WO2001093419A1

WO2001093419A1 - Output stage protection circuit for power amplifier

Info

Publication number: WO2001093419A1
Application number: PCT/KR2001/000782
Authority: WO
Inventors: Bumjong Ko
Original assignee: Individual
Current assignee: Individual
Priority date: 2000-05-30
Filing date: 2001-05-15
Publication date: 2001-12-06
Anticipated expiration: 2002-11-30
Also published as: KR100394171B1; KR20010108619A; AU5684601A

Abstract

An output stage protection circuit for power amplifiers is disclosed. The protection circuit in which currents flowing in output stage elements flow through a load resistor comprises a detection part and a control part. The detection part detects an overcurrent at the output stage, and the control part controls the currents flowing in the output stage elements in response to the detected overcurrent. The detection part includes a voltage division circuit for dividing a voltage dropped at the load resistor of the output stage, and the control part including a reference voltage generating part controls the current flowing in the output stage elements so that the divided voltage is maintained at the same level as a reference voltage by comparing the divided voltage with the reference voltage.

Description

OUTPUT STAGE PROTECTION CIRCUIT FOR POWER AMPLIFIER

Technical Field

The present invention relates in general to an output stage protection circuit for power amplifiers, and more particularly to a protection circuit having a control part comprising a comparator or a high speed operational(OP) amplifier, which prevents output stage elements of a power amplifier from breaking when an overcurrent flows or overload is detected at the output stage.

Background Art

As well known to those skilled in the art, the output stage of power amplifiers typically includes elements such as a transistor, an FET(Field Effect Transistor) and an IGBT(Insulated Gate Bipolar Transistor). The power amplifiers are typically classified into four degrees of A, B, C, and H according to their operating capacities, and classified into push-pull amplifiers or SEPP(Single-Ended Push-Pull) amplifiers according to their constructions. In any case, the power amplifiers inevitably require an output stage protection circuit.

The breakage of the elements of the power amplifier's output stages may result from an unstable circuit construction or operational instability of the output stages. However, such breakage is mainly due to an electric overload such as an excessively high voltage, current and power, or a short-circuit of an output terminal. In this regard, various methods for protecting the output stage elements from such breakage have been proposed and used.

One of the conventional protecting methods is to reduce the drive power, thus preventing an overdrive when a load is short-circuited. However, such a method is problematic in that it only reduces the output power of the amplifier. Another method is to shut down the output stage when a collector current of the output stage is greater than a normal level, thus preventing the collector current from flowing in the output stage.

Fig. 1 is a circuit diagram showing a conventional output stage protection circuit for power amplifiers. Referring to Fig. 1, a power amplifier including complementarily coupled transistors Q4 and Q5 is connected to a protection circuit having transistors Q6 and Q7. The above protection circuit is operated as follows. When an overcurrent flows through the transistors Q4 and Q5, the overcurrent is detected by resistors R3 and R6. At this time, the transistors Q6 and Q7 are turned on, such that the path of the current flowing through the transistors Q4 and Q5 is changed to another path including the transistors Q6 and Q7. Such an overcurrent may flow through the transistors Q4 and Q5 when the output terminal OUT is short-circuited to the ground, or when a speaker with an impedance lower than a normal level is connected to the output terminal OUT.

Referring to Fig. 1, when an overload is detected in the output stage elements, the protection circuit can restrict the flow of overcurrent effectively in the case of a constant load variation, but it cannot effectively protect the output stage elements in the case of an instantaneous output signal. Further, when the output stage is short-circuited without an output signal, the protection circuit can effectively protect the output stage elements with respect to an instantaneous short-circuit of the output stage, while it results in a breakage of the output stage elements due to an excessively high Pc(power consumption of a collector) during a constant short-circuit of the output stage.

Further, since the protection circuit including the transistors Q6 and Q7 has a response time longer than tsc (short circuit withstand time) of the output stage elements, the output stage cannot be effectively protected. Referentially, tsc of the IGBT, which is recently popular used as an output stage element, is several micro-seconds remarkably shorter than that of a conventional transistor or FET. For example, tsc of HGTG20N60C3, the IGBT made by Intersil co.Ltd., is about 4μs.

Further, as recent semiconductor techniques have been developed, the expected life span of the semiconductor devices has become semipermanent. However, it is impossible to prevent the semiconductor devices from being broken due to an overcurrent or an electric short in its physical characteristics.

Disclosure of Invention

Therefore, the present invention has been made in view of the above problems, and it is an object of the present invention to provide an output stage protection circuit for power amplifiers, which detects currents flowing through output stage elements, restricts the currents to a level lower than a predetermined level when an overload is detected in the output stage, and restricts the currents within a safe range before breakage of the output stage elements when the output stage is short-circuited.

