UMZCH protection system. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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UMZCH protection system. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Transistor power amplifiers

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Currently, any modern audio frequency power amplifier (UMZCH) contains a system for protecting the output stage (VC) from overcurrent in the event of a short circuit in the load (short circuit) or low resistance of the acoustic system (AS). The same system protects the speakers from a constant voltage at the output of the UMZCH and fluctuations in infrasonic frequencies. In addition, high-quality UMZCH provides for a delay in connecting the speakers to the UMZCH output (for the duration of transients), which is necessary to protect the speakers from clicks when turned on, as well as automatically disconnecting the UMZCH from the mains in case of any malfunction.

One of the simplest and most common ways to protect speakers from a constant component at the output of the UMZCH is to connect a fuse in series with the speakers. In the presence of a constant voltage at the output of the UMZCH, a direct current flows through the voice coil of the dynamic head (DG), which is most often low-frequency, galvanically connected to the output of the UMZCH. If the current is sufficient to blow the fuse, then the AC is disconnected from the UMZCH. However, such an easy way, of course. not optimal, since before the AC fuse burns out, it is under constant voltage for some time. To reduce the operating time, the rated current of the fuse should be three times less than the current that burns it, and several times less than the maximum current that the AC can withstand.

At first glance, there are no particular problems here, since in the event of a breakdown of one of the transistors of the UMZCH output stage, the output will have a voltage close to the VC supply voltage. So, at a voltage of 32 V, the current through the AC with a nominal resistance of 4 ohms will be about 8 A. and the 2-amp fuse will successfully complete its task. But what if the output is not 32 V at all, but, say, 7 V? In this case, the 2-amp fuse will not disconnect the speakers from the UMZCH, and the DG voice coil will gradually warm up, which can lead to its failure.

In addition, the protection of speakers with the help of fuses has thermal, harmonic and intermodulation distortions, which degrade the quality indicators of the entire UMZCH [1]. These distortions can be minimized by using high rated fuses, but then the protection becomes ineffective. In addition, this method does not protect the speakers from infrasonic vibrations, which can damage the diffusers of the DG.

Another way to protect the speakers is the use of special electronic circuits that quickly determine the presence of a constant voltage or oscillations of infrasonic frequencies at the output of the UMZCH and turn off the speakers. However, it may happen that if the VC fails (when the AC protection system is recorded from the same power source as the VC) due to a “power supply voltage drop, the AC protection system will not work, but this drawback can be eliminated by using a separate source power supply for the protection system.

As for the protection of the VC from overcurrent, the same two methods are possible here: fuses and electronic circuits. However, attempts to protect semiconductor devices with fuses are useless: a typical semiconductor will fail from overcurrent long before the fuse melts, only high-speed electronic circuits can provide reliable overload protection.

But from all of the above, it does not follow that you need to forget about fuses. Fuses are desirable in the secondary circuit of a power transformer in order to protect against overheating when shorted in a bridge rectifier. Mains fuses are mandatory. The mains and secondary fuses must be slow so that they do not blow out during surges due to the charging of storage capacitors and the starting current of the transformer when the power is turned on.

It should also be mentioned about the fight against starting currents UMZCH. To this end, in powerful UMZCH, soft start systems (SPP, Soft Start) are increasingly being used. The purpose of a soft start is to reduce the starting current, extend the life of the mains switch contacts, and avoid unnecessary blowing of the mains fuses.

In medium power amplifiers, NTC can be implemented using a negative temperature coefficient (NTC) resistor. connected in series with the primary winding of the mains transformer. When the amplifier is turned on, as the thermistor heats up, its resistance decreases from its initial, relatively large, value to almost zero within a few tenths of a second, thereby limiting the current surge. The advantage of this solution is the use of only one additional element. At the same time, the main drawback of the SPP circuit based on the NTC resistor is the slow cooling of the thermistor after the UMZCH is turned off. Therefore, when the amplifier is turned on again immediately after turning it off, the NTC resistor does not have time to cool down, and the current surge is only partially smoothed out.

