Until consideration use the intersection of LED and driver change topological use invariable current source that LED support for into the intersection of AC and input voltage, it is helpful to employ LED and divide into three kinds of power level: 1 Low-powered application. Demand to input and is lower than 20W, such as lamp strip, R light and substitute of the incandescent lamp; 2 Application of medium-sized power. Input is at most 50W, such as flush light of the ceiling and L light; 3 Heavy-duty application. Demand to input and is higher than 50W, such as the light of the sign or lantern. The designer faces different challenge associations within the range of these three kinds of power, including the space, efficiency, design complexity, power factor, average invalid time mean-time-to-failure for cost, installation LED driver, MTTF And reliability, the aforesaid is only some of a great deal of challenges. What this text will recommend using within the range of these three kinds of basic power is basically topological to design the challenge in order to tackle.

The low-powered solution faces the lighting lamp application of small dimension, the design that the application demands to install LED driver is small, reach the stable ray radiation by controlling the electric current flowing through LED, and have high efficiency and low cost. In order to accord with ” Star Energy Star of the energy ” The planning of the light fixture requires, the power factor of the lamps and lanterns of the house must be greater than or equal to 0.7, and is greater than the business application of 5W to the input power, the power factor must be greater than or equal to 0.9.

1 Need the intersection of LED and driver isolate, step down controller to be topological to have the lowest BOM cost, therefore is a low-cost solution that can consider. 1 Fig. isolate, step down topological example, including power factor correction and ability of adjusting luminance, there is a magnetic element only ‘ A simple inductance And a single MOSFET / diode is right, used for stepping down power conversion. If the input voltage is higher than LED and support the required output voltage, it is the best choice that this is topological.

*1 Fig. with PFC to isolate, step down converter *

Until consideration use the intersection of LED and driver change topological use invariable current source that LED support for into the intersection of AC and input voltage, it is helpful to employ LED and divide into three kinds of power level: 1 Low-powered application. Demand to input and is lower than 20W, such as lamp strip, R light and substitute of the incandescent lamp; 2 Application of medium-sized power. Input is at most 50W, such as flush light of the ceiling and L light; 3 Heavy-duty application. Demand to input and is higher than 50W, such as the light of the sign or lantern. The designer faces different challenge associations within the range of these three kinds of power, including the space, efficiency, design complexity, power factor, average invalid time mean-time-to-failure for cost, installation LED driver, MTTF And reliability, the aforesaid is only some of a great deal of challenges. What this text will recommend using within the range of these three kinds of basic power is basically topological to design the challenge in order to tackle.

The low-powered solution faces the lighting lamp application of small dimension, the design that the application demands to install LED driver is small, reach the stable ray radiation by controlling the electric current flowing through LED, and have high efficiency and low cost. In order to accord with ” Star Energy Star of the energy ” The planning of the light fixture requires, the power factor of the lamps and lanterns of the house must be greater than or equal to 0.7, and is greater than the business application of 5W to the input power, the power factor must be greater than or equal to 0.9.

1 Need the intersection of LED and driver isolate, step down controller to be topological to have the lowest BOM cost, therefore is a low-cost solution that can consider. 1 Fig. isolate, step down topological example, including power factor correction and ability of adjusting luminance, there is a magnetic element only ‘ A simple inductance And a single MOSFET / diode is right, used for stepping down power conversion. If the input voltage is higher than LED and support the required output voltage, it is the best choice that this is topological.

*1 Fig. with PFC to isolate, step down converter *

While needing to isolate LED driver, a good topological choice is regulating in the primary-side primary-side regulated, PSR Anti exciting and topological; Fig. 2 is an PSR anti exciting LED driver example. Not needing secondary end to feedback, can lower costs, therefore this topological component number is less, can realize the good invariable current control. Control device can integrated by reducing BOM number and reducing the space of printed-circuit board MOSFET. Because does not need to use and can be improved while used in the opto-isolator PSR anti exciting reliability that the second order feedbacksing.

*The primary-side of Fig. 2 regulates the converter *

Anti exciting and topological to PSR, turn on the mode Discontinuous Conduction Mode, DCM in discontinuity It is a first-selected work pattern, because it can regulate and export better. The typical waveform is shown as in Fig. 3.

*Fig. 3 DCM anti exciting converter waveform *

When PSR LED driver works in invariable voltage regulation mode, during electric current discharge time Tdis of the inductor, the sum of output voltage and diode forward voltage drop is reflected to the end of auxiliary coil. Because diode forward voltage drop reduces as reduce through the output current of the diode, at the end of Tdis of discharge time of the diode, the voltage of the auxiliary coil reflected the output voltage. The voltage of auxiliary coil of the port of the end is sampled through the discharge time in the diode, obtain information of output voltage.

