TWI407676B - Control circuit for detecting the output current of the power converter circuit - Google Patents

Abstract

A control circuit of a power converter circuit of the present invention has the feedback control by detecting the output current. The power converter circuit includes a transformer and a power switch. The control circuit generates a reference voltage of the output current according to a feedback control voltage, and compares a detection voltage of the output current with the reference voltage of the output current so as to generate a turn-off signal of the power switch. In addition, the control circuit converts the reference voltage of the output current to a current signal according to a reflection voltage of the transformer, and generates the feedback control voltage according to the voltage value of the current signal.

Description

Control circuit for detecting output current of power converter circuit

The invention relates to a power converter circuit, in particular to a control circuit for detecting an output current of a power converter circuit.

Please refer to FIG. 1. FIG. 1 is a schematic diagram of a prior art power converter circuit 10. The power converter circuit 10 is a flyback power converter circuit including a transformer 11, a power switch 12, a diode 13 and a capacitor 14. The transformer 11 includes a primary side coil NP, a secondary side coil NS, and an auxiliary side coil NA. The primary side coil NP is electrically connected to the input voltage VIN, and the auxiliary side coil NA is electrically connected to the resistor 15 and the resistor formed by the resistor 16. The road is used to generate the reflected voltage VDET of the transformer 11. The power switch 12 is controlled by a drive voltage VPWM for adjusting the output voltage of the power converter circuit 10 and the output current.

Please refer to FIG. 2, and FIG. 2 is an operational waveform diagram of the power converter circuit of FIG. When the driving voltage VPWM turns on the power switch 12 (logic high level), the primary side current IP is generated, and the energy is stored in the transformer 11, and the waveform of the power converter circuit at this stage is as shown in the period T1 of FIG. Show. The peak IP1 of the primary current IP can be expressed as equation (1):

Where LP is the inductance value of the primary side coil NP of the transformer 11, and TON is the conduction time of the driving voltage VPWM.

When the driving voltage VPWM turns off the power switch 12 (logic low level), the energy stored in the transformer 11 is released to the secondary side of the transformer 11, and is discharged to the output of the power converter circuit via the diode 13 to generate The secondary side current IS, the waveform of the power converter circuit at this stage is as shown in the period T2 of Fig. 2. The peak ISSW of the secondary side current IS can be expressed as Equation (2), and the discharge time TDS of the transformer 11 can be expressed as Equation (3), which also represents the discharge time of the secondary side current IS:

Where VO is the output voltage of the power converter circuit, VF is the forward bias of the diode 13, and LS is the inductance of the secondary side coil NS of the transformer 11.

At the same time, an auxiliary voltage VAUX is generated on the auxiliary side coil NA of the transformer 11, and the relationship between the auxiliary voltage VAUX and the reflected voltage VDET can be expressed as equation (4):

At the same time, the energy stored in the transformer 11 will charge the parasitic capacitance CJ of the power switch 12, and a voltage VDS will be generated at both ends of the parasitic capacitance CJ, which can be expressed as equation (5):

Wherein VIN is the input voltage of the power converter circuit, and TNA, TNP, and TNS are the number of turns of the auxiliary side coil NA, the primary side coil NP, and the secondary side coil NS of the transformer 11, respectively, and R1 and R2 are the resistor 15 and the resistor, respectively. The resistance value of 16.

When the energy stored in the transformer 11 is completely released, the secondary side current IS drops to zero, and the waveform of the power converter circuit at this stage is as shown in the period T3 of Fig. 2. The voltage VDS drops to the valley voltage at the end of the period TQ, and the falling slope of the voltage VDS is determined by the resonance frequency of the inductance of the primary side coil NS and the capacitance of the power switch 13. When the voltage VDS begins to drop, the auxiliary voltage VAUX begins to decrease. The auxiliary voltage VAUX is related to the voltage VDS and can be expressed by the formula (6):

Therefore, the discharge time TDS in the equation (2) can be obtained from the falling edge of the driving voltage VPWM to the falling corner of the auxiliary voltage VAUX.

