Variable bleeder circuit

A bleeder circuit includes an input current sense circuit, coupled to one of first and second input terminals of a driver circuit, to output a bleeder on/off signal in response to an input current through the first and second input terminals of the driver circuit. A variable current circuit is coupled between the first and second input terminals of the driver circuit and coupled to the input current sense circuit. The variable current circuit is coupled to conduct a bleeder current between the first and second input terminals in response to the bleeder on/off signal. A current scaling circuit is coupled to the variable current circuit to output a current scale signal which is received by the variable current circuit in response to a shutdown signal. The shutdown signal is representative of a conduction angle.

BACKGROUND INFORMATION

Field of the Disclosure

The present invention relates generally to power converters. More specifically, examples of the present invention are related to lighting systems including dimming circuitry.

Background

Electronic devices use power to operate. Power is generally delivered through a wall socket as high voltage alternating current (ac). A device typically referred to as a power converter can be utilized in lighting systems to convert the high voltage ac input into a well regulated direct current (dc) output through an energy transfer element. Switched mode power converters are commonly used to power many of today's electronics due to their high efficiency, small size, and low weight. During operation, a switch included in the power converter is used to provide the desired output by varying (1) the duty cycle (the ratio of the on time of the switch to the total switching period), (2) the switching frequency, or (3) the number of pulses per-unit-time of the switch.

In one type of dimming for lighting applications, a dimmer circuit disconnects a portion of the ac input voltage to limit the amount of voltage and current supplied to an incandescent lamp. This is generally known as phase dimming because it is often convenient to designate the position of the missing voltage in terms of a fraction of the ac input voltage (as measured in degrees). In general, the ac input voltage is a sinusoidal waveform and the period of the ac input voltage is referred to as a full line cycle.

While phase dimming may work well in some applications (for example, with incandescent lamps), in other applications, phase dimming may be less desirable due to the stringent power requirements of modern electronic devices.

DETAILED DESCRIPTION

Reference throughout this specification to “one embodiment”, “an embodiment”, “one example” or “an example” means that a particular feature, structure or characteristic described in connection with the embodiment or example is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment”, “in an embodiment”, “one example” or “an example” in various places throughout this specification are not necessarily all referring to the same embodiment or example. Furthermore, the particular features, structures or characteristics may be combined in any suitable combination and/or subcombination in one or more embodiments or examples. Particular features, structures or characteristics may be included in an integrated circuit, an electronic circuit, a combinational logic circuit, or other suitable components that provide the described functionality. In addition, it is appreciated that the figures provided herewith are for explanation purposes only and are not necessarily drawn to scale. Furthermore, embodiments/examples in this application refer to different pieces of circuitry responding to a “logic high” or “logic low” signal in a particular way; however, one skilled in the art will appreciate that the same piece of circuitry may be configured to respond the same way to the opposite signal (e.g., a piece of circuitry that turns on in response to a logic high signal, may be configured to turn on in response to a logic low signal or vice versa).

Although phase angle dimming works well with incandescent lamps, certain types of phase angle dimming may create problems for light emitting diode (LED) systems driven by a switched mode power converter. Unless a power converter is specially designed for an LED lamp, a phase angle dimmer circuit may produce unacceptable results such as flickering or “pop-on” of the LED system. In some instances, flickering may be attributed to a TRIAC dimmer circuit losing power (and failing to function) as a result of the low-power requirement of the LED system. Pop-on arises when the dimmer circuit is set above its existing state to produce light output at initial turn-on; the difference between the initial turn-on setting and the existing setting may be referred to as “pop”. Pop-on may reduce the overall efficiency of the lighting system. Accordingly, it is generally advantageous to have a circuit that eliminates flicker and pop-on in LED lighting systems. As will be shown, power converters utilizing bleeder circuits may help mitigate these issues.

