V to 10V, Triac dimmer||DCM||QR flyback close to ZVS, PWM with PSR||No||8-pin SOIC|
|International Rectifier||IRS2983||>0,90||Yes||120V to 300V||<10W||≤90%||With extra components||CCM or DCM||Flyback or buck-boost converter with PSR||No||SO8N|
|NXP||SSL21084T||>0,90||<30%||Yes/No||170V to 260V||<15W||≤85%||With extra components||BCM||QR buck or isolated flyback, PWM with PSR||600V||SO12|
|NXP||SSL2129/AT||>0,90||<20%||Yes/No||85V to 265V||<25W||≤95%||No||BCM||QR buck or isolated flyback, PWM with PSR||No||SO8|
|ON Semiconductor||NCL30002||>0,90||No||90V to 264V||<20W||≤84%||With extra components||CRM||ZCD buck converter||No||SOIC-8 NB|
|ON Semiconductor||NCL30000||>0,97||<10%||Yes||90V to 305V||<17,5W||≤85%||With extra components||CRM||ZCD flyback with opto feedback||No||SOIC-8 NB|
|Richtek||RT7302||>0,96||<12%||Yes/No||80V to 300V||<60W||≤94%||PWM dimmable||CRM||QR flyback or buck-boost (non isolated) PWM with PSR||No||SOP-8|
|Richtek||RT7304||>0,96||<12%||Yes/No||80V to 300V||<40W||≤91%||No||CRM||QR flyback or buck-boost (non isolated) PWM with PSR||No||SOT-23-6|
|STMicroelectronics||HVLED807PF||>0,94||<20%||Yes||90V to 265V||<7W||≤84%||Triac dimmer only 115V||BCM||QR flyback close to ZVS, PWM with PSR||800V||SO16N|
|STMicroelectronics||HVLED815PF||>0,94||<20%||Yes||90V to 265V||<15W||≤84%||Triac dimmer only 115V||BCM||QR Flyback close to ZVS, PWM with PSR||800V||SO16N|
Table 2: a selection of ICs suitable for high power factor, low power LED lamps
(Dual-stage conversion – in which the first stage controls the power factor only and feeds a high DC supply to the second conversion stage – is also found, but normally at power levels above the 25W maximum examined in this article.)
Methods for Power Factor Control
The devices shown in Table 2 adopt a variety of techniques for correcting the normally low power factor of non-linear AC-DC power supplies. The basic requirement for a high power factor is to draw current in parallel with the rectified voltage’s sine waveform. The various techniques used to control the MOSFET’s switching depend on the power level and on the technology that each IC manufacturer has developed. Each has its own advantages and disadvantages.
Continuous Current (or Conduction) Mode (CCM) is normally used when the load is >300W. Here, the frequency is fixed and the duty cycle is modulated. In its favor, it offers stable operation at light loads, and it smoothes peak currents easily, and as it has a low ripple current, flickering is negligible. The main drawback is that a larger inductor is needed – about double the size of the inductor required in boundary-mode topologies. In addition, the high reverse-recovery losses in the diode impair the system’s efficiency.
Critical Conduction Mode (CRM), Boundary Conduction Mode (BCM) or Transition Mode (TM) may all be used when the load is <300W. These methods feature a variable switching frequency with a fixed on-time.
While in CCM the current through the inductor is continuous, in CRM, BCM and TM each new switching period is initiated when the inductor current returns to zero. In other words, these techniques are at the boundary between continuous conduction and discontinuous conduction.
The advantage of CRM, BCM and TM is that the MOSFET is turned on when the current is at or near to zero, which reduces switching losses and generates low EMI. In addition, diode reverse recovery is eliminated and a fast recovery diode is not required, and the circuit can use a small and cheap inductor.
On the other hand, the system’s higher peak current at the transformer produces a higher switching ripple on the output, adversely affecting EMI.
Discontinuous Conduction Mode (DCM) has a fixed relation between frequency and on-time; both parameters are adjusted relative to the load. This method provides for a higher power factor and lower THD than CRM, BCM or TM. But the peak current for any given load is even higher than in CRM, BCM and TM mode, generating higher levels of EMI.
Highly integrated AC-DC LED driver ICs with PFC for <25W designs are readily available today. The decision to add PFC to an LED lamp design adds a negligible extra amount to the bill-of-materials cost, yet delivers great benefits in energy savings and power quality, and in preparedness for new European regulation.
All the featured devices in Table 2 are excellent. The designer’s choice will often come down to a preference for CRM, BCM, TM or DCM as the correction method. But all these integrated devices provide for a low component count and low system cost; some also support LED dimming.