Patent Publication Number: US-2013229215-A1

Title: Variable Resistance for Driver Circuit Dithering

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority to U.S. Patent Application No. 61/606,286 entitled “Variable Resistance for Driver Circuit Dithering”, filed Mar. 2, 2012, the entirety of which is incorporated herein by reference for all purposes. 
    
    
     BACKGROUND 
     Electronic circuits such as power supplies and drivers are widely used to power and control electrical circuits and devices such as lighting circuits with light emitting diodes (LEDs) and light dimming circuits. However, switching elements in power supplies and drivers can cause electromagnetic interference (EMI), causing problems for nearby electrical devices. Such switching elements can also reduce the efficiency and power factor of electrical circuits. 
     SUMMARY 
     A dithering circuit is disclosed which may be used for example to vary a control resistance used to set the frequency and/or duty cycle of a switching circuit, such as a switching circuit in a power supply, a switching circuit in an LED driver, a clock, essentially any circuit that uses a timing resistor, etc. An example LED driver that benefits from a dithering circuit provides power for LED lighting systems using pulse control of a switch to adjust load current and/or voltage. The LED driver sets the frequency of the pulse signal used to control the switch based on an impedance value set by an external resistor. The dithering circuit may be used in place of or in conjunction with the external resistor to vary the frequency of the pulse signal, spreading the frequency of the noise or EMI generated by the switch and reducing its affects. 
     This summary provides only a general outline of some particular embodiments. Many other objects, features, advantages and other embodiments will become more fully apparent from the following detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A further understanding of the various embodiments may be realized by reference to the figures which are described in remaining portions of the specification. In the figures, like reference numerals may be used throughout several drawings to refer to similar components. 
         FIG. 1  depicts a block diagram of a dimming driver with a dither circuit in accordance with some embodiments of the invention; 
         FIG. 2  depicts a schematic of a dimming driver with a dither circuit in accordance with some embodiments of the invention; 
         FIG. 3  depicts a schematic of a variable resistance circuit that may be used as a dither circuit in accordance with some embodiments of the invention; 
         FIG. 4  depicts a graph of a reference current generated at the input of a current mirror in the variable resistance circuit of  FIG. 3 ; 
         FIG. 5  depicts a schematic of a variable resistance circuit that may be used as a dither circuit in accordance with some embodiments of the invention, with a voltage source illustrating the connection of a frequency control device; 
         FIG. 6  depicts a graph of a reference current generated at the output of the variable resistance circuit of  FIG. 5  with a first voltage level generated by the frequency control device; 
         FIG. 7  depicts a graph of a current generated at the output of the variable resistance circuit of  FIG. 5  with a second voltage level generated by the frequency control device; 
         FIG. 8  depicts a schematic of a variable resistance circuit that may be used as a dither circuit and connected in parallel with a frequency control resistor, and depicting a test resistor and voltage source illustrating the connection of a frequency control device; 
         FIG. 9  depicts a graph of the current across the frequency control resistor of  FIG. 8 ; 
         FIG. 10  depicts a graph of the total current through the frequency control device, including the reference current from the variable resistance circuit and the current across the frequency control resistor of  FIG. 8 ; 
         FIG. 11  depicts a dither circuit including a waveform source and current mirror, connected to a frequency control device in parallel with a frequency control resistor; 
         FIG. 12  depicts a dither circuit including a waveform source and current mirror, connected to a frequency control device in series with a frequency control resistor; 
         FIG. 13  depicts a dither circuit including a waveform source and current mirror, connected to a pulse generator used to control a power control switch; 
         FIG. 14  depicts a graph of the current through the power control switch of  FIG. 13 , illustrating the frequency variation caused by the dither circuit; 
         FIG. 15  depicts a NAND-based pulse generator connected to a dither circuit, with a voltage source representing a frequency control device; 
         FIG. 16  depicts an inverter-based pulse generator connected to a dither circuit, with a voltage source representing a frequency control device; and 
         FIG. 17  depicts a graph of the current through the frequency control device of  FIGS. 15 and 16 . 