FIG. 1is a functional block diagram of one example of a lighting system100including an example variable bleeder circuit104. As shown, lighting system100includes a driver circuit106coupled to drive a load108with an output voltage VO116and an output current IO118. In one example, driver circuit106includes a switched mode power converter (not shown), and load108includes one or more light emitting diodes (LEDs). Driver circuit106has an input with a first input terminal109and a second input terminal111; both terminals are coupled to an input105to receive an input voltage VIN112and an input current IIN119. In one example, input voltage VIN112is received from a rectifier circuit114and a dimmer circuit102. The dimmer circuit102is coupled to receive an ac line voltage VAC110between terminals101and103. Dimmer circuit102may be external to driver circuit106. The input voltage VIN112is positive with respect to the input return149. In one example, dimmer circuit102may be a TRIAC dimmer circuit or a thyristor dimmer circuit, which may add high frequency transitions to input voltage VIN112by removing portions of the ac line voltage VAC110.

Lighting system100also includes variable bleeder circuit104including a first terminal121coupled to the first input terminal109of driver circuit106, and a second input terminal131coupled to the second input terminal111of driver circuit106. The variable bleeder circuit104includes a third terminal129coupled to receive the shutdown signal128. In various examples, variable bleeder circuit104may be implemented as a monolithic integrated circuit, as discrete electrical components, or as a combination of discrete and integrated components, in accordance with the teachings of the present invention.

Variable bleeder circuit104includes a variable current circuit122, and a current scaling circuit124. The variable bleeder circuit104also includes an optional input current sense circuit120and an optional bleeder protection circuit126. Both of these optional features will be discussed here, in connection withFIG. 1; however, embodiments without these optional features will be discussed in greater detail in connection withFIG. 5. In some examples, the input current sense circuit120may be combined with the variable current circuit122. If present, the input current sense circuit120may be coupled between first input terminal109and second input terminal111of driver circuit106.

The variable current circuit122is coupled to conduct the bleeder current IB115between first input terminal109and second input terminal111. In the depicted example, the bleeder on/off signal125and the current scale signal123, control the amount of bleeder current IB115through variable current circuit122. If both the bleeder on/off signal125and the current scale signal123are logic low, no bleeder current IB115flows between first input terminal109and second input terminal111. If the bleeder on/off signal125is logic high and the current scale signal123is logic low, a first value IBLof bleeder current IB115flows between first input terminal109and second input terminal111. If both the bleeder on/off signal125and the current scale signal123are logic high, a second a second value IBHof bleeder current IB115flows between first input terminal109and second input terminal111. The second value IBHof bleeder current IB115is greater than the first value IBLof bleeder current IB115.

The input current sense circuit120is coupled to output the bleeder on/off signal125to the variable current circuit122, in response to the input current IIN119. The bleeder on/off signal125indicates if the input current IIN119has fallen to a value which is less than a threshold input current ITH. If the input current IIN119is lower than ITH, then the bleeder on/off signal is logic high; if the input current IIN119is greater than or equal to ITH, then the bleeder on/off signal is logic low. When bleeder on/off signal125is logic high, the variable current circuit122is enabled, and when bleeder on/off signal125is logic low, the variable current circuit122is disabled.

The current scaling circuit124is coupled to receive a shutdown signal128. In one example, if the conduction angle is less than a threshold conduction angle ALTH, then shutdown signal128is logic high and if the conduction angle is equal to or greater than ALTH, then the shutdown signal128is logic low. The value of ALTHmay be predefined and may be measured in degrees. In one example, the ALTHis thirty degrees; however, the value of ALTHmay be any value depending on the requirements of the lighting system.

In one example, if the shutdown signal128is logic low then the current scaling circuit124may be disabled, and if the shutdown signal128is logic high then the current scaling circuit124may be enabled. Furthermore, if the variable current circuit122is enabled but the current scaling circuit124is disabled, then the variable current circuit122may conduct a bleeder current of a lower value IBLbecause only the input current sense circuit120is enabled (in other words, the variable current circuit122is only receiving the bleeder on/off signal125and not both the bleeder on/off signal125and the current scale signal123). If the variable current circuit122is enabled and the current scaling circuit124is also enabled, then the variable current circuit122may conduct a bleeder current of higher value IBH. With either higher value IBHor lower value IBLof bleeder current IB115, a sufficient holding current is drawn by input current IIN119to prevent a switch in dimmer circuit102from opening. This may help prevent unwanted flickering in an LED lamp driven by driver circuit106, in accordance with the teachings of the present invention.

