Patent Publication Number: US-2022232682-A1

Title: Driving circuit and associated lamp

Description:
FIELD 
     Embodiments of the present disclosure generally relate to a driving circuit for an LED lamp, and an LED lamp comprising the driving circuit. 
     BACKGROUND 
     Due to the characteristics of light load and negative temperature of LEDs, a driving circuit dedicated to LED loads is typically required to drive the LED loads. 
     In general, there are two driving schemes for LED loads: a switched mode driving and a linear driving. A switched mode driving provides good current control accuracy and high overall efficiency. As an example, CN104427721B discloses a switched mode LED driving circuit that provides an LED driving circuit capable of supplying power to a microcomputer in a standby state in which the LEDs are not illuminated, the LED driving circuit is capable of, upon the input of a standby signal, outputting an output voltage lower than the voltage required for illuminating the LEDs through constant voltage control signals. 
     Whilst a linear driving is a simplest and most direct driving manner with the advantages including simple structures and low cost. Common linear driving circuits for LED loads are typically low-side driving, and in order to achieve higher power factor, capacitors in parallel with LED loads are typically large. Furthermore, in this linear driving scheme, a VDD supply dedicated to an auxiliary load circuits typically needs to be connected to the mains output via a diode and a capacitor. 
     SUMMARY 
     One of the objects of the present disclosure is at least to overcome the problem existing in the driving circuit of the prior art, i.e., a long delay from a standby mode to a minimum light emitting level of e.g., a linear driving circuit or a switching mode power supply driving circuit. Preferably, it can also solve the problem of high standby power consumption of the linear driving circuit. The basic idea of the present invention is that, under a standby mode, a linear power supply directly charges an output capacitor from a mains supply, to enable a voltage of the output capacitor lower than the turn-on voltage of the LED but above a lowest voltage, such that when switching from a standby mode to an illumination mode, the output capacitor can be quickly charged to illuminate the LED. It should be understood that word “directly” used herein means that the capacitor is connected to the mains supply via a linear driver and is charged by the mains with its frequency, rather than to the mains supply via a high frequency switching mode power supply. 
     In accordance with a first aspect of the present application, it provides a driving circuit. The driving circuit comprises inputs connected to a mains supply; outputs connected to an LED load; an output capacitor (C 1 ) connected in parallel with the LED load; an LED driving current source connected to the outputs, and configured to convert the mains supply at the inputs to current at the outputs in an illumination mode, such that the current flows through the LED load and charges the output capacitor (C 1 ); and a control circuit configured to receive a standby signal to enable a standby mode, and control the mains supply to linearly charge the output capacitor (C 1 ) in the standby mode, wherein an output voltage at the outputs is lower than a turn-on voltage of the LED load and higher than a preset lowest voltage. 
     In the first aspect, the output voltage of the outputs is controlled lower than the turn-on voltage of the LED load and above a preset lowest voltage, which enables the output capacitor to be quickly charged to the turn-on voltage of the LED when switching from the standby mode to a minimum light emitting level. This greatly reduces the delay of switching from the standby mode to the minimum light emitting level. The application of this principle possesses novelty in the case that the driving circuit linearly charges the output capacitor directly through the mains supply. 
     In some embodiments, the LED driving current source comprises a linear current source that converts the mains supply at the inputs to current at the outputs with a controlled impedance in an illumination mode, such that the current flows through the LED load and charges the output capacitor. In the embodiment, a linear current source is a driving current source for the LEDs and meanwhile charges the output capacitor separately in the standby mode (in the case that LED stops illuminating). When the LED driving circuit is in the standby mode, the control circuit is configured to enable the linear current source to operate to charge the output capacitor if the amplitude of the mains supply is equal to the output voltage at the outputs, and if not, to control the linear current source to turn off. In the embodiments, the switch elements in the linear current source can operate at zero or lower voltage drop and stop operating when the voltage drop is high, thereby greatly reducing the standby power consumption of the LED driving circuit. 
