Minimizing bond wire power losses in integrated circuit full bridge CCFL drivers

[0001] This Application claims the benefit of U.S. Provisional Patent Application No. 60/603,409 filed Aug. 20, 2004, and U.S. Provisional Patent Application No. 60/603,958 filed Aug. 23, 2004, each of which is incorporated by reference in its entirety.

[0002] Circuits are used to drive a load by supplying a potential across the load. Many loads are driven with alternating current in order to modulate the power delivered to the load. Power inverters are often used to generate such alternating current. One type of power inverter is the full bridge circuit. Some full bridge circuits use fast-switching transistors in order to produce alternating current of high frequency.

[0003] One type of load that can be driven by a full bridge power inverter is a fluorescent lamp. Compared to incandescent lamps, fluorescent lamps are more efficient and emit less heat. Thus, fluorescent lamps may be more useful in situations in which batteries are being used to power the lamp. Fluorescent lamps that can be driven by such a power inverter include by way of example but not limitation the cold cathode fluorescent lamp (CCFL), the external electrode fluorescent lamp (EEFL), the flat fluorescent lamp (FFL), and other fluorescent lamps. The power inverter may also be used to drive banks of lamps.

[0004] CCFLs are commonly used in notebook computers as a backlight for a liquid crystal display (LCD). Portable notebook computer systems, for example, place increasing demands on higher efficiency, smaller size, lower costs, and increased battery life. A critical system that affects this is the power required by the display system. CCFLs are often used in such a display system because CCFLs are efficient and have a low heat emission, rugged electronics, and a long service life. Furthermore, CCFLs, and fluorescent lamps in general, emit light over a broad area and may contribute to even brightness across a notebook computer display screen. Driving a fluorescent lamp differentially (e.g., at both ends) can further improve evenness in brightness.

[0005] Today most of the CCFLs used in notebook computers are driven by a full bridge power supply that drives a magnetic step up transformer to apply the high voltage required by the CCFL. In this manner a notebook supply with a typical voltage of 7 to 22 V can tightly regulate a 600 VRMS voltage to the CCFL in an efficient manner. Full bridge power supplies in this application are typically made up of switches connected to one another and to other components of the circuit by bond wires. Parasitic losses occur in the bond wires and switches due to their resistance. Battery life in notebook computers can be prolonged by a reduction of these parasitic losses.

[0006] MP1010, MP1011, and MP1015 are manufactured by Monolithic Power Systems. These may be the only commercially available CCFL drivers that integrate the power transistors and control circuitry as of the filing date of this application.

[0007] The following embodiments and aspects thereof are described and illustrated in conjunction with systems, tools, and methods that are meant to be exemplary and illustrative, not limiting in scope. In various embodiments, one or more of the above-described problems have been reduced or eliminated, while other embodiments are directed to other improvements.

[0008] A technique for reducing parasitic losses in a circuit involves arranging a switching network serially. An example of a system according to the technique involves a serially arranged network of four switches. By way of example but not limitation, the system may include five pins: Two pins are coupled to a power supply, two pins are coupled to a load, and one pin is grounded. The system may operate in one of two active phases, or a rest phase. In one active phase, a controller may direct current to flow from a first power supply pin to the ground pin, driving a load with a first potential thereby. In the other active phase, the controller may direct current to flow from a second power supply pin to the ground pin, driving a load with a second potential thereby. The first and second potentials may be of opposite polarity. The controller may alternate between active phases with a rest phase during which resonant current passes through the ground pin.

[0009] A folded full bridge apparatus constructed according to the technique may include a power source, multiple switches and a controller. The switches may be arranged in series in order to minimize parasitic losses in connections between the switches. The middle two switches may be connected between ground and a load. The switches at each end may be connected between a power source and the load. A controller may be coupled to the switches to control the opening and closing of the switches to drive the load with a first potential and a second potential thereby.

[0010] A method according to the technique may produce a voltage waveform that has three phases. The first phase may include driving a load with a first current through the ground pin. The third phase may include driving a load with a second current through the ground pin.

