JP2015095964A - Output current detection method of flyback converter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an output current detection method capable of detecting an output current of a flyback converter.SOLUTION: The method of detecting an output current Iof a flyback converter includes the steps of: detecting a voltage Uof a switching node in order to acquire a term Tof a switching cycle of the flyback converter and a term Tof an OFF stage of the switching cycle; and calculating the output current Ion the basis of an expression I=k×T/T(k=constant). According to this method, it is enough for a user only to detect the output current Iof the switching cycle of the flyback converter. Therefore, the output current Iof the flyback converter can be acquired by the expression without detecting any power of the flyback converter while using a detection resistor.

Description

The present invention relates to an output current detection method for a flyback converter, and more particularly to an output current detection method for a flyback converter in a discontinuous mode without reducing the efficiency of the flyback converter.

  Flyback converters are typically used as low power AC / DC power converters. Referring to FIG. 8, the flyback converter includes a primary circuit 30 and a secondary circuit 40. The primary circuit 10 includes a rectifier unit 31, a primary coil 32, a primary switch 33, a PWM control unit 34, and a primary capacitor 35.

The rectifier unit 31 is a full bridge circuit having two input terminals and two output terminals. The input terminal is configured to be connectable to _AC power VAC. The rectifier unit 31 pulsates the _AC power VAC. The DC power is converted to DC power, and the DC power is transmitted to the output terminal. The primary coil 32 and the primary switch 33 are connected in series between the output terminals of the rectifier unit 31. The PWM control unit 34 is connected to the primary switch 33 and controls on / off switching of the primary switch 33. The primary capacitor 35 is connected in series between the output terminals of the rectifier unit 31 in order to reduce ripple caused by pulsating DC power.

  The secondary circuit 40 includes two output terminals, a secondary coil 41, a diode 42, and a secondary capacitor 43. The output terminal is configured to be connectable to an electronic device. The secondary coil 41 has two ends. The diode 42 has an anode and a cathode. The anode of the diode 42 is connected to one end of the secondary coil 41, and the cathode of the diode 42 and the other end of the secondary coil 41 are respectively connected to the output terminal of the secondary circuit 40. Connected. The secondary capacitor 43 is connected in series between the output terminals of the secondary circuit 40.

The flyback converter connected to an electronic device, an AC power V _AC is turned on, the primary coil 32 takes the pulsating DC power from the rectifier unit 31, and outputs a voltage U _P. Secondary coil 41 is induced by the voltage _{U P,} and outputs the induced voltage _{U S.} Induced voltage U _S is filtered and the diode 42 by the secondary condenser, is rectified and supplies the output voltage U _OUT connected to the electronic device.

In general, the output voltage U _OUT of the flyback converter is controlled to a predetermined output voltage, but the output current I _OUT of the flyback converter is not fixed depending on the state of the electronic device connected to the flyback converter. The electronic device is usually connected to the flyback converter via a cable having an internal resistance. The internal resistance of the cable causes cable loss. In order to compensate the cable loss and adjust the output voltage U _OUT and protect the connected electronic device, it is necessary to monitor the output current I _OUT . In order to detect the output current I _OUT of the flyback converter, a power detection resistor 44 is usually used. The resistor 44 is connected in series to an electronic device connected to the flyback converter, and the output current I _OUT can be obtained by measuring the voltage difference between the two ends of the resistor 44. However, when the output current I _OUT flows through the resistor 44, the resistor 44 causes a power loss, and the efficiency of the flyback converter is reduced.

JP 2006-121885 A

  A main object of the present invention is to provide an output current detection method for a flyback converter that can detect the output current of the flyback converter without reducing the efficiency of the flyback converter.