In general, the power converter circuit must provide electrical isolation between the primary side and the secondary side. If the secondary side feedback circuit is used to detect the output voltage, an optical coupler is required. On the other hand, since the controller of the power converter circuit uses the auxiliary side coil of the transformer to supply power, the voltage of the auxiliary side has a certain ratio with the voltage of the secondary side. Therefore, the controller of the power converter circuit adjusts the output voltage of the power converter circuit according to the voltage of the auxiliary side and the output current can eliminate the optical coupler and the feedback circuit of the secondary side.

Accordingly, it is an object of the present invention to provide a control circuit for detecting an output current of a power converter circuit.

The invention provides a control circuit for a power converter circuit, the power converter circuit comprising a power switch and a transformer, the control circuit comprising an output current control unit, a leading edge shielding unit, a comparator, and a first magnetic A hysteresis comparator, a first voltage to current converter, and a first operational amplifier. The output current control unit generates a reference voltage of an output current according to a feedback control voltage. The leading edge shielding unit is electrically connected to the power switch to generate a detection voltage of an output current. The comparator is electrically connected to the output current control unit and the leading edge shielding unit for comparing the detected voltage of the output current with a reference voltage of the output current to generate a cutoff signal. The first hysteresis comparator is configured to receive a reflected voltage of the transformer and compare the reflected voltage with a first reference voltage to generate a first comparison voltage. The first voltage-to-current converter is electrically connected to the output current control unit for converting the reference voltage of the output current into a first current signal according to the first comparison voltage. The first operational amplifier is electrically connected to the voltage to current converter for comparing the voltage value of the first current signal with a second reference voltage to generate the feedback control voltage.

Please refer to FIG. 3, which is a schematic diagram of the power converter circuit 20 of the present invention. The power converter circuit 20 is a flyback power converter circuit including a transformer 21, a diode 23, a capacitor 24, and a controller 30. The transformer 21 includes a primary side coil NP, a secondary side coil NS, and an auxiliary side coil NA. The primary side coil NP is electrically connected to the input voltage VIN, the secondary side coil NS is electrically connected to the diode 23 and the capacitor 24 for generating an output voltage, and the auxiliary side coil NA can generate the auxiliary voltage VAUX and is electrically connected to The diode 31 and the capacitor 32 are used to provide the power required by the controller 30. The controller 30 includes six pins of VDD, SWD, GND, ZCD, IST, ISN, and the like. The pin ZCD is electrically connected to the voltage dividing circuit formed by the resistor 25 and the resistor 26 for receiving the reflected voltage VDEF of the transformer 21. The pin IST is electrically connected to the capacitor 33 and the resistor 34 for controlling the output current. The pin ISN is electrically connected to the resistor 35 for detecting the output current. The pin SWD is electrically connected to the primary side coil NP for adjusting the output voltage of the transformer 21 and the output current.

Please refer to FIG. 4, which is a circuit block diagram of the controller 30 of the present invention. The controller 30 includes a comparator 41, a first operational amplifier 42, a second operational amplifier 43, a leading edge blanking (LEB) unit 44, an output current control unit 45, a first hysteresis comparator 51, and a timing unit 52. The SR flip-flop 61, the buffer 62, the power switch 63, the voltage to current converter 71, the capacitor 72, the resistor 73, the hysteresis comparator 74, the switch 75, the inverse OR gate 76, and the capacitor 77. The leading edge shielding unit 44 is electrically connected to the power switch 63 for preventing an unstable output current from being detected to generate a detection voltage of the output current, and the output current control unit 45 generates a reference of the output current according to the feedback control voltage VCMP. The comparator 41 is electrically connected to the output current control unit and the leading edge shielding unit for comparing the detection voltage of the output current with the reference voltage of the output current to generate the cutoff signal SOFF. On the other hand, the voltage to current converter 71 receives the ZCD voltage to generate the current signal I2, the resistor 73 receives the current signal I2 to generate the reflected voltage received by the pin ZCD, and the first hysteresis comparator 51 reflects the reflected voltage with the first reference voltage (0.4). V) performing a comparison to generate a first comparison voltage V1, and the timing unit 52 provides a timing signal according to the first comparison voltage V1 to generate a conduction number SON. The set (SET) terminal of the SR flip-flop 61 is electrically connected to the timing unit 52. The reset (RESET) terminal of the SR flip-flop 61 is electrically connected to the comparator 41, and the output of the SR flip-flop 61 is based on the cutoff signal. The SOFF and the conduction signal number SON generate a driving signal, and the driving signal generates the driving voltage VPWM of the power switch 62 through the buffer 62.

