NL1043346B1 - Improved electronic control circuit for LEDs and similar power devices. - Google Patents

Abstract

Improved electronic control circuit (7,8) which comprises a power supply (V+,V-) and an active current limiting circuit (3,4) and a thermal circuit (9) which provides a thermal management control, comrpising: - a voltage divider (10) comprising a thermistor (R1) connected in series with a second resistor (R2); - a capacitor (Cl) connected in parallel over the second resistor (R2); and, - a first transistor (T1) connected with its base (B) between the thermistor (R1) and the second resistor (R2), which controls the flow of current (M) from its collector (C) towards its emitter (E), wherein the flow of current (M) from the collector (C) towards the emitter (E) of the first transistor (T1) defines a temperature dependent voltage set point (SP) for the active current limiting circuit (3,4).

Description

Inproved electronic control circuit for LEDs and similar pover devices. The present invention relates to an improved electronic control circuit for LEDs and similar powsr devices, in particular the present invention relates Le such an improved electronic control circuit which maintains an 0 optimum energy efficiency of the concerned LED or power

JX device, which efficiency is highly dependent on the operating temperature. Furthermore, an improved electronic control circuit of the iS invention 1s realized with a minimal amount of electronic components. The problem solved by the present invention can be summarized as follows, A light-emitting diode (LED) is a semiconductor light source which emits light when current flows through it. Blectrong in the semiconductor recombine with electron holes, releasing energy in the form of photons. Due to the semiconductor properties, also some heat is produced, partly released, but some heat stays accumulated. 39 This can lead Lo the so-called phenomenon of “thermal vonawayY,

indeed, when a LED junction gets warmer, the current that is running through it at a given voltage will increase, The increased current in turn heats the Junction further, and he problem of heatirc ats Worse 3 and the problem of heating gets worse.

Eventually, if nothing limits the current, the Junction will fail due to overheating.

16 In order to avoid this phenomenon of thermal runaway, it is important to use some current Idniting circuit even in cases wherein already a regulated or controlled voitage source is applied.

Another important parameter that influences the performance and the lifetime of LEDs and equipment that contains LEDs is the temperature itself.

LED performance largely depends on the ambient temperature ir 4 Ry > 2b of the operating environment and/or as a consequence on the properties of the applied thermal management system. Overdriving a LED in high ambient temperatures may result in overheating the LED package, eventually leading to 23 device failure, An adequate heat sink or thermal management system ig needed zo maintain long life.

Still another parameter that is important to take into account is the applied Voltage, since LEDs are sensitive to this applied Voltage.

LEDs must be supplied with a voltage above their threshold voltage and with a current below their rating.

3 Current and lifetime change greatly with a small change in the voltage applied.

LEDs and circuitry or equipment that contain LEDs thus require a current-regulated supply.

According to the state of the art some solutions are known, but these are still prone to improvement, For low cost solutions, a current limiting resistor is the typical solution.

Such kinds of circcuitry known from thes prior art are represented in figures 1 and 2.

In particular, figure 1 represents a known circuit 1 which is applied at the “high side" HS of a LED or a series of LEDs LEDL, wherein a current Limiting resistor R4 is placed in series between the positive voltage pole V+ of a power supply and the LED or series of LEDs LED controlled by the circuitry, while figure 2 represents known circuitry 2 which is applied at the “low side” L3 of a LED or series of LEDs LED, wherein a current limiting resister R4 is placed in series between the LED or series of LEDs LED controlled by the circuitry 2 and the ground or negative voltage pole 38 V- of a power supply.

It iz clear that these known solutions are of a very basic kind. More developed known circuits apply a current source which is realized by vging on one or more transistors.

Figure 3 represents a typical circuit 3 applied at the high side HS of a LED or series of LEDs LED], whersin the main load current I flows from the positive voltage pole V+ from i the collector side to the emitter side of a transistor T3, serving as a main load current valve, through a resistor EH4 into the LED or LEDs LEDL towards the ground or negative vole V- of the power supply.