In the output current detection method of the flyback converter according to the present invention, the flyback converter is:
1) A rectifier unit having two input terminals and two output terminals, the input terminal being configured to be connectable to AC power, converting AC power into pulsating DC power, and transmitting the DC power to the output terminal A rectifier unit;
A primary coil having two ends, wherein one end of the primary coil is connected to one of the output terminals of the rectifier unit;
A primary switch connected to the other end of the primary coil and the other output terminal of the rectifier unit;
Flyback converter, and ON phase, a PWM control unit having a OFF stage, the built-in operating cycle to provide a switching cycle consists of three stages of circuit disconnection phase (dead Stage), the period of the switching cycle T _SW A PWM control unit for setting the OFF stage period of the switching cycle of the flyback converter to T _OFF ;
A primary circuit having a primary capacitor connected in series between the two output terminals of the rectifier;
2) two output terminals configured to be connectable to an electronic device;
A secondary coil having two ends, one end of the secondary coil being connected to one of the output terminals of the secondary circuit;
A diode having an anode and a cathode, where the anode is connected to the other end of the secondary coil, the cathode is connected to the other output terminal of the secondary circuit, and the connection node between the diode and the secondary coil is a switching node. A secondary circuit having a diode; and a secondary capacitor connected in series between the two output terminals of the secondary circuit;
The method for detecting the output current I _OUT of the flyback converter according to the present invention includes the following steps.

Detecting the voltage at the switching node to obtain the cycle T _SW of the switching cycle of the flyback converter and the cycle T _OFF of the OFF stage of the switching cycle;
A method of detecting an output current of a flyback converter, comprising: calculating the output current I _OUT from the period T _SW and the period T _OFF by the formula I _OUT = k · T ² _OFF / T _SW (k is a constant).

By using the method for detecting the output current I _OUT of the flyback converter according to the present invention, the user only detects the voltage of the switching node 24 and the cycle T _SW of the flyback converter and the switching cycle OFF. The stage period T _OFF can be obtained, and then the output current I _OUT of the flyback converter can be obtained using an equation without also detecting the flyback converter power with a sensing resistor.

1 is a circuit diagram of a first embodiment of a flyback converter according to the present invention. A is a waveform chart of the current of the primary coil of the flyback converter of FIG. B is a waveform chart of the voltage of the primary coil of the flyback converter of FIG. C is a waveform chart of the voltage at the switching node of the flyback converter of FIG. 1 and the output voltage of the flyback converter of FIG. D is a waveform chart of the output current of the secondary coil of the flyback converter of FIG. FIG. 2 is an operation circuit diagram of the flyback converter of FIG. 1 and shows an operation at an ON stage of a switching cycle of the flyback converter. FIG. 2 is an operation circuit diagram of the flyback converter of FIG. 1 and shows an operation at an OFF stage of a switching cycle of the flyback converter. FIG. 2 is an operation circuit diagram of the flyback converter of FIG. 1 and shows an operation in a circuit disconnection stage of a switching cycle of the flyback converter. It is a circuit diagram of the 2nd example of the flyback converter concerning the present invention. A is a waveform chart of the current of the secondary coil of the flyback converter of FIG. B is a waveform chart of the voltage of the secondary coil of the flyback converter of FIG. C is a waveform chart of the voltage at the switching node of the flyback converter of FIG. 6 and the output voltage of the flyback converter of FIG. D is a waveform chart of the output current of the secondary coil of the flyback converter of FIG. 1 is a circuit diagram of a flyback converter having a sensing resistor according to the prior art. FIG.

  Referring to FIG. 1, the flyback converter according to the first embodiment of the present invention includes a primary circuit 10 and a secondary circuit 20. The primary circuit 10 includes a rectifier unit 11, a primary coil 12, a primary switch 13, a PWM control unit 14, and a primary capacitor 15.

Rectifier unit 11 is a full bridge rectifier circuit having two input terminals and two output terminals, the input terminal is configured to be connectable to an AC power V _AC, rectifier unit 11, the AC power V _AC, pulsating DC It converts into electric power and transmits the direct-current power to the output terminal. The primary coil 12 and the primary switch 13 are connected in series between the output terminals of the rectifier unit 11. The PWM control unit 14 is electrically connected to the primary switch 13 and controls switching of the primary switch 13. That is, the PWM control unit 14 has a built-in operation cycle which is a switching cycle given to the flyback converter by three stages of an ON stage, an OFF stage, and a circuit disconnection stage. A primary capacitor 15 is connected in series between the output terminals of the rectifier unit 11 in order to reduce ripple caused by pulsating DC power.