     
    
    
     DESCRIPTION 
     A dithering circuit is disclosed which may be used for example to vary a control resistance to set the frequency and/or duty cycle of a switching circuit in a power supply, for example, an LED driver, a fluorescent lamp driver, a general lighting driver, a current or voltage controlled power supply, etc. An example LED driver that benefits from a dithering circuit provides power for LED lighting systems using pulse control of a switch to adjust load current and/or voltage. The LED driver sets the frequency of the pulse signal used to control the switch based on an impedance value set by an external resistor which is sometimes referred to, in general, as a timing or frequency resistor. The dithering circuit may be used in place of or in conjunction with the external resistor to vary the frequency of the pulse signal, spreading the frequency of the noise or EMI generated by the switch and reducing its affects. 
     Examples of LED drivers that may incorporate a dithering circuit disclosed herein include those in U.S. patent application Ser. No. 12/422,258, filed Apr. 11, 2009 for a “Dimmable Power Supply”, and in U.S. patent application Ser. No. 12/776,409, filed May 9, 2010 for a “LED Lamp with Remote Control”, which are incorporated herein by reference for all purposes. Such a driver provides power for lights such as LEDs of any type and other loads. The lighting driver may be dimmed or otherwise controlled externally, for example by controlling a line voltage supplying the lighting driver, or internally, for example using a wireless controller to command internal dimming circuits, etc. The current and/or voltage to a load is adjusted using a switch to pass or block input current, controlled by a variable pulse signal. 
     Turning to  FIG. 1 , a block diagram of a dimming driver with a dither circuit in accordance with some embodiments of the invention. The dimming driver with dither circuit  10  is powered in some embodiments by an AC input  12 , for example by a 50 or 60 Hz sinusoidal waveform of 120 V or 240 V RMS or higher such as that supplied to commercial and residential facilities by municipal electric power companies. The dimming driver can also be supplied with a direct current (DC) voltage/current/power supply. It is important to note, however, that the dimming driver with dither circuit  10  is not limited to any particular power input. Furthermore, the voltage applied to the AC input  12  may be externally controlled, such as in an external dimmer (not shown) that reduces the voltage. The AC input  12  is connected to a rectifier  14  to rectify and invert any negative voltage component from the AC input  12 . Although the rectifier  14  may filter and smooth the power output  16  if desired to produce a DC signal, this is not necessary and the power output  16  may be a series of rectified half sinusoidal waves at a frequency double that at the AC input  12 , for example 100 or 120 Hz. A variable pulse generator  20  is powered by the power output  16  from the AC input  12  and rectifier  14  to generate a train of pulses at output  22 . The pulse width of the pulses in output  22  is controlled in the variable pulse generator  22  by load current detector  24  based on load current levels. Various implementations of pulse width control including pulse width modulation (PWM) by frequency, analog and/or digital control may be used to realize the pulse width control. Other features such as soft start, delayed start, instant on operation, etc. may also be included if deemed desirable, needed, and/or useful. Output driver  30  produces a current through the load  26 , with the current levels adjusted by the pulse width at the output  22  variable pulse generator  22 . The load current is monitored by the load current detector  24  and may also be monitored by a master load current detector sensor. Such a sensor may be, but is not limited to, a sense resistor, a sense transformer, a winding on a transformer or inductor, sensing via passive and/or active components, etc. 
     A dither circuit  40  is provided to vary the frequency and/or duty cycle of the variable pulse generator  20 , spreading noise such as EMI from the dimming driver with dither circuit  10  over a wider range of frequencies to reduce its effect. Less noise is generated at the original non-dithered frequency, because the circuit operation is shifted across the dithered range of spread frequencies and spends less time operating at the non-dithered frequency or at any single frequency. The term “dither” is used herein to refer to variation in the frequency and/or duty cycle of the output of the pulse generator, which may be random, pseudo-random, or have any other shifting variation. 
     Turning to  FIG. 2 , a schematic of an embodiment of a dimming driver with dither circuit  100  is illustrated in accordance with some embodiments of the invention. An AC input  112  is converted to a DC supply  116  by rectifier  114 . As noted above, the dimming driver with dither circuit  100  is not limited to this particular example power configuration. A switch  120  controls current from DC supply  116  to a load  122 . The load  122  is connected in parallel with, for example, a capacitor  124  which is optional in some embodiments of the present invention. An optional load current sense resistor  126  can be connected in series with the load  122 . An inductor  130  is connected in series with load  122  and capacitor  124  to store energy as current flows from DC supply  116  through the load  122 , when the switch  120  is on. A diode  132  is connected to make a loop including load  122  and inductor  130 , allowing energy stored in inductor  130  to produce a current through load  122  when switch  120  is off. 
