Lamp driver with voltage sensing

Abstract

The lamp driver comprises a transformer, a primary side circuit and a secondary side circuit, the primary side circuit comprising a half-bridge circuit and a sensor circuit for measuring the conduction time or switching frequency of one of the transistors of the half-bridge circuit, the measured conduction time providing an estimate of the output voltage.

Description

[Technical field]

[0001] The present invention relates to a lamp driver with sensing of the voltage across the lamp. [Background technology]

[0002] LEDs are increasingly becoming the dominant lighting technology in residential and commercial applications.

[0003] LEDs are current driven devices, and therefore LED drivers are often designed to provide a regulated output current. However, if the driver attempts to provide a regulated current into an open circuit (e.g., no LED device connected or with an open circuit failure mode), the voltage will rise to unacceptable levels.

[0004] In the case of a DALI driver, it is necessary to have something that indicates the LED voltage levels and various other parameters to the DALI microcontroller.

[0005] As LED drivers, isolated drivers with a non-isolated primary side and an isolated secondary side are often used. Isolation is provided, for example, by a transformer forming the output stage of the LED driver. If the voltage is sensed directly on the secondary side, the voltage information needs to be provided across the isolation barrier. This can be achieved by using a dedicated feedback microprocessor and an optocoupler isolation interface. In another example, an isolated winding can be used to transfer information about the LED voltage level to the non-isolated side of the driver. However, these two structures are expensive. Summary of the Invention [Problem to be solved by the invention]

[0006] It would be desirable to be able to determine the output voltage based on measurements on the primary side, however, due to leakage inductance within the transformer, measurements of the voltage across a winding on the non-isolated primary side of the transformer may not be a sufficiently accurate indicator of the LED voltage level. [Means for solving the problem]

[0007] The invention is defined by the claims.

[0008] According to an example according to an aspect of the present invention, Transformer and a primary circuit including a primary winding of the transformer; a secondary side circuit including a secondary winding of the transformer, the secondary side circuit being isolated from the primary side circuit, the secondary side circuit being for providing an output voltage and an output current to a lamp load, The primary side circuit is a half-bridge circuit including a high-side transistor and a low-side transistor, the high-side transistor and the low-side transistor having a node between the high-side transistor and the low-side transistor; a controller for controlling switching of the high-side transistor and the low-side transistor; a sensor circuit for measuring a conduction time or a switching frequency of the high-side transistor or the low-side transistor; A lamp driver is provided, in which the controller is adapted to derive an estimate of the output voltage from the measured conduction time or the switching frequency.

[0009] The lamp driver can derive an estimate of the output voltage based on primary side sensing. The invention is based on the realization that when the output voltage increases, the switching frequency of the converter decreases and vice versa. By measuring the conduction time or the switching frequency of one of the transistors, a good indication of the output voltage can be obtained.

[0010] The controller may be configured to compensate for non-linearities in a primary (bus) voltage, the output current, and / or the transfer function.

[0011] The controller may, for example, be adapted to drive each of the high-side and low-side transistors with a fixed 50% duty cycle, and the control of the high-side and low-side switches thus performs frequency control rather than duty cycle control, where the resulting frequency is directly correlated to the conduction duration of any of the transistors.

[0012] The controller preferably further comprises: a predetermined transfer function relating the conduction time or the switching frequency to the output voltage; the output current, and / or An estimate of the output voltage is derived from one or more supply voltages in the primary side circuit.

[0013] Thus, the controller can take into account the operating characteristics of the driver (primary side voltage, output current) when determining the output voltage.

[0014] The lamp driver may further comprise an isolated feedback circuit for monitoring the output current and providing a feedback signal to the primary side circuit. Thus, the output current can also be used to improve voltage estimation. A current feedback mechanism is already necessary, for example, to perform output current regulation.

[0015] The sensor circuit may include a sense capacitor charged by a gate drive signal of the high side transistor or the low side transistor, and a peak detection circuit including a diode connected between the sense capacitor and a peak signal storage capacitor.

[0016] The conduction time is represented by the peak voltage stored on a capacitor which is charged during the conduction time. Using peak detection, a DC voltage can be generated which can be easily monitored by a microprocessor on the non-isolated side of the driver.

[0017] The sensor circuit may include a charge transistor between the gate of the high side transistor or the gate of the low side transistor and the sense capacitor, the charge transistor being turned on by a high gate drive signal of the high side transistor or the low side transistor, such that when the high side transistor or the low side transistor is turned on, a capacitor is charged by a conductive path through the charge transistor.

