# Patent

## Title
Wide input voltage range power factor correction circuit

## Abstract

A boost circuit is used for power factor correction (PFC). In a low power application, transition mode control is utilized. However, switching frequency varies with different input voltages, and over a wide input voltage range, the switching frequency can become too high to be practical. To address this issue, a boost circuit is provided whose effective inductance changes as a function of input voltage. By changing the inductance, control is exercised over switching frequency.

## Background
PRIORITY CLAIM
This application is a continuation of United States Application for patent Ser. No. 13/023,072 filed Feb. 8, 2011 which claims priority from U.S. Provisional Application for Pat. No. 61/304,696 filed Feb. 15, 2010, the disclosures of which are hereby incorporated by reference.


BACKGROUND
1. Technical Field
The present invention relates generally to boost circuits.
2. Description of Related Art
Reference is now made to FIG. 1 which shows a circuit diagram of a prior art boost circuit. The circuit uses, for example, an STMicroelectronics L6561 or L6562 Transition Mode Power Factor Correction (PFC) Controller (see FIG. 2 for a block diagram and see also the STMicroelectronics L6561 and L6562 Data Sheets, the disclosures of which are hereby incorporated by reference). For low power (<250 W) applications, the boost circuit operates in transition mode (boundary mode between continuous current mode and discontinuous current mode) and is used for power factor correction.
In each switching cycle the output transistor Q1 is turned on and the current in the inductor (actually the voltage across sense resistor R1) is compared to a current command (internal voltage). The output transistor Q1 is turned off when the peak current equals the current command. In the transition mode of operation, the output transistor Q1 is turned on again when the current is approximately equal to zero. For each cycle the current command is the error voltage (the output of an error amplifier sensing the boosted output voltage) times the instantaneous line (input) voltage (sensed at the Vmult input). In this way the peak current in each cycle is proportional to the instantaneous line voltage and the resulting line current is in phase with the line voltage.
With reference to FIG. 2 and the block diagram of the PFC controller, an RS flip-flop controls the gate driver that drives the gate of the transistor Q1. This flip-flop is set (to turn on the output) by a zero current detector, and reset (to turn off the output) by the comparator comparing the instantaneous current (CS) to the output of the multiplier. The inputs to the multiplier are the instantaneous line voltage (MULT) and the output of the error amplifier (COMP, compensation pin). The error amplifier compares the feedback from the boosted output voltage (on INV) to an internal reference and works to regulate the output voltage.
One feature of transition mode PFC is variable switching frequency. The frequency F is given by the following equation:




  
    
      
        F
        =
        
          
            1
            
              2
              *
              L
              *
              Pin
            
          
          *
          
            
              
                Vrms
                2
              
              *
              
                (
                
                  Vo
                  -
                  
                    
                      2
                    
                    *
                    
                      Vrms
                      2
                    
                    *
                    
                      sin
                      ⁡
                      
                        (
                        θ
                        )
                      
                    
                  
                
                )
              
            
            Vo
          
        
      
    
    
      
        (
        1
        )
      
    
  




where L=the inductance of L1; Pin=input power; Vrms=input voltage RMS value; Vo=output voltage of the PFC boost converter; and θ=electrical angle for the sinusoidal waveform.