     In some embodiments, the linear current source further comprises a PNP transistor, wherein an emitter of the PNP transistor is connected to a positive output of a rectifier bridge, a collector of the PNP transistor is connected to a positive output of the driving circuit. In the embodiments, with the PNP transistor, the cost of the driving circuit can be greatly reduced. 
     In some embodiments, the linear current source further comprises an NPN transistor, wherein a collector of the NPN transistor is connected to a base of the PNP transistor, an emitter of the NPN transistor is connected to a negative output of the rectifier bridge, and the base of the NPN transistor is connected to the control circuit. In the embodiments, the NPN transistor can be used in cascade with the PNP transistor as a linear current source, thereby effectively reducing the cost of the driving circuit. 
     In an alternative embodiment, the LED driving current source comprises a switching mode power supply, the driving circuit further comprises a standby linear current source different from the switching mode power supply. In the illumination mode, the switching mode power supply converts the mains supply at the inputs to current at the outputs, such that the current flows through the LED load and charges the output capacitor, the standby linear current source is turned off; and in the standby mode, the switching mode power supply is turned off, the standby linear current source connects the output capacitor to the input, and the output capacitor and the switching mode power supply are decoupled, the standby linear current source controls the mains supply to linearly charge the output capacitor. 
     This embodiment provides an alternative embodiment in which a commonly used switching mode power supply that supplies LEDs, such as an AC/DC PFC converter, is disabled under a standby mode without charging the output capacitor, which reduces the loss of the switching mode power supply. The voltage of the output capacitor is maintained by a different linear current source directly in accordance with the mains supply, resulting in lower cost and power consumption. 
     Further, an anode of the output capacitor is connected to the input that provides a positive voltage, and a cathode is connected to a current terminal of the standby linear power source, the other current terminal of the standby linear power supply is connected to the input that provides a negative voltage. This provides a connection between the output capacitor and the standby linear power supply throughout the circuit. 
     Further, the switching mode power supply comprises a buck converter, a boost converter, a buck-boost converter or a flyback converter, wherein an input thereof is connected to the inputs and an output thereof is connected to the output capacitor. It will be appreciated that the concept of the present invention is also applicable to other types of switching power supplies. 
     Further, the driving circuit further comprises a voltage detecting circuit for detecting a voltage on the output capacitor, the standby linear power supply is turned off when the voltage on the output capacitor is higher than the turn-on voltage of LED load and the standby linear power supply is enabled when the voltage on the output capacitor is lower than the lowest voltage. This embodiment can minimize the power loss when charging the output capacitor and improve the standby efficiency. 
     The voltage detecting circuit comprises a Zener diode connected to a cathode of the output capacitor, the Zener diode is reversely biased and connected to the control electrode of the standby linear power supply, the current terminal of the standby linear power supply is connected in series with the output capacitor via a charging resistor at the inputs. This embodiment provides a voltage detecting circuit composed of a plurality of basic electronic components, which does not require a complicated microcontroller, an integrated comparator, etc., and therefore low cost is realized. 
     In some embodiments, the driving circuit further comprises an auxiliary load circuit which operates according to the voltage at the positive output of the driving circuit and is configured to generate a standby signal or a dimming signal based on a user&#39;s input. In some embodiments, the voltage at the positive output of the driving circuit can be directly used as a VDD supply voltage of the auxiliary load circuit, in which the output voltage is reused without additional circuitry to generate the VDD supply voltage directly from the mains supply. This makes the power supply structure of the auxiliary load circuit more compact. 
     In some embodiments, the auxiliary load circuit comprises a power supply circuit and a radio frequency circuit, wherein the power supply circuit generates a supply voltage from the output voltage, the radio frequency circuit is configured to receive the supply voltage from the power supply circuit and transmit the standby signal or the dimming signal to the control circuit. In these embodiments, the RF circuit can be kept running at a lower power consumption, and the RF circuit can receive a remote control signal from the user, which makes the control means of the driving circuit more abundant, which is advantageous for intelligent home design. 
     In some embodiments, the driving circuit further comprises a current sensing device connected in series with the LED load and configured to sense current flowing through the LED load. In these embodiments, the driving circuit can thereby perform constant current control for the light output of the LED load based on the sensed current signal. 
