Power supply protection circuits. Short-circuit protection on field-effect transistor. Adding realism to the security system

Power Good Signal

When we turn on, the output voltages do not immediately reach the desired value, but after about 0.02 seconds, and to prevent the supply of reduced voltage to the PC components, there is special signal"power good", also sometimes called "PWR_OK" or simply "PG", which is applied when the voltages at the +12V, +5V and +3.3V outputs reach the correct range. To supply this signal, a special line is allocated on the ATX power connector connected to (No. 8, gray wire).

Another consumer of this signal is the undervoltage protection circuit (UVP) inside the power supply, which will be discussed later - if it is active from the moment it is turned on on the power supply, it will simply not allow the computer to turn on, immediately turning off the power supply, since the voltages will obviously be below nominal. Therefore, this circuit is turned on only when the Power Good signal is applied.

This signal is supplied by a monitoring circuit or a PWM controller (pulse width modulation used in all modern switching power supplies, which is why they got their name, the English abbreviation is PWM, familiar from modern coolers - to control their rotation speed supplied to them the current is modulated in a similar way.)

Power Good signal delivery diagram according to ATX12V specification.
VAC is the incoming alternating voltage, PS_ON# is the "power on" signal, which is sent when the power button on the system unit is pressed. "O/P" is an abbreviation for "operating point", i.e. working value. And PWR_OK is the Power Good signal. T1 is less than 500 ms, T2 is between 0.1 ms and 20 ms, T3 is between 100 ms and 500 ms, T4 is less than or equal to 10 ms, T5 is greater than or equal to 16 ms and T6 is greater than or equal to 1 ms.

Undervoltage and overvoltage protection (UVP/OVP)

Protection in both cases is implemented using the same circuit that monitors the output voltages +12V, +5V and 3.3V and turns off the power supply if one of them is higher (OVP - Over Voltage Protection) or lower (UVP - Under Voltage Protection ) a certain value, which is also called the “trigger point”. These are the main types of protection that are currently present in virtually all devices; moreover, the ATX12V standard requires OVP.

A bit of a problem is that both OVP and UVP are typically configured with trigger points too far from the nominal voltage value and in the case of OVP this is a direct match to the ATX12V standard:

Those. you can make a power supply with an OVP trigger point of +12V at 15.6V, or +5V at 7V and it will still be compatible with the ATX12V standard.

This will produce, say, 15V instead of 12V for a long time without triggering the protection, which can lead to failure of PC components.

On the other hand, the ATX12V standard clearly stipulates that output voltages should not deviate more than 5% from the nominal value, but the OVP can be configured by the power supply manufacturer to operate at a deviation of 30% along the +12V and +3.3V lines and 40% - along the +5V line.

Manufacturers select the values ​​of the trigger points using one or another monitoring chip or PWM controller, because the values ​​of these points are strictly defined by the specifications of a particular chip.

As an example, let's take the popular PS223 monitoring chip, which is used in some that are still on the market. This chip has the following trigger points for OVP and UVP modes:

Other chips provide a different set of trigger points.

And once again we remind you how far from normal voltage values ​​OVP and UVP are usually configured. In order for them to work, the power supply must be in a very difficult situation. In practice, cheap power supplies that do not have other types of protection besides OVP/UVP fail before OVP/UVP is triggered.

Overcurrent Protection (OCP)

In the case of this technology (the English abbreviation OCP is Over Current Protection) there is one issue that should be considered in more detail. According to the international standard IEC 60950-1, no single conductor in computer equipment must carry more than 240 Volt-Amps, which is the case with DC gives 240 watts. The ATX12V specification includes a requirement for overcurrent protection on all circuits. In the case of the most loaded 12V circuit, we get a maximum permissible current of 20Amps. Naturally, such a limitation does not allow the production of a power supply with a power of more than 300 Watt, and in order to get around it, the +12V output circuit began to be divided into two or more lines, each of which had its own overcurrent protection circuit. Accordingly, all power supply pins that have +12V contacts are divided into several groups according to the number of lines, in some cases they are even color-coded in order to adequately distribute the load across the lines.

However, in many cheap power supplies with stated two +12V lines, in practice only one current protection circuit is used, and all +12V wires inside are connected to one output. In order to implement adequate operation of such a circuit, the current load protection is triggered not at 20A, but at, for example, 40A, and the limitation of the maximum current on one wire is achieved by the fact that in a real system the +12V load is always distributed among several consumers and even more wires.