The secondary circuit 20 includes two output terminals, a secondary coil 21, a diode 22, and a secondary capacitor 23. The output terminal is configured to be connectable to the electronic device. The secondary coil 21 has two ends, represents the inductance of the secondary coil 21 in L _S. The diode 22 has an anode and a cathode. The anode of the diode 22 is connected to one end of the secondary coil 21, and the cathode of the diode 22 and the other end of the secondary coil 21 are connected to the output terminal of the secondary circuit 20, respectively. Is done. The forward voltage of the diode 22 represented by _{U D.} The secondary capacitor 23 is connected in series between the output terminals of the secondary circuit 20. A node between the secondary coil 21 and the diode 22 becomes a switching node 24 of the flyback converter.

  The working state of the flyback converter at different stages of the switching cycle is described below.

Referring to FIGS. 2A to 2D, the vertical axis of FIG. 2A represents the current of the primary coil 12. The vertical axis of FIG. 2B shows the voltage _{U P} of the primary coil 12. The vertical axis of FIG. 2C represents the voltage U _S of the switching node 24, the output voltage U _OUT is a predetermined output voltage of the flyback converter secondary circuit. The vertical axis of FIG. 2D shows an output current _{I S} of the secondary coil 21. All horizontal axes show time. The period of the switching cycle is _TSW . The period of the ON stage is _TON . The cycle of the OFF stage is T _OFF . The period of the circuit disconnection stage is T _dead .

Further, from FIG. 2A-2D, the period _{T SW, T ON, T OFF} , T dead is determined based on the voltage _{U S} of the switching nodes 24, i.e., the period _{T SW, T ON, T OFF} , T dead is it will be appreciated that be obtained by detecting the voltage U _S of the switching node 24.

In order to obtain the first period T _{OFF, the} constant voltage U _S1 , and the second period T _SW , the following steps can be performed.
(A) by detecting the voltage _{U S,} comprising the steps of: voltage _{U S} acquires the output voltage _{U OUT} is greater than the first period, the first period and _{T OFF,} the voltage U during the first period _{T OFF} there in the step of acquiring the second period until the beginning of and (b) following the first period _{T OFF} following the same first period _{T OFF} from the beginning of the first period _{T OFF;} the _S step a constant voltage _{U S1} The step of setting the second cycle to _TSW .

Furthermore, the period from the end of the first period _{T OFF} until the end of the next first period _{T OFF} following the same first period _{T OFF} is set to _{T SW.}

The operation when the flyback converter is in the ON stage will be described with reference to FIG. 3. The primary switch 13 is turned on by the PWM control unit 14, and the rectifier unit 11 forms a primary loop together with the primary coil 12 and the primary switch 13. The rectifier unit 11 outputs a current _IP that flows through the primary loop. Current _{I P} gradually increases from 0A in ON stage, the primary coil 12 is charged by the current _{I P.} The secondary capacitor 23 forms a secondary loop with the connected electronic device, and releases the stored energy. The secondary capacitor 23 outputs a current I _OUT that flows through the secondary loop. The secondary capacitor 23 outputs the output voltage U _OUT to the electronic device connected to the flyback converter.

The operation when the flyback converter is in the OFF stage will be described with reference to FIG. 4 as well. The primary switch 13 is turned off by the PWM control unit 14 and the primary loop is released. The primary coil 12, including the release of stored energy in the ON stage, and outputs a voltage U _P. Secondary coil 21 of the secondary circuit 20 is induced by the voltage _{U P,} and outputs the induced voltage _{U S1} to the switching node 24. Diode 22 becomes conductive by the voltage _{U S1,} the secondary coil 21 outputs an induced current _{I S,} the voltage obtained by subtracting the forward voltage _{U D} from the voltage _{U S1} as the output voltage _{U OUT (U} S1 -U _D = U _OUT ). The induced current I _S decreases from the high current I _S1 to 0 A at the OFF stage, and the waveform of the induced current I _S is a triangular wave. Induced current _{I S} is shunted to the output current _{I OUT} and capacitor current _{I C1,} the output current _{I OUT} and capacitor current _{I C1,} respectively, and an electronic device connected to a flyback converter, it flows to the secondary condenser 23. Secondary capacitor 23 is charged by current I _C1 .

