LED driver circuit with inrush current limiting

Abstract

The LED driver circuit uses a current-limiting resistor in series with the LED driver. A transistor in parallel with the current-limiting resistor is used to short-circuit the current-limiting resistor when the transistor is activated, and is activated by the control circuit when a threshold voltage is reached at the input to the transistor control circuit. The current-limiting resistor is used not only at startup but may also be used at the start of each AC cycle, in particular when a phase-cut dimming input signal is received.

Description

Technical Field

[0001] The present invention relates to a circuit for limiting inrush current to a load. The present invention is of particular interest for LED driver circuits that can be driven by a phase cut dimmer.

Background Art

[0002] High-efficiency LED lamps that need to operate with various dimmers require additional circuitry to enable the lamp to be driven by the signal generated by the dimmer.

[0003] An LED lamp driver has, for example, an AC input, a rectifier, and a switch-mode converter such as a boost converter driven by a rectified voltage.

[0004] One approach to enable compatibility with a phase cut dimmer is to provide an RC latch network on the AC input or the (rectified) DC bus. The latch circuit enables better compatibility with the phase cut dimmer by drawing sufficient current at the edge of the main power supply (when the dimmer turns on). The latch circuit also attenuates the oscillations of the main power supply current that can lead to an unwanted switch-off of the dimmer.

[0005] Limiting the inrush current to the LED lamp is also important. This is usually achieved with a rather high ohmic resistor in series with the lamp to prevent a large inrush current, especially when the lamp driver has a power factor correction (PFC) boost converter. Limiting the inrush current is important to limit the switching current of the lighting switch so that more lamps can be added.

[0006] The series resistor is short-circuited by the field-effect transistor (FET) once the circuit reaches a stable state, and therefore provides increased resistance only during the period when inrush current is present. To control the FET, the rectified mains voltage is detected, for example, by a high-resistance voltage divider, and the FET is switched on when sufficient voltage is available. When the FET is conducting, the input current does not pass through the input resistor, thus eliminating losses. In some cases, a limiting Zener diode is added to protect the gate voltage of the FET. [Overview of the project] [Problems that the invention aims to solve]

[0007] The problem with this circuit is that if the FET is not completely turned off because the gate of the FET is not fully discharged, a high inrush current can still flow. This can occur when a phase-cut dimming input is present.

[0008] Therefore, improvements to the inrush current limiting circuit are necessary. [Means for solving the problem]

[0009] The present invention is defined by the claims.

[0010] According to an example of a certain aspect of the present invention, An input for receiving an AC input signal, A rectifier for rectifying the aforementioned AC input signal, An LED driver supplied by the rectified AC input signal, wherein the output of the LED driver is for driving a load, A current-limiting resistor for connecting in series with the aforementioned LED driver, A transistor in parallel with the current-limiting resistor, wherein the transistor short-circuits the current-limiting resistor when the transistor is activated, An LED driver circuit is provided, which includes an inrush current control circuit coupled to the AC input signal, the inrush current control circuit configured to control the transistor so that it is turned on and off in response to the AC input signal, which includes a phase-cut AC dimming input signal.

[0011] In this configuration, the current-limiting resistor may be used not only at startup but also at the start of each AC cycle. This occurs particularly when a phase-cut dimming input signal is received. Specifically, the inrush current control circuit is configured to discharge the control voltage applied to the transistor during dimming to below the transistor's threshold voltage, and therefore, when the next rising edge of the rectified phase-cut dimming signal is applied to the LED driver circuit, the transistor is open. Consequently, the current-limiting resistor then limits the rising edge of the input voltage for each AC cycle.

[0012] Preferably, the inrush current control circuit is directly coupled to the AC input signal. In other words, the inrush current control circuit is coupled to the AC input signal to receive the AC input signal, i.e., the AC input signal before rectification, rather than the rectified version of the AC input signal.

[0013] Preferably, the LED driver circuit is for receiving the AC input signal, which includes a phase-cut AC dimming input signal.

[0014] The LED driver circuit thereby provides effective limitation of the initial inrush current when the circuit is first powered on, and also provides effective limitation of recurring inrush current when a dimmer is connected.

[0015] The inrush current control circuit is, for example, A transistor control circuit, comprising a transistor control circuit for operating the transistor when a threshold voltage is reached at the input to the control circuit, The present invention provides a voltage divider for the AC input signal, wherein the output of the voltage divider is supplied to the input of the transistor control circuit.

[0016] By using a voltage divider from the AC side of the circuit (e.g., a phase-cut dimming signal before rectification) to control the use of the current-limiting resistor, the current-limiting resistor can be used not only at startup but also at the start of each AC cycle.

