WO2013093708A1

WO2013093708A1 - Power converter with lower power rating than load power

Info

Publication number: WO2013093708A1
Application number: PCT/IB2012/057162
Authority: WO
Inventors: Ulrich Boeke
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2011-12-20
Filing date: 2012-12-11
Publication date: 2013-06-27

Abstract

The present invention relates to a power converter (22) for the use e.g. as a light emitting diode (LED) driver that offers a cost saving feature. This feature is achieved since the converter's (22) rated power can be significant smaller than the load power. In practice this is achieved by supplying a load (R_L), e.g., a string of LED's, with the sum of a converter input current (I₁) plus a converter output current (I₂).

Description

POWER CONVERTER WITH LOWER POWER RATING THAN LOAD POWER

FIELD OF THE INVENTION

The present invention relates to a power converter circuit and power conversion method for supplying a load with a voltage or current without electrical insulation between DC input and DC output voltage.

BACKGROUND OF THE INVENTION

A simple way to generate a regulated DC supply voltage or DC supply current is to use a linear regulator, but linear regulators waste energy as they operate by dissipating excess power as heat. Buck converters, on the other hand, can be remarkably efficient (95% or higher for integrated circuits). Thus, regulated or controlled DC voltages or DC currents are typically realized by such buck converters which are characterized to generate an output voltage smaller than their input voltage. A buck converter with regulated load current may be used e.g. to supply power to light emitting diodes (LED's).

Fig. 3 depicts a buck converter that controls the supply voltage V₂ of a load R_L. TO achieve this, a control unit 20 controls switching of a current supplied from a voltage source 30 via an inductor to the load R_L. When the switched S is closed the energy stored in inductor L and capacitor C is increased. When the switch S is open, energy stored in the inductor L drives a current through the diode D and charges the capacitor C.

Flyback converters can be also used for DC/DC conversion with galvanic isolation between the input and output. More precisely, the flyback converter is a buck- boost converter where an energy storage inductor has two windings, so that charging and discharging time intervals can be adapted to input and output voltage with different winding numbers. Patent US 6 304 464 discloses an example of a flyback converter used as a driver for light emitting diodes (LEDs).

Forward converters can be also used for DC/DC conversion with galvanic isolation between the input and output. More precisely, a forward converter is a buck converter where a transformer is added to an energy storage inductor, so that charging and discharging time intervals can be adapted to input and output voltage with different transformer winding numbers. Patent US 6 490 184 discloses an example of a forward converter.

Load-resonant converters can be also used for DC/DC conversion with galvanic isolation between the input and output. The publication by R. L. Steigerwald "A comparison of half-bridge resonant converter topologies" IEEE Transactions on Power Electronics, Vol. 3, No. 2, April 1988 discloses examples of load-resonant converters.

Fig. 4 shows a circuit diagram of a conventional power converter 10 which processes 100 % of load power. It is a characteristic of known buck or flyback converters that they process 100 % of the load power by means of two power semiconductors (not shown), a power inductor (not shown) and an output filter capacitor (not shown). The following equation (1) describes that the rated power P_c of a converter is equal to the product of efficiency η times input current Ii times input or supply voltage Vs equal to output or load current I_L times output voltage V₂.

P_c = r| x I₁ x V_s = I_L x V₂ (1)

In a specific example the supply voltage is the output voltage of a mains rectifier and power factor correction circuit that is typically V_s = 400 V DC. The load is a series connection of 100 LED's e.g. in a lighting application. The load current is in this example II = 125 mA. The load voltage is V₂ = 320 V resulting in a load power and rated power P_c of 40 W.

Due to their non-linear and temperature dependent V-I characteristic, LEDs shall be supplied by a power converter with a regulated output current. A conventional power converter for the above exemplary application could be the buck converter. Thus, the components of the buck converter for this example need to be designed for a rated power P_c of 40 W, which makes it costly and bulky.

