Multi-mode switching control circuit and method for improving light load efficiency in switching power supplies

1. A circuit for transitioning between a discontinuous conduction mode (DCM) and a fixed frequency continuous conduction mode (CCM) of a power converter, said power converting having a driver receiving PWM signals and controlling a switching stage including a control switch and a sync switch connected at a common switching node for driving a load, the circuit comprising:

a PWM modulator for providing PWM signals; and

a mode selector for receiving a duty cycle value D, a preset switching period T SET , and a duty cycle at a critical conduction point D MIN having a switching frequency equal to the preset switching period T SET and providing an on-time T ON of the control switch and a switching period T S to the PWM modulator, wherein

if the duty cycle value D is greater than the duty cycle at a critical conduction point D MIN , the PWM modulator will provide the PWM signals to operate the switching stage in the CCM with constant-frequency duty cycle control, and

if the duty cycle value D is less than duty cycle at a critical conduction point D MIN , the PWM modulator will provide the PWM signals to operate the switching stage in the DCM by turning off the sync switch when a load current becomes negative.

2. The circuit of claim 1 , wherein for the PWM modulator to operate in the CCM the mode selector sets the switching period T S to equal the preset switching period T SET and the on-time T ON is set to equal a product of the duty cycle value D and the preset switching period T SET .

3. The circuit of claim 1 , wherein for the PWM modulator to operate in the DCM the mode selector sets the on-time T ON to the product of the duty cycle at a critical conduction point D MIN and the preset switching period T SET .

4. The circuit of claim 3 , wherein switching period T S is computed as

T

S

=

D

MIN

·

T

SET

D

.

5. The circuit of claim 4 , wherein the mode selector directs the PWM modulator to modulate a switching frequency at light load instead of the duty cycle D, thereby regulating the output voltage.

6. The circuit of claim 5 , wherein the mode selector comprises:

an A/D converter for processing an input voltage V in ;

a first arithmetic unit for dividing a reference voltage by the input voltage V in to calculate a minimum duty cycle D MIN ;

a first logic unit for comparing the input duty cycle D with the minimum duty cycle D MIN ;

a second arithmetic unit for multiplying the minimum duty cycle D MIN with the preset switching period T SET and dividing the product by the duty cycle D;

a first multiplexer for using the result of the first logic unit to select between the duty cycle D and the minimum duty cycle D MIN ;

a second multiplexer for using the result of the first logic unit to select between the result of the second arithmetic unit and the preset switching period T SET ;

and an arithmetic unit for providing the on-time of the control switch T ON by multiplying the selection of the first multiplexer and the preset switching period T SET .

7. The circuit of claim 3 , wherein the PWM modulator has an ability to change the on-time and the switching period during a switching cycle.

8. The circuit of claim 7 , wherein the PWM modulator comprises:

a latch for providing the PWM signals;

a first counter having enable and set clear controls for receiving and counting a first clock signal to produce a first signal, the first counter starting counting when the first clock signal is received;

a second counter having enable and set clear controls for receiving and counting a second clock signal to produce a second signal;

a first comparator for clearing the first counter when the first signal equals the switching period T S , thereby implementing control of the switching period T S ;

a second comparator for comparing the first signal to zero and setting the latch to high when the first signal is zero;

a third comparator for comparing the first signal to T ON [m+1 . . . n] and providing a third signal to enable the second counter when the first signal is equal T ON [m+1 . . . n] when the second counter is enabled;

a fourth comparator for comparing the second signal to T ON [0 . . . m] and re-setting the latch when the second signal is equal to T ON [0 . . . m], thereby implementing the T ON control; and

a NAND circuit for performing a nand operation on the third and PWM signals to clear the second counter.

9. The circuit of claim 1 , wherein the CCM is used when the load current of the power converter is high and allows a fast transient response, higher efficiency, and a narrower-spread noise spectrum and the DCM reduces circulation energy and associated losses of the power converter.

10. The circuit of claim 1 , further comprising a compensator for receiving an output voltage V O , a load current I L , and a reference voltage and computing a duty cycle value D for the mode selector, wherein the same compensator is used in CCM and DCM.

11. The circuit of claim 10 , wherein the duty cycle value D is a result of a division of an on-time of the control switch T ON divided by a switching period T S .

12. The circuit of claim 1 , wherein the on-time of the control switch T ON and the switching period T S are variable.

13. A multi-mode switching control circuit comprising:

a first sub-circuit configured to determine an ON time of a duty cycle of a switched system controlled by said multi-mode switching control circuit;

a second sub-circuit configured to determine a frequency of said duty cycle;

said multi-mode switching control circuit configured to modulate said ON time in a fixed frequency mode if said duty cycle is greater than a minimum duty cycle;

said multi-mode switching control circuit configured to modulate said frequency in a variable frequency mode if said duty cycle is less than said minimum duty cycle.

14. The multi-mode switching control circuit of claim 13 , wherein said switched system is coupled to said multi-mode switching control circuit through a pulse width modulator configured to receive an output from said multi-mode switching control circuit and to provide a pulse width modulated signal to said switched system in response to said output.

15. The multi-mode switching control circuit of claim 13 , wherein said switched system comprises a power converter.

16. The multi-mode switching control circuit of claim 15 , wherein said fixed frequency mode results in said power converter operating in a continuous conduction mode (CCM).

17. The multi-mode switching control circuit of claim 15 , wherein said variable frequency mode results in said power converter operating in a discontinuous conduction mode (DCM).

18. A method for use by a multi-mode switching control circuit, said method comprising:

determining a minimum duty cycle of a switched system controlled by said multi-mode switching control circuit;

modulating an ON time of a duty cycle of said switched system in a fixed frequency mode if said duty cycle is greater than said minimum duty cycle;

modulating a frequency of said duty cycle in a variable frequency mode if said duty cycle is less than said minimum duty cycle.

19. The method of claim 18 , wherein said switched system comprises a power converter.

20. The method of claim 19 , wherein said fixed frequency mode results in said power converter operating in a continuous conduction mode (CCM) and said variable frequency mode results in said power converter operating in a discontinuous conduction mode (DCM).