Apparatus and method for fixed-frequency control in a switching power supply

1. A fixed-frequency switching power supply comprising:

N switches coupled to a D-C power source and configured to receive an input energy therefrom, wherein N is a positive integer, and wherein said N switches are configured to divide said input energy into N phases;

N inductances, wherein each of said N inductances is coupled to one of said N switches;

a capacitance coupled to each of said N inductances and coupled to a dynamic load, wherein said capacitance and said N inductances are together configured to provide an output energy to said dynamic load;

a monitor circuit coupled to said capacitance and configured to monitor said output energy; and

a control circuit coupled to said monitor circuit, configured to derive a first state variable of said power supply from said monitor circuit, to synthesize a second state variable of said power supply from said first state variable, and to switch said N switches at a substantially constant switching frequency in response to said first and second state variables.

2. A power supply as claimed in claim 1 wherein.

said input energy is D-C energy in a first form comprising an input current at an input voltage;

said output energy is D-C energy in a second form comprising an output current at an output voltage; and

said input voltage does not equal said output voltage.

3. A fixed-frequency switching power supply comprising:

N switches coupled to a D-C power source and configured to receive an input energy therefrom, wherein N is a positive integer, and wherein said N switches are configured to divide said input energy into N phases;

N inductances, wherein each of said N inductances is coupled to one of said N switches;

a capacitance coupled to each of said N inductances and coupled to a dynamic load, wherein said capacitance and said N inductances are together configured to provide an output energy to said dynamic load;

a monitor circuit coupled to said capacitance and configured to monitor said output energy; and

a control circuit coupled to said monitor circuit and configured to switch said N switches at a substantially constant switching frequency in response to said monitor circuit, wherein said control circuit comprises:

a first state-variable generator coupled to said monitor circuit and configured to generate a first state variable;

a second state variable generator coupled to said first state-variable generator and configured to synthesize a second state variable;

a sliding-surface generator coupled to said monitor circuit and configured to generate a single sliding surface for said N phases in response to said first and second state variables;

a pulse-width-modulation (PWM) generator coupled to said sliding-surface generator and configured to translate said single sliding surface into a stream of switching pulses at said substantially constant switching frequency; and

a phase selector coupled to said N switches, coupled to said PWM generator, and configured to switch said N switches at said substantially constant switching frequency in response to said stream of switching pulses so that each of said N switches effects one of said N phases.

4. A power supply as claimed in claim 3 wherein said sliding surface has a value αx 1 +x 2 , wherein:

α is a constant having a range, 0≦α≦(τ)−1, where τ is a time constant having a value RC, where R is a value of a resistive component of said load; and C is a value of said capacitance;

x 1 is a first two state variable and has a value V Ref −V Out , where V Ref is a value of a reference voltage, and V Out is a value of an output voltage of said power supply; and

x 2 is a second state variable and is a derivative of said first state variable over time.

5. A power supply as claimed in claim 3 wherein:

said first state variable is a difference between a voltage of said output energy and a fixed reference voltage equal to an idealized output voltage of said power supply; and

said second state variable is a derivative of said first state variable over time.

6. A power supply as claimed in claim 3 wherein:

said second state-variable generator is an inductive current generator configured to synthesize N synthesized inductive currents from said first state variable; and

said PWM generator comprises a current-balance control coupled to said sliding-surface generator, coupled to said inductive current generator, and configured to adjust said comprehensive feedback signal so that N inductive currents are rendered substantially equal in response to said N synthesized inductive currents, wherein each of said N inductive currents is a current through one of said N inductances.

7. A power supply as claimed in claim 3 wherein:

said PWM generator is configured to form a variable window for said sliding surface, and

said phase selector is configured to switch said N switches at said switching frequency as determined by said variable window.

8. A power supply as claimed in claim 7 wherein said switching frequency is determined by said variable window and said load.

9. A power supply as claimed in claim 8 wherein:

a change in said dynamic load produces a first marginal change in said switching frequency; and

said PWM generator produces a second marginal change in said switching frequency in opposition to said first marginal change.

