WO2018038681A1

WO2018038681A1 - A multi-channel driver circuit and method for leds

Info

Publication number: WO2018038681A1
Application number: PCT/SG2017/050413
Authority: WO
Inventors: Cikai YE; Ramprakash KATHIRESAN; Pritam Das; Thomas Guenter REINDL
Original assignee: National University Of Singapore
Priority date: 2016-08-22
Filing date: 2017-08-22
Publication date: 2018-03-01

Abstract

A multi-channel driver circuit for LEDs and a method of multi-channel driving LEDs. The method comprises limiting a peak current on a secondary side of a non-resonant DC-DC circuit for protection of the LEDs; and controlling an average current on the secondary side of the DC-DC circuit for dimming of respective groups of the LEDs.

Description

A MULTI-CHANNEL DRIVER CIRCUIT AND METHOD FOR LEDs

FIELD OF INVENTION

The present invention relates broadly to a multi-channel driver circuit and method for light emitting devices (LEDs).

BACKGROUND

There has been a growing impetus to reduce energy consumption by urban buildings so as to reduce the carbon foot print of a city. This has led to research and development of "NET ZERO Buildings" wherein a building or a group of buildings will produce their own energy from renewable energy sources installed on site and will have highly energy efficient smart appliances which are operated optimally so as to reduce electrical power demand. One of the key electrical loads in buildings is lighting. It constitutes about 10%~12% of the total electrical loads in a commercial building.

It has been found that LEDs are 3 times more efficient than CFL (Compact Fluorescent Lamp) in converting electrical energy to light and also their disposal is easier than mercury filled CFL lamps. Optimized lighting with LED, precisely conforming to the exact needs for lumens in a certain zone in a building depending on available natural light and occupancy and other factors can significantly reduce the electrical energy demands for lighting in a load. However, the key difficulty with LEDs is that they require DC current for their operation and controlling their luminous intensity which enforces available electrical energy from the utility mains or renewable energy to be converted into a constant current source. From this stems the importance in developing the power electronics involved in driving LEDs.

According to Fig. 1(a), an LED driver 100 typically has AC Mains 102 input, that can vary from 90Vac to 265Vac. The AC Mains 102 passes through an electro-magnetic interference (EMI) filter 104, a rectifier 106 and power factor correction (PFC) boost circuit 108 which accomplishes input PFC) and also boosts input AC to 400V DC output voltage. This DC voltage feeds a housekeeping flyback converter 110 with an isolated 12VDC output for low- dropout (LDO) 112. The function of the LDO 112 is to produce a 3.3VDC for micro- controller unit (MCU) 114 supply. Another function of the 400VDC is powering the isolated DC-DC converter 116. In this part of the circuit, the 400VDC bus is converted to a high frequency bi-polar AC waveform by properly gating ON and OFF the MOSFET switches 118; this high frequency AC voltage is incident across the primary of the high frequency transformer 120 that provides galvanic isolation of the output LEDs 122 from the 400VDC bus. The voltage of secondary rectifies 124 the high frequency AC to produce a DC voltage to drive the LED lamps 122. The control of MOSFET switches 118 depends on the peak of the transformer 120 primary current which is proportional to the LED current or the Pulse- width modulation (PWM) frequency. Meanwhile, the Touch & Dim interface 126 is used to control the brightness of LED lamps 122.

According to Fig. 1(b), the 400 VDC is from the output of PFC boost circuit 108. Due to the large capacitors CI and C2 of equal capacitances, the voltage across each of them is 200 VDC. When the switch Ql is on and Q2 is off, the voltage between the two transformers 130a, b is 200V. At this moment, the transformer primary current increases due to the positive voltage across the combined leakage inductance of the transformer primary. When Ql is off and Q2 is on, the voltage between two transformers 130a, b is -200V. At this moment, the current reduces due to the negative voltage. After the current reduces to zero, it will increase in the negative direction. As a result of this charging and discharging of the primary leakage inductor, energy is transferred from the primary side to secondary side of the transformers 130a, b. The rectifiers 136a, b (symmetric voltage doubler) at the secondary side will rectify the transformer secondary voltage to DC and supply the LED lamps e.g. 138. It should be noted that each switch Ql, Q2 is ON for half of the total switching cycle.

By changing the frequency of the gating waveform of the switches Ql, Q2, the peak current of primary side of the transformers 130a, b is varied, which leads to the variation in the energy transferred through the transformers 130a, b and hence the average current fed to the output LEDs e.g. 138 whose light intensity can hence be varied. By increasing the frequency of switches Ql, Q2, the charging time of the primary inductor reduces. As a result, the peak of the transformer primary current reduces and hence the average LED currents reduce, and vice versa.

On the other hand PWM dimming can also be realized just by activating and deactivating the primary side switches Ql, Q2 at a frequency lower than the switching frequency of these switches Ql, Q2 so that the average LED current in each channel will be proportional to the duty ratio of the low frequency waveform.

The key advantages of the above mentioned multi-channel single stage LED driver are:

1) Truly single stage converter which reduces additional active and passive devices resulting in higher reliability, reduced cost and increased electrical efficiency

2) The unique non-resonant energy conversion by energizing and de-energizing the high leakage inductance of the frequency transformer results in globally asymptotic stability of the converter. This is essential for reliable operation during PWM dimming of the converter wherein the primary side switches will operate at a fixed switching frequency when activated by a PWM waveform of certain duty ratio and whose frequency is much less than the switching frequency.

3) Wide load range zero voltage switching (ZVS) of primary side devices will be realized using the extremums of the transformer primary current. 4) The converter will always operate in the lagging current mode wherein the zero crossings of the transformer primary current will always lag those of the output voltage at the output of the half -bridge converter.

5) No high voltage DC blocking capacitor is necessary due to symmetry of the half bridge used here

On the other hand, this multi-channel LED driver has one drawback which is dimming of the 8 channel LEDs occurs together and equally, not selectively for each channel. In a building where the light needs to be optimized depending on the ambient light, occupancy and user needs, then selective dimming of individual channels or group of channels is important.

According to Fig. 2, in one possible solution, the light sensor / photodetector 200 returns a feedback to the control block 202 which sends PWM signals with adjustable ratio to 3 power converters 203-205 to control three lamps Lampl to Lamp3.

This solution realizes the function of varying R(red) G(green) B(blue) brightness selectively. However, the product consists of one controller 202 and three power converters 203-205 for three lamps Lampl to Lamp3, which increases the size and total cost, and meanwhile reduces its efficiency.

