US20150263510A1 - Control Methods for Over Voltage Protection and Relevant Power Controllers - Google Patents

Control Methods for Over Voltage Protection and Relevant Power Controllers Download PDF

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US20150263510A1
US20150263510A1 US14/203,757 US201414203757A US2015263510A1 US 20150263510 A1 US20150263510 A1 US 20150263510A1 US 201414203757 A US201414203757 A US 201414203757A US 2015263510 A1 US2015263510 A1 US 2015263510A1
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voltage
current
valley
direct
input voltage
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US9148013B1 (en
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Chen-Lun Yang
Ren-Yi Chen
Shun-Chin Chou
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Nanjing Greenchip Semiconductor Co Ltd
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Grenergy Opto Inc
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    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
    • H02H7/00—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions
    • H02H7/10—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions for converters; for rectifiers
    • H02H7/12—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions for converters; for rectifiers for static converters or rectifiers
    • H02H7/125—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions for converters; for rectifiers for static converters or rectifiers for rectifiers
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M1/00—Details of apparatus for conversion
    • H02M1/32—Means for protecting converters other than automatic disconnection
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M3/00—Conversion of dc power input into dc power output
    • H02M3/02—Conversion of dc power input into dc power output without intermediate conversion into ac
    • H02M3/04—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters
    • H02M3/10—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • H02M3/145—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal
    • H02M3/155—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only
    • H02M3/156—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only with automatic control of output voltage or current, e.g. switching regulators
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
    • H02M7/02—Conversion of ac power input into dc power output without possibility of reversal
    • H02M7/04—Conversion of ac power input into dc power output without possibility of reversal by static converters
    • H02M7/06—Conversion of ac power input into dc power output without possibility of reversal by static converters using discharge tubes without control electrode or semiconductor devices without control electrode
    • H05B33/0887—
    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
    • H05B45/00—Circuit arrangements for operating light-emitting diodes [LED]
    • H05B45/30—Driver circuits
    • H05B45/37—Converter circuits
    • H05B45/3725—Switched mode power supply [SMPS]
    • H05B45/375—Switched mode power supply [SMPS] using buck topology
    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
    • H05B47/00—Circuit arrangements for operating light sources in general, i.e. where the type of light source is not relevant
    • H05B47/20—Responsive to malfunctions or to light source life; for protection
    • H05B47/24—Circuit arrangements for protecting against overvoltage
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M1/00—Details of apparatus for conversion
    • H02M1/0067—Converter structures employing plural converter units, other than for parallel operation of the units on a single load
    • H02M1/007—Plural converter units in cascade
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02B—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO BUILDINGS, e.g. HOUSING, HOUSE APPLIANCES OR RELATED END-USER APPLICATIONS
    • Y02B20/00—Energy efficient lighting technologies, e.g. halogen lamps or gas discharge lamps
    • Y02B20/30—Semiconductor lamps, e.g. solid state lamps [SSL] light emitting diodes [LED] or organic LED [OLED]

