WO2000038304A1

WO2000038304A1 - Switched-mode power supply with a dummy load

Info

Publication number: WO2000038304A1
Application number: PCT/EP1999/009353
Authority: WO
Inventors: Abraham L. Melse; Joan W. Strijker
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 1998-12-18
Filing date: 1999-12-01
Publication date: 2000-06-29

Abstract

A switched-mode power supply (PS) generates a voltage (VA, VO1; VO2) for supplying power to a load. When the power supplied drops below a particular minimal value, the stabilizing control loop of the switched-mode power supply (PS) is no longer able to stabilize the voltage (VA, VO1; VO2). To prevent the switched-mode power supply (PS) from supplying a non-stabilized voltage (VA, VO1; VO2), a dummy load (DL) which has a controlled impedance is connected to receive the voltage (VA, VO1; VO2). The impedance of the dummy load (DL) is controlled to vary in accordance with the power quantity supplied by the switched-mode power supply (PS). When the power supplied decreases, the impedance of the dummy load (DL) is decreased. Thus, at low output powers, the impedance is low enough to obtain a stabilized voltage (VA, VO1; VO2), and at high output power the impedance is high to minimize the dissipation in the dummy load (DL).

Description

Switched-mode power supply with a dummy load.

The invention relates to a switched-mode power supply as defined in the precharacterizing part of claim 1. The invention further relates to a display apparatus with such a switched-mode power supply, and to a battery-charging device comprising such a switched- mode power supply.

US 5,341,179 discloses as prior art a switched-mode power supply with a "TN mode" during which power is supplied to TV (Television) circuits and to BS (Direct Broadcasting Satellite) circuits. In a "BS mode", a dummy load is connected to the TV power source +130V line to prevent a no-load state in the stabilization control. Furthermore, measures are disclosed to improve the power efficiency without the use of the dummy load.

It is, inter alia, an object of the invention to provide a switched-mode power supply using a dummy load and having an improved efficiency.

To this end, a first aspect of the invention provides a switched-mode power supply as claimed in claim 1. A second aspect of the invention provides a display apparatus with a switched-mode power supply as claimed in claim 5. A third aspect of the invention provides a battery-charging device comprising a switched-mode power supply as claimed in claim 6. Advantageous embodiments are defined in the dependent claims.

The switched-mode power supply according to the invention generates a voltage which may be supplied to a power consumption circuit requiring well stabilized supply voltage, even if the power required by the power consumption circuit varies over a large range. In accordance with the invention, the switched-mode power supply comprises a measurement and control circuit for generating a power signal or output power quantity which indicates an output power supplied by the switched-mode power supply to control an impedance of a dummy load coupled to the voltage in accordance with the power signal. In the prior art, the switched-mode power supply has two modes. In a first mode, a high output power has to be supplied to loading circuits, in a second mode, a low output power has to be supplied. The stabilizing control of the switched-mode power supply is not able to stabilize the output voltage when the output power drops below a certain minimal value which is higher than the low output power. A solution is found by connecting a dummy load to the stabilized output voltage when the second mode is selected and the low output power is supplied. The dummy load has an impedance which is low enough to obtain the required minimal load on the switched-mode power supply to obtain a total output power larger than the minimal value. In practical situations, the impedance of the dummy load should be selected to be low enough to cope with tolerances in the power consumption of the circuits which are active in the second mode. This gives rise to dissipation in the dummy load, which is larger than is necessary. Furthermore, for example, in different types of televisions, different circuits may be active in the second mode. To obtain a minimal waste of energy in the dummy load, the dummy load should have different values in different types of televisions, which gives rise to an undesired diversity. Moreover, when the switched-mode power supply is used to charge a battery, for example in a shaver, the load current may vary during the charging operation. Such battery- operated apparatus often have an indicator which indicates the charge in the battery. It should be prevented that the voltage supplied by the switched-mode power supply to the battery varies to such an extent that it gives an incorrect indication of the charge in the battery, even when the apparatus is switched on or off during the charging operation.

In accordance with the invention, the measurement of the output power of the switched-mode power supply allows a decrease of the impedance of the dummy load in accordance with a decrease of the output power requested by the loading circuits. In this way, the total load of the switched-mode power supply does not decrease below the certain minimum value, while dissipation in the dummy load can be minimized. Low impedance of the dummy load only occurs at very low output power in the loading circuits. At a higher output power in the loading circuits, the impedance of the dummy load is higher. The value of the dummy load may be proportional to the power supplied.