[0011] The proposed circuits can offer, among other advantages, minimizing parasitic bond wire losses in drivers, such as by way of example but not limitation CCFL full bridge drivers, or increasing battery lifetime. These and other advantages of the present invention will become apparent to those skilled in the art upon a reading of the following descriptions and a study of the several figures of the drawings.

[0026] In the following description, several specific details are presented to provide a thorough understanding of embodiments of the invention. One skilled in the relevant art will recognize, however, that the invention can be practiced without one or more of the specific details, or in combination with other methods, components, materials etc. In other instances, well-known structures, materials, implementations or operations are not shown or described in detail to avoid obscuring aspects of various embodiments of the invention.

[0027] FIG. 1 depicts an example of a system 100 for differentially driving a load 106. The system 100 includes an application circuit 102, a resonant tank module 104, and a load 106. The application circuit 102 is coupled to the resonant tank module 104. The resonant tank module 104 is coupled at both ends of a load 106. In operation, the application circuit 102 produces a square waveform voltage and the resonant tank 104 converts the square wave voltage into two out of phase analog signals used to differentially drive the load 106.

[0028] FIGS. 2A and 2B depict an example of an application circuit system 200. The system 200 includes a resonant tank module 204, a CCFL 206, a fixed frequency inverter module 208, and other exemplary components. In the example of FIG. 2B, the resonant tank module 204 is coupled to the CCFL 206 at two ends of the CCFL 206. The fixed frequency inverter 208 is represented in FIG. 2B as well as in FIG. 2A in order to enhance clarity. The fixed frequency inverter 208 is coupled to the resonant tank module 204.

[0029] In the example of FIG. 2B, the resonant tank module 204 includes a transformer 222 and capacitor 224. The capacitor 224 is coupled to the primary windings of the transformer 222. FIG. 2B depicts a non-limiting example of the resonant tank module 204, which may include different components or configurations in alternative embodiments.

[0030] In the example of FIG. 2B, the CCFL 206 includes a lamp. In alternative embodiments, the lamp may be a cold cathode fluorescent lamp (CCFL), an external electrode fluorescent lamp (EEFL), a flat fluorescent lamp (FFL), or some other lamp. In alternative embodiments, the load could include a bank of CCFLs, a bank of EEFLs, a bank of FFLs, a bank of some combination of lamps, or any other load. Thus, the CCFL 206 is a specific, non-limiting example of a load.

[0031] In the example of FIGS. 2A and 2B, the fixed frequency inverter module 208 includes pins 210, 212, 214, 216, and 218. Pin 210 is coupled to the transformer 222 in the resonant tank 204. Pin 212 and pin 214 are coupled to a power source. Pin 216 is coupled to the capacitor 224 in the resonant tank 204. Pin 218 is coupled to ground. The fixed frequency inverter module 208 is a non-limiting example of an inverter module. FIGS. 3A and 3B depict pin arrangements in two alternative fixed frequency inverters 308-A and 308-B. Fixed frequency inverters 308A and 308B are alternative examples that are also non-limiting. Other fixed frequency inverters could be used in alternative embodiments for the same effect. Indeed, the system 200 need not even include a fixed-frequency inverter 208 if some other logic can be used to replicate the desired functionality of such a device.

[0032] It may be noted that in the example of FIGS. 2A and 2B, the fixed frequency inverter module 208 includes pins for receiving lamp current feedback (LI), lamp current voltage feedback (LV), and full bridge output bootstrap pins (BSTR and BSTL) for providing gate bias for the driver of internal power transistors. The capacitors respectively between BSTL and pin 210 and between BSTR and pin 216 are charged through an internal switch when the pin 210 or the pin 216 are low. LI, LV, BSTR, and BSTL are optional.

[0033] FIGS. 2A and 2B are intended to illustrate a specific, non-limiting example of the system 100 (FIG. 1). Other examples could include different components or configurations.