Please refer to Figure 5, and Figure 5 is the voltage waveform diagram of Figure 4. When the power switch 63 is turned on, the primary side coil NP of the transformer 21 gradually has a current flowing, and energy is stored in the transformer 21 at this time. Since the polarity of the primary side coil NP and the secondary side coil NS are opposite, the diode 23 is reversely biased at this time, so no energy is transferred to the load, and the output energy is supplied from the capacitor 24. When the driving voltage VPWM turns on the power switch 63 (logic high level), that is, the primary side current IP is generated, the energy is stored in the transformer 21, and the magnetic flux density is increased from the residual magnetization to the operating peak, so the magnetic flux voltage VFX It starts to rise from a voltage of 1V. The ZCD voltage can be used to indicate the reflected voltage of the transformer when the power switch 63 is turned off. When the driving voltage VPWM turns off the power switch 63 (logic low level), the energy stored in the transformer 21 is released to the secondary side of the transformer 21, and is discharged to the output terminal of the power converter circuit via the diode 23, resulting in The secondary side current IS, at which time the ZCD voltage is converted to a positive value, and the magnetic flux density is lowered from the operating peak to the original residual magnetization, so the magnetic flux voltage VFX starts to drop, and the switch 75 is turned on to start sampling the reflected voltage. When the magnetic flux voltage VFX falls below the voltage of 1.1 V, the switch 75 is turned off, so the reflected voltage at this time is held as the feedback voltage VFB. When the energy stored in the transformer 21 is completely released, the secondary side current IS drops to zero. As the secondary side current IS disappears, the ZCD voltage begins to drop and oscillates around the zero voltage, and by the control of the timing unit 52, the power switch 63 will be turned on again after the ZCD voltage has elapsed for a predetermined time. Therefore, the output current of the transformer 21 can be calculated based on the discharge time TDS of the transformer 21 and the secondary side current IS, and feedback control can be performed.

Please refer to Figures 3 to 5 again. During the period in which the power switch 63 is turned on, the primary side current IP increases linearly. When the power switch 63 is turned off, the primary side current IP will drop to zero, at which time the magnetic flux voltage VFX begins to decrease, the polarity of the voltage on the transformer 21 will reverse, and the diode 23 will be turned on, which will be stored in the transformer. Energy is transferred to capacitor 24 and to the load. During the turn-off of the power switch 63, the secondary side current IS drops from a maximum value to zero. Therefore, the controller 30 will be held as the feedback voltage VFB according to the reflected voltage when the secondary side current IS drops to zero, and the first voltage to current converter during the period in which the secondary side current IS falls from the maximum value to zero. The reference voltage I* of the output current is converted into a current signal I1 to generate an IST voltage, and the feedback control voltage VCMP is generated by the first operational amplifier 42 and the second operational amplifier 43.