FIGS. 3-4 provide examples of the switching frequency at different input voltages. For example, Vrms=108V in FIG. 2, and Vrms=380V in FIG. 3. The other conditions are: Pin=150 W, L=1 mH and Vout=580V. In FIG. 3, the minimum switching frequency is 25 kHz and the maximum switching frequency is 35 kHz. In FIG. 4, the minimum switching frequency is 30 kHz and the maximum switching frequency is 400 kHz.
For high input voltage Vrms=380V, and an output voltage Vo=580V, an 800V MOSFET would be needed for transistor Q1. Switching this MOSFET at 400 kHz will cause excessive switching loss, and thus is impractical. Therefore, the same boost circuit cannot operate over such a wide range of input voltage. In response, manufacturers instead specifically design products for high voltage inputs (such as, for example, a 340-380 VAC input).
U.S. Pat. No. 5,383,109, the disclosure of which is hereby incorporated by reference, teaches a solution which involves changing inductance for different configurations through a switching operation. U.S. Pat. No. 6,690,589, the disclosure of which is hereby incorporated by reference, teaches an interleaved set of master and slave controller units. U.S. Pat. No. 7,313,007, the disclosure of which is hereby incorporated by reference, teaches simultaneous control of several power converters.
Additional disclosure concerning the configuration and operation of power factor correction circuits of the type shown in FIG. 1 is provided in the STMicroelectronics Application Note entitled “Circuits For Power Factor Correction With Regards To Mains Filtering” by J. M. Bourgeois, incorporated herein by reference.
Additional disclosure concerning use of the STMicroelectronics L6561 Transition Mode Power Factor Correction (PFC) Controller in a power factor correction circuit of the type shown in FIG. 1 is provided in STMicroelectronics Application Notes AN966 and AN1089, entitled “L6561, Enhanced Transition Mode Power Factor Corrector” and “Control Loop Modeling of L6561-Based TM PFC,” respectively, by C. Adragna. Application Notes AN966 and AN1089 are incorporated herein by reference.
Additional disclosure concerning use of the STMicroelectronics L6561 and L6562 Transition Mode Power Factor Correction (PFC) Controllers in a power factor correction circuit of the type shown in FIG. 1 is provided in STMicroelectronics Application Note AN1757, entitled “Switching From the L6561 to the L6562” by L. Salati, and incorporated herein by reference.
SUMMARY
A boost circuit is provided whose effective inductance changes as a function of input voltage. By changing the inductance, control is exercised over switching frequency.
In an embodiment, a boost circuit comprises: a variable inductance circuit selectively presenting at least a first inductance value and a second inductance value; a switching circuit coupled between the variable inductance circuit and a reference node; a controller operable to generate control signaling for controlling actuation of the switching circuit to boost an input voltage applied to the variable inductance circuit; and a control circuit configured to sense the input voltage and select in response thereto the use of one of the first and second inductance values.
In an embodiment, a boost circuit comprises: an inductance circuit selectively presenting a first inductance value and a second inductance value; a controller operable to boost an input voltage applied to the first and second inductors as a function of a boost inductance of the inductance circuit; and a selection circuit operable to select the first inductance value as the boost inductance in a first mode as select the second inductance value as the boost inductance in a second mode.
In another embodiment, a boost circuit presents a boost inductance to assist in boosting an input voltage, said boost circuit operable in at least a first mode of operation and a second mode of operation, wherein when in the first mode of operation the boost inductance is selected to have a first inductance value and wherein when in the second mode of operation the boost inductance is selected to have a second inductance value, said boost circuit further including a mode selection circuit operable to sense a value dependent on the input voltage and control selection of one of the first and second modes of operation in response to said sensed value.



BRIEF DESCRIPTION OF THE DRAWINGS
A more complete understanding of the method and apparatus of the present invention may be acquired by reference to the following Detailed Description when taken in conjunction with the accompanying Drawings wherein:
FIG. 1 is a circuit diagram of a prior art boost circuit;
FIG. 2 is a block diagram of a controller used in the circuit of FIG. 1;
FIGS. 3 and 4 illustrate the relationship between switching frequency and input voltage for the circuit of FIG. 1; and
FIG. 5 is a circuit diagram of an embodiment of a boost circuit with a circuit inductance that can be selectively doubled;
FIG. 6 is a circuit diagram of an alternate embodiment of the boost circuit illustrated in FIG. 5;
FIG. 7 is a circuit diagram of an alternate embodiment of the boost circuit illustrated in FIG. 5;
FIG. 8 is a circuit diagram of an embodiment of a boost circuit providing three modes of operation;
FIG. 9 is a circuit diagram of an alternate embodiment of the boost circuit illustrated in FIG. 8; and
FIG. 10 is a circuit diagram of an alternate embodiment of the boost circuit illustrated in FIG. 8.



