Classifications

• • 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
  • H02M3/22—Conversion of dc power input into dc power output with intermediate conversion into ac
  • H02M3/24—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters
  • H02M3/28—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac
  • H02M3/325—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal
  • H02M3/335—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal using semiconductor devices only
  • H02M3/33507—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal using semiconductor devices only with automatic control of the output voltage or current, e.g. flyback converters
  • H02M3/33523—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal using semiconductor devices only with automatic control of the output voltage or current, e.g. flyback converters with galvanic isolation between input and output of both the power stage and the feedback loop
• • 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
  • H02M1/0003—Details of control, feedback or regulation circuits
  • H02M1/0025—Arrangements for modifying reference values, feedback values or error values in the control loop of a converter

Definitions

• the invention relates to a method for operating a switching converter according to the so-called current-mode control, wherein a PWM controller, a current measuring signal for determining the turn-off of a converter switching element is supplied.
• the invention relates to a switching converter for carrying out the method.
• switching converter types such as boosters, buckets, flyback converters, flux converters, etc. They all have the principle in common, an input-side voltage by means of a converter switching element in faster
• a desired output voltage is provided at the output of the switching converter by means of a voltage regulator.
• a deviation of the actual voltage from the target voltage causes a change in the output signal of this voltage regulator.
• This output signal is in the current mode control proportional to a current value, the increasing current through the coil or the primary winding of the switching converter during a turn-on of the converter switching element should reach.
• the switching converter usually comprises a PWM controller, which on the one hand the transmitted output signal of the voltage regulator and on the other hand, a current measurement signal is supplied in proportion to the rising coil or primary winding current.
• the PWM controller compares these two signals and turns off the converter switching element if they match.
• the voltage output applied to the COMP input of the PWM controller is divided into one third of the transmitted output signal of the voltage regulator via an internal voltage divider of the PWM controller.
• the divided voltage is compared via an internal comparator of the PWM controller with the applied as voltage at the current measurement input (I-SENSE input) current measurement signal. If the current sense signal is greater than the divided voltage, the internal comparator tilts and the converter switching element is turned off.
• a current limitation is usually provided to protect the components for the rising current.
• this current limiting is accomplished by means of an internal Zener diode (e.g., breakdown voltage IV) which is arranged in parallel with the flow resistance of the internal voltage divider.
• an internal Zener diode e.g., breakdown voltage IV
• the internal Zener diode for current limiting has a tolerance of about 20% (for example, breakdown voltage 0.9V - 1.1V). This means that during a switching cycle of the switching converter of the rising coil or
• the invention has for its object to provide an improvement over the prior art for a method of the type mentioned.
• a known from the prior art switching converter should be further developed in such a way that it is suitable for carrying out the method according to the invention.
• this object is achieved by a method according to claim 1 and a switching converter according to claim 7.
• the method according to the invention provides that a voltage which is proportional to the current measuring signal is compared with a comparison voltage by means of a comparator and that the converter switching element is switched off when the comparison voltage is exceeded. It is therefore not the internal Zener diode of the PWM controller, but used in addition to the PWM controller arranged comparator for current limiting. This comparator is much cheaper than the extra cost of a PWM controller with a correspondingly accurate internal current limit. In this case, the comparator is given a precise reference voltage as a reference.
• the inventive method is implemented with a switching converter comprising a PWM controller for controlling a converter switching element, wherein a current measuring signal for determining the turn-off of the converter switching element is applied to a current measuring input of the PWM controller.
• a comparator is arranged, whose first input is connected to a measuring point at which a voltage proportional to the current measurement voltage is connected.
• the second input of the comparator is connected to a reference voltage source and at the comparator output is an output signal for switching off the converter switching element.
