Patent Publication Number: US-2022239230-A1

Title: Isolated conversion apparatus with magnetic bias balance control and method of controlling the same

Description:
BACKGROUND 
     Technical Field 
     The present disclosure relates to a conversion apparatus with magnetic bias balance control and a method of controlling the same, and more particularly to an isolated conversion apparatus with magnetic bias balance control and a method of controlling the same. 
     Description of Related Art 
     The statements in this section merely provide background information related to the present disclosure and do not necessarily constitute prior art. 
     Please refer to  FIG. 1A , which shows a circuit diagram of a conventional full-bridge phase-shift converter. Due to the difference of the hardware circuit or the signals of controlling control switches Q 1 -Q 4  with different duty cycles, the average positive and negative voltage across the transformer  12  is not zero. Therefore, the phenomenon that the average amount of magnetizing current is not zero (referred to as magnetic bias) occurs so that the magnetizing inductance of the transformer  12  saturates and the inductance value decreases rapidly, causing the risk of excessive current on the primary side of the converter. 
     In the prior art, a capacitor C is usually connected in series on the primary side of the transformer  12  to balance the positive and negative half-cycle voltage of the transformer  12  to avoid the occurrence of magnetic bias. However, this control method requires adding a capacitor C to the circuit, which will relatively increase the circuit cost and circuit volume. Another common solution is shown in  FIG. 1B , where a current sensor CT is added to the full-bridge circuit and the current-peak control method is performed. This control method will make the half cycle of the magnetic bias close the switch duty cycle earlier due to the higher current so that the product of voltage and time on the side of the magnetic bias can be reduced to achieve the effect of balancing the magnetic bias. However, this control method needs to add the current sensor CT in the circuit, which also increases the circuit cost and circuit volume. 
     Accordingly, the isolated conversion apparatus with magnetic bias balance control and the method of controlling the same are provided to achieve the effect of balancing the magnetic bias without adding the isolation capacitor and the current sensor. 
     SUMMARY 
     In order to solve the above-mentioned problems, the present disclosure provides an isolated conversion apparatus with magnetic bias balance control. The isolated conversion apparatus includes an isolated converter, a controller, and a magnetic bias balance circuit. The isolated converter includes a transformer with a primary side, and the primary side has a primary-side winding and at least one switch bridge arm. The controller is coupled to the at least one switch bridge arm, and provides a PWM (pulse width modulation) signal group to control the at least one switch bridge arm. The magnetic bias balance circuit is coupled to two ends of the primary-side winding and the controller, and provides a compensation voltage to the controller according to an average voltage value of a winding voltage across the two ends of the primary-side winding. The controller adjusts a duty cycle of the PWM signal group according to the compensation voltage. 
     In order to solve the above-mentioned problems, the present disclosure provides a method of controlling magnetic bias balance of an isolated conversion apparatus. The isolated conversion apparatus includes an isolated converter, and the isolated converter includes a transformer and at least one switch bridge arm coupled to a primary side of the transformer. The method includes steps of: (a) providing a PWM (pulse width modulation) signal group to control switching the at least one bridge arm so that the isolated converter converting an input voltage into an output voltage, (b) providing a compensation voltage corresponding to a magnetic bias of the transformer according to a winding voltage across two ends of a primary-side winding of the transformer, and (c) adjusting a duty cycle of the PWM signal group according to the compensation voltage to correct the magnetic bias. 
     Accordingly, the isolated conversion apparatus samples the winding voltage across the two ends of the primary-side winding of the transformer, the filter circuit acquires the average voltage value of the winding voltage, and the controller controls the average voltage value to be zero so as to correct the magnetic bias. The magnetic bias compensation of the transformer can be simply completed without using the control method of adding the current sensor and without adding the isolation capacitor, thereby reducing the circuit volume and the circuit cost. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the present disclosure as claimed. Other advantages and features of the present disclosure will be apparent from the following description, drawings and claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The present disclosure can be more fully understood by reading the following detailed description of the embodiment, with reference made to the accompanying drawing as follows: 
         FIG. 1A  is a circuit diagram of a conventional full-bridge phase-shift converter. 
         FIG. 1B  is a block diagram of a peak current control. 