Definitions

  • the present disclosure relates generally to over voltage protection for power converters.
  • Power converters which usually supply power to appliances used in daily life, need to be equipped with protection mechanism to prevent abnormal satiations from damaging users or surroundings.
  • a power converter that powers light emitting diodes for lighting must have over-voltage protection (OVP) so as to avoid over voltage occurring in its outputs, which might cause electric shock to human beings if touched.
  • OVP over-voltage protection
  • FIG. 1 demonstrates a conventional power converter 10 .
  • Bridge rectifier 12 provides full-wave rectification to alternative-current (AC) mains voltage V AC to generate rectified direct-current (DC) input voltage V IN and a ground line.
  • Power converter 10 is a buck converter having LED module 14 as a load, which connects in series with a primary winding PRM in a transformer, between DC input voltage V IN and the ground line.
  • Power controller 17 has a power switch 18 , which, when turned on (as being in a conduction state), energizes primary winding PRM and conducts a driving current to illuminate LED module 14 .
  • LED module 14 If a LED open event happens to LED module 14 , meaning that at least one LED in LED module 14 is open or cannot conduct current, driving voltage V LED could rocket if there is no corresponding protection mechanism, or OVP, built in power controller 17 .
  • the two end terminals of LED module 14 which meanwhile has a drop voltage the same with the rocket-high driving voltage V LED , could cause severe electric shock to anyone whoever touches them, endangering human beings.
  • Power controller 17 in FIG. 1 detects driving voltage V LED , through the help from the combination of node VOP, voltage divider 22 and secondary winding SEC.
  • the voltage across primary winding PRM is about the summation of driving voltage V LED and the forward voltage of wheel diode 16 , and the voltage across secondary winding SEC is in proportion to that across primary winding PRM. Accordingly, in case that the voltage at node VOP exceeds a certain limit when the transforming de-energizes, it implies driving voltage V LED is somehow over high, and, responsively, power controller 17 could continuously turn off power switch 18 to stop power conversion of power converter 10 , achieving OVP.
  • FIG. 1 demonstrates a conventional power converter
  • FIG. 2 demonstrates a power converter according to embodiments of the invention
  • FIG. 3 demonstrates a power controller and some peripheral devices
  • FIG. 4 shows some signal waveforms of signals in FIGS. 2 and 3 .
  • FIG. 2 demonstrates power converter 60 according to embodiments of the invention, which uses an inductor L to replace the transformer in FIG. 1 .
  • Power converter 60 could achieve OVP without a costly and bulky transformer. This does not mean a power converter according to the invention must not have a transformer.
  • Some embodiments of the invention might use one winding of a transformer to be an inductor.
  • Resistors 64 and 66 for voltage dividing are connected in series between DC input voltage V IN and a ground line GND, where the joint therebetween provides a detection voltage V PF which is therefore a scaled version of DC input voltage V IN .
  • power controller 62 which could be in form of an integrated circuit, operates power converter 60 substantially in boundary mode.
  • One operation mode is called discontinuous conduction mode (DCM), referring to that an inductor in a power converter is operated to empty completely the energy stored therein every time when a new switching cycle starts.
  • Another operation mode is continuous conduction mode (CCM), referring to that a power converter is operated to start a new switching cycle while the inductor has not emptied the energy stored.
  • Boundary mode operates a power converter in a way between DCM and CCM, generally referring to that a new switching cycle starts right after the inductor just empties the energy stored.
  • Inductor L starts to increase its stored energy when the power switch in power controller 62 is turned on, and the voltage V L and the current I L of inductor L shall follow the relationship presented as the following equation (I).
  • V L *T ON L L *I L ,
  • V L and I L denote the voltage drop across inductor L and the current through inductor L; L L the inductance of inductor
  • I CS-PEAK is about 0 when DC input voltage V IN is the same with driving voltage V LED , and inductor L cannot be energized.