In accordance with the present invention, the above and other objects can be accomplished by the provision of an output stage protection circuit for a power amplifier in which currents flowing in output stage elements flow through a load resistor, comprising a detection part for detecting an overcurrent at the output stage; and a control part for controlling the currents flowing in the output stage elements in response to the detected overcurrent, wherein the detection part includes a voltage division circuit for dividing a voltage dropped at the load resistor of the output stage, and the control part including a reference voltage generating part controls the current flowing in the output stage elements so that the divided voltage is maintained at the same level as a reference voltage by comparing the divided voltage with the reference voltage.

Preferably, the control part uses a high speed comparator or high speed OP amplifier having a response time shorter than a short circuit withstand time of the output stage elements. Preferably, the power supply voltage of the control part is applied from a main power supply using a constant current source and a zener diode, or an independent power supply. Further, the control part can be integrated with an IC package.

Brief Description of Drawings

The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

Fig. 1 is a circuit diagram of a conventional output stage protection circuit for power amplifiers;

Figs. 2 through 4 are circuit diagrams of an output stage protection circuit for power amplifiers according to the preferred embodiment of this invention;

Fig. 5 is a circuit diagram showing an independent power supplying circuit of the output stage protection circuit of this invention; and

Fig. 6 is a circuit diagram of a power amplifier to which the output stage protection circuit is adapted.

Best Modes for Carrying out the Invention

As will be described below, the output stage protection circuit according to the present invention can be adapted to all kinds of power amplifiers. Hereinafter, an output stage protection circuit used with a complementary power amplifier will be described as a preferred embodiment of this invention.

Further, the preferred embodiment is an example of output stage protection circuits used with complementary SEPP power amplifiers. The complementary SEPP power amplifier includes an upper output stage and a lower output stage, wherein the upper and lower output stages employ elements such as transistors or FETs with the same electric characteristics and different polarities.

Figs. 2 through 4 are circuit diagrams of an output stage protection circuit for power amplifiers according to the present invention. Fig. 2 is an output stage protection circuit using a high speed comparator, and Fig. 3 is an output stage protection circuit using a conventional comparator. Fig. 4 is an output stage protection circuit using a high speed operational (OP) amplifier. Referring to Fig. 2, the circuit employs a comparator designated by the reference character "LM319", and is used for protecting the output stage transistors Q4 and Q5. The LM319 is a high speed comparator with a response time of about 80ns, which is much shorter than that of transistors as well as IGBTs. Additionally, "AD8598" with a response time of about 7ns can be used as the high speed comparator. Referring to Fig. 2, the output stage protection circuit for power amplifiers comprises a detection part 300, an upper control part 100, a lower control part 200, and a power supplying part 400. The detection part 300 detects an overcurrent and overload at the output stage. The control parts 100, 200 control currents flowing in the output stages in response to a detected overcurrent. The power supplying part 400 supplies a power voltage to the protection circuit. The power amplifier shown in Fig. 2 is a complementary SEPP power amplifier, and then the detection part 300, the control parts 100, 200, and the power supplying part 400 are divided into upper and lower parts, respectively.

First, the construction and operation of the detection part 300 is described in detail. The construction and operation of the detection part 300 is similar to the conventional protection circuit. Specifically, resistors R9, R10 branched from the final output stage in the upper part are grounded through a diode D7. Similar to the upper part, resistors Rl l, R12 branched from the final output stage in the lower part are grounded through a diode D8. The detection part 300 with the upper and power parts is a typical voltage divider, of which the divided voltage from between the resistors R9, R10 is applied to a fifth pin of the comparator U1A and a ninth pin of the comparator U1B in the upper and lower control parts 100, 200. When the output terminal OUT is short-circuited to a ground, a very high current flows through output stage transistors Q4, Q5, such that the voltages dropped at the load resistors R15, R16, respectively, are increased. The voltages dropped at the resistors R15, R16 are respectively divided by the resistors R9, RIO, and the diode D7, and by the resistors R12, Rl l, and the diode D8, prior to being applied to the comparators UIA, U1B.

Next, the construction and operation of the control parts 100, 200 are described in detail. Each of the upper control part 100 and the lower control part 200 employs a high speed comparator UIA, U IB. Because there are not complementary type comparators, a PΝP transistor Q6 is additionally connected to the output terminal of the lower comparator U1B. Typically, a conventional comparator or an OP amplifier is not designed in complementary-type, however, if the circuit of this invention is integrated into a hybrid IC package, a desired output stage protection circuit can be effectively realized only by compact wiring outside the IC package. A fourth pin (positive input terminal) of the upper comparator UIA and a tenth pin(negative input terminal) of the lower comparator U1B are terminals for receiving a reference voltage. Referring to Fig. 2, the upper reference voltage generating part is a voltage divider including resistors R3, R4, R5, and a diode D5, which are connected to an upper power voltage. Further, the lower reference voltage generating circuit is a voltage divider including resistors R6, R7 and a diode D6, which are connected to a lower power voltage. The reference voltage is a factor of determining a control current when the protection circuit of this invention is activated. The reference voltage can be generated by another construction as well as the circuit shown in Fig. 2. In this case, because the reference voltage is below several volt(V), maintenance of a precise voltage must be considered while designing the protection circuit.

