IP protection with the help of an analog multiplier KR525PS2. Scheme, description

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IP protection with the help of an analog multiplier KR525PS2. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Radio amateur designer

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In many power supplies, the regulating element is protected from exceeding the permissible current and temperature. In the author's opinion, these measures are not enough, and he proposes to protect the control transistor from exceeding the permissible power, taking into account the case temperature. The article considers a variant of such protection of an unstabilized power supply using an analog multiplier KR525PS2. Stabilized sources can also be equipped with such a node.

The power dissipated by the control transistor of the power supply (PSU) is equal to the product of the collector current (load current) and the voltage drop in the collector-emitter section. In laboratory PSUs (with adjustable output voltage), with a low output voltage and a high load current, the control transistor can generate power that exceeds the allowable for a particular device. Such PSUs usually provide only current protection. However, it is not able to protect the regulating element when the load current is less than the protection operation value, and the power released on the regulating transistor exceeds the allowable one.

The question arises, how to protect the control transistor from exceeding the permissible power dissipation? There is a domestic chip KR525PS2, which performs the operation of multiplying two analog parameters. If signals proportional to the collector current of the regulating transistor and the voltage in the collector-emitter section are applied to its inputs, then the output voltage will be proportional to their product. Thus, on the basis of this microcircuit, it is possible to assemble a protection unit against exceeding the permissible power by the regulating transistor.

A diagram of such a protection node in an unstabilized PSU is shown in the figure.

IP protection using an analog multiplier KR525PS2

Input X of the DA1 microcircuit is supplied with voltage from the base of the regulating transistor VT3, and input Y is part of the voltage from the current sensor R5, proportional to the load current (the voltage at the base of the regulating transistor is equal to the voltage at its emitter minus 0,6 V).

A comparator is made on the op-amp DA2. Its non-inverting input is supplied with voltage from the output of the DA1 multiplier, and the inverting input is supplied with an exemplary voltage from the resistive divider R15R16RK1. When the voltage at the non-inverting input is greater than at the inverting one, a signal is sent from the output of the op-amp to the trinistor VS1. It opens and connects the base of the transistor VT5 to a common wire through the VD1 diode. Transistor VT1, followed by transistors VT2 and VT3 are closed. At the same time, the HL1 LED turns on, signaling the PSU is turned off and the load is de-energized. Thermistor RK1 acts as a temperature sensor for the body of the transistor VT3 (it is fixed on the body of the transistor or on a heat sink near the body).

The protection unit is fed from the additional winding of the network transformer T1. On diodes VD6-VD9 and capacitors C2, C3, a rectifier is assembled, and on zener diodes VD10, VD11 and resistors R10, R11 - the simplest bipolar stabilizer.

The security node is set up as follows. First, trimming resistors R12-R14 correct the zero voltage at the multiplier. At zero voltages at the inputs X and Y (they are connected to a common wire), the trimmer resistor R14 sets zero voltage at the output Z. Then, by applying a voltage of a few volts to the input X (there is still zero voltage at the input Y), the trimmer resistor R13 is set zero voltage at output Z. And finally, a similar operation is carried out for input Y. After that, the multiplier adjustment is considered complete.

Then, based on the allowable power dissipated by the regulating transistor and the allowable current, the collector-emitter voltage is calculated. For the KT819G transistor, with a permissible power of 60 W and a current of 15 A, this value is 4 V. At a maximum current of 15 A, the voltage drop across the current sensor is 3,6 V. These voltages (3,4 and approximately 2,2 ... 3,6, 7 V - this value depends on the position of the trimmer resistor R6) is fed to the corresponding (X and Y) inputs of the multiplier, having previously disconnected them from the resistors R7 and R15, and the comparator is activated, the trinistor and the LED are turned on. This may require the selection of the resistor RXNUMX.

The presence of a thermistor RK1 with a negative TCR leads to the fact that with an increase in the temperature of the transistor case, the resistance of the thermistor decreases, therefore, the voltage at the inverting input of the op-amp will decrease. And since the allowable power dissipated by the transistor decreases with increasing case temperature, in this case the protection unit should operate at a lower power.

Such protection can be applied in almost any power supply unit with a regulating transistor. However, it should be remembered that the voltage at the inputs of the multiplier should not exceed 10 V. Therefore, in cases where the voltage on the regulating transistor can exceed this value, it must be applied to the X input of the multiplier through a resistive divider.

Author: A.Partin, Ekaterinbur

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