In the embodiment of the present invention, the output current control unit 45 generates a reference voltage I* of the output current according to the feedback control voltage VCMP as shown in the formula (7):

I*= MAX {( VCMP -0.6)/4,0.25} (7)

When the reflected voltage is greater than the first reference voltage (0.4V), the transformer 21 generates a secondary side current IS during which the first voltage to current converter 46 converts the reference voltage I* of the output current into a first comparison voltage V1. The current signal I1, the current signal I1 is output to the pin IST, the capacitor 33 is used to average the current signal I1, and the resistor 34 receives the current signal I1 to generate the IST voltage. The first operational amplifier 42 compares the IST voltage with a second reference voltage (3V) to generate a feedback control voltage VCMP. Since the IST voltage is proportional to the reference voltage I* of the output current, the output current of the transformer 21 can be controlled by adjusting the resistance value of the resistor 34. On the other hand, the voltage-to-current converter 71 receives the ZCD voltage to generate a current signal I2, the capacitor 72 receives the current signal I to generate a magnetic flux voltage VFX, and the magnetic flux voltage VFX is used to indicate a change in the flux density of the transformer 21. The hysteresis comparator 74 compares the magnetic flux voltage VFX with the third/fourth reference voltage 1V/1.1V to generate a second comparison voltage V2, and the inverse gate 76 receives the V2 comparison voltage and the driving signal to generate the switching signal SSW. The switch 75 samples the reflected voltage according to the switching signal SSW as the feedback voltage VFB, and holds the feedback voltage VFB by the capacitor 77. The second operational amplifier 43 compares the feedback voltage VFB with a second reference voltage (3V) to generate a feedback control voltage VCMP. Therefore, the controller 30 of the present invention performs feedback control based on the output current of the transformer 21 and the output voltage.

In summary, the control circuit of the power converter circuit of the present invention can detect the output current for feedback control. In an embodiment of the invention, the power converter circuit includes a power switch and a transformer, the control circuit includes an output current control unit, a leading edge shielding unit, a comparator, a first hysteresis comparator, and a A first voltage to current converter and a first operational amplifier. The output current control unit generates a reference voltage of the output current according to a feedback control voltage, and the comparator compares the detection voltage of the output current generated by the leading edge shielding unit with a reference voltage of the output current to generate the power. The cutoff signal of the switch. The first hysteresis comparator generates a first comparison voltage according to the reflected voltage of the transformer, and the first voltage to current converter converts the reference voltage of the output current into a current signal according to the first comparison voltage, the first The operational amplifier generates the feedback control voltage based on the voltage value of the current signal. In addition, the controller of the power converter circuit of the present invention detects the output voltage of the power converter circuit according to the voltage of the auxiliary side coil of the transformer, thereby eliminating the optical coupler and the feedback circuit of the secondary side.

The above are only the preferred embodiments of the present invention, and all changes and modifications made to the scope of the present invention should be within the scope of the present invention.

10, 20. . . Power converter circuit

11, 21. . . transformer

12, 63. . . Power switch

13, 23, 31. . . Dipole

14, 24, 32, 33, 72, 77. . . capacitance

15, 16, 25, 26, 34, 35, 73. . . resistance

30. . . Controller

41. . . Comparators

42. . . First operational amplifier

43. . . Second operational amplifier

44. . . Leading edge shielding unit

45. . . Output current control unit

46. . . First voltage to current converter

51. . . First hysteresis comparator

52. . . Timing unit

61. . . SR flip-flop

62. . . buffer

63. . . Power switch

71. . . Second voltage to current converter

74. . . Second hysteresis comparator

75. . . switch

76. . . Reverse or gate

ZCD, IST, ISN, VDD, SWD, GND. . . stitch

NP. . . Primary side coil

NS. . . Secondary side coil

NA. . . Auxiliary side coil

VIN. . . Input voltage

VAUX. . . Auxiliary voltage

VDET. . . Reflected voltage

VPWM. . . Driving voltage

CJ. . . Parasitic capacitance

VDS. . . Power switch signal

IP. . . Primary current

IS. . . Secondary current

I1, I2. . . Current signal

SON. . . Communication number

SOFF. . . Cutoff signal

SSW. . . Switch signal

V1, V2. . . Comparison voltage

VPWM. . . Driving voltage

VFB. . . Feedback voltage

VFX. . . Flux voltage

VCMP. . . Feedback control voltage

Figure 1 is a schematic diagram of a prior art power converter circuit.

Fig. 2 is an operational waveform diagram of the power converter circuit of Fig. 1.

Figure 3 is a schematic diagram of the power converter circuit of the present invention.