     The switch  120  is controlled by pulses at an output  133  of a variable pulse generator  134 . The on-time and/or off-time of the pulses from the variable pulse generator  134  may be adjusted based on the current through the load  122 , measured by load current detector  136  based on load current sense resistor  126 . The dimming driver with dither circuit  100  may be dimmed by an external dimmer, controlled by the voltage level at DC supply  116  as represented by a reference current from a reference current generator  140 . The dimming driver with dither circuit  100  may also be dimmed by an internal dimmer that adjusts the reference current from reference current generator  140  based on any suitable control input. The on-time and/or off-time of the pulses from the variable pulse generator  134  may be also be adjusted based on the input current through the switch  120 , measured for example using a current sense resistor  144 . 
     Components of the dimming driver with dither circuit  100  may be powered by any suitable power source, such as from the DC supply  116  via a power supply  142 . 
     The frequency of the pulses at the output  133  of the variable pulse generator  134  is set in some embodiments by a resistor  150 , with the variable pulse generator  134  applying a test voltage to the resistor  150  and basing the frequency on the current through the resistor  150 . A dither circuit  152  is used in conjunction with or to replace the resistor  150 , varying the resistance to dither the frequency of the pulses at output  133  of variable pulse generator  134 . 
     Turning to  FIG. 3 , a dither circuit  300  produces a variable resistance at an output  302 . An integrator  306  and comparator  304  produce a triangle wave or sawtooth wave  308  such as that illustrated in  FIG. 4  at the input  310  to a current mirror  312 . The integrator  306  includes an op-amp  316  with a feedback capacitor  320  and resistor  322  connected to the inverting input, forming an RC network. The capacitor  320  is charged and discharged over time, depending on whether the signal applied to the resistor  322  is high or low. Notably, any other suitable circuit may be used in place of the integrator  306  to produce a triangle wave or sawtooth wave, and other embodiments perform dithering in other manners than the triangle wave or sawtooth wave. The comparator  304 , based on op-amp  324 , toggles the state of the signal applied to resistor  322  by comparing the output of op-amp  316  in integrator  306  with a reference voltage provided by a potentiometer  326 , or a voltage divider or other variable impedance or other voltage source. When the output of op-amp  316  in integrator  306  with a reference voltage rises to a level established by reference source  326 , the comparator  304  turns off the signal applied to resistor  322  and the waveform  308  begins to fall. When the output of op-amp  316  in integrator  306  with a reference voltage falls to a level established by reference source  326 , the comparator  304  turns on the signal applied to resistor  322  and the waveform  308  begins to rise. 
     Current mirror  312  controls the current through resistor  314 , used to set the effective impedance of the frequency input (also referred to herein as an impedance input) to the pulse generator (e.g.,  134 ). Resistor  314  may be connected alone to the frequency input of the pulse generator (e.g.,  134 ), or in parallel or in series with an external resistor (e.g.,  150 ) connected to the frequency input of the pulse generator (e.g.,  134 ). The current from the output of op-amp  316  in integrator  306  through resistor  332  and the diode-connected transistor of current mirror  312  controls the current through resistor  314  at dither circuit output  302 . 
     The dither circuit  300  may be powered by any suitable power supply  330 , such as a power supply (e.g.,  142 ) that derives power from DC supply  116  or AC input  112 . In other embodiments, the dither circuit  300  is powered by other sources such as a tag-along inductor coupled to inductor  130 , a battery, solar power source, mechanical or thermal power source, etc, or any combination of these, etc. 
     The dither circuit  300  can be used to modulate the current used for example to set the frequency of variable pulse generator  134  in dimming driver with dither circuit  100 , without interfering with the voltage level applied by the variable pulse generator  134 . The resistor  314  may be used in place of resistor  150  of dimming driver with dither circuit  100 , or may be connected in series or in parallel or in other combinations with resistor  150 . 