In one example, the shutdown signal128is an external signal. In other examples, the shutdown signal128is not an external signal and may result from a conduction angle detection circuit integrated with the variable bleeder circuit104.

The variable bleeder circuit104also includes optional bleeder protection circuit126coupled to receive the shutdown signal128and the input voltage VIN112. The bleeder protection circuit126is also coupled to output a bleeder bypass signal127to the current scaling circuit124in response to the shutdown signal128and the input voltage VIN112. Under certain conditions, such as an open load condition (not shown), the shutdown signal may become erroneously logic high (falsely indicating that the conduction angle is low). In this situation, the bleeder protection circuit126can disable the current scaling circuit124by making the bleeder bypass signal127logic high. In other words, the bleeder protection circuit126can either enable or disable the current scaling circuit124in response to the shutdown signal128and the input voltage VIN112. If the shutdown signal128is logic high and VIN112is greater than or equal to the bleeder protection voltage threshold VBTH, then the bleeder bypass signal127is logic high. If the shutdown signal128is logic high but VIN112is lower than VBTHthen the bleeder bypass signal127is logic low. The current scaling circuit124is enabled when the bleeder bypass signal127is logic high, and the current scaling circuit124is disabled when the bleeder bypass signal127is logic low. If the shutdown signal128is logic low, then the current scaling circuit124is disabled, and the variable current circuit122will conduct a bleeder current of lower value IBL(provided the input current sense circuit is enabled). Thus, the bleeder protection circuit126prevents the variable current circuit122from erroneously conducting a bleeder current of higher value IBH.

FIG. 2Aillustrates an example waveform200A of an ac line voltage VAC210received by the dimmer circuit.FIG. 2Billustrates an example rectified waveform200B of an input voltage VIN212received from a dimmer circuit (such as a TRIAC dimmer circuit) by a driver circuit of a lighting system. In the depicted example, ac line voltage VAC210is an ac input voltage (a sinusoidal waveform with a line cycle period TAC228). The line cycle period TAC228of the ac line voltage VAC210may also be referred to as a full line cycle period.FIG. 2Aalso shows a half line cycle TAC/2230, which is half of the line cycle period TAC228. As shown, half line cycle TAC/2230is the length of time between consecutive zero crossings of ac line voltage VAC210.

Referring briefly now back toFIG. 1, dimmer circuit102disconnects and reconnects the ac line voltage VAC110from the first input terminal109of driver circuit106. In leading edge dimming, when the ac line voltage VAC110crosses the zero voltage, dimmer circuit102disconnects the ac line voltage VAC110from first input terminal109. Thus, the ac line voltage VAC110is disconnected from the driver circuit106and variable bleeder circuit104. After a given amount of time, dimmer circuit102reconnects ac line voltage VAC110to first input terminal109of driver circuit106and to variable bleeder circuit104. However, one skilled in the art will appreciate that dimmer circuit102may also be a trailing edge dimmer. In trailing edge dimming, the dimmer circuit102connects the ac line voltage VAC110to the first input terminal109when the ac line voltage VAC110crosses zero voltage, and disconnects the ac line voltage VAC110after a given amount of time. Referring now toFIG. 1andFIG. 2B, the dimmer circuit102removes a portion of each half line cycle TAC/2230of ac line voltage VAC210to limit the amount of voltage and current supplied by the driver circuit106to the load108.

As shown inFIG. 2B, input voltage VIN212is substantially zero when the dimmer circuit102has disconnected the ac line voltage VAC210from first input terminal109. Once the dimmer circuit102reconnects the ac line voltage VAC210to first input terminal109, the voltage waveform of input voltage VIN212substantially follows the ac line voltage VAC210. Edges223of input voltage VIN212result during each half line cycle TAC/2230from the high frequency transitions223caused by dimmer circuit102disconnecting and reconnecting ac line voltage VAC210.

The amount of dimming corresponds to the length of time during which the dimmer circuit102disconnects the ac line voltage VAC210from first input terminal109of the input of driver circuit106. It is noted that dimmer circuit102also includes an input (not shown), which provides dimmer circuit102with information regarding the amount of desired dimming.

