IE84852B1 - Frequency feedforward for constant light output in backlight inverters - Google Patents

Abstract

ABSTRACT In a method and system for an improved inverter providing power to a load, the inverter receives a direct current (DC) input. A plurality of switches included in the inverter are controlled by a plurality of control signals to generate an alternating current (AC) output in response to the DC input. A zero crossing of the AC output current provided by the plurality of switches is detected, and the controlling of the plurality of switches is delayed in response to the zero crossing. The amount of delay is adjusted responsive to a change in the DC input to effectively maintain a constant switching frequency of the inverter. The delayed AC output current of the plurality of switches is filtered to provide power to the load.

Description

Frequency Feedforward for Constant Light Output in Backlight Inverters Dell Products LP FREQUENCY FEEDFORWARD FOR CONSTANT LIGHT OUTPUT IN BACKLIGHT INVERTERS Background ‘ The present disclosure relates generally to the field of power supplies for information handling systems. and more particularly to techniques for efficiently providing power to drive a discharge lamp, such as a cold cathode fluorescent lamp (CCFL).

As the value and use of information continues to increase, individuals and businesses seek additional ways to acquire. process and store information. One option available to users is information handling systems. An information handling _system (‘lHS') generally processes, compiles, stores, and/or communicates information or data for business, personal. or other purposes thereby allowing users to take advantage of the value of the information. Because technology and information handling needs and requirements vary between different users or applications. information handling systems may also vary regarding what information is handled, how the information is handled, how much information is processed, stored, or communicated, and how quickly and efficiently the information may be processed, stored, or communicated. The variations in information handling systems allow for information handling systems to be general or configured for a specific user or specific use such as financial transaction processing, airline reservations. enterprise data storage, or global communications. In addition, . information handling systems may include a variety of hardware and software components that may be configured to process, store, and communicate information and may include one or more computer systems, data storage systems, and networking systems.

Liquid crystal display (LCD) panel based display devices have been commonly utilized in many IHS systems due to their compact size, and low power consumption. Although there are different types of backlights (e.g., light sources including a discharge lamp), which are currently used for backlighting the latest LCD panels, the CCF L (also known as cold cathode fluorescent tube (CCFT)) is most commonly used. Circuits for supplying power to CCFL's generally require a controllable alternating current (AC) power supply and a feedback loop to accurately monitor the current in the lamp in order to maintain operating stability of the circuit and to have an ability to vary the lamp brightness. Such circuits typically generate a high voltage to initially turn on the CCFL and then lower the voltage when current begins to flow through the lamp. For example, Monolithic Power Systems, lnc., . ‘ located at 933 University Ave. Building D. Los Gatds. CA 95032, usA, provides a MP1015 power circuit chip for driving the CCFL.

Such circuits also typically include an inverter circuit to convert a direct current (DC) voltage received as an input to a regulated AC voltage generated as an output. Inverter circuits typically include a controller component. such as a pulse width modulator (PWM) based controller. Various well-known inverter circuit ' configurations or "topologies" include a Royer converter, full-bridge or half—bridge inverters. ‘ The CCFL power consumption may account for a significant portion (e.g., up to 50% in some cases) of the IHS system power requirement, especially for portable systems. Therefore, there is a considerable amount of interest to achieve advantages in extending battery life and reducing re-charge frequency by improving the efficiency of power supplies configured to provide power to the CCFL.

In traditional inverter based power circuits, changes in input voltage causes a decrease in power conversion efficiency. For example, when an adapter is unplugged/plugged into a portable IHS system such as a notebook computer, the voltage to the LCD backlight inverter varies from AC adapter voltage (approximately between 18V and 22V) to the battery voltage (approximately between 9V and 17V).

The varying voltage causes a noticeable change in the LCD brightness level, which is often perceived as a flicker.

Brightness output from the CCFL is a function of operating frequency. FIG. 1A illustrates a graphical relationship between LCD brightness (shown on Y-axis) versus frequency (shown on X-axis) measurements for a commercially available CCFL. Curves 110, 120 and 130 are shown for lamp currents of 6 mA, 5 mA and 4 mA.