This current flow I starts as soon as the Voltage V+ is high encugh in order to cause a current J to flow through a registor R3 towards the base side of the transistor T3, When the current flow I increases, the voltage Vm at the high side of resistor R4 increases and as a conseguence a second transistor TZ, which serves as a monitoring device for the LED current, will eventually be turned on.

This second transistor T2 is connected with its base at 28 this high side Vm of resistor R4, causing a part of the current J through resistor H3 to flow as a current flow K from the low side of resistor R3 through the collector side of the second transistor T2 towards its emitter side.

As a consequence, the flow K towards the base of the first transistor is limited, closing somewhat the main current valve, and the main corrent flow I will drop until the

Voltage drop over resistor R4 is sufficiently decreased in order to turn off the second transistor T2. In that way the current flowing through the LED or LEDs $5 LEDL is limited, Figure 4 zllustrates a similar circuit 4, this time applied at the low side of the LEDL.

HW Although such circuitry 3 and 4 is much more advanced, none of these sclutions has however an active electrical thermal management system or circuit, According to the state of the art some LED drivers are available which incorporate an active electrical thermal nanagemant solution, but these solutions reguire a thermal connection between the driver and the driven LED string. This is not a solution for long wired LED strings.

In particular, known solutions with a thermal management system are based on the so-called “Load Balancing” or “Current Mirror” principle.

23 Figures 5 and & illustrate circultry % and 6 of this kind, respectively applied at the high side HS and the low side LS of a LED or series of LEDs LEDL. in these solutions there is a resistor HI which is made thermally dependent, also called a thermistor, which is for example an NIC (“negative temperature coefficient”) type resistor RI.

However, in this solution the current is mirrored by using a set point "VB", which creates a constant energy loss.

3 The current flowing through diodes Dl and DE respectively is not used for the purpose of lighting, but solely for defining the current set point VB. This is a waste of energy.

3 The present invention aims at an improved electronic cirvuit for LEDs and similar power Devices which does not show one or more of the above mentioned disadvantages and possibly also other disadvantages of the known electronic clrouitny, In particular, it is an ain of the invention to provide electronic control circuitry for LEDs or power devices which comprises a thermal management system that is applicable for use in long wired LED strings and which XD ensures an optimal energy efficiency of the controlled LEDs or power device.

Another aim of the invention is to provide such circuury comprising a minimum of electronic components, which can be 24 easily integrated in LED lighting devices and which can be produced at a very low cost in high guantities as well as in smaller guentities, To these aims, the invention relates to an improved electronic control circuit for a LED or a series of LEDs and/or for similar power devices, which comprises a power supply and an active current limiting circuit which limits the current flowing through the LED or LEDs or power device, wherein tbe electronic control circuit additionally comprises a thermal cilrouit which provides a thermal management control. which is connected in parallel on the active current limiting circuit and which comprises the following components: - a voltage divider comprising a thermistor connected in series with a second resistor: -= & capacitor connected in parallel over the second i} resistor: and, ~ a first transistor connected with its base between the thermistor and the second resistor , which controls the flow of current from its collector towards its emitter, the collector and emitter connected in parallel over the voltage divide; and wherein the flow of current from the collector towards the emitter of the first transistor defines a temperature dependent voltage set point for the active current limiting circuib.

A great advantage of such an improved electronic control circuit in accordance with the invention is that is easy Lo make, has a very limited amount of components and provides an efficient control system which keeps the flowing main current limited also when the ambient temperature Ls varying considerably, With the intention of better showing the characteristics of the invention, hereafter, as an example without any 3 restrictive character whatsoever, some preferred embodiments of electronic control circuits according to the ë invention are described, with reference to the accompanying illustrations, wherein: figures 1 te §& illuslrate different circuits known according to the state of the art: and, figures 7 and 8 represent electronic control circuits according to the invention; respectively applied at the high side and the low side of a series of LEDs.

