# Patent

## Title
Digital latch control circuit for over-voltage protection and power converter using the control circuit

## Abstract

An AC/DC power supply with over-voltage protection includes a voltage converting circuit and a digital latch control circuit. The voltage converting circuit has a first-side winding, a second-side winding, and an auxiliary winding for providing a supply voltage according to the AC input voltage. The digital latch control circuit is coupled to the voltage converting circuit and utilized for latching a voltage level of the supply voltage at a first predetermined level according to an over-voltage protection (OVP) trigger signal, where the voltage converting circuit is disabled when the voltage level is latched at the first predetermined level.

## Background
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a latch control circuit, and more particularly, to a digital latch control circuit applied for over-voltage protection.
2. Description of the Prior Art
Generally speaking, a power converter utilizes an over-voltage protection scheme to prevent an internal high voltage signal from exceeding a particular voltage level. Please refer to FIG. 1, which is a schematic diagram of a conventional power converter 100. The power converter 100 is a flyback converter comprising a bridge rectifier 105, a transformer TX1 having a primary-side winding LP, second-side winding LS, and an auxiliary winding Laux, a diode D1, capacitors C1 and C2, resistors R1, R2, Rp, and Rn, transistors Q1 and Q2, and a de-glitch circuit 110. Those skilled in this art should appreciate circuit design of this flyback converter, and detailed description for the circuit design of the power converter 100 is not illustrated for brevity. The above-mentioned over-voltage protection scheme applied to the power converter 100 is for detecting whether the level of the supply voltage VCC at the capacitor C2 becomes over higher, for avoiding damage to internal circuit elements within the power converter 100. If the power converter 100 does not include the resistors Rp and Rn and the transistors Q1 and Q2, the primary-side winding LP of the transformer TX1 does not transfer energy from the AC input voltage VAC to the secondary-side winding LS when detecting the supply voltage VCC becoming overly high (i.e., an abnormal condition). The auxiliary winding Laux cannot obtain the energy coming from the AC input voltage VAC, so the level of the supply voltage VCC decreases. The primary-side winding LP transfers the energy from the AC input voltage VAC to the secondary-side winding LS until the supply voltage VCC becomes equal to or lower than a voltage level at which the internal circuit elements within the power converter 100 can work normally. But, if the abnormal condition still exists, the level of the supply voltage VCC will increase and become overly high again. In other words, if an abnormal condition to the power converter 100 still exists, the level of the supply voltage VCC will repeatedly exceed the threshold and drop lower.
The power converter 100 may include resistors Rp and Rn and transistors Q1 and Q2 for solving the above-described problems. With regards to other circuit elements within the power converter 100, the resistors Rp and Rn and the transistors Q1 and Q2 are externally coupled circuit elements. When detecting that the supply voltage VCC becomes overly high, an over-voltage protection trigger signal OVPtrigger is triggered and then sent to the base terminal (i.e., node N1) of transistor Q2 to increase the voltage level at node N1 and turn on transistor Q2. Since the transistor Q2 becomes conductive, the voltage level at node N2 decreases so that transistor Q1 then also becomes conductive. In this situation, although the level of the over-voltage protection trigger signal OVPtrigger is not kept at a high logic level, the voltage level at node N1 increases since the transistor Q1 becomes conductive. As a result, the transistors Q1 and Q2 are eventually fully turned on, and the level of the supply voltage VCC becomes lower and is maintained at a lower level, which is caused by a voltage division resulting from the AC input voltage VAC passing through diodes of the bridge rectifier 105, the resistor R1, and an impedance formed by the resistors Rp and Rn being connected in parallel. Therefore, the level of the supply voltage VCC will not repeatedly become cycle overly higher and then lower.
The power converter 100, however, still has some drawbacks. The transistors Q1 and Q2 and the resistors Rp and Rn form an analog circuit, the transistors Q1 and Q2 are not conductive under normal condition, and nodes N1 and N2 are seen as high impedence points. In this situation, the impedences seen from nodes N1 and N2 are very high. If noise arises, it can easily make the transistors Q1 and Q2 become conductive so that the power converter 100 operates erroneously. In order to solve this problem, it is necessary to add a de-glitch circuit (e.g., the de-glitch circuit 110 of the power converter 100 shown in FIG. 1) at node N1. Doing so, however, increases production costs and need larger circuit area. In addition, since the de-glitch circuit 110 has to operate when the power converter 100 powers on, additional current must be provided for the de-glitch circuit 110 when the power converter 100 powers on.
SUMMARY OF THE INVENTION
According to embodiments of the present invention, a power converter with over-voltage protection comprises a voltage converting circuit and a digital latch control circuit. The voltage converting circuit includes a transformer having a primary-side winding, a secondary-side winding, an auxiliary winding, where the auxiliary winding is utilized for providing a supply voltage. The digital latch control circuit is coupled to the voltage converting circuit and is utilized for latching a voltage level of the supply voltage at a first predetermined level when receiving an over-voltage protection trigger signal. The voltage converting circuit is disabled when the voltage level is latched at the first predetermined level.
These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram of a conventional power converter.
FIG. 2 is a schematic diagram of an AC-to-DC power converter according to an embodiment of the present invention.
FIG. 3 is a diagram illustrating normal operation and latch operation of the digital latch control circuit shown in FIG. 2.
FIG. 4 is a timing diagram of the normal operation and latch operation of the digital latch control circuit shown in FIG. 2.
FIG. 5 is a diagram illustrating a waveform of the supply voltage VCC shown in FIG. 2.
FIG. 6 is a schematic diagram of a power converter according to another embodiment of the present invention.