When working in invariable current control mode, can estimate the output current while using the drain current IPEAK and inductive current discharge time Tdis of crest value, because the output current is the same as mean value of the electric current of the diode under the stable condition. Adopt and fly and promise TRUECURRENT that the semiconductor innovates? Technology, can control invariable Tap accurately.

PSR topological efficiency can be up to 85%. As an example, consider the application of 8.4W, it is 1.32W that the total power consumption of LED driver is measured when 85VAC inputs. The expenditure lossed, maximum comes from the voltage transformer, estimate it as 0.55W, and then buffer circuit are show as in Fig. 2, diode and parallel resistance and electric capacity connect in series, bridge on the voltage transformer primary coil ,Its loss is 0.31W, the loss of MOSFET is 0.26W, and export and commutate 0.20W with bridge rectifier to loss.

2 Voltage transformer and buffer circuit are usually mainer power dissipation assemblies, because from voltage transformer leaks the inductance, need buffer circuit to prevent the voltage from exerting pressure on MOSFET, if has not noticed these two design respects, printed-circuit board and input EMI electric-wave filter can become the prominent power dissipation source.

Overall 1.32W losses the important source that may seem it is not a power consumption, but in one low-powered the intersection of LED and driver, LED support close to driver, make, design generate heat overall load power including driver loss. Heat transfer will not choose to force cooling blast, therefore the example quoted above must be used can be from the high-efficient lamps and lanterns which conduct 8.4W power in semiconductor and electricity device, in order to maintain reliability. If the heat-dissipating solution can not equilibrate this power and combine low temperature of the holding element, then, using the electrolytic capacitor will reduce the mean operating time between failures MTTF designed .

The medium-sized power solution still requires small volume design and power factor correction. Efficiency and reliability remain important design constraint within the range of this power. It is that the single stage is anti exciting and topological in power factor correction to be workable and good and topological, as shown in Fig. 4.

*Fig. 4. Single stage PFC anti exciting converter *

The single stage is designed to reduce the component number and not need to input the large volume capacitor, not only has saved the design space but also lowered costs. The ones that are used in correcting control of the power factor are anti exciting, use the second order to feedback. Adopt the anti exciting topological design of these medium-sized power, can realize the efficiency up to 84%. Because topological adopts the anti exciting way, in driver design of this LED, voltage transformer and buffer circuit remain the master work and consume the source of dawdling. In the medium-sized power range, have increased the power consumption of the buffer circuit as compared to high power level, because the buffer circuit losses and leaks the proportional quadratic product of the crest current in inductance and MOSFET with the voltage transformer. In this medium-sized power design, the size of the voltage transformer is increasing, and the crest current in MOSFET is increasing.

The high-power solution pays close attention to optimal efficiency and reliability, reasonable cost and less BOM number. Recommend using two grades type LED driver. First step is used for power factor correction, and then DC-DC changes grade to regulate invariable Tap. First step can adopt the control device the same as the medium-sized power range single stage PFC anti exciting converter design. In order to reduce the component number in this two-step method, on first step, the control device has integrated some components and characteristics.

Recommend two kinds of secondary DC-DC converters to choose here: Quasi-resonance is anti exciting, used in the application less than 100W, or LLC is topological, used for being higher than the application of 100W. Anti exciting scheme can reach reasonable efficiency, topological to choose as to LLC, it a complicated one topological very. Turn on the voltage of electric capacity while opening through reducing, QR is topological to reduce the switching loss correlated to MOSFET output capacitance. QR topological MOSFET soft switch reduced EMI too. However, topological to LLC, better efficiency attributes to the zero voltage switch zero voltage switching of MOSFET ,And can use and keep holdup small-scaly Capacitor. Can realize the efficiency up to 92% in this two-step method. Fig. 5 and Fig. 6 have revealed QR and LLC are topological. Please notice the intersection of LLC and circuit in the 6 Fig. use to leak inductance and magnetization inductance so as to set up the intersection of LLC and acceptor voltage transformer.

*Two-step PFC QR anti exciting example of Fig. 5 *

*Two-step PFC LLC example of Fig. 6 *

High-power employs and usually uses many strings LED. Fig. 6 has shown equilibrating through different LED a bunch of electric current supported with secondary control device.

Conclusion

This text directs against the driver application of off lining LED of three kinds of different power ranges, it is topological to recommend different one. According to the power level of increasing LED load, the converter, PFC single stage are anti exciting after proposing stepping down, and two-step PFC is anti exciting, and proposed or LLC scheme that QR is anti exciting subsequently. Topological schemes of each kind of recommendation are considered on the basis of available design space, efficiency requirement, reliability, cost and design complexity of installing LED driver,etc., can preferably meet the above-mentioned constraint.