     In some embodiments, a positive output of the rectifier bridge is connected to the control circuit via a sensing resistor to supply the control circuit with power and simultaneously detect a change in the voltage amplitude at the positive output of the rectifier bridge. 
     In some embodiments, the positive output of the driving circuit is electrically connected to the control circuit, to detect the output voltage at the positive output of the driving circuit. In these embodiments, the control circuit can monitor the voltage change at the output of the drive circuit so that a constant voltage output at the outputs can be achieved in the standby mode. 
     In accordance with a second aspect of the present application, there is provided a lamp, which comprises a driving circuit described in embodiments in the above first aspect. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the figures, similar/same reference numerals generally refer to the similar/same parts throughout the different views. The drawings are not necessarily to scale to emphasize the illustration of the principles of the invention. In the figures: 
         FIG. 1  shows a schematic diagram of a linear driving circuit common in the prior art. 
         FIG. 2  shows a schematic diagram of a linear driving circuit in accordance with an embodiment of the present invention; and 
         FIG. 3  shows a circuit including a switching mode power supply and a linear power supply in accordance with another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Various embodiments of the present disclosure will be described in detail below with reference to the drawings. One or more examples of embodiments are illustrated by the figures. The examples are provided by way of illustration of the disclosure, and are not intended to limit the invention. For example, features illustrated or described as part of one embodiment may be used in another embodiment to create a further embodiment. These or other modifications and variations are intended to be included within the scope and spirit of the disclosure. 
     In order to more clearly understand the advantages of the linear driving circuit of the present disclosure, the structure of a common linear driving circuit in the prior art will be first described below. 
       FIG. 1  shows a schematic diagram of a linear driving circuit common in the prior art. As shown in  FIG. 1 , the exemplary linear driving circuit  100 ′ includes inputs  11 ′,  12 ′ and outputs  21 ′,  22 ′, wherein the inputs  11 ′,  12 ′ are connected to mains  10 ′, the outputs  21 ′,  22 ′ supply power to an LED load  70 ′ via a rectifier bridge  20 ′. 
     As an example, in  FIG. 1 , the LED load  70 ′ includes three LEDs in series. However, in other embodiments, an LED load  70 ′ may include more or fewer LEDs in series or in parallel. 
     The linear driving circuit  100 ′ may further include a rectifier bridge  20 ′ including four diodes D 1 ′-D 4 ′ for rectifying the mains supply received from the inputs  11 ′,  12 ′ to generate a rectified DC output. 
     An output capacitor C 1 ′ is connected in parallel with the LED load  70 ′ to smooth the input to the LED load  70 ′. In addition, a control circuit  30 ′ is connected to a positive output of the rectifier bridge  20 ′ via a resistor R 1 ′, thereby supplying power to the control circuit  30 ′. 
     The linear driving circuit  100 ′ may further include an auxiliary load circuit  80 ′. The auxiliary load circuit  80 ′ can be connected to the positive output of the rectifier bridge  20 ′ via a diode D 5 ′, a resistor R 2 ′ and a capacitor C 2 ′ to provide a Vdd power supply to the auxiliary load circuit  80 ′. The auxiliary load circuit  80 ′ can generate an operation signal such as a standby signal or a dimming signal to the control circuit  30 ′ based on user&#39;s input, wherein the standby signal can be used to stop the LED load from emitting light; and the dimming signal can be used to dim the LED load to output light of a desired characteristic. In some embodiments, the dimming signal can be used as the standby signal when the duty cycle of the dimming signal is zero. 
     In some embodiments, the auxiliary load circuit  80 ′ may include a switching mode power supply circuit  81 ′ and a radio frequency circuit  82 ′, wherein the switching mode power supply circuit  81 ′ may be used to supply power to the radio frequency circuit  82 ′, and the radio frequency circuit  82 ′ may be used to transmit an operation signal such as a standby signal or a dimming signal to the control circuit  30 ′ based on the user&#39;s input (e.g., a remote control signal), and the control circuit  30 ′ can transmit a control signal to a linear current source  60 ′ based at least on the standby signal or the dimming signal, thereby controlling the ON and OFF of the linear current source  60 ′. 