DETAILED DESCRIPTION
Please refer to FIG. 2. FIG. 2 is a schematic diagram of an AC-to-DC power converter 200 according to an embodiment of the present invention. As shown in FIG. 2, the power converter 200 is a flyback converter and comprises a voltage converting circuit 205 and a digital latch control circuit 210. The voltage converting circuit 205 includes a bridge rectifier 206, an input filter capacitor C1, a transformer TX2, resistors R1 and R2, a diode D1, and a capacitor C2. The bridge rectifier 206 is utilized for rectifying an AC input voltage VAC to a pulse DC voltage. The pulse DC voltage is filtered by the input filter capacitor C1 and then a DC input voltage is generated. The transformer TX2 includes a primary-side winding LP, a secondary-side winding LS, and an auxiliary winding Laux, where operations of the primary-side winding LP and the secondary-side winding LS are not detailed for brevity. Briefly, the voltage converting circuit 205 is used for converting the input voltage VAC into an output voltage Vout and an output voltage VCC; the output voltage VCC is provided by the auxiliary winding Laux, resistor R2, diode D1, and the capacitor C2. When the voltage level of the output voltage Vout is higher than a protection threshold level, an over-voltage detection circuit (not shown in this figure) in the power converter 200 generates an over-voltage protection trigger signal OVPtrigger.
The purpose of the digital latch control circuit 210 is to latch the level of the supply voltage VCC at a first predetermined level V1 when receiving the over-voltage protection trigger signal OVPtrigger. When the level of the supply voltage VCC is latched at the first predetermined level V1, the voltage converting circuit 205 is disabled and therefore cannot operate. After users plug the power converter 100 in again, the abnormal condition is resolved.
The power converter 200 further comprises a voltage regulator, for example, a linear voltage regulator 2105. The linear voltage regulator 2105 is used for converting the supply voltage VCC into a converted voltage VCC′, which is lower than the supply voltage VCC. The digital latch control circuit 210 is utilized for latching the voltage level of the converted voltage VCC′ at a second predetermined level V2 in a digital manner, to latch the level of the supply voltage VCC at the first predetermined level V1. Compared to the conventional power converter 100 implemented with analog circuit elements for achieving the latch function, the digital latch control circuit 210 of this invention can be implemented by digital circuit elements for achieving the latch function, and the circuit elements can be low voltage elements, thereby reducing the circuit costs. Particularly, the digital latch control circuit 210 includes a complementary switch unit SW formed by transistors Q21 and Q22, a resistor unit 2130, and a control module. The resistor unit 2130 includes a transistor Q3 and a resistor R3, and the control module is formed by a NOR gate 2115, a D-type flip flop (DFF) 2120, and an inverter 2125. The complementary switch unit SW selectively turns on one of the transistors Q21 and Q22 according to a control signal Sc. When the transistor Q21 is conductive, the transistor Q22 becomes non-conductive; otherwise, the transistor Q22 becomes conductive when the transistor Q21 is non-conductive. The control module generates the control signal Sc to control the complementary switch unit SW according to the over-voltage protection trigger signal OVPtrigger, the converted voltage VCC′ corresponding to the supply voltage VCC, and the voltage V′ at drain terminals of the transistors Q1 and Q2. When the control signal Sc controls the transistor Q21 to become conductive and controls the transistor Q22 to become non-conductive, the voltage level of the supply voltage VCC is latched at the first predetermined level V1 because of voltage division.
Please refer to FIG. 3 in conjunction with FIG. 4. FIG. 3 is a diagram illustrating normal operation and digital latch operation of the digital latch control circuit 210 shown in FIG. 2 respectively. FIG. 4 is a timing diagram of the normal operation and latch operation of the digital latch control circuit 210 shown in FIG. 2. As shown in the top half of FIG. 3, since no over-voltages appear at the supply voltage VCC during normal operation of the digital latch control circuit 210, the over-voltage protection trigger signal OVPtrigger continues to maintain at a low logic level before timing t1 shown in FIG. 4. The NOR gate 2115 outputs a reset control signal Sreset having a high logic level to the reset input terminal CL of the DFF 2120. Consequently, the DFF 2120 outputs an output signal VQ having the low logic level to the inverter 2125 according to the received reset control signal Sreset through the data output terminal Q. The inverter 2125 then inverts the output signal VQ to obtain a control signal Sc having a high logic level, where under this condition the control signal Sc controls the transistor Q22 to become conductive, but controls the transistor Q21 to be non-conductive. Since the supply voltage VCC is not connected to any path connected to ground level, the voltage converting circuit 205 operates normally.