The key drawbacks of this solution are:

1) Large number of active and passive components owing to application of separate controlling converters at the input of every LED channel

2) Reduced efficiency because of multiple energy conversion stages

3) Increased cost

For isolated selective dimming LED drivers, typically, flyback and resonant DC-DC stages are used for isolation. A flyback based isolation topology, which maintains a constant average transformer primary current by varying the frequency and duty ratio of MOSFETs on the primary side, is presented in Chin Chang, Yorktown Heights; Subramanian Muthu, Tarrytown; Gert W. Bruning, Sleepy Hollow, all of NY: White light-emitting-diode lamp driver based on multiple output converter with output current mode control, United States Patent. Patent No.: US 6,369,525 B l . Average current of each LED channel remains constant when they are ON. The MOSFETs on the secondary side control the duty ratio of each channel to control the selective brightness. However, this flyback based topology only controls the average current on the primary side of the transformer, and the pulse of primary current may trigger the single -pulse over-current of the LEDs and can lead to a failure of the devices, see e.g. "Pulsed Over-Current Driving of Cree Xlamp LEDs: Information and Cautions," [Online] Available:http://www. cree.com/led- components/media/documents/XLampPulsedCurrent.pdf. Also, the isolated resonant DC-DC converter proposed in W. Feng, F. C. Lee and P. Mattavelli, "Optimal Trajectory Control of LLC Resonant Converters for LED PWM Dimming," in IEEE Trans, on Power Electron., vol. 29, no. 2, pp. 979-987, Feb. 2014, has the limitation of PWM dimming since the dynamic oscillation in the resonant tank during each dimming cycle will trigger the high current spike which may damage the LEDs as mentioned before. While secondary current control can be used to eliminate the current spike, fixed switching frequency variable on time control is applied to control the brightness of resonant DC -DC converter. However, the on state LED peak current control is hard to achieve because very fast control of resonant converter during LED ON, which is of the order of few tens of us time, is not possible due to dynamic oscillations during frequency variations. An isolated Quasi-two-stage (PFC and resonant DC- DC) topology proposed in J. Zhang, T. Jiang, and X. Wu, "A high-efficiency quasi-two-stage LED driver with multichannel outputs," IEEE Transactions on Industrial Electronics, vol. 64, no. 7, pp. 5875-5882, 2017, and in T. Jiang, J. Zhang, K. Sheng, and Z. Qian, "High- efficiency quasi-two-stage converter with current sharing for multi-channel LED driver," in Future Energy Electronics Conference (IFEEC), 2013 1st International, pp. 311-315, IEEE, 2013, can achieve the target of selective dimming meanwhile eliminating the current spike. However, this topology uses two secondary side DC buses obtained from the resonant DC- DC stage for LED current and dimming control. An auxiliary Buck-Boost stage including one buck-boost control chip, one filter inductor, one MOSFET and one diode, has to be added for each channel to achieve the selective dimming.

On the other hand, a switched capacitor based selective dimming LED driver presented in J. Liu, W. Sun, and J. Zeng, "Precise current sharing control for multi-channel LED driver based on switch-controlled capacitor," IET Power Electronics, vol. 10, no. 3, pp. 357-367, 2017, uses a resonant DC-DC stage for isolation and each two channels of LEDs can achieve selective dimming. However, in addition to needing a large number of active and passive components owing to application to achieve selective dimming, e.g. four diodes, one MOSFET, one filter inductor and several capacitors on the secondary side of DC-DC stage for each two channels, the primary side peak current of the transformer is not controlled, which leads to decreased lifetime as mentioned above.

Embodiments of the present invention provide seek to address at least one of the above problems.

SUMMARY

In accordance with a first aspect of the present invention, there is provided a multi-channel driver circuit for LEDs comprising a flyback circuit; a non-resonant DC-DC circuit; an AC- DC boost circuit supply for the flyback circuit and the DC-DC circuit; and a control circuit configured to limit peak current on a secondary side of the DC -DC circuit for protection of the LEDs and to control an average current on the secondary side of the DC-DC circuit for dimming of respective groups of the LEDs.

In accordance with a second aspect of the present invention, there is provided a method of multi-channel driving LEDs comprising limiting a peak current on a secondary side of a non- resonant DC-DC circuit for protection of the LEDs; and controlling an average current on the secondary side of the DC-DC circuit for dimming of respective groups of the LEDs. BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be better understood and readily apparent to one of ordinary skill in the art from the following written description, by way of example only, and in conjunction with the drawings, in which:

Fig. 1(a) shows a schematic drawing illustrating an existing LED driver.

Fig. 1(b) shows a schematic circuit diagram of an existing LED driver.

Fig. 2 shows a schematic drawing illustrating another existing LED driver.

Fig. 3 shows a schematic circuit diagram of a multi-channel LED driver with selective dimming according to an example embodiment.

Fig. 4(a) shows a detail of Fig. 3 during an operation scenario.

Fig. 4(b) shows a detail of Fig. 3 during another operation scenario.

Fig. 4(c) shows a programming procedure for the embodiment of Fig. 3.

Fig. 4(d) shows a sketch of the relationship of primary side current and switching states of secondary side switches in the embodiment of Fig. 3.

Fig. 5 shows a schematic circuit diagram of a multi-channel LED driver with selective dimming according to an example embodiment.

Fig. 6 shows a schematic circuit diagram of a multi-channel LED driver with selective dimming according to an example embodiment.

Fig. 7(a) shows a detail of Fig. 6 during an operation scenario.

Fig. 7(b) shows a sketch of the relationship of primary side current and switching states of secondary side switches in the embodiment of Fig. 6.

Fig. 8 shows a schematic circuit diagram of a multi-channel LED driver with selective dimming according to an example embodiment.

Fig. 9 shows a schematic circuit diagram of a multi-channel LED driver with selective dimming according to an example embodiment.

Fig. 10(a) shows a programming procedure for the embodiment of Fig. 9.

Fig. 10(b) shows a sketch of the relationship of primary side current and switching states of secondary side switches in the embodiment of Fig. 9.

Fig. 11 shows a schematic circuit diagram of a multi-channel LED driver with selective dimming according to an example embodiment. Fig. 12 shows a schematic circuit diagram of a multi-channel LED driver with selective dimming according to an example embodiment.

Fig. 13 shows a sketch of the relationship of primary side current and switching states of secondary side switches in the embodiment of Fig. 12.

Fig. 14 shows a schematic circuit diagram of a multi-channel LED driver with selective dimming according to an example embodiment.

Fig. 15 shows a schematic circuit diagram of a multi-channel LED driver with selective dimming according to an example embodiment.

Fig. 16 shows a schematic circuit diagram of a multi-channel LED driver with selective dimming according to an example embodiment.

Fig. 17 shows a flow chart illustrating a method of driving LEDs according to an example embodiment.

Fig. 18 shows a schematic drawing of a non-limiting commercial application example for the LED drivers according to example embodiments.

DETAILED DESCRIPTION

Example embodiment of the present invention described herein provide multi-channel LED driver structures with selective dimming that can have the advantages of low cost, high power and high efficiency. Example embodiment 1

A first example embodiment of a multi-channel LED driver 300 is shown in Fig. 3 with selective dimming.