  • Bridge rectifier 12 causes DC input voltage V IN to follow the absolute value of AC mains voltage V AC if DC input voltage V IN is about less than that absolute value. That absolute value has no influence to DC input voltage V IN nevertheless if DC input voltage V IN exceeds that absolute value. Accordingly, when that absolute value is less than driving voltage V LED , DC input voltage V IN will have the same voltage as that of driving voltage V LED because inductor L stops energizing at the same condition. When that absolute value exceeds driving voltage V LED , DC input voltage V IN is about the same as that absolute value. It can be concluded that the local minimum of DC input voltage V IN should be about the same as driving voltage V LED . A local minimum of DC input voltage V IN happens in a valley of the waveform of DC input voltage V IN .
  • One embodiment of the invention detects a local minimum of DC input voltage V IN to decide whether to trigger OVP.
  • Power controller 62 in FIG. 2 determines the occurrence of a local minimum of DC input voltage V IN by detecting current-sense signal V CS .
  • DC input voltage V IN seems to be in a valley and have a local minimum if current-sense signal V CS continues to be about 0 (or less than a predetermined value V CS-REF ) for a predetermined period of time.
  • DC input voltage V IN is in a valley, it could be used to represent driving voltage V LED .
  • Power controller 62 compares detection voltage V PF with a reference voltage for OVP (V OVP-REF ). If DC input voltage V IN is having a local minimum and detection voltage V PF exceeds reference voltage V OVP-REF driving voltage V LED is deemed to be over high and, in response, power controller 62 provides an OVP signal S Protection to stop the power conversion of power converter 60 .
  • FIG. 3 demonstrates power controller 62 and some peripheral devices.
  • Power controller 62 has, but is not limited to have, valley detector 79 , OVP comparator 82 , ramp-signal generator 84 , logics 83 and 88 , etc.
  • Valley detector 79 includes valley comparator 80 and delay time generator 81 .
  • Valley comparator 80 compares current-sense signal V CS with a predetermined reference V CS-REF which is 50 mV in one embodiment. If the input of delay time generator 81 indicates that current-sense signal V CS has been less than predetermined reference V CS-REF for a predetermined period of time T OVP-DELAY , delay time generator 81 makes its output 1 in logic, meaning the occurrence of a local minimum of DC input voltage V IN .
  • logic 83 sends out OVP signal S Protection with logic 1 to stop the power conversion of power converter 60 , thereby driving voltage V LED being prevented from going higher.
  • Ramp-signal generator 84 generates ramp signal V RAMP , whose slope is determined by a peak value of detection voltage V PF .
  • the peak value of detection voltage V PF can be sensed or recorded by power controller 62 , and it represents a swing magnitude of AC mains voltage V AC .
  • the higher the peak value of detection voltage V PF the higher the slope of ramp signal V RAMP .
  • Both ramp signal V RAMP and a compensation signal V COMP are forwarded to two inputs of comparator 86 . For instance, ramp signal V RAMP starts to ramp up at the same time when power switch 18 is turned on. Once ramp signal V RAMP exceeds compensation signal V COMP , comparator 86 makes logic 86 to turn off power switch 18 .
  • Ramp-signal generator 84 and comparator 86 together can determine the ON time T ON of power switch 18 during which it is turned on.
  • FIG. 4 shows some signal waveforms of signals in FIGS. 2 and 3 .
  • AC mains voltage V AC has for example a sinusoidal waveform with a swing magnitude of 110V and a frequency of 60 Hz. Shown in FIG. 4 is also DC input voltage V IN , whose local minimums occur in valleys and always even with driving voltage V LED .
  • Detection voltage V PF is in proportion to DC input voltage V IN .
  • LED module 14 mistakenly becomes open since time t LED-OPEN . Accordingly, as an open LED module 14 does not consume electric power and the switching of power switch 18 continues the power conversion, driving voltage V LED ramps up after time t LED-OPEN .
  • a first valley VL 1 occurs in the waveform of DC input voltage V IN , shown in FIG. 4 .
  • OVP is not triggered though because detection voltage V PF has not exceeded reference voltage V OVP-REF .
  • a second valley VL 2 occurs in the waveform of DC input voltage V IN .
  • detection voltage V PF has exceeded reference voltage V OVP-REF .
  • V OVP-REF reference voltage
  • Current-sense signal V CS becomes a constant 0V after time t OVP because power switch 18 is constantly turned off.
  • FIG. 2 shows power converter 60 , which needs only an inductor and is capable of achieving OVP. Power converter 60 could render a product with more market competitiveness in view of its compact size and low cost.