FIG. 3Aillustrates timing diagram300A. Timing diagram300A shows example waveforms of input voltage VIN312and input current IIN314of a lighting system100, which includes the variable bleeder circuit104(with optional bleeder protection circuit126and optional input current sense circuit120, see e.g., the embodiment depicted inFIG. 4). Conversely,FIG. 3Billustrates timing diagram300B showing example waveforms received by a driver circuit of a lighting system without a variable bleeder circuit. To help explain the advantages conferred by variable bleeder circuit104, the description ofFIG. 3Amay be found immediately following the description ofFIG. 3B.

InFIG. 3B, the input voltage VIN312is substantially zero at the beginning of the half line cycle TAC/2330. When the dimmer circuit102reconnects the ac line voltage VAC110, the input voltage VIN312increases quickly at high frequency transition (edge)316, and substantially follows the voltage of ac line voltage VAC110for the remainder of the half line cycle316. In some examples of leading edge dimming, at the beginning of the half line cycle TAC/2330, the input current IIN314is also substantially zero until the dimmer circuit fires. Once the dimmer circuit102fires, the input current IIN314also increases quickly such that there is a high frequency transition (edge) of input current IIN314. Without the inclusion of variable bleeder circuit104, the input current IIN314rings (oscillates several times). This may be due in part to inductive and capacitive elements included in driver circuit106. If the input current IIN314falls below the holding current of the dimmer circuit before the end of the half line cycle TAC/2330, or before the input voltage VIN312has reached zero, the dimmer circuit may prematurely turn off and cause flickering in the load.

InFIG. 3A, examples in accordance with teachings of the present invention may reduce the ringing of the input current IIN314. Similar toFIG. 3B, the input voltage VIN312inFIG. 3Ais substantially zero until the dimmer circuit fires. Once the dimmer circuit fires, the input voltage VIN312increases rapidly (high frequency transition) and substantially follows the ac line voltage VAC110. The input current IIN314is also substantially zero until the dimmer circuit102reconnects the ac line voltage VAC110. Once the dimmer circuit102reconnects the ac line voltage VAC110, the input current IIN314also increases quickly (high frequency transition). However, the inclusion of variable bleeder circuit104inFIG. 3Areduces ringing (current oscillations) and helps to prevent the input current IIN314from falling below ITH318. Thus, input current IIN314is held above the holding current of dimmer circuit102, in accordance with the teachings of the present invention.

FIG. 3Cillustrates a timing diagram300C depicting example waveforms of bleeder current IB115through a variable bleeder circuit104(including optional bleeder protection circuit126and optional input current sense circuit120). Referring to bothFIG. 3BandFIG. 3C, at time tX1324, when the input current is lower than the holding current ITH, the bleeder current IB115gradually starts increasing. At time tX2326, the bleeder current IB115reaches a value IBL332, the lower of two values of bleeder current IB115. The bleeder current IB115remains substantially the same until time tX3328. At time tX3328, the shutdown signal may be logic high indicating that the conduction angle is below ALTH. Therefore, the bleeder current IB115is increased to a value IBH334, in accordance with the teachings of the present invention.

FIG. 3Dillustrates timing diagram300D. Timing diagram300D shows example waveforms of input voltage VIN352and input current IIN354of lighting system100including a variable bleeder circuit104(without optional bleeder protection circuit126, and without optional input current sense circuit120, see e.g., the embodiment depicted inFIG. 5). To help explain the advantages conferred by variable bleeder circuit104, the description ofFIG. 3Dmay be found immediately following the description ofFIG. 3E.

As illustrated inFIG. 3E, the input voltage VIN352is substantially zero at the beginning of the half line cycle TAC/2350. When the dimmer circuit102reconnects the ac line voltage VAC110, the input voltage VIN352increases quickly (high frequency transition356) and substantially follows the voltage of ac line voltage VAC110for the remainder of the half line cycle TAC/2350. At the beginning of the half line cycle TAC/2350, the input current IIN319is also substantially zero until the dimmer circuit fires. Once the dimmer circuit102fires, the input current IIN354also increases. Without the inclusion of variable bleeder circuit104, the input current IIN354may ring (oscillate several times). As explained earlier with respect toFIG. 3B, the ringing may be partially due to inductive and capacitive elements included within driver circuit106. Further, if the input current IIN354falls below the holding current of the dimmer circuit before the end of the half line cycle TAC/2350, or before the input voltage VIN352has reached zero, the dimmer circuit may prematurely turn off and cause flickering in the load driven by driver circuit.