[0017] This circuit can be used with or without a dimmer circuit. If a dimmer circuit is not connected, the circuit limits the initial inrush current. After startup, the transistor is kept on continuously, and therefore the current limiting resistor is short-circuited, which does not affect the efficiency of the converter. If a dimmer is present in the circuit, the LED driver circuit limits the initial / startup inrush current, as before, but the LED driver circuit also limits the inrush current each time the dimmer is turned on. This occurs every half-cycle of the mains power supply because it generates recurring inrush currents.

[0018] Accordingly, the present invention implements a cycle-by-cycle inrush current limiting function when a dimmer is added. The inrush current limiting component (of the inrush current control circuit) can also be used to implement the function of an RC latch, thus eliminating the need for additional components for the RC latch.

[0019] The control circuit may include an RC filter, which comprises a capacitor and a resistor connected in parallel to each other, and a Zener diode connected in parallel to the RC filter, between the control terminal of the transistor and the ground terminal.

[0020] The aforementioned RC filter functions as a latch.

[0021] A capacitor is provided, for example, at the input to the LED driver circuit. This functions as an EMI filter.

[0022] The voltage divider has, for example, a first resistor between the first terminal of the input and the input to the control circuit, and a second resistor between the second terminal of the input and the input to the control circuit. The first resistor and the second resistor have, for example, equal values.

[0023] The voltage divider has, for example, a first unidirectional device between the first terminal of the input and the input to the transistor control circuit, and a second unidirectional device between the second terminal of the input and the input to the transistor control circuit.

[0024] The LED driver generally has a boost converter.

[0025] The present invention also provides a lighting circuit having an LED driver circuit as defined above, and an LED device driven by the LED driver circuit.

[0026] These and other aspects of the present invention will be described and clarified by reference to the following embodiments.

Brief Description of the Drawings

[0027] For a better understanding of the present invention, and to more clearly show how the present invention can be implemented, reference is now made, by way of example only, to the accompanying drawings. [Figure 1] A known LED driver circuit with an inrush current limiter to provide higher efficiency is shown. [Figure 2] The waveforms occurring during operation of the circuit of FIG. 1 are shown. [Figure 3] A first example of an LED driver circuit according to the present invention is shown. [Figure 4] A second example of an LED driver circuit according to the present invention is shown. [Figure 5] The first waveform occurring during operation of the circuit of FIG. 4 with a non-dimming input is shown. [Figure 6] The second waveform occurring during operation of the circuit of FIG. 4 with a phase cut dimming input is shown. [Figure 7] A third example of an LED driver circuit according to the present invention is shown. [Modes for carrying out the invention]

[0028] The present invention will be described with reference to the figures.

[0029] The detailed descriptions and specific examples illustrate exemplary embodiments of the apparatus, systems, and methods, but are for illustrative purposes only and should not be used to limit the scope of the invention. These and other features, aspects, and advantages of the apparatus, systems, and methods of the invention will be better understood from the following description, the appended claims, and the appended drawings. The figures are for illustrative purposes only and are not drawn to scale. The same reference numerals are used throughout the figures to indicate the same or similar parts.

[0030] The present invention provides an LED driver circuit that uses an LED driver and a current-limiting resistor in series. A transistor in parallel with the current-limiting resistor is used to short-circuit the current-limiting resistor when the transistor is activated and is activated by the control circuit when a threshold voltage is reached at the input to the transistor control circuit. The current-limiting resistor may be used not only at startup but also at the start of each AC cycle, in particular when a phase-cut dimming input signal is received.

[0031] Figure 1 shows a known LED driver circuit.

[0032] The AC input is represented by a voltage source V1. This may be the mains power signal, or the mains power signal that has been cut off by a phase-cut dimmer.

[0033] The AC signal is rectified by a rectifier REC. The rectified voltage is supplied as input to the LED driver DRV, which has, for example, a switch-mode power converter. A capacitor C1 at the input to the LED driver provides an EMI filter function. The LED driver DRV is a boost converter, such as a power factor correction boost converter.

[0034] The output of the LED driver DRV is connected to a load, which is an LED device schematically shown as a single LED, LED1.

[0035] The LED driver is in series with the Rinrush current-limiting resistor. The LED device can also be considered to be in series with the current-limiting resistor, as it is connected to the output of the LED driver. Therefore, the LED drive current generated by the LED driver flows from the positive rectified input voltage terminal, through the LED device, through the current-limiting resistor, and to ground.

