# Patent

## Title
Full-bridge circuit

## Abstract

A full-bridge circuit for a heat dissipation system. The heat dissipation system further includes a fan and a control circuit. The control circuit outputs a first control signal and a second control signal. The full-bridge circuit includes a first control line having a first switch and a third switch serially coupled, a second control line includes a second switch and a fourth switch serially coupled, and a induction coil having one terminal coupled between the first and third switches and the other terminal coupled between the second and fourth switches. The ON/OFF states of the first and second switches are controlled by the first and second control signals respectively. The ON/OFF states of the third and fourth switches are controlled by two driving voltage sources respectively.

## Background
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a full-bridge circuit, and in particular to a full-bridge circuit employed in a direct-current fan.
2. Brief Discussion of the Related Art
FIG. 1a is a block diagram of a conventional heat dissipation system. FIG. 1b shows a circuit of the full-bridge circuit in FIG. 1a. As shown in FIG. 1a, the heat dissipation system comprises a fan 10, a control circuit 11, and a full-bridge circuit 12. The full-bridge circuit 12 comprises switches Q1 to Q4 and an induction coil L. When the switches Q1 and Q4 are turned on, the switches Q2 and Q3 are turned off and a current path of an input voltage Vin includes the switch Q1, the induction coil L, and the switch Q4. Conversely, when the switches Q1 and Q4 are turned off, the switches Q2 and Q3 are turned on and a current path of the input voltage Vin includes the switch Q2, an induction coil L, and the switch Q3.
The switches Q1 to Q4 of the full-bridge circuit 12 are respectively controlled by four control signals output from node A1 to A4 of the control circuit 11. The control signals from the nodes A1 and A4 are in phase and out of phase with the control signals from the nodes A2 and A3. To ensure that the switch Q3 is turned off when the switch Q1 is turned on, a high-level control signal from the node A1 turns on the switch Q1 and a switch S1. Thus residual current of the switch Q3 is grounded, preventing false operation of the switch Q3. Similarly, a high-level control signal from the node A2 turns on the switch Q2 and a switch S2. Thus, that residual current of the switch Q4 is grounded, preventing false operation of the switch Q4.
The control circuit 11 is typically a micro-processor control unit (MCU) for outputting the four control signals A1 to A4. As the largest output voltage of a MCU is 5V, the lower bridge, the switches Q3 and Q4, of the full-bridge circuit 12 must substantially composed of low voltage elements, such as 4.5V metal oxide semiconductor field effect transistors (MOSFETs). The low voltage elements however have weak anti-noise ability resulting in false operation due to floating voltage in the circuit. As the number of available low voltage elements is limited, effective cost reduction is difficult. Moreover, four control signals are required from a MCU to control switches Q1 to Q4 of the full-bridge circuit 12, resulting in complicated circuitry.
Thus, a full-bridge circuit employed by a heat dissipation system for improving anti-noise ability and simplification of the full-bridge circuit are desirable.
SUMMARY OF THE INVENTION
Full-bridge circuits are provided. Some embodiments of the full-bridge circuit comprise two control signals utilized for controlling all switches therein to simplify the circuitry. The switches can comprises various types of elements, including low cast elements reducing in circuit cost.
Some embodiments of a full-bridge circuit are employed in heat dissipation systems. Some embodiments of a heat dissipation system comprise a control circuit outputting a first control signal and a second control signal to the full-bridge circuit. The first and second control signals are out of phase.
The full-bridge circuit comprises a first control line, a second control line, and an induction coil. The first control line comprises a first switch and a third switch serially connected. The second control line, parallel with the first control line, comprises a second switch and a fourth switch serially connected. The induction coil has one terminal coupled between the first and third switches and the other terminal coupled between the second and fourth switches. The ON/OFF states of the first and second switches are controlled by the first and second control signals respectively and the third and fourth switches are high voltage switches.
An input terminal of the third switch is coupled to a fifth switch in parallel and to a first resistor and a first driving voltage source in series. The ON/OFF state of the fifth switch is controlled by the first control signal. The ON/OFF state of the third switch is controlled by the first driving voltage source according to the ON/OFF state of the fifth switch.
An input terminal of the fourth switch is coupled to a sixth switch in parallel and to a second resistor and a second driving voltage source in series. The ON/OFF state of the sixth switch is controlled by the second control signal. The ON/OFF state of the fourth switch is controlled by the second driving voltage source according to the ON/OFF state of the sixth switch.
Further scope of the applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.



BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will become more fully understood from the subsequent detailed description and the accompanying drawings, which are given by way of illustration only, and thus are not limitative of tile present invention, and wherein:
FIG. 1a is a block diagram of a conventional heat dissipation system.
FIG. 1b shows a circuit of a conventional full-bridge circuit.
FIG. 2 shows an embodiment of a full-bridge circuit according to the invention.
FIG. 3 shows an embodiment of an other full-bridge circuit according to the invention.



DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Full-bridge circuits for heat dissipation system are provided. An embodiment of a full-bridge circuit comprises a fan and a control circuit. The fan dissipates heat for an object. The control circuit outputs control signals to control ON/OFF states of switches within the full-bridge circuit. For example, a micro-processor control unit (MCU) outputs control signals. In other words, blocks of some embodiments of the heat dissipation system are as shown in FIG. 1a. 
Referring to FIG. 2, like reference numbers are used to designate like parts throughout the various views and illustrative embodiments. An embodiment of a full-bridge circuit 22 receives two control signals to control ON/OFF states of switches therein. The two control signals are a first control signal from a node A1 and a second control signal from a node A2. The first and second control signals are out of phase.
As shown in FIG. 2, the full-bridge circuit 22 comprises a first control line, a second control line, and an induction coil L. The first control line is comprises a first switch Q1 and a third switch Q5 and the second control line parallel to the first control line comprises a second switch Q2 and a fourth switch Q6. One terminal of the induction coil L is coupled between the first and third switches Q1 and Q5, and the other terminal is coupled between the second and fourth switches Q2 and Q6.
The ON/OFF state of the first switch Q1 is controlled by the first control signal from the node A1 and that of the second switch Q2 is controlled by the second control signal from the node A2. The third and fourth switches Q5 and Q6 are controlled by first and second driving voltage sources Vcc1 and Vcc2 respectively. The third and fourth switches Q5 and Q6 are not controlled by control signals from an MCU, and they can be high voltage elements, such as standard level MOSFETs. The first and second switches Q1 and Q2 are controlled by control signals from an MCU, and logical level MOSFETs can be chosen to serve as the first and second switches Qi and Q2.
In order to control the ON/OFF state of the third switch Q5, an input terminal of the third switch Q5 is coupled to a fifth switch S1 in parallel and to a first resistor R1 and the first driving voltage source Vcc1 in series. The ON/OFF state of the fifth switch S1 is controlled by the first control signal from the node A1. According the ON/OFF state of the fifth switch S1, the first driving voltage source Vcc1 is directly connected to ground or turns on the third switch Q5. For example, the first driving voltage source Vcc1 and the third switch Q5 are connected to ground when the fifth switch S1 is turned on, and the third switch Q5 is turned on by the first driving voltage source Vcc1 when the fifth switch S1 is turned off. Additionally, an input terminal of the fourth switch Q6 is coupled to a sixth switch S2 in parallel and to a second resistor R2 and the second driving voltage source Vcc2 in series. The ON/OFF state of the sixth switch S2 is controlled by the second control signal from the node A2. According the ON/OFF state of the sixth switch S2, the second driving voltage source Vcc2 is directly connected to the ground or is used to turn on the fourth switch Q6.