However, as shown inFIG. 3D, the inclusion of variable bleeder circuit104may reduce ringing and help prevent the input current IIN354from falling below a threshold input current ITH357(which keeps the input current IIN354above the holding current of dimmer circuit102), in accordance with teachings of the present invention. Furthermore, the input current IIN354may be scaled in response to the input voltage VIN352falling below a low input voltage threshold VLTH358.

FIG. 3Fillustrates timing diagram300F. Timing diagram300F shows example waveforms of bleeder current IB115of the variable bleeder circuit104(without optional bleeder protection circuit126, and without optional input current sense circuit120, see e.g.FIG. 5). Referring to bothFIG. 3EandFIG. 3F, at time tX1364, when the input voltage VIN352is lower than VLTH358, the bleeder current IB115gradually starts increasing. At time tX2366, the bleeder current may reach a maximum value. After time tX2366, the bleeder current IB115, may substantially follow the input voltage VIN352for the remaining portion of the half line cycle.

FIG. 4is a schematic400illustrating a variable bleeder circuit404which is an example of the variable bleeder circuit104included in the lighting system100ofFIG. 1, in accordance with the teachings of the present invention. The variable bleeder circuit404depicts an embodiment of the disclosure that includes optional pieces of circuitry (i.e., bleeder protection circuit426, and input current sense circuit420), along with pieces of circuitry common to other embodiments (i.e., current scaling circuit424, and variable current circuit422).

The input current sense circuit420is included in variable bleeder circuit404and is coupled to one of first and second input terminals409and411respectively of the driver circuit (not shown). The input current sense circuit420is coupled to output a bleeder on/off signal425in response to the input current IIN419. The variable current circuit422is coupled between first input terminal409and second input terminal411of driver circuit406and conducts a bleeder current IB415between the first input terminal409and the second input terminal411in response to the bleeder on/off signal425. Additionally, the variable current circuit422is coupled to conduct either a higher value IBHor a lower value IBLof the bleeder current IB415, in response to a current scale signal423. With bleeder current IB415flowing between first input terminal409and the second input terminal411, the input current IIN419is greater than or equal to the holding current of the dimmer. Keeping the input current IIN419above the holding current may prevent a switch in dimmer circuit402from turning off prematurely, and reduce unwanted flickering in LED lamps.

In the illustrated example, input current sense circuit420includes a current sense transistor Q1442(hereafter Q1442), a current sense resistor R2436(hereafter R2436), a resistor R1434, a resistor R3438, a capacitor C2440, and a diode D2444. The R2436is coupled to sense the input current IIN419. The first input terminal411and control terminal of Q1442are coupled to the R2436, hereafter R2436. In one example, R2436is coupled to the control terminal of Q1442through the resistor R3438. An anode of the diode D2444is coupled to a first terminal of Q1442. The cathode of the diode D2444is coupled to produce the bleeder on/off signal425. The capacitor C1432, diode D1430, and the resistor R1434, are also coupled to the output of diode D2444and the Q1442.

In the illustrated example, Q1442is an NPN bipolar transistor, with the R2436coupled between the base and emitter. The base to emitter voltage of the Q1442may be referred to as VSENSE(not shown), and the current through R2436may be referred to as ISENSE(not shown). The value of ISENSEmay be substantially given by—

The values of resistors R2436and R3438are selected so when the input current TIN419is greater than or equal to ITH, ISENSEproduces enough voltage across resistor R2436(and at the control terminal of the Q1442), to fully turn on or keep the Q1442in saturation. In other words, the control terminal of the Q1442is logic high. When the Q1442is in saturation, the anode of diode D2444is pulled low and the diode D2444is reverse biased. Accordingly, the bleeder on/off signal425is logic low and the variable current circuit422is disabled.