[0034] In operation, in a first phase a voltage enters the fixed frequency inverter module 208 at pin 214. The voltage is converted to, by way of example but not limitation, a square wave signal which is output at pin 216. The square wave signal is received at the capacitor 224, which facilitates conversion of the square wave signal into an analog signal. The analog signal is received at the primary windings of the transformer 222, passed to the secondary windings of transformer 222, and is used to drive the CCFL 206 at a first end of the CCFL 206. The signal from the primary windings of the transformer 222 is received at the pin 210, and passed to pin 218, which is grounded.

[0035] In operation, in a second phase a voltage enters the fixed frequency inverter module 208 at pin 212, and the current flow path for a second phase is from pin 212 to pin 210 to the transformer 222 to the capacitor 224 to pin 216 to and to pin 218, which is grounded. In operation, a rest phase current flow path is from pin 210, to the transformer 222, to the capacitor 224, to pin 216, and vice versa. Advantageously, the rest phase current does not flow through pin 218, thus eliminating power loss on a wire bond within Pin 218.

[0036] FIGS. 4A and 4B depict an example of an application circuit system 400. The system 400 includes a fixed-frequency inverter-driven CCFL module 402 and other circuit components. In the example of FIGS. 4A and 4B, the module 402 includes a resonant tank 404, a CCFL 406, a switch network 410, a control logic 450, and a parasitic capacitance compensation module 480. The resonant tank module 404 is coupled to the CCFL 406 and to the parasitic capacitance compensation module 480. The switching network 410 is coupled to the resonant tank module 404. The resonant tank module 404 is similar to the resonant tank module 204 (FIG. 2B) described previously. The CCFL 406 is similar to the CCFL 206 (FIG. 2B) described previously.

[0037] In the example of FIG. 4B, the switching network 410 includes switches 430A, 430B, 430C, and 430D arranged in series (hereinafter collectively referred to as the switches 430). Switch 430A is coupled to a voltage input from a power source (not shown) and to the resonant tank 404. Switch 430B is coupled to the resonant tank 404 and to ground. Switch 430C is coupled to a voltage input from a power source (not shown) and to the resonant tank 404. Switch 430D is coupled to the resonant tank and to ground. FIG. 4B depicts a non-limiting example of the switching network 410, which may include other components or configurations in alternative embodiments.

[0038] In the example of FIG. 4B, the control logic 450 is coupled to the switches 430 in the switching network 410. The control logic 450 may include any logic effective to open and close switches in the switching network 410 in a manner that is described later with reference to FIGS. 5 to 9.

[0039] In the example of FIG. 4B, the parasitic capacitance compensation module 480 includes a capacitor and a resistance. The parasitic capacitance compensation module 480 is an optional component, examples of which are described in a co-pending patent application Ser. No. 11/198,029 entitled METHOD AND APPARATUS FOR DRIVING DISCHARGE LAMPS IN A FLOATING CONFIGURATION, which was filed Aug. 5, 2005, and which is incorporated herein by reference.

[0040] In operation, the control logic 450 directs the switches 430 in the switching network 410 to be opened and closed in particular configurations to produce a square waveform voltage signal. The square waveform signal travels from the switching network 410 to the resonant tank 404. The resonant tank 404 produces an analog current from the square waveform signal, which drives the CCFL 406.

[0041] FIG. 5 depicts an example of an application circuit system 500 with serially arranged transistors. The example of FIG. 5 includes a load 506, four transistors 530A, 530B, 530C, and 530D (hereinafter referred to collectively as the transistors 530), five pads 532-1, 532-2, 532-3, 532-4, and 532-5 (hereinafter referred to collectively as the pads 532), five bond wires 534-1, 534-2, 534-3, 534-4, and 534-5 (hereinafter referred to collectively as the bond wires 534), five pins 536-1, 536-2, 536-3, 536-4, and 536-5 (hereinafter referred to collectively as the pins 536), a battery 540, and a control logic 550. The load 506 is coupled to pin 536-2 and pin 536-4. The transistor 530A is coupled to pads 532-4 and 532-5. The transistor 530B is coupled to pads 532-3 and 532-4. The transistor 530C is coupled to pads 532-1 and 532-2. The transistor 530D is coupled to pads 532-2 and 532-3. Pad 532-1 is coupled to bond wire 534-1, which is coupled to pin 536-1. Pad 532-2 is coupled to bond wire 534-2, which is coupled to pin 536-2. Pad 532-3 is coupled to bond wire 534-3, which is coupled to pin 536-3. Pad 532-4 is coupled to bond wire 534-4, which is coupled to pin 536-4. Pad 532-5 is coupled to bond wire 534-5, which is coupled to pin 536-5. The pins 536-1 and 536-5 are coupled to the battery 540. The control logic 550 is coupled to the transistors 530. The configuration shown is a non-limiting example. Other examples could include different components or configurations.

