Patent Publication Number: US-6703796-B2

Title: Power supply and inverter used therefor

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a power supply system, and particularly to a structure suitable for multiple ranges of input voltage, which integrates a rectifier/filter&#39;s circuitry and a converter&#39;s circuitry with an inverter to reduce volume and increase power efficiency. 
     2. Description of the Related Art 
     Household power supply typically ranges from 90-132 Vac and 180-264 Vac. However, in current LCD monitors, a DC source with lower voltage than the power supply is used to power all circuits, e.g. the video control circuit, except that the discharge lamp for illumination is powered by an AC source with higher voltage than the power supply. For example, a mono-lamp notebook requires about 7-21 Vdc while a multi-lamp LCD monitor requires the rated voltage about 12 or 15 Vdc. Also, the monitor requires more than 1000 Vac to drive a cold cathode fluorescent lamp (CCFL) for illumination. Therefore, to meet the above requirements, a typical power supply system, as shown in FIG. 1, must include an AC source input from a socket passing through a rectifier/filter  11 , a fly-back converter  12 , a DC-AC inverter  13  and a buck regulator  14  to provide the lamp(s) with AC power and other elements of the display system with DC power. As such, the typical power supply system must convert between AC and DC in too many stages, which causes inconvenience and inefficiency. In current products, the rectifier/filter  11  and the fly-back converter  12  are combined together to form an additional adapter  10 , which is further connected to the inverter  13  and the buck regulator  14  via additional connectors and cables (not shown). Accordingly, such a product carries power efficiency only to about 70%, high production costs and larger dimensions. 
     SUMMARY OF THE INVENTION 
     Therefore, an object of the invention is to provide a power supply with reduced dimensions and increased power efficiency without the need of an additional adapter. The power supply for powering a system having a lamp includes a rectifier/filter, a DC-DC converter and a DC-AC inverter. The rectifier/filter has an input terminal for inputting AC voltage in order to convert the input AC voltage into DC voltage. The DC-DC converter and DC-AC inverter are parallel to each other with one end concurrently connected to the rectifier/filter&#39;s output and the other end respectively outputting the power required by the system. As such, DC-DC converter reduces the converted DC voltage to the lower DC voltages to power all circuits except for the lamp, and DC-AC inverter converts the converted DC voltage to a higher AC voltage output to drive the lamp. 
     Accordingly, the inventive power supply can directly integrate the rectifier/filter, converter and inverter to increase power efficiency. Moreover, components with lower rated power can be used and the power supply can be arranged on a single circuit board. Therefore, the volume is reduced and the component cost and assembling cost are both lowered. 
     A further object of the invention is to provide an inverter for driving a discharge lamp, the inverter including: two switches, a driver for driving the two switches alternately turned on, a transformer, a sampling circuit for obtaining the current value through the lamp and outputting a feedback signal, a PWM control circuit for controlling the duty cycle of the driver according to the feedback signal, a voltage detection circuit for outputting a control signal according to the DC voltage received by the inverter, and an impedance adjustment circuit for adjusting the equivalent impedance value of the inverter according to the control signal. 
     Accordingly, the inventive inverter can change the frequency-to-impedance curves through the impedance adjustment circuit&#39;s adjustment when the input voltage is higher. Therefore, the operating frequency of the inverter will not change remarkably with the increasing input voltage. The invention thus ensures a longer lifespan of the lamp and avoids the temperature-increasing problem due to the skin effect on the wires during high-frequency operation to thereby reduce the converting loss. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be apparent by referring to the following detailed description of a preferred embodiment with reference to Accompanying drawings, wherein: 
     FIG. 1 shows a block diagram of a typical power supply system; 
     FIG. 2 shows a block diagram of an inventive power supply system; 
     FIG. 3 shows a block diagram of an inverter in FIG. 2 according to the invention; 
     FIG. 4 shows two impedance-frequency curves illustrating the impedance switching of the impedance adjustment circuit in FIG. 3; 
     FIG. 5 is an embodiment of the circuit in FIG. 3 according to the invention; 
     FIG. 6 is a second embodiment of the impedance adjustment circuit in FIG. 5; 
     FIG. 7 is a third embodiment of the impedance adjustment circuit in FIG. 5; and 
     FIG. 8 is a fourth embodiment of the impedance adjustment circuit in FIG.  5 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following similar function elements are denoted by the same reference numerals. 