figure 7 illustrates a first embodiment of an improved slectronic control circuit 7 for a series of LEDs LEDL, which comprises a DC voltage power supply indicated by V+ and V- and an active current limiting circuit 3 cf a type as has already been described in the introduction with respect to figure 3. 13 This current limiting circuit 3 limits the main current I flowing through the LEDs LED, According to the invention the electronic control circuit 7 additionally comprises a thermal circuit S which provides a thermal management control and which is connected in parallel on the active current limiting circuit 3, This thermal circuit 9 comprises the following components: 24 = a voltage divider 10 comprising an NTC thermistor Rl, of which the resistance decreases when the temperature increases and which is connected in series with a second resistor R2; - a vapacitor Ol connected in parallel over the second 32 resistor RZ: and, - a first transistor T1 connected with its base B between the thermistor RI and the second resistor R2, which controls the flow of current M from its collector € towards its emitter EB. The collector C and emitter E of the first transistor Ti are connected in parallel over the voltage divider 10. Furthermore, the flow of current M from the collector € towards the emitter B of the first transistor Tl defines a temperature dependent voltage set point SP for the active B current limiting circuit 3.

The active current limiting circuit 3 comprises as in the case of figure 3 a second transistor TZ which serves as a steering device for adjusting the amount of current let through by a third transistor T3.

This third transistor T3 serves as a valve for defining the main load current I flowing through the LEDs LEDI or other power device in other cases, The base B of the second transistor T2 is connected to the high voltage side Vm of a sensing resister R4 through which the main load current I is flowing.

24 The collector CT and emitter EB of the second Lransistor T2 are connected in parallel on a pair of comnections defined by the base B of the third transistor TI and the low voltage side of the sensing resistor R4.

3% A third resistor R3 ig provided between the collector C and the base B of the third transistor T3,

The thermal circuit 9 is connected in parallel by means of the pair of contacts formed by the collector © and the emitter B of the first transistor Ul on the pair of contacts formed by the collector C and the emitter EB of the SS second transistor T2. In the case of figure 7 the improved electronic control circuit 7 is connected at a high voltage side HS of the LEDs LEDL to be controlled, while in figure § the improved 1 electronic control circult 8 is connected at a low voltage side LS of the LEDs LED! to be controlled.

In a preferred embodiment the thermistor Rl is a NIC {negative temperature coefficient} resistor Ri. 15% The here provided solutions are based on a Current Limiting schematic using NPN transistors, however a similar version with PNP transistors can according to the invention be made, Both a Low Side and High Side version have been designed, whichever is best suitable for the thermal connection of the LED or LEDS LEDIL.

The thermal monitoring / sensing part, represented by the thermal circuit 8, should be placed at the same PCB as close to the LED or LEDS LEDL as possible, The main advantages of such an improved electronic control cirouit 7 or & according to the invention are: 39 - It can be easily integrated in each LED bulb il “ It can be used for both type of LED's: which have a thermal connection to the positive supply, and others which have a thermal connection to the negative supply. - Additional amount of components and additional costs 3 compared Lo existing circuitry is very low {one capacitor Cl, 2 resistors Rl and RZ and one transistor Tl}. -— The current used by the thermal management circuit 9 will also flow through the LED. ig - No energy is wasted. - It extends LED lifetime. - It can be set to any thermal management scheme, Circuit legend: Cl = capacitor, to stop possible osgillation of TL.

RI = NTC resistor, defines the gain of thermal management {amplification when activated). R2 = registor, defines the thermal activation setpoint {a temperature}. R3 = pull-up or starter resistor, without this resistor the circuit cannot Vstart*, R4 = registor, defines the desired LED current setpoint by I=D,6/R4, Tl = NPN transistor, the LED temperature monitoring device, 24 T2 = NPN transistor, the LED current monitoring device.

T3 = NPN transistor, the main current valve.

The circuit 7 or 8 works as follows: When first powered on, resistor RS enables T3 to provide 3 current to the LED LEDI.

When Loo much current is drawn by the LED LEDL, 7T2 will be turned on and wiil pull the

17% voltage at base of T3 lower; which in turn will close transistor T3. As such, T2 and T3 will maintain an equilibrium over a large voltage range.