         FIG. 2  is a block circuit diagram of an isolated conversion apparatus with magnetic bias balance control according to the present disclosure. 
         FIG. 3  is a block circuit diagram of a controller and a magnetic bias balance circuit according to the present disclosure. 
         FIG. 4A  is a block circuit diagram of an isolated converter according to a first embodiment of the present disclosure. 
         FIG. 4B  is a block circuit diagram of the isolated converter according to a second embodiment of the present disclosure. 
         FIG. 5A  is a schematic waveform diagram of a positive magnetic bias correction of an isolated conversion apparatus according to a first embodiment of the present disclosure. 
         FIG. 5B  is a schematic waveform diagram of a negative magnetic bias correction of the isolated conversion apparatus according to a second embodiment of the present disclosure. 
         FIG. 6A  is a flowchart of a method of controlling magnetic bias balance of the isolated conversion apparatus according to the present disclosure. 
         FIG. 6B  is a flowchart of a method of adjusting a duty cycle of the isolated conversion apparatus according to the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made to the drawing figures to describe the present disclosure in detail. It will be understood that the drawing figures and exemplified embodiments of present disclosure are not limited to the details thereof. 
     Please refer to  FIG. 2 , which shows a block circuit diagram of an isolated conversion apparatus with magnetic bias balance control according to the present disclosure. An input end  1 A of the isolated conversion apparatus  1  receives an input voltage Vin, converts the input voltage Vin into an output voltage Vo, and supplies power to a load  2  through an output end  1 B. The isolated conversion apparatus  1  includes an isolated converter  10 , a controller  20 , and a magnetic bias balance circuit  30 . The isolated converter  10  includes a transformer  12 , at least one switch bridge arm  14 , and a secondary-side circuit  16 . A primary side of the transformer  12  includes a primary-side winding  122 , and a secondary side of the transformer  12  includes a secondary-side winding  124 . The switch bridge arm  14  is coupled to the input end  1 A and the primary-side winding  122 , and the secondary-side circuit  16  is coupled to the secondary-side winding  124  and the output end  1 B. The controller  20  is coupled to the switch bridge arm  14  and the output end  1 B, and provides a PWM (pulse width modulation) signal group PWM to control the switch bridge arm  14  according to a feedback signal Sf transmitted from the output end  1 B. The magnetic bias balance circuit  30  is coupled to two ends of the primary-side winding  122  and the controller  20 , and provides a compensation voltage Vc corresponding to a magnetic bias of the transformer  12  to the controller  20  according to a winding voltage Vw across the two ends of the primary-side winding  122 . The controller  20  adjusts a duty cycle of the PWM signal group PWM according to the compensation voltage Vc to correct the magnetic bias of the transformer  12 , thereby achieving the effect of magnetic bias balance. 
     Please refer to  FIG. 3 , which shows a block circuit diagram of a controller and a magnetic bias balance circuit according to the present disclosure, and also refer to  FIG. 2 . The controller  20  includes a first operational circuit  22 , a voltage controller  24 , and a PWM circuit  26 . The first operational circuit  22  is coupled to the output end  1 B, and provides a first error value Ve 1  to the voltage controller  24  according to a difference between the feedback signal Sf and the reference voltage Vref. The voltage controller  24  is a compensator for general feedback control, receives the first error value Ve 1  to generate a voltage control signal Sv according to the first error value Ve 1 , and provides the voltage control signal Sv to the PWM circuit  26 . The PWM circuit  26  generates the PWM signal group PWM according to the voltage control signal Sv, and provides the PWM signal group PWM to the switch bridge arm  14  so as to regulate/stabilize a voltage value of the output voltage Vo by switching the switch bridge arm  14 . In one embodiment, the internal structure of the controller  20  is only the most basic feedback control structure, and it is not limited that the controller  20  can only be implemented with this circuit structure. 