1) Hardware Design Analysis:

Based on the high frequency multi-channel LED driver 300 shown in Fig. 3, this embodiment uses two high frequency transformers Tl and T2 of a non-resonant DC-DC circuit to control the brightness of each of the two groups 1 and 2 of LED channels selectively by switching ON and OFF two MOSFETs QS 1 and QS2 on the secondary side. The supply voltages for two optocoupler MOSFET driver ICs 302a, b are derived from auxiliary windings 304a, b on the secondary side of the respective transformers Tl and T2.

According to Fig. 3, in order to control the brightness of the two groups 1 and 2 of LEDs selectively,

MOSFET QS 1 is placed in series between nodes PAl-1 and PA 1-2 for LED group 1

MOSFET QS2 is placed in series between nodes PA2-1 and PA2-2 for LED group 2 For the high frequency multi-channel LED driver 300, the voltage at the output of each of the 4-channel LEDs in group 1 and 2 is floating with respect to control ground, hence an optocoupler MOSFET driver IC 302a, b is advantageously used for isolation and driving the MOSFETs QS l and QS2. However, the supply voltage of this IC preferably is much lower than the LED channel voltages. In this example embodiment one solution is to add an auxiliary winding 304a, b on the secondary side of the high frequency transformers Tl, T2. The voltage produced from these windings 304a, b is rectified to give rise to a low voltage DC output which is used to power the optocoupler MOSFET drive IC 302a, b thru suitable connections as shown in Fig. 3.

According to this method, the average current of groups 1 and 2 of LED channels to be dimmed simultaneously is sensed and fed back to the controller 308 which controls the duty ratio of the gating pulses of the MOSFETs QS l & QS2 via the optocoupler MOSFET drive IC 302a, b to realize selective dimming for two different groups 1 and 2 of LEDs.

As a result, a low cost, high power and high efficiency LED converter with two groups of selectively dimmed LED is advantageously achieved.

2) Control algorithm analysis:

Current in each of the two groups 1 and 2 of LED channels are sensed using resistive sensing 310a, b and fed back to the controller 308. For this embodiment, two example types of algorithms are proposed to realize selective dimming for two groups 1 and 2 of LEDs as follows: a) Algorithm 1: primary side frequency control and secondary side LED current control

Primary side frequency control analysis:

This example control algorithm aims to keep the peak current of each group of LEDs equal when both secondary MOSFETs QS l and QS2 are ON. The control algorithm is essentially control of duty ratio of QS l & QS2 switched at a frequency much lower than that of Ql Q2 while the switching frequency of Ql Q2 will be varied when either of QS l or QS2 or both of them will be ON.

When MOSFETs both QS 1 and QS2 are ON, the structure of the high frequency transformers Tl and T2 can be analyzed as shown in Fig 4(a).

According to Fig. 4(a),

Note:

The primary current of transformers Tl and T2 increases from 0 to peak in quarter period. Within this quarter period, the primary voltage between the two transformers Tl and T2 keeps constant as 200V or -200V according to the state of Ql & Q2.

When MOSFET QS l is ON and QS2 is OFF, both

flow through the magnetization inductor

In this situation, according to Fig. 4(b),

Comparing formula (1) and (2), it can be found that if keeping the switching frequency of Ql

Q2 constant, which means keeping

constant, both

will decrease when

MOSFET QS l is ON and QS2 is OFF. Meanwhile,

decrease, and

increases. This is reasonable according to the above formulas. Therefore, when MOSFET QS l is ON and QS2 is OFF, in order to maintain

constant when both QSl & QS2 are ON, one solution is to increase

which means to decrease the switching frequency of Ql and Q2. The situation is the same when MOSFET QS l is OFF, and QS2 is ON.

As a result, a typical example of the relationship between secondary MOSFET QS 1 & QS2 state and switching frequency of Ql & Q2 according to an example embodiment is shown in the following table:

Table 1

Note that for above example, the values of some parameters are as follows:

Table 2 In this method,

remains constant, and hence the peak current of each group 1 and 2 of LEDs remains constant when they are in the ON state, because it is proportional to

Secondary side LED current control analysis:

This example control algorithm aims to keep sampled average currents of two groups of LEDs the same as their respective set values.

According to the primary frequency control algorithm,

is constant, and hence the peak current of each group 1 and 2 of LEDs will remain constant when they are in the ON state. Therefore, the average current of each group of LEDs is

Due to the constant

are proportional to respective

Hence, the average current of each group 1 and 2 of LEDs varies according to the secondary MOSFET QS 1 or QS2 duty ratio.

Note:

The detailed algorithm is described with reference to Fig. 4(c), in which:

ILED1&ILED2: Respective sampled average currents of LED Groups 1&2

ILEDlsp & ILED2sp: Respective reference values of average currents for LED

Groups 1&2 Maintain switching frequencies of Ql Q2 at (For example 80KHz) at the beginning Set the switching frequencies of QS 1 QS2 at (For example 2KHz) such that

According to Fig. 4(c), the programming procedure in an example embodiment is described as follows:

Programming procedure:

Power on the LED driver and wait for all initialization ready. Execute Step 1 to estimate if ILED1 is equal to ILEDlsp. If they are equal, execute Step 3 directly; if not, execute Step 2 first to adjust MOSFET QS1 duty ratio to reach the target of ILEDl=ILEDlsp, and then execute Step 3. According to the estimation of Step 3, if ILED2 is equal to ILED2sp, execute Step 1 directly; if not, execute Step 4 first to adjust MOSFET QS2 duty ratio to reach the target of ILED2=ILED2sp, and then execute Step 1. This secondary side LED current control loop, it realizes the function of keeping sampled average currents of two groups of LEDs 1 and 2 the same as their set points.

In conclusion, the primary side frequency control aims to keep the peak current of each group of LEDs 1 and 2 constant when they are in the ON state, and the secondary side LED current control aims to vary average current of each group of LEDs according to the duty ratios of the secondary MOSFETs QS 1 & QS2. The goal of these two control algorithms is that the average current in each of the group of LEDs will be maintained at the respective reference values for dimming control.

b) Algorithm 2: primary side peak current control and secondary side LED current control

Primary side peak current control analysis:

This example control algorithm is to maintain the peak current at the primary side of the transformers Tl and T2 at a constant value and so will be the peak current of each group of LEDs when they are ON (explained in example embodiment 1 - Algorithm 1).

In this control algorithm, switching frequency of Ql Q2 is varied depending on the peak current at the primary side of the transformers Tl and T2.

With reference to Fig. 4(d), the primary peak current is set at a constant value C. When Ql is on and Q2 is off, the primary current of transformers Tl and T2 increases due to the positive voltage across the combined leakage inductance of the transformer primary. Once this current arrives at C, change Ql & Q2 state so that Ql is turned OFF and Q2 is turned ON. At this moment, the primary current reduces due to the negative voltage, i.e. after this current reduces to 0, it will increase in the negative direction. Once the absolute value of this negative current arrives at C, the states of Ql & Q2 are again changed. At this stage, the primary current increases to 0, and then increases to be a positive. Once it arrives at C again, change Ql & Q2 state etc..