     In some embodiments, the linear current source  60 ′ may be formed by a cascade of two low-side switch elements Q 1 ′ and Q 2 ′, and one side of the linear current source  60 ′ is connected to a negative terminal of the LED load  70 ′ to control current through the LED load  70 ′, the other side is connected to a negative output of the rectifier bridge  20 ′ via a current sense resistor R 3 ′. A node between the current sense resistor R 3 ′ and the linear current source  60 ′ is coupled to a control circuit that senses the current flowing through the LED load  70 ′ and the sensed current signal is fed back to a control circuit  30 ′. 
     In  FIG. 1 , by way of example only, the two low-side switch elements Q 1 ′, Q 2 ′ are both NPN transistors, wherein a base of Q 1 ′ is controlled by a control signal output by the control circuit  30 ′, and an emitter of Q 1 ′ is connected to a base of Q 2 ′, a collector of Q 1 ′ is connected to a collector of Q 2 ; the collector of Q 2 ′ is connected to the current sense resistor R 3 ′. 
     In an operation state, the driving circuit  100 ′ may enter a normal operation mode, a standby mode, or a shutdown mode based on an operation signal (such as a standby signal or a dimming signal) generated by the auxiliary load circuit  80 ′. 
     In the normal operation mode, the control circuit  30 ′ may generate a switch control signal for the switch element Q 1 ′ to realize a dimming control of the LED load  70 ′ based on a modulation signal (e.g., a dimming signal) from the auxiliary load circuit  80 ′ and a current feedback signal from the current sensing resistor R 3 ′. 
     In the standby mode, the control circuit  30 ′ is configured to turn off the two low-side switch elements Q 1 ′ and Q 2 ′ in response to the standby signal. In the standby mode, the LED load  70 ′ stops emitting light. 
     In real life, there is often a need to switch from the standby mode to a minimum light emitting level of the LED load  70 ′. However, when a switching from the above standby mode to the minimum light emitting level of the LED load  70 ′ is requested by a user, the above-described driving circuit  100 ′ may have some problems. 
     To be specific, in order to achieve high power, the capacitor C 1  is generally required to be large. When the driving circuit  100 ′ is in the standby mode for a long time, the capacitor C 1  is generally fully discharged due to the presence of leakage current. As a result, when switching from the standby mode described above to the minimum light emitting level of the LED load  70 ′, the capacitor C 1  may take a considerable long time to be charged to a turn-on voltage that is required by the LED load  70 ′, due to the function of the linear current source  60 ′. That is, in the case of the above-described driving circuit  100 ′, the switching from the above standby mode to the minimum light emitting level of the LED load  70 ′ may require a relatively long delay, which is highly undesirable. On the other hand, if in order to shorten the above-described delay, the driving circuit  100 ′ is required to keep the switching element Q 2 ′ in operation during the standby mode, so as to maintain the charging current to the capacitor C 1 , which however results in an increase of the standby power consumption. This is also not desired. 
     Accordingly, it is an object of the present disclosure to provide a linear driving circuit capable of overcoming the technical problems of the above-described driving circuit  100 ′ while the cost of the linear driving circuit would not be increased. 
       FIG. 2  shows a schematic diagram of a linear driving circuit  100  in accordance with an embodiment of the present invention. Similar to the linear driving circuit  100 ′ described above, the linear driving circuit  100  also includes inputs  11  and  12  and outputs  21  and  22 , wherein the inputs  11 ,  12  are connected to the mains  10 , and the outputs  21  and  22  supply power to the LED load  70  via a rectifier bridge  20 . 
     By way of example only, the LED load  70  shown in  FIG. 2  also includes three LEDs in series. However, in other embodiments, the LED load  70  can include more or fewer LEDs in series or in parallel. 