DETAILED DESCRIPTION OF THE DRAWINGS
From the frequency equation:




  
    
      
        F
        =
        
          
            1
            
              2
              *
              L
              *
              Pin
            
          
          *
          
            
              
                Vrms
                2
              
              *
              
                (
                
                  Vo
                  -
                  
                    
                      2
                    
                    *
                    
                      Vrms
                      2
                    
                    *
                    
                      sin
                      ⁡
                      
                        (
                        θ
                        )
                      
                    
                  
                
                )
              
            
            Vo
          
        
      
    
    
      
        (
        1
        )
      
    
  




it is recognized that, for the same input power, input voltage and output voltage, if the inductance L is doubled, the switching frequency F will be halved. Embodiments disclosed herein provide a boost circuit including a functionality of changing, and in specific examples selectively doubling, the value of the inductance L.

Reference is now made to FIG. 5 which shows a circuit diagram 500 of an embodiment of a boost circuit with a circuit inductance that can be selectively doubled. The circuit of FIG. 5 is similar in configuration to the circuit of FIG. 1. However, the FIG. 5 circuit includes both a first inductor L1 (having a first inductance) and a second inductor L2 (having a second inductance). In a preferred implementation, the first inductance equals the second inductance. The first and second inductors may be wound on separate magnetic cores. Alternatively, the first and second inductors may be wound on the same magnetic core.
In each switching cycle the output transistors Q1 and Q2 are turned on and the current in the inductors (actually the voltage across sense resistor R1) is compared to a current command (internal voltage). The output transistors Q1 and Q2 are turned off when the peak current equals the current command. In the transition mode of operation, the output transistors Q1 and Q2 are turned on again when the current is approximately equal to zero. For each cycle the current command is the error voltage (the output of an error amplifier sensing the boosted output voltage) times the instantaneous line (input) voltage (sensed at the Vmult input). In this way the peak current in each cycle is proportional to the instantaneous line voltage and the resulting line current is in phase with the line voltage.
The circuit includes voltage divider 502 used to generate a comparator input voltage Vin from the input voltage Vrms in accordance with the following equation:




  
    
      
        Vin
        =
        
          
            
              R
              ⁢
              
                
              
              ⁢
              10
            
            
              
                R
                ⁢
                
                  
                
                ⁢
                9
              
              +
              
                R
                ⁢
                
                  
                
                ⁢
                10
              
            
          
          *
          
            2
          
          *
          Vrms
        
      
    
    
      
        (
        2
        )
      
    
  




where Vrms is the RMS value of the input voltage.