Moreover, sometimes you can figure out whether a particular power supply unit uses separate current protection for each +12V line only by disassembling it and looking at the number and connection of shunts used to measure the current (in some cases, the number of shunts may exceed the number of lines, since multiple shunts can be used to measure current on one line).

Another interesting point is that, in contrast to over/undervoltage protection, the permissible current level is regulated by the power supply manufacturer by soldering resistors of one or another value to the outputs of the control microcircuit. And on cheap power supplies, despite the requirements of the ATX12V standard, this protection can only be installed on the +3.3V and +5V lines, or absent altogether.

Over Temperature Protection (OTP)

As its name suggests (OTP - Over Temperature Protection), overheat protection turns off the power supply if the temperature inside its case reaches a certain value. Not all power supplies are equipped with it.

In power supplies, you may see a thermistor attached to the heatsink (although in some power supplies it may be soldered directly to the printed circuit board). This thermistor is connected to the fan speed control circuit and is not used for overheating protection. In power supplies equipped with overheating protection, two thermistors are usually used - one to control the fan, the other to actually protect against overheating.

Short Circuit Protection (SCP)

Short Circuit Protection (SCP) is probably the oldest of these technologies because it is very easy to implement with a couple of transistors, without using a monitoring chip. This protection is necessarily present in any power supply and turns it off in the event of a short circuit in any of the output circuits, in order to avoid a possible fire.

Integrated circuit (IC) KR142EN12A is adjustable stabilizer voltage compensation type in the KT-28-2 housing, which allows you to power devices with a current of up to 1.5 A in the voltage range 1.2...37 V. This integrated stabilizer has thermally stable current protection and output short circuit protection.

Based on the KR142EN12A IC, you can build adjustable block power supply, the circuit of which (without transformer and diode bridge) is shown in Fig.2. The rectified input voltage is supplied from the diode bridge to capacitor C1. Transistor VT2 and chip DA1 should be located on the radiator.

Heat sink flange DA1 is electrically connected to pin 2, so if DAT and transistor VD2 are located on the same heatsink, then they need to be isolated from each other.

In the author's version, DA1 is installed on a separate small radiator, which is not galvanically connected to the radiator and transistor VT2. The power dissipated by a chip with a heat sink should not exceed 10 W. Resistors R3 and R5 form a voltage divider included in the measuring element of the stabilizer. A stabilized negative voltage of -5 V is supplied to capacitor C2 and resistor R2 (used to select the thermally stable point VD1). In the original version, the voltage is supplied from the KTs407A diode bridge and the 79L05 stabilizer, powered from a separate winding of the power transformer.

For guard from closing the output circuit of the stabilizer, it is enough to connect an electrolytic capacitor with a capacity of at least 10 μF in parallel with resistor R3, and shunt resistor R5 with a KD521A diode. The location of the parts is not critical, but for good temperature stability it is necessary to use the appropriate types of resistors. They should be located as far as possible from heat sources. The overall stability of the output voltage consists of many factors and usually does not exceed 0.25% after warming up.

After switching on and warming up the device, the minimum output voltage of 0 V is set with resistor Rao6. Resistors R2 ( Fig.2) and resistor Rno6 ( Fig.3) must be multi-turn trimmers from the SP5 series.

Possibilities the current of the KR142EN12A microcircuit is limited to 1.5 A. Currently, there are microcircuits on sale with similar parameters, but designed for a higher current in the load, for example LM350 - for a current of 3 A, LM338 - for a current of 5 A. Recently on sale imported microcircuits from the LOW DROP series (SD, DV, LT1083/1084/1085) appeared. These microcircuits can operate at a reduced voltage between input and output (up to 1... 1.3 V) and provide a stabilized output voltage in the range of 1.25...30 V at a load current of 7.5/5/3 A, respectively . Closest in parameters domestic analogue type KR142EN22 has a maximum stabilization current of 7.5 A. At the maximum output current, the stabilization mode is guaranteed by the manufacturer at an input-output voltage of at least 1.5 V. The microcircuits also have built-in protection against excess current in the load of the permissible value and thermal protection against overheating of the case . These stabilizers provide output voltage instability of 0.05%/V, output voltage instability when the output current changes from 10 mA to a maximum value of no worse than 0.1%/V. On Fig.4 shows a power supply circuit for a home laboratory, which allows you to do without transistors VT1 and VT2, shown in Fig.2.