     The magnetic bias balance circuit  30  includes a sampling circuit  32  and an offset compensation circuit  34 . The sampling circuit  32  is coupled to the two ends of the primary-side winding  122 . The offset compensation circuit  34  is coupled to the sampling circuit  32  and outputs the compensation voltage Vc to the PWM circuit  26  of the controller  20 . In one embodiment, the sampling circuit  32  is a differential filter circuit, and the differential filter circuit (i.e., the sampling circuit  32 ) includes an operational amplifier  322 , a filter circuit  324 , a first voltage divider circuit  326 , and a second voltage divider circuit  328 . The operational amplifier  322  has a first input end I 1 , a second input end I 2 , and an output end O, and the output end O of the operational amplifier  322  is coupled to the offset compensation circuit  34 . A first end of the filter circuit  324  is coupled to the two ends of the primary-side winding  122 , and a second end of the filter circuit  324  is coupled to the first voltage divider circuit  326  and the second voltage divider circuit  328 . The filter circuit  324  includes a first filter resistor Rf 1 , a second filter resistor Rf 2 , and a filter capacitor Cf. A first end of the first filter resistor Rf 1  is coupled to a first end of the primary-side winding  122 , and a second end of the first filter resistor Rf 1  is coupled to the first voltage divider circuit  326 . A first end of the second filter resistor Rf 2  is coupled to a second end of the primary-side winding  122 , and a second end of the second filter resistor Rf 2  is coupled to the second voltage divider circuit  328 . A first end of the filter capacitor Cf is coupled to the second end of the first filter resistor Rf 1 , and a second end of the filter capacitor Cf is coupled to the second end of the second filter resistor Rf 2 . 
     The first voltage divider circuit  326  includes a first resistor R 1  and a second resistor R 2  connected to the first resistor R 1  in series. The first resistor R 1  is coupled to the first end of the filter capacitor Cf, and two ends of the second resistor R 2  are respectively coupled to the first input end I 1  and the output end O of the operational amplifier  322 . The second voltage divider circuit  328  includes a third resistor R 3  and a fourth resistor R 4  connected to the third resistor R 3  in series. The third resistor R 3  is coupled to the second end of the filter capacitor Cf, and two ends of the fourth resistor R 4  are respectively coupled to the second input end I 2  and a negative end. The sampling circuit  32  filters, averages, and gains the winding voltage Vw through the filter circuit  324 , the operational amplifier  322 , the first voltage divider circuit  326 , and the second voltage divider circuit  328  to generate an average voltage Va corresponding to an average voltage value of the winding voltage Vw. In one embodiment, the implementation of the sampling circuit  32  is only a schematic analog circuit, and it is not limited to only being implemented with the circuit structure of the differential filter circuit in  FIG. 3 . An example of another possible implementation may acquire the average voltage values of the winding voltage Vw in the positive half cycle and the negative half cycle. Afterward, the average voltage value of the positive half cycle is subtracted from the average voltage value of the negative half cycle to acquire an average voltage Va corresponding to an average voltage value of the winding voltage Vw for the offset compensation circuit  34  to control the difference to be zero. Such an implementation is advantageous for implementation using a processor or a microcontroller. In other words, as long as the circuit or method corresponding to the average voltage value of the winding voltage Vw can be acquired, it can be applied to the sampling circuit  32  of the present disclosure. 
     The offset compensation circuit  34  is a proportional integral controller, and the proportional integral controller (i.e., the offset compensation circuit  34 ) includes a second operational circuit  342  and a proportional integral unit  344 . The second operational circuit  342  provides a second error value Ve 2  according to a voltage difference between the average voltage Va and a zero voltage Vz. In particular, the zero voltage Vz represents a target value under the magnetic bias balance, and it is usually a reference voltage of 0 volt. The proportional integral unit  344  receives the second error value Ve 2 , generates a compensation voltage Vc corresponding to a direction and a magnitude of the magnetic bias according to the second error value Ve 2 , and provides the compensation voltage Vc to the PWM circuit  26  so that the PWM circuit  26  adjusts the duty cycle of the PWM signal group PWM to correct the magnetic bias according to the compensation voltage Vc. In one embodiment, the implementation of the offset compensation circuit  34  is only a preferred implementation, and is not limited to only being implemented in the form of the proportional integral controller of  FIG. 3 . In other words, any controller structure that can generate the compensation voltage Vc according to the average voltage Va and the zero voltage Vz to make the average voltage Va close to zero volt can be applied to the offset compensation circuit  34  of the present disclosure. The offset compensation circuit  34  may be also implemented by a processor or a microcontroller with digital control. 