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  • Engineering & Computer Science (AREA)
  • Power Engineering (AREA)
  • Rectifiers (AREA)

Abstract

Disclosure includes a control method for over voltage protection, suitable for a power converter. The power converter has a rectifier for rectifying an alternative-current voltage into a direct-current input voltage. The direct-current input voltage is sensed and determined whether it locates in a valley. A detection voltage is provided, capable of representing the direct-current input voltage. The detection voltage is compared with a predetermined reference voltage. When the detection voltage exceeds the predetermined reference voltage and the direct-current input voltage is in the valley, an OVP signal is provided to stop the power conversion of the power converter.

Description

    BACKGROUND
  • The present disclosure relates generally to over voltage protection for power converters.
  • Power converters, which usually supply power to appliances used in daily life, need to be equipped with protection mechanism to prevent abnormal satiations from damaging users or surroundings. For example, a power converter that powers light emitting diodes for lighting must have over-voltage protection (OVP) so as to avoid over voltage occurring in its outputs, which might cause electric shock to human beings if touched.
  • FIG. 1 demonstrates a conventional power converter 10. Bridge rectifier 12 provides full-wave rectification to alternative-current (AC) mains voltage VAC to generate rectified direct-current (DC) input voltage VIN and a ground line. Power converter 10 is a buck converter having LED module 14 as a load, which connects in series with a primary winding PRM in a transformer, between DC input voltage VIN and the ground line. Power controller 17 has a power switch 18, which, when turned on (as being in a conduction state), energizes primary winding PRM and conducts a driving current to illuminate LED module 14. When power switch 18 is turned off (as being in a non-conduction state), primary winding PRM starts to release its stored energy to generate another driving current, which passes wheel diode 16 to keep LED module 14 illuminating. Current-sense resistor 20 provides to power controller 17 current-sense signal VCS, a representative of the current passing through power switch 18.
  • If a LED open event happens to LED module 14, meaning that at least one LED in LED module 14 is open or cannot conduct current, driving voltage VLED could rocket if there is no corresponding protection mechanism, or OVP, built in power controller 17. The two end terminals of LED module 14, which meanwhile has a drop voltage the same with the rocket-high driving voltage VLED, could cause severe electric shock to anyone whoever touches them, endangering human beings.
  • Power controller 17 in FIG. 1 detects driving voltage VLED, through the help from the combination of node VOP, voltage divider 22 and secondary winding SEC. When the transformer de-energizes to release its stored energy, the voltage across primary winding PRM is about the summation of driving voltage VLED and the forward voltage of wheel diode 16, and the voltage across secondary winding SEC is in proportion to that across primary winding PRM. Accordingly, in case that the voltage at node VOP exceeds a certain limit when the transforming de-energizes, it implies driving voltage VLED is somehow over high, and, responsively, power controller 17 could continuously turn off power switch 18 to stop power conversion of power converter 10, achieving OVP.
    • BRIEF DESCRIPTION OF THE DRAWINGS
  • Non-limiting and non-exhaustive embodiments of the present invention are described with reference to the following drawings. In the drawings, like reference numerals refer to like parts throughout the various figures unless otherwise specified. These drawings are not necessarily drawn to scale. Likewise, the relative sizes of elements illustrated by the drawings may differ from the relative sizes depicted.
  • The invention can be more fully understood by the subsequent detailed description and examples with references made to the accompanying drawings, wherein:
  • FIG. 1 demonstrates a conventional power converter;
  • FIG. 2 demonstrates a power converter according to embodiments of the invention;
  • FIG. 3 demonstrates a power controller and some peripheral devices; and
  • FIG. 4 shows some signal waveforms of signals in FIGS. 2 and 3.
    •  DETAILED DESCRIPTION
  • Even power converter 10 in FIG. 1 achieves OVP, it is a costly and bulky solution because of the necessity of the transformer composed of at least primary winding PRM and secondary winding SEC. A transformer, which has more than one windings coupled to each other, normally costs and occupies more than an inductor with only one winding does.
  • FIG. 2 demonstrates power converter 60 according to embodiments of the invention, which uses an inductor L to replace the transformer in FIG. 1. Power converter 60 could achieve OVP without a costly and bulky transformer. This does not mean a power converter according to the invention must not have a transformer. Some embodiments of the invention might use one winding of a transformer to be an inductor.
  • Resistors 64 and 66 for voltage dividing are connected in series between DC input voltage VIN and a ground line GND, where the joint therebetween provides a detection voltage VPF which is therefore a scaled version of DC input voltage VIN.
  • In the embodiment of FIG. 2, power controller 62, which could be in form of an integrated circuit, operates power converter 60 substantially in boundary mode. One operation mode is called discontinuous conduction mode (DCM), referring to that an inductor in a power converter is operated to empty completely the energy stored therein every time when a new switching cycle starts. Another operation mode is continuous conduction mode (CCM), referring to that a power converter is operated to start a new switching cycle while the inductor has not emptied the energy stored. Boundary mode operates a power converter in a way between DCM and CCM, generally referring to that a new switching cycle starts right after the inductor just empties the energy stored.
  • Inductor L starts to increase its stored energy when the power switch in power controller 62 is turned on, and the voltage VL and the current IL of inductor L shall follow the relationship presented as the following equation (I).

