# Patent

## Title
High efficiency power converter

## Abstract

A high efficiency power converter comprises a boost converter for converting an input voltage to a first voltage on a first output, a buck converter for converting the input voltage to a second voltage on a second output, a linear regulator for converting the first voltage to a third voltage on the second output when the second voltage is lower than a first threshold, and a voltage detector for detecting the input voltage for preventing a reverse current flowing from the second output to the buck converter when the input voltage is lower than a second threshold.

## Background
FIELD OF THE INVENTION
The present invention is related generally to power conversion and more particularly, to a high efficiency power converter.
BACKGROUND OF THE INVENTION
For portable electronic products, alkaline and lithium batteries are generally used as the power sources. However, the battery has a tendency of gradually decreased voltage as the use time increases. To retain a stable output voltage, a two-stage power converter is typically employed, by which the input voltage supplied by the battery or batteries is stepped up first and then stepped down to the desired level.
FIG. 1 shows a conventional two-stage power converter 10, which comprises a boost converter 12 cascaded with a buck converter 14. Generally, the battery voltage Vbat is provided by two alkaline batteries or one lithium battery. The battery voltage Vbat is converted by the boost converter 12 to generate a first output voltage Vout1, and then the first output voltage Vout1 is converted by the buck converter 14 to generate a second output voltage Vout2. Unfortunately, the boost converter 12 and the buck converter 14 each has the conversion efficiency of about 90%, and therefore, the total efficiency of the two-stage power conversion is only about 81% or less. Namely, the overall efficiency of the two-stage power converter is reduced due to the cascaded conversion.
Therefore, it is desired a high efficiency power converter.
SUMMARY OF THE INVENTION
One object of the present invention is to provide a high efficiency power converter.
In a high efficiency power converter, according to the present invention, a boost converter is used to convert an input voltage to a first voltage on a first output, a buck converter is used to convert the input voltage to a second voltage on a second output, and a linear regulator is coupled between the first and second outputs to convert the first voltage to a third voltage on the second output when the second voltage is lower than a first threshold. The buck converter comprises a switch coupled between the input voltage and the second output, an error amplifier for generating an error signal in response to a difference between the voltage on the second output and a reference voltage, and a logic circuit responsive to the error signal for switching the switch. A voltage detector is further comprised in the power converter to detect the input voltage, so as to prevent a reverse current flowing from the second output to the buck converter when the input voltage is lower than a second threshold.
Since the power conversion is implemented by single conversion stage, either with the boost converter or with the buck converter, in association with the linear regulator to generate the third voltage in replace of the second voltage when the second voltage is lower than the first threshold, the overall efficiency of the power conversion is improved up to about 90% or more.



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 simplified schematic illustration of a conventional two-stage power converter;
FIG. 2 shows a power converter of the present invention having two alkaline batteries as its power source; and
FIG. 3 shows a power converter of the present invention having a lithium battery as its power source.