At normal temperatures, transistor Tl will be turned off and does not affect the current limiting properties of the T2 and T3 transistor pair, EL and E&2 work together as a voltage divider 10 which 0 define both the temperature set point SP and the temperature gain. When the temperature rises, the NTC resistor Rl will lower in resistance.

Through the RIJRZ divider 10, this will increase the voltage at the base of transistor Ti. Once this voltage reached the 0.6V threshold of the first rransistor Tl, this transistor T1 will pull the base of T3 lower ~ reducing the current through the LEDs LEDL. This will continue until a new equilibrium is reached.

. 23 The presen: invention 1s an ne way limited Lo the embodimencs of an improved electronic control circuit 7 or & in accordance with the invention described above and represented in the drawings, but such an improved elactronic control circuit 7 or & may be realised in 38 different shapes and dimensions, without departure from the Scope of the invention.

Claims (10)

Conclusions. i. Improved electronic control circuit {7,8} for an LED or a series of LED's {LEDI} and / or for equal current devices, requiring a power supply (V +, Vr} and an active current limiting circuit (3, 4) limiting the current {1} flowing through the LED or LEDs (LEDI) or power supply, characterized in that the electronic control circuit additionally comprises a thermal circuit (9) providing thermal management control, connected in parallel with the active current limiting circuit (5,4) and that includes the following components: ~ a voltage divider {10} comprising a thermistor {R1} connected in series with a second resistor (RZ): ~ a capacitor {Cl} connected in parallel across the second resistor (RE): and, ~ a first transistor {Di} connected to iin base {EB} between the thermistor {Ri} and the second resistor {(R2}, which controls the flow of current {M} from its collector {Cy to its emitter {BE}, the collector {CC} and emitter {5} in parallel connected across the voltage divider (10%; and wherein the current of current {My from the collector {C} to the emitter (8) of the first transistor {Ti} defines a variable frequency dependent voltage set point {SP} for the active current limiting circuit (3, ä}., Improved electronic control circuit {7.8} according to claim 1, characterized in that the active current limiting circuit (3,4) comprises a second transistor {T2) which serves as a control device for adjusting the amount of current passed through said third transistor {T3). , which serves as a valve for de-activating the peak load current (I) flowing through the LED or LEDs (LED) or other power supply device. Improved electronic driver circuit {7,8} according to claim Z, characterized in that the base (B) of the two-way transistor {02} is connected to the high voltage side (Vm) of a detection resistor (R4) through which the main load current (I ) Flowing, An improved electronic control circuit (7, 8) according to claim 2 or 3, characterized in that the collector (CO) and emitter (E} of the second transistor (TZ) are connected in parallel on a pair of connections defined by the base { B} of the third transistor {13} and the low voltage side of the derection resistance (Rá}, Improved electronic control circuit (7,8) according to any one of claims 2 to 4, characterized in that a third resistor (R3} is arranged between the collector 2) and the base {B} of the third transistor {D3} . &. Improved electronic control circuit (7,8) according to any of the preceding claims, characterized in that the thermal circuit (9) is connected in parallel by means of the pair of contacts formed by the collector {C) and the emitter {B} of the first transistor {Ti} on the pair of contacts formed by the collector {CY and the emitter (Fy of the second transistor (2), 7. Improved electronic control circuit {7,8} according to one of the foregoing variants, characterized in that the improved electronic control circuit {7,8} is connected to a Low-voltage side (LB) of the LED or LEDs (LEDI) or the hetreifende Le operate power supply. Improved electronic control circuit {7,8} according to any one of claims 1 to ©, characterized in that the improved electronic control circuit {7,83} is connected to a high voltage side (HS) of the LED or LEDs {LEDL} cf the relevant power supply to operate. 3. Improved electronic control circuit {7,8} according to any one of the preceding claims. characterized in that the thermistor at NTC is (negative temperature viscosity) resistance {R1}. Improved electronic control circuit 17.5} according to any one of the preceding claims, characterized in that it comprises one or more LEDs on a PCB (Mprintplaat®) and that the thermal circuit (9) or the entire improved electronic control circuit is provided. (7,8) is located on the same circuit board. 348