With this primary peak current control, the peak of transformer primary current is maintained at a constant value C and hence peak current in every LED channel will be constant when they are in the ON state (i.e. same effect on the peak current in every LED channel as in example embodiment 1 - Algorithm 1).

Note:

LED peak current depends on the constant C.

Secondary LED current control analysis:

The same as example embodiment 1 - Algorithm 1 - secondary side LED current control. Therefore, average current of each group of LEDs can be varied depending on duty ratio of QS 1 & QS2 respectively, for dimming control.

Example embodiment 2

A multi-channel LED driver 500 with selective dimming according to another example embodiment is shown in Fig. 5.

1) Hardware Design Analysis:

This example embodiment uses two high frequency transformers Tl and T2 of a non- resonant DC-DC circuit to control the brightness of each of the two groups 1 and 2 of LED channels selectively by switching ON and OFF two MOSFETs QS 1 and QS2 on the secondary side. The supply voltages for two optocoupler MOSFET driver ICs 502a,b are derived from two auxiliary windings (not shown) on the secondary side of the flyback transformer 504.

According to Fig. 5, the hardware design is similar to example embodiment 1. The difference between example embodiment 1 and example embodiment 2 is

• The power supply solution of optocoupler MOSFET driver ICs 502a,b.

For example embodiment 2, two voltages produced by two auxiliary windings on the secondary side of the flyback transformer 504 are rectified to give rise to two low voltage DC outputs 15V_DC1 506a and 15V_DC2 506b, which are used to power two respective optocoupler MOSFET driver ICs 502a,b thru suitable connections as shown in Fig. 5.

2) Control algorithm analysis:

The same control algorithm analysis as for example embodiment 1 can be applied to example embodiment 2.

Example embodiment 3

Another embodiment of a multi-channel LED driver 600 with selective dimming is shown in Fig. 6. 1) Hardware Design Analysis:

This embodiment uses two high frequency transformers Tl and T2 of a non-resonant DC-DC circuit to control the brightness of each of the two groups 1 and 2 of LED channels selectively by switching ON and OFF two MOSFETs QS l and QS2 on the secondary side. The supply voltages for two optocoupler MOSFET driver ICs 602a,b are derived from auxiliary windings 604a,b on the secondary side of the respective transformers Tl and T2.

According to Fig. 6, the power supply solution of optocoupler MOSFET driver ICs 602a,b is the same as in example embodiment 1. The difference of hardware between example embodiment 1 and example embodiment 3 is

• The placement of secondary MOSFETs QS 1 & QS2.

For example embodiment 3,

MOSFET QS l is placed in parallel between nodes PB l-1 and PB 1-2 for LED group 1;

MOSFET QS2 is placed in parallel between nodes PB2-1 and PB2-2 for LED group 2.

The average current of a group 1 and 2 of LED channels to be dimmed simultaneously is sensed and fed back to the controller which controls the duty ratio of the gating pulses of the MOSFETs QS l & QS2 via the optocoupler MOSFET driver ICs 602a,b to realize selective dimming for two different groups 1 and 2 of LEDs.

2) Control algorithm analysis:

For this embodiment, two types of example algorithms are proposed to realize selective dimming for two groups 1 and 2 of LEDs as follows: a) Algorithm 1: primary side frequency control and secondary side LED current control Primary side frequency control analysis:

This example control algorithm aims to keep the peak current of each group 1 and 2 of LEDs equal when both secondary MOSFETs QS l and QS2 are OFF. The control algorithm is essentially control of duty ratio of QSl & QS2 switched at a frequency much lower than that of Ql Q2 while the switching frequency of Ql Q2 will be varied when either of QS l or QS2 or both of them will be OFF.

This control algorithm is similar to example embodiment 1 - Algorithm 1 - Primary side frequency control. The difference of this algorithm between example embodiment 1 and example embodiment 3 is

• The relationship between secondary MOSFETs QS1&QS2 state and switching frequency of Q1&Q2.

When MOSFETs both QSl and QS2 are OFF, the analysis of primary current of high frequency transformers Tl and T2 is identical to embodiment 1 - Fig. 4(a).

When MOSFET QS l is OFF and QS2 is ON, according to Fig. 7(a), because MOSFET QS2 leads to short of Secondary side circuit of Transformer T2, the voltage of secondary side of

Transformer T2 is close to zero. As a result,

is closed to zero, which means there is no current through

Therefore, the mathematical formulas of high frequency transformers can be described as follows:

Comparing formula (1) and (3), it can be found that if keeping the switching frequency of Ql Q2 constant, which means keeping

constant, both

will increase when MOSFET

QS l is OFF and QS2 is ON. Meanwhile,

increase, and

is close to zero. This is reasonable according to the above formulas. Therefore, when MOSFET QS l is OFF and

QS2 is ON, in order to remain

constant when both QSl & QS2 are OFF, one solution is to decrease which means to increase the switching frequency of Ql Q2. The situation is the same when MOSFET QS 1 is ON, and QS2 is OFF.

As a result, a typical example of the relationship between secondary MOSFET state and switching frequency of Ql & Q2 is shown in the following table:

Table 3

Note that for above example, the values of some parameters are the same as shown in Table 2.

In this method,

remains constant, and hence the peak current of each group of LEDs remains constant when they are in the ON state, because it is proportional to

(explained in embodiment 1).

Also for this embodiment, the advantage of

remaining constant is that when one of secondary MOSFETs is ON and

would remain unchanged,

and hence the peak current of LEDs would instead increase, which leads to an undesirable decreasing LED life time due to higher peak current through LEDs.

Secondary side LED current control analysis:

This example control algorithm aims to keep sampled average currents of two groups of LEDs the same as their respective set values. It is similar to example embodiment 1 - Algorithm 1 - Secondary side LED current control. The difference of these two algorithms is

• The relationship between average current of each group 1 and 2 of LEDs and duty ratio of QS 1 & QS2

For example embodiment 3,

In conclusion, the primary side frequency control aims to keep the peak current of each group 1 and 2 of LEDs constant when they are in the ON state, and secondary side LED current control aims to vary average current of each group 1 and 2 of LEDs according to the duty ratios of the secondary MOSFETs QS 1 & QS2. The goal of these two control algorithms is that the average current in each of the group of LEDs will be maintained at the respective reference values, for dimming control;.

Primary side peak current control analysis:

This example control algorithm is to maintain the peak current at the primary side of the transformers Tl and T2 at a constant value and so will be the peak current of each group 1 and 2 of LEDs when they are ON (explained in example embodiment 1 - Algorithm 1). It is similar to example embodiment 1 - Algorithm 2 - Primary side peak current control. The difference of these two algorithms is

• The Relationship between secondary MOSFETs QS 1&QS2 state and switching frequency of Q1&Q2.

