Abstract:
The invention comprises a dimming switch for use with a string of light emitting diodes (LEDs). The dimming switch comprises a bipolar junction transistor (BJT) driven in a cascode scheme. The dimming switch also comprises circuitry to offset the current that drives the base of the BJT to provide a controlled amount of current to the LEDs when the dimming input signal is high.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority from U.S. Provisional Application No. 61/907,819, filed on Nov. 22, 2013, and titled “Cascode-Type Dimming Switch Using BJT,” which is incorporated herein by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The invention comprises a dimming switch for use with a string of light emitting diodes (LEDs). The dimming switch comprises a bipolar junction transistor (BJT). The dimming switch also comprises control circuitry to offset the current that drives the base of the BJT to provide a controlled amount of current to the LEDs when the dimming input signal is high. The invention is useful for numerous applications, including LCD panel backlighting. 
     BACKGROUND OF THE INVENTION 
     The prior art includes some solutions for dimming switches for LEDs. These solutions typically utilize field effect transistors (FETs) to control the current driven through the LEDs. BJTs have not been used in this context because of two drawbacks. First, BJTs have a slow switching speed compared to FETs. Second, because the current through the base of a BJT is added to the current through the emitter of the BJT, the current through the LEDs (which would be connected to the collector of the BJT) cannot be properly sensed and controlled. However, high-voltage BJTs are cheaper and easier to manufacture than FETs, and the overall cost of a dimming switch could be improved by using BJTs instead of FETs. 
     A dimming switch for LEDs that utilizes a FET and BJT in a cascode configuration can combine the benefits of the fast switching speed of an FET and the low cost of a BJT. However, the source current of the low-voltage FET in a cascode configuration will not be an accurate representation of the current through the LEDs connected to the BJT collector, as the source current of the FET will be offset by base current of the high-voltage BJT. Therefore, a current sense resistor placed in series with the FET source will produce an erroneous sense voltage. What is needed is control circuitry for driving a BJT-FET cascode while negating the effect of the BJT base current in the current sense resistor. 
     SUMMARY OF THE INVENTION 
     The invention comprises a dimming switch for use with a string of LEDs. The dimming switch comprises a BJT and FET in a cascode configuration. The dimming switch also comprises circuitry to offset the current that drives the base of the BJT to provide a controlled amount of current to the LEDs when the dimming input signal is high. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts an embodiment of a dimming switch for LEDs. 
         FIG. 2  depicts certain current characteristics of different nodes within the embodiment of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 1  depicts lighting system  10  comprising string of LEDs  100  and dimming switch  20 . Dimming switch  20  in this example is a pulse width modulation (PWM) dimming switch. 
     Dimming switch  20  comprises FET  104  acting as an emitter driver switch controlled by an input dimming signal DIM (which is a PWM signal) and a high-voltage BJT  110 . Dimming switch  20  is connected in series with string of LEDs  100 . The current magnitude through string of LEDs  100  is controlled by a current feedback amplifier  105  receiving a current sense signal from a resistor  106  and regulating the current sense signal to a reference voltage REF. 
     Base current i b  is supplied to BJT  110  by a current mirror circuit  101  in proportion with a reference current k 1 *i b  programmed by a current reference network such as resistor-capacitor (RC) network  107 . Control switch  103  enables the reference current k 1 *i b  when the dimming signal DIM is received. In this example, control switch  103  comprises inverter  116 , transistor  117 , and transistor  118  in the configuration shown in  FIG. 1 . One of ordinary skill in the art will appreciate that other types of logic gates can be used instead of inverter  116 . 
     A second proportional current k 2 *ib generated by current mirror circuit  101  offsets the reference voltage REF to cancel the effect of the base current i b  in the current i LED  through string of LEDs  100 . The offset of the reference voltage REF is generated at a resistor  108  by passing the current k 2 *ib through resistor  108 . 
     A diode  102  facilitates fast turn off of the BJT  110 . 
     Dimming switch  20  shown in  FIG. 1  can be characterized as a means for driving current through a string of LEDs  100  in response to a dimming input signal. 
     Operation of lighting system  10  will now be described with reference to  FIGS. 1 and 2 . In  FIG. 2 , an exemplary pulse of the input signal DIM is depicted. In lighting system  10 , dimming is achieved through the pulse width modulation input signal DIM. Lower light will be emitted from string of LEDs  100  if the width of the pulses of input signal DIM is reduced, or if the frequency of the pulses within input signal DIM is reduced. 
     When input signal DIM is low, the output of inverter  116  is high. As a result, transistor  117  is turned off, and transistor  118  is turned on. Any charge stored on capacitor  112  is removed through resistor  111  and through transistor  118  to ground. FET  104  is turned off, and no current flows through the emitter of BJT  110 . Any residual current in LED string  100  flows through the base of BJT  110  and through diode  102 . 
     When input signal DIM goes high, such as at time t 1 , the output of inverter  116  goes low. As a result, transistor  117  is turned on, and transistor  118  is turned off. FET  104  is turned on, and current i e  is drawn through the emitter of BJT  110 . Current k 1 *i b  is drawn through transistors  113  and  117  and charges capacitor  112 . As shown in  FIG. 2 , the current i b  initially spikes as the capacitor begins to charge and eventually plateaus once the capacitor is charged and the current k 1 *i b  flows through resistor  111 . Current mirror  101  generates current k 2 *i b  through resistor  108  and current i b  into the base of BJT  110 . Here, k 1  and k 2  are based on the relative matching characteristics of transistors  113 ,  114 , and  115 . Arbitrary values for k 1  and k 2  can be selected. Ideally, k 1  and k 2 &gt;&gt;1 to achieve low current consumption. In this embodiment, cancellation of the base current i b  assumes that R 106 =k 2 *R 108 . 
     In BJT  110 , the current i e  through the emitter will equal the sum of i LED  through string of LEDs  100  (which is the current through the collector of BJT  110 ) and i b  (which is the current through the base of BJT  110 ). As shown in  FIG. 2 , the magnitude of i LED  ideally will be unaffected by the base current i b  and will have a fast response to the input signal DIM. Current feedback amplifier  105  adds the offset of k 2 *i b *R 108  into the reference REF path, and the turn-on process is accelerated by capacitor  112  and the turn-off process is accelerated by diode  102  and the cascode connection of FET  104  and BJT  110 . 
     Current feedback amplifier  105  will seek to maintain a voltage of REF+k 2 *i b *R 108  at the inverting node, which will develop at resistor  106 . The current through resistor  106  therefore will be i e =(REF+k 2 *i b *R 108 )/R 106 =(REF/R 106 )+i b . Hence the LED current iLED=i e −i b =REF/R 106 , is unaffected by the base current i b . The current through the emitter, i e , is also shown in  FIG. 2 . As can be seen, dimming switch  20  corrects for the current i b  that flows into the base of BJT  110 . 
     Thus, the embodiment comprises a dimming switch using a cascode connection of a BJT  110  and a source driver MOSFET  104  where the current through the string of LEDs  100  is not affected by the current driving the base of the BJT.