As shown in bottom half of FIG. 3, when an over-voltage appears at the supply voltage VCC, the digital latch control circuit 210 performs a digital latch operation (after time t1). The over-voltage results in the over-voltage protection trigger signal OVPtrigger having a short pulse with a high logic level, and this causes the NOR gate 2115 to output the reset control signal Sreset having the low logic level. After the DFF 2120 receives the reset control signal Sreset having the low logic level, and the over-voltage protection trigger signal OVPtrigger having a short pulse with high logic level is received at the clock input terminal of the DFF 2120 itself, the DFF 2120 transmits the converted voltage VCC′ from the data input terminal D to the output terminal as the output signal VQ. The converted voltage VCC′ is obtained through the linear voltage regulator 2105 from the supply voltage VCC and has a lower level; the level of the converted voltage VCC′, however, is still considered a high logic level for the DFF 2120 and inverter 2125. Therefore, the level of the control signal Sc outputted by the inverter 2125 switches from a high to a low logic level such that the transistor Q21 becomes conductive and the transistor Q22 does not become conductive. In this situation, since the AC input voltage VAC is connected to the ground level via the diodes, the resistor R1, the linear voltage regulator 2105, the transistors Q21, and the resistor unit 2130, the converted voltage VCC′ latches at the second predetermined level V2 based on voltage division result, and the supply voltage VCC latches at the first predetermined level V1. The design of the first predetermined level V1 is to disable the voltage converting circuit 205 and make it incapable of operating. Thus, it is necessary for a user to plug the power converter 200 into AC power again to resolving the abnormal condition. Accordingly, when using the power converter 200, when detecting that the voltage converting circuit 205 is disabled, the user can deduce that an abnormal condition to the power converter 200 occurred and the user should plug the power converter 200 into AC power again to resolve the abnormal condition.
To summarize, in this embodiment, when an over-voltage appears in the supply voltage VCC, the DFF 2120 is triggered by the short pulse of the over-voltage protection trigger signal OVPtrigger. This causes the output signal of the DFF 2120 to be latched at a predetermined level to thereby achieve the purpose for latching the supply voltage VCC at the first predetermined level V1. The above-mentioned operation is namely the digital latch operation.
When the digital latch control circuit 210 latches the supply voltage VCC at the first predetermined level V1, in order to maintain the operations of the digital circuits within the digital latch control circuit 210, it is still necessary for the converted voltage VCC′ to be higher than a specific level at least. If the resistor R3 for voltage division has a fixed resistance and a manufacturing process variation of the transistor Q21 is introduced, after the voltage division, it cannot be assured that the voltage level at node N′, i.e. the voltage level V′ at the drain terminal of the transistor Q22, is maintained at the high logic level to ensure performing the digital latch operation. Therefore, in this embodiment, the transistor Q3 acts as a variable resistor for voltage division.
When the power converter 200 just turns on, there is a possibility that the transistors Q21 and Q3 are non-conductive. The resistor R3 having smaller resistance can ensure that the voltage level at node N′ (i.e. the voltage level V′ at the drain terminal of the transistor Q22) can be maintained at the low logic level, to prevent the digital latch control circuit 210 from operating erroneously. Accordingly, in this embodiment, the resistor unit 2130 implemented with the transistor Q3 and resistor R3 is a preferred choice. In addition to the above-described operation, the resistor unit 2130 further acts as a de-glitch circuit. The transistor Q3 and resistor R3 form a variable resistor. Hence, if any noise occurs, the impedance seen from node N′ is a small value before the digital latch control circuit 210 latches the supply voltage VCC at the first predetermined level V1. Therefore, a voltage variation due to noise can be discharged rapidly through the resistor unit 2130, thereby preventing the digital latch control circuit 210 from operating erroneously.
Please refer to FIG. 5. FIG. 5 is a diagram illustrating the waveform of the supply voltage VCC shown in FIG. 2. As shown in FIG. 5, at time t1, an over-voltage appears in the supply voltage VCC. At time t2, the supply voltage VCC is latched at the first predetermined level V1. During times t3 and t4, a user plugs the power converter 200 into AC power again. After time t4, the power converter 200 operates normally, since the originally abnormal condition has been resolved. In addition, the digital latch control circuit 210 is implemented by a digital circuit. The problem of a high impedance resulting from the conventional latch control circuit being an analog circuit does not occur in this embodiment. Therefore, the power converter 200 has excellent de-glitching capability. It is not required for the power converter 200 to use another de-glitch circuit for de-glitching during start-up, avoiding the necessity to provide additional currents during start-up.
Furthermore, the linear voltage regulator 2105 is an optional circuit element. The voltage regulator 2105 is removable in another embodiment. Please refer to FIG. 6. FIG. 6 is a schematic diagram of a power converter 600 according to another embodiment of the present invention. In this embodiment, even though the power converter 600 does not include a linear voltage regulator, the power converter 600 can still latch onto the supply voltage VCC of FIG. 6 at a specific level. Therefore, this also falls within the scope of the present invention. Since the elements and related circuit connections of the power converter 600 are identical to those of the power converter 200 shown in FIG. 2 except the linear voltage regulator 2105, a person skilled in this art should appreciate detailed operation of the power converter 600 after reading the above-mentioned disclosure; further description for the operation of the power converter 600 is therefore not detailed here for brevity.
Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention.

