Smooth switching circuit for the amplifier. Soft-start on MOSFET and power switch for ULF and other devices. Slow start circuit

Smooth power-on circuit (soft start or step-by-step) for a low-frequency power amplifier or other device. This simple device can improve the reliability of your radio equipment and reduce network interference when turned on.

Schematic diagram

Any power supply for radio equipment contains rectifying diodes and high-capacity capacitors. At the initial moment of turning on the mains power, a pulse current jump occurs while the filter capacitors are being charged.

The amplitude of the current pulse depends on the capacitance value and voltage at the rectifier output. So, at a voltage of 45 V and a capacitance of 10,000 μF, the charging current of such a capacitor can be 12 A. In this case, the transformer and rectifier diodes operate briefly in short-circuit mode.

To eliminate the danger of failure of these elements by reducing the inrush current at the time of initial switching on, the circuit shown in Figure 1 is used. It also allows you to lighten the modes of other elements in the amplifier during transient processes.

Rice. 1. Schematic diagram of smooth switching on of a power source using a relay.

At the initial moment, when power is applied, capacitors C2 and C3 will be charged through resistors R2 and R3 - they limit the current to a value that is safe for the rectifier parts.

After 1...2 seconds, after capacitor C1 is charged and the voltage on relay K1 increases to a value at which it will operate and, with its contacts K1.1 and K1.2, will bypass limiting resistors R2, R3.

The circuit uses a relay RES47 RF4.500.407-00 (RF4.500.407-07 or others) with a rated operating voltage of 27 V (winding resistance 650 Ohms; current switched by contacts can be up to 3 A). In fact, the relay operates already at 16...17 V, and resistor R1 is selected as 1 kOhm, and the voltage across the relay will be 19...20 V.

Capacitor C1 type K50-29-25V or K50-35-25V. Resistors R1 type MLT-2, R2 and R3 type S5-35V-10 (PEV-10) or similar. The values of resistors R2, R3 depend on the load current, and their resistance can be significantly reduced.

Improved device circuit

The second diagram shown in Fig. 2, performs the same task, but makes it possible to reduce the dimensions of the device by using a timing capacitor C1 of smaller capacity.

Transistor VT1 turns on relay K1 with a delay after capacitor C1 (type K53-1A) is charged. The circuit also allows, instead of switching secondary circuits, to provide a stepwise voltage supply to the primary winding. In this case, you can use a relay with only one group of contacts.

Rice. 2. Improved circuit diagram of smooth switching on of the UMZCH power supply.

The value of resistance R1 (PEV-25) depends on the load power and is selected such that the voltage in the secondary winding of the transformer is 70 percent of the rated value when the resistor is turned on (47...300 Ohms). Setting up the circuit consists of setting the delay time for turning on the relay by selecting the value of resistor R2, as well as selecting R1.

In conclusion

The given circuits can be used in the manufacture of a new amplifier or in the modernization of existing ones, including industrial ones.

Compared to similar devices for two-stage supply voltage given in various magazines, those described here are the simplest.

Original source: unknown.

M. SIRAZETDINOV, Ufa
Radio, 2000, No. 9

When assembling powerful ULFs, the question always arises about protection against impulse overloads at the moment of switching on. As a rule, the output stage of any powerful amplifier is powered from a bipolar source in which very large capacitors are installed (up to 10,000 μF and sometimes even higher). When the power supply is turned on, a very large charging current begins to flow through them, which creates a significant load on the power source itself, and this is also not very good for the output stage...

The way out is the so-called “soft” start: smooth supply of mains voltage to the mains transformer. Quite a lot of devices have been considered in the literature and another of them is presented here.

Its main distinctive feature is that here the increase in mains voltage occurs really smoothly, and not stepwise as in many similar devices.

Device diagram for soft switching on ULF

Fundamental circuit diagram of the UMZCH “soft” power-on device shown in the figure. Transistor VT1 through the diode bridge VD1-VD4 is connected in series with the primary winding of transformer T1 of the power supply. The choice of a MOSFET with an insulated gate is due to the high input impedance of its control circuit, which reduces power consumption.