Instead of the DA1 KR142EN12A microcircuit, the KR142EN22A microcircuit was used. This is an adjustable stabilizer with a low voltage drop, which allows you to obtain a current of up to 7.5 A in the load. For example, the input voltage supplied to the microcircuit is Uin = 39 V, output voltage at the load Uout = 30 V, current at the load louf = 5 A, then the maximum power dissipated by the microcircuit at the load is 45 W. Electrolytic capacitor C7 is used to reduce output impedance at high frequencies, and also reduces noise voltage and improves ripple smoothing. If this capacitor is tantalum, then its nominal capacity must be at least 22 μF, if aluminum - at least 150 μF. If necessary, the capacitance of capacitor C7 can be increased. If the electrolytic capacitor C7 is located at a distance of more than 155 mm and is connected to the power supply with a wire with a cross-section of less than 1 mm, then an additional electrolytic capacitor with a capacity of at least 10 μF is installed on the board parallel to the capacitor C7, closer to the microcircuit itself. The capacitance of filter capacitor C1 can be determined approximately at the rate of 2000 μF per 1 A of output current (at a voltage of at least 50 V). To reduce the temperature drift of the output voltage, resistor R8 must be either wire-wound or metal-foil with an error of no worse than 1%. Resistor R7 is the same type as R8. If the KS113A zener diode is not available, you can use the unit shown in Fig.3. The author is quite satisfied with the protection circuit solution given in, as it works flawlessly and has been tested in practice. You can use any power supply protection circuit solutions, for example those proposed in. In the author’s version, when relay K1 is triggered, contacts K 1.1 close, short-circuiting resistor R7, and the voltage at the power supply output becomes 0 V. Printed circuit board The power supply unit and the location of the elements are shown in Fig. 5, appearance BP - on Fig.6.

Many homemade units have the disadvantage of lacking protection against power reverse polarity. Even an experienced person can inadvertently confuse the polarity of the power supply. And there is a high probability that after this Charger will fall into disrepair.

This article will discuss 3 options for reverse polarity protection, which work flawlessly and do not require any adjustment.

Option 1

This protection is the simplest and differs from similar ones in that it does not use any transistors or microcircuits. Relays, diode isolation - that’s all its components.

The scheme works as follows. The minus in the circuit is common, so the positive circuit will be considered.

If there is no battery connected to the input, the relay is in the open state. When the battery is connected, the plus is supplied through the diode VD2 to the relay winding, as a result of which the relay contact closes and the main charging current flows to the battery.

At the same time, the green LED indicator lights up, indicating that the connection is correct.

And if you now remove the battery, then there will be voltage at the output of the circuit, since the current from the charger will continue to flow through the VD2 diode to the relay winding.

If the connection polarity is reversed, the VD2 diode will be locked and no power will be supplied to the relay winding. The relay will not work.

In this case, the red LED will light up, which is intentionally connected incorrectly. It will indicate that the polarity of the battery connection is incorrect.

Diode VD1 protects the circuit from self-induction that occurs when the relay is turned off.

If such protection is introduced into , it’s worth taking a 12 V relay. The permissible current of the relay depends only on the power . On average, it is worth using a 15-20 A relay.

This scheme still has no analogues in many respects. It simultaneously protects against power reversal and short circuit.

The operating principle of this scheme is as follows. During normal operation, the plus from the power source through the LED and resistor R9 opens the field-effect transistor, and the minus through the open junction of the “field switch” goes to the output of the circuit to the battery.

When a polarity reversal or short circuit occurs, the current in the circuit increases sharply, resulting in a voltage drop across the “field switch” and across the shunt. This voltage drop is enough to trigger the low-power transistor VT2. Opening, the latter closes the field-effect transistor, closing the gate to ground. At the same time, the LED lights up, since power for it is provided by the open junction of transistor VT2.

Due to its high response speed, this circuit is guaranteed to protect for any problem at the output.

The circuit is very reliable in operation and can remain in a protected state indefinitely.

This is special simple circuit, which can hardly even be called a circuit, since it uses only 2 components. This is a powerful diode and fuse. This option is quite viable and is even used on an industrial scale.

Power from the charger is supplied to the battery through the fuse. The fuse is selected based on the maximum charging current. For example, if the current is 10 A, then a 12-15 A fuse is needed.

The diode is connected in parallel and closed when normal operation. But if the polarity is reversed, the diode will open and a short circuit will occur.

And the fuse is the weak link in this circuit, which will burn out at the same moment. After this you will have to change it.

The diode should be selected according to the datasheet based on the fact that its maximum short-term current was several times greater than the fuse combustion current.

This scheme does not provide 100% protection, since there have been cases when the charger burned out faster than the fuse.