10. A power supply as claimed in claim 3 wherein:

said phase selector collectively switches said N switches at said substantially constant switching frequency; and

said phase selector individually switches each of said N switches at substantially N −1 times said substantially constant switching frequency.

11. A method of operating a fixed-frequency switching power supply, said method comprising:

receiving an input energy from a D-C power source;

providing an output energy to a dynamic load;

monitoring said output energy;

deriving a first state variable of said power supply in response to said monitoring activity;

synthesizing a second state variable of said power supply in response to said deriving activity;

producing a stream of switching pulses at a substantially constant frequency in response to said deriving and synthesizing activities;

switching N switches in response to said stream of switching pulses, where N is a positive integer; and

effecting N phases of said input D-C energy with said N switches.

12. A method as claimed in claim 11 wherein said deriving activity comprises:

generating a fixed reference voltage; and

subtracting an output voltage of output energy from said fixed reference voltage.

13. A method as claimed in claim 11 wherein said synthesizing activity comprises extracting a derivative of said first state variable over time.

14. A method of operating a fixed-frequency switching power supply, said method comprising:

receiving an input energy from a D-C power source;

providing an output energy to a dynamic load;

monitoring said output energy;

producing a stream of switching pulses at a substantially constant frequency in response to said monitoring activity, wherein said producing activity comprises;

generating a sliding surface from no more than two state variables of said power supply;

forming a variable window for said sliding surface; and

translating said sliding surface into a stream of switching pulses;

switching N switches in response to said stream of switching pulses, where N is a positive integer; and

effecting N phases of said input D-C energy with said N switches.

15. A method as claimed in claim 14 wherein said generating activity generates said sliding surface for said N phases, said sliding surface having a value αx 1 +x 2 , where:

α is a constant not less than zero;

x 1 is a first state variable of said power supply; and

x 2 is a second state variable of said power supply.

16. A method as claimed in claim 14 additionally comprising:

generating signals corresponding to inductive currents through each of N inductances in response to said monitoring activity, wherein said signals are N synthesized inductive currents; and

adjusting said sliding surface for each of said N phases so that said N synthesized inductive currents are rendered more closely equal.

17. A method as claimed in claim 11 additionally comprising:

producing a first marginal change in said switching frequency in response to a change in said load; and

producing a second marginal change in said switching frequency in opposition to said first marginal change.

18. A system comprising:

a D-C power source;

a dynamic load; and

a fixed-frequency sliding-mode switching power supply having N phases, where N is a positive integer, and configured to receive an input energy from said D-C power source, to convert said input energy into an output energy, and to provide said output energy to said dynamic load, said power supply comprising:

N switches coupled to said D-C power source, wherein each of said N switches effects one of said N phases;

N inductances, wherein each of said N inductances is coupled to one of said N switches;

a capacitance coupled to said N inductances and said dynamic load;

a monitor circuit coupled to said capacitance and configured to monitor said output energy; and

a sliding mode control circuit coupled to said monitor circuit, having a sliding surface generator configured to generate a single sliding surface for said N phases as said feedback signal, and configured to switch said N switches at a substantially constant switching frequency in response to said monitor circuit.

19. A system as claimed in claim 18 wherein said sliding-mode control circuit comprises:

a constant-frequency control coupled to said sliding-surface generator, configured to translate said sliding surface into a stream of switching pulses at a substantially constant switching frequency, and comprising:

a reference generator configured to generate a reference frequency;

a frequency comparator configured to compare said switching frequency to said reference frequency to produce a frequency error; and

a variable-window generator configured to form a variable window for said sliding surface in response to said error frequency; and

a phase selector coupled to said N switches, coupled to said constant-frequency control, and configured to sequentially switch said N switches at said substantially constant switching frequency in response to said stream of switching pulses, wherein:

said phase selector switches said N switches in response to said sliding surface, said variable window, and an impedance of said dynamic load;

said dynamic load produces a first marginal change in said switching frequency in response to a change in said impedance; and

said constant-frequency control adjusts said variable window to produce a second marginal change in said switching frequency in opposition to said first marginal change.