A comparator 504 functions to compare the input voltage Vin to a reference voltage Vref and generate a control signal applied to the control terminal of the transistor Q3. The transistor Q3 functions to control whether transistor Q2 can be activated by the PFC controller 506. When transistor Q3 is turned on, the control terminal of transistor Q2 is driven to ground thus blocking the controller 506 from being able to turn transistor Q2 on.
When Vin<Vref, the transistor Q3 is turned off. Thus, both transistor Q1 and transistor Q2 are switched in the circuit in response to the gate signal output by the controller 506. The effective inductance of the circuit is thus L/2 (where L is the inductance of both the first inductor L1 and second inductor L2). In other words, in this mode, the first inductor L1 and second inductor L2 are coupled in parallel with each other.
When Vin>Vref, the transistor Q3 is turned on. Thus, transistor Q2 is disabled by transistor Q3 in the circuit. The effective inductance of the circuit is thus L (where L is the inductance of the first inductor L1). In other words, in this mode, only the first inductor L1 contributes its inductance to the circuit.
By turning on and off of transistor Q3, the effective inductance of the boost PFC circuit is changed. The operating frequency, consistent with equation (1), is also changed. When the input voltage is excessive (Vin>Vref), the effective inductance of the circuit is increased and this prevents the switching frequency from becoming too high.
The operation of transistor Q3 in FIG. 5 is controlled by the comparator 504 receiving the reference voltage Vref at a first input, and the voltage divided input voltage Vin. When Vin>Vref, the output of the comparator 504 switches to logic high so as to turn on transistor Q3 (and turn off transistor Q2) setting the inductance at L. Conversely, when Vin<Vref, the output of the comparator 504 switches to logic low so as to turn off transistor Q3 (and allow transistors Q1 and Q2 to both be turned on) setting the inductance at L/2.
It will be understood that the first inductor L1 and second inductor L2 need not have the same inductance L.
It will further be understood that alternative mechanisms exist for controlling the operation of transistor Q3. For example, as shown in FIG. 6, a microcontroller 600 could be used to control the operation of transistor Q3.
It will further be understood that the functionality for controlling the operation of transistor Q3 and the PFC functionality provided, for example, by the L6562 controller, could be implemented in a single controller circuit 700 as shown in FIG. 7.
The inductances L1 and L2 can be selected to provide for efficient operation of the circuit when the Vrms input voltage is potentially at two distinct levels. For example, it is common for AC mains voltage to be available at 120V, 240V and 347V. The circuit as described can be configured with inductances for L1 and L2 such that the effective inductance L in one mode is optimized for a 120 VAC input and in a second mode is optimized for a 240 VAC input. More specifically, a smaller effective inductance for the lower 120 VAC input and a larger effective inductance for the higher 240 VAC input. The connection of 120 VAC versus 240 VAC at the Vrms input is detected through the voltage divided input voltage Vin. Alternatively, the inductance values can be selected for optimization at other voltage levels.
Reference is now made to FIG. 8. FIG. 8 is similar to FIG. 5 except that an additional comparator 802, transistor circuit Q4, and logic circuit 804 have been added. The additional comparator 802 functions to compare the input voltage Vin to a second reference voltage Vref2 and generate a first signal applied to a first input of the logic circuit 804. The other comparator 806 functions to compare the input voltage Vin to a first reference voltage Vref1 and generate a second signal applied to a second input of the logic circuit 804.
The logic circuit 804 functions to logically combine the first and second signals to implement, in conjunction with the two comparators 802 and 806, a window comparator circuit 810. The design of the logic circuit 804 to implement the window comparison functionality is well within the capabilities of one skilled in the art, and numerous option for logic circuit design are available for selection in implementing the functionality. The window comparator circuit 810 generates a first control signal applied to the control terminal of the transistor Q4. The transistor Q4 functions to control whether transistor Q1 can be activated by the PFC controller 812. When transistor Q4 is turned on, the control terminal of transistor Q1 is driven to ground thus blocking the controller 812 from being able to turn transistor Q1 on. The window comparator circuit 810 further generates a second control signal applied to the control terminal of the transistor Q3. The transistor Q3 functions to control whether transistor Q2 can be activated by the PFC controller 812. When transistor Q3 is turned on, the control terminal of transistor Q2 is driven to ground thus blocking the controller 812 from being able to turn transistor Q2 on.