[0018] The sensor circuit may further include a discharge path including a diode from the sense capacitor to a gate of the high side transistor or the low side transistor.

[0019] The capacitor then discharges to the forward voltage drop of the diode, whose temperature dependent forward drop can be used to provide temperature compensation of the charge transistor, i.e. the temperature dependence of the base-emitter voltage of the charge transistor.

[0020] The sensor circuit is for example for measuring the conduction time or switching frequency of the low-side transistor, which allows for simpler and less expensive monitor circuitry than for the high-side transistor.

[0021] The primary circuit may further comprise a capacitor between the node and the primary winding, the capacitor and the primary winding forming or forming part of a resonant LLC circuit.

[0022] The resonant LLC circuit is a known low-cost driver circuit.

[0023] The present invention relates to a lamp driver as defined above; Also provided is a lighting system having an LED device including the lamp load.

[0024] The lighting system is, for example, integrated into a lighting fixture.

[0025] The present invention relates to performing half-wave rectification by controlling switching of high-side and low-side transistors in a primary circuit on the primary side of a transformer of the lamp driver; providing an output voltage and an output current to a lamp load using a secondary side circuit on a secondary side of the transformer; measuring a conduction time or a switching frequency of the high-side transistor or the low-side transistor; and deriving an estimate of the output voltage from the measured conduction time.

[0026] The method further comprises: a predetermined transfer function relating the conduction time or the switching frequency to the output voltage; the output current, and The method may include deriving an estimate of the output voltage from one or more supply voltages in the primary side circuit.

[0027] The method may, for example, comprise the step of measuring the conduction time of the low-side transistor.

[0028] These and other aspects of the invention will be elucidated and elucidated with reference to the following embodiments. [Brief description of the drawings]

[0029] For a better understanding of the present invention, and to show more clearly how the same may be carried into effect, reference will now be made, by way of example only, to the accompanying drawings in which: [Figure 1] 1 shows a simplified block diagram of a known driver device utilizing an LLC converter; [Diagram 2] 2 shows an example of a circuit according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0030] The present invention will be described with reference to the drawings.

[0031] It should be understood that the detailed description and specific examples, while indicating exemplary embodiments of the devices, systems and methods, are for purposes of illustration only and are not intended to limit the scope of the invention. These and other features, aspects and advantages of the devices, systems and methods of the present invention will become better understood from the following description, appended claims and accompanying drawings. It should be understood that the figures are merely schematic and are not drawn to scale. It should also be understood that the same reference numerals are used throughout the figures to denote the same or similar parts.

[0032] Figure 1 shows a schematic block diagram of a known driver device utilizing an LLC converter, generally indicated at 10. Figure 1 corresponds to a driver device as more fully described in WO 2014 / 060899, which may be referenced for further details.

[0033] The driver device 10 is connected to a voltage source 12 providing a supply voltage V10. The driver device 10 is connected to the voltage source 12 by input terminals 14, 16. The driver device 10 converts the input voltage V10 into an output voltage V20 for powering a load 18, which in the particular embodiment shown in FIG. 1 is formed as an LED unit 18.

[0034] The driver device 10 comprises two controllable switches 20, 22 and an electromagnetic conversion unit 24 (e.g., LLC circuit) for converting an input voltage V10 into an output voltage V20 for powering a load 18. The input voltage V10 is a DC voltage or a rectified voltage rectified by a rectifier (not shown) connected to an AC voltage source. The controllable switches 20, 22 are connected in series with each other and in parallel with the input terminals 14, 16. The controllable switches 20, 22 are connected with each other to form a half-bridge circuit, and a node 26 between the controllable switches 20, 22 forms an output terminal of the half-bridge circuit. The driver device 10 comprises a controller 28 for controlling the controllable switches 20, 22.

[0035] The controller 28 alternates the controllable switches 20, 22 to supply an AC voltage to the node 26 and to the electromagnetic conversion unit 24 as described below. For simplicity, the controller 28 may also be considered to comprise a DALI microcontroller to which the output voltage is reported.

[0036] The node 26 of the half-bridge circuit is connected to an electromagnetic conversion unit 24. The electromagnetic conversion unit 24 is formed by a resonant capacitor 30 and a transformer having a primary winding 32, two secondary windings 34, 36, and an electromagnetic coupling member 38 for coupling the primary winding 32 and the secondary windings 34, 36. It should be noted that instead of using two windings with single sided rectification, it is also possible to have one winding with a bridge rectifier. The primary winding 32 and the resonant capacitor 30 are connected in series with each other, and the resonant capacitor 30 may be connected to the node 26 as shown in FIG. 1 or to the input terminal 16.