     Please refer to  FIG. 4A , which shows a block circuit diagram of an isolated converter according to a first embodiment of the present disclosure, and also refer to  FIG. 2  to  FIG. 3 . The isolated converter  10  of the isolated conversion apparatus  1  is a full-bridge converter, and therefore the switch bridge arm  14  includes a first bridge arm  142  and a second bridge arm  144 . The first bridge arm  142  has a first switch Q 1  and a second switch Q 2  connected to the first switch Q 1  in series. The second bridge arm  144  is connected to the first bridge arm  142  in parallel, and the second bridge arm  144  has a third switch Q 3  and a fourth switch Q 4  connected to the third switch Q 3  in series. The first end of the primary-side winding  122  is coupled to a node between the first switch Q 1  and the second switch Q 2 , and the second end of the primary-side winding  122  is coupled to a node between the third switch Q 3  and the fourth switch Q 4 . The PWM signal group PWM has a first control signal S 1  of controlling the first switch Q 1 , a second control signal S 2  of controlling the second switch Q 2 , a third control signal S 3  of controlling the third switch Q 3 , and a fourth control signal S 4  of controlling the fourth switch Q 4 . The controller  20  provides control signals S 1 -S 4  to respectively switch the switches Q 1 -Q 4  according to the feedback signal Sf transmitted from the output end  1 B so that the isolated converter  10  converts the input voltage Vin into the output voltage Vo. The magnetic bias balance circuit  30  is coupled to the two ends of the primary-side winding  122 , and provides the compensation voltage Vc corresponding to the magnetic bias of the transformer  12  to the controller  20  according to the winding voltage Vw, and therefore the controller  20  adjusts the duty cycle of the control signals S 1 -S 4  to correct the magnetic bias of the transformer  12 , thereby achieving the effect of magnetic bias balance. 
     Please refer to  FIG. 5A  and  FIG. 5B , which show schematic waveform diagrams of a positive magnetic bias correction and a negative magnetic bias correction of the isolated conversion apparatus according to a first embodiment of the present disclosure, and also refer to  FIG. 2  to  FIG. 4A . The compensation voltage Vc provided by the magnetic bias balance circuit  30  corresponds to a magnetic bias direction of the transformer  12 , and the magnetic bias direction is a direction of positive half-cycle bias or a direction of negative half-cycle bias. As shown in  FIG. 5A , when the magnetic bias direction of the transformer  12  is positive, an effective duty cycle of the winding voltage Vw in the positive half cycle is greater than that in the negative half cycle. At this condition, the compensation voltage Vc provided by the magnetic bias balance circuit  30  is positive (i.e., greater than 0 volt) according to the winding voltage Vw. The controller  20  decreases (in the direction of an arrow A) the duty cycle of the first control signal S 1  and the duty cycle of the fourth control signal S 4  according to the positive compensation voltage Vc so as to correct the magnetic flux that wants to bias to the positive value. The effective duty cycle refers to the duty cycle when the winding voltage Vw actually has a voltage. Take the full-bridge converter shown in  FIG. 4A  as an example, the control signals S 1 , S 4  can be synchronously turned on and turned off, and the effective duty cycle is equal to the duty cycle of the control signal S 1  or the duty cycle of the control signal S 4 . If the phase shift control is used, since the control signals S 1 , S 4  will not be synchronized, there will be voltage on the winding only when the control signals S 1 , S 4  are simultaneously in high level, and therefore the effective duty cycle is the time when the control signals S 1 , S 4  overlap in high level. 
     On the contrary, please refer to  FIG. 5B . When the magnetic bias direction of the transformer  12  is negative, the effective duty cycle of the winding voltage Vw in the negative half cycle is greater than that in the positive half cycle. At this condition, the compensation voltage Vc provided by the magnetic bias balance circuit  30  is negative (i.e., less than 0 volt) according to the winding voltage Vw. The controller  20  decreases (in the direction of an arrow A) the duty cycle of the second control signal S 2  and the duty cycle of the third control signal S 3  according to the negative compensation voltage Vc so as to correct the magnetic flux that wants to bias to the negative value. In one embodiment, the correspondence between the positive and negative values of the winding voltage Vw and the compensation voltage Vc is only an example, and it can also be conversely that the winding voltage Vw with a larger positive half-cycle effective duty cycle corresponds to a negative compensation voltage Vc. As long as an effective duty cycle of the half cycle with larger effective duty cycle can be decreased. 