When the input current IIN419is less than ITH, the ISENSEdoes not produce enough voltage across R2436to turn on the Q1442. In other words, the control terminal of the Q1442is logic low and the transistor Q1442is turned off. Accordingly, the anode of output diode D2444is high and forward biased, making the bleeder on/off signal logic high. When diode D2444is not conducting, the input current sense circuit420turns the bleeder on/off signal425logic low and disables the variable current circuit422. When diode D2444is conducting, the input current sense circuit420turns the bleeder on/off signal425logic high and enables the variable current circuit422. Further, diode D2444may be used to ensure that current flows in one direction (from the input current sense circuit420to the variable current circuit422).

Variable current circuit422includes a transistor Q2450, a resistor R4448, and a resistor R5452. The variable current circuit422is coupled to conduct the bleeder current IB415between input terminals409and411of the driver circuit (not shown), in response to the bleeder on/off signal425and the current scale signal423. One end of the resistor R4452is coupled to the first input terminal409of driver circuit406. The other end of resistor R4452is coupled to a first terminal of the transistor Q2450. A second terminal of the transistor Q2450is coupled to the second input terminal411of driver circuit406, and a control terminal of the transistor Q2450is coupled to receive the bleeder on/off signal425. One end of the resistor R5452is coupled to the control terminal of the transistor Q2450, and the other end of resistor R5452is coupled to the second input terminal411.

If the bleeder on/off signal425is logic low, then the transistor Q2450is off and the value of bleeder current IB415is substantially zero. If the bleeder on/off signal425is logic high, then the transistor Q2450is on and conducts bleeder current IB415. As will be explained later, when the bleeder on/off signal425is logic high, the transistor Q2450may operate either in a linear regime or a saturation regime (in response to the current scale signal423).

Transistor Q2450may be a NPN bipolar transistor, or a PNP bipolar transistor. However, one skilled in the art will appreciate that other transistors may be used, such as metal-oxide-semiconductor field-effect transistors (MOSFETs), junction gate field-effect transistors (JFETs), or insulated gate bipolar transistors (IGBTs). The bleeder current IB415may be substantially equal to the current provided by bleeder on/off signal425multiplied by the beta of transistor Q2450.

The current scaling circuit424is coupled to receive the shutdown signal128. The output of the current scaling circuit424is coupled to the control terminal of transistor Q2450as the current scale signal423. The current scaling circuit424includes a current scale resistor R6456and a diode D3454. In one example, the current scaling circuit424is coupled to vary the bleeder current IB415through the variable current circuit422in response to the shutdown signal128and the bleeder bypass signal427. One end of the current scale resistor R6456is coupled to receive the shutdown signal128and the other end of current scale resistor R6456is coupled to the anode of the diode D3454. The cathode of diode D3454is coupled to the control terminal of the transistor Q2450.

The transistor Q2450is controlled by both bleeder on/off signal425via the input current sense circuit420and current scale signal423via the current scaling circuit424. If the shutdown signal128is logic low, then the voltage across the resistor R6456is not high enough to forward bias the diode D3454. Subsequently, the current scale signal423is logic low. If the shutdown signal128is logic high, then the voltage across the resistor R6456is large enough to forward bias the diode, and the current scale signal423becomes logic high. Accordingly, the Q2450is fully turned and operates in the saturation regime. If the bleeder on/off signal425is logic high but if the shutdown signal128is logic low, then the transistor Q2450is partially turned on and operates in the linear regime. Thus, the transistor Q2450conducts a bleeder current of a lower value IBL. If the bleeder on/off signal425is logic high and the shutdown signal128is also logic high, transistor Q2450is fully turned on and operates in the saturation regime. Thus, the transistor Q2450conducts a bleeder current of a higher value IBH. The transistor Q2450is substantially controlled by the current scaling circuit424when the shutdown signal128is high. In summary, if the conduction angle is equal to or greater than ALTH, then the transistor Q2450is only partially turned on and may conduct a bleeder current of a lower value IBL; if the conduction angle is lower than ALTH, then the transistor Q2450is fully turned on and conducts a bleeder current of higher value IBH.