[0037] The electromagnetic conversion unit 24 comprises a measuring device 40 for providing one or more feedback signals. The measuring device 40 may be based on a primary side control or on a secondary side control.

[0038] 1, the measuring device 40 may be for measuring the primary voltage V12 at the primary winding 32. As an alternative, the measuring device may be arranged to measure the capacitor voltage V30 at / across the resonant capacitor 30. As a further example, the measuring device may be arranged to measure a combination of the capacitor voltage V30 and the primary voltage, for example the voltage at / across the resonant capacitor 30 and the primary winding 33.

[0039] A measuring device 40 may be connected to the series connection of the primary winding 32 and an additional inductance (not shown) to measure the corresponding primary voltage.

[0040] In other embodiments of the measurement device 40, the primary side current may be measured (e.g., using a resistor in series with a controllable switch or by measuring the primary current in the primary winding) and / or the secondary side voltage may be measured by utilizing an auxiliary winding and a voltage measurement circuit coupled to the auxiliary winding (as described above). However, this increases the complexity and cost of the circuit.

[0041] In all cases, the measurement device 40 is connected to the controller 28 for providing corresponding measurement signals to the controller 28 for controlling the controllable switches 20,22.

[0042] The driver device 10 further includes a feedback circuit for monitoring the output current and providing a feedback signal to the primary side circuit. The feedback circuit includes a comparison circuit 48 for comparing the sensed output current ILED with a target current IREF. The result of the comparison forms a feedback signal that is provided to the primary side using an optocoupler 49, although other isolated communication mechanisms may be used. The feedback signal is used by the controller 28 to control switching such that the desired regulated current is provided to the lamp load.

[0043] In the case shown in FIG. 1, the primary winding 32 is connected to the primary ground 41. In another example, the resonant capacitor 30 may be connected to the primary ground 41 and the primary winding 32 may be connected to the node 26. The primary voltage V12 and the primary current 112 in the primary winding 32 are converted into two secondary voltages V14, V16 and two secondary currents 114, 116 provided by the secondary windings 34, 36, respectively. The secondary windings 34, 36 are connected to the load 18 via the diodes 42, 44 and the output capacitor 46 to provide the load 18 with a DC output current and a DC voltage as the output voltage V20, respectively, for powering the load. The secondary ground 47 may be connected to the primary ground 41 or may be isolated from the primary ground 41.

[0044] The controllable switches 20, 22 form a half-bridge circuit. The controllable switches 20, 22 are alternately switched to provide a pulsed (e.g., square wave) voltage at node 26. The output voltage V20 and secondary currents 114, 116 depend on the waveform of the primary voltage V12 and can be controlled by the switching frequency of the controllable switches 20, 22 and the duty cycle of the on-time of the controllable switches 20, 22. The controller 28 receives a measurement signal from the measurement device 40 and controls the controllable switches 20, 22 based on the measurement signal.

[0045] Further details of the known operation of the circuit can be found in WO 2014 / 060899.

[0046] It is well known to operate a resonant LLC converter with a fixed 50% duty cycle, where the operating frequency is adapted in response to a feedback signal, for example to provide a regulated output current.

[0047] Known circuits do not allow for accurate measurement of the output voltage based solely on measurements on the primary side.

[0048] The present invention is based on the realization that when the LED voltage increases, the switching frequency of the electromagnetic conversion unit 24 (e.g. LLC converter) decreases. A good indication of the LED voltage can be obtained, for example by measuring the conduction time of the low-side transistor. Compensation can be made for the primary bus voltage, the LED current, and / or the nonlinearity of the transfer function.

[0049] The present invention therefore provides a lamp driver comprising a transformer 24, a primary side circuit and a secondary side circuit. The primary side circuit comprises a half-bridge circuit and a sensor circuit 50 for measuring the conduction time of one of the transistors of the half-bridge circuit. The measured conduction time provides an estimate of the output voltage.

[0050] Figure 2 shows an example of a circuit according to the invention, with the same reference numbers as in Figure 1 being used for the same components.

[0051] The circuit includes a lamp driver having a transformer 24, a primary side circuit including a transformer primary winding 32, and a secondary side circuit including transformer secondary windings 34, 36, isolated from the primary side circuit, for providing an output voltage and an output current to a lamp load.

[0052] Again, the primary side circuit includes a half bridge circuit having a high side transistor 20 and a low side transistor 22 with a node 26 between the high side transistor 20 and the low side transistor 22. A controller 28 controls the switching of the high side and low side transistors.