     FIG. 2 is a block diagram of an inventive power supply system. In FIG. 2, the power supply includes: a rectifier/filter  21 , a DC-DC converter  22  and a DC-AC inverter  23 . As shown in FIG. 2, the rectifier/filter  21  has an input terminal connected to an alternating current (AC) source for converting the input AC voltage (generally, household power is 90-132 Vac or 180-264 Vac) into the direct current (DC) voltage to be output (e.g., the voltage at the node M is 120-190 Vdc or 250-380 Vdc). The DC-DC converter  22  and the DC-AC inverter  23 , other than a typical three-stage power supply system, are connected in parallel and have one end concurrently connected to the rectifier/filter&#39;s output so as to reduce the number of stages from the input AC voltage to the desired output voltage and raise the power efficiency up to 80%. It means that, with respect to power efficiency, the inventive configuration is about 10% higher than normal. In such a configuration, the DC-DC converter  22  reduces DC voltage output generated by the rectifier/filter  21  to the lower DC voltage to power all circuits except for the lamp. The DC-AC inverter  23  converts DC voltage output into a higher AC voltage output to drive the lamp. For example, the converter  22  provides a 12 Vdc and/or a 5 Vdc to the circuits of an LCD, or even to a PC. As well, the inverter  23  provides the AC driving voltage to a CCFL with more than 1000 Vac. The inverter is described in detail as follows. 
     FIG. 3 shows a block diagram of the inverter  23  in FIG.  2 . In FIG. 3, the inverter  23  applied to drive the lamp  37  includes: switches  33  (including a first switch M 1  and a second switch M 2 ), a high side driver (HSD)  31 , a low side driver (LSD)  32 , a transformer T 3 , an impedance adjustment circuit  34 , a voltage detection circuit  36 , a sampling circuit  38  and a pulse width modulation (PWM) control circuit  30 . 
     As shown in FIG. 3, the HSD  31  and the LSD  32  are respectively coupled to the control input of the first switch M 1  and the second switch M 2  so as to drive the two switches M 1 , M 2  to be alternately turned on with a driving frequency. Therefore, DC voltage (i.e., DC voltage fed into the node M of FIG. 2) from the input terminal Vin is converted by switching between the switches M 1 , M 2  into a square-wave AC to feed into the primary side of the transform T 3 . The transformer T 3  steps up and filters the square-wave AC to output a sine-wave AC with about more than 1000 V in order to drive the lamp  37  coupled to the secondary side of the transformer T 3 . The sampling circuit  38  is coupled to one end of the lamp  37  to detect the current through the lamp  37  and output a feedback signal to the PWM control circuit  30 . The PWM control circuit  30  controls the duty cycles of the HSD  31  and the LSD  32  according to the feedback signal so as to regulate the brightness of the lamp  37 . The impedance adjustment circuit  34  is coupled between the primary side of the transformer T 3  and the voltage detection circuit  36 . The voltage detection circuit  36  compares DC voltage Vin input to the inverter  23  with a predetermined reference voltage Vref and controls the impedance switching of the impedance adjustment circuit  34  based on the comparison result. As such, the impedance value of the impedance adjustment circuit  34  is changed and the equivalent impedance value observed at the primary side of the transformer T 3  is changed. 
     The PWM control circuit  30  in FIG. 3 can also be replaced by, for example, a frequency modulation control circuit, which controls the switching frequency of the HSD  31  and the LSD  32  according to the feedback signal to reach the goal of the lamp brightness adjustment. 
