Protective circuit for an electronic device

The invention relates to a protective circuit for an electronic device, which is located between a DC voltage source and the operating voltage input of the device, and consists of the series connection of a diode and a limiting resistor.

A large number of electronic devices, for example also switched-mode power supplies, are fed from a direct voltage source, for example a battery, or from a local direct voltage network. When connecting the device to the DC voltage source, incorrect polarity can lead to the destruction of expensive components of the device, so that polarity reversal protection is provided, for example in the form of a diode or a diode bridge. Furthermore, connecting the device to the DC voltage source - due to inductive and / or capacitive components - can lead to very high inrush currents, which can also destroy components. This can be done by fast fuses, e.g. Fuses can be prevented in many cases, but frequent replacement of the fuses is labor-intensive, so that a limiting resistor, in particular a thermistor, is used to limit the inrush current.

Protective circuits of this type, consisting for example of a diode and an NTC thermistor, generate permanent losses during operation, which reduce the overall efficiency of the device. With an input current of the device of 2A and when using a diode and a 1 ohm limiting resistor, the additional permanent losses are approx. 5 watts.

One object of the invention is to create a protective circuit which results in significantly lower permanent losses.

This object is achieved with a protective circuit of the type mentioned, in which, according to the invention, the diode and the limiting resistor are bridged by a controlled switch, and a voltage monitor is connected to an operating voltage of the device, which is used to output a switching signal that closes the switch when a predetermined one is reached Minimum value of the operating voltage is set up.

Thanks to the invention, the protective circuit is only in operation when it is actually needed, namely during the switch-on process. The permanent losses of the protective circuit are only determined by the resistance of the switch in its closed state and are up to two orders of magnitude below those of the known protective circuits.

The monitored operating voltage is advantageously the supply voltage of the device.

If the device is a power supply unit, the monitored voltage can expediently be its output voltage.

A variant with very low permanent losses is characterized in that the controlled switch is formed by a relay with a normally open contact. It can be recommended that an excitation winding of the relay is in series with the collector-emitter path of a control transistor and this is controlled by a voltage comparator for voltage monitoring.

If the device is a power supply unit, it is recommended that an excitation winding of the relay is connected to the output voltage, whereby the relay itself contains the voltage monitoring.

In another variant, the controlled switch is formed by a switching transistor. As a result, more influence can be exerted on the switching process. The switching transistor can be controlled, for example, by a voltage comparator for voltage monitoring.

To limit the maximum input current that occurs, it can be provided that the output of the voltage comparator is fed to the switching transistor via a delay element.

If there is a low-impedance damping resistor in series with the controlled switch, slightly higher losses are accepted, but disturbances during the switching process are kept particularly low.

Galvanic isolation is recommended, especially for power supply units, in that an optocoupler is connected between the voltage monitor and the controlled switch.

The invention together with further advantages is explained in more detail below with reference to exemplary embodiments which are illustrated in the drawing. 1 shows a protective circuit according to the invention in a basic representation,

AT 408 494 B

2 shows a protective circuit according to the invention using a relay,

3 shows a protection circuit according to the invention using a switching transistor,

Fig. 4 shows a protection circuit according to the invention for a power supply using a relay, and

5 shows a protection circuit according to the invention for a power supply unit using an optocoupler and a switching transistor.

According to Fig. 1, an electronic device G from a DC voltage source U._v are supplied. Between the device G and the DC voltage source U_v there is a protective diode D, here in the positive line_v in series with a current limiting resistor R_v. This series circuit D / R_v is bridged by the normally open contact of a controlled switch S, a voltage monitor W being provided for controlling the switch. This voltage monitor W monitors an operating voltage U_B. of the device, for example its input voltage, and is set up, when an adjustable minimum value U_M.this operating voltage U_B. to issue a Schaltsignai s to the controlled switch S.

If when connecting the device G to the DC voltage source U_v a wrong polarity is chosen, the diode D_v no current will flow, and therefore the operating voltage U_B. remain at zero or never reach their predetermined minimum value, and the switch S remains open. If the polarity is correct when the device G is connected, a current flows through the diode D._v, which is caused by the limiting resistor R_v is limited to such a value that neither for the device G nor for the DC voltage source U_v an impermissibly high current surge occurs. Such current surges are to be expected especially when operating voltages are filtered through capacitors of high capacitance. Since the limiting resistor R_v If enough current flows into the electronic device G, the monitored operating voltage U_B. rise, and - as soon as they reach their predetermined minimum value U_M. has reached - the monitoring W will emit a switching signal s to the switch S, which now closes, so that the voltage drops that have been present at the diode D up to that point_v or at the limiting resistor R_v disappear, and the corresponding power loss no longer comes into play.