The control unit consists of circuits that generate voltage at the gate of transistor VT1, and an electronic switch on transistors VT2, VT3. The first circuit is formed by elements VD5, C1, R1 - R3, VD7, C4, which set the initial voltage at the gate of transistor VT1. The second includes elements VD8, R4, R5, C2, SZ, which ensure a smooth increase in voltage at the gate of transistor VT1. Zener diode VD6 limits the voltage at the gate of transistor VT1 and protects it from breakdown.

In the initial state, the capacitors of the control unit circuits are discharged, therefore, at the moment of closing the contacts of the mains switch SB1, the voltage at the gate of transistor VT1 relative to its source is zero and there is no current in the source-drain circuit. This means that the current in the primary winding of transformer T1 and the voltage drop across it are also zero. With the arrival of the first positive half-cycle of the mains voltage, capacitor C1 begins to charge through the circuit VD5, VD3 and during this half-cycle it is charged to the amplitude value of the mains voltage.

Zener diode VD7 stabilizes the voltage on the divider R2R3. The voltage on the lower arm of trimming resistor R3 in the circuit determines the initial gate-source voltage of transistor VT1, which is set close to the threshold value of 2...4 V. After several periods of mains voltage, current pulses flowing through capacitor C2 will charge it to voltage exceeding the cutoff voltage of transistor VT3.

The electronic switch on transistors VT2, VT3 closes, and the capacitor SZ begins to charge through the circuit VD8, R4, R5, R3, VD3. The gate-source voltage of transistor VT1 is determined at this time by the sum of the voltage on the lower arm of resistor R3 and the gradually increasing voltage on capacitor SZ. As this voltage increases, transistor VT1 opens and the resistance of its source-drain channel becomes minimal. Accordingly, the voltage on the primary winding of transformer T1 smoothly increases almost to the value of the mains voltage. A further increase in the gate-source voltage of transistor VT1 is limited by the zener diode VD6. In steady state, the voltage drop across the diodes of the bridge VD1-VD4 and transistor VT1 does not exceed 2...3 W, so this practically does not affect the further operation of the UMZCH power supply. The duration of the most severe operating mode of transistor VT1 does not exceed 2...4 s, so the power dissipated by it is small. Capacitor C4 eliminates voltage ripple at the gate-source junction of transistor VT1. created by pulses of the charging current of the capacitor SZ on the lower arm of resistor R3.

An electronic switch on transistors VT2, VT3 quickly discharges the capacitor SZ after turning off the UMZCH power supply or during short-term interruptions in the power supply and prepares the control unit for re-switching on.

The author's version of the protection device uses an imported capacitor manufactured by Gloria (C1), as well as domestic ones: K53-1 (C2, C4) and K52-1 (SZ). All fixed resistors are MLT, trimming resistor R3 is SP5-3. Transistor KP707V (VT1) can be replaced with another, for example. KP809D. It is important that the resistance of its channel in the open state is minimal, and the maximum source-drain voltage is at least 350 V. Instead of the KT3102B (VT2) transistor, it is permissible to use KT3102V and KT3102D, and instead of KP103I (VTЗ) - KP103Zh.

Transistor VT1 is equipped with a small heat sink with an area of 10...50 cm 2.

Setting up the device consists of selecting the optimal position of the trimmer resistor R3. Initially, it is installed in the lower (according to the diagram) position and connected through a high-resistance divider to the primary winding of the transformer

T1 oscilloscope. Then the contacts of switch SB1 are closed and, by moving the slider of resistor R3. observe the process of increasing voltage amplitude on the primary winding of the transformer. The engine is left in a position in which the time interval between turning on SB1 and the beginning of the increase in the voltage amplitude on the T1 winding is minimal. If necessary, you should select the capacitance of the capacitor SZ.

The device was tested with a prototype UMZCH, similar in structure to the amplifier described in the article by A. Orlov “UMZCH with single-stage voltage amplification” (see “Radio”. 1997, No. 12, pp. 14 - 16). The voltage surge at the output of the UMZCH when the power supply was turned on did not exceed 1.5 V

This simple device can improve the reliability of your radio equipment and reduce network interference when turned on.