Bottom line

From an efficiency point of view, the first scheme is better than the others. But from the point of view of versatility and speed of response, the best option is scheme 2. Well, the third option is often used on an industrial scale. This type of protection can be seen, for example, on any car radio.

All circuits, except the last one, have a self-healing function, that is, operation will be restored as soon as the short circuit is removed or the polarity of the battery connection is changed.

Attached files:

How to make a simple Power Bank with your own hands: diagram of a homemade power bank

Modern power switching transistors have very low drain-source resistances when on, which ensures low voltage drop when large currents pass through this structure. This circumstance allows the use of such transistors in electronic fuses.

For example, the IRL2505 transistor has a drain-source resistance, with a source-gate voltage of 10V, only 0.008 Ohms. At a current of 10A, the power P=I² R will be released on the crystal of such a transistor; P = 10 10 0.008 = 0.8 W. This suggests that at a given current the transistor can be installed without using a radiator. Although I always try to install at least small heat sinks. In many cases, this allows you to protect the transistor from thermal breakdown in emergency situations. This transistor is used in the protection circuit described in the article “”. If necessary, you can use surface-mounted radioelements and make the device in the form of a small module. The device diagram is shown in Figure 1. It was calculated for a current of up to 4A.

Electronic fuse diagram

In this circuit, a field-effect transistor with a p channel IRF4905 is used as a key, having an open resistance of 0.02 Ohm, with a gate voltage = 10V.

In principle, this value also limits the minimum supply voltage of this circuit. With a drain current of 10A, it will generate a power of 2 W, which will entail the need to install a small heat sink. The maximum gate-source voltage of this transistor is 20V, therefore, to prevent breakdown of the gate-source structure, a zener diode VD1 is introduced into the circuit, which can be used as any zener diode with a stabilization voltage of 12 volts. If the voltage at the input of the circuit is less than 20V, then the zener diode can be removed from the circuit. If you install a zener diode, you may need to adjust the value of resistor R8. R8 = (Upit - Ust)/Ist; Where Upit is the voltage at the circuit input, Ust is the stabilization voltage of the zener diode, Ist is the zener diode current. For example, Upit = 35V, Ust = 12V, Ist = 0.005A. R8 = (35-12)/0.005 = 4600 Ohm.

Current-voltage converter

Resistor R2 is used as a current sensor in the circuit, in order to reduce the power released by this resistor; its value is chosen to be only one hundredth of an Ohm. When using SMD elements, it can be composed of 10 resistors of 0.1 Ohm, size 1206, with a power of 0.25 W. The use of a current sensor with such a low resistance entailed the use of a signal amplifier from this sensor. The DA1.1 op amp of the LM358N microcircuit is used as an amplifier.

The gain of this amplifier is (R3 + R4)/R1 = 100. Thus, with a current sensor having a resistance of 0.01 Ohm, the conversion coefficient of this current-voltage converter equal to one, i.e. One ampere of load current is equal to a voltage of 1V at output 7 DA1.1. You can adjust the Kus with resistor R3. With the indicated values ​​of resistors R5 and R6, the maximum protection current can be set within.... Now let's count. R5 + R6 = 1 + 10 = 11kOhm. Let's find the current flowing through this divider: I = U/R = 5A/11000Ohm = 0.00045A. Hence, the maximum voltage that can be set at pin 2 of DA1 will be equal to U = I x R = 0.00045A x 10000 Ohm = 4.5 V. Thus, the maximum protection current will be approximately 4.5A.

Voltage comparator

A voltage comparator is assembled on the second op-amp, which is part of this MS. The inverting input of this comparator is supplied with a reference voltage regulated by resistor R6 from stabilizer DA2. Non-inverting input 3 of DA1.2 is supplied with amplified voltage from the current sensor. The comparator load is series circuit, optocoupler LED and damping adjustment resistor R7. Resistor R7 sets the current passing through this circuit, about 15 mA.

Circuit operation

The scheme works as follows. For example, with a load current of 3A, a voltage of 0.01 x 3 = 0.03V will be released at the current sensor. The output of amplifier DA1.1 will have a voltage equal to 0.03V x 100 = 3V. If in this case, at input 2 of DA1.2 there is a reference voltage set by resistor R6, less than three volts, then at the output of comparator 1 a voltage will appear close to the supply voltage of the op-amp, i.e. five volts. As a result, the optocoupler LED will light up. The optocoupler thyristor will open and bridge the gate of the field-effect transistor with its source. The transistor will turn off and turn off the load. Return the diagram to the initial state You can use the SB1 button or turn the power supply off and on again.