In an embodiment of the invention as claimed in claim 2, the dummy load comprises a main current path of a controllable switch. The controllable switch has a control input for controlling the impedance of the main current path of the controllable switch. In a preferred embodiment, the controllable switch is a FET. The control circuit generates a voltage varying with the power measure. In this way, it is possible to gradually increase the impedance of the controllable switch when the output power supplied to the loading circuits increases. It is possible to arrange an impedance in series with the main path of the controllable switch. The gradual increase might be in steps if the measurement and control circuit comprises digital circuits. For example, a microprocessor might be used to control the impedance of the dummy load in small steps in accordance with the power measure.

In an embodiment of the invention as claimed in claim 3, the controllable switch is controlled to obtain a high impedance when the output power is higher than a predetermined power set value. The predetermined power set value may be selected to be equal to or somewhat higher than the minimal output power at which the switched-mode power supply is still stabilizing its output voltage or voltages. Below the power set value, the impedance of the controllable switch decreases in accordance with the output power.

In an embodiment of the invention as claimed in claim 4, the dummy load has a high impedance during at least part of the non-conductivity period of the rectifier diode. In this way, a ripple on the stabilized voltage is minimized, because the dummy load does not load the stabilized voltage during the non-conductivity period of the rectifier diode. In a preferred embodiment, the dummy load is disconnected from the stabilized voltage during the total non- conductivity period of the rectifier diode.

These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter. In the drawings: Fig. 1 shows a switched-mode power supply with a measurement and control circuit according to an embodiment of the invention,

Fig. 2 shows waveforms of signals occurring in the switched-mode power supply shown in Fig.l,

Fig. 3 shows a switched-mode power supply with a measurement and control circuit according to another embodiment of the invention,

Fig. 4 shows a display apparatus with a switched-mode power supply according to the invention, and

Fig. 5 shows a battery charger apparatus with a switched-mode power supply according to the invention.

Fig. 1 shows a switched-mode power supply with a measurement and control circuit 1 according to an embodiment of the invention. A transformer Tl has a primary winding LPl, a secondary winding LSI, and an auxiliary winding LA. A series arrangement of the primary winding LPl and a switching element SI receives an input voltage VII. The input voltage VII may be a rectified mains voltage. The switching element SI may be a bipolar transistor, a FET (Field Effect Transistor), or any other suitable semiconductor device. The dots near the windings of the transformer Tl indicate respective polarities of the windings. A series arrangement of a rectifier diode DOl and a smoothing capacitor COl is connected to the secondary winding LSI. A secondary voltage VOl is generated across the smoothing capacitor COl. A series arrangement of a rectifier diode DA and a smoothing capacitor CA is connected across the auxiliary winding LA. An auxiliary voltage VA is generated across the smoothing capacitor CA. This auxiliary voltage VA may be used to supply circuits at the primary side of the transformer Tl. The secondary voltage VOl supplies power to a circuit or an apparatus which needs a well-stabilized voltage over a large range of output power.

A feedback circuit 2 comprises an operational amplifier (further referred to as opamp) or comparator 20 to compare the auxiliary voltage VA with a sawtooth or triangle- shaped oscillator waveform VO. It is assumed that the output signal DR of the opamp 20 closes the switching element SI during an on-time when the oscillator waveform VO exceeds the level of the auxiliary voltage VA. The switching element SI is open during an off-time when the level of the oscillator waveform VO is below the level of the auxiliary voltage VA. The ratio between the on-time and the off-time is referred to as the delta. It is further assumed that the switched-mode power supply is in a stable situation wherein the delta is in conformance with the output power. When the output power decreases, the secondary voltage VOl and the auxiliary voltage VA increase as the delta is not yet adapted. The increased auxiliary voltage VA causes a decreasing delta as the oscillator waveform VO exceeds the level of the auxiliary voltage VA for a shorter time. A new stable situation will be reached wherein the delta is again in conformance with the output power. In this way, the auxiliary voltage VA is stabilized and thus also the secondary voltage VOl is stabilized.

The load DL comprises a series arrangement of a bipolar transistor TR1 and a resistor Rl. The load DL is connected in parallel with the smoothing capacitor CA. The load need not comprise the resistor Rl. Any other device with controllable impedance may replace the bipolar transistor TR1. An opamp 18 has a non-inverting input + to receive the auxiliary voltage VA and an inverting input - receiving a reference voltage VR to supply a control signal CS to the control input (base) of the bipolar transistor TR1. When the output power decreases, the auxiliary voltage VA increases and the conductivity of the bipolar transistor TR1 increases to lower the impedance of the dummy load DL. The reference voltage VR may be selected in such a way that when the output power is above a certain value (the auxiliary voltage VA is below a certain value), the dummy load DL has a very high impedance (the bipolar transistor TR1 is non-conductive). When the auxiliary voltage VA increases above the reference voltage VR due to a decreasing output power, the impedance of the dummy load DL is gradually increased to prevent a no-load condition and yet obtain a minimal dissipation in the dummy load DL.