[0042] In operation, the transistors 530 act as switches. The control logic 550 is effective to open and close each of the transistors 530 in a manner discussed in more detail later with reference to FIGS. 7A to 7C.

[0043] FIG. 6 depicts an example of an application circuit system 600 with serially arranged switches. In the example of FIG. 6, the system 600 includes a load 606, switches 630A, 630B, 630C, and 630D (hereinafter collectively referred to as the switches 630), a power source 640, and a controller 650. The switches 630 may include any applicable switching mechanism that allows opening and closing of the switch. The load 606 is coupled to each of the switches 630. The controller 650 is coupled to each of the switches 630. The power source 640 is operationally connected to the switches 630A, 630C and the switches 630B, 630D are operationally connected to ground. In addition, the switch 630A is serially coupled to the switch 630B, the switch 630B is serially coupled between the switch 630A and the switch 630D, and the switch 630D is serially coupled between the switch 630B and the switch 630C. The configuration shown is a non-limiting example. Other examples could include different components or configurations.

[0044] In operation, the controller 650 is effective to open and close the switches 630 in order to produce a square wave voltage signal from the power source 640. The square wave voltage signal may be converted into, by way of example but not limitation, an analog signal for driving the load 606. Three exemplary configurations of the switches 630 with respect to being open and closed according to an aspect of an embodiment are discussed in more detail later with reference to FIGS. 7A, 7B, and 7C.

[0045] FIGS. 7A, 7B, and 7C depict switch configurations in first, second, and third respective phases. The example configurations of switches in FIGS. 7A, 7B, and 7C include a load 706, switches 730A, 730B, 730C, and 730D (hereinafter collectively referred to as the switches 730), bond wires 734-1, 734-2, 734-3, 734-4, and 734-5 (hereinafter collectively referred to as the bond wires 734), and a power source 740. The switches 730 are serially arranged and may be similar to the switches 630 (FIG. 6) described previously. The bond wire 734-1 is coupled between the power source 740 and the switch 730C. The bond wire 734-2 is coupled between the load 706 and the switches 730C, 730D. The bond wire 734-3 is coupled between ground and the switches 730D, 730B. Bond wire 734-4 is coupled between the load 706 and the switches 730B, 730A. Bond wire 734-5 is coupled between the power source 740 and the switch 730A. The configuration shown is a non-limiting example. Other examples could include different components or configurations.

[0046] In operation, a square voltage waveform is produced by cycling the switches between three configurations in the following order: (A-D), (B-D), (B-C), and (B-D). FIG. 8 depicts an example of a voltage waveform 800 produced when switch configurations change as shown in FIGS. 7A, 7B, and 7C. FIG. 7A depicts the (A-D) configuration that corresponds to the (A-D) portion of the voltage waveform 800. FIG. 7B depicts the (B-D) configuration that corresponds to the (B-D) portion of the voltage waveform 800. FIG. 7C depicts the (B-C) configuration that corresponds to the (B-C) portion of the voltage waveform 800.

[0047] In the example of FIG. 7A, switch 730A is closed, 730B is open, 730C is open, and 730D is closed. In this configuration, a signal will travel from the power source 740 through bond wire 734-5, switch 730A, bond wire 734-4, load 706 (driving the load 706 thereby), bond wire 734-2, switch 730D, and bond wire 734-3 to ground.