     Similarly, the output capacitor C 1  is connected in parallel with the LED load  70  to smooth input to the LED load  70 . In addition, a control circuit  30  is also coupled to a positive output of a rectifier bridge  20  via a resistor R 1  to supply power to the control circuit  30 . 
     However, unlike the driving circuit  100 ′ of  FIG. 1 , the driving circuit  100  includes a high-side linear current source  60 , one side of which is connected to a positive output of the rectifier bridge  20  and the other side of which is connected to a positive terminal of the LED load  70 . In addition, the linear current source component  60  is also connected to the control circuit  30 , such that the linear current source component  60  can be controlled by the control circuit  30 . The function of the linear current source  60  is to control the output current of the linear current source  60  via a variable impedance. 
     By way of example only, in some embodiments, the linear current source component  60  may be an amplifier formed by two bipolar transistor switch elements Q 1  and Q 2 , where Q 1  may be an NPN transistor, Q 2  may be a PNP transistor, and an emitter of Q 2  is connected to an positive output of the rectifier bridge  20 , a collector of Q 2  is connected to a positive output  21  of the driving circuit  100 , a base of Q 2  is connected to a collector of Q 1 , and a base of Q 1  is connected to the control circuit  30 , an emitter of Q 1  is connected to a negative output of the rectifier bridge  20 . Therefore, the base current of the transistor switch element Q 1  can be controlled by the control circuit  30 , thereby the collector-emitter current of the transistor switch element Q 1  is controlled, which in turn is the base current of the transistor Q 2 , and finally the collector-emitter current of the transistor switch element Q 2  is controlled. For example, when operated under a linear mode, the collector-emitter current of the transistor Q 2  is β times the base current. In these embodiments, the use of a transistor switch element can effectively reduce cost of the linear driving circuit. However, it is to be appreciated that other linear current sources may be employed in other embodiments. For example, a MOSFET can be used to replace a bipolar junction transistor. 
     In addition, the driving circuit  100  may further include a current sensing device. In the example of  FIG. 2 , the current sensing device may include a current sensing resistor R 3  connected in series with the LED load  70  for sensing current flowing through the LED load and the sensed current signals are fed back to the control circuit  30 . In a normal operation mode, the control circuit  30  can control the current flowing through the LEDs according to the feedback sensed current signals. 
     In some embodiments, the control circuit  30  may be a linear control circuit, which may further reduce the cost of the driving circuit. 
     In some embodiments, the control circuit  30  can be further connected to the output of the driving circuit via an electrical connection line  32  so as to monitor the voltage at the output  21  of the driving circuit. It will be understood that since capacitor C 1  is in parallel with the LED load  70  and the negative terminal of capacitor C 1  is grounded via a resistor R 3 , in a standby mode voltage V 2  at the output  21  of the detected driving circuit (or the positive terminal of capacitor C 1 ) can then be considered equal to the voltage across C 1 . 
     In some embodiments, the linear driving circuit  100  can also include an auxiliary load circuit  80 . As an example of the auxiliary load circuit  80 , the auxiliary load circuit  80  may include, for example, a power supply circuit  81  and a radio frequency circuit  82 , wherein the power supply circuit  81  may be used to supply power to the radio frequency circuit  82 . In particular, the power supply circuit  81  is a switching mode power supply circuit, for example, a buck converter circuit. Alternatively, the power supply circuit  81  may be a further linear circuit such as an LDO (Low DropOut) circuit, or the like. The radio frequency circuit  82  can be used to transmit an operational signal such as a standby signal or a dimming signal to the control circuit  30  in accordance with user&#39;s input (e.g., a remote control signal). Specific radio frequency signals can be transmitted using Bluetooth, WiFi or Zigbee, and so on. The function of the standby signal is to stop the LED load from emitting light and enable the driving circuit into a standby mode; and the dimming signal is to dim the LED load in the normal/illuminating mode of the LED to output light of a desired characteristic. Note that although in the above description, the standby signal and the dimming signal are distinguished, it will be understood that in some embodiments, the dimming signal can also be used as a standby signal when duty cycle of the dimming signal is zero. 