• a signal present at the output of the comparator which exceeds an internal current limit value of the PWM controller, is fed to the PWM controller for determining the switch-off times of the converter switching element.
• the internal current limit value defines the breakdown voltage of the internal Zener diode of the PWM controller.
• the switching converter is advantageously designed such that the output of the comparator is connected via a diode to the current measuring input of the PWM controller.
• the signal proportional to the rising coil or primary winding current is present, for example, as a voltage dropped across a shunt resistor at the comparator input. If the voltage regulator allows maximum power, the current increases until the voltage dropping across the shunt resistor reaches the value of the reference voltage. The output of the comparator changes from low to high, exceeding the tolerance limit of the breakdown voltage of the internal zener diode, causing the internal comparator of the PWM controller to tilt caused and subsequently the converter switching element is turned off.
• An improved method provides that if the comparison voltage is exceeded at the output of the
• Comparator applied high signal is supplied to an auxiliary switching element, which switches a control signal for driving the converter switching element in such a way that the converter switching element is turned off.
• the comparator voltage is present at the positive input of the comparator and the comparator voltage at the negative input of the comparator, and the output of the comparator is connected to the control terminal of the auxiliary switching element for switching over a control signal supplied to the converter switching element.
• an internal delay (about 200 ns) of the PWM controller is eliminated.
• the converter switching element thus switches off as soon as the additionally arranged comparator tilts, and not only when the internal comparator of the PWM controller registers an exceeding of the current limit.
• a positive feedback by means of a resistor and a capacitor at the positive input of the comparator ensures that the comparator remains stable for a longer than the delay of the PWM controller time span and there is no reconnection of the converter switching element.
• a further improvement of the method provides that when the comparison voltage is exceeded, a low signal present at the output of the comparator is fed to a first auxiliary switching element, which has a
• Control voltage for driving the converter switching element switches in such a way that the converter switching element is turned off.
• the low signal is supplied to a second auxiliary switching element, by means of which the PWM controller an auxiliary signal which exceeds an internal current limit value of the PWM controller is supplied to determine the switch-off of the converter switching element.
• the negative input of the comparator is proportional to the current measurement signal voltage and the positive
• Control voltage of the converter switching element is switched.
• an auxiliary signal is switched to the current measuring input of the PWM controller, wherein this auxiliary signal exceeds the tolerance upper limit of the internal current limit of the PWM controller.
• the auxiliary signal is derived from a reference voltage provided by the PWM controller, wherein a value above the tolerance upper limit of the breakdown voltage of an internal Zener diode of the PWM controller is set, for example by means of a voltage divider. Moreover, it is favorable if the comparison voltage is derived by means of a voltage divider from a reference voltage provided by the PWM controller.
• the flyback converter shown in Figure 1 comprises in a known manner a transformer having a primary winding Wl and a counter-wound secondary winding W2.
• a converter switching element S1 designed as an insulating layer field-effect transistor (MOS-FET) switches an intermediate circuit voltage formed from an alternating voltage to the primary winding W1.
• MOS-FET insulating layer field-effect transistor
• the gate terminal G of the converter switch Sl is connected to the switching signal output OUT of a PWM controller 1 (eg Unitrode Uc3842).
• the PWM controller 1 is supplied via a COMP terminal, which is transmitted by means of an optocoupler 3 output signal Reg of the secondary side voltage regulator 4.
• This voltage regulator 4 forms the desired signal for the current mode control by means of an adjustable Zener diode (eg REF 2.5V) from a rectified and smoothed output voltage Ua.
• the current measuring signal compared.
• the current measurement signal is formed by means of a shunt resistor R4, which is arranged between the source terminal S of the converter switching element Sl and a reference potential MP.
• the voltage dropping across this shunt resistor R4 is switched to the current measuring input I-SENSE via a first resistor Rl. If the rising current measurement signal reaches the predetermined desired value, the converter switching element S1 is switched off.
• FIG. 1 An embodiment of the solution according to the invention for avoiding these problems is shown in FIG.
• the construction of the flyback converter shown in FIG. 1 is supplemented by an additional comparator 2.