     The dither circuit  300  may be adapted to generate any desired waveform, including single or multiple, simple or complex waveforms, or random or pseudo-random waveforms. The current mirror  312  and other components of the dither circuit  300  is not limited to bipolar junction transistors (BJTs) but may comprise N-channel metal oxide semiconductor field effect transistors (MOSFETs), P-Channel MOSFETs, NPN bipolar junction transistors (BJTs), PNP BJTs, junction FETs, heterojunction bipolar transistors (HBTs), high electron mobility transistors (HEMTs), modulation doped transistors (MODFETs), any other type of transistor, appropriate three terminal devices, op amps, etc. The dither circuit  300  and transistors therein can be made of any material or materials including, but not limited to, silicon (Si), silicon carbide (SiC), silicon germanium (SiGe), gallium arsenide (GaAs)-based, gallium nitride (GaN)-based, indium phosphide (InP)-based, silicon on insulator (SOI), any combination of binary, ternary, etc. compounds, etc. The dither circuit  300  may be made or incorporated into an integrated circuit, and can be made of discrete or integrated components. 
     Various embodiments of a dither circuit may be used to generate any suitable current waveform, using any suitable technique. For example, a digital to analog converter (DAC) may be used to generate a current waveform. Single or multiple waveforms may be used and may be summed, multiplied, divided, added, subtracted, etc. in the time, frequency, amplitude, etc. domains. The dither circuit  300  may be used at any practical frequency—low or high. The dither circuit  300  may yield a waveform at a single frequency or at multiple frequencies, with constant or varying frequencies. 
     Turning to  FIG. 5 , the dither circuit  300  is depicted with a voltage source  330  representing or illustrating the connection of a frequency control device, such as the frequency setting component of variable pulse generator  134 . The voltage source  330  represents the voltage applied by variable pulse generator  134  to resistor  150  and/or dither circuit  300 . The current waveforms  340 ,  342  illustrated in  FIGS. 6 and 7  are generated using two different voltages from voltage source  330 , demonstrating the substantially voltage-independent current modulation. The current waveforms of  FIGS. 6 and 7  are measured at output  302  of dither circuit  300 . 
     Turning to  FIG. 8 , dither circuit  300  is connected with output resistor  314  in parallel with external frequency control resistor  150 . A small test resistor  336  is included to illustrate current waveforms at various circuit nodes. In  FIG. 9 , the constant current  344  across resistor  150  is illustrated. In  FIG. 10 , the modulated current  346  at node  338  between test resistor  336  and voltage source  334  is illustrated, or the total current including the modulated current from dither circuit  300  through resistor  314  and the constant current through resistor  150 . 
     Turning to  FIG. 11 , a dither circuit  400  is depicted including a waveform source  402  and current mirror  404 , connected to a frequency control device  406  in parallel with a frequency control resistor  410 . The waveform source  402  may comprise any suitable circuit or device to generate a modulated current at the output  412 , with any suitable dithering waveform, from the triangle wave illustrated in  FIG. 4  to other simple or complex waveforms with constant or varying frequency also including, but not limited to, pseudo-random, random, noise, noise of any kind and type, etc. . . . The frequency control device  406  may comprise a portion of a variable pulse generator  134  in a dimming driver with dither circuit  100 , for example, used to apply a voltage and to set the frequency of output pulses based on the resulting current. 
     Turning to  FIG. 12 , a dither circuit  420  is depicted including a waveform source  422  and current mirror  424 , connected to a frequency control device  426  in place of or in series with or an external frequency control resistor. The waveform source  422  may comprise any suitable circuit or device to generate a modulated current at the output  430 , with any suitable dithering waveform, from the triangle wave illustrated in  FIG. 4  to other simple or complex waveforms with constant or varying frequency including, but not limited to, any pseudo-random, random, noise, etc. types. The frequency control device  426  may comprise a portion of a variable pulse generator  134  in a dimming driver with dither circuit  100 , for example, used to apply a voltage and to set the frequency of output pulses based on the resulting current. 