  • V L *T ON =L L *I L,

  • (V IN −V LED)*T ON =L L *I CS-PEAK  (I),
  • where VL and IL denote the voltage drop across inductor L and the current through inductor L; LL the inductance of inductor
  • L; TON the duration or the ON time when the power switch in power controller 62 is turned on; and ICS-PEAK the peak current flowing through current-sense resistor 20.
  • It can be derived from equation (I) that ICS-PEAK is about 0 when DC input voltage VIN is the same with driving voltage VLED, and inductor L cannot be energized. Bridge rectifier 12 causes DC input voltage VIN to follow the absolute value of AC mains voltage VAC if DC input voltage VIN is about less than that absolute value. That absolute value has no influence to DC input voltage VIN nevertheless if DC input voltage VIN exceeds that absolute value. Accordingly, when that absolute value is less than driving voltage VLED, DC input voltage VIN will have the same voltage as that of driving voltage VLED because inductor L stops energizing at the same condition. When that absolute value exceeds driving voltage VLED, DC input voltage VIN is about the same as that absolute value. It can be concluded that the local minimum of DC input voltage VIN should be about the same as driving voltage VLED. A local minimum of DC input voltage VIN happens in a valley of the waveform of DC input voltage VIN.
  • One embodiment of the invention detects a local minimum of DC input voltage VIN to decide whether to trigger OVP.
  • Power controller 62 in FIG. 2 determines the occurrence of a local minimum of DC input voltage VIN by detecting current-sense signal VCS. For example, DC input voltage VIN seems to be in a valley and have a local minimum if current-sense signal VCS continues to be about 0 (or less than a predetermined value VCS-REF) for a predetermined period of time. When DC input voltage VIN is in a valley, it could be used to represent driving voltage VLED.
  • Power controller 62 compares detection voltage VPF with a reference voltage for OVP (VOVP-REF). If DC input voltage VIN is having a local minimum and detection voltage VPF exceeds reference voltage VOVP-REF driving voltage VLED is deemed to be over high and, in response, power controller 62 provides an OVP signal SProtection to stop the power conversion of power converter 60.
  • FIG. 3 demonstrates power controller 62 and some peripheral devices. Power controller 62 has, but is not limited to have, valley detector 79, OVP comparator 82, ramp-signal generator 84, logics 83 and 88, etc.
  • Valley detector 79 includes valley comparator 80 and delay time generator 81. Valley comparator 80 compares current-sense signal VCS with a predetermined reference VCS-REF which is 50 mV in one embodiment. If the input of delay time generator 81 indicates that current-sense signal VCS has been less than predetermined reference VCS-REF for a predetermined period of time TOVP-DELAY, delay time generator 81 makes its output 1 in logic, meaning the occurrence of a local minimum of DC input voltage VIN.
  • If a local minimum of DC input voltage VIN occurs and OVP comparator 82 deems detection voltage VPF exceeding reference voltage VOVP-REF, logic 83 sends out OVP signal SProtection with logic 1 to stop the power conversion of power converter 60, thereby driving voltage VLED being prevented from going higher.
  • Ramp-signal generator 84 generates ramp signal VRAMP, whose slope is determined by a peak value of detection voltage VPF. For example, the peak value of detection voltage VPF can be sensed or recorded by power controller 62, and it represents a swing magnitude of AC mains voltage VAC.In one embodiment, the higher the peak value of detection voltage VPF, the higher the slope of ramp signal VRAMP. Both ramp signal VRAMP and a compensation signal VCOMP are forwarded to two inputs of comparator 86. For instance, ramp signal VRAMP starts to ramp up at the same time when power switch 18 is turned on. Once ramp signal VRAMP exceeds compensation signal VCOMP, comparator 86 makes logic 86 to turn off power switch 18. Ramp-signal generator 84 and comparator 86 together can determine the ON time TON of power switch 18 during which it is turned on.
  • FIG. 4 shows some signal waveforms of signals in FIGS. 2 and 3. AC mains voltage VAC has for example a sinusoidal waveform with a swing magnitude of 110V and a frequency of 60 Hz. Shown in FIG. 4 is also DC input voltage VIN, whose local minimums occur in valleys and always even with driving voltage VLED. Detection voltage VPF is in proportion to DC input voltage VIN.
  • In FIG. 4, it is supposed that LED module 14 mistakenly becomes open since time tLED-OPEN. Accordingly, as an open LED module 14 does not consume electric power and the switching of power switch 18 continues the power conversion, driving voltage VLED ramps up after time tLED-OPEN.
  • A first valley VL1 occurs in the waveform of DC input voltage VIN, shown in FIG. 4. In the meantime, OVP is not triggered though because detection voltage VPF has not exceeded reference voltage VOVP-REF.
  • Following the first valley VL1, a second valley VL2 occurs in the waveform of DC input voltage VIN. Meanwhile, detection voltage VPF has exceeded reference voltage VOVP-REF. At time tOVP which is the moment when current-sense signal VCS has continued to be about 0, or less than reference VCS-REF for a predetermined period of time TOVP-DELAY OVP is triggered. Current-sense signal VCS becomes a constant 0V after time tOVP because power switch 18 is constantly turned off.
  • Different from FIG. 1, which needs a bulky and costly transformer, FIG. 2 shows power converter 60, which needs only an inductor and is capable of achieving OVP. Power converter 60 could render a product with more market competitiveness in view of its compact size and low cost.
  • While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.

