BRIEF DESCRIPTION OF DRAWINGS
These and other objects, features and advantages of the present invention will become apparent to those skilled in the art upon consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings, in which:
FIG. 1 shows a conventional PWM power converter;
FIG. 2 shows various signals in the circuit of FIG. 1;
FIG. 3 shows an embodiment according to the present invention;
FIG. 4 shows an embodiment for the PWM power converter of FIG. 3;
FIG. 5 is a flow chart according to the present invention for the process of modulating the duty of a PWM signal;
FIG. 6 shows how the duty of a PWM signal is changed by changing the slope of the linear oscillating ramp signal RAMP;
FIG. 7 shows another embodiment according to the present invention; and
FIG. 8 shows an embodiment of the PWM power converter of FIG. 7.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 3 shows an embodiment according to the present invention, in which the PWM power converter 200 is similar to the conventional PWM power converter 100 of FIG. 1. The difference is that the PWM power converter 200 has a duty information circuit 210 to extract a duty information from the PWM signal and transform the duty D of the PWM signal to a linearly dependent voltage V(D), a feed forward circuit 212 to transform the voltage V(D) to a feed forward signal FF, and a ramp generator 204 to produce a linear oscillating ramp signal RAMP for the PWM comparator 206 according to an oscillator signal OSC provided by an oscillator 214 and the feed forward signal FF. In this embodiment, the slope of the linear oscillating ramp signal RAMP is modulated by the feed forward signal FF, namely, the duty D of the PWM signal, and hence the slope of the linear oscillating ramp signal RAMP will change with the duty D of the PWM signal.
Because the slope of the linear oscillating ramp signal RAMP is modulated by the duty D which is directly extracted from the PWM signal, the transient response of the PWM power converter 200 is improved. Besides, because the slope of the linear oscillating ramp signal RAMP can be changed, the modulation gain can be changed accordingly. When the duty D decreases with the decrease of the output voltage Vout, the slope of the linear oscillating ramp signal RAMP is increased to decrease the modulation gain, and hence improve the stability. Specifically, the foregoing control process does not need to monitor the supply voltage Vin.
FIG. 4 shows an embodiment for the duty information circuit 210, the feed forward circuit 212, and the ramp generator 204 of FIG. 3. In the duty information circuit 210, a current source 2102 provides a charging current Iosc, a switch 2104 is switched by the PWM signal to charge the capacitor C4 with the charging current Iosc to produce the voltage V(D), and a sample and hold circuit 2122 samples the voltage V(D) to produce a sampled voltage for the feed forward circuit 212. The sampled voltage from the duty information circuit 210 is filtered out its noises by a low pass filter (LPF) 2124, and an analog divider and current transformer 2126 divides the filtered voltage by a constant and transforms that into a current I. In this embodiment, the feed forward signal FF is the current I. In the ramp generator 204, two transistors m1 and m2 form a current mirror to mirror the current I to charge a capacitor C5, and a switch 2042 is switched by the oscillator signal OSC of the oscillator 214, such that the capacitor C5 produces the linear oscillating ramp signal RAMP. Briefly, a greater duty D of the PWM signal results in a longer charging time for the capacitor C4, and so produces a higher voltage V(D); a higher voltage V(D) results in a lower current I, and so causes the capacitor C5 to be charged slower, i.e., the slope of the linear oscillating ramp signal RAMP becomes smaller.
In further detail, let t represent the rising time of the linear oscillating ramp signal RAMP, and T represent the switching period of the PWM signal. As mentioned in the foregoing description, the duty D of the PWM signal is equal to Vout/Vin, and therefore
t=D×T. [Eq-1]
The current Iosc charges the capacitor C4 to produce the voltage
which shows that the voltage V(D) is linearly dependent on the duty D. Then, by the analog divider and current transformer 2126, the voltage V(D) is divided by a constant A and transformed into the current
which shows that the current I is inversely proportion to the voltage V(D) and is a function of the duty D. By charging the capacitor C5 with the current I, the linear oscillating ramp signal RAMP has the amplitude
which shows that Vramp is also a function of the duty D. Since the frequency of the linear oscillating ramp signal RAMP is determined by the oscillator 214, the slop of the linear oscillating ramp signal RAMP is also inversely proportional to the duty D. Hence, the modulation gain is
Vin/Vramp=(Iosc×C5×Vout)/(C4×A). [Eq-5]
If the output voltage Vout is constant, the modulation gain is constant too. In addition, the loop gain is proportional to
(Vin/Vramp)×(Vref/Vout)=(Iosc×C5×Vref)/(C4×A), [Eq-6]
and is therefore a constant, independent on the supply voltage Vin and the output voltage Vout.
Alternatively, depending on the duty D, the duty information circuit 210 may change the charging current Iosc, instead of the charging time, so as to produce the voltage V(D) linearly dependent on the duty D. In another embodiment, the feed forward circuit 212 may employ an integrator or other circuits, instead of analog divider, to produce the current I as a function of the duty D.
