Low consumption converter directly connectable to the mains

Abstract

A converter that is directly connectable to an AC power source (e.g., the mains) includes a rectifier stage for rectifying a network voltage, a power factor correction pre-regulating circuit supplied with the rectified network voltage for producing a DC voltage of a predetermined nominal value on an output node, and a DC-DC converter. The DC-DC converter may be supplied on an input node thereof with the DC voltage of the predetermined nominal value for producing a regulated DC voltage on an output node thereof. The DC-DC converter may use a clock whose frequency is selected between at least one low and one high value by a selection signal. Furthermore, the converter may also include a stand-by circuit for producing the selection signal based upon the current delivered to the load.

Description

FIELD OF THE INVENTION

[0001] The present invention relates to the field of electronic circuits, and, more particularly, to converters, adapters, battery chargers and similar circuits. Specifically, the invention relates to a low consumption converter directly connectable to the mains or an AC power source.

BACKGROUND OF THE INVENTION

[0002] Power supplies typically include a DC-DC converter coupled to an AC power source (e.g., the mains) through one or more stages. In a pulse width modulation (PWM) switching converter, a square wave drives the control terminal of a power switch and determines whether it is conductive or not conductive. The output voltage is increased by increasing the duration of the phase of conduction of the switch, and decreased by increasing the duration of the phase during which there is no conduction. Thus, the output voltage is controlled by varying the duty cycle of the driving square wave.

[0003] When the power switch is a MOS transistor, a non-negligible amount of power is spent to periodically charge the gate of the switching transistor. Power dissipation increases with an increase in the switching frequency and noticeably affects the overall efficiency of the converter. In particular, such a power dissipation lowers the efficiency of the converter, which is particularly true when the load is relatively small. For this reason, typical prior art devices provide for a lowered switching frequency when supplying a relatively small load.

[0004] In the following description reference will be made to a two stage converter, as illustrated in FIG. 1, because of its far greater diffusion than other types of converters. Yet, the following considerations are equally applicable to a converter with a number of stages greater than two.

[0005] As illustrated in FIG. 1, a two stage converter may include a rectifier coupled to an AC power source, a power factor correction pre-regulating circuit PFC supplied with the rectified voltage and producing a DC voltage of a certain nominal value. Further, a DC-DC converter is input with the nominal DC voltage and controls a load. The DC-DC converter may be based on a control scheme of any suitable kind (e.g., PWM, quasi resonant, resonant, etc.).

[0006] A well known solution to reduce energy consumption under relatively small or null load conditions includes reducing the switching frequency of the switches of the PFC stage or of the DC-DC stages, separately. Even if the energy savings that can be obtained thereby is not negligible, the energy consumption remains conspicuously large in the case of systems that remain in a stand-by state or supply a relatively small load for a long time.

[0007] Several converters which attempt to address this problem are known in the art. Such converters show enhanced performance in terms of energy savings because, under conditions of relatively small load, the PFC pre-regulating circuit is automatically set to a low consumption (quiescent) condition. On the contrary, when the load increases the PFC resumes its normal functioning. This provides for an increased output power requisite.

[0008] By turning off the PFC, the voltage on the bulk capacitor Co that couples the PFC to the converter drops from the level of the regulated voltage to the lower rectified and filtered network voltage. Considering the fact that the leakage current of the capacitor increases as the applied voltage increases approximately according to the following formula:

I.sub.leak.ltoreq.0.02*C(.mu.F)*V.sub.R(V)+15 .mu.A,

[0009] the turning off of the PFC reduces capacitor losses. Moreover, lowering the voltage on the capacitor Co and the voltage supplied to the converter reduces the switching losses of the converter and the PFC.

[0010] U.S. Pat. No. 5,903,138 discloses a two-stage switching regulator that operates in one of four functioning modes, selected according to load conditions. A drawback of this regulator is the fact that it requires a relatively complicated logic circuit for switching from one functioning mode to another. Moreover, such a regulator may change its functioning mode even if the load remains constant, and it may cause the generation of electric noise at audible frequencies.