     Please refer to  FIG. 4B , which shows a block circuit diagram of the isolated converter according to a second embodiment of the present disclosure, and also refer to  FIG. 2  to  FIG. 4A  and  FIG. 5A  and  FIG. 5B . The difference between the isolated conversion apparatus  1 ′ of this embodiment and the isolated conversion apparatus  1  shown in  FIG. 4A  is that the isolated converter  10 ′ of the isolated conversion apparatus  1 ′ is a half-bridge converter. The switch bridge arm  14  includes a first bridge arm  142  and a capacitor assembly  146 . The first bridge arm  142  has a first switch Q 1  and a second switch Q 2  connected to the first switch Q 1  in series. The capacitor assembly  146  is connected to the first bridge arm  142  in parallel, and the capacitor assembly  146  has a first capacitor C 1  and a second capacitor C 2  connected to the first capacitor C 1  in series. The first end of the primary-side winding  122  is coupled to a node between the first switch Q 1  and the second switch Q 2 , and the second end of the primary-side winding  122  is coupled to a node between the first capacitor C 1  and the second capacitor C 2 . The PWM signal group PWM has a first control signal S 1  of controlling the first switch Q 1  and a second control signal S 2  of controlling the second switch Q 2 . The controller  20  provides the control signals S 1 , S 2  to respectively switch the switches Q 1 , Q 2  according to a feedback signal Sf transmitted from the output end  1 B so that the isolated converter  10  converts the input voltage Vin into the output voltage Vo. The magnetic bias balance circuit  30  provides the compensation voltage Vc corresponding to the magnetic bias of the transformer  12  to the controller  20  according to the winding voltage Vw so that the controller  20  adjusts the duty cycle of the control signals S 1 , S 2  to correct the magnetic bias of the transformer  12 , thereby achieving the effect of magnetic bias balance. In one embodiment, the waveform of correcting the magnetic bias by the isolated conversion apparatus  1 ′ is similar to that shown in  FIG. 5A  and  FIG. 5B . When the duty cycle of the winding voltage Vw in the positive half cycle is greater, the duty cycle of the first control signal S 1  is decreased; when the duty cycle of the winding voltage Vw in the negative half cycle is greater, the duty cycle of the second control signal S 2  is decreased. 
     In one embodiment, the secondary-side circuit  16  may be a full-bridge circuit shown in  FIG. 4A  and  FIG. 4B , and it can also be a center-tapped rectifier circuit, which can be implemented in accordance with the actual requirements of the circuit. Although the magnetic bias balance circuit  30  can also use the voltage of the secondary-side winding  124  to control the magnetic bias balance, the structure of the secondary-side winding  124  will be different (such as a single-winding structure or a center-tapped structure) due to the different circuit types of the secondary-side circuit  16 . Therefore, the magnetic bias balance circuit  30  cannot be universally applied to all the circuit structures of the secondary-side circuit  16 . Especially in the center-tapped winding, the parameters of the two windings cannot be made completely the same, which will result in the effect that even if the magnetic bias balance is controlled, the complete magnetic bias balance cannot be achieved. The advantage of the magnetic bias balance circuit  30  of the present disclosure using the primary-side winding  122  to control the magnetic bias balance is that regardless of whether the switch bridge arm  14  on the primary side is one bridge arm or two bridge arms (as shown in  FIG. 4A  and  FIG. 4B ), the magnetic bias balance circuit  30  is universal to achieve the effect of increasing the convenience of use. Moreover, the primary-side winding  122  has only a single winding, and the magnetic bias balance control of the present disclosure can achieve the effect of complete magnetic bias balance. 
     Accordingly, the isolated conversion apparatus  1  samples the winding voltage Vw across the two ends of the primary-side winding  122  of the transformer  12 , the filter circuit (i.e., the sampling circuit  32 ) acquires the average voltage value of the winding voltage Vw, and the controller (i.e., the offset compensation circuit  34 ) controls the average voltage value to be zero so as to correct the magnetic bias. The average voltage value of the winding voltage Vw is equal to the product of the voltage and the time (effective duty cycle) so that the magnetic bias compensation of the transformer  12  can be simply completed without using the control method of adding the current sensor and without adding the isolation capacitor, thereby reducing the circuit volume and the circuit cost. 