For this embodiment, the sketch of this relationship is shown in Fig. 7(b).

From Fig. 7(b) it can be found that the switching frequency of Q1&Q2 increases with the shorting of LED group 1 or group 2 as explained in example embodiment 3 - Algorithm 1 - Primary side frequency control.

Secondary side LED current control analysis:

The same as example embodiment 3 - Algorithm 1 - Secondary side LED current control.

Example embodiment 4

Another embodiment of a multi-channel LED driver 800 with selective dimming is shown in Fig. 8.

1) Hardware Design Analysis:

This embodiment uses two high frequency transformers Tl and T2 of a non-resonant DC-DC circuit to control the brightness of each of the two groups 1 and 2 of LED channels selectively by switching ON and OFF two MOSFETs QS 1 and QS2 on the secondary side. The supply voltages for two optocoupler MOSFET driver ICs 802a,b are derived from two auxiliary windings (not shown) on the secondary side of the flyback transformer 804.

According to Fig. 8, the hardware design is similar to example embodiment 3. The difference between example embodiment 3 and example embodiment 4 is · The power supply solution of optocoupler MOSFET driver ICs 802a,b.

For example embodiment 4, the optocoupler MOSFET driver ICs 802a,b power supply solution is the same as for example embodiment 2.

2) Control algorithm analysis:

The same control algorithm analysis as for example embodiment 3 can be applied to example embodiment 4.

Example embodiment 5

Another embodiment of a multi-channel LED driver 900 with selective dimming is shown in Fig. 9.

1) Hardware Design Analysis:

This embodiment uses two high frequency transformers Tl and T2 of a non-resonant DC-DC circuit to control the brightness of each of the four groups 1 to 4 of LED channels selectively by switching ON and OFF four MOSFETs QS 1, QS2, QS3 and QS4 on the secondary side. The supply voltages for four optocoupler MOSFET driver ICs 902a-d are derived from auxiliary windings 904a-d on the secondary side of the transformers Tl and T2.

According to Fig. 9, the power supply solution of optocoupler MOSFET driver ICs 902a-d is the same as for example embodiment 3. However, for this embodiment, two auxiliary windings 904a,b and 904c ,d are added on the secondary side of each high frequency transformer to produce two low voltage DC outputs which are used to power the optocoupler MOSFET driver ICs 902a-d thru suitable connections as shown in Fig. 9.

Another difference of hardware between example embodiment 3 and example embodiment 5 is

• The grouping mode of e.g. 8 channels of LEDs

For this embodiment, each 2-channel LEDs are set as one group 1-4. Two groups 1,2 and 3,4 of LEDs are powered by one high frequency transformer, respectively.

Another difference of hardware between example embodiment 3 and example embodiment 5 is • The placement of secondary MOSFETs QS 1, QS2, QS3 and QS4.

For example embodiment 5,

MOSFET QS 1 is placed in parallel between nodes PCl-1 and PC 1-2 for LED group 1;

MOSFET QS2 is placed in parallel between nodes PC 1-2 and PC 1-3 for LED group 2;

MOSFET QS3 is placed in parallel between nodes PC2- 1 and PC2-2 for LED group 3 ;

MOSFET QS4 is placed in parallel between nodes PC2-2 and PC2-3 for LED group 4.

In this embodiment, the average current of a group of LED channels to be dimmed simultaneously is sensed and fed back to the controller 906 which controls the duty ratio of the gating pulses of the MOSFETs QS1, QS2, QS3 and QS4 via the optocoupler MOSFET driver ICs 902a-d to realize selective dimming for four different groups LEDs.

2) Control algorithm analysis:

For this embodiment, two example types of algorithms are proposed to realize selective dimming for four groups 1-4 of LEDs as follows: a) Algorithm 1: primary side frequency control and secondary side LED current control

Primary side frequency control analysis:

This example control algorithm aims to keep the peak current of each group of LEDs equal when all secondary MOSFETs QS 1, QS2, QS3 & QS4 are OFF. The control algorithm is essentially control of duty ratio of QS1, QS2, QS3 & QS4 switched at a frequency much lower than that of Ql & Q2 while the switching frequency of Ql & Q2 will be varied when some of or all of secondary MOSFETs QS1, QS2, QS3 & QS4 will be OFF.

This control algorithm is similar to example embodiment 3 - Algorithm 1 - Primary side frequency control. The difference of this algorithm between example embodiment 3 and example embodiment 5 is

• The relationship between Secondary MOSFETs QS 1, QS2, QS3 & QS4 state and switching frequency of Q1&Q2.

As analysed in example embodiment 3 - Algorithm 1, compared to the state of all of Secondary MOSFETs QS 1, QS2, QS3 & QS4 OFF, when some of them are ON,

will increase, and hence the peak current of LEDs increases (explained in example embodiment 1). Therefore, when some of Secondary MOSFETs QS 1, QS2, QS3 & QS4 are ON, in order to maintain constant when all of Secondary MOSFETs QS l, QS2, QS3 & QS4 are OFF, one solution is to decrease

which means to increase the switching frequency of Ql Q2.

For this embodiment, a typical example of the relationship between secondary MOSFET state and switching frequency of Ql & Q2 is shown in the following table:

Note that for the above example, the values of some parameters are the same as in Table 2.

In this method, ^l2p remains constant, and hence the peak current of each group 1-4 of LEDs remains constant when they are in the ON state (explained in example embodiment 1). Secondary side LED current control analysis:

This control algorithm aims to keep sampled average currents of four groups 1-4 of LEDs the same as their respective set values.

According to the primary side frequency control algorithm,

is constant, and hence, the peak current of each group 1-4 of LEDs will remain constant when they are in the ON state. Therefore, the average current of each group of LEDs is

Due to the constant

are proportional to respective

Hence, the average current of each group 1-4 of LEDs varies according to the secondary MOSFET QS 1, QS2, QS3 & QS4 duty ratio.

Note:

The detailed algorithm is described with reference to Fig. 10(a).

Note:

ILED1, ILED2, ILED3 & ILED4: Respective sampled average currents of LED

Groups 1, 2, 3&4

ILEDlsp, ILED2sp, ILED3sp & ILED4sp: Respective reference values of average currents for LED

Groups 1, 2, 3&4 According to Fig. 10(a), the programming procedure is described as following: Programing procedure:

Power on the LED driver and wait for all initializations ready. Execute Step 1 to estimate if ILED1 is equal to ILEDlsp. If they are equal, execute Step 3 directly; if not, execute Step 2 first to adjust MOSFET QS 1 duty ratio to reach the target of ILEDl=ILEDlsp, and then execute Step 3. According to the estimation of Step 3, if ILED2 is equal to ILED2sp, execute Step 5 directly; if not, execute Step 4 first to adjust MOSFET QS2 duty ratio to reach the target of ILED2=ILED2sp, and then execute Step 5. According to the estimation of Step 5, if ILED3 is equal to ILED3sp, execute Step 7 directly; if not, execute Step 6 first to adjust MOSFET QS3 duty ratio to reach the target of ILED3=ILED3sp, and then execute Step 7. According to the estimation of Step 7, if ILED4 is equal to ILED4sp, execute Step 1 directly; if not, execute Step 8 first to adjust MOSFET QS4 duty ratio to reach the target of ILED4=ILED4sp, and then execute Step 1. As this secondary LED current control loop, it realizes the function of keeping sampled average currents of four groups of LEDs the same as their set points.