## Claims

1. A power converter with over-voltage protection, comprising:
a voltage converting circuit having a transformer, the transformer including a primary-side winding, a secondary-side winding, and an auxiliary winding where the auxiliary winding is utilized for providing a supply voltage; and
a digital latch control circuit, coupled to the voltage converting circuit, for latching a voltage level of the supply voltage at a first predetermined level in a digital manner according to an over-voltage protection trigger signal;
wherein the voltage converting circuit is disabled when the voltage level is latched at the first predetermined level.

2. The power converter of claim 1, wherein the digital latch control circuit comprises:
a complementary switch unit, having a first transistor and a second transistor, the complementary switch unit selectively turning on one of the first and second transistors according to a control signal;
a resistor unit, coupled to ground level and the second terminals of the first and second transistors; and
a control module, coupled to the resistor unit and the complementary switch unit, for generating the control signal to control the complementary switch unit according to the over-voltage protection trigger signal, the supply voltage, and a voltage level at the second terminals of the first and second transistors;
wherein the voltage level of the supply voltage is latched at the first predetermined level when the control module controls status of the first transistor to become conductive and status of the second transistor not to become conductive.

3. The power converter of claim 2, wherein the control module comprises:
a logic gate, coupled to the second terminals of the first and second transistors, for receiving the voltage level at the second terminals of the first and second transistors and the over-voltage protection trigger signal to output a reset control signal; and
a flip flop, having a reset input terminal coupled to the logic gate for receiving the reset control signal, a data input terminal for receiving the supply voltage, a clock input terminal for receiving the over-voltage protection trigger signal, and a data output terminal for generating an output signal corresponding to the control signal.

4. The power converter of claim 2, wherein the resistor unit comprises:
a third transistor, having a first terminal, a second terminal, and a control terminal, wherein the first terminal of the third transistor is coupled to the second terminals of the first and second transistors and the second and control terminals of the third transistor are coupled to the ground level.