In a preferred embodiment of this implementation, the first inductor L1 and second inductor L2 do not have same inductance values. By properly choosing the Vref1 and Vref2 values, along with properly configuring the logic circuit 804, a number of different effective inductance values can be provided.
The operation of transistors Q3 and Q4 in FIG. 8 are controlled in response to the comparison of the input voltage Vin to the first and second reference voltages (Vref1 and Vref2). The first comparator 806 receives the first reference voltage Vref1 and the voltage divided input voltage Vin. The second comparator 802 receives the second reference voltage Vref2 and the voltage divided input voltage Vin. The logic circuit 804 responds to the results of the two comparisons to generate the first and second control signals.
When Vin<Vref1, the window comparator circuit 810 generates control signals to turn off both transistor Q3 and transistor Q4. Thus, both transistor Q1 and transistor Q2 are switched in the circuit in response to the gate signal output by the controller 812. The effective inductance of the circuit is thus L1*L2/(L1+L2) (where L1 is the inductance of the first inductor L1 and L2 is the inductance of the second inductor L2). In other words, in this mode, the first inductor L1 and second inductor L2 are coupled in parallel with each other. If the inductance L2=1⁄2 L1, then the effective inductance in this mode of operation is 1⁄3 L1.
When Vref1<Vin<Vref2, the window comparator circuit 810 generates control signals to turn off transistor Q3 and turn on transistor Q4. Thus, transistor Q1 is disabled by transistor Q4 in the circuit. The effective inductance of the circuit is thus L2 (where L2 is the inductance of the second inductor L2). In other words, in this mode, only the second inductor L2 contributes its inductance to the circuit. If the inductance L2=1⁄2 L1, then the effective inductance in this mode of operation is 1⁄2 L1.
When Vin>Vref2, the window comparator circuit 810 generates control signals to turn on transistor Q3 and turn off transistor Q4. Thus, transistor Q2 is disabled by transistor Q3 in the circuit. The effective inductance of the circuit is thus L1 (where L1 is the inductance of the first inductor L1). In other words, in this mode, only the first inductor L1 contributes its inductance to the circuit.
FIG. 9 shows an alternative implementation wherein a programmable microcontroller 900 is provided to generate the gate drive signals controlling the operation of transistors Q3 and Q4 in response to the input voltage Vin. In one mode, the programmable microcontroller 900 turns off both transistors Q3 and Q4. In this mode, the effective inductance of the circuit is L1*L2/(L1+L2), or 1⁄3 L1 where L2=1⁄2 L1. In another mode, the programmable microcontroller 900 turns off transistor Q3 and turns on transistor Q4. In this mode, the effective inductance of the circuit is L2, or 1⁄2 L1 where L2=1⁄2 L1. In yet another mode, the programmable microcontroller 900 turns on transistor Q3 and turns off transistor Q4. In this mode, the effective inductance of the circuit is L1.
FIG. 10 shows another alternative implementation wherein the functionality for controlling the operation of transistors Q3 and Q4 as well as the PFC functionality are implemented in a single controller circuit 1000. Three different modes of operation, like those described above with respect to FIGS. 8-9, can be provided.
The existence of three modes provides the circuits of FIGS. 8-10 with additional flexibility and options for optimization to at least three different Vrms input voltages (such as, 120V, 240V and 347V). The circuit as described can be configured with inductances for L1 and L2 such that the effective inductance L in one mode is optimized for a 120 VAC input, in a second mode is optimized for a 240 VAC input, and in a third mode is optimized for a 347 VAC input. More specifically, a smaller effective inductance for the 120 VAC input and a middle effective inductance for the 240 VAC input and a higher effective inductance for the 347 VAC input. The connection of 120 VAC, 240 VAC or 247 VAC at the Vrms input is detected through the voltage divided input voltage Vin. Alternatively, the inductance values can be selected for optimization at other voltage levels.
Although preferred embodiments of the method and apparatus of the present invention have been illustrated in the accompanying Drawings and described in the foregoing Detailed Description, it will be understood that the invention is not limited to the embodiments disclosed, but is capable of numerous rearrangements, modifications and substitutions without departing from the spirit of the invention as set forth and defined by the following claims.