Power conversion efticiency of an inverter is a function of the input voltage.

FIG. 1B illustrates a graphical relationship between a change in efficiency (shown on Y-axis) versus input voltage (shown on X-axis) measurements for a commercially available inverter.

FlG. 10 illustrates a graphical relationship between a change in frequency (shown on Y-axis) versus input voltage (shown on X-axis) measurements for a commercially available inverter. Thus. frequency of the backlight inverter changes as a function of input voltage causing the brightness change and the change in frequency causes a decrease in the inverter efficiency.

Therefore. a need exists for improved efficiency of the power circuits providing power to the CCFL. More specifically, a need exists to develop tools and techniques for improving the efficiency of inverters under changing voltage and frequency conditions. Accordingly, it would be desirable to provide tools and techniques for an improved inverter of an IHS absent the disadvantages found in the prior methods discussed above.

Summary One embodiment accordingly provides a pulse start component operable to receive a first input indicative of a primary current. The pulse start component generates a first output responsive to a zero crossing of the tirst input. A pulse duration component is operable to receive a second input indicative of a load current and a third input indicative of a direct current (DC) input voltage. The pulse duration component is operable to generate a second output responsive to the second and third inputs. A time delay component is operable to receive thefirst output and the DC input voltage. The time delay component introduces a time delay to generate a ‘delayed first output A logic component is operable to receive the delayed first output and the second output to generate a plurality of control signals. A plurality of switches are operable to ‘convert the DC input to the primary current in response to receiving the plurality of control signals. A filtering component is provided to filter the primary current for generating the load current.

Several advantages are achieved by the method and system according to the illustrative embodiments presented herein. The embodiments advantageously provide for a system and method for an improved inverter, because the inverter includes a variable time delay mechanism to advantageously operate the inverter at a constant switching frequency. The efficiency of the inverter is improved by maintaining the constant switching frequency. In addition, the inverter has the ability to accommodate varying DC input voltages and changing pulse widths.

V, The present invention will be described, by way of example, with reference to the accompanying drawings, in which: FIG. 1A described hereinabove, illustrates a graphical relationship between LCD brightness versus frequency measurements for a commercially available CCFL. according to prior art; FIG. 18 described herelnabove, illustrates a graphical relationship between a change in efficiency versus input voltage measurements for a commercially available inverter, according to prior art; FIG. 1C described hereinabove. illustrates a graphical relationship between ’ a change in frequency versus input voltage measurements for a commercially available inverter, according to prior art; FIG. 2 illustrates an inverter for providing power to a lead, according to an embodiment; FIG. 3A illustrates waveforms associated with an inverter having no time delay FIG. 38 illustrates waveforms associated with an inverter having a time delay, according to an embodiment; FIG. 4A illustrates detail of a time delay component 230 of FIG. 2, according to one embodiment; FIG. 4B illustrates a graphical relationship between the time delay id 316 of FIG. 3B and the DC input voltage 201 of FIG. 2, according to one embodiment; FIG. 5 is a flow chart illustrating a method for improving efficiency of an inverter, according to an embodiment; and FIG. 6 illustrates a block diagram of an information handling system having an improved inverter, according to an embodiment.

Detailed Description Novel features believed characteristic of the present disclosure are set forth in the appended claims. The disclosure itself, however. as well as a preferred mode of use, various objectives and advantages thereof. will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings. The functionality of various circuits, devices or components described herein may be implemented as hardware (including discrete components. integrated circuits and systems-on-a-chip), firmware (including application specific integrated circuits and programmable chips) and/or software or a combination thereof, depending on the application requirements.

Changes in DC input voltage causes a change in frequency for the traditional inverter circuits. thereby causing a change in the brightness level of the CCFL. More importantly, the efficiency of traditional inverter circuits providing power to the CCFL decreases as DC input voltage increases. It would be desirable to improve the efficiency of inverters under changing voltage and frequency conditions.

The problem of varying backlight operating frequency due to changes in the DC in put voltage may be advantageously eliminated by adding a frequency feed-forward term. This technique provides a constant frequency over the DC input voltage range and improves efficiency over the range of DC input voltages.