[0011] U.S. Pat. No. 5,726,871 discloses a power supply circuit for a video display capable of reducing power consumption. To this end, the functioning of its power factor correction is controlled by an external microcomputer depending on functioning conditions of the video display.

[0012] Furthermore, U.S. Pat. No. 5,960,207 discloses a power supply including a power factor correction and a controller that disables the power factor correction when the power supply is operating in a low power mode. Monitoring of the load is carried out in the secondary circuit of the power supply which is isolated from the primary circuit according to safety rules. Therefore, the control command produced by the controller for disabling the power factor correction must be transmitted to the primary circuit of the power supply using a device appropriate to keep such an isolation, such as an optoelectronic switching coupler.

SUMMARY OF THE INVENTION

[0013] In view of the foregoing background, it is therefore an object of the invention to provide a converter directly connectable to an AC power source (e.g., the mains) that allows a significant reduction of power dissipation by turning off its PFC when the load is smaller than a certain threshold, substantially avoiding the risk of generating electrical noise at audible frequencies.

[0014] It is another object of the invention to provide such a converter which has a relatively simple control circuit and that may be realized in an integrated form.

[0015] This and other objects, features, and advantages in accordance with the present invention are provided by a converter that is directly connectable to the mains that includes a rectifier stage for rectifying a network voltage and a power factor correction pre-regulating circuit supplied with the rectified network voltage for producing a DC voltage of a certain nominal value on an output node. The converter further includes a DC-DC converter supplied on an input node thereof with the DC voltage of the nominal value for producing a regulated DC voltage on an output node thereof. The DC-DC converter may use a clock whose frequency is selected between at least one low and one high value by a selection signal. A standby circuit may also be included for producing the selection signal based upon the current delivered to the load.

[0016] One advantageous feature of the converter of the invention is that it may include a control circuit having a comparator for receiving the selection signal and generating a disabling signal for the power factor correction pre-regulating circuit. The disabling signal is generated so long as the selection signal assumes a value corresponding to the low frequency value of the driving clock.

[0017] The power factor correction pre-regulating circuit may include a correction circuit that receives at an input thereof a signal representing the desired nominal DC voltage and produces a correction signal, and a power device driver that is supplied with the rectified network voltage and receives as inputs an enabling signal and the correction signal. The power device driver produces the nominal DC voltage on an output node thereof so long as the enabling signal is disabled. The power factor correction pre-regulating circuit may also include an enabling circuit for producing the enabling signal when the voltage on a control node is zero. Moreover, the power factor correction may be turned on or off by electrically isolating or coupling the control node to a reference voltage using a switch driven by the disabling signal.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The various aspects and advantages of the invention will become more evident through a detailed description of the invention with reference to the attached drawings, in which:

[0019] FIG. 1 is a schematic block diagram of a two stage converter according to the present invention;

[0020] FIG. 2 is a schematic block diagram illustrating two possible embodiments of the invention;

[0021] FIG. 3 is a schematic block diagram of the prior art L6561 PFC pre-regulating circuit shown in FIG. 2; and

[0022] FIG. 4 is a schematic block diagram of the prior art L5991 DC-DC converter shown in FIG. 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] A converter according to the invention may be realized with a common DC-DC converter, a power factor correction pre-regulating circuit, and a control circuit for controlling the functioning of the PFC of the converter. Typically, DC-DC converters function at a clock frequency selectable between at least one low and one high value. The low frequency is used in small load conditions for reducing the power consumption. The DC-DC converter selects the clock frequency by providing a selection signal, which depends on the load conditions, to its oscillator.

[0024] The control circuit of the converter of the present invention may be realized in integrated form relatively easily and without using external devices, such as optoelectronic switching couplers, because it utilizes the selection signal for disabling the PFC. Two possible embodiments of the invention which conveniently use commercially available components (i.e., an L6561 PFC pre-regulating circuit and an L5991 DC-DC converter, both manufactured by STMicroelectronics S.r.l., assignee of the present invention), are shown in FIG. 2. Schematic block diagrams of the L6561 and L5991 devices are illustratively shown in FIGS. 3 and 4, respectively.