FIG. 3 is an embodiment of an other full-bridge circuit. An embodiment of a full-bridge circuit shown in FIG. 3 is same as those previously shown with the exception of the operation and connection of the third and fourth switches Q5 and Q6. For the third switches Q5, the first driving voltage source Vcc1 is coupled to a first diode D1 and a first capacitor C1. The first diode D1 has an anode coupled to die first driving voltage source Vcc1 and a cathode coupled to a first resistor R1. The first capacitor C1 has one terminal coupled between the first diode D1 and the first resistor R1 and the other terminal coupled to ground. For the fourth switches Q6, the second driving voltage source Vcc2 is coupled to a second diode D2 and a second capacitor C2. The second diode D2 has an anode coupled to the second driving voltage source Vcc2 and a cathode coupled to a second resistor R2. The second capacitor C2 has one terminal coupled between the second diode D2 and the second resistor R2 and The other terminal coupled to the ground.
As the third and fourth switches Q5 and Q6 of the lower bridge are controlled by the first and second driving voltage sources Vcc1 and Vcc2 respectively, and not by control signals from an MCU, in some embodiments of the full-bridge circuit, standard level MOSFETs, high voltage elements, and the like can be used. For example, the third and fourth switches Q5, and Q6 can be switch elements driven by 5V to 12V, which provides higher anti-noise ability than elements driven by 4.5V. Thus, false operation caused by floating voltage in the circuit may be more readily prevented.
Moreover, there are various low cost elements, such as logical level MOSFETs and standard level MOSFETs, can be used as lower bridge switches.
In some embodiments of a full-bridge circuit of the invention, two switches of one control line are controlled by one control signal and two switches of the other control line are controlled by the other control signal. In other words, four switches are controlled by only two control signals, simplifying the circuitry.
As the third and fourth switches Q5 and Q6 of the lower bridge are high voltage elements and the fifth and sixth switches S1 and S2 are low voltage elements, turn on time for the third and fourth switches Q5 and Q6 is lengthy and a short time is required to ground the third and fourth switches Q5 and Q6. Thus, shorts cannot occur between the lower bridge and upper bridge of the circuit because the switches of the lower bridge are turned on slowly and turned off rapidly. The coupling of the first and second capacitors C1 and C2 and the third and fourth switches Q5 and Q6 allows the transistors to remain on for a short time when the voltage sources stops supplying voltage. Thus the fan is braked and stops rotating.
Finally, while the invention has been described by way of preferred embodiment, it is to be understood that the invention is not limited thereto. On the contrary, it is intended to cover various modifications and similar arrangements as would be apparent to those skilled in the art. Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.