     FIG. 4 shows two impedance-frequency curves illustrating the impedance switching of the impedance adjustment circuit in FIG.  3 . As shown in FIG. 4, as the power supply shown in FIG. 2 has an input power Vin from 90 to 132 Vac, DC voltage 120-190 Vdc converted from the input Vin is detected by the voltage detection circuit  36 . At this point, the impedance adjustment circuit  34  is controlled so that the inverter  23  is operated at the impedance Z 1 . As such, the operating frequency ranges between f 1  and f 3 , wherein f 1  responds to the 120 Vdc input voltage and f 3  responds to the 190 Vdc input voltage. As the power supply shown in FIG. 2 has an input power Vin from 180 to 264 Vac, DC voltage 250-380 Vdc converted from the input Vin is detected by the voltage detection circuit  36 . At this point, the impedance adjustment circuit  34  is controlled so that the inverter  23  is operated at the impedance Z 2 . As such, the operating frequency ranges between f 2  and f 4 , wherein f 2  responds to the 250 Vdc input voltage and f 4  responds to the 380 Vdc input voltage. Accordingly, the inventive operation ranges between f 1  and f 4 , which in practice ranges between about 50 kHz and about 65 kHz. Contrarily, the conventional inverter is not provided with voltage detection circuit  36  and the impedance adjustment circuit  34  and thus has no impedance switching function. In such case, when the power supply has an input power Vin from 180 to 264 Vac, the operating frequency is ranged between f 5  and f 6 , wherein f 5  responds to the 250 Vdc input voltage and f 6  responds to the 380 Vdc input voltage. As such, obviously, when the input voltage is higher, the inverter may be operated at high frequency (about 80 kHz), the operating frequency range is more varied and thus easily causes skin effect. The problem can be solved with the use of the inventive inverter, which can switch the impedance-frequency curve from Z 1  to Z 2  when the input voltage is higher, to operate in a relatively narrow operating frequency range, thereby reducing the skin effect. In addition, due to the narrow frequency variation, the life of the lamp is prolonged. Moreover, because the switching frequency of the switch  33  is lowered, the entire circuit is reduced in temperature and further reduced in power loss so as to increase efficiency. In the above description, the input voltage range 120-380 Vdc is only used for illustration and is not intended to be limiting. Those familiar with the prior art can change the input voltage range according to needs. Further, the voltage detection circuit  36  may be modified to detect the external AC input voltage in FIG. 2 and accordingly output a control signal to the impedance adjustment circuit  34 . 
     FIG. 5 is an embodiment of the circuit in FIG. 3 according to the invention. In FIG. 5, the PWM control circuit  30  can be implemented by any known technique in the prior art. As shown in FIG. 5, in order to increase the driving ability of the signal PWM 2 , the switches Q 14  and Q 18  in the HSD  31  are implemented to be alternately turned on to produce a square-wave output. The square-wave signal provides a driving signal to the switch M 1  after passing through a capacitor C 56  and an isolating driving transformer T 4 . The switching speed of the switch M 1 , driven by the driving signal, can be increased via the circuit with a switch Q 13 , a resistor R 44 , a resistor R 77 , a resistor R 88  and a capacitor C 8 . Similarly, switch Q 19 , resistor R 95 , and diode D 3  in the LSD  32  can speed up the switching of switch M 2 . Switches  33  include the first and second switches M 1  and M 2 , which are respectively driven by the HSD  31  and LSD  32 . Switches M 1  and M 2  are alternately turned on with an operating frequency so as to convert the input DC voltage Vin into a square-wave output. The square-wave signal is input to the primary side of the transformer T 3  and then stepped up and filtered by the transformer T 3  to produce a sine-wave outputfor driving the lamp  37  coupled to the secondary side of the transformer T 3 . A capacitor C 35  is connected in parallel with the secondary side of the transformer T 3  to adjust the resonant curve. A capacitor C 67  is connected in series with one end of the lamp  37  to reduce the influence of the LCD panel&#39;s characteristics. The feedback circuit  38  couples to the other end of the lamp  37 . The feedback circuit  38 , which is coupled to the other end of the lamp  37 , includes a pair of diodes D 5  and D 8  for filtering the AC signal to produce a signal with only the positive sine-wave remaining and a sampling resistor R 100  for sampling the current value through the lamp  37  and converting it into a voltage form as a feedback signal FB output to the PWM control circuit  30 . The circuit  30  outputs the signals PWM 2  and PWM 1  according to the feedback signal FB to control the duty cycles of the HSD  31  and the LSD  32 , respectively. Therefore, the lamp&#39;s brightness can be regulated. 