In an embodiment of the invention, the stabilization of the auxiliary voltage VA and thus the output voltage VOl is improved by disconnecting the dummy load DL from the auxiliary voltage VA during at least part of the period when the rectifier diode DA does not conduct. Therefore, the switched-mode power supply further comprises an opamp or comparator 16 for comparing the voltage VLA across the auxiliary winding LA with a reference level VD. The reference level VD is selected in such a way that the output signal CD of the opamp 16 indicates that the voltage VLA has such a polarity that the rectifier diode DA is not conductive. For example, the reference level VD is selected to be zero so as to detect when the voltage VLA is negative and the rectifier diode DA is unable to conduct. The output signal CD may directly command the opamp 18 to open the bipolar transistor TR1 during the period when the voltage VLA is negative. Alternatively, for example, the output signal CD may modulate the reference level VR to become high during this period. It is also possible to detect when the voltage VLA is positive and to allow the transistor TR1 to have a low impedance only when the voltage VLA is positive. But, as is clear from the voltage waveform of the voltage VLA shown in Fig. 2, this may lead to an undesired activation of the transistor TR1 during positive periods of the ringing occurring from t4 to t5. Therefore, a circuit 17 may be added with an input receiving the output signal CD from the opamp 16, an input receiving the drive signal DR from the opamp 20, and an output to supply a control signal CE to the opamp 18. The drive signal DR indicates the on-time tl to t2 of the switching element SI. The rectifier diodes DOl and DA start conducting substantially immediately after the instant t2 at which the switching element SI is switched off. The circuit 17 generates an active control pulse CE starting related to this instant t2 and lasting until the instant when the voltage VLA drops below the reference level VD for the first time at instant t3 as indicated by the control signal CD. In this way, the positive parts of the ringing in the voltage VLA do not influence the control signal CE. For example, the circuit 17 may be a set-reset flipflop which is set with the edge of the drive signal DR at instant t2 and reset by the edge of the control signal CD at instant t3.

In the embodiment of the invention shown in Fig. 1, the measurement and control circuit 1 comprises the opamp 16, the circuit 17 and the opamp 18. In Fig.l, the voltage VOl is stabilized by stabilizing the voltage VA. It is also possible to directly stabilize the voltage VOl by feeding back the voltage VOl to the non- inverting input of the feedback circuit 2 via a mains-separated element. The mains-separated element may be an optocoupler or a pulse transformer. If a pulse transformer is used, the circuits have to be adapted in known manner to convert the value of the voltage VI into pulses supplied to a primary winding of the pulse transformer and to convert the pulses supplied by the secondary winding of the pulse transformer to a level to be supplied to the non-inverting input of the opamp 20.

Fig. 2 shows waveforms of signals occurring in the switched-mode power supply shown in Fig.l. Fig. 2A shows the drive signal DR supplied to the control input of the switching element SI. The switching element SI is closed from tl to t2 to generate an increasing current in the primary winding LPl. The voltages across the auxiliary winding LA and the secondary winding LSI are negative and thus the rectifier diodes DA and DOl are non-conductive. At instant t2, the switching element SI is opened, causing the voltages across the primary winding LPl, the auxiliary winding LA, and the secondary winding LSI to inverse polarity. The rectifier diodes DA and DOl start conducting and the energy in the primary winding LPl is transferred to the loads at the auxiliary winding LA and the secondary winding LSI. At instant t3, the switching element SI and both the rectifier diodes DA and DOl are non-conductive, all transformer windings LPl, LSI, and LA are floating and an oscillation starts. The oscillation lasts until the instant tl' at which the switching element SI is closed again to start a next cycle.