Any power supply for radio equipment contains rectifying diodes and high-capacity capacitors. At the initial moment of turning on the mains power, a pulse current jump occurs - while the filter capacitors are being charged. The amplitude of the current pulse depends on the size of the capacitance and the voltage at the output of the rectifier. Thus, at a voltage of 45 V and a capacitance of 10,000 μF, the charging current of such a capacitor can be 12 A. In this case, the transformer and rectifier diodes operate briefly in short-circuit mode.

To eliminate the danger of failure of these elements by reducing the inrush current at the time of initial switching on, the one shown in Fig. is used. 1.7 diagram. It also allows you to lighten the modes of other elements in the amplifier during transient processes.

Rice. 1.7

At the initial moment, when power is applied, capacitors C2 and SZ will be charged through resistors R2 and R3 - they limit the current to a value that is safe for the rectifier parts.

After 1...2 seconds, after the capacitor C1 is charged and the voltage on the relay K1 increases to a value at which it will operate and bypass the limiting resistors R2, R3 with its contacts K1.1 and K1.2.

The device can use any relay with an operating voltage lower than that at the output of the rectifier, and resistor R1 is selected so that the “excess” voltage drops across it. The relay contacts must be designed for the maximum current operating in the amplifier's power supply circuits. The circuit uses a relay RES47 RF4.500.407-00 (RF4.500.407-07 or others) with a rated operating voltage of 27 V (winding resistance 650 Ohms; current switched by contacts can be up to 3 A). In fact, the relay operates already at 16...17 V, and resistor R1 is selected as 1 kOhm, and the voltage across the relay will be 19...20 V.

Rice. 1.8

The second diagram shown in Fig. 1.8, performs the same task, but allows you to reduce the size of the device by using a timing capacitor C1 of smaller capacity. Transistor VT1 turns on relay K1 with a delay after capacitor C1 (type K53-1A) is charged. The circuit also allows, instead of switching secondary circuits, to provide a stepwise voltage supply to the primary winding. In this case, you can use a relay with only one group of contacts.

Setting up the circuit consists of setting the delay time for turning on the relay by selecting the value of resistor R2, as well as selecting R1.

The given circuits can be used in the manufacture of a new amplifier or in the modernization of existing ones, including industrial ones.

Compared to similar devices for two-stage supply voltage given in various magazines, those described here are the simplest.

These two are power device circuits with a toroidal transformer. Typically the starting (inrush) current is very high for a short period while the smoothing capacitors are charging. This is a kind of stress for capacitors, rectifier diodes and the transformer itself. Also at such a moment the fuse may blow.

The soft start circuit is designed to limit the starting current to an acceptable level. This is achieved by connecting the transformer to the mains supply through a resistor, which is connected for a short time using a relay.

The circuits combine soft start and push-button control, thus creating a ready-made module that can be used in power amplifiers or in conjunction with other electrical appliances.

Description of soft start circuits

The first circuit is built on CMOS logic chips (4027), and the second on the NE556 integrated circuit, which consists of 2 combined in one package.

As for the first circuit, it uses a JK flip-flop connected as a T flip-flop.

T-flip-flop is a counting flip-flop. The T-trigger has one counting (clocking) input and one synchronizing one.

When J2 is pressed, the trigger state changes. When transitioning from the off state to the on state, the signal is transmitted through a resistor and capacitor to the second part of the circuit. There, the second JK flip-flop is connected in an unusual way: the reset pin is driven high, and the SET pin is used as an input.

In the truth table, you will find that when the reset pin is high, all other inputs are ignored except the SET pin. When the SET pin is high, the output is also high in reverse.

Resistor R6 and capacitor C6 are used to delay the signal at the moment of switching on. With the values indicated in the diagram, the delay is 1 second. If necessary, parameters R6 and C6 can change the delay time. Diode VD2 bypasses resistor R6, as a result of which when turned off, the relay turns off without delay.

The second circuit uses a dual NE556 timer. The first timer is used as a push-button switch, and the second as a switch associated with the delay created by elements R5, VD2 and C6.

Resistors R8 - R10 have a resistance of 150 Ohms and a power of 10W. They are connected in parallel resulting in a 50 Ohm resistor with a power of 30 W. On the PCB, two of them are located next to each other, and the third is in the middle on top of them. The power of transformer Tr1 is about 5 W with a voltage in the secondary winding of 12-15 V. Connector J1 is used if 12 volt power is needed for other external devices.