     Please refer to  FIG. 6A , which shows a flowchart of a method of controlling magnetic bias balance of the isolated conversion apparatus according to the present disclosure, and also refer to  FIG. 2  to  FIG. 5B . The method of controlling magnetic bias balance is suitable for controlling the isolated converter  1  with the transformer  12 , and the primary side of the transformer  12  includes at least one switch bridge arm  14 . The method of controlling magnetic bias balance includes the following steps. First, a PWM (pulse width modulation) signal group is provided to control switching the bridge arm so that the isolated converter converting an input voltage into an output voltage (S 100 ). The controller  20  provides the PWM signal group PWM to control the switch bridge arm  14  according to a feedback signal Sf transmitted from the output end  1 B of the isolated conversion apparatus  1  so that the isolated conversion apparatus  1  converts the input voltage Vin into the output voltage Vo. Afterward, a compensation voltage corresponding to a magnetic bias of the transformer is provided according to a winding voltage across two ends of a primary-side winding of the transformer (S 200 ). The sampling circuit  32  filters, averages, and gains the winding voltage Vw through the filter circuit  324 , the operational amplifier  322 , the first voltage divider circuit  326 , and the second voltage divider circuit  328  to generate an average voltage Va corresponding to an average voltage value of the winding voltage Vw. The second operational circuit  342  provides a second error value Ve 2  according to a voltage difference between the average voltage Va and a zero voltage Vz. The proportional integral unit  344  generates a compensation voltage Vc corresponding to a direction and a magnitude of the magnetic bias according to the second error value Ve 2 , and provides the compensation voltage Vc to the PWM circuit  26 . Finally, the duty cycle of the PWM signal group is adjusted according to the compensation voltage to correct the magnetic bias (S 300 ). The PWM circuit  26  adjusts the duty cycle of the PWM signal group PWM according to the compensation voltage Vc to correct the magnetic bias. 
     Please refer to  FIG. 6B , which shows a flowchart of a method of adjusting a duty cycle of the isolated conversion apparatus according to the present disclosure, and also refer to  FIG. 2  to  FIG. 6A . The step (S 300 ) includes the following steps. The compensation voltage corresponding to the magnetic bias direction is provided (S 320 ). The compensation voltage Vc provided by the magnetic bias balance circuit  30  corresponds to a magnetic bias direction of the transformer  12 , and the magnetic bias direction is a direction of positive half-cycle bias or a direction of negative half-cycle bias. Afterward, the duty cycle of the control signal corresponding to the magnetic bias direction is decreased according to the compensation voltage corresponding to the magnetic bias direction (S 340 ). When the isolated converter  10  of the isolated conversion apparatus  1  is a full-bridge converter and the magnetic bias direction of the transformer  12  is positive, an effective duty cycle of the winding voltage Vw in the positive half cycle is greater than that in the negative half cycle. At this condition, the compensation voltage Vc provided by the magnetic bias balance circuit  30  is positive according to the winding voltage Vw. The controller  20  decreases the duty cycle of the first control signal S 1  and the duty cycle of the fourth control signal S 4  according to the positive compensation voltage Vc, and otherwise, the controller  20  decreases the duty cycle of the second control signal S 2  and the duty cycle of the third control signal S 3 . 
     When the isolated converter  10  of the isolated conversion apparatus  1  is a half-bridge converter and the magnetic bias direction of the transformer  12  is positive, the duty cycle of the winding voltage Vw in the positive half cycle is greater than that in the negative half cycle. At this condition, the compensation voltage Vc provided by the magnetic bias balance circuit  30  is positive according to the winding voltage Vw. The controller  20  decreases the duty cycle of the first control signal S 1  according to the positive compensation voltage Vc, and otherwise, the controller  20  decreases the duty cycle of the second control signal S 2 . 
     Although the present disclosure has been described with reference to the preferred embodiment thereof, it will be understood that the present disclosure is not limited to the details thereof. Various substitutions and modifications have been suggested in the foregoing description, and others will occur to those of ordinary skill in the art. Therefore, all such substitutions and modifications are intended to be embraced within the scope of the present disclosure as defined in the appended claims.