It should be noted in setting the reference voltage that the control current of the control part is higher than a bias current. If the control current is lower than the bias current, a DC drift is generated in the output stage elements. On the other hand, if the control current is excessively high, during an activating of the protection circuit, the power consumption of the output stage exceeds the allowable Pc. At this time, if the output stage maintains a short-circuit state constantly, the protection circuit cannot protect the output stage, thereby breaking the output stage elements. For example, providing that both the resistance values of the resistors R15, R16 are 0.33Ω, the bias current is 100mA, and the reference voltage is 0.1V, the control current of only 300mA flows in the circuit when the protection circuit is activated. Further, if the power supply voltage is 100V, the Pc of the output stage reaches 30 W, thus protecting the output stage during a constant short of the output stage.

Hereinafter, the control operation of the protection circuit in the case of short- circuit of the output stage is described in detail. When a short-circuit is detected at the output stage(for example, referring to Fig. 2, when the output terminal OUT is short- circuited to the ground terminal), since the diodes D7 and D8 of the detection part 300 are connected to each other, the voltage dropped at the resistors R15 or R16 is applied to each input terminal of the control parts 100 and 200(respectively, the fifth pin of the comparator UIA, or the ninth pin of the comparator U1B). Then, a current corresponding to the reference voltage flows through the transistors Q4, Q5, thus protecting the output stage elements. The protecting operation of the protection circuit is described according to various cases as follows.

First, when a short-circuit of the output stage is detected without an output signal thereof, a current corresponding to a reference voltage flows through the resistor R15 or R16 in the upper or lower output stage. For example, if the reference voltage is 0.1V and the resistance values of the resistors R15, R16 are 0.33Ω, the current of 300mA flows in the resistor R15 or R16.

Next, when the short-circuit of the output stage is detected during a generation of output signal, a control speed of the control part 100 or 200 is faster than t_sc of the output stage elements, thus restricting the current flowing the resistor R15 or R16 within a safe range, without a breakage of the output stage elements.

Further, even if a signal is applied to the output stage in state of a short-circuit of the output stage, the control speed of the control part is faster than a rise time of the current flowing through the output stage elements, thus restricting the maximum current flowing in elements within the control current.

Moreover, a protecting operation against an overcurrent is described. When an overload is detected, the output stage voltage added to the voltage dropped at the resistor R15 is detected and divided by the resistors R9 and RIO, while the output stage voltage added to the voltage dropped at the resistor R16 is detected and divided by the resistors Rl l and R12. Thereby, the output power of the output stage can be limited. For example, providing that the power supply voltage Vcc is ±60V, resistance values of the resistors R9 and R12 are 2kΩ, resistance values of the resistors R15 and R16 are 0.33Ω, and the reference voltage is 0.1V, the power Po of the output stage is calculated as following Equation 1,

V²

2R ^{L J} wherein "r" is an internal resistance of the output stage elements, and the current I and the voltage V are related to each other as follows: / = A — Po , V = -JPoR . In equation

1, if a load resistance RT S 8Ω, a power Po is 150W and the current I is 4.2A. On the other hand, if R_L is 4Ω, power Po is 300 W and current I is 8.6A. Further, if the R is

2Ω, the power Po is 600W and the current I is 17A. Accordingly, if using a load(e.g. speaker) of 2Ω in a power amplifier designed to have a load of 4Ω, the output stage elements will be broken due to an overcurrent during an output of a maximum power.

Accordingly, it should be noted that the maximum power must be restricted when such an overload is detected at the output stage.

Fig. 3 is a circuit diagram of a protection circuit using a conventional comparator, and Fig. 4 is a protection circuit using a conventional high speed OP amplifier. The basic constructions of the protection circuits shown in Figs. 3 and 4 remain the same as the protection circuit of Fig. 2. Here, the term "conventional comparator" is defined as a comparator with a response time of hundreds of ns. For example, the comparator "LM393" has a response time of about 300ns. Referring to Fig. 3, the protection circuit compensates for the slow response time of the comparator LM393 by adding transistors Q6, Q7 and Q8 to the protection circuit.

The protection circuit using the high speed OP amplifier as shown in Fig. 4 is not substantially different from the protection circuit shown in Fig. 3 in its construction. The high speed OP amplifier may be selected from LM6172(slew rate = 750), AD8012(slew rate = 2500), and OPA2658(slew rate =1700).