The operation when the flyback converter is in the circuit disconnection stage will be described with reference to FIG. 5 as well. Since the primary switch 13 is still OFF, the primary loop is still released. The primary coil 12 emits the remaining energy, the voltage U _P ringing (ring). Also ringing voltage U _S of the switching node 24, the diode 22 stops conducting, the secondary coil 21 does not output the induced current I _S. The secondary capacitor 23 forms a secondary loop again with the electronic device connected to the flyback converter, and releases the stored energy. The secondary capacitor 23 outputs an output current I _OUT that flows through the secondary loop. The secondary capacitor 23 also outputs the output voltage U _OUT to the electronic device connected to the flyback converter.

  After completion of the circuit disconnection phase, the primary switch 13 is turned on by the PWM control unit 14, the ON phase starts again, and the switching cycle of the flyback converter is continued.

A method for detecting the output current I _OUT of the flyback converter according to the present invention will be described using the following mathematical development.

The output voltage U _OUT is controlled to a predetermined output voltage, and the total output energy E _OUT of a single switching cycle of the flyback converter is U _{OUT 'I OUT'} T _SW . Figures 2C and 2D, only outputs a current I _S is the secondary coil 21 is OFF stage, i.e., the secondary coil 21, in the OFF phase of the flyback converter of a switching cycle that only releases energy Will understand. The energy E _S released by the secondary coil 21 in the OFF stage of the switching cycle is U _{S1 ′ I Save ′} T _OFF , and I _Save is the average induced current I _S of the secondary coil 21 in the OFF stage. Furthermore, the diode 22, and because the forward voltage _{U D,} the OFF phase of the switching cycle, consumes diode energy _{E D,} consumed diode energy _{E D} is _{U D'I Save'T OFF} (E _D = U _{D 'I Save'} T _OFF ). Energy _{E S} emitted by the coil 21 is equal to the sum of the output energy _{E OUT} and diode energy _{E D} in duty cycle:
E _S = E _OUT + E _D = U _{S1 'I Save'} T _OFF = U _{OUT 'I OUT'} T _SW + U _{D 'I Save'} T _OFF
(U _S1 -U _D ) I _{Save '} T _OFF = U _{OUT' I OUT '} T _SW

From FIG. 2D, it can be seen that in the OFF phase, the induced current I _S drops from the high current I _S1 to zero.
dI _S1 / dt = U _S1 / Ls
dI _S1 = U _S1 / Ls′dt
I _S1 = U _S1 / Ls′T _OFF

Then, the waveform of the induced current I _S is triangular. That is, the average current I _Save is expressed by the following formula.
I _Save = 1 / 2′I _S1 = 1 / 2′U _S1 / Ls′T _OFF .
(U _S1 -U _D ) '1 / 2'I _S1' T _OFF = 1 / 2'U _S1 / Ls'T _OFF = U _{OUT 'I OUT'} T _SW
(U _S1 -U _D ) 'U _S1 T ² _OFF / 2L _S = U _{OUT' I OUT '} T _SW
I _OUT = (U _S1 −U _D ) ′ U _S1 T ² _OFF / 2L _S U _OUT T _SW

Here, (U _S1 -U _D ) is equal to U _OUT . Therefore,
The _{I OUT = (U S1 -U D} )'U S1 T 2 OFF / 2L S U OUT T SW = U S1 T 2 OFF / 2L S T SW.

Here, U _S1 and Ls are all known constants.
The above formula is also expressed by the following formula.
I _OUT = U _S1 T ² _OFF / 2L _S T _SW = k′T ² _OFF / T _SW
Here, k is a constant and is equal to U _S1 / 2Ls.

The method for detecting the output current I _OUT of the flyback converter according to the present invention described with reference to FIG. 1 includes the following steps.
And the period _{T SW} of (a) a flyback converter of a switching cycle, in order to obtain the period _{T OFF} of the OFF phase of the switching cycle, step detects the voltage _{U S} of the switching node 24; in and (b) the following formula Calculating the output current I _OUT based on the output current I _OUT ;
I _OUT = k · T ² _OFF / T _SW

  Furthermore, with reference to FIG. 6, the flyback converter which concerns on 2nd Example is demonstrated. The difference between this flyback converter and the first embodiment is that the cathode of the diode 22 is connected to one end of the secondary coil, and the anode of the diode 22 and the other end of the secondary coil 21 are respectively output from the secondary circuit 20. It is to be connected to the terminal. A node between the secondary coil 21 and the diode 22 becomes a switching node 24 of the flyback converter.