[0048] In the example of FIG. 7B, switch 730A is open, switch 730B is closed, switch 730C is open, and switch 730D is closed. This configuration is a “rest” configuration that is depicted as (B-D) in FIG. 8. At rest, there may be resonant current that flows through the bond wires 734-2, 734-3, and 734-4. Notably, the resonant current only flows through the bond wire 734-3 to reach ground.

[0049] In the example of FIG. 7C, switch 730A is open, switch 730B is closed, switch 730C is closed, and switch 730D is open. In this configuration, a signal will travel from the power source 740 through bond wire 734-1, switch 730C, bond wire 734-2, load 706 (driving the load 706 thereby), bond wire 734-4, switch 730B, and bond wire 734-3 to ground.

[0050] In an alternative embodiment, other waveforms may be created using a variable number of phases, as depicted in FIG. 9. A square voltage waveform is produced as shown in FIG. 9 by cycling the switches between four configurations in the following order: (B-D), (A-D), (A-C), and (B-C). The (B-C) configuration is obtained by having switch 630A open, switch 630B closed, 630C closed, and 630D open. Other waveforms and switch configurations could be used in alternative aspects or embodiments.

[0051] FIG. 10A to 10E depict an example of a system 1000 and various alternative load modules. In the example of FIG. 10A, the system 1000 includes a load module 1006, switches 1030A, 1030B, 1030C, and 1030D (hereinafter collectively referred to as the switches 1030), wire bonds 1034-1, 1034-2, 1034-3, 1034-4, and 1034-5 (hereinafter collectively referred to as the wire bonds 1034), a power source 1040, and a controller 1050. The wire bonds 1034 couple power devices to external circuit elements, including the load module 1006, the power source 1040, and ground.

[0052] In operation, the control schemes or drive waveforms associated with the system 1000 can be similar to the waveforms shown in FIGS. 8 and 9, or can be some other waveform. In the resting phase (B-D), the switches 1030D and 1030B conduct the resting phase current. The current does not need to flow through ground and wire bond 1034-3. Therefore, loss in the wire bond 1034-3 is eliminated. The system 1000 can drive a load module 1006 for different applications. FIGS. 10B to 10E depict examples of load module 1006 configurations.

[0053] In the example of FIG. 10B, the load module 1006 includes a thermal electric cooler (TEC) 1042. Inductors 1044 and capacitors 1046 form the low pass filter to filter out both differential and common mode noises on the terminals of TEC 1042 while the optional capacitor 1048 further attenuates the differential mode noise.

[0054] In the example of FIG. 10C, the load module 1006 includes a class D audio amplifier 1052. The components depicted in the example of FIG. 10C are similar to those of FIG. 10B. In addition, for class D audio amplifiers used in the portable devices such as cell phones, the capacitors 1046 and 1048 may or may not be used and the inductors 1044 can be replaced by bead inductors (not shown) or removed completely.

[0055] In the example of FIG. 10D, the load module 1006 includes a motor 1054.

[0056] In the example of FIG. 10E, the load module 1006 includes a transformer 1062 with center-tapped rectifier configuration. Other isolated dc/dc topologies include, by way of example but not limitation, full bridge secondary rectifier, current doubler rectifier, multiple transformer rectifier, and other topologies. The transformer 1062 is used to step up or down a square wave voltage applied on the primary winding. Diodes 1064 rectify the secondary winding voltages. Inductor 1066 and capacitor 1068 filter out the ac ripple and provide an essential dc voltage to power the load 1070. The load module 1006 may be effective in isolated dc/dc applications.

