RU2523021C1 - Voltage switch with over current protection - Google Patents

Abstract

FIELD: electricity.

SUBSTANCE: device includes electronic switch with MOS structure, supplying power to load unit. Electronic switch device and a sequence of resistor and thermal resistor connected to common point of switch and load unit reduce error of relay element actuation level generation with hysteresis that control switching load power supply unit on and off with the help of first and second AND elements. In case of current overload, power is disconnected from load unit.

EFFECT: reduced weight and dimensions of voltage switch, increased precision during changes in open electronic switch depending on temperature.

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Description

The present invention relates to the field of electronic technology and can be used in switched power supplies with overcurrent protection, mainly in control systems of spacecraft.

Known voltage switch with overcurrent protection [1], containing an electronic switch, first and second relay elements, current sensor, trigger, load unit, operational amplifier, voltage adjuster, transistor.

A disadvantage of the known device is its complexity and the use of a current sensor (shunt). When switching high currents, considerable power is allocated to the shunt, which leads to an increase in the mass and dimensions of the voltage switch due to the installation of a metal element that removes heat from the shunt, designed to remove heat of high power.

The closest technical solution to the proposed device is a voltage switch with overcurrent protection, described in [2]. The known voltage switch contains a series-connected current sensor, an electronic switch and a load unit, as well as a relay element, a trigger.

A disadvantage of the known device is that it uses a current sensor to implement its main function. When switching large currents in the chains of their flow, significant interference occurs. To obtain a reliable level of operation of the relay element, it is necessary that the level of the useful signal exceeds the level of the interference signal. This requires increasing the ohmic resistance of the current sensor, and this leads to a significant increase in the dissipated power on it and, as a consequence, to an increase in mass and dimensions. For reliable operation of the relay element that turns off the switch when an overload current occurs, the value of the useful input signal of the relay element should be at the level of 100-300 mV. So, when switching current I _H = 50A and when the resistance of the current sensor (shunt) r _w = 4 mOhm, the heat release of the shunt will be 10 watts. To remove such heat from the shunt, a metal element of considerable weight and dimensions is required, which increases the mass and dimensions of the voltage switch.

The objective of the invention is to reduce the mass and dimensions of the voltage switch and improve accuracy when changing the resistance of the electronic switch in the open state depending on the temperature.

This task is achieved in that the voltage switch with overcurrent protection contains an input bus, a first element And, a relay element with hysteresis, the inverse output of which is connected to the first input of the first element And, and an electronic switch and a load block connected in series, while the electronic the switch is made in the form of an electronic key with a MOS structure, and a second element And, an electronic key and a resistor and a thermistor connected in series, are added to the voltage switch, a common point s is connected to the input of the relay element with hysteresis, the inverse output of which is connected to the first input of the second element And, the second input of which is connected to the input bus and the second input of the first element And, the output of the second element And is connected to the control input of the electronic key, the output of which is connected to the resistor , the output of the first element And is connected to the control input of the electronic switch, the common point of which with the load unit is connected to the signal input of the electronic key.

Figure 1 shows a block diagram of a voltage switch with overcurrent protection. In this diagram: 1 - input bus, 2 - first I element, 3 - electronic switch, 4 - load block, 5 - second I element, 6 - electronic key, 7 - relay element with hysteresis, 8 - resistor, 9 - thermistor .

The input bus 1 is connected to the second inputs of the first 2 and second 5 AND elements, the first inputs of which are connected to the inverse output of the relay element with hysteresis 7. The electronic switch 3 and the load unit 4 are connected in series, while their common point is connected to the signal input of the electronic key 6 the output of which is connected to the resistor 8 and the thermistors 9 in series, the common point of which is connected to the control input of the relay element with hysteresis 7. The output of the first element And 2 is connected to the control input ele ctron switch 3, the output of the second element And 5 is connected to the control input of the electronic key 6.

The voltage switch with overcurrent protection operates as follows. As an electronic switch 3, it is assumed to use an electronic key (transistor) with a MOS structure. A feature of such an element is the ability to switch high currents, while switching different currents, the ohmic resistance of an open electronic switch (transistor) is practically independent of the current value and amounts to a small amount (units of mOhm). In addition, transistors with a MOS structure change their resistance depending on the transition temperature, moreover, this change is usually linear in nature.