     However, unlike the auxiliary load circuit  80 ′ in  FIG. 1 , the auxiliary load circuit  80  may be directly connected to the positive output  21  of the driving circuit  100  without through any diode element or capacitive element, and runs according to the voltage at the positive output  21 . The reason for this connection of the auxiliary load circuit  80  is that the auxiliary load circuit  80  can share the capacitive element C 1  such that the auxiliary load circuit  80  does not need a capacitive element dedicated thereto. 
     The operating principle of the driving circuit  100  according to the embodiments of the present disclosure is described below. 
     In a normal operation mode, the control circuit  30  turns on the switch elements Q 1  and Q 2  in a linear current source component  60 , and normal operating current can flow through the LED load  70 , thereby enables LED load  70  to emit light. At this time, the voltage across the capacitor C 1  is maintained above the turn-on voltage of the LED load  70 . The current sense resistor R 3  can sense the current flowing through the LED load  70  and the sensed current signals are fed back to the control circuit  30 . Thus, the control circuit can control the linear current source component  60  based on the sensed current signals such that the linear current source component  60  can adjust the output current via a variable impedance, so as to achieve constant light output of the LED load  70 . 
     In the normal operation mode, the control circuit  30  may further control current drawn from the linear current source component  60  based on the dimming signals from the auxiliary load circuit  80 , so as to dim the LED load  70 . 
     In this normal operation mode, the power supply circuit  81  also draws power from the outputs of the linear current source component  60 . Since control of the linear current source  60  is based on the sensing of the LED current, the power supply circuit  81  does not affect the supply of constant current from the linear current source  60  to the LEDs. 
     However, once the control circuit  30  receives a standby signal from the auxiliary load circuit  80 , the control circuit  30  will control the driving circuit  100  to enter a standby mode. 
     In a standby mode, the control circuit  30  will first turn off the linear current source  60 , or increase its impedance to decrease input current, resulting in that voltage of the capacitor C 1  drops below the turn-on voltage of the LED load  70  and then the LED load  70  does not emit any light. 
     As stated before, for the driving circuit  100 ′ of the prior art, if the driving circuit  100 ′ stays in this standby mode for a long time, the voltage across the capacitor C 1  might be discharged to a relatively low voltage (or even 0), causing a problem that the switching from a standby mode to a minimum light emitting level of the LED load would take a rather long start time (also known as “delay”). In order to prevent this problem, the control circuit  30  in the present disclosure is configured to control the impedance of the linear current source  60  in the standby mode such that output voltage at the output  21  is lower than turn-on voltage of the LED load  70  but above a set lowest voltage. 
     Merely as an example, in some embodiments, the lowest voltage may be preset to be in a range of 50%-90% of the turn-on voltage of the LED load  70 , for example, 60%, 70% or 80%. In other embodiments, the lowest voltage may be lower than 50% or slightly higher than 90%. Therefore, the lowest voltage is selectable, which makes it possible to adjust the standby power consumption of the driving circuit. 
     In order to achieve the desired output voltage at the output  21  of the driving circuit  100  as described above, the connection line  32  can sense the output voltage and control the impedance of the linear current source  60  accordingly such that the output voltage is within a desired range. 
     In some preferred embodiments, to reduce loss of the linear current source  60 , voltage drop of the linear current source  60  should be controlled at a lower value. The control circuit  30  can compare a periodic voltage amplitude V 1  from the rectified mains supply at the positive output of the rectifier bridge  20  detected via the resistor R 1  with voltage V 2  obtained at the output  21  of the driving circuit (or the positive terminal of the capacitor C 1 ) via the electrical connection line  32 , and only when the voltage amplitude V 1  is equal to the output voltage V 2 , the linear current source is then controlled to operate; otherwise, the linear current source is turned off. In these embodiments, as the control circuit  30  operates only when the periodic voltage amplitude V 1  is equal to the output voltage V 2 , thus the switching element Q 2  of the linear voltage source  60  can be activated with zero voltage drop or low voltage, which significantly reduces power consumption in the standby mode. The word “equal” herein covers the scenario that the voltage amplitude V 1  is slightly larger than the output voltage V 2 , to allow the input voltage to charge the output capacitor C 1 , and the voltage difference therebetween applied to the linear current source  60  is small, for example, less than 20V. 