• the negative input of the comparator 2 is supplied with a reference voltage (eg 0.8V).
• the comparison voltage is conducted by means of a voltage divider formed from two resistors R2, R3 from a reference voltage available at a reference voltage output VREF of the PWM controller 1.
• At the positive terminal of the comparator 2 is that at the source terminal S of the converter switching element Sl tapped current measurement signal (ie, the voltage dropped across the shunt resistor R4). If the current measurement signal exceeds the value of the comparison voltage, the output signal of the comparator 2 changes from low to high.
• the high value of the output signal supplied via a diode to the current measuring input I-SENSE of the PWM controller 1 exceeds the value
• Zener diode which determines the internal current limit value of the PWM controller (e.g., 0.8V ⁇ 0.9V - 1.1V).
• the effective current limit value is thus determined by means of the predetermined reference voltage.
• the source terminal S of the converter switching element Sl continues to be connected via the first resistor Rl to the current measurement input I-SENSE of the PWM controller, so that at a target current specification of the voltage regulator 4 below the predetermined by the comparison voltage effective current limit value, the current mode control unaffected by the additional Comparator 2 works.
• a further embodiment of the invention illustrated in FIG. 3, provides for the arrangement of an auxiliary switching element S2, which is controlled by means of comparator 2.
• the comparison voltage is predetermined according to the above-described embodiment of the invention at the negative input of the comparator 2.
• the positive input is connected via a fifth resistor R5 to the source terminal S of the converter switching element Sl.
• the comparator 2 tilts from low to high when the voltage applied to the shunt resistor R4 exceeds the comparison voltage. As before, via a diode Dl the
• Output signal of the comparator 2 whose high value exceeds the tolerance upper limit of the internal current limitation of the PWM Controller 1, led to the current measurement input I-SENSE of the PWM controller 1.
• the output of the comparator 2 is connected to the gate of the auxiliary switching element.
• the auxiliary switching element switches from low to high as a result of the signal change.
• the source terminal of the auxiliary switching element S2 is connected to a connection point between the switching signal output OUT of the PWM controller 1 and the gate terminal G of the converter switching element Sl and the drain terminal of the auxiliary switching element S2 is connected to the reference potential MP.
• a resistor RO is arranged between the connection point and the switching signal output OUT.
• auxiliary switching element S2 thus causes the switching signal at the gate terminal G of the converter switching element Sl drops and the converter switching element Sl is turned off.
• the internal response delay (about 200ns) of the PWM controller is thus eliminated.
• the auxiliary switching element S2 remains switched on until the PWM controller 1 shuts off the switching signal at the switching signal output OUT as a result of exceeding the internal current limit at the current measuring input I-SENSE, ie during the mentioned response delay.
• the comparator 2 tilts from high to low.
• the output of the comparator 2 is in this case connected to the base terminal of a first PNP transistor
• Auxiliary switching element S3 connected.
• the signal change from high to low causes the first auxiliary switching element S3 to be switched on, as a result of which the switching signal at the gate connection G of the converter switching element S1 is again drawn to the reference potential MP; the converter switching element Sl is turned off.
• the output of the comparator 2 is connected to the gate terminal of a second auxiliary switching element S4 designed as a MOSFET.
• the source terminal of this auxiliary switching element S4 is connected to a reference voltage and the drain terminal is connected to the current measuring input I-SENSE of the PWM controller 1.
• Gate terminal is arranged a sixth resistor R6.
• a control voltage of the second auxiliary switching element S4 drops as soon as the output of the comparator 2 changes from high to low.
• the current measuring input I-SENSE of the PWM controller 1 is supplied with a voltage which exceeds the tolerance upper limit of the internal current limiting and thus leads to switching off at the switching signal output OUT.

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• Engineering & Computer Science (AREA)
• Power Engineering (AREA)
• Dc-Dc Converters (AREA)

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• 2007
  • 2007-12-20 AT AT0207207A patent/AT506273B1/de not_active IP Right Cessation
• 2008
  • 2008-10-17 US US12/809,188 patent/US8416589B2/en not_active Expired - Fee Related
  • 2008-10-17 WO PCT/EP2008/064009 patent/WO2009080382A1/de active Application Filing
  • 2008-10-17 EP EP08865225A patent/EP2223417A1/de not_active Withdrawn
  • 2008-10-17 CN CN200880121690.9A patent/CN101933218B/zh not_active Expired - Fee Related

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