Electricity: power supply or regulation systems – Output level responsive – Using a three or more terminal semiconductive device as the...
Reexamination Certificate
2002-12-13
2004-03-30
Han, Jessica (Department: 2838)
Electricity: power supply or regulation systems
Output level responsive
Using a three or more terminal semiconductive device as the...
C323S283000
Reexamination Certificate
active
06713995
ABSTRACT:
This application incorporates by reference Taiwan application Ser. No. 091108569, filed on Apr. 25, 2002.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates in general to a voltage regulator, and more particularly to a pulse width modulation (PWM) voltage regulator.
2. Description of Related Art
As more and more powerful and innovative portable electronic devices are being developed, the efficiency of the switching power converters for supplying power to the portable electronic devices becomes more significant. In order to improve the power converting efficiency, the pulse width modulation (PWM) power converter has been widely applied to various electronic products. Referring to
FIG. 1
, a conventional PWM voltage regulator is illustrated. Suppose a load terminal voltage Vt between a load terminal of the PWM voltage regulator and the ground has a voltage rating of 5V. When a reduced load is connected to the load terminal, the load terminal voltage Vt will increase slightly to 5.01 V, for example. In order to stabilize the load terminal voltage Vt to the voltage rating, a switching device, such as a transistor Q, can be used to adjust the input current fed to the load. After the effective value of the input current is reduced, the load terminal voltage Vt will be reduced to the voltage rating so as to achieve voltage regulation. In order to implement the foregoing function, the control circuit
100
can be connected to the load terminal and the transistor Q. The control circuit
100
has an error amplifier
110
for detecting an error between the load terminal voltage Vt and a reference voltage Vr, in which the reference voltage level Vr can be set to 5V, that is, the voltage rating at the load terminal. In addition, an error signal er with respect to this error is produced. The oscillator
130
can produce an oscillating signal at a constant frequency, such as a 200-KHz sawtooth signal. The oscillating signal is also fed into a modulation signal generator
120
, such as a comparator. After the modulation signal generator
120
receives the error signal er and the oscillating signal, a modulation signal ms will be generated according to these two signals, and the modulation signal ms will be fed to a driving device
140
. Then, the driving device
140
makes the conduction period for the transistor Q shorter according to the modulation signal ms. As a result, the effective value of the current flowing through the transistor Q will be reduced, and the load terminal voltage Vt will return to the voltage rating.
Referring to
FIG. 2
, a schematic diagram illustrates a relation among the error signal, the oscillating signal, and the modulation signal. The oscillating signal, for example, has a frequency of 200 KHz and a sawtooth waveform. The modulation signal generator
120
can convert the relation between the error signal er and the oscillating signal into the modulation signal ms. When the error signal er is greater than the oscillating signal, the modulation signal ms can be set to a high level. When the error signal er is less than the oscillating signal, the modulation signal ms can be set to a low level. Therefore, if the load terminal voltage level Vt is higher than the reference voltage level Vr, the error signal er is lowered, so that the modulation signal ms has a shorter duration at the high level. Since the driving device
140
is to drive the transistor Q based on the modulation signal ms, the conduction period for the transistor Q is shortened. As a result, the effective value of the current flowing into the load is reduced, and the load terminal voltage level Vt is reduced. Since this is a closed loop control, the load terminal voltage level Vt can be stabilized at the voltage rating, that is, Vr=Vt.
As described above, the modulation signal ms results from the comparison of the error signal er and the oscillating signal, and therefore the frequencies of the modulation signal ms and the oscillating signal are the same. Since the oscillating signal has a constant frequency and a sawtooth waveform, the modulation signal ms has a constant frequency also, but only the pulse width would vary as the load varies. In other words, no matter whether a light load or a heavy load is connected to the PWM regulator, the PWM regulator uses a signal at a constant frequency, whose pulse width may change with the load, to switch on and off the transistor Q. Since the power consumption for the light load is rather small, the conduction period for the transistor Q is relatively shorter and the power transmitted during the conduction period is less. However, while the PWM regulator operates at light load under the PWM mode, the transistor Q even has a switching rate of 200,000 times per second, if the oscillating signal is set to 200 KHz, for example. There is power loss when the transistor Q is switched on and off and as a result, the efficiency of the power conversion decreases significantly. In order to solve the issue of poor efficiency under the PWM mode at light load, a PWM/PFM dual mode converter has therefore been developed. The PWM/PFM dual mode converter enters the PWM mode at heavy load, but enters the pulse frequency modulation (PFM) mode at light load. The PFM uses a modulation signal with the same pulse width but a changeable frequency to drive the transistor Q, so that the efficiency of power conversion is improved. This manner can solve the problem of poor efficiency of the power conversion at light load. However, unwanted noise would occur due to vibration of the inductance and the voltage effect from the ceramic capacitor, when the operation mode is switched to the PFM mode at light load and the frequency of the modulation signal is within the range of about 20 Hz to 20 KHz. In this situation, it would cause an increase of the ripple voltage output due to the decrease of the working frequency.
Due to the foregoing reasons, how to improve the efficiency of power conversion with respect to the light load condition and further prevent the noise is then a very important issue to be solved.
SUMMARY OF THE INVENTION
It is therefore an objective of the invention to provide a voltage regulator with pulse width modulation in dual frequencies, so that the efficiency of power conversion can be effectively improved for the light load condition, and the occurrence of noise can be prevented also.
In accordance with the foregoing and other objectives of the invention, the invention provides a voltage regulator with pulse width modulation in dual frequencies, which is briefly described as follows.
The voltage regulator with pulse width modulation in dual frequencies takes the PWM voltage regulation mode using a different frequency according to the degree of loading effect on the voltage regulation. In order to determine the degree of loading effect, a heavy-load reference voltage and a light-load reference voltage can be predetermined. When the error signal is greater than the heavy-load reference voltage, the load is a heavy load and a modulation signal at a higher frequency, such as 200 KHz, is used to drive the transistor. Conversely, when the error signal is less than the light-load reference voltage, the load is a light load and a modulation signal at a lower frequency, such as 20 KHz, is used to drive the transistor. In this way, the efficiency of power conversion is improved.
The voltage regulator with pulse width modulation in dual frequencies includes a transistor switcher, an error amplifier, a dual frequency signal generator, a modulation signal generator, a load sensing device, an OR operator, an AND operator, and a driving device. The dual frequency signal generator is used for generating a high frequency signal and a low frequency signal. The modulation signal generator can receive the high frequency signal and the error signal and then can modulate the two signals into a modulation signal, in which the frequencies for the high frequency signal and the modulation signal are the same. On the other hand, the load sensing device can determ
Han Jessica
Quanta Computer Inc.
Rabin & Berdo P.C.
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