The circuit of FIG. 4 is a preferred embodiment of producing the linear oscillating ramp signal RAMP, and FIG. 5 provides a general process of producing the linear oscillating ramp signal RAMP from the duty D of the PWM signal. At step 300, a duty information is extracted from the PWM signal to provide the duty D. At step 310, the duty D is transformed into a voltage V(D) or a current I(D) whose value is determined by the duty D. At step 320, the voltage V(D) or the current I(D) is transformed into a duty dependent function f(D), such as linear function, piecewise linear function, logarithm function, exponential function, power function, trigonometric function, and any other non-linear function of the duty D. At step 330, the linear oscillating ramp signal RAMP is produced according to the function f(D), such that the slope of the linear oscillating ramp signal RAMP is determined by the value of the function f(D). In another embodiment, it may directly determine the slope of the linear oscillating ramp signal RAMP from the duty D, without the transform process shown in FIG. 5.
FIG. 6 shows how the duty D of the PWM signal is changed by changing the slope of the linear oscillating ramp signal RAMP. When the supply voltage Vin becomes higher or the output voltage Vout becomes lower, the slope of the linear oscillating ramp signal RAMP becomes steeper, and therefore the error signal EO and the linear oscillating ramp signal RAMP will approach each other faster, which causes them to cross with each other earlier, and so the PWM signal changes earlier from high to low, in other words, the duty D becomes smaller. On the contrary, when the slope of the linear oscillating ramp signal RAMP becomes smaller because the supply voltage Vin decreases or the output voltage Vout increases, as shown by the dash line of FIG. 6, the error signal EO and the linear oscillating ramp signal RAMP will approach each other slower, which causes them to cross with each other later, and therefore, the PWM signal changes later from high to low, in other words, the duty D is greater. In steady state, the error signal EO will keep stable.
In the above embodiments, the duty D of the PWM signal is modulated by modulating the slope of the linear oscillating ramp signal RAMP. One skilled in the art should know that it could be replaced by feeding forward the duty D to modulate the voltage level of the error signal EO to achieve the same effect, and FIG. 7 provides such an embodiment. In a PWM power converter 400, a circuit 410 extracts a duty information from the PWM signal and produces a feed forward signal FF to combine with the error signal EO, so as to modulate the voltage level of the error signal EO, and therefore to modulate the duty D of the PWM signal. In one embodiment, the feed forward signal FF is a product of the duty D and a modulation coefficient β, and a combiner 412 subtracts the feed forward signal FF from the error signal EO to produce a modulated error signal
EO′=EO−FF=EO−β×D. [Eq-7]
The modulated error signal EO′ is thus changed with the duty D. Then, a PWM modulator 406 compares the modulated error signal EO′ with a constant slope linear oscillating ramp signal RAMP to produce the PWM signal. The modulated error signal EO′ equals to the product of the amplitude Vramp of the linear oscillating ramp signal RAMP and the duty D, that is
EO′=Vramp×D. [Eq-8]
From the equations Eq-7 and Eq-8, it may have the error signal
Besides, the modulation gain Vin/Vramp equals Vout/EO, so it can conduct the modulation gain from the equation Eq-9
Let Vramp+β be proportional to 1/D, i.e.
Vramp+β=α(1/D), [Eq-11]
where α is a constant. From the equations Eq-10 and Eq-11, the modulation gain will be
Therefore, if the output voltage Vout is constant, the modulation gain is constant too. In addition, the loop gain is proportional to
and is thus a constant, independent on the supply voltage Vin and the output voltage Vout. From the equation Eq-11, it may conduct the modulation coefficient
β=α(1/D)−Vramp [Eq-14]
FIG. 8 shows an embodiment for the circuit 410 of FIG. 7. In this embodiment, a duty information circuit 414 transforms the duty D of the PWM signal into a linearly dependent voltage V(D), and a feed forward circuit 416 produces the feed forward signal FF according to the operation of the voltage V(D) and the amplitude Vramp of the linear oscillating ramp signal RAMP. In another embodiment, the feed forward signal FF may be produced according to a linear function, a piecewise linear function, a logarithm function, an exponential function, a power function, a trigonometric function, or any other non-linear function of the duty D.
As known by one skilled in the art, it may combine the feed forward signal FF with the reference signal Vref or the feedback signal Vfb alternatively, in order to modulate the error signal EO. Therefore, only by modifying some parts of the circuit of FIG. 7 or FIG. 8, it may have the same effect as illustrated by the foregoing embodiments.
As shown in the above embodiments, direct extracting of the duty information from the PWM signal to produce a feed forward signal to modulate the input of the PWM comparator can speed up the transient response of the PWM power converter and improve the stability of the PWM loop.
While the present invention has been described in conjunction with preferred embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and scope thereof as set forth in the appended claims.
Patent Application
20080062731