Relays K1 and K2 are 12V, the contact groups of which must be designed for switching 220V / 16A. The value of fuse F1 must be selected in accordance with the device that will be connected to the soft starter module.

Both circuits have been tested on a breadboard and both work, but the second circuit is susceptible to interference if the wire going to the button is long enough, which in turn causes false switching.

Most resistors, capacitors and diodes are SMD. Lately I've been using more and more SMD elements in designs because there's no need to drill holes. If you decide to use either of these two PCBs, check them carefully because they have not been tested.

(unknown, downloads: 1,192)

One of the most important problems that arise when designing radio equipment is the problem of ensuring its reliability. The solution to this problem is based on the optimal design of the device and good adjustment during its manufacture. However, even in an optimally designed and adjusted device there is always a danger of its failure when the mains power is turned on. This danger is greatest for equipment with high power consumption - an audio frequency power amplifier (AMP).
The fact is that at the moment the mains power is turned on, the elements of the UMZCH power supply experience significant pulsed current overloads. The presence of discharged high-capacity oxide capacitors (up to tens of thousands of microfarads) in rectifier filters causes an almost short circuit of the rectifier output at the moment the power is turned on.
So, according to the data, with a supply voltage of 45V and a filter capacitor capacity of 10,000 μF, the charging current of such a capacitor at the moment the power is turned on can reach 12A. Almost at this moment, the power supply transformer operates in short circuit mode. The duration of this process is short, but it is quite sufficient under certain conditions to damage both the power transformer and the rectifier diodes.
In addition to the power supply, the UMZCH itself experiences significant overloads when the power is turned on. They are caused by non-stationary processes that arise in it due to the establishment of current and voltage modes of active elements and the slow activation of built-in feedback systems. And the higher the rated supply voltage of the UMZCH, the greater the amplitude of such overloads and, accordingly, the higher the likelihood of damage to the amplifier elements.
Of course, attempts have been made before to protect the UMZCH from overloads when turning on the power. A device was proposed that protected the amplifier from overloads, made in the form of a powerful bipolar supply voltage stabilizer, which, when turned on, initially supplied the amplifier with a voltage of +10 and -10V, and then gradually increased it to the nominal value of +32 and -32V. According to the author of this device, it made it possible to significantly improve the reliability of the UMZCH and abandon the use of traditional systems for protecting speaker systems from overloads when turning on the power.
Despite the undeniable advantages of this device, it also has disadvantages - the device protected only the UMZCH, but left its power supply unprotected; due to the complexity of its own design, it was in itself unreliable.
We present to your attention a simple and reliable device for “soft” power-on of the UMZCH, which protects both the UMZCH itself and its power supply from overloads. It is available for production even to a novice radio designer and can be used both in the development of new types of radio equipment and in the modernization of existing ones, including industrial production.
Operating principle
The principle of operation of the device is a two-stage supply of supply voltage to the primary winding of the transformer of the UMZCH power supply. A powerful ballast resistor is connected in series to the primary winding circuit of the power supply transformer (Fig. 1). The value of its resistance is calculated in accordance with the overall power of the transformer so that when turned on, the alternating current voltage on the primary winding is approximately half the mains voltage.
Then, at the moment of switching on, both the alternating voltage of the secondary windings of the transformer and the supply voltage of the UMZCH will be two times less. Due to this, the amplitudes of current and voltage pulses on the elements of the rectifier and UMZCH are sharply reduced. Unsteady processes at a reduced supply voltage proceed significantly “softer”.
Then, a few seconds after turning on the power, the ballast resistor R1 is closed by contact group K1.1 and the full mains voltage is supplied to the primary winding of the power transformer. Accordingly, they are restored to the nominal voltage values of the power supply.
By this time, the rectifier filter capacitors are already charged to half the nominal voltage, which eliminates the occurrence of powerful current pulses through the secondary windings of the transformer and the rectifier diodes. In the UMZCH by this time, the non-stationary processes are also completed, the feedback systems are turned on, and the supply of full supply voltage does not cause any overloads in the UMZCH.
