Control method for buck-boost power converters

1. A method comprising:

generating a first ramp signal for controlling a buck converter portion of a buck-boost converter generating a second ramp signal for controlling a boost converter portion of the buck-boost converter;

comparing the first ramp signal and the second ramp signal to a control signal;

controlling a state of a high-side switch of the buck converter portion using the comparing the first ramp signal to the control signal and a state of a low-side switch of the boost converter portion using the comparing the second ramp signal to the control signal;

comparing a current flowing through an inductor of the buck-boost converter to a current threshold, wherein the current threshold is a negative value; and

terminating a switching cycle of the buck-boost converter based upon the comparing the current flowing through the inductor to the current threshold, wherein the current flowing through the inductor changes direction twice during the switching cycle.

2. The method of claim 1 , further comprising:

adding an offset into the second ramp signal;

in response to an output voltage greater than an input voltage of the buck-boost converter, turning off the low-side switch of the boost converter portion, turning on a high-side switch of the boost converter portion and resetting the second ramp signal when the second ramp signal is equal to the control signal; and

in response to the output voltage greater than the input voltage of the buck-boost converter, turning off the high-side switch of the boost converter portion and resetting the first ramp signal when the current flowing through the inductor is equal to the current threshold.

3. The method of claim 1 , further comprising:

adding a delay time at an end of a switching cycle.

4. The method of claim 3 , further comprising:

configuring the buck-boost converter to operate at a fixed switching frequency by adjusting the delay time.

5. The method of claim 1 , further comprising:

adding an offset into the second ramp signal;

in response to an output voltage approximately equal to an input voltage of the buck-boost converter, turning off the low-side switch of the boost converter portion, turning on a high-side switch of the boost converter portion and resetting the second ramp signal when the second ramp signal is equal to the control signal;

in response to the output voltage approximately equal to the input voltage of the buck-boost converter, turning off the high-side switch of the buck converter portion, turning on a low-side switch of the buck converter portion and resetting the first ramp signal when the first ramp signal is equal to the control signal; and

in response to the output voltage approximately equal to the input voltage of the buck-boost converter, turning off the low-side switch of the buck converter portion and the high-side switch of the boost converter portion, and turning on the high-side switch of the buck converter portion and the low-side switch of the boost converter portion when the current flowing through the inductor is equal to the current threshold.

6. The method of claim 1 , further comprising:

adding an offset into the second ramp signal;

in response to an input voltage greater than an output voltage of the buck-boost converter, turning off the high-side switch of the buck converter portion, turning on a low-side switch of the buck converter portion and resetting the first ramp signal when the first ramp signal is equal to the control signal; and

in response to the input voltage greater an output voltage of the buck-boost converter, turning off the low-side switch of the buck converter portion and turning on the high-side switch of the buck converter portion when the current flowing through the inductor is equal to the current threshold.

7. The method of claim 6 , wherein:

the offset is greater than the control signal.

8. The method of claim 1 , further comprising:

adding an offset into the first ramp signal;

in response to an output voltage greater than an input voltage of the buck-boost converter, turning off the low-side switch of the boost converter portion, turning on a high-side switch of the boost converter portion and resetting the second ramp signal when the second ramp signal is equal to the control signal;

in response to the output voltage greater than the input voltage of the buck-boost converter, turning off the high-side switch of the buck converter portion, turning on a low-side switch of the buck converter portion and resetting the first ramp signal when the first ramp signal is equal to the control signal; and

in response to the output voltage greater than the input voltage of the buck-boost converter, turning off the high-side switch of the boost converter portion and the low-side switch of the buck converter portion, and turning on the low-side switch of the boost converter portion and the high-side switch of the buck converter portion when the current flowing through the inductor is equal to the current threshold.

9. The method of claim 1 , further comprising:

adding an offset into the first ramp signal;

in response to an output voltage approximately equal to an input voltage of the buck-boost converter, turning off the low-side switch of the boost converter portion, turning on a high-side switch of the boost converter portion and resetting the second ramp signal when the second ramp signal is equal to the control signal;

in response to the output voltage approximately equal to the input voltage of the buck-boost converter, turning off the high-side switch of the buck converter portion, turning on a low-side switch of the buck converter portion and resetting the first ramp signal when the first ramp signal is equal to the control signal; and

in response to the output voltage approximately equal to the input voltage of the buck-boost converter, turning off the low-side switch of the buck converter portion and the high-side switch of the boost converter portion, and turning on the high-side switch of the buck converter portion and the low-side switch of the boost converter portion when the current flowing through the inductor is equal to the current threshold.