Fig. 6 is a circuit diagram of a power amplifier to which the output stage protection circuit is adapted. Referring to Fig. 6, four pairs of transistors are

^■ complementarily combined to each other at an output stage. A case of restricting the output power Po to 300W at the load resistor R_L of 2Ω is described as follows. Providing that in the upper part, a total current I is 12 A, voltage V is 25V, and each current of the transistors is 3A, an emitter voltage of each transistor is calculated to be 25+(3*0.33)V«26V, and then a voltage across the resistor R27 is close to 0.9V, and a voltage across the resistor R28 is close to 25.1V. Thereby, if the resistors R27 and R28 are set to 2kΩ, 56kΩ, respectively, the maximum power at a load resistor of 2Ω can be restricted to 300W.

Hereinafter, the construction and operation of the power supplying part is described in detail. In the output stage of the protection circuit, the output stage is grounded to a reference potentiate. g. GND). Two methods may be used to supply a power voltage to the output stage protection circuit.

One of the two methods is to use an independent power supply separate from a main power supply as shown in Fig. 5. Referring to Fig. 5, the protection circuit employs an independent power supply 400 for the control parts 100, 200. Especially, the independent power supply 400 comprises a transformer TI, a rectifying diode D9 and smoothing capacitors C4, C5, and generates the separate supply voltages with a positive voltage and a negative voltage. The separate voltages are provided to the control parts 100, 200.

The other method of supplying the power voltage is to use a main power supply as shown in Figs. 2 through 4. Referring to the drawings, a main power supply circuit consists of constant current sources 1, 2 and zener diodes Dl, D2. Thereby, the power supply voltages of the control parts 100, 200 are supplied through the zener diodes Dl, D2. According to the present invention, even if the potential of the output stage varies, the output stage is grounded, thus supplying a constant voltage to the control parts. The method of Figs 2 through 4 is more suitable in a practical point of view.

As apparent from the above description, the present invention provides an output stage protection circuit for power amplifiers, which detects currents flowing through output stage elements, restricts the currents to a level lower than a predetermined level when an overload is detected in the output stage, and restricts the current within a safe range before a breakage of the output stage elements occurs when the output stage is shorted, thereby safely protecting the output stage against the overcurrent or a short- circuit of the output stage. Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.

Claims

What Is Claimed Is:

1. An output stage protection circuit for power amplifiers in which currents flowing in output stage elements flow through a load resistor, comprising: a detection part for detecting an overcurrent at the output stage; and a control part for controlling the currents flowing in the output stage elements in response to the detected overcurrent, wherein the detection part includes a voltage division circuit for dividing a voltage dropped at the load resistor of the output stage, and the control part including a reference voltage generating part controls the current flowing in the output stage elements so that the divided voltage is maintained at the same level as a reference voltage by comparing the divided voltage with the reference voltage.

2. The protection circuit as set forth in Claim 1, wherein the control part is a high speed comparator or high speed OP amplifier having a response time shorter than a short circuit withstand time of the output stage elements.

3. The protection circuit as set forth in Claim 1 or 2, wherein a power voltage for the control part is supplied from an independent power supply separate from a main power supply.

4. The protection circuit as set forth in Claim 1 or 2, wherein a power voltage for the control part is supplied from at least one constant current source for converting a main power supply voltage into a constant current and at least one zener diode for voltage control.

5. An output stage protection circuit for power amplifiers in which upper and lower output stages are complementarily connected, and currents flowing in the output stages elements flow through load resistors, comprising: a detection part for detecting an overcurrent at the upper or lower output stage; an upper control part for controlling the currents, flowing in the upper output stage elements, in response to the detected overcurrent at the upper output stage; and ' a lower control part for controlling the currents, flowing in the lower output stage elements, in response to the detected overcurrent at the lower output stage, wherein the detection part includes a voltage division circuit for dividing voltages dropped at the load resistors of the output stages, and the upper and lower control parts include a reference voltage generating part, respectively, and are complementarily connected to each other, and control the currents flowing in the output stage elements so that the divided voltages are maintained at the same level as a reference voltage by comparing the divided voltages with the reference voltage.

6. The protection circuit as set forth in Claim 5, wherein the upper and the lower control parts are high speed comparators or high speed OP amplifiers having a response time shorter than a short circuit withstand time of the output stages elements.

7. The protection circuit as set forth in Claim 6, wherein the lower control part further includes a transistor for complementarily connecting the lower control part to the upper control part.

8. The protection circuit as set forth in any one of Claims 5 to 7, wherein a power voltage for the upper and lower control parts is supplied from an independent power supply separate from a main power supply.

9. The protection circuit as set forth in any one of Claims 5 to 7, wherein a power voltage of the upper and lower control parts is supplied from at least one constant current source for converting a main power supply voltage into a constant current and at least one zener diode for voltage control.

10. The protection circuit as set forth in Claim 5 or 6, wherein the upper and lower control parts are integrated into an IC package.