In conclusion, the primary side frequency control aims to keep the peak current of each group of LEDs constant when they are in the ON state, and the secondary side LED current control aims to vary the average current of each group 1-4 of LEDs according to the duty ratios of the secondary MOSFETs QS 1, QS2, QS3 & QS4. The goal of these two control algorithms is that the average current in each of the group 1-4 of LEDs will be maintained at the respective reference values, for dimming control.

> Primary side peak current control analysis:

This example control algorithm is to maintain the peak current at the primary side of the transformers Tl and T2 at a constant value and so will be the peak current of each group 1-4 of LEDs when they are ON (explained in example embodiment 3 - Algorithm 1). It is similar to example embodiment 3 - Algorithm 2 - Primary side peak current control. The difference of these two algorithms is

• The relationship between secondary MOSFETs QS 1 QS2 QS3 & QS4 state and switching frequency of Q1&Q2.

For this solution, the sketch of this relationship is shown in Fig. 10(b). From Fig. 10(b), it can be found that the switching frequency of Q1&Q2 increases with the shorting groups 1-4 of LEDs increasing as explained in example embodiment 5 - Algorithm 1 - Primary side frequency control.

Secondary side LED current control analysis:

The same control analysis as for example embodiment 5 - Algorithm 1 - Secondary side LED current control can be applied in Algorithm 2.

Example embodiment 6

Another embodiment of a multi-channel LED driver 1100 with selective dimming is shown in Fig. 11.

1) Hardware Design Analysis:

This embodiment uses two high frequency transformers Tl and T2 of a non-resonant DC-DC circuit to control the brightness of each of the four groups 1-4 of LED channels selectively by switching ON and OFF four MOSFETs QS1, QS2, QS3 and QS4 on the secondary side. The supply voltages for four optocoupler MOSFET driver ICs 1102a-d are derived from four auxiliary windings 1103a-d on the secondary side of the flyback transformer 1104.

According to Fig. 11, the hardware design is similar to example embodiment 5. The difference between example embodiment 5 and example embodiment 6 is

• The power supply solution of optocoupler MOSFET driver ICs 1102a-d

For example embodiment 6, the power supply solution of optocoupler MOSFET driver ICs 1102a-d is the same as for example embodiment 2. However, for this embodiment, four auxiliary windings 1103a-d are added on the secondary side of the flyback transformer 1104 to produce four low voltage DC outputs 15V_DC1, 15V_DC2, 15V_DC3 and 15V_DC4 which are used to power the four respective optocoupler MOSFET driver ICs 1102a-d thru suitable connections as shown in Fig. 11.

2) Control algorithm analysis:

The same control algorithm analysis as for example embodiment 5 can be applied to example embodiment 6. Example embodiment 7

Another embodiment of a multi-channel LED driver 1200 with selective dimming is shown in Fig. 12.

1) Hardware Design Analysis:

This embodiment uses four high frequency transformers Tl, T2, T3 and T4 of a non-resonant DC-DC circuit to control the brightness of each of the four groups 1-4 of LED channels selectively by switching ON and OFF four MOSFETs QS 1, QS2, QS3 and QS4 on the secondary side. The supply voltages for four optocoupler MOSFET driver ICs 1202a-d are derived from auxiliary windings 1204a-d on the secondary side of the respective transformers Tl, T2, T3 and T4.

According to Fig. 12, the power supply solution of optocoupler MOSFET driver ICs 1202a-d is the same as for example embodiment 1. However, for this embodiment, four auxiliary windings 1204a-d are added on the secondary side of four respective high frequency transformers Tl, T2, T3 and T4 to produce four low voltage DC outputs which are used to power four respective optocoupler MOSFET driver ICs 1202a-d thru suitable connections as shown in Fig. 12.

Another difference of hardware between example embodiment 1 and example embodiment 7 is

• The grouping mode of e.g. 8 channels of LEDs

For this embodiment, each 2-channel LEDs are set as one group powered by one high frequency transformer. Four groups of LEDs 1-4 are powered by four respective high frequency transformers Tl, T2, T3 and T4.

Another difference of hardware between example embodiment 1 and example embodiment 7 is · The placement of secondary MOSFETs QS 1, QS2, QS3 and QS4.

For example embodiment 7,

MOSFET QS 1 is placed in series between nodes PDl-1 and PD1-2 for LED group 1;

MOSFET QS2 is placed in series between nodes PD2-1 and PD2-2 for LED group 2;

MOSFET QS3 is placed in series between nodes PD3-1 and PD3-2 for LED group 3;

MOSFET QS4 is placed in series between nodes PD4-1 and PD4-2 for LED group 4.

In this embodiment, the average current of a group 1-4 of LED channels to be dimmed simultaneously is sensed and fed back to the controller 1206 which controls the duty ratio of the gating pulses of the MOSFETs QSl, QS2, QS3 and QS4 via the optocoupler MOSFET driver ICs 1202a-d to realize selective dimming for four different groups 1-4 of LEDs.

2) Control algorithm analysis:

Primary side frequency control analysis:

This example control algorithm aims to keep the peak current of each group 1-4 of LEDs equal when all secondary MOSFETs QS l, QS2, QS3 & QS4 are ON. The control algorithm is essentially control of duty ratio of QS l, QS2, QS3 & QS4 switched at a frequency much lower than that of Ql Q2 while the switching frequency of Ql Q2 will be varied when some of secondary MOSFETs or all of them will be ON.

This control algorithm is similar to example embodiment 1 - Algorithm 1. The difference of this algorithm between example embodiment 1 and example embodiment 7 is

• The relationship between secondary MOSFETs QS l, QS2, QS3 & QS4 states and switching frequency of Q1&Q2

As analysed in example embodiment 1 - Algorithm 1, compared to the state of all of

Secondary MOSFETs QS l, QS2, QS3 & QS4 are ON, when some of them are OFF,

will decrease, and hence the peak current of LEDs decreases (explained in embodiment 1). Therefore, when some of Secondary MOSFETs QS l, QS2, QS3 & QS4 are OFF, in order to maintain constant when all of Secondary MOSFETs are ON, one solution is to increase which means to decrease the switching frequency of Ql Q2.

Table 5

Note that for above example, the values of some parameters are as shown in Table 2.

In this method, ^l2p remains constant, and hence the peak current of each group 1-4 of LEDs remains constant when they are in the ON state (explained in example embodiment 1).