## Claims

1. A full-bridge circuit for a heat dissipation system having a control circuit which outputs a first control signal and a second control signal to the full-bridge circuit, the full-bridge circuit comprising:
a first control line having a first switch and a third switch serially connected;
a second control line parallel with the first control line, having a second switch and a fourth switch serially connected; and
an induction coil having one terminal coupled between the first and third switches and the other terminal coupled between the second and fourth switches,
wherein the ON/OFF states of the first and second switches are controlled by the first and second control signals respectively;
wherein an input terminal of the third switch is coupled to a fifth switch in parallel and to a first resistor and a first driving voltage source in series, the ON/OFF state of the fifth switch is controlled by the first control signal, and the ON/OFF state of the third switch is controlled by the first driving voltage source and the first resistor according to the ON/OFF state of the fifth switch; and
wherein an input terminal of the fourth switch is coupled to a six switch in parallel and to a second resistor and a second driving voltage source in series, the ON/OFF state of the sixth switch is controlled by the second control signal, and the ON/OFF state of the fourth switch is controlled by the second driving voltage source and the second resistor according to the ON/OFF state of the sixth switch.

2. The full-bridge circuit as claimed in claim 1, wherein the first and second control signals are out of phase.

3. The full-bridge circuit as claimed in claim 1, wherein the third and fourth switches are high voltage switches.

4. The full-bridge circuit as claimed in claim 1, wherein the control circuit is coupled to a fan.

5. The full-bridge circuit as claimed in claim 1, wherein the control circuit is a micro-processor control unit.

6. The full-bridge circuit as claimed in claim 1, wherein the third switch is a standard level metal oxide semiconductor field effect transistor.

7. The full-bridge circuit as claimed in claim 1, wherein the fourth switch is a standard level metal oxide semiconductor field effect transistor.

8. The full-bridge circuit as claimed in claim 1, wherein the first switch is a logical level metal oxide semiconductor field effect transistor.

9. The full-bridge circuit as claimed in claim 1, wherein the second switch is a logical level metal oxide semiconductor field effect transistor.

10. The full-bridge circuit as claimed in claim 6, wherein voltage from the first driving voltage source is between 5 V to 12 V.

11. The full-bridge circuit as claimed in claim 7, wherein voltage from the second driving voltage source is between 5 V to 12 V.

12. The full-bridge circuit as claimed in claim 4, further comprising:
a first diode having an anode coupled to the first driving voltage source and a cathode coupled to the first resistor; and
a first capacitor having one terminal coupled between the first diode and the first resistor and the other terminal coupled to a ground thereby stopping rapid rotation when the first driving voltage source does not supply voltage.

13. The full-bridge circuit as claimed in claim 12, further comprising:
a second diode having an anode coupled to the second driving voltage source and a cathode coupled to the second resistor; and
a second capacitor having one terminal coupled between the second diode and the second resistor and the other terminal coupled to a ground thereby stopping rapid rotation when the first driving voltage source does not supply voltage.

14. A fill-bridge circuit for a heat dissipation system having a control circuit which outputs a first control signal and a second control signal to the full-bridge circuit, the full-bridge circuit comprising:
a first control line having a first switch and a third switch serially connected;
a second control line parallel with the first control line, having a second switch and a fourth switch serially connected, wherein the ON/OFF states of the first and second switches are controlled by the first and second control signals respectively;
an induction coil having one terminal coupled between the first and third switches and the other terminal coupled between the second and fourth switches;
a first diode having an anode coupled to a first driving voltage source and a cathode coupled to a first resistor; and
a first capacitor having one terminal coupled between the first diode and the first resistor and the other terminal coupled to a ground thereby stopping rapid rotation when the first driving voltage source does not supply voltage;
wherein an input terminal of the third switch is coupled to a fifth switch in parallel, the ON/OFF state of the fifth switch is controlled by the first control signal, and the ON/OFF state of the third switch is controlled by the first driving voltage source according to the ON/OFF state of the fifth switch; and
wherein an input terminal of the fourth switch is coupled to a six switch in parallel, the ON/OFF state of the sixth switch is controlled by the second control signal, and the ON/OFF state of the fourth switch is controlled by the second driving voltage source according to the ON/OFF state of the sixth switch.

15. The full-bridge circuit as claimed in claim 14, wherein the control circuit is coupled to a fan.

16. The full-bridge circuit as claimed in claim 14, wherein the control circuit is a micro-processor control unit.

17. The full-bridge circuit as claimed in claim 14, wherein the third and fourth switches are standard level metal oxide semiconductor field effect transistors.

18. The full-bridge circuit as claimed in claim 14, wherein the first and second switches are logical level metal oxide semiconductor field effect transistors.

19. The full-bridge circuit as claimed in claim 14, wherein voltage from the first driving voltage source is between 5 V to 12 V, and voltage from the second driving voltage source is between 5 V to 12 V.

20. The full-bridge circuit as claimed in claim 14, further comprising:
a second diode having an anode coupled to the second driving voltage source and a cathode coupled to the second resistor; and
a second capacitor having one terminal coupled between the second diode and the second resistor and the other terminal coupled to a ground thereby stopping rapid rotation when the first driving voltage source does not supply voltage.