     In other applications, the variable resistance circuit may be used in or incorporate or be incorporated into, for example but not limited to, noise sources, waveform generators (i.e., triangle, sine, sawtooth, pulse, square, AM, FM, etc. and combinations of these waveforms), semiconductor-based noise sources, microcontrollers, microprocessors, field programmable gate arrays (FPGAs), complex logic devices (CLDs), application specific integrated circuits (ASICs), analog and digital circuits and logic, shift registers, and may include pickups or sensors of RF and other EM, audible noise, mechanical and vibration noise, optical and photo input, etc. and any combinations of these. 
     The variable resistance circuit can be used as an “add on” feature to existing circuits, ICs, clocks, etc, and can have multiple embodiments of the present invention on the same circuit, sub circuit, subsystem, system, product, etc. 
     The variable resistance can be used for/with, for example, (but not limited to) power supplies, lighting including general lighting, light emitting devices (LEDs) and/or organic LEDs (OLEDs), fluorescent lighting, high intensity drivers, ballasts, power supplies, etc., communications, control electronics including lighting control, general electronics, etc. The variable resistance can be smart, intelligent, adaptable, programmable, etc. The variable resistance can used with discontinuous conduction mode (DCM), continuous conduction mode (CCM), critical conduction mode (CRM), resonant conduction mode, Cuk, SEPIC, etc. The variable resistance circuit can be used where voltage of the resistor (timing) element may be unknown or changing, etc. 
     Turning to  FIG. 13 , a pulse generator with dither circuit  500  is depicted including a waveform source  502  connected through a current mirror  504  to a pulse generator  506 , in this case a  555  timer. The pulse generator  506  is used to control a power control switch  510 . A load  514  and main power input may be connected in series with the power control switch  510 , with load  514  used to set the effective impedance of the frequency input (also referred to herein as an impedance input) to the pulse generator (e.g.,  134 ). Load  514  may be connected alone to the frequency input of the pulse generator (e.g.,  134 ), or in parallel or in series with an external resistor (e.g.,  150 ) connected to the frequency input of the pulse generator (e.g.,  134 ). Alternatively, power control switch  510  may correspond with output driver  30  in dimming driver  10 . The pulse generator and dither circuit  500  may be adapted to generate a current waveform  512  such as that illustrated in  FIG. 14  at the control input  514  of power control switch  510 . Notably, current waveform  512  has a varying frequency caused by the dithering circuit including the waveform source  502  and current mirror  504 . 
     Turning to  FIG. 15 , a NAND-based pulse generator  600  (which also may be made of other digital and related elements including, but not limited to, inverter-based, NOR-based, or other logic gate based pulse generator, etc.) is depicted with a dither circuit  602  including a waveform generator  604  and a current mirror  606 . A variable frequency square wave  608  as illustrated in  FIG. 17  is generated at output node  610 , which may be used to control a power control switch such as output driver  30  of dimming driver  10  or switch  120  of dimming driver with  100 . 
     Turning to  FIG. 16 , an inverter-based pulse generator  700  is depicted with a dither circuit  702  including a waveform generator  704  and a current mirror  706 . The variable frequency square wave  608  as illustrated in  FIG. 17  may be generated at output node  710 , which may be used to control a power control switch such as output driver  30  of dimming driver  10  or switch  120  of dimming driver with  100 . (Although subtle, the frequency of square wave current waveform  608  varies, and dither circuits  602  and  702  may be adapted to vary the frequency to any extent desired.) Furthermore, current waveform  608  may be any waveform including, but not limited to, for example a triangle wave, random wave, noise, sine, sawtooth, etc. 
     The present invention can be used in high power factor (PF) circuits with or without dimming including triac, forward and reverse dimmers, 0 to 10 V dimming, powerline dimming, wireless and other wired dimming, DALI dimming, PWM dimming, DMX, etc., as well as any other dimming and control protocol, interface, standard, circuit, arrangement, hardware, etc. 
     The example embodiments disclosed herein illustrate certain features of the present invention and not limiting in any way, form or function of present invention. Note that linear or switching voltage or current regulators or any combination can be used in the present invention and other elements/components can be used in place of the diodes, etc. The present invention can also include passive and active components and circuits that assist, support, facilitate, etc. the operation and function of the present invention. Such components can include passive components such as resistors, capacitors, inductors, filters, transformers, diodes, other magnetics, combinations of these, etc. and active components such as switches, transistors, integrated circuits, including ASICs, microcontrollers, microprocessors, FPGAs, CLDs, programmable logic, digital and or analog circuits, and combinations of these, etc. and as also discussed below. 