Fig. 3 shows a switched-mode power supply with a measurement and control circuit 1 according to another embodiment of the invention. A transformer T2 has a primary winding LP2 and a secondary winding LS2. A series arrangement of the primary winding LP2 and a switching element S2 receives a DC input voltage VI2. The input voltage VI2 may be a rectified mains voltage. The switching element S2 may be a bipolar transistor, a FET, or any other suitable semiconductor device. The dots near the windings of the transformer T2 indicate respective polarities of the windings. A series arrangement of a rectifier diode DO2 and a smoothing capacitor CO2 is connected to the secondary winding LS2. A secondary voltage VO2 is generated across the smoothing capacitor CO2. A feedback circuit 2 receives the secondary voltage VO2 to supply a drive signal DR to a control input of the switching element S2 for controlling on and/or off-times of the switching element S2 to stabilize the output voltage VO2. The feedback circuit 2 may comprise an opamp as shown in Fig. 1. The feedback circuit 2 may further comprise an element for transferring information while providing mains separation, for example an optocoupler. A dummy load DL is arranged in parallel with the smoothing capacitor CO2. In this embodiment, the dummy load DL is a field effect transistor (FET) TR2. An opamp 13 measures the voltage across the diode DO2 caused by the current flowing through diode DO2 to supply a power measure signal PM to a drive circuit 14. Instead of the measurement of the current through the rectifier diode DO2, it is also possible to measure the current through a resistor (not shown) arranged in such a way that the current supplied to a load connected to the secondary voltage VO2 flows through the resistor. The drive circuit 14 supplies an output signal with a voltage level varying in accordance with the power measure signal PM to the control input of the FET TR2 via a switch 15. The switch 15 is closed when the power measure signal PM indicates that the voltage across the rectifier diode DO2 has such a polarity that the rectifier diode DO2 is conductive. In this way, during the time when the rectifier diode DO2 is conductive, the impedance of the FET TR2 is controlled to vary in accordance with the power supplied. The impedance of the FET TR2 is high during the period of time when the switch 15 is open. It is possible to omit the switch 15. In this case, the dummy load DL loads the secondary voltage VO2 also during the time when the rectifier diode DO2 is non-conductive and the ripple of the secondary voltage VO2 increases.

The measurement and control circuit 1 may further comprise a comparator 12 with a first input to receive the power measure signal PM and a second input to receive a predetermined power preset value PS. The output of the comparator 12 commands the drive circuit 14 to drive the FET TR2 in such a way that the FET TR2 has a high impedance as long as the power measure signal PM indicates that the power supplied is larger than the power preset value PS. The impedance of the FET is controlled to decrease with decreasing supplied power when the power measure signal PM indicates that the power supplied is smaller than the power preset value PS.

In the embodiment of the invention shown in Fig. 3, the measurement and control circuit 1 comprises the comparator 12, the opamp 13, the driver circuit 14, and the switch 15. Fig. 4 shows a display apparatus with a switched-mode power supply according to the invention. The display apparatus comprises a display device DD with a display screen DS, a scan circuit SC, a circuit CL , and a switched-mode power supply PS. The display device DD is shown as a cathode ray tube to which a deflection coil LD is coupled. However, the invention is not restricted to a particular display device DD. The scan circuit SC receives synchronizing information SI of an input signal to supply position-determining signals PDS to the display device DD for displaying the input signal at a desired position on the display screen DS. For example, when the display device DD is a cathode ray tube, the scan processor SC comprises a known synchronization circuit and a known deflection circuit. The synchronization circuit may comprise a phase-locked loop to generate internal synchronizing pulses locked to the synchronizing information SI. The internal synchronizing pulses determine scan and flyback periods of the scan current generated by the deflection circuit in the deflection coil LD.

For example, the circuit CIR may be a microcomputer for the control of the display apparatus, a video-processing circuit, a circuit for processing satellite signals, or a combination of several of such functions. The switched-mode power supply PS supplies a stabilized voltage VS1 to the circuit CIR and a stabilized voltage VS2 to the scan circuit SC. The switched-mode power supply PS may be configured as shown in Fig. 1 or 3. One of the stabilized voltages VS1, VS2 is the secondary voltage VOl or VO2 shown, the other one of the stabilized voltages VS 1, VS2 is generated by a further secondary winding of the transformer Tl or T2 (not shown). In the normal operating mode of the display apparatus, both the circuit CIR and the scan circuit SP are active and a large power has to be supplied by the switched-mode power supply PS. In another mode of the display apparatus, only the circuit CLR is active, and the switched-mode power supply PS has to supply a low output power. In accordance with an aspect of the invention, the switched-mode power supply PS is able to stabilize the stabilized voltages VS1 and VS2 very well over a very large range of output powers without dissipating more than necessary in the dummy load DL.