[0036] A current-limiting resistor can be short-circuited by a transistor called Minrush, so that the series current flows through the transistor instead. This results in lower losses but does not provide current-limiting functionality. When the current-limiting resistor is not short-circuited, it provides current-limiting functionality, particularly to limit the inrush current that occurs when the circuit is started up. This functionality consumes power, and therefore, after startup, this functionality is disabled by activating the transistor Minrush to provide a parallel short.

[0037] The transistors are controlled by a transistor control circuit that has a parallel RC filter formed by a parallel capacitor C10 and resistor R11, both of which are in parallel with Zener diode D10. The output of the transistor control circuit is connected to the gate of the transistor. The transistor control circuit is supplied with a rectified input voltage to the LED driver via resistor R10.

[0038] Resistor R10 has high ohmic resistance (e.g., 6 MΩ), and therefore, together with Zener diode D10, it forms a high-resistance voltage divider. Zener diode D10 protects the gate voltage of the FET. Alternatively, a simple resistor divider may be present.

[0039] Figure 2 shows the waveforms generated during the operation of the circuit in Figure 1.

[0040] The top pane displays the sinusoidal AC main power input signal 10 and the phase-cut dimming signal 12.

[0041] The second pane shows the gate voltage applied to the transistor. Plot 20 is for the full AC mains power signal, and plot 22 is for the phase-cut mains power signal.

[0042] The third pane shows the current 30 flowing through the transistor. The high spikes are those for the phase-cut dimming input. They coincide in timing with the sharp edges of the phase-cut dimming signal.

[0043] The bottom pane shows the current of 40 flowing through the current-limiting resistor. In both cases (dimmed and undimmed), the transistor remains on after the initial startup phase.

[0044] When a phase-cut dimming signal is received, the LED driver stops operating at a certain input voltage, but voltage remains across capacitor C10 in the transistor control circuit, and therefore the transistor Minrush remains on. Consequently, current continues to flow through the FET and not through the inrush resistor. As a result, the input current becomes much higher, as indicated by the large current spike.

[0045] The present invention provides a modified circuit example in which, in the case of a phase-cut dimming signal, the gate of a transistor is discharged below its threshold during each cycle. In this method, the transistor is open when the edge of the phase-cut dimming signal is reached, and a current-limiting resistor performs a current-limiting function each time there is a rising edge of the input voltage.

[0046] Figure 3 shows a first example of an LED driver circuit according to the present invention, in which the load, preferably the LED device, is omitted for clarity.

[0047] The same components as in Figure 1 are assigned the same reference numerals and perform the same function.

[0048] Therefore, in this case as well, the circuit has an AC input source V1 that is rectified by a rectifier REC that supplies power to the LED driver DRV (the main power impedance is also shown as R30). The same current limiting resistor Rinrush and transistor Minrush are shown. The transistor is controlled by an inrush current control circuit IRC.

[0049] The inrush current control circuit (IRC) generates an input signal to the transistor control circuit for controlling the transistor Minrush in a different way than the circuit in Figure 1. In particular, the inrush current control circuit is connected to the AC input signal (L,N), and when a dimmer is connected, the inrush current control circuit controls Minrush so that the transistor is turned on and off each cycle. When a dimmer is not connected, the transistor Minrush is always on.

[0050] Figure 4 shows an example of implementing the LED driver circuit from Figure 3 (in this case, as before, the LED device is omitted for clarity).

[0051] The same components as in Figure 3 are assigned the same reference numerals and perform the same function.

[0052] Therefore, in this case as well, the circuit has an AC input source V1 which is rectified by a rectifier REC that supplies power to the LED driver DRV. The inrush current control circuit includes a transistor control circuit as shown in Figure 1, which has a capacitor C10, a resistor R11, and a Zener diode D10.

[0053] The modification compared to Figure 1 is the use of AC resistors R20 and R21 to form the input signal to the transistor control circuit. These resistors form a voltage divider (R20, R21) across the AC input signal (i.e., on the input side of the rectifier), and the output of the voltage divider (R20, R21) is supplied to the input of the control circuit.

[0054] The effect of this change is that when a non-dimmable input signal is received, the gate voltage to the transistor remains high, but when a phase-cut dimmable input is received, the gate voltage drops below the transistor's threshold voltage for every half-cycle of the mains power input.

[0055] Figure 5 shows the circuit waveform for a non-dimmable input 10. In this case as before, the gate voltage applied to the transistor is shown as plot 20, the current flowing through the transistor is shown as plot 30, and the current flowing through the current-limiting resistor is shown as plot 40.

[0056] As you can see, the current-limiting resistor is only used during startup, and after that, the transistor remains on.

[0057] Figure 6 shows the circuit waveform for the phase-cut dimming input 12. In this case as before, the gate voltage applied to the transistor is shown as plot 22, the current flowing through the transistor is shown as plot 30, and the current flowing through the current-limiting resistor is shown as plot 40.