In the General case, the voltage drop U _K on the open electronic switch 3 can be represented as

$$U_{\mathrm{TO}}=r_{\mathrm{TO}}{I}_H(\mathrm{one})$$

where I _H is the switched load current, r _K is the resistance of the electronic switch 3 in the open (on) state. The resistance of the electronic switch r _K can be represented as

$$r_{\mathrm{TO}}=r_0(\mathrm{one}+K_{\mathrm{one}}\Delta t)(2)$$

where r ₀ is the resistance of the electronic switch 3 at a temperature t ₀ , K ₁ is the temperature coefficient of resistance change r _K , Δt is the temperature difference relative to t ₀ .

We will assume that the electronic switch 3 is in the open (on) state if a positive signal U2 from the output of the first element And 2 (U ₂ = 1) is supplied to its control input, and in the closed (off) state if U ₂ = 0 . We also believe that in the initial state the signal at the inverse output of the relay element with hysteresis 7 U ₇ = 1. In this case, when the signal on the input bus 1 U _BX = 1, the electronic switch 3 is in the open (on) state, when U _BX = 0, the electronic switch 3 is in the closed (off) state. As follows from the structural diagram of figure 1, the electronic key 6 operates synchronously with the electronic switch 3 (the electronic key 6 is open if the signal from the output of the second element 5 And U ₅ = 1, the electronic key 6 is closed if the signal U ₅ = 0, and its output signal is U ₆ = 0).

Let the response level of the relay element with hysteresis 7 is chosen equal to U _P. The voltage at the input of the relay element 7 is denoted by U _X. The stresses U _X and U _K are related by

$$U_X=U_{\mathrm{TO}}{R}_t/(R_{\mathrm{one}}+R_t)(3)$$

where R _t is the resistance of the thermistor, R ₁ is the resistance of the resistor.

When the voltage drops on the switch 3 U _K , at which U _X = U _P , the relay element will trigger with hysteresis 7 (the hysteresis of the relay element 7 is chosen equal to U _P ). The output signal of the relay element 7 U ₇ = 0 and the output signals of the first 2 and second 5 elements And will be equal to U ₂ = 0 and U ₅ = 0, respectively. These signals turn off the electronic switch 3 and the electronic key 6. The electronic switch 3 removes voltage from the load unit 4.

The voltage U _P is selected from the condition of the specified current I _H , at which it is necessary to disconnect the voltage from the load unit 4, and the known resistance of the electronic switch with a MOS structure of 3 r _K in accordance with (1). If, for example, R ₁ = R _t0 , r _K = 4 mOhm and the tripping current I _H = 50 A, then U _P is chosen equal to 0.1 V. If the load current I _H exceeds 50 A, then the electronic switch 3 will turn off the voltage from the load unit 4. When the power is disconnected from the load unit, the electronic switch 6 is also turned off, which removes the input voltage from the relay element with hysteresis 7 (U ₆ = 0), which allows the relay element 7 to be in the off state.

To compensate for the change in resistance r _K , depending on the temperature, we choose a thermal resistance with a negative coefficient of temperature change K ₂ , i.e. we will assume that

$$R_t=R_{t0}(\mathrm{one}-K_2\Delta t)(\mathrm{four})$$

where R _t0 is the value of thermal resistance at a temperature t ₀ . The voltage U _K can be represented as

$$U_{\mathrm{TO}}=U_0(\mathrm{one}+K_{\mathrm{one}}\Delta t)(5)$$

where U ₀ is the voltage at the open switch 3 at a temperature t ₀ and a trip current I _H.

To fully compensate for the temperature change in the resistance r _K, it is necessary to fulfill the condition

$$U_X=U_{\mathrm{TO}}{R}_t/(R_{\mathrm{one}}+R_t)=U_0{R}_{t0}(R_{\mathrm{one}}+R_{t0})=U_{X0}=0.5U_0=U_P(6)$$

From (6), taking into account (4) and (5), we have

$$K_2=2K_{\mathrm{one}}/(\mathrm{one}+2K_{\mathrm{one}}\Delta t)(7)$$

As follows from (7), in order to completely compensate for the temperature change in the resistance r _K, it is necessary to change the temperature coefficient of the thermistor 9 K ₂ depending on the temperature Δt. It is not possible to find such a thermistor. Let us estimate the error of the proposed scheme provided that K _{2 is} selected at Δt = 0.5 (Δt _MAX + Δt _MIN ) where Δt _MAX and Δt _MIN are the maximum and minimum possible overheating of the electronic switch 3.