According to one embodiment, in a method and system for an improved inverter providing power to a load, the inverter receives the DC input. A plurality of switches included in the inverter are controlled by a plurality of control signals to generate the AC output in response to the DC input. A zero crossing of an AC output current provided by the plurality of switches is detected, and the controlling of ' the plurality of switches is delayed in response to the zero crossing. The amount of delay is adjusted responsive to a change in the DC input to effectively maintain a constant switching frequency of the inverter. The delayed AC output current provided by the plurality of switches is filtered to provide powerto the load.

FIG. 2 illustrates an improved inverter 200 for providing power to a load 290, according to one embodiment. The inverter 200 includes the following components: a) a pulse start component 210, b) a pulse duration component 220, c) a time delay component 230, cl) a logic component 240. e) a plurality of switches 250, and f) a. filter component 260. ‘ in one embodiment, the filter component 260 includes a transformer device having a primary section)262 eiectro-magnetically coupled to a secondary section 264. The primary section 262 is electrically coupled to the plurality of the switches 250. A primary current 263 flows through the primary section 262 and a primary voltage (not shown) is measured across a pair of terminals 268 of the primary section 262. The secondary section 264 is electrically coupled to the load 290. A secondary current 265, which is also the load current, flows through the secondary section 264. In one embodiment, the load 290 is the CCFL.

The pulse start component 210 is operable to receive a first input 212 indicative of the primary current 263. The pulse start component 210 generates a first output 214 responsive to a zero crossing of the primary current 263. That is, the first output 214 is generated when a value of the first input 212 is equal to zero.

The pulse duration component 220 is operable to receive a second input 222 indicative of the secondary current 265, which is also the load current. and a third input 224 indicative of a direct current (DC) input voltage 201. The DC input voltage 201 may also be referred to as a DC bus. The DC input voltage 201 may vary between a battery voltage (approximately between 9V and 17V) and an AC adapter voltage (approximately between 18V and 22V). The pulse duration component 220 generates a second output 226 responsive to the second 222 and third 224 inputs.

The time delay component 230 is placed between the pulse start component 210 and the logic component 240 to advantageously impose a time delay on the first output 214. The time delay component 230 receives the first output 214 and the third input 224 (indicative of the DC input voltage 201), introduces atime delay to generate a delayed first output 232. in one embodiment, a variable value for the time delay is selectable to advantageously maintain a D constant switching frequency of the inverter 200, thereby improving the efficiency of the inverter 200. in one embodiment, a particular value of the time delay is selectable by the time delay component 230 to maintain a constant switching frequency of 68 kHz, in response to a change in the value of the DC input voltage 201. The range for selecting a particular value for the constant switching frequency may generally vary from 40 to 160 kHz. Additional details of the timing aspects oi the time delay are described in FlG.'s 3A and 33. Further detail of the structure of the time delay component 230 and the selection of a particular value for the time delay are described in F lG.'s 4A and 48.

The logic component 240 is operable to receive the delayed first output 232 and the second output 226 to generate a plurality of control signals 242 for controlling the plurality of switches 250. The plurality of switches 250 control the flow of current from the DC input voltage 201 source to the filter component 260.

The plurality of switches 250 thus control the magnitude and direction of the primary current 263 and hence the secondary current 265 and the current flowing through the load 290.

In one embodiment. each control signal included in the plurality of control signals 242 controls a corresponding switch included in the plurality of switches 250.

Each control signal controls the corresponding control switch by placing it in an ON or OFF state, and by controlling a time period the switch remains in the ON or OFF state. In one embodiment, each of the plurality of switches 2530 is a MOSFET‘ device.

The plurality of switches 250 may be configured in a variety of configurations such as half-bridge and full~bridge. In the depicted embodiment, the plurality of switches is configured as a full~bridge circuit that includes four switches. in this embodiment, the plurality of control signals 242 include four control signals operable to control the corresponding four switches. The plurality of switches 250 are electrically coupled to the primary section 262 by the pair of terminals 268. An AC output measured across a pair of terminals 252 of the plurality of switches 250 has a constant switching frequency caused by imposing the time delay.