Secondary side LED current control analysis:

This example control algorithm aims to keep sampled average currents of four groups 1-4 of LEDs the same as their respective set values. It is similar to example embodiment 5 - Algorithm 1 - Secondary side LED current control. The difference of these two algorithms is

• The relationship between average current of each group 1-4 of LEDs and duty ratio of QS 1 QS2 QS3 & QS4

For example embodiment 7,

In conclusion, the primary side frequency control aims to keep the peak current of each group 1-4 of LEDs constant when they are in the ON state, and the secondary side LED current control aims to vary the average current of each group 1-4 of LEDs according to the duty ratios of the secondary MOSFETs QS l QS2 QS3 & QS4. The goal of these two control algorithms is that the average current in each of the group 1-4 of LEDs will be maintained at the respective reference values, for dimming control.

Primary side peak current control analysis:

This example control algorithm is to maintain the peak current at the primary side of the transformers Tl T2 T3 and T4 at a constant value and so will be the peak current of each group 1-4 of LEDs when they are ON (explained in example embodiment 1 - Algorithm 1). It is similar to example embodiment 1 - Algorithm 2 - Primary side peak current control. The difference of these two algorithms is

• The relationship between secondary MOSFETs QS l QS2 QS3 & QS4 state and switching frequency of Q1&Q2.

For this embodiment, the sketch of this relationship is shown in Fig. 13. From Fig. 13, it can be found that the switching frequency of Q1&Q2 decreases with the disconnecting groups 1- 4 of LEDs increasing as explained in example embodiment 7 - Algorithm 1 - Primary side frequency control.

Secondary side LED current control analysis:

The same control analysis as for example embodiment 7 - Algorithm 1 - Secondary LED side current control can be applied.

Example embodiment 8

Another embodiment of a multi-channel LED driver 1400 with selective dimming is shown in Fig. 14. 1) Hardware Design Analysis:

This embodiment uses four high frequency transformers Tl, T2, T3 and T4 of a non-resonant DC-DC circuit to control the brightness of each of the four groups 1-4 of LED channels selectively by switching ON and OFF four MOSFETs QS 1, QS2, QS3 and QS4 on the secondary side. The supply voltages for four optocoupler MOSFET driver ICs 1402a-d are derived from four auxiliary windings 1403a-d on the secondary side of the flyback transformer 1404.

According to Fig. 14, the hardware design is similar to example embodiment 7. The difference between example embodiment 7 and example embodiment 8 is · The power supply solution of optocoupler MOSFET driver ICs 1402a-d

For example embodiment 8, the power supply solution of optocoupler MOSFET driver ICs 1402a-d is the same as for example embodiment 6.

2) Control algorithm analysis:

The same control algorithm analysis as for example embodiment 7 can be applied to example embodiment 8.

Example embodiment 9

Another embodiment of a multi-channel LED driver 1500 with selective dimming is shown in Fig. 15.

1) Hardware Design Analysis:

This embodiment uses four high frequency transformers Tl, T2, T3 and T4 of a non-resonant DC-DC circuit to control the brightness of each of the four groups 1-4 of LED channels selectively by switching ON and OFF four MOSFETs QS 1, QS2, QS3 and QS4 on the secondary side. The supply voltages for four optocoupler MOSFET driver ICs 1502a-d are derived from auxiliary windings 1504a-d on the secondary side of the respective transformers T1, T2, T3 and T4.

According to Fig. 15, the power supply solution of optocoupler MOSFET driver ICs 1502a-d is to the same as for example embodiment 7. The difference of hardware between example embodiment 7 and example embodiment 9 is

• The placement of secondary MOSFETs QS 1, QS2, QS3 and QS4.

For example embodiment 9,

MOSFET QS 1 is placed in parallel between PEl-1 and PE1-2 for LED group 1; MOSFET QS2 is placed in parallel between PE2-1 and PE2-2 for LED group 2;

MOSFET QS3 is placed in parallel between PE3-1 and PE3-2 for LED group 3;

MOSFET QS4 is placed in parallel between PE4-1 and PE4-2 for LED group 4.

In this embodiment, the average current of a group 1-4 of LED channels to be dimmed simultaneously is sensed and fed back to the controller 1506 which controls the duty ratio of the gating pulses of the MOSFETs QS1, QS2, QS3 and QS4 via the optocoupler MOSFET driver ICs 1502a-d to realize selective dimming for four different groups 1-4 of LEDs.

2) Control algorithm analysis:

The same control algorithm as for example embodiment 5 can be applied to example embodiment 9.

Example embodiment 10

Another embodiment of a multi-channel LED driver 1600 with selective dimming is shown in Fig. 16.

1) Hardware Design Analysis:

This embodiment uses four high frequency transformers Tl, T2, T3 and T4 of a non-resonant DC-DC circuit to control the brightness of each of the four groups 1-4 of LED channels selectively by switching ON and OFF four MOSFETs QS 1, QS2, QS3 and QS4 on the secondary side. The supply voltages for four optocoupler MOSFET driver ICs 1602a-d are derived from four auxiliary windings 1603a-d on the secondary side of the flyback transformer 1604.

According to Fig. 16, the hardware design is similar to example embodiment 9. The difference between example embodiment 9 and example embodiment 10 is · The power supply solution of optocoupler MOSFET driver ICs 1602a-d

For example embodiment 10, the power supply solution of optocoupler MOSFET driver IC 1602a-d is to the same as for example embodiment 6.

2) Control algorithm analysis:

The same control algorithm analysis as for example embodiment 5 can be applied to example embodiment 10. In one embodiment, a driver circuit for LEDs is provided comprising a flyback circuit; a non- resonant DC-DC circuit; an AC-DC boost circuit supply for the flyback circuit and the DC- DC circuit; and a control circuit configured to limit peak current on a secondary side of the DC-DC circuit for protection of the LEDs and to control an average current on the secondary side of the DC -DC circuit for dimming of the LEDs.

The control circuit may omprise a plurality of MOSFET driver ICs. A power supply for the MOSFET driver ICs may comprise at least one auxiliary winding on a secondary side of at least one DC-DC circuit transformer. A power supply for the MOSFET driver ICs may comprise at least two auxiliary windings on a secondary side of a flyback circuit transformer.

The control circuit may be configured for frequency control on a primary side of the DC -DC circuit for limiting the peak current on the secondary side of the DC-DC circuit.

The control circuit may be configured for peak current control on a primary side of the DC- DC circuit for limiting the peak current on the secondary side of the DC-DC circuit.

The driver circuit may comprise two DC-DC circuit transformers.

The driver circuit may comprise two MOSFET devices configured for selectively disconnecting respective ones of two groups of the LEDs.

The driver circuit may comprise two MOSFET devices configured for selectively shorting respective ones of two groups of the LEDs.

The driver circuit may comprise four MOSFET devices configured for selectively shorting respective ones of four groups of the LEDs.