10. The method of claim 1 , further comprising:

adding an offset into the first ramp signal;

in response to an input voltage greater than an output voltage of the buck-boost converter, turning off the low-side switch of the boost converter portion, turning on a high-side switch of the boost converter portion and resetting the second ramp signal when the second ramp signal is equal to the control signal;

in response to the input voltage greater than the input voltage of the buck-boost converter, turning off the high-side switch of the buck converter portion, turning on a low-side switch of the buck converter portion and resetting the first ramp signal when the first ramp signal is equal to the control signal; and

in response to the input voltage greater than the input voltage of the buck-boost converter, turning off the low-side switch of the buck converter portion and the high-side switch of the boost converter portion, and turning on the high-side switch of the buck converter portion and the low-side switch of the boost converter portion when the current flowing through the inductor is equal to the current threshold.

11. A method comprising:

detecting an input voltage and an output voltage of a power converter comprising a buck converter portion and a boost converter portion;

comparing a first ramp signal and a second ramp signal to a control signal;

controlling a state of a high-side switch of the buck converter portion using the comparing the first ramp signal to the control signal and a state of a low-side switch of the boost converter portion using the comparing the second ramp signal to the control signal;

determining an operation mode transition based upon a ratio of the input voltage to the output voltage;

comparing a current flowing through an inductor of the power converter to a current threshold; and

terminating a switching cycle based upon the comparing the current flowing through the inductor to the current threshold.

12. The method of claim 11 , further comprising:

adding an offset into the second ramp signal; and

configuring the power converter such that:

the high-side switch of the buck converter portion is always on when the output voltage is greater than the input voltage.

13. The method of claim 11 , further comprising:

adding an offset into the second ramp signal;

configuring the power converter such that:

the high-side switch of the buck converter portion is turned off and a low-side switch of the buck converter portion is turned on when the first ramp signal is equal to the control signal; and

the low-side switch of the boost converter portion is turned off and a high-side switch of the boost converter portion is turned on when the second ramp signal is equal to the control signal.

14. The method of claim 11 , further comprising:

adding an offset into the second ramp signal; and

configuring the power converter such that:

a high-side switch of the boost converter portion is always on when the input voltage is greater than the output voltage.

15. The method of claim 11 , wherein:

adding an offset into the first ramp signal;

configuring the power converter such that:

the high-side switch of the buck converter portion is turned off and a low-side switch of the buck converter portion is turned on when the first ramp signal is equal to the control signal; and

the low-side switch of the boost converter portion is turned off and a high-side switch of the boost converter portion is turned on when the second ramp signal is equal to the control signal.

16. The method of claim 15 , further comprising:

turning off the low-side switch of the buck converter portion and the high-side switch of the boost converter portion, and turning on the high-side switch of the buck converter portion and the low-side switch of the boost converter portion when the current flowing through the inductor is equal to the current threshold.

17. The method of claim 15 , further comprising:

under a light load operating condition, turning off the high-side switch of the boost converter portion when the current flowing through the inductor is equal to the current threshold; and

turning off the low-side switch of the buck converter portion after a delay time from the turning off the high-side switch of the boost converter portion.

18. The method of claim 11 , wherein:

the current threshold is a negative value.

19. A converter comprising:

a buck converter portion comprising a first high-side switch and a first low-side switch connected in series across an input capacitor;

a boost converter portion comprising a second high-side switch and a second low-side switch connected in series across an output capacitor;

an inductor coupled between the buck converter portion and the boost converter portion; and

a controller configured to:

generate a first ramp signal for controlling the buck converter portion;

generate a second ramp signal for controlling the boost converter portion;

compare the first ramp signal and the second ramp signal to a control signal;

control a state of the first high-side switch using the comparing the first ramp signal to the control signal and a state of the second low-side switch using the comparing the second ramp signal to the control signal;

compare a current flowing through the inductor to a current threshold; and

terminate a switching cycle based upon comparing the current flowing through the inductor to the current threshold.

20. The converter of claim 19 , wherein the controller is configured to:

receive an output voltage of an error amplifier, wherein the error amplifier has a first input coupled to an output voltage of the converter and a second input connected to a reference voltage;

compare an error amplifier output voltage to a voltage threshold;

configure the converter to operate in a buck operating mode when the voltage threshold is greater than the error amplifier output voltage, wherein the boost converter portion operates under a minimum duty cycle;

configure the converter to operate in a boost operating mode when the error amplifier output voltage is greater than the voltage threshold and the output voltage of the converter is greater than an input voltage of the converter, wherein the first high-side switch of the buck converter portion is always on; and

configure the converter to operate in a buck-boost operating mode when the error amplifier output voltage is greater than the voltage threshold and the output voltage of the converter is approximately equal to the input voltage of the converter.