The variable bleeder circuit404may also include an optional bleeder protection circuit426. The example bleeder protection circuit426includes a transistor Q3460, an input voltage sense resistor R7458, a resistor R8466, and a capacitor C3464. The bleeder protection circuit426is coupled to sense the input voltage VIN112and the shutdown signal128. The bleeder protection circuit426is coupled to output bleeder bypass signal427to the current scaling circuit424.

A first terminal of transistor Q3460is coupled to receive the shutdown signal128. A second terminal of transistor Q3460is coupled to input terminal411of the driver circuit. A control terminal of transistor Q3460is coupled to sense the input voltage VIN412via resistor R7458. The values of resistors R7458and R8462are chosen such that the turn-on voltage of the transistor Q3460is substantially equal to the VBTH. In operation, if the shutdown signal128is logic high (indicating that the conduction angle is lower than ALTH), and if the input voltage VIN412is lower than VBTH, then the control terminal of the transistor Q3460is low and the transistor Q3460is turned off. Accordingly, the anode of diode D3454is high, and diode D3454is forward biased, making the current scale signal logic high. Subsequently, variable current circuit422conducts bleeder current of higher value IBH. Conversely, if the shutdown signal128is logic high and the input voltage VIN412is greater than or equal to VBTHthen the control terminal of transistor Q3460becomes high, and the transistor Q3460is fully turned on (operating in saturation). This further makes the anode of diode D3454logic low, reverse biasing the diode D3454. When the diode D3454is reverse biased, the transistor Q2450changes from saturation operation to linear operation. Subsequently, the bleeder current through the variable current circuit422is decreased from a higher value to IBHto a lower value IBL. Thus, the bleeder protection circuit426may protect the variable bleeder circuit from conducting higher value of bleeder current in case of an open load condition. The capacitor C3464is a bypass capacitor.

FIG. 5is a schematic500illustrating a variable bleeder circuit504which is an example of the variable bleeder circuit104included in the lighting system100ofFIG. 1, in accordance with the teachings of the present invention. The example variable bleeder circuit504depicts an embodiment without optional bleeder protection circuit126and without optional input current sense circuit120.

The variable bleeder circuit504is coupled to receive an input voltage VIN512from a rectifier (not shown) at the terminals501and503. The input voltage VIN512is positive with respect to the input return549. The variable bleeder circuit504is coupled to receive an input current IIN519in the direction shown. The variable bleeder circuit504may be coupled to a driver circuit (not shown) via a first input terminal509and a second input terminal511.

In the depicted example, the variable bleeder circuit504includes a variable current circuit522and a current scaling circuit524. The variable bleeder circuit504may be implemented as a monolithic integrated circuit, with discrete electrical components, or a combination of discrete and integrated components. The variable current circuit522is coupled to conduct a bleeder current IB515between the first input terminal509and the second input terminal511of the driver circuit506in response to the input voltage VIN512, in accordance with the teachings of the present invention.

The variable current circuit522includes a transistor Q4542, a resistor R10536, a resistor R11538, a resistor R12540, an op-amp530, a capacitor C4532, and a resistor R9534. One end of the resistor R10536, is coupled to a first terminal of the transistor Q4542. A second terminal of transistor Q4542is coupled to one end of the resistor R11538at a first node N1545. A control terminal of transistor Q4542is coupled to an output531of an op-amp530. The other end of the resistor R11538is coupled to one end of the resistor R12540at a second node N2555. A second end of the resistor R12540is coupled to the second input terminal511of the driver circuit (not shown). The op-amp530is coupled as an error amplifier. The non-inverting input537of op-amp530is coupled to receive a reference voltage VREF535. A capacitor C4532is coupled in the feedback path of the op-amp530in such a way that one end of the capacitor C4532is coupled to the inverting terminal533of the op-amp530, and the other end of the capacitor C4532is coupled to the output531of the op-amp530. One end of resistor R9534is coupled to the inverting terminal of the op-amp530, and the other end of resistor R9534is coupled to the second terminal of transistor Q4542at a third node N3547.

In the illustrated example, the current scaling circuit524includes a current scaling circuit resistor R13548. One end of the resistor R13548is coupled to receive the input voltage VIN512via the first input terminal509. The other end of the resistor R13548is coupled to resistor R12540at a second node N2555. The current scaling circuit524is coupled to output a current scale signal523at the second node N2555. In one example, the current scale signal555is a voltage signal. The current scale resistor R13548forms a potential divider circuit with the resistor R12540.