[0053] The primary circuit further includes a resonant capacitor 30 between node 26 and a primary winding 32. The resonant capacitor 30 and the primary winding 32 form, or form part of, a resonant LLC circuit.

[0054] According to the invention, there is a sensor circuit 50 for measuring the conduction time or switching frequency of either the high side transistor 20 or the low side transistor 22. In the example shown, the measurement circuit is associated with the low side transistor 22 because this provides a simpler circuit implementation (since the gate-source voltage is referenced to ground). However, since both transistors have the same 50% duty cycle, the conduction time of either transistor can be used.

[0055] In the example below, the controller 28 derives an estimate of the output voltage from the measured conduction time. This estimate is therefore based on primary side sensing, eliminating the need for an additional isolated feedback path (i.e., in addition to the current feedback paths 48, 49). The voltage sensing is based on the recognition that when the output voltage increases, the switching frequency of the converter decreases. By measuring the conduction time or switching frequency of one of the transistors, a good indication of the output voltage can be obtained.

[0056] The output voltage may be derived from the conduction time or switching frequency using a predetermined transfer function relating the conduction time or switching frequency to the output voltage. However, this predetermined transfer function also depends on the output current ILED and / or the supply voltage (VBUS) in the primary circuit. Therefore, one or both of these parameters may be used as additional inputs to the controller 28 to make a more accurate estimate of the output voltage.

[0057] The sensor circuit 50 in this example comprises a sense capacitor C1 which is charged by the gate drive signal GL of the low side transistor (in this example).

[0058] Charging of the sense capacitor C1 is done through a transistor 60 (a bipolar transistor in this example). The base voltage is defined by a resistor divider R4, R5 from the gate drive signal GL. The transistor 60 acts as a constant current source that charges the sense capacitor C1. The transistor 60 is turned on by a high gate drive signal GL as a result of the base voltage created by the voltage divider R4, R5. The sense capacitor C1 is then charged through the feed resistor R3.

[0059] The gate voltage is at a constant level, and therefore during a high gate voltage, the base voltage is constant, and therefore the emitter voltage is constant. As a result, there is a constant voltage across the supply resistor R3, and therefore a constant supply current. The voltage reached across the sense capacitor C1 is therefore a linear function of the charging time (assuming that the sense capacitor C1 is discharged to a constant starting voltage).

[0060] The sensing capacitor C1 is connected to a peak detection circuit that includes a diode D1 connected (forward) between the sensing capacitor C1 and the peak signal storage capacitor C2. Thus, the peak voltage across the sensing capacitor C1 (i.e. the voltage reached by the sensing capacitor C1 while it is charging) is fed to the capacitor C2, which is prevented by the diode D1 from discharging into the sensing capacitor C1.

[0061] A peak signal storage capacitor C2 is in parallel with a discharge resistor R1. The sense capacitor C1 is charged and discharged during each switching cycle of the half-bridge circuit. The time constant established by the resistor-capacitor (RC) circuit C2, R1 is much longer than the conduction time of the low-side transistor 22. For example, the high-side and low-side transistors are typically switched at frequencies of kHz, whereas the time constant may be of the order of 1 second.

[0062] The peak voltage across the sense capacitor C1 indicates the conduction time (for the previous time period related to the time constant of R1 and C1). The detected peak signal micro_Vled is provided to the controller 28.

[0063] The sensor circuit 50 further includes a discharge path including a diode D2 and a resistor R2 from the sense capacitor C1 to the gate of the low-side transistor. During the dead time and conduction time of the high-side transistor 20, the sense capacitor C1 is discharged by the gate driver (of the low-side transistor 22) through resistor R2 until the forward voltage drop of the diode D2 is reached. Discharging the sense capacitor C1 through the gate driver of the low-side resistor, rather than, for example, directly to ground, advantageously prevents the occurrence of an unintentional short circuit of any gate voltage supplied to the low-side transistor 22 through transistor 60 (for example, through a diode-resistor series pair virtually connected between the sense capacitor C1 and ground).

[0064] The temperature dependence of the base-emitter voltage of transistor 60 (e.g., approximately −2 mV / K), which affects the constant current generated by the current source circuit of transistor 60, is partially compensated for by the temperature dependence of the forward voltage drop through diode D2 (also approximately −2 mV / K).

[0065] Vcc_micro is the DALI microcontroller supply voltage. Capacitor C1 is connected to this supply voltage through diode D3. Once the voltage on the sense capacitor C1 exceeds this level by an amount equivalent to the forward voltage drop of diode D3, the voltage on the sense capacitor C1 cannot rise any further. Thus, the maximum voltage on micro_Vled is clamped to Vcc_micro plus the forward voltage drop of diode D3 minus the forward drop of D1. Thus, the maximum voltage is clamped near Vcc_micro.