Fig. 5 shows a battery-charging apparatus with a switched-mode power supply according to the invention. A switched-mode power supply PS generates a secondary voltage VB to supply power to a battery BAT and a battery-operated apparatus BO. For example, the battery-operated apparatus BO is a cordless telephone, a shaver, or another home appliance. Furthermore, an indicator circuit BI may indicate the actual amount of charge in the battery BAT. The switched-mode power supply PS has to supply a power varying over a very wide range. For example, when an empty battery BAT is being charged and the apparatus is active, a high power has to be supplied. When the battery BAT is nearly fully charged and the apparatus is inactive, a very low output power has to be supplied. It is important that the voltage VB is very well stabilized. A first reason is that the indicator circuit BI might otherwise indicate the incorrect charge situation of the battery, which would confuse the user.

It would be harmful for the lifetime of the battery BAT if the user started a charge operation when the battery BAT is not yet fully discharged, or when the user stopped charging the battery BAT before it has been fully charged. A second reason is that, for certain types of batteries BAT, the voltage VB during the last part of a charging cycle wherein a very low charging current occurs, has to be within narrow limits to avoid deterioration or even explosion of the battery BAT.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. Instead of varying the impedance of the dummy load DL gradually in dependence on the output power supplied, the impedance may also be varied in steps. Although the embodiments are limited to flyback converters, the invention is also applicable to other types of switched-mode power supplies.

In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word "comprising" does not exclude the presence of elements or steps other than those listed in a claim. The invention can be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In the device claim enumerating several means, several of these means may be embodied by one and the same item of hardware.

Claims

CLAIMS:

1. A switched-mode power supply comprising a winding (LA; LS2) for generating a voltage (VA, VO2), and a dummy load (DL) coupled to the winding (LA; LS2), characterized in that the switched-mode power supply further comprises measurement and control means (1) for supplying a control signal (CS) to the dummy load (DL) to control an impedance of the dummy load (DL) so as to vary in accordance with an output power quantity supplied by the switched-mode power supply.

2. A switched-mode power supply as claimed in claim 1, characterized in that the dummy load (DL) comprises a controllable switching element (TRl; TR2) with a main path coupled to the winding (LA; LS2), and a control input for receiving the control signal (CS) to control an impedance of the switching element (TRl; TR2) so as to gradually vary in accordance with the output power quantity.

3. A switched-mode power supply as claimed in claim 1, characterized in that the measurement and control means (1) comprises: a comparator (12) for comparing the output power quantity with a power set value (PS) to supply an output signal, and a driver circuit (14) receiving the output power quantity and the output signal to supply the control signal (CS) for controlling the dummy load (DL): so as to obtain a high impedance when the output power is larger than the power set value

(PS), and to gradually decrease the impedance in accordance with the decreasing output power when the output power is lower than the power set value (PS).

4. A switched-mode power supply as claimed in claim 1, characterized in that the switched-mode power supply further comprises a rectifier element (DA; DO2) coupled to the winding (LA; LS2) for supplying said voltage (VA; VO2), and in that the measurement and control means (1) further comprises means (13; 16, 17) for supplying a timing signal (PM; CE) indicating a conductivity or a non-conductivity period of the rectifier diode (DA; DO2) for controlling the dummy load (DL) to obtain a high impedance during at least part of the non-conductivity period of the rectifier diode (DA; DO2).

5. A display apparatus comprising: a display device (DD) with a display screen (DS), a scan processor (SC) for receiving synchronizing information (SI) of an input signal to supply position-determining signals (PDS) to the display device (DD) for displaying the input signal at a desired position on the display screen (DS), and a switched-mode power supply (PS) comprising: a winding (LA, LSI; LS2) for generating a supply voltage (VA, VOl; VO2; VS1) for a circuit (CIR) of the display device (DI) , and a dummy load (DL) coupled to said winding (LA, LSI; LS2), characterized in that the switched-mode power supply (PS) further comprises measurement and control means (1) for supplying a control signal (CS) to the dummy load (DL) to control an impedance of the dummy load (DL) to vary in accordance with an output power quantity supplied by the switched-mode power supply (PS).

6. A battery-charging device comprising a switched-mode power supply (PS) for supplying a stabilized supply voltage (VOl; VO2; VB) to the battery (BAT), the switched- mode power supply (PS) comprising: a winding (LSI; LS2) for generating said supply voltage (VOl; VO2; VB), and a dummy load (DL) coupled to said winding (LSI; LS2) or to another winding (LA) of the power supply (PS), characterized in that the switched-mode power supply (PS) further comprises measurement and control means (1) for supplying a control signal (CS) to the dummy load (DL) to control an impedance of the dummy load (DL) to vary in accordance with an output power quantity supplied by the switched-mode power supply (PS).