[0025] The L6561 device includes a correction circuit supplied with the rectified network voltage Vcc and by a power device driver DRIVER. The correction circuit receives at an input INV a signal representing the desired nominal DC voltage produced by the PFC and generates a correction signal. The power device driver DRIVER is supplied with the voltage Vcc and produces a DC voltage of a certain nominal value, adjusted as a function of the correction signal, on an output node GD. The L6561 device also includes an enabling circuit DISABLE that disables the power device driver DRIVER when the voltage on the control node ZCD is zero. In this case, the PFC may be turned off simply by grounding the node ZCD. This can be done using the control circuit illustrated in FIG. 2a and connected between the two commercially available devices.

[0026] The transistor 1a, as is the case with the transistor 2a, is coupled to a Zener diode and acts as a comparator. This comparator produces, on the base of transistor 1b (2b), a zero collector-emitter voltage when the voltage on the node ST-BY is greater than or equal to 5V, and a voltage suitable to set the transistor 1b (2b) in a conduction state on ST-BY when a voltage lower than 5V is present. This collector-emitter voltage is the disabling signal. The transistor 1b is a switch driven by the disabling signal that isolates or grounds the node ZCD of the L6561 device.

[0027] An alternative way of turning off the L6561 device includes disconnecting it from the supply line in the way shown in FIG. 2b. In this second embodiment the switch 2b driven by the disabling signal couples the L6561 device to the rectified network voltage Vcc. The commercial L5991 device, whose block diagram is shown in FIG. 4, has a logic circuit regulating the PWM driving signal which receives on the input node ISEN a signal representing the current delivered to the load. On the node DC it receives a signal representing the desired duty-cycle, and on the node RCT it receives a clock signal provided by an external oscillator. The load is coupled to the output nodes OUT and PGND.

[0028] The L5991 device also has a stand-by circuit STAND-BY producing a selection signal on the node ST-BY. This reduces the frequency of the external oscillator that generates the clock signal whenever the load drops below a certain pre-established threshold. Even when the PFC is in a low consumption mode, the L5991 device works at a certain frequency which does not vary in as much as the load does not overcome the pre-established threshold. Working at an established frequency, the risk of generating noise at acoustic frequencies is prevented, or at least noticeably reduced.

[0029] Such a stand-by circuit is typically present in several other commercially available converters for reducing the switching losses when the load is relatively small. The improved converter of the invention can be relatively easily realized by inputting the selection signal, which is already generated by such commercially available converters, to any one of the two control circuits of FIG. 2.

Claims

That which is claimed is:

1. A converter directly connectable to the mains comprising a rectifier stage of the network voltage, a power factor correction pre-regulating circuit (L6561) supplied with the rectified network voltage (Vcc) and producing a DC voltage of a certain nominal value on an output node (GD), a DC-DC converter (L5991} supplied on an input node (Vc) with said DC voltage of a certain nominal value and producing a regulated DC voltage on an output node (OUT), said DC-DC converter using a clock whose frequency is selected between at least a low and a high value by a selection signal (ST-BY), a stand-by circuit (STAND-BY) producing said selection signal (ST-BY) in function of the current delivered to the load, characterized in that it comprises a control circuit comprising a comparator input with said selection signal (ST-BY) and generating a disabling signal of said power factor correction pre-regulating circuit (L6561) as long as said selection signal (ST-BY) assumes a value corresponding to the low frequency value of said clock.

2. The converter of claim 1, wherein said power factor correction pre-regulating circuit (L6561) comprises a correction circuit that receives at an input (INV) a signal representing the desired nominal DC voltage and produces a correction signal, a power device driver (DRIVER) supplied with the rectified network voltage (Vcc) and receiving as input an enabling signal and said correction signal, producing said nominal DC voltage on said output node (GD) as long as said enabling signal is disabled, an enabling circuit (DISABLE) producing said enabling signal when the voltage on a control node (ZCD) is null, characterized in that said control circuit further comprises a switch driven by said disabling signal electrically isolating or coupling to a reference voltage said control node (ZCD).

3. The converter according to claim 1 wherein said control circuit comprises a switch driven by said disabling signal electrically connecting to or disconnecting from said rectified network voltage (Vcc) said power factor correction pre-regulating circuit (L6561).