DETAILED DESCRIPTION OF THE INVENTION
FIG. 2 shows a power converter 30, according to the present invention, that uses two alkaline batteries as its power source. In the power converter 30, the input voltage Vbat supplied by the two alkaline batteries is 3.2V, and is converted by a boost converter 32 to generate a first voltage of 3.3V on a first output Vout1. The level of the first voltage Vout1 is set up by selecting the parameters of the inductor L1, capacitors C1 and C2, and resistors R1 and R2 in the boost converter 32. In the boost converter 32, the resistors R1 and R2 connected in series between the first output Vout1 and ground GND serve as a voltage divider for dividing the first voltage Vout1 to generate a feedback voltage VFB1 for an error amplifier 3204 to compare with a reference voltage Vref1, so as to generate an error signal EA1 upon which a logic circuit 3206 will generate a signal to switch a transistor 3210 by a driver 3208 to regulate the first voltage Vout1 at 3.3V. The error signal EA1 may be compensated additionally by a compensation network 3212 consisting of a capacitor C3 and a resistor R3 in such a manner that the phase of the error signal EA1 will not shift. The input voltage Vbat of 3.2V is also provided to a buck converter 34 to generate a second voltage of 1.8V on a second output Vout2. The level of the second voltage Vout2 is set up by selecting the parameters of the inductor L2, capacitors C4 and C5, and resistors R4 and R5 in the buck converter 34. In the buck converter 34, the resistors R4 and R5 connected in series between the second output Vout2 and ground GND serve as a voltage divider for dividing the second voltage Vout2 to generate a feedback voltage VFB2 for an error amplifier 3404 to compare with a reference voltage Vref2, so as to generate an error signal EA2 upon which a logic circuit 3406 will generate a signal to switch transistors 3412 and 3414 by drivers 3408 and 3410 to regulate the second voltage Vout2 at 1.8V. Likewise, the error signal EA2 may be compensated by a compensation network 3416 consisting of a capacitor C6 and a resistor R6 in such a manner that the phase of the error signal EA2 will not shift. In this embodiment, the buck converter 34 is designed with a synchronous voltage converter scheme for illustration, however the power converter of the present invention may be implemented with asynchronous buck converter in some other embodiments as well.
As the use time of the two alkaline batteries increases, the input voltage Vbat reduces gradually, and until the input voltage Vbat is lower than 1.8V, the second voltage Vout2 generated by the buck converter 34 could not be regulated at 1.8V any more. For the power converter 30 still to operate normally when the input voltage Vbat is lower than 1.8V, a low dropout (LDO) regulator 36 or other type of linear regulator is further coupled between the first output Vout1 and the second output Vout2. In the LDO regulator 36, resistors R7 and R8 are connected in series between the second output Vout2 and ground GND to serve as a voltage divider for dividing the voltage on the second output Vout2 to generate a voltage VD for a comparator 3604 to compare with a reference voltage Vref3, and when the second voltage Vout2 generated by the buck converter 34 is lower than a threshold, for example 1.75V, the comparator 3604 generates a signal to turn on a transistor 3602 connected between the first output Vout1 and the second output Vout2. After the transistor 3602 turns on, the first voltage Vout1 of 3.3V generated by the boost converter 34 is converted by the LDO regulator 36 to generate a third voltage Vout3 of 1.75V on the second output Vout2. As a result, the power converter 30 may operate normally even in the case that the input voltage Vbat supplied by the two alkaline batteries decreases under the threshold 1.8V, and therefore the overall efficiency of the power converter 30 is improved.
On the other hand, as the use time of the two alkaline batteries increases, the input voltage reduces gradually until the input voltage Vbat lower than the voltage on the second output Vout2, due to the parasitic diode of the transistor 3412, as shown by dashed line in FIG. 2, a reverse current Ib will be generated to flow from the second output Vout2 toward the input 33 of the buck converter 34 if the transistor 3412 is on. To prevent the generation of the reverse current Ib, a voltage detector 38 is coupled to the input 33 and the buck converter 34 for detecting the input voltage Vbat. In the voltage detector 38, resistors R9 and R10 are connected in series between the input 33 and ground GND to serve as a voltage divider for dividing the input voltage Vbat to generate a voltage VA for a comparator 3804 to compare with a reference voltage Vref4. When the input voltage Vbat is lower than a threshold, for example 1.9V, the comparator 3804 will generate a signal to turn on a transistor 3802 connected between the output of the error amplifier 3404 in the buck converter 34 and ground GND to direct the error signal EA2 to ground GND, such that the transistor 3412 turns off, and hence the reverse current Ib will not be generated.
FIG. 3 shows a power converter 40 using a lithium battery as its power source. In this embodiment, the input voltage Vbat supplied by the lithium battery is 4.2V, and the boost converter 32, the buck converter 34, the LDO regulator 36 and the voltage detector 38 that are employed in the power converter 30 shown in FIG. 2 are also comprised hereof. By selecting the parameters of the inductor L1, the capacitors C1 and C2 and the resistors R1 and R2, the voltage on the first output Vout1 is set to be the sum of 3.2V and the minimum LDO dropout voltage for the LDO regulator 36 to operate in higher efficiency region. The error amplifier 3204 compares the feedback voltage VFB1 proportional to the first voltage Vout1 with the reference voltage Vref1 to generate the error signal EA1, and the logic circuit 3206 responds to the compensated signal of the error signal EA1 compensated by a compensation network 3212 to switch the transistor 3210 to regulate the first voltage Vout1 at 4.2V.
Similarly, in the buck converter 34, by selecting the parameters of the inductor L2, the capacitors C4 and C5 and the resistors R4 and R5, the input voltage Vbat is converted to a second voltage of 3.3V at the second output Vout2, the error amplifier 3404 compares the feedback voltage VFB2 proportional to the second voltage Vout2 with the reference voltage Vref2 to generate the error signal EA2, and the logic circuit 3206 responds to the compensated signal of the error signal EA2 compensated by a compensation network 3416 to switch the transistors 3412 and 3414 to regulate the second voltage Vout2 at 3.3V.
In the LDO regulator 36, the comparator 3604 compares the voltage VD proportional to the second voltage Vout2 with the reference voltage Vref3 to generate a signal to switch the transistor 3602. When the second voltage Vout2 generated by the buck converter 34 is lower than the threshold 3.2V, the comparator 3604 will turns on the transistor 3602, so as to convert the first voltage of 4.2V generated by the boost converter 32 to a third voltage Vout3 of 3.2V on the second output Vout2. Therefore, the power converter 40 may operate normally as well.
In the voltage detector 38, the comparator 3804 compares the voltage VA proportional to the input voltage Vbat with the reference voltage Vref4 to generate a signal to switch the transistor 3802. When the input voltage Vbat is lower than the threshold 3.3V, the output generated by the comparator 3804 will turn on the transistor 3802 to direct the error signal EA2 to ground GND. The transistor 3412 is thus turned off to prevent the generation of a reverse current Ib.
In addition, some other embodiments may further comprise another buck converter 42 connected with the input voltage Vbat for other requirements, as shown by dashed line in FIG. 3.
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.

## Claims

1. A high efficiency power converter comprising:
a boost converter for converting an input voltage to a first voltage on a first output;
a buck converter concurrently operable with the boost converter to convert the input voltage to a second voltage on a second output, the second voltage being different in value from the first voltage;
a linear regulator coupled between the first and second outputs for converting the first voltage to a third voltage on the second output when the second voltage is lower than a first threshold; and
a voltage detector for detecting the input voltage for preventing a reverse current flowing from the second output to the buck converter when the input voltage is lower than a second threshold.

2. The power converter of claim 1, wherein the buck converter comprises:
an error amplifier for generating an error signal in response to a difference between the voltage on the second output and a reference voltage;
a switch coupled between the input voltage and the second output; and
a logic circuit responsive to the error signal for switching the switch.

3. The power converter of claim 2, wherein the buck converter further comprises a compensation network for compensating the error signal.

4. The power converter of claim 2, wherein the voltage detector comprises:
a voltage divider for dividing the input voltage to thereby generate a fourth voltage;
a comparator for comparing the fourth voltage with a second reference voltage to thereby generate a comparison signal; and
a second switch controlled by the comparison signal for directing the error signal to a reference ground.

5. The power converter of claim 1, wherein the linear regulator comprises an LDO regulator.