     In some alternative embodiments, in a standby mode, the control circuit  30  may activate the linear current source  60  only when it is detected that the voltage V 2  at the output drops to a preset lowest voltage or below, such that the linear current source  60  charges the capacitor C 1 , thereby enabling the voltage V 2  at the output  21  of the driving circuit to rise above the preset lowest voltage. Then, the linear current source  60  can be turned off after a predetermined time so that the voltage V 2  at the output  21  of the driving circuit is not higher than or equal to the turn-on voltage of the LED load  70 , thereby keeping the LED load  70  in standby state without being illuminated. In these embodiments, switch element Q 2  in the linear current source  60  can still be activated with a small voltage drop, which significantly reduces the power consumption in a standby mode. 
     In still other alternative embodiments, in the standby mode, the control circuit  30  may control the linear current source to operate only when the voltage amplitude V 1  at the positive output of the rectifier bridge  20  detected via the resistor R 1  is equal to a preset lowest voltage, so as to charge the capacitor C 1 ; otherwise, the linear current source is controlled to be turned off. Likewise, in these embodiments, switching element Q 2  in linear current source  60  can also be activated with a small voltage drop, which also significantly reduces the power consumption in a standby mode. Note that: in this case, the electrical connection line  32  for detecting the voltage V 2  at the output  21  of the driving circuit (or the positive terminal of the capacitor C 1 ) can be omitted. 
     Through the description of the operating state in the standby mode described above, the control circuit  30  can repeatedly turn on and turn off the linear current source  60  in a manner similar to hiccup/burst, and the voltage V 2  at the output  21  of the driving circuit is substantially steadily kept lower than the turn-on voltage of the LED load  70 , but above a preset lowest voltage. In other words, in the standby mode, the voltage V 2  at the output  21  of the driving circuit can be in a substantially constant voltage state. 
     Further, it will be understood that the driving circuit  100  of the present disclosure may have the following beneficial effects. 
     First, in the standby mode, the voltage V 2  at the output  21  of the driving circuit  100  may be substantially kept at a constant voltage slightly lower than the turn-on voltage of the LED load  7 . When the user switches from the standby mode to a minimum light emitting level of the LED load, the voltage across the capacitor C 1  can quickly reach the turn-on voltage of the LED load  70  after the linear current source  60  is turned on, thereby realizing the minimum light emitting level of the LED load  70 . Therefore, as compared with the driving circuit  100 ′ of the prior art, the delay for switching from the standby mode to the minimum light emitting level of the LED load is greatly shortened, which remarkably improves the performance of the linear driving circuit. 
     Second, in the standby mode, the switch element Q 2  in the linear current source  60  can be operated with a small voltage drop, even a zero voltage drop, which also significantly reduces the standby power consumption of the driving circuit. 
     Third, in the standby mode, the substantially constant voltage V 2  at the output of the above driving circuit  100  is obviously very suitable as a VDD power supply for the auxiliary load circuit, which helps to simplify the circuit and reduces the cost. 
     Fourth, the auxiliary load circuit  80  is directly connected to the outputs of the driving circuit  100 , which allows the auxiliary load circuit  80  to share the capacitance C 1  of the LED load  70 . Therefore, a diode D 5 ′, a resistor R 2 ′ and a capacitor C 2 ′ are eliminated as compared with the driving circuit  100 ′ of the prior art. This obviously further simplifies the circuit and reduces the cost. 
     In the above embodiments, during standby the linear power supply is used to charge the output capacitor directly from the mains supply, while during illumination the linear circuit is still used. In an alternative embodiment to be described below, when used with a switching mode power supply commonly seen in the industry, the linear power supply can be turned off by the switching mode power supply during an illumination mode; and the switching mode power supply itself can be turned off during standby, thereby avoiding the low efficiency of the switching mode power supply at extreme-low output power, but enabling the dedicated standby linear power supply different from the switching mode power supply to linearly charge the output capacitor directly from the mains supply. 