However, it would be desirable to reduce the cost and size and increase efficiency of power converters and specifically of LED driver circuits by having them adapted to process less than 100 % of the load power. SUMMARY OF THE INVENTION

An object of the present invention is to provide a power converter circuit and power conversion method, by means which the rated power of an employed power converter can be reduced.

This object is achieved by a power converter circuit as claimed in claim 1 and by a power conversion method as claimed in claim 9.

Accordingly, the converter rated power can be significant smaller than the load power, since the load, e.g. a string of LED' s, is supplied with the sum of the converter input current plus the converter output current.

According to a first aspect, the power converter may be adapted to supply the load device with a regulated supply voltage or a regulated supply current. Thus, although the rated power of the converter is reduced, the load device can still be supplied with a regulated supply voltage or current.

According to a second aspect which can be combined with the first aspect, the load device may comprises series connected light emitting diodes or parallel connected light emitting diodes or a combination of series and parallel connected light emitting diodes. Thereby, various LED applications can be supplied with the required regulated supply voltage or current, while the power processed by the employed converter can be reduced.

According to a third aspect which can be combined with the first or second aspect, the power converter may comprises a flyback converter, wherein the summing element may be adapted to supply the load device with the sum of the flyback converter input current and the flyback converter output current. Flyback converter with lower rated power are available at high quantities from various other applications and can now be advantageously used for applications with higher power requirements.

According to a fourth aspect which can be combined with the first or second aspect, the power converter may comprise a forward converter, wherein the summing element may be adapted to supply the load device with the sum of the forward converter input current and the forward converter output current. Hence, forward converters with less rated power can now be advantageously used for applications with higher power requirements. According to a fifth aspect which can be combined with the first or second aspect, the power converter may comprise a load-resonant converter, wherein the summing element may be adapted to supply the load device with the sum of the load-resonant converter input current and the load-resonant converter output current. Hence, load- resonant converters with less rated power can now be advantageously used for applications with higher power requirements.

According to a sixth aspect which can be combined with any one of the first to fifth aspects, the summing element is a simple circuit node where the converter input current and the converter output current are combined.

Further advantageous embodiments are defined below.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described, by way of example, based on embodiments with reference to the accompanying drawings, wherein:

Fig. 1 shows a schematic circuit diagram of a converter circuit according to a first embodiment;

Fig. 2 shows a schematic circuit diagram of a flyback converter circuit

according to a second embodiment;

Fig. 3 shows a schematic circuit diagram of a conventional buck converter circuit; and

Fig. 4 shows a schematic circuit diagram of a conventional converter circuit.

DESCRIPTION OF PREFERRED EMBODIMENTS

Various embodiments of the present invention will now be described based on a DC to DC converter circuit which may be used for LED driver circuits.

In order to reduce the rated power of power converters, used for example in

LED driver circuits, power converter circuits according to the following embodiments are adapted to process less than 100 % of the power supplied to their load. This is achieved by supplying the load with the sum of input plus output current of the converter. Fig. 1 shows a schematic power converter circuit according to a first embodiment with a power converter 22, where the load current II can be calculated according to the following equation (2):

II = Ii + I₂ (2)

A further characteristic of the power converter circuit of the first embodiment is that the supply voltage Vs is the sum of converter input voltage Vi plus the load voltage V₂. Thus the load voltage is smaller than the supply voltage.

The rated power of that converter is the load power P_L multiplied by the ratio of converter input voltage Vi to supply voltage Vs.

P_C = P_L x V!/Vs (4)

An advantage of the reduced rated power will now be explained based on the initial example. As described initially, the supply voltage was the output voltage of a mains rectifier and power factor correction circuit that is typically Vs = 400 V DC. The load was the series connection of 100 LED's e.g. in a lighting application. The load current was in the initial example II = 125 mA. The load voltage was V₂ = 320 V resulting in a load power of 40 W.