When the mains power is turned off, the contacts K1.1 open, the ballast resistor is again connected in series with the primary winding of the transformer, and the entire cycle can be repeated. The “soft” power-on device itself consists of a transformerless power supply, a timer loaded on an electromagnetic relay. The design of the device and the modes of its elements are selected taking into account the maximum margin of reliability in operation. Its diagram is shown in Fig. 1.
When the UMZCH power supply is supplied by switch SB 1 with mains voltage through current-limiting elements R2 and C2, it is simultaneously supplied to a bridge rectifier assembled on diodes VD1 - VD4. The rectified voltage is filtered by capacitor SZ, limited by zener diode VD5 to a value of 36V and supplied to a timer made on transistor VT1. The current flowing through resistors R4 and R5 charges capacitor C4, when a voltage of approximately 1.5 V is reached on it, transistor VT1 goes into the open state - relay K1 is activated and contacts K1.1 bypass the ballast resistor R1.
The design of the device uses a sealed electromagnetic relay RENZZ version RF4.510.021 with an operating voltage of 27V and an operating current of 75 mA. It is possible to use other types of relays that allow switching an inductive AC load with a frequency of 50 Hz and at least 2A, for example, REN18, REN19, REN34.
A transistor with a large value of the current transfer coefficient parameter - KT972A - was used as VT1. It is possible to use the KT972B transistor. In the absence of the indicated transistors, transistors with a pnp conductivity structure are suitable, for example, KT853A, KT853B, KT973A, KT973B, but only in this case the polarity of all diodes and capacitors of this device should be reversed.
Fig.2.
In the absence of transistors with a high current transfer coefficient, you can use a composite transistor circuit of two transistors according to the circuit shown in Fig. 2. Any silicon transistors with a permissible collector-emitter voltage of at least 45V and a sufficiently large current gain, for example, types KT5OZG, KT3102B, can be used as VT1 in this circuit. As a transistor VT2 - medium power transistors with the same parameters, for example, KT815V, KT815G, KT817V, KT817G or similar to them. The connection of the composite transistor option is made at points A-B-C of the main circuit of the device.
In addition to KD226D diodes, the device can use KD226G, KD105B, KD105G diodes. An MBGO type capacitor with an operating voltage of at least 400V is used as capacitor C2. The current-limiting circuit R2C2 is rated to provide a maximum AC current of approximately 145 mA, which is sufficient when using an electromagnetic relay with a trip current of 75 mA.
For a relay with an operating current of 130 mA (REN29), the capacitance of capacitor C2 will need to be increased to 4 μF. When using a relay of the REN34 type (operation current 40 mA), a capacitance of 1 μF is sufficient. In all options for changing the capacitance of the capacitor, its operating voltage must be at least 400 V. In addition to metal-paper capacitors, good results can be obtained by using metal-film capacitors of the types K73-11, K73-17, K73-21, etc.
A PEV-25 vitrified wire resistor is used as ballast resistor R1. The indicated rated power of the resistor is designed for use in conjunction with a power transformer having an overall power of about 400 W. For a different value of overall power and half the voltage of the first stage, the resistance of resistor R1 can be recalculated using the formula:
R1 (Ohm) = 48400/Slave (W).
Settings
Adjusting the device comes down to setting the timing of the timer to delay the activation of the second stage. This can be done by selecting the capacitance of capacitor C5, so it is advisable to compose it from two capacitors, which will facilitate the adjustment process.
Note: In the original version of the device, there is no fuse in the power circuit. In normal operation, it is, of course, not required. But emergency situations can always arise - short circuits, breakdowns of elements, etc. the author himself argues for the need to use his design in just such a situation, then the role of the protective element is taken over by resistor R2, it heats up and burns out.
The use of a fuse-link in emergency situations is quite justified. It is cheaper, easier to purchase, and the response time is so much shorter that other elements do not have time to heat up and cause any additional damage. And finally, this is a generally accepted, proven many times over method of protecting devices from the possible consequences of hardware malfunctions.
M. Korzinin
Literature:
1. Sukhov N. UMZCH of high fidelity. - Radio, 1989, No. 6,7.
2. Kletsov V. Low-frequency amplifier with low distortion. - Radio, 1983, No. 7, pp. 51 - 53; 1984, No. 2, pp. 63, 64.