It should be mentioned at this point that the limiting resistor R_v can be omitted if the forward resistance of the diode D_v has a value that is high enough for the current limit desired in each case

In a practical implementation according to FIG. 2, the monitoring circuit W is between a direct voltage U_v and the input operating voltage U_B. of a device G switched. The series circuit of a diode D is again located in the positive line_v and a limiting resistor R_v, this series circuit being bridged by the normally open contact a of a relay A, and - in this exemplary embodiment - a damping resistor R in series with the normally open contact a_d lies. A comparator K is provided for voltage monitoring. The positive input of this comparator K is the operating voltage U, taken from a voltage divider R1 / R2 via a resistor Re_B. of the device G is supplied with a proportional voltage, whereas the voltage of a Zener diode D is applied to the negative input of the comparator K_z as reference voltage. The current through the Zener diode D_z is by means of a resistor R_z generated. The output of the comparator K, which here has a resistor R in its feedback branch_r has, is via a limiting resistor R_a the base of a transistor T_a supplied, in the collector circuit of which the excitation winding A of the relay A, a is located, whereas the emitter of the transistor T is located_a is on the negative lead.

When the device G is connected to the supply voltage, correct polarity is assumed, a resistor R_v Limited current flow into the device G and its operating voltage U_B. increase as well as the voltage at the positive input of the comparator K, and when a value is reached which the voltage of the Zener diode D_z exceeds, the comparator K will switch through the transistor T._a and energize the coil of relay A, a. The normally open contact a closes, and the operating current for the device G now flows through the damping resistor R._d. This damping resistor is intended to avoid extreme voltage peaks and thus high-frequency interference, but can also be omitted in many cases.

Assuming a continuous current for device G of 2A and a contact resistance of the relay of 10m Ohm, the result is a continuous power loss of 40mW. If you agree against it

AT 408 494 B the same current of 2A the loss at the diode D._v with 0.7 V x 2A, i.e. 1.4 W and uses a limiting resistor, for example a thermistor with an operating resistance of 1 ohm, this results in an ohmic loss of 4 W and a total continuous power loss of 5.4 W. This power loss is two orders of magnitude greater than when the protective circuit according to the invention is used.

A variant of the invention using a switching transistor T._s is shown in FIG. In principle, this circuit is similar to that of FIG. 2, but the relay contact is through the EmitterKol lektor path of the switching transistor T._s replaced, the switching transistor from the output of the comparator K via a resistor R_t and a series resistor R_b at its base T_sis controlled, with a resistor R in the collector_c lies. As soon as the operating voltage Ub after connecting the device a certain, through the Zener diode D₂ or the voltage divider R1 / R2 has reached a predefinable value, the switching transistor T_s switched through, and it bridges the diode D._v and the series resistor R_v. Here, too, the resistor R is used_vthe avoidance of extreme current peaks. FIG. 3 also shows how, for example, “slow” switching of the transistor can be achieved in order to additionally limit the maximum input current. The resistor R shown here is used for this purpose_t together with a capacitor C_t, the switching delay or the increase in the current through the transistor T_s, by the time constant of the RC element R, / C_t is determined. This RC element can of course also be omitted, in which case the capacitor Ct and the resistor R._tis replaced by a short circuit.

The embodiment according to FIG. 4 shows an application example in which the device G is a power supply unit, for example a switched-mode power supply unit, which is supplied by a DC voltage source U._v, for example an intermediate circuit voltage, is supplied and a DC output voltage U_A. owns. The series circuit of the diode D is in turn in the positive input line of the device G in a known manner_vand the protective resistance R_v arranged, and this series circuit is bridged by a normally open contact a of a relay A, a. The excitation winding A of the relay is connected to the DC voltage output of the power supply unit G with a target output voltage U._A.. The relay or the relay winding is dimensioned so that when a certain value is reached, the output voltage U_A. the relay picks up, and the contact a closes, and the series circuit Rv / D_v bridged. In addition to the low contact resistance of the normally open contact a, the use of a relay A, a also has the advantage of galvanic isolation, which can be required, for example, in a power supply unit.

Another variant of the invention according to FIG. 5 also shows galvanic isolation, but - similar to FIG. 3 - a switching transistor T_s is used. In the protective circuit according to FIG. 5, the output voltage of a power supply unit G is monitored and fed to the positive input of a comparator K via a voltage divider R1 / R2. The Zener voltage of a diode D is applied to the negative input of the comparator K_z with a series resistor R_z, and the comparator K, which has a feedback resistor R_r owns, controls via a resistor R_O the diode of an optocoupler O on. The collector C and emitter E of the transistor belonging to the optocoupler are connected to correspondingly labeled points C and E on the input side of the power supply unit G, and a base current flows through a resistor R when the optocoupler is switched through_b, the transistor of the optocoupler, and into the base of the switching transistor T_swho is now switching through.