In industrial and amateur radio equipment, current limiting stages are widely used, in which a powerful resistor is connected in series with the primary winding of the power transformer to combat the current surge. After some time, this resistor is shunted by the relay contacts [2J. In this case, the disadvantage of the NTC resistor circuit is not present, but the complexity of the current surge suppression circuit increases, as does its cost. To prevent large inductive transients that occur when the transformer is connected to the mains, a circuit of a series-connected resistor and capacitor is placed in parallel with the primary winding of the transformer or the contacts of the mains switch [3, 4].

The UMZCH protection system, the diagram of which is shown in fig. 1 is built with the above remarks in mind. Unlike the protection scheme from [5], it is simpler. The protection system is powered by a separate power source (PS), made on the elements T1, VD19, C13. The same IP acts as a standby voltage source (12 V), which is necessary to power the switching circuit (DD2, K1, SB1, etc.), which allows you to turn on / off the UMZCH by pressing just one button without fixing. Due to this, it becomes possible to control the state of the amplifier by applying a single pulse to pin 1 of the XP5 plug, for example, from a remote control system.

(click to enlarge)

When the device is connected to the network, the standby voltage +12 V from the output of the rectifier VD19, C13 is supplied to the D-trigger DD2, which is set to "11" using the chain C19-H0. This state corresponds to a voltage of about +12 V at pin 2, which maintains the VT7 transistor in the closed state. Therefore, the voltage on the turnip winding K1 is zero, the contacts K1.1 and K1.2 are open, and the UMZCH is de-energized. When the SB1 button is pressed briefly at pin 3 DD2, a short pulse is generated that changes the state of DD2 ("0th at pin 2 DD2). Transistor VT7 opens and switches relay K1, the relay contacts close and connect the UMZCH to the network. Parallel to the contacts repeK1.1 and K1.2. 21, the R15-C22 and R16-CXNUMX chains are included, which dampen the transients that occur when the power transformer is turned on.

When power is applied to the soft start circuit (R20, SYU, VD16, VT6, K2, VD17, R23 ... R25), the SU capacitor is slowly charged (approximately 0,5 ... 1 s). As soon as the voltage on the SU becomes sufficient to open VT6, relay K2 is activated and shunts the composite powerful resistor R23 with its contacts. ..R25. serving to dampen the inrush current when the UMZCH is turned on.

At the same time, +12 V is supplied to the remaining nodes of the circuit. On the elements of R3. R4, C1. C2, VT1, VT3 (R5, R6, C3, C4. VT2, VT4) assembled a two-threshold comparator, on the elements R3, C1, R4, C2 (R5, C3. R6. C4) - an infra-low frequency filter. The threshold voltages are approximately +0.65 V and -0,65 V. The constant component or voltage of the infrasonic oscillations at the output of the UMZCH is compared with these threshold values. When the threshold level is exceeded, one of the transistors is unlocked, as a result of which the capacitor C6 is discharged.

Capacitor C6 is also discharged in the event of operation of the current protection VK (VD1 ... VD8. R7 ... R10, VU1, VU2). The current protection threshold can be adjusted by changing the resistance R7 (R9). With the indicated ratings, the current protection operates at a voltage between contacts 1.2 - 3, 4 ХРЗ (ХР4) of about 6 V, which corresponds to a current of 6 A (if 0,47 Ohm resistors are installed in the emitter or source circuit of the VK transistors). To exclude the operation of current protection at signal peaks, it has some inertia.

Since at the moment of switching on, due to transients in the UMZCH, a constant component with a level exceeding the threshold value (0,65 V) may appear at the output, it is necessary to block the operation of the system for disconnecting the amplifier from the mains (DD1.1, DD1.2, DD1.4. 14). For this, a chain R8-C8 is provided. Until the voltage at C1 reaches level "4" (about 4 s), the operation of the trip circuit is blocked. In the case when the duration of transients when the UMZCH is turned on exceeds 14 s, the time constant R8-CXNUMX should be increased.

The acoustic system is connected to the UMZCH output with a delay of about 12s, which is enough to completely end the transient processes in the UMZCH. The delay time is determined by the circuit time constant R7-CXNUMX. The AU is disconnected from the UMZCH in cases where the current protection of the VC is triggered or the constant component at the output of the UMZCH exceeds the threshold value.

Author: M. Shushnov, Novosibirsk

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