[0066] The present invention takes advantage of the fact that when the output voltage is constant, the LLC frequency changes only slightly. Although there is a change in frequency in response to a change in LED current (e.g., due to a change in current setting), this is small compared to the frequency difference caused by a change in voltage. Since the change in frequency caused by the voltage is dominant, and the controller knows the current setting, the small change in frequency caused by the current can be compensated for in the controller.

[0067] Therefore, the switching frequency is a function of the LED voltage level (higher LED voltage results in lower switching frequency and lower LED voltage results in higher switching frequency).

[0068] The LED voltage levels are on the secondary side, whereas the DALI microcontroller to which the LED voltage levels need to be supplied is on the primary side. Crossing the mains isolation to provide voltage feedback is achieved in a very cost-effective manner by sensing the switching frequency or conduction time of the high-side or low-side half-inverter transistors.

[0069] The above example times the conduction time by charging a capacitor with a constant current and measuring the resulting voltage, however other methods for measuring duration may be employed or other methods for measuring frequency may be used.

[0070] The circuit of Figure 2 forms a lighting system when provided with an LED device as a load, such a lighting system being integrated, for example, in a luminaire.

[0071] Those skilled in the art can understand and effect variations to the disclosed embodiments in the practice of the claimed invention, from a study of the drawings, the specification and the appended claims. In the claims, the word "comprises" does not exclude other elements or steps, and the singular form "a" does not exclude a plurality.

[0072] 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.

[0073] Please note that when the term "adapted to" is used in the claims or specification, the term "adapted to" is intended to be equivalent to the term "configured to."

[0074] Any reference signs in the claims should not be construed as limiting the scope.

Claims

1. Transformer and a primary circuit including a primary winding of the transformer; a secondary side circuit including a secondary winding of the transformer, isolated from the primary side circuit, the secondary side circuit adapted to supply an output voltage and an output current to a lamp load, The primary side circuit is a half-bridge circuit including a high-side transistor and a low-side transistor, the high-side transistor and the low-side transistor having a node between the high-side transistor and the low-side transistor; a controller adapted to control switching of the high side transistor and the low side transistor; and a sensor circuit adapted to measure a conduction time or a switching frequency of the high-side transistor or the low-side transistor; the controller is adapted to derive an estimate of the output voltage from the measured conduction time or the switching frequency; the controller is adapted to derive an estimate of the output voltage using a predetermined transfer function relating the conduction time or the switching frequency to the output voltage together with a supply voltage in the primary side circuit and / or the output current; The lamp driver, wherein the controller is adapted to drive each of the high-side and low-side transistors with a fixed 50% duty cycle.

2. 2. The lamp driver of claim 1, further comprising an isolation feedback circuit adapted to monitor the output current and provide a feedback signal to the primary side circuit.

3. 3. The lamp driver of claim 1, wherein the sensor circuit comprises a sensing capacitor charged by a gate drive signal of the high-side transistor or the low-side transistor, and a peak detection circuit including a diode connected between the sensing capacitor and a peak signal storage capacitor.

4. 4. The lamp driver of claim 3, wherein the sensor circuit comprises a charging transistor between a gate of the high-side transistor or a gate of the low-side transistor and the sensing capacitor, the charging transistor being turned on by a high gate drive signal of the high-side transistor or the low-side transistor.

5. 5. A lamp driver as claimed in claim 3 or 4, wherein the sensor circuit further comprises a discharge path including a diode from the sense capacitor to the gate of the high side transistor or the low side transistor.

6. 6. A lamp driver as claimed in any one of claims 1 to 5, wherein the primary circuit further comprises a capacitor between the node and the primary winding, the capacitor and the primary winding forming or forming part of a resonant LLC circuit.

7. A lamp driver according to any one of claims 1 to 6, and an LED device including the lamp load.

8. 8. The lighting system of claim 7, incorporated in a lighting fixture.

9. providing a pulsed voltage by controlling switching of a high-side transistor and a low-side transistor in a primary circuit on the primary side of a transformer of the lamp driver; providing an output voltage and an output current to a lamp load using a secondary side circuit on a secondary side of the transformer; measuring a conduction time or a switching frequency of the high-side transistor or the low-side transistor; deriving an estimate of the output voltage using a predetermined transfer function relating the conduction time or the switching frequency to the output voltage together with a supply voltage in the primary side circuit and / or the output current; and driving each of the high side transistor and the low side transistor with a fixed 50% duty cycle.

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