Let 20 ° C≤Δt≤40 ° C. We determine the value of K ₂ for Δt = 30 ° C. Suppose that when Δt changes from 0 ° C to 100 ° C, the resistance r _K linearly changes twice, i.e. r _K = 2r ₀ . From (2) K ₁ = 0.01. From (7) at Δt = 30 ° CK ₂ = 0.0125. We choose a thermistor with such a temperature coefficient. Let us determine the error δ _{1 of the} formation of the current cutoff level by the voltage switch proposed by the minimum temperature Δt _MIN = 20 ° C. At Δt = 20 ° C from (5) U _K1 = 1.2U ₀ . From (4), R _t1 = 0.75R ₀ . From (6), U _X1 = 0.514U ₀ . The error δ _{1 is} defined as

$${\delta }_{\mathrm{one}}=(U_{X\mathrm{one}}-U_{X0})/U_{X0}(8)$$

The error δ ₁ from (8) is 0.028 or 2.8%.

We determine the error δ _{2 of the} formation of the current cutoff level proposed by the voltage switch at the maximum temperature Δt _MAX = 40 ° C. From (5), U _K2 = 1.4U ₀ . From (4), R _t2 = 0.5R ₀ . From (6), U _X2 = 0.47 U ₀ . The error δ _{2 is} defined as

$${\delta }_{\mathrm{one}}=(U_{X0}-U_{X2})/U_{X0}(9)$$

The error δ ₂ from (9) is 0.06 or 6%.

The error in the formation of the signal U _K at Δt = 20 ° C from (1) and (2) is 20%, at Δt = 40 ° C it is 40%. The proposed scheme allows to reduce the error in the formation of the level of voltage disconnection from the load unit 4 during overcurrent, respectively, to 2.8% and 6%.

Compared with the known voltage switch [2], the present invention allows to reduce the mass and dimensions of the switch by removing the requirements for heat removal from the current sensor, which is absent in the proposed circuit. In a known circuit, when using a current sensor with r _w = 4 mΩ at a current of 50A, a power of 10 W is dissipated. To remove heat of 10 W, a metal exhaust surface of 2 dm ² is required. With a permissible overheating at the current sensor at 30 ° C compared to the ambient temperature, a heat sink with a heat sink surface of 100 × 200 mm will be required. When using an aluminum plate with a thickness of 5 mm as a heat sink, the heat sink mass will be 250 g, which is a significant value for one switch. When using electronic switches in control systems, for example, spacecraft, the additional mass is a significant drawback.

The proposed set of features in the solutions considered by the authors was not found to solve the problem and does not follow explicitly from the prior art, which allows us to conclude that the technical solution meets the criteria of "novelty" and "inventive step". As elements for the implementation of the device, logical elements And, for example, 564 series, standard relay elements, electronic switches with a MOS structure, for example, type 2P7161 B, electronic keys, 564 series, resistors and thermistors can be used.

Literature

1. RF patent No. 2258302, Cl. H03K 08/17, 2005

2. RF patent No. 2208291, Cl. H03K 08/17, 2003

Claims (1)

A voltage switch with overcurrent protection, comprising an input bus, a first element And, a relay element with hysteresis, the inverse output of which is connected to the first input of the first element And, and an electronic switch and a load unit connected in series, characterized in that the electronic switch is made in in the form of an electronic key with a MOS structure, and in addition, a second element And, an electronic key and a resistor and a thermistor connected in series with a common point with connected to the input of the relay element with hysteresis, the inverse output of which is connected to the first input of the second element And, the second input of which is connected to the input bus and the second input of the first element And, the output of the second element And is connected to the control input of the electronic key, the output of which is connected to the resistor, the output of the first element And is connected to the control input of the electronic switch, the common point of which with the load unit is connected to the signal input of the electronic key.