The rated power level of the power converter 22 of the first embodiment can be reduced in the example above to 8W using the proposed power conversion principle. The reduction of the rated power level is achieved by operating the power converter 22 with an input voltage of just Vi = 80V in this example that is the difference of supply voltage Vs = 400V and the load voltage V_L = 320V. The converter output current is just I₂ = 25 mA. Thus, cost of the output diode D can be reduced compared with an output diode of a buck converter. High efficiency can be achieved with the proposed converter topology since the converter rated power and related power loss is only a part of the application rated power that determines total system efficiency.

Fig. 2 shows a schematic circuit diagram of a realization of the proposed power conversion circuit by means of a flyback converter according to a second embodiment. An inductor L has two windings nl and n2. The converter output current I₂ flows via a switch S on/off-controlled by a control circuit 12 and via rectifier diode D, and is summed at a circuit or summing node 40 with the input current Ii to form the load current II which flows through the load DL which is a series connection of LEDs. When the switch S is opened by the control unit 12, the magnetic field of the inductor L generates the current I₂ flowing via the diode D to charge a parallel output capacitor C to the output voltage V₂. Thereby, the components of the proposed flyback converter circuit according to the second embodiment can be designed for lower power levels, of e.g. 8 W considering the example above.

In summary, the present invention relates to a power converter for use e.g. as a light emitting diode (LED) driver that offers higher efficiency and lower costs. This is achieved since the converter's power can be smaller than the load power. In practice this is achieved by supplying a load, e.g., a string of LED's, with the sum of a converter input current plus a converter output current.

While the invention has been illustrated and described in detail in the drawings and the foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive. The invention is not limited to the disclosed embodiments. From reading the present disclosure, other modifications will be apparent to persons skilled in the art. Such modifications may involve other - readily available - elements and circuit configurations for summing the converter input and output currents, which may be used instead of or in addition to features already described in the above embodiments. In particular, summing of currents may be achieved by semiconductor or resistor elements and other power converters, such as load resonant converters may be used.

Variations to the disclosed embodiments can be understood and effected by those skilled in the art, from a study of the drawings, the disclosure and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality of elements or steps. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Any reference signs in the claims should not be construed as limiting the scope thereof.

Claims

CLAIMS:

1. A converter circuit comprising:

a) an input terminal for inputting an input current (Ii);

b) an output terminal for supplying a load current (I_L) to a load device

(RL; D_L);

c) a power converter (22) for receiving said input current (Ii) and for generating an output current (I₂) to be supplied to said output terminal; and

d) a summing element (40) for summing said input current (Ii) and said output current (I₂) to generate said load current (I_L).

2. The converter circuit according to claim 1, wherein said power converter (22) is adapted to supply said load device (R_L; D_l) with a regulated supply voltage (V₂) or a regulated supply current (I₂).

3. The converter circuit according to claim 1, wherein said load device (D_L) comprises series connected light emitting diodes or parallel connected light emitting diodes or a combination of series and parallel connected light emitting diodes.

4. The converter circuit according to claim 1, wherein said power converter (22) comprises a flyback converter and wherein said summing element (40) is adapted to supply said load device (R_L; D_l) with the sum of the flyback converter input current (Ii) and the flyback converter output current (I₂).

5. The converter circuit according to claim 1, wherein said power converter (22) comprises a forward converter and wherein said summing element (40) is adapted to supply said load device (R_D; D_l) with the sum of the forward converter input current and the forward converter output current.

6. The converter circuit according to claim 1, wherein said power converter (22) comprises a load-resonant converter and wherein said summing element (40) is adapted to supply said load device (R_D; D_l) with the sum of the load-resonant converter input current and the load-resonant converter output current.

7. The converter circuit according to claim 1, wherein summing element comprises a circuit node (40).

8. The converter circuit according to claim 1, wherein said power converter (22) comprises a switching controller (12) for controlling a switch (S) which is adapted to switch on and off said output current (I₂).

9. A power conversion method comprising:

a) inputting an input current (Ii) to a power converter (22); b) supplying a load current (I_L) to a load device (R_L; D_L); c) generating by a power converter (22) an output current (I₂); and d) summing said input current (Ii) and said output current (I₂) to generate said load current (I_L).