5. The power converter of claim 2, wherein the resistor unit comprises:
a resistor, coupled to the second terminals of the first and second transistors.

6. The power converter of claim 1, further comprising:
a voltage regulator, coupled to the voltage converting circuit, for converting the supply voltage into a converted voltage being lower than the supply voltage;
wherein the digital latch control circuit is coupled to the voltage regulator for latching the converted voltage at a second predetermined level so as to latch the voltage level of the supply voltage at the first predetermined level.

7. The power converter of claim 1, wherein the voltage regulator is a linear voltage regulator.

8. A digital latch control circuit for over-voltage protection, an auxiliary winding of a power converter being utilized for providing a supply voltage, the digital latch control circuit being used for latching a voltage level of the supply voltage at a first predetermined level according to an over-voltage protection trigger signal, and the digital latch control circuit comprises:
a complementary switch unit, having a first transistor and a second transistor, the complementary switch unit selectively turning on one of the first and second transistors according to a control signal;
a resistor unit, coupled to ground level and the second terminals of the first and second transistors; and
a control module, coupled to the resistor unit and the complementary switch unit, for generating the control signal to control the complementary switch unit according to the over-voltage protection trigger signal, the supply voltage, and a voltage level at the second terminals of the first and second transistors;
wherein the voltage level of the supply voltage is latched at the first predetermined level when the control module controls status of the first transistor to become conductive and status of the second transistor not to become conductive.

9. The digital latch control circuit of claim 8, wherein the control module comprises:
a logic gate, coupled to the second terminals of the first and second transistors, for receiving the voltage level at the second terminals of the first and second transistors and the over-voltage protection trigger signal to output a reset control signal; and
a flip flop, having a reset input terminal coupled to the logic gate for receiving the reset control signal, a data input terminal for receiving the supply voltage, a clock input terminal for receiving the over-voltage protection trigger signal, and a data output terminal for generating an output signal corresponding to the control signal.

10. The digital latch control circuit of claim 8, wherein the resistor unit comprises:
a third transistor, having a first terminal, a second terminal, and a control terminal, the first terminal of the third transistor being coupled to the second terminals of the first and second transistors, the second and control terminals of the third transistor being coupled to the ground level.

11. The digital latch control circuit of claim 8, wherein the resistor unit comprises:
a resistor, coupled to the second terminals of the first and second transistors.

12. A power converter, comprising:
a voltage converting circuit, for converting a power source voltage into a first output voltage and a second output voltage; and
a digital latch control circuit having a plurality of digital logic gates powered by the second output power, the digital latch control circuit latching a voltage level of the second output voltage at a first predetermined level according to an over-voltage protection trigger signal, and the first predetermined level being capable of maintaining the operation of the digital logic gates;
wherein the over-voltage protection trigger signal indicates that a voltage level of the first output voltage is higher than a protection threshold level.

13. The power converter of claim 12, wherein the digital latch control circuit comprises:
a complementary switch unit, having a first transistor and a second transistor, the complementary switch unit selectively turning on one of the first and second transistors according to a control signal;
a resistor unit, coupled to ground level and the second terminals of the first and second transistors; and
a control module, coupled to the resistor unit and the complementary switch unit, for generating the control signal to control the complementary switch unit according to the over-voltage protection trigger signal, the supply voltage, and a voltage level at the second terminals of the first and second transistors;
wherein the voltage level of the supply voltage is latched at the first predetermined level when the control module controls status of the first transistor to become conductive and status of the second transistor not to become conductive.

14. The power converter of claim 13, wherein the control module comprises:
a logic gate, coupled to the second terminals of the first and second transistors, for receiving the voltage level at the second terminals of the first and second transistors and the over-voltage protection trigger signal to output a reset control signal; and
a flip flop, having a reset input terminal coupled to the logic gate for receiving the reset control signal, a data input terminal for receiving the supply voltage, a clock input terminal for receiving the over-voltage protection trigger signal, and a data output terminal for generating an output signal corresponding to the control signal.

15. The power converter of claim 12, wherein the resistor unit comprises:
a third transistor having a first terminal, a second terminal, and a control terminal, wherein the first terminal of the third transistor is coupled to second terminals of the first and second transistors, and the second and control terminals of the third transistor are coupled to ground level.

16. The power converter of claim 12, wherein the resistor unit comprises:
a resistor, coupled to second terminals of the first and second transistors.