FIG. 3A illustrates waveforms associated with the inverter 200 having no time delay. in this illustration, the time delay is not present or has a value of zero.

A The first output 214 (not shown) is the same as the delayed first output 232 (not shown). Also. the primary voltage across the pair of terminals 268 substantially tracks the AC output 252 of the plurality of switches 250. Frequency of the inverter 200 is determined indirectly by pulse width of voltage waveform of the output across terminals 252. The polarity of the pulse may be positive, negative or equal to zero.

At low input voltages, the pulse width is long compared to the high input voltages, when the pulse width is short. That is, in order to deliver the same power to the lead 290 the duty cycle decreases. In the illustration, the pulse starts when the inverter circuit 200 detects a zero crossing 301 of the primary current 263 since there is no time delay component 230 or the time delay is zero. if the pulse varies in width, due to achanging value of the DC input voltage 201 , the subsequent zero crossing for the next pulse varies as well. thereby causing a variable switching frequency.

At time t = to 310, the output across the terminals 252 increases from an initial value V0 312 (eg, 0 volts) to an increased value of V2 314 volts. When a selective number of the plurality of switches 250 are turned ON in response to a change in the output across the tenninals 252, the primary current 263 ramps up from an initial value lo (e.g.. 0 amperes) to a maximum value of '2 due to more voltage being available to change the current in the transformer leakage inductance, since V = L'(di/dt). The time duration for which the primary current 263 continues to ’ increase is a to" 312 period of the duty cycle. At t = [1 320. the output across the terminals 252 decreases to V0. The primary current 263 decays to la. The time duration for which the piimary current 263 continues to decrease is a ton: 314 period of the duty cycle. At t = t; 330. a zero crossing of the primary current 263 is detected by the pulse start component 210. In response to the zero crossing, the plurality of the switches 250 are placed in an ON or OFF state and the cycle is repeated. In this illustration, the pulse width (e.g., ton 312 + ion: 314) varies as the DC input voltage 201 varies. Thus, it would be desirable to provide a constant switching frequency to improve the efficiency of the inverter 200.

FIG. 3B illustrates waveforms associated with the inverter 200 having a time delay, according to an embodiment. In this embodiment. the time delay component 230 (not shown) introduces (or imposes) a time delay ta 316 in changing the state of a switch included the plurality of switches 250 (not shown) in response to the zero crossing 301. That is, the time delay is 316 is imposed from the zero crossing time t2 330 to a change in the state of the switch time t; 340. As described earlier, an amount or a value of the time delay id 316 imposed varies with the DC input voltage 201. The introduction of the variable time delay t., 316 causes a subsequent zero crossing of the first input 212 to remain fixed relative to a previous zero crossing thereby advantageously generating a constant switching frequency, ‘ e.g., 68 KHz. Operating the inverter 200 at the constant switching frequency advantageously improves the efficiency, as described in FlG.'s 1A and 1B. The time delay td 316 causes an adjustment of the time period top; 314 to maintain the constant switching frequency.

FIG. 4A illustrates detail of the structure of the time delay component 236 of FIG. 2, according to one embodiment. In the depicted embodiment, the time delay component 230 includes: 1) a transconductance amplifier 410, 2) an inverter 420, 3) a MOSFET switch 430, 4) a capacitance 440 and 5) a comparator 450. As described in FIG. 2, the time delay component 230 is placed between the pulse start component 210 and the logic component 240 to advantageously impose a time delay on the first output 214. The output of the time delay component 230 is the delayed first output 232.

The transconductance amplifier 410 acts as a programmable current source with the following equation: low = (Va, - V.N)*gm, where Vm 412 represents a standard reference, and tom 414 the current output. in the depicted embodiment.