Claims (12)

What is claimed is:
1. A control method for over-voltage protection, suitable for a power converter with a rectifier for rectifying an alternative-current voltage to provide a direct-current input voltage, the control method comprising:
determining whether the direct-current input voltage is in a valley where a local minimum of the direct-current input voltage occurs;
providing a detection voltage representing the direct-current input voltage;
comparing the detection voltage with a predetermined reference voltage; and
providing an OVP signal to stop a power conversion of the power converter when the detection voltage exceeds the predetermined reference voltage and the direct-current input voltage is in the valley.
2. The control method as claimed in claim 1, wherein the power converter includes an inductor, a load capable of emitting light, and a power switch, connected in series, and the step of determining comprises:
providing a current-sense signal, a representative of a current through the power switch; and
comparing the current-sense signal with a predetermined value; and
after the current-sense signal is less than the predetermined value, determining that the direct-current input voltage is in the valley.
3. The control method as claimed in claim 2, comprising:
after the current-sense signal continues to be less than the predetermined value for a predetermined period of time, determining that the direct-current input voltage is in the valley.
4. The control method as claimed in claim 1, wherein the power converter includes an inductor, a load capable of emitting light, and a power switch, connected in series, the power converter further includes a wheel diode with two terminals coupled to the direct-current input voltage and the power switch respectively.
5. The control method as claimed in claim 1, further comprising:
providing a ramp signal with a slope;
sensing and recording a peak value of the detection voltage; and
determining the slope in accordance with the peak value.
6. The control method as claimed in claim 5, further comprising:
determining an ON time of a power switch based on the ramp signal and a compensation signal.
7. A power controller suitable for a power converter, which comprises a rectifier for rectifying an alternative-current voltage to provide a direct-current input voltage, the power controller comprising:
a valley detector for determining whether the direct-current input voltage is in a valley;
an over-voltage comparator, for comparing a detection voltage with a predetermined reference voltage, wherein the detection voltage is capable of representing the direct-current input voltage; and
a logic coupled to the over-voltage comparator and the valley detector, for providing an OVP signal to stop a power conversion of the power converter when the detection voltage exceeds the predetermined reference voltage and the direct-current input voltage is in the valley.
8. The power controller as claimed in claim 7, wherein power converter is a buck converter including a power switch and an inductor, and the valley detector comprises:
a valley comparator for comparing a current-sense signal with a predetermined value;
wherein the current-sense signal is a representative of a current through the power switch.
9. The power controller as claimed in claim 8, wherein the valley detector further comprises:
a delay time generator, coupled to the valley comparator, for providing an output signal indicating that the direct-current input voltage is in a valley after the current-sense signal has been less than the predetermined value for a predetermined period of time.
10. A power converter, comprising:
a rectifier for rectifying an AC mains voltage to provide a direct-current input voltage and a ground line; and
a power controller, comprising:
a valley detector for determining whether the direct-current input voltage is in a valley; and
an over-voltage comparator, for comparing a detection voltage with a predetermined reference voltage, wherein the detection voltage is capable of representing the direct-current input voltage;
wherein the power controller stops a power conversion of the power converter when the detection voltage exceeds the predetermined reference voltage and the direct-current input voltage is in the valley.
11. The power converter as claimed in claim 10, further comprising:
a buck converter with an inductor and a power switch connected in series between the direct-current input voltage and the ground line;
wherein the valley detector comprises a valley comparator for comparing a current-sense signal with a predetermined value, and the current-sense signal is a representative of a current through the power switch.
12. The power converter as claimed in claim 10, wherein the valley detector further comprises a delay time generator coupled to the valley comparator for providing an output signal indicating that the direct-current input voltage is in a valley after the current-sense signal has been less than the predetermined value for a predetermined period of time.
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