## Claims

1. A boost circuit, comprising:
a variable inductance circuit selectively presenting at least a first inductance value and a second inductance value;
a switching circuit coupled between the variable inductance circuit and a reference node;
a controller operable to generate control signaling for controlling actuation of the switching circuit to boost an input voltage applied to the variable inductance circuit; and
a control circuit configured to sense the input voltage and select in response thereto the use of one of the first and second inductance values.

2. The circuit of claim 1, wherein the control circuit comprises a comparator circuit configured to compare a voltage derived from the input voltage to a reference voltage and generate a control signal for selecting one of the first and second inductance values.

3. The circuit of claim 1, wherein the variable inductance circuit comprises a first inductor and a second inductor; and wherein the switching circuit comprises a first switch coupled between the first inductor and the reference node and a second switch coupled between the second inductor and the reference node.

4. The circuit of claim 3, wherein the first and second inductors have a same inductance.

5. The circuit of claim 3, wherein the control circuit comprises a third switch coupled between a control terminal of the first switch and the reference node, said third switch activated in response to said sensed input voltage to preclude boost use of said first inductor.

6. The circuit of claim 5, wherein the control circuit comprises a comparator circuit configured to compare a voltage derived from the input voltage to a reference voltage and generate a control signal for application to control actuation of the third switch.

7. The circuit of claim 5, wherein the control circuit comprises a fourth switch coupled between a control terminal of the second switch and the reference node, said fourth switch activated in response to said sensed input voltage to preclude boost use of said second inductor.

8. The circuit of claim 7, wherein the control circuit comprises a comparator circuit configured to compare a voltage derived from the input voltage to at least one reference voltage and generate control signals for application to control actuation of the third and fourth switches.

9. A boost circuit, comprising:
an inductance circuit selectively presenting a first inductance value and a second inductance value;
a controller operable to boost an input voltage applied to the first and second inductors as a function of a boost inductance of the inductance circuit; and
a selection circuit operable to select the first inductance value as the boost inductance in a first mode and select the second inductance value as the boost inductance in a second mode.

10. The circuit of claim 9, wherein the selection circuit comprises a comparator configured to compare a voltage derived from the input voltage to a reference voltage and select one of the first and second modes in response to said comparison.

11. The circuit of claim 9, wherein the inductance circuit includes: a first inductor coupled in series with a first switch between the input voltage and a reference node and second inductor coupled in series with a second switch between the input voltage and the reference node.

12. The circuit of claim 11, wherein the selection circuit comprises a third switch coupled between a control node of the first switch and the reference node, said third switch actuated in response to said comparison to preclude boost use of the first inductor.

13. The circuit of claim 12, wherein the selection circuit comprises a comparator configured to compare a voltage derived from the input voltage to a reference voltage and actuate the first switch in response to said comparison.

14. The circuit of claim 12, wherein the selection circuit further comprises a fourth switch coupled between a control node of the second switch and the reference node, said fourth switch actuated in response to said comparison to preclude boost use of the second inductor.

15. The circuit of claim 14, wherein the selection circuit comprises a comparator configured to compare a voltage derived from the input voltage to at least one reference voltage and actuate the first and second switches in response to said comparison.

16. A boost circuit presenting a boost inductance to assist in boosting an input voltage, said boost circuit operable in at least a first mode of operation and a second mode of operation, wherein when in the first mode of operation the boost inductance is selected to have a first inductance value and wherein when in the second mode of operation the boost inductance is selected to have a second inductance value, said boost circuit further including a mode selection circuit operable to sense a value dependent on the input voltage and control selection of one of the first and second modes of operation in response to said sensed value.

17. The boost circuit of claim 16, wherein the boost circuit includes a first inductor and a second inductor, and wherein the boost inductance first inductance value is associated with the first inductor and wherein the boost inductance second inductance value is associated with a combination of the first and second inductors.

18. The boost circuit of claim 17, wherein the combination is a parallel combination.

19. The boost circuit of claim 17, further including a first switch coupled in series with the first inductor and a second switch coupled in series with the second inductor, said mode selection circuit disabling said second switch in the first mode of operation, said mode selection circuit enabling said second switch in the second mode of operation.

20. The boost circuit of claim 16, comprising a variable inductance circuit presenting the first inductance value in the first mode of operation and presenting the second inductance value in the second mode of operation.