VTN 416 is representative of the system power source (PWR_SRC) (not shown). e.g., AC adapter or battery. Generally V," 416 is lower than VR1 412. When Vm 416 is at its minimum value (corresponding to a low voltage on PWR_SRC) lo." 414 is at it's maximum. When Vm 416 is at its maximum value (corresponding to a high voltage on PWR_SRC) IM414 is at it's minimum. low 414 charges the capacitor 440. A small value of the tour 414 charges the capacitor 440 at a slower rate than a larger value of the low 414. When the voltage on the capacitor 440 goes above V32 418 (another reference voltage) the output, which is the delayed first output 232 goes high. If Vm 416 is at its minimum value and the flrst output 214 transitions from low to high the MOSFET 430 turns off and the capacitor 440 voltage starts charging from ’ 0V. low 414 will charge the capacitor 440 at it's maximum rate. This results in a minimum delay time between the first output 214 transitioning from low to high to the delayed flrst output 232 going low to high. In another scenario, when Vm 416 is at it's maximumvalue, this results in low 414 being at,it's minimum value further resulting in a maximum delay time between the first output 214 transitioning from low to high to the delayed first output 232 going low to high. When the first output 214 transitions low the MOSFET 430 turns on and the capacitor 440 is discharged at a rate independent of loin 414. Although this illustrative embodiment has been shown and described, a wide range of modification, change and substitution is contemplated. For example, the time delay component 230 may be implemented by other means such as programmable logic chips.

FIG. 4B illustrates a graphical relationship between the time delay t., 316 of F IG. 3B (shown on Y-axis) and the DC input voltage 201 of FlG. 2 (shown on X- axis), according to one embodiment. in the depicted embodiment. the time delay t., 316 is variable. A particular value of the time delay id 316 is selectable responsive to the DC input voltage 201. That is, the graph defines the particular value required for the time delay t.; 316 for a selected value of the DC input voltage 201. The particular value for the time delay t, 316 causes an adjustment of the top; 314 time Period (not shown) to maintain a constant switching frequency of 68 KHZ.

F lG. 5 is a flow chart illustrating a method for improving efliciency of the inverter 200 providing power to the lead 290, according to an embodiment. in step 510, the DC input 201 is received by the plurality of switches 250. In step 520, the plurality of switches 250 are controlled by the plurality of control signals 242 so as to generate the AC output across the terminals 252, in response to the DC input 201. in step 530, the output across the tenninals 252 of the plurality of switches 250 is filtered by the filter component 260 to generate a filtered AC output to power the lead 290. In step 540, a zero crossing of the AC output across the tenninals 252 is detected by the pulse start component 210. In step 550, the controlling of the plurality of switches 250. is delayed by ta 316 to effectively maintain a constant switching frequency of the inverter 200 in response to a change in the DC input 201.

Various steps described above may be added, omitted, combined, altered, or performed in different orders. For example, steps 540 and 550 may be performed before step 530 in one embodiment.

For purposes of this disclosure, an IHS may include any instmmentality or aggregate of instrumentalities operable to compute, classify, prpcess, transmit, receive, retrieve. originate, switch, store, display, manifest. detect. record, reproduce, handle, or utilize any form of information, intelligence, or data for business, scientific, control, or other purposes. For example, the IHS may be a personal computer, Including notebook computers, personal digital assistants, cellular phones, gaming consoles, a network storage device, or any other suitable device and may vary in size, shape, performance, functionality, and price.

The IHS may include random access memory (RAM), one or more processing resources such as a central processing unit (CPU) or hardware or software control logic, ROM, and/or other types of nonvolatile memory. Additional components of the IHS may include one or more disk drives, one or more network ports for communicating with extemal devices as well as various input and output (l/O) devices. such as a keyboard, a mouse, and a video display. The IHS may also "include one or more buses operable to transmit communications between the various hardware components.

FIG. 6 illustrates a block diagram of an information handling system 600 having an improved inverter, according to an embodiment. The information handling system 600 includes a processor 610. a system random access memory (RAM) 620 (also referred to as main memory), a non-volatile ROM 622 memory, a display device 605, a keyboard 625 and an l/0 controller 640 for controlling various other input/output devices. It should be understood that the term "infonnation handling system" is intended to encompass any device having a processor that executes instructions from a memory medium. The IHS 600 is shown to include a hard disk drive 630 connected to the processor 610 although some embodiments may not include the hard disk drive 630. The processor 610 communicates with the system components via a bus 650, which includes data, address and control lines. in one embodiment, the IHS 600 may include multiple instances of the bus 650. A ‘communications controller 645, such as a network interface card, may be connected to the bus 650 to enable information exchange between the IHS 600 and other ‘devices (not shown). in one embodiment, a power supply system (not shown) providing power to the IHS. 600 incorporates the inverter 200 (not shown) described in FIG. 2. in this embodiment. the display device 605 may include a CCFL representing the load 290.