The voltage VN2at the node N2555may be given by—

The reference voltage VREF535may be chosen by design. Because of the op-amp action, the voltage VINV539at the inverting terminal is maintained substantially equal to VREF535. If VR9is assumed to be the voltage across the resistor R9534, and if IR9is assumed to be the current through the resistor R9534, then voltage VN1at node N1545may be given by—
VN1=VREF−VR9(2)
VR9=IR9R9  (3)

If VN4is assumed to be the voltage at the node N4551and XC4is assumed to be the capacitive reactance of the capacitor C4532, then the current through resistor R9may be substantially given by equation—

From equations 2, 3, and 4 above, it may be understood that the voltage VN1at node N1545may also be substantially constant and independent of the input voltage VIN512.

However, as the input voltage VIN512varies then, the voltage across the resistors R12540and R13548also varies; this may cause the voltage VN2at the node N2555to change (as shown by equation 1). Since VN1is substantially constant, one end of resistor R11538is maintained at a constant voltage while the voltage at the other end of resistor R11538may vary. It may be appreciated that this varying voltage across the resistor R11538may draw more current through the transistor Q4542. Accordingly, if the input voltage VIN512increases then the bleeder current IB115will decrease, and if the input voltage VIN512decreases, then the bleeder current IB115will increase. In other examples, other circuitry such as peak detectors, comparators, logic gates may be included as part of the variable bleeder circuit. In some examples, the bleeder current may be increased as the input voltage increases and the bleeder current may be decreased as the input voltage decreases.

Transistor Q4542may be an NPN bipolar transistor or a PNP bipolar transistor. However, one of ordinary skill in the art will appreciate that other transistors may be used, such as metal-oxide-semiconductor field-effect transistors (MOSFETs), junction gate field-effect transistors (JFETs), or insulated gate bipolar transistors (IGBTs).

FIG. 6is a flow chart600illustrating an example process for scaling the bleeder current in response to sensing a low input current and/or a low conduction angle, consistent with an embodiment of a variable bleeder circuit including optional pieces of circuitry bleeder protection circuit426, and input current sense circuit420(see e.g.,FIG. 4).

After starting at block601, block602illustrates checking if the input current IINis greater than ITH. If the input current IINis equal to greater than ITH, the process proceeds to the beginning of block602. If the input current IINis less than ITH, the process proceeds to the block603.

At block603the bleeder current is maintained at a lower value IBL. The process then checks if IINis equal to or greater than ITH. If IINis equal to or greater than ITH, the process will go back to the beginning of block602, otherwise the process will proceed to block604.

Block604illustrates checking if the conduction angle is greater than or equal to a threshold conduction angle ALTH. If the conduction angle is equal to or greater than ALTH, then the process proceeds to block603. If the conduction angle is less than ALTH, then the process proceeds to block605.

At block605if the input voltage is equal to or greater than a bleeder protection voltage threshold voltage VBTH, the process proceeds to block603. If the input voltage is less than the bleeder protection voltage threshold voltage VBTH, the process proceeds to block606.

At block606the bleeder current is maintained at a higher bleeder current value IBH. At the end of block606, the process goes back to block601.

FIG. 7is a flow chart700illustrating an example process for scaling the bleeder current in response to sensing a low input voltage, constant with an embodiment of the disclosure where the variable bleeder circuit does not include optional bleeder protection circuit126, and does not include optional input current sense circuit120(see e.g.,FIG. 5).

Starting at block701, block702illustrates checking if the input voltage VINis greater than or equal to zero. If the input voltage VINis greater than zero, then the process proceeds to block703.

Block703illustrates checking if the input voltage is greater than or equal to VLTH. If the input voltage is greater than or equal to VLTH, then the process proceeds to block704. If the input voltage is lower than VLTH, then the process proceeds to block705.

At block704the bleeder current may be maintained at a lower value IBL. At the end of block704, the process goes back to block701.

At block705the bleeder current may be maintained at a higher value IBH. At the end of block705, the process goes back to block701.