     The present invention can be used in power supplies, drivers, ballasts, etc. with or needing high power factor (PF) and/or lower THD circuits with or without dimming including triac, forward and reverse dimmers, 0 to 10 V dimming, powerline dimming, wireless and other wired dimming, DALI dimming, PWM dimming, DMX, etc., as well as any other dimming and control protocol, interface, standard, circuit, arrangement, hardware, etc. 
     The present invention is, likewise, not limited in materials choices including semiconductor materials such as, but not limited to, silicon (Si), silicon carbide (SiC), silicon on insulator (SOI), other silicon combination and alloys such as silicon germanium (SiGe), etc., diamond, graphene, gallium nitride (GaN) and GaN-based materials, gallium arsenide (GaAs) and GaAs-based materials, etc. The present invention can include any type of switching elements including, but not limited to, field effect transistors (FETs) such as metal oxide semiconductor field effect transistors (MOSFETs) including either p-channel or n-channel MOSFETs, junction field effect transistors (JFETs), metal emitter semiconductor field effect transistors, etc. again, either p-channel or n-channel or both, bipolar junction transistors (BJTs), heterojunction bipolar transistors (HBTs), high electron mobility transistors (HEMTs), unijunction transistors, modulation doped field effect transistors (MODFETs), etc., again, in general, n-channel or p-channel or both, vacuum tubes including diodes, triodes, tetrodes, pentodes, etc. and any other type of switch, etc. The present invention can, for example, be used with continuous conduction mode (CCM), critical conduction mode (CRM), discontinuous conduction mode (DCM), etc., of operation with any type of circuit topology including but not limited to buck, boost, buck-boost, boost-buck, cuk, etc., SEPIC, flyback, etc. In addition, the present invention does not require any additional special isolation or the use of an isolated power supply, etc. The present invention applies to all types of power supplies and sources and the respective power supply(ies) can be of a constant frequency, variable frequency, constant on time, constant off time, variable on time, variable off time, etc. Other forms of sources of power including thermal, optical, solar, radiated, mechanical energy, vibrational energy, thermionic, etc. are also included under the present invention. The present invention may be implemented in various and numerous forms and types including those involving integrated circuits (ICs) and discrete components and/or both. The present invention may be incorporated, in part or whole, into an IC, etc. The present invention itself may also be non-isolated or isolated, for example using a tag-along inductor or transformer winding or other isolating techniques, including, but not limited to, transformers including signal, gate, isolation, etc. transformers, optoisolators, optocouplers, etc. 
     The present invention can be used with a buck, a buck-boost, a boost-buck and/or a boost, flyback, or forward-converter design etc., topology, implementation, etc. 
     Other embodiments can use comparators, other op amp configurations and circuits, including but not limited to error amplifiers, summing amplifiers, log amplifiers, integrating amplifiers, averaging amplifiers, differentiators and differentiating amplifiers, etc. and/or other digital and analog circuits, microcontrollers, microprocessors, complex logic devices, field programmable gate arrays, etc. 
     The present invention includes other implementations that contain various other control circuits including, but not limited to, linear, square, square-root, power-law, sine, cosine, other trigonometric functions, logarithmic, exponential, cubic, cube root, hyperbolic, etc. in addition to error, difference, summing, integrating, differentiators, etc. type of op amps. In addition, logic, including digital and Boolean logic such as AND, NOT (inverter), OR, Exclusive OR gates, etc., complex logic devices (CLDs), field programmable gate arrays (FPGAs), microcontrollers, microprocessors, application specific integrated circuits (ASICs), etc. can also be used either alone or in combinations including analog and digital combinations for the present invention. The present invention can be incorporated into an integrated circuit, be an integrated circuit, etc. 
     While detailed descriptions of one or more embodiments of the invention have been given above, various alternatives, modifications, and equivalents will be apparent to those skilled in the art without varying from the spirit of the invention. Therefore, the above description should not be taken as limiting the scope of the invention, which is defined by the appended claims.