[0058] The gate voltage is periodically reduced below the gate threshold to turn off the transistor. Therefore, there is a repeating period during which no current flows through the transistor. Similarly, there is a current spike in the current flowing through the current-limiting resistor, timed to coincide with the sharp edges of the phase-cut dimming signal. In this way, the current flows first through the current-limiting resistor and then through the transistor for every half-cycle of the phase-cut dimming input.

[0059] Figure 4 shows additional RC networks Rdamp1 and Cdamp1. These are known latch circuits added to the bus voltage to attenuate the network to prevent errors (false signals).

[0060] This latch circuit introduces extra losses. However, in the circuit of Figure 4 (without the latch circuit), the network of C1 and Rinrush exists each time transistor Minrush1 opens, and this can perform the same role in attenuating the dimming. Therefore, since C1 and Rinrush perform the desired function, the latch circuits Rdamp1 and Cdamp1 are not necessary.

[0061] The circuit in Figure 4 uses resistor dividers R20 and R21 from the input side of the main power supply to the inrush current control circuit.

[0062] Figure 7 shows an alternative circuit design using diode voltage divider circuits D20, D21 connected to the inrush current control circuit via an input resistor R30. Preferably, the inrush current control circuit has a transistor control circuit for activating the transistor when a threshold voltage is reached at the input to the transistor control circuit. The inrush current control circuit has a first unidirectional device D20 between the first input terminal and the input to the transistor control circuit, and a second unidirectional device D21 between the second input terminal and the input to the transistor control circuit. Preferably, the unidirectional devices are diodes. The cathode of the diodes may be coupled to the transistor control circuit.

[0063] Therefore, in summary, the above inrush current limiting circuit enables a highly efficient design of a phase-cut dimmable driver, providing effective limiting of the initial inrush current after power-on and effective limiting of recurrent inrush current when a dimmer is connected. A typical RC latch circuit is not required, and therefore the overall circuit size is reduced.

[0064] A person skilled in the art will be able to understand and achieve, in carrying out the claimed invention, variations to the disclosed embodiments by studying the drawings, specification and appended claims. In the claims, the word “has” does not exclude other elements or steps, and singular nouns do not exclude plural nouns.

[0065] The mere fact that certain means are mentioned in different dependent claims does not mean that combinations of these means cannot be used to one's advantage.

[0066] No reference numeral in the claims should be construed as limiting the scope.

Claims

1. An input for receiving an AC input signal, A rectifier for rectifying the aforementioned AC input signal, An LED driver supplied by the rectified AC input signal, wherein the output of the LED driver is for driving a load, A current-limiting resistor for connecting in series with the aforementioned LED driver, A transistor in parallel with the current-limiting resistor, wherein the transistor short-circuits the current-limiting resistor when the transistor is activated, An LED driver circuit having an inrush current control circuit coupled to the AC input signal, the inrush current control circuit configured to control the transistor so that the transistor is turned on and off by the AC input signal.

2. The inrush current control circuit, A transistor control circuit, comprising a transistor control circuit for operating the transistor when a threshold voltage is reached at the input to the transistor control circuit, The LED driver circuit according to claim 1, further comprising a voltage divider between the ends of the AC input signal, wherein the output of the voltage divider is supplied to the input of the transistor control circuit.

3. The LED driver circuit according to claim 2, wherein the transistor control circuit comprises an RC filter including a capacitor and a resistor connected in parallel to each other between the control terminal and the ground terminal of the transistor, and a Zener diode in parallel with the RC filter.

4. The LED driver circuit according to claim 2 or 3, wherein the voltage divider has a first resistor between the first terminal of the input and the input to the transistor control circuit, and a second resistor between the second terminal of the input and the input to the transistor control circuit.

5. The LED driver circuit according to claim 4, wherein the first resistor and the second resistor have equal values.

6. The LED driver circuit according to any one of claims 1 to 5, further comprising a capacitor at the input to the LED driver.

7. The LED driver circuit according to any one of claims 1 to 6, wherein the LED driver has a boost converter.

8. The inrush current control circuit, A transistor control circuit, wherein the transistor is activated when a threshold voltage is reached at the input to the transistor control circuit, and The LED driver circuit according to claim 1, further comprising a first unidirectional device between the first terminal of the input and the input to the transistor control circuit, and a second unidirectional device between the second terminal of the input and the input to the transistor control circuit.

9. An LED driver circuit according to any one of claims 1 to 8, A lighting circuit having an LED device driven by the aforementioned LED driver circuit.

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