Figures 7A-7D, the period _{T SW, T ON, T OFF} , since _{T dead} is determined by the voltage _{U S} of the switching nodes 24, by detecting the voltage _{U S} of the switching node 24, the period _T SW, T It will be understood that _ON , T _OFF , and T _dead can be obtained.

The first cycle T _{OFF, the} constant voltage U _S1 , and the second cycle T _SW can be acquired by performing the following steps.
By detecting (a) a voltage _{U S,} comprising the steps of: voltage _{U S} obtain the following first period zero voltage (ground voltage), the first period is set to _{T OFF,} during the first period _{T OFF} step voltage _{U S} is constant voltage _{U S1;} and (b) obtaining a second period until the beginning of the first period _{T OFF,} which from the beginning of the next subsequent to the first period _{T OFF} of the first period _{T OFF} And the second period is _TSW .

Furthermore, the period from the end of the first period _{T OFF} until the end of the next first period _{T OFF} following the same first period _{T OFF} is set to _{T SW.}

A method for detecting the output current I _OUT of the flyback converter according to the present invention shown in FIG. 6 will be described using the following mathematical development.
The output voltage U _OUT is controlled to a predetermined output voltage, and the total output energy E _OUT of a single switching cycle of the flyback converter is U _{OUT 'I OUT'} T _SW . Figures 7C and FIG. 7D, the secondary coil 21 only output current I _S in OFF stage, i.e., the secondary coil 21 will only emit energy in OFF phase of the flyback converter of a switching cycle You will understand that. The energy E _S released by the secondary coil 21 in the OFF stage of the switching cycle is −U _{S1 ′ I Save ′} T _OFF , and I _Save is the average induced current I _S of the secondary coil 21 in the OFF stage. . Furthermore, the diode 22, and because the forward voltage _{U D,} the OFF phase of the switching cycle, consumes diode energy _{E D,} is consumed diode energy _{E D = U D'I Save'T} OFF. Energy _{E S} emitted by the coil 21 is equal to the sum of the output energy _{E OUT} and diode energy _{E D} in duty cycle:
_{E S = E OUT + E D} = -U S1'I Save'T OFF = U OUT'I OUT'T SW + U D'I Save'T OFF
(-U _S1 -U _D ) I _{Save '} T _OFF = U _{OUT' I OUT '} T _SW

From FIG. 7D, it can be seen that in the OFF stage, the induced current I _S decreases from the high current I _S1 to 0A.
dI _S1 / dt = −U _S1 / Ls
dI _S1 = −U _S1 / Ls′dt
I _S1 = −U _S1 / Ls′T _OFF
Then, the waveform of the induced current I _S is a triangle. That is, the average current I _Save is expressed by the following formula.
I _Save = 1 / 2′I _S1 = 1 / 2′−U _S1 / Ls′T _OFF .
_{(-U S1 -U D) '1 /} 2'I S1'T OFF = 1 / 2'-U S1 / Ls'T OFF'T OFF = U OUT'I OUT'T SW
^{_{(-U S1 -U D) '-}} U S1 T 2 OFF / 2L S = U OUT'I OUT'T SW
I _OUT = (U _S1 −U _D ) ′ U _S1 T ² _OFF / 2L _S U _OUT T _SW

Here, (U _S1 + U _D ) is equal to −U _OUT . Therefore,
I _OUT = (U _S1 + U _D ) ′ U _S1 T ² _OFF / 2L _S U _OUT T _SW = −U _S1 T ² _OFF / 2L _S T _SW

Here, -1 / _2, and _{U S1,} Ls are all known constants.
The above formula is also expressed by the following formula.
Formula I _OUT = −U _S1 T ² _OFF / 2L _S T _SW = k′T ² _OFF / T _SW
Here, k is a constant and is equal to -U _S1 / 2Ls.
The method for detecting the output current I _OUT of the flyback converter according to the present invention described with reference to FIG. 6 includes the following steps.
And the period _{T SW} of (a) a flyback converter of a switching cycle, in order to obtain the period _{T OFF} of the OFF phase of the switching cycle, step detects the voltage _{U S} of the switching node 24; in and (b) the following formula Calculating the output current I _OUT based on the output current I _OUT ;
I _OUT = k · T ² _OFF / T _SW

As described above, in the method for detecting an output current I _OUT of the flyback converter according to the present invention, the user detects the voltage U _S of the switching node 24, then, a flyback converter with the above formula Output current I _OUT can be obtained. That is, the output current I _OUT of the flyback converter can be acquired without being detected by the detection resistor.

