1. Field of the Invention
The present invention relates generally to a circuitry for supplying a load with an output current, particularly to a control device for constant current loads with piezoelectric transformer.
2. Description of the Related Art
In many technical applications, it is required to supply electric loads, such as light-emitting diodes, fluorescent lamps or accumulators with a current.
Present simple approaches for operating such loads are, for example, series resistors. Thus, series resistors are frequently used, particularly in the context of light-emitting diodes. The disadvantage of such approaches is a comparatively high power dissipation and the fact that the current flowing through the load also changes with changing input voltage. A constant current regulator can, for example, provide a solution for the mentioned disadvantages. Analog regulators, such as linear regulators, have the advantage of a very low interference emission, but again have the disadvantage of comparatively high power dissipation.
Switched regulators, such as “hard-”, which means non-resonant, switching converters, offer the advantage of a high efficiency, but emit a stronger interference spectrum (for example compared to analog regulators or linear regulators, respectively). In other words, with hard- or non-resonant switching converters, an interference voltage occurs in a conducted way and by free emission. Conventionally, said interference emission has to be suppressed or filtered out, respectively, by further circuit complexity.
Resonant flyback converters (also known as “soft” switching converter) have also the advantage of high efficiency and preferably only emit a low interference (noise) spectrum. However, resonant flyback converters are more expensive in terms of circuit engineering, since they require additional resonance elements.
A resonant arrangement for driving antiparallel LED chains or chains of light-emitting diodes or luminescent diodes is illustrated in U.S. Pat. No. 6,853,150 B2, wherein a resonance half bridge with an inductance and a capacitance is used. The antiparallel LED chains as load are again separated from each other by decoupling capacities, in order to balance voltage differences between the chains. Thus, a number of passive devices are given at least by the inductance and the capacitance. Thus, when only one antiparallel LED chain is used, the number of passive devices is determined to be at least two. Additionally, there are decoupling capacitances in every antiparallel LED chain branch.
Above a power of about 1 watt, at least one inductance has to be used as energy storage for current regulation in all switched or clocked regulators, respectively. The inductance makes the regulator bulky and expensive, respectively, both in size and costs. Thus, it is desirable to reduce the number of passive devices.
In the following, known converters will be described with regard to FIGS. 1a, 1b, 2 and 3. Thus, FIGS. 1a and 1b show circuit diagrams of flyback converters according to the prior art. In other words, a flyback converter is shown in FIGS. 1a and 1b as hard-switching regulator with a switch S. The converter according to FIG. 1a is designated by 100 in its entirety and the converter according to FIG. 1b is designated by 150 in its entirety. A flyback converter as hard-switching regulator with a switch according to FIG. 1a or 1b has the disadvantage that a current with high frequency is interrupted towards the input, so that a high-frequency input interference-spectrum is significant and has to be suppressed with additional filtering effort (at least in interference-sensitive applications).
Further, converters or regulators according to FIG. 1A or 1B for driving constant current loads, such as light-emitting diodes (LEDs) have the disadvantage that a filter capacitor Cout on the output side and a fast rectifier diode Dout have to be used to generate a voltage across the light-emitting diodes (LEDs) as a constant current load (KS). If the light-emitting diodes (LEDs) were connected directly to the output of the inductance Ls, and an alternating voltage were applied, then, a resulting reverse voltage across the light-emitting diodes (LEDs) with uninterrupted current flow would become at least equal to the input voltage Uin. Since light-emitting diodes (LEDs) have no high reverse disruptive strength, the shown arrangement is usually not practicable.
FIG. 2 shows a circuit diagram of a Buck converter according to the prior art. The Buck converter according to FIG. 2 is designated by 200 in its entirety, and comprises an input voltage source for providing an input voltage Uin, a switch S, a diode Dout, an inductance Ls, a capacitance Cout as well as a constant current load KS consisting of a series connection of light-emitting diodes (LEDs), wherein the mentioned elements are connected to each other in the way shown in FIG. 2. The Buck converter 200 has the same disadvantages as the flyback converters 100, 150 shown with regard to FIGS. 1a and 1b. In other words, the discussion with regard to the problems of the reverse voltage occurring at the light-emitting diodes to the flyback converters 100, 150 apply also with regard to the Buck converter shown in FIG. 2. Further, the Buck converter has the additional disadvantage that the input voltage Uin always has to remain higher than the sum of forward voltages of the light-emitting diode of the LED chain.
Further, it should be noted that the circuitry 150 according to FIG. 1b has the advantage compared to the circuitry 100 according to FIG. 1a, that controlling the switch S can be performed without additional circuits (such as, for example, bootstrap circuits) from the input source (with the input voltage Uin).
FIG. 3 shows a circuit diagram of a boost converter according to the prior art. The boost converter according to FIG. 3a is designated by 300 in its entirety and comprises an input voltage source providing an input voltage Uin, an inductance Ls, a switch S, a diode Dout, a capacitance Cout as well as a constant current load KS, which is formed, for example, by a series connection of light-emitting diodes (LEDs). The mentioned circuit elements are connected or coupled, respectively, in the way shown in FIG. 3.
The circuit 300 according to FIG. 3 can cause improved smoothing of the input current as boost converter (compared to the circuitries shown in FIGS. 1a, 1b and 2). Additionally, with uninterrupted current flow, usage of the LED chain KS as rectifier without additional smoothing capacitor Cout would be possible. A reverse disruptive strength of the LED chain would additionally be sufficient to operate the light-emitting diodes (LEDs) in reverse direction, since a voltage would be equal to zero in an on state of the switch S. However, the circuit or circuitry 300 has the disadvantage that it is not short-circuit-proof, so that in the case of a short circuit across the LED chain, the input source (providing the input voltage Uin) is not protected from overcurrent. A corrective is provided, for example by an additional limiting resistor in an input circuit, which causes, additionally, increased losses, even during normal operation. Alternatively, a fast fuse can be used, which again causes additional costs and irreversible failure in the case of a short circuit.
Further, all flyback converters shown with regard to FIGS. 1a, 1b, 2 and 3 have the disadvantage that they have to be regulated by a feedback of the output current or output voltage to an approximately constant output power and thus to a constant current in the light-emitting diodes (LEDs) or other electrical loads. Therefore, a resistor divider for detecting the input and output voltage or a sense resistor is usually required for detecting the output current.
Additionally, the boost converter (for example the converter according to FIG. 3) has the disadvantage that additional elements for damping short-term overvoltages (for example for dealing with a load drop or load dump, respectively) are required in vehicle networks in order to avoid an overload at the electrical loads (for example at the LED chain).