The inverter 200 (not shown) may be configured to provide power to the display device 605.

The processor 610 is operable to execute the computing instructions and/or operations of the IHS 600. The memory medium, e.g.. RAM 620, preferably _ stores instructions (also known as a "software program") for implementing various embodiments of a method in accordance with the present disclosure. in various embodiments the one or more software programs are implemented in various ways. including procedure—based techniques, component-based techniques. and/or object- oriented techniques, among others. Specific examples include assembler, 0, XML, C++ objects. Java and Microsoft Foundation Classes (MFC).

Although illustrative embodiments have been shown and described. a wide range of modification. change and substitution is contemplated in the foregoing disclosure and in some instances, some features of the embodiments may be employed without a corresponding use of other features. For example, the frequency feedforward technique described herein may be applied to apulse width feedforward control scheme as well. Also, while the frequency feedforward technique is advantageously applied to reduce power consumptionin portable lHS. such a scheme may be applied to reduce the regulation requirements of an intermediatezAC powered (or otherwise powered) DC stage as well. The technique may be used to remove the flicker effect from low frequency ripple on the preceding DC stage. For example, if there was a 60 Hz ripple on the DC, this technique may be used to reduce the impact on the LCD intensity. This advantageously leads to cost reductions in filter and/or control elements.

Claims (14)

Claims

1. An inverter for providing power to a load, the inverter comprising: a pulse start component operable to receive a first input indicative of a primary current, the pulse start component generating a first output responsive to a zero crossing of the first input; a pulse duration component operable to receive asecond input indicative of a load current and a third input indicative of a direct current (DC) input voltage, the pulse duration component being operable to generate a second output responsive to the second and third inputs; a time delay component operable to receive the flrst output and the DC input voltage, the time delay component introducing a time delay to generate a delayed first output; a logic component operable to receive the delayed first output and the second output to generate a plurality of control signals: ‘a plurality of switches operable to convert the DC input to the primary current in response to receiving the plurality of control signals; and a filtering component operable to filter the primary current for generating the load current.

2. The inverter of claim 1. wherein the filtering component includes a transformer having a primary section electro~magnetically coupled to a secondary section. wherein the primary current flows through the primary section, and wherein the primary section is electrically coupled to the plurality of the switches and the secondary section is electrically coupled to the load. ' I

3. The inverter of claim 1, or claim 2, wherein the load is a cold cathode fluorescent lamp (CCFL).

4. The inverter of any one of the preceding claims, wherein the time delay is variable, a value of the time delay being selectable responsive to the DC input voltage. ‘

5. The inverter of claim 4, wherein the value increases as the DC input increases.

6. The inverter of any one of the preceding claims, wherein the logic component imposes the time delay to assert at least one control signal for controlling a corresponding switch following the zero crossover.

7. The inverter of any one of the preceding claims, wherein the plurality of switches are configured as a full—bridge.

8. The inverter of claim 7, wherein an output of the full-bridge has a constant switching frequency caused by imposing the time delay.

9. The inverter of any one of the preceding claims, wherein each control signal of the plurality of control signals controls a corresponding switch by placing the switch in an ON or OFF state, and by controlling a time period in the ON state.

10. The inverter of claim 9, wherein the time delay is imposed from the zero crossing to a change inthe state of the switch.

11. The inverter of claim 9 or claim 10 wherein the time delay causes an adjustment of the time period in the OFF state to maintain a constant switching frequency.

12. The inverter of any one of the preceding claims, wherein imposing the time delay causes a subsequent zero crossing of the first input to remain fixed relative to a previous zero crossing thereby generating a constant switching frequency.

13. An information handling system (lHS) comprising: a display device; and an inverter according to any one of the preceding claims operable to provide power to the display device.

14. The system of claim 13, wherein the display device includes a cold cathode fluorescent lamp (CCFL).