Claims (9)

An output current detection method for a flyback converter,
And the period T _SW of the flyback converter of a switching cycle, in order to obtain the period T _OFF of the OFF phase of the switching cycle, and detecting a voltage of said switching nodes of the flyback converter;
A step of calculating an output current I _OUT of the flyback converter based on the following equation:
I _OUT = k · T ² _OFF / T _SW
Where
k: constant T _OFF : cycle of the OFF stage of the switching cycle of the flyback converter L _S : inductance of a secondary coil of the flyback converter T _SW : cycle of the switching cycle of the flyback converter
The step of detecting the voltage at the switching node of the flyback converter includes:
By detecting the voltage of the switching node, obtaining a first period T _OFF where the voltage of the switching node is greater than U _OUT , the voltage of the switching node during the first period T _OFF is a constant voltage U A step of _S1 ,
Claim 1, characterized by comprising, a step of acquiring the second period _{T SW} until the beginning of the next first period _{T OFF} following the same first period _{T OFF} from the beginning of the first period _{T OFF} The output current detection method of the described flyback converter.
The step of detecting the voltage at the switching node of the flyback converter includes:
By detecting the voltage of said switching nodes, comprising the steps of voltage of the switching node to obtain a first period T _OFF of less than or equal to zero voltage, the first period T voltage constant voltage of said switching nodes in _OFF U A step of _S1 ,
Claim 1, characterized by comprising, a step of acquiring the second period _{T SW} until the beginning of the next first period _{T OFF} following the same first period _{T OFF} from the beginning of the first period _{T OFF} The output current detection method of the described flyback converter.
The step of detecting the voltage at the switching node of the flyback converter includes:
Detecting a voltage of the switching node to obtain a first period T _{OFF in} which the voltage of the switching node is greater than U _OUT , wherein the voltage of the switching node in the first period T _OFF is a constant voltage a step which is U _S1,
Claim 1, characterized by comprising, a step of acquiring the second period _{T SW} from the end of the first period _{T OFF} until the end of the next first period _{T OFF} following the same first period _{T OFF} The output current detection method of the described flyback converter.
The step of detecting the voltage at the switching node of the flyback converter includes:
Detecting a voltage of the switching node to obtain a first period T _{OFF in} which the voltage of the switching node is less than zero voltage, and the voltage of the switching node in the first period T _OFF is a constant voltage U A step of _S1 ,
Claim 1, characterized by comprising, a step of acquiring the second period _{T SW} from the end of the first period _{T OFF} until the end of the next first period _{T OFF} following the same first period _{T OFF} The output current detection method of the described flyback converter.
The constant k is equal to U _S1 / 2Ls, the constant voltage U _S1 is the voltage of the switching node of the flyback converter in the OFF stage of the switching cycle of the flyback converter, and the inductance L _S is 3. The flyback converter output current detection method according to claim 2, wherein the output current is an inductance of the secondary coil of the flyback converter.
The constant k is equal to U _S1 / 2Ls, the constant voltage U _S1 is the voltage of the switching node of the flyback converter in the OFF stage of the switching cycle of the flyback converter, and the inductance L _S is 5. The flyback converter output current detection method according to claim 4, wherein the output current is an inductance of the secondary coil of the flyback converter.
The constant k is equal to −U _S1 / 2Ls, the constant voltage U _S1 is a voltage of the switching node of the flyback converter in the OFF stage of the switching cycle of the flyback converter, and the inductance L _S is 4. The flyback converter output current detection method according to claim 3, wherein the flyback converter has an inductance of the secondary coil.
The constant k is equal to −U _S1 / 2Ls, the constant voltage U _S1 is a voltage of the switching node of the flyback converter in the OFF stage of the switching cycle of the flyback converter, and the inductance L _S is 6. The output current detection method for a flyback converter according to claim 5, wherein the output current is an inductance of the secondary coil of the flyback converter.

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