[0057] FIGS. 11A to 11D depict an example of a circuit 1100 with a dual output configuration. In the example of FIG. 11A, the circuit 1100 includes load modules 1106-1 and 1106-2, switches 1130A, 1130B, 1130C, and 1130D (hereinafter collectively referred to as the switches 1130), wire bonds 1134-1, 1134-2, 1134-3, 1134-4, and 1134-5 (hereinafter collectively referred to as the wire bonds 1134), a power source 1140, and a controller 1150. The wire bonds 1134 couple the switches 1130 to other circuit elements including the load modules 1106-1 and 1106-2, the power source 1140, and ground. The controller 1150 may include a first output control logic 1172-1 and a second output control logic 1172-2 for respectively controlling the load modules 1106-1 and 1106-2. In alternative embodiments, more a configuration with more than two outputs and load modules could be used.

[0058] FIG. 11B depicts an example of a circuit that includes dual step down synchronous buck converters. Inductors 1144 and capacitors 1146 filter a square wave into the essential dc voltage V to power respective loads 1142.

[0059] FIGS. 11C and 11D show two exemplary operation waveforms: in phase operation in the example of FIG. 11C and out of phase operation in the example of FIG. 11D. The power loss reduction on the wire bond 1134-3 can be realized in several ways. When one of the loads sinks current and the other sources current, the bond wire (or circuit trace) 1134-3 only conducts the difference between these two currents. Thus the conduction loss is substantially reduced. A typical example for such applications includes the termination supply for the double date rate (DDR) memory, which must sink or source the current. Even if both load devices draw dc current in the same direction, the wire bond 1134-3 only conducts one inductor current in the shaded area shown in FIG. 11C and FIG. 11D and thus the proper phasing technique shown in FIG. 11D can also reduce the dc conductor loss.

[0060] FIG. 12 depicts an example of a circuit 1200 having connected dual outputs. The example of FIG. 12 is similar to that of FIG. 11A, where both outputs are connected together to power a single load. In the example of FIG. 12, the circuit 1200 includes load 1206, switches 1230A, 1230B, 1230C, and 1230D (hereinafter collectively referred to as the switches 1230), wire bonds 1234-1, 1234-2, 1234-3, 1234-4, and 1234-5 (hereinafter collectively referred to as the wire bonds 1234), a power source 1240, inductors 1244, a capacitor 1246, and a controller 1250. This configuration may be used to deliver high current output for, by way of example but not limitation, CPUs or memory banks in a computer. In an embodiment that adopts, for example, the out of phase operation shown in FIG. 11D, the anti-phase technique can substantially reduce power losses in a wire bond or circuit trace 1234-3.

[0061] FIG. 13 depicts an example of a circuit 1300 for dual output dc/dc voltage conversion applications. In the example of FIG. 13, the circuit 1300 includes loads 1306-1 and 1306-2, switches 1330A, 1330B, 1330C, and 1330D (hereinafter collectively referred to as the switches 1330), wire bonds 1334-1, 1334-2, 1334-3, 1334-4, and 1334-5 (hereinafter collectively referred to as the wire bonds 1334), power sources 1340-1 and 1340-2, inductors 1344-1 and 1340-2, capacitors 1346-1 and 1346-2, and a controller 1350.

[0062] In the example of FIG. 13, in an embodiment, the switches 1330A and 1330B, inductor 1342-1 and capacitor 1346-1 form a synchronous boost converter that receives input from power source 1340-1 and produces output for the load 1306-1. Power switches 1330C and 1330D, inductor 1342-2, and capacitor 1346-2 form the synchronous buck converter that steps down the voltage receiving from power source 1340-2 and provides output to load 1306-2. When switches 1330D and 1330B conduct at the same time, the difference between the two inductor currents flows through 1334-3, thus reducing the power loss on 1334-3. With proper phase technique in the driving waveforms, the common conduction time for 1330D and 1330B are maximized to minimize the power loss on 1334-3.

[0063] It will be appreciated to those skilled in the art that the preceding examples and embodiments are exemplary and not limiting to the scope of the present invention. It is intended that all permutations, enhancements, equivalents, and improvements thereto that are apparent to those skilled in the art upon a reading of the specification and a study of the drawings are included within the true spirit and scope of the present invention. It is therefore intended that the following appended claims include all such modifications, permutations and equivalents as fall within the true spirit and scope of the present invention.