The driver circuit may comprise four DC-DC circuit transformers.

The driver circuit may comprise four MOSFET devices configured for selectively disconnecting respective ones of four groups of the LEDs.

Each group may comprise one or more of the LEDs.

Figure 17 shows a flow chart 1700 illustrating a method of driving LEDs according to an example embodiment. At step 1702, a peak current on a secondary side of a non-resonant DC-DC circuit is limited for protection of the LEDs. At step 1704, an average current on the secondary side of the DC-DC circuit is controlled for dimming of the LEDs.

The method may comprise using a plurality of MOSFET driver ICs. The method may comprise supplying power for the MOSFET driver ICs using at least one auxiliary winding on a secondary side of at least one DC-DC circuit transformer. The method may comprise supplying power for the MOSFET driver ICs using at least two auxiliary windings on a secondary side of a flyback circuit transformer.

The method may comprise controlling a frequency on a primary side of the DC-DC circuit for limiting the peak current on the secondary side of the DC-DC circuit.

The method may comprise controlling a peak current on a primary side of the DC -DC circuit for limiting the peak current on the secondary side of the DC-DC circuit.

The method may comprise using two DC-DC circuit transformers.

The method may comprise using two MOSFET devices for selectively disconnecting respective ones of two groups of the LEDs.

The method may comprise using two MOSFET devices for selectively shorting respective ones of two groups of the LEDs.

The method may comprise using four MOSFET devices configured for selectively shorting respective ones of four groups of the LEDs.

The method may comprise using four DC-DC circuit transformers.

The method may comprise using four MOSFET devices for selectively disconnecting respective ones of four groups of the LEDs.

The method may comprise four MOSFET devices configured for selectively shorting respective ones of four groups of the LEDs.

Each group may comprise one or more of the LEDs.

Commercial applications of embodiments of the present invention.

One non-limiting commercial application example for the LED drivers with the requirement of selective dimming according to example embodiments is shown as Fig. 18.

According to this indoor illumination design application example, the illumination in a room 1800 prefers to keep at 500-600 lux. Therefore, the lamps e.g. lamp 4 installed near the window 1802 should preferably have less brightness than the lamps e.g. lamps 1-3 installed away from the window 1802 to save energy. That is, LED lamp 4 supplies less brightness than the other LED lamps 1-3 due to the sunshine entering the window 1802 as shown in Fig. 18. However, of the 4 LED lamps are to be supplied by one LED driver 1804, which is preferred due to e.g. cost considerations, the LED driver 1804 according to example embodiment preferably has the function of selective dimming for different groups of LEDs, i.e. the different LED lamps 1-4 as schematically shown in Fig. 18. It will be appreciated by a person skilled in the art that numerous variations and/or modifications may be made to the present invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects to be illustrative and not restrictive. Also, the invention includes any combination of features, in particular any combination of features in the patent claims, even if the feature or combination of features is not explicitly specified in the patent claims or the present embodiments.

Claims

1. A multi-channel driver circuit for LEDs comprising:

a flyback circuit;

a non-resonant DC-DC circuit;

an AC-DC boost circuit supply for the flyback circuit and the DC-DC circuit; and a control circuit configured to limit peak current on a secondary side of the DC -DC circuit for protection of the LEDs and to control an average current on the secondary side of the DC-DC circuit for dimming of respective groups of the LEDs.

2. The driver circuit of claim 1, wherein the control circuit comprises a plurality of MOSFET driver ICs.

3. The driver circuit of claim 2, wherein a power supply for the MOSFET driver ICs comprises at least one auxiliary winding on a secondary side of at least one DC-DC circuit transformer.

4. The driver circuit of claim 2, wherein a power supply for the MOSFET driver ICs comprises at least two auxiliary windings on a secondary side of a flyback circuit transformer.

5. The driver circuit of any one of claims 1 to 4, wherein the control circuit is configured for frequency control on a primary side of the DC -DC circuit for limiting the peak current on the secondary side of the DC-DC circuit.

6. The driver circuit of any one of claims 1 to 4, wherein the control circuit is configured for peak current control on a primary side of the DC-DC circuit for limiting the peak current on the secondary side of the DC-DC circuit.

7. The driver circuit of any one of the preceding claims, comprising two DC -DC circuit transformers.

8. The driver circuit of any one of the preceding claims, comprising four DC-DC circuit transformers.

9. The driver circuit of claim 7, comprising two MOSFET devices configured for selectively disconnecting respective ones of two groups of the LEDs.

10. The driver circuit of claim 7, comprising two MOSFET devices configured for selectively shorting respective ones of two groups of the LEDs.

11. The driver circuit of claim 7, comprising four MOSFET devices configured for selectively shorting respective ones of four groups of the LEDs.

12. The driver circuit of claim 8, comprising four MOSFET devices configured for selectively disconnecting respective ones of four groups of the LEDs.

13. The driver circuit of claim 8, comprising four MOSFET devices configured for selectively shorting respective ones of four groups of the LEDs.

14. A method of multi-channel driving LEDs comprising:

limiting a peak current on a secondary side of a non-resonant DC-DC circuit for protection of the LEDs; and

controlling an average current on the secondary side of the DC-DC circuit for dimming of respective groups of the LEDs.

15. The method of claim 14, comprising using a plurality of MOSFET driver ICs.

16. The method of claim 15, comprising supplying power for the MOSFET driver ICs using at least one auxiliary winding on a secondary side of at least one DC-DC circuit transformer.

17 The method of claim 15, comprising supplying power for the MOSFET driver ICs using at least two auxiliary windings on a secondary side of a flyback circuit transformer.

18. The method of any one of claims 14 to 18, comprising controlling a frequency on a primary side of the DC-DC circuit for limiting the peak current on the secondary side of the DC-DC circuit.

19. The method of any one of claims 14 to 18, comprising controlling a peak current on a primary side of the DC-DC circuit for limiting the peak current on the secondary side of the

DC-DC circuit.

20. The method of any one of claims 14 to 19, comprising using two DC-DC circuit transformers.

21. The method of any one of claims 14 to 20, comprising using four DC-DC circuit transformers.

22. The method of claim 20, comprising using two MOSFET devices for selectively disconnecting respective ones of two groups of the LEDs.

23. The method of claim 20, comprising using two MOSFET devices for selectively shorting respective ones of two groups of the LEDs.

24. The method of claim 20, comprising using four MOSFET devices for selectively shorting respective ones of four groups of the LEDs.

25. The method of claim 21, comprising using four MOSFET devices for selectively disconnecting respective ones of four groups of the LEDs.

26. The method of claim 21, comprising four MOSFET devices configured for selectively shorting respective ones of four groups of the LEDs.

27. The driver circuit of any one of claims 1 to 13, wherein each group comprises one or more of the LEDs.

28. The method of any one of claims 14 to 26, wherein each group comprises one or more of the LEDs.