Figure 4 is a block diagram of the controller of the present invention.

Figure 5 is a voltage waveform diagram of Figure 4.

30. . . Controller

41. . . Comparators

42. . . First operational amplifier

43. . . Second operational amplifier

44. . . Leading edge shielding unit

45. . . Output current control unit

46. . . First voltage to current converter

51. . . First hysteresis comparator

52. . . Timing unit

61. . . SR flip-flop

62. . . buffer

63. . . Power switch

71. . . Second voltage to current converter

72, 77. . . capacitance

73. . . resistance

74. . . Second hysteresis comparator

75. . . switch

76. . . Reverse or gate

I1, I2. . . Current signal

SON. . . Communication number

SOFF. . . Cutoff signal

SSW. . . Switch signal

V1, V2. . . Comparison voltage

VPWM. . . Driving voltage

VFB. . . Feedback voltage

VFX. . . Flux voltage

VCMP. . . Feedback control voltage

ZCD, ISN, IST, SWD. . . stitch

Claims (9)

A control circuit for a power converter circuit, the power converter circuit comprising a power switch and a transformer, the control circuit comprising: an output current control unit for generating a reference voltage of an output current according to a feedback control voltage; a leading edge blanking (LEB) unit electrically connected to the power switch to generate a detection voltage of an output current; a comparator electrically connected to the output current control unit and the leading edge shielding unit The detection voltage of the output current is compared with a reference voltage of the output current to generate a cutoff signal; a first hysteresis comparator is configured to receive a reflected voltage of the transformer, and the reflected voltage is Comparing a first reference voltage to generate a first comparison voltage; a first voltage to current converter electrically connected to the output current control unit for using a reference voltage of the output current according to the first comparison voltage Converting to a first current signal; and a first operational amplifier electrically coupled to the voltage to current converter for A current signal of the voltage value is compared with a second reference voltage to generate the voltage feedback control. The control circuit of the power converter circuit of claim 1, wherein the reflected voltage is provided by one of the auxiliary side coils of the transformer. The control circuit of the power converter circuit of claim 1, further comprising: a first resistor electrically connected to the first voltage to current converter for receiving the first current signal to generate the first The voltage value of the current signal. The control circuit of the power converter circuit of claim 1, further comprising: a timing unit electrically connected to the first hysteresis comparator for generating a pilot communication number according to the first comparison voltage; The reverse device has a set (SET) terminal electrically connected to the timing unit, a reset (RESET) terminal electrically connected to the comparator, and an output terminal for generating a drive according to the cutoff signal and the pilot signal number And a buffer electrically connected to the output of the SR flip-flop and the power switch. The control circuit of the power converter circuit of claim 4, further comprising: a second voltage to current converter for receiving the reflected voltage to generate a second current signal; a first capacitor, electrically connected The second voltage to current converter is configured to receive the second current signal to generate a magnetic flux voltage; a second hysteresis comparator electrically connected to the first capacitor for using the magnetic flux voltage Comparing a third reference voltage and a fourth reference voltage to generate a second comparison voltage; a switch electrically connected to the second voltage to current converter for determining the driving signal and the second comparison voltage Sampling the reflected voltage; and a second capacitor electrically coupled to the switch for maintaining the reflected voltage to generate a feedback voltage. The control circuit of the power converter circuit of claim 5, further comprising: a reverse or gate electrically connected to the second hysteresis comparator for receiving the second comparison voltage and the driving signal to generate a switch Signal. The control circuit of the power converter circuit of claim 5, further comprising: a second operational amplifier electrically connected to the switch for comparing the feedback voltage with the second reference voltage to generate the Feedback control voltage. The control circuit of the power converter circuit of claim 5, further comprising: a second resistor electrically connected to the second voltage to current converter for receiving the second current signal to generate the reflected voltage . The control circuit of the power converter circuit of claim 1, wherein when the reflected voltage is greater than the first reference voltage, the first voltage-to-current converter converts the reference voltage of the output current into the first current signal .