     Referring specifically to  FIG. 3 , a driving circuit is coupled to a mains supply  38 . The driving circuit includes a rectifier bridge  32 , a switching mode power supply  34 , an output capacitor C 1  and a standby linear current source  36 . The switching mode power supply  34  is a buck converter, including a switch S 1 , an inductor L, and a freewheeling diode D as shown in connection in the figure. It will be appreciated that other types of switch power supplies/converters are also applicable. 
     The driving circuit also includes a standby linear current source  36  that essentially includes a linear transistor Q 2  that is different from the switching mode power supply  34 . 
     In an illumination mode, the switching mode power supply  34  converts the mains supply at its inputs to current at its outputs, such that the current flows through the LED load  70 , and charges the output capacitor C 1 , the standby linear current source  36  is turned off. 
     In a standby mode, the switching mode power supply  34  is turned off, e.g. by turning off a switch S 1 . The standby linear current source  36  connects the output capacitor C 1  to the input, and the output capacitor C 1  is decoupled from the switching mode power supply  34  (since the switch S 1  has been turned off, the switching mode power supply is no longer operated to charge the output capacitor C 1 ), the standby linear current source  36  controls the mains supply to linearly charge the output capacitor C 1 . 
     Specifically, the output capacitor C 1  has an anode connected to said input that provides a positive voltage, and a cathode connected to a current terminal of a transistor Q 2  of the standby linear power supply. Another current terminal of the transistor Q 2  of the standby linear power supply is coupled to said input that provides a negative voltage. 
     The operation of the standby linear current source is realized by a voltage detecting circuit, which is configured to detect the voltage of the output capacitor C 1 . If the voltage of the output capacitor C 1  is higher than turn-on voltage of the LED load (meaning that the switching mode power supply is operating), the standby linear power supply  36  is turned off, and the transistor Q 2  is turned off; if the voltage of the output capacitor C 1  is lower than the lowest voltage (meaning entering a standby state), the standby linear power supply  36  is enabled. The operation of the switching mode power supply  34  can be controlled by a microcontroller, which is not shown. 
     More specifically, the voltage detecting circuit includes a Zener diode X 1  connected to the cathode of the output capacitor C 1 , and the Zener diode X 1  is reversely biased and connected to a control electrode/a base of the transistor Q 2  of the standby linear power source  34 , a collector and a emitter of transistor Q 2  of the standby linear power supply are connected in series with the output capacitor C 1  through the charging resistor R 2  to said inputs. 
     When the voltage of the output capacitor C 1  is lower than a minimum value, the divided voltage on the Zener diode X 1  will become large and break it down, and the base of the transistor Q 2  will get a high voltage, so the transistor Q 2  is turned on, the mains supply thus charges the capacitor C 1  via the rectifier bridge  32 , the capacitor C 1 , the resistor R 2 , and the transistor Q 2  until the capacitor C 1  is charged above the minimum value. Thereafter, the Zener diode X 1  is turned off, charging is suspended until the capacitor C 1  is discharged by other auxiliary modules being supplied by the capacitor C 1 , or slowly discharged by itself or through R 1  to below the minimum value, and then the above process repeats. 
     The present invention has been described and illustrated in detail in the drawings and the foregoing description. The description and description are to be considered as illustrative and exemplary but not restrictive. The invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and practiced by those skilled in the art by studying the figures, disclosure and attached claims. 
     In the claims, the word “comprising” does not exclude other elements and an indefinite article “a” or “an” does not exclude plurality. A single element or other unit may fulfill the functions of the various items set forth in the claims. The mere fact that certain measures are recited in mutually different embodiments or dependent claims does not indicate a combination of these measures cannot be used to advantage. The scope of protection of the present application covers any possible combination of the various features recited in the various embodiments or the dependent claims, without departing from the spirit and scope of the invention. 
     Any reference signs in the claims should not be construed as limiting the scope of the invention.