     The voltage detection circuit  36  has two input terminals, one for the input voltage Vin of the inverter, the other for a predetermined reference voltage Vref. The circuit  36  mainly includes a comparator OP, wherein the voltage Vin is fed into the non-inverted input terminal of the comparator OP and the voltage Vref is fed into the inverted input terminal. The impedance adjustment circuit  34  mainly includes a first capacitor C 97  and a second capacitor C 52  connected in parallel, one of the connection point of the capacitors C 97  and C 52  connected to the primary side of the transformer and a control switch Q 15  connected in series with the second capacitor C 52 . The control switch Q 15  has a control input terminal coupled to the output of the voltage detection circuit  36 . As such, when the voltage Vin is higher than the predetermined reference voltage Vref, the comparator OP will output a high voltage so that a switch Q 17  connected to its output terminal is turned on and outputs a control signal to turn on the switch Q 15  in the impedance adjustment circuit  34 . In such a situation, the equivalent impedance of the circuit  34  is equal to the equivalent impedance of the parallelly-connected capacitors C 97  and C 52 , which leads to the curve Z 2  case as shown in FIG.  4 . Conversely, when the voltage Vin is lower than the predetermined reference voltage Vref, the switch Q 15  will not turn on. The equivalent impedance of the circuit  34  is equal to the equivalent impedance of the capacitor C 97 , which leads to the curve Z 1  case as shown in FIG.  4 . Accordingly, frequency-impedance curve switching is achieved so that the inverter is operated in a small varying bandwidth. 
     Preferably, the voltage detection circuit  36  also includes a hysteresis circuit mainly consisting of a switch Q 39  and a resistor R 22   k  to adjust the switching threshold of the control switch Q 15 . For example, in the case of the switching voltage designed in the external input AC voltage of the inventive power supply at 150 Vac, when the input voltage has a small change about 150 Vac, the switch Q 15  may generate an error action. This can be solved by the hysteresis circuit. The reason is, for example, in a step-up situation, the hysteresis circuit shifting the threshold from 150 to 160 Vac so that the switch Q 15  is turned on only at the voltage above 160 Vac. Also, in a step-down situation, the hysteresis circuit shifts the threshold from 150 to 140 Vac so that the switch Q 15  is turned off only at voltage below 140 Vac. 
     The embodiment is only for illustration, and is not intended to be limiting, and other modification is allowable to those familiar with the prior art. For example, as shown in FIG. 6, the impedance adjustment circuit can be the series connection of first and second inductors L 61  and L 62 . The second inductor L 62  is connected in parallel with the control switch Q 15 . In addition, the series connection can be replaced by using an inductor L 7  connected in parallel with the primary side of the transformer T 3  and the inductor L 7  is connected in series with the switch Q 15 , as shown in FIG.  7 . Further, the switch Q 15  can directly couple to the primary winding of the transformer T 3  so as to change the equivalent impedance by changing the coil number of the primary side of the transformer T 3  according to the on/off status of the switch Q 15 , as shown in FIG.  8 . 
     In the preferred embodiment of FIG. 5, according to the invention, the switches  33  are provided in a half-bridge configuration, but the full-bridge and the push-pull configurations are also suitable for the invention. The switches M 1  and M 2  can be implemented by a MOS FET or any other type of transistor. Driving circuit  31  and  32  is only an example of explanation, and modification is adapted to meet the practical requirements. Further, the impedance adjustment circuit  34  can also be coupled to the secondary side of the transformer T 3  even though it appears on the primary side of the transformer T 3  in FIG.  5 . That is, when the impedance adjustment circuit  34  is coupled between the capacitor C 35  and the ground, the frequency-impedance curve switching effect is also achieved. 
     Although the invention has been described in its preferred embodiment, it is not intended to limit the invention to the precise embodiment disclosed herein. Those who are skilled in this technology can still make various alterations and modifications without departing from the scope and spirit of this invention. Therefore, the scope of the invention shall be defined and protected by the following claims and their equivalents.