Specifications concerning current limiting in telecommunications equipment can be found in the European telecommunications standard ETS 300 132-2.

From the prior art it is known an advantageous inrush current limiting protection circuit, which is shown in Fig. 1. The protection circuit includes a current supply part 1, defined by a dotted line in the drawing, comprising a resistor R2 and a volt- age limiting reverse biased voltage-regulator diode Dl. The protection circuit further includes a current sensing part 2 comprising a current sensing resistor R1 and a bipolar transistor Ql. The voltage across the current sensing resistor RI, which preferably has a resistance as low as possible, say 0.1 Q, is used to limit the inrush current by utilizing the threshold voltage of the bipolar transistor Q1 in such a manner that as the voltage of resistor R1 starts to rise, the threshold voltage of transistor Ql is exceeded and transistor Q1 starts to conduct. The conductive state of transistor Q1 is further used to control a current regulating MOSFET (Metal Oxide Semiconductor Field Effect Transistor) M1. The control circuit of transistor MI also comprises a current shut-off part 3 in which the current is shut off after a short circuit has occurred. It relies on shut-off control by transistor Q2 based on the voltage across the current regulating transistor MI at the coupling point of resistors R3 and R4. The resistance of resistor R3 is about five times that of resistor R4. The purpose of capacitor Cl is to produce a delay of about 1.5 ms for the current shut- off.

From the prior art it is also known a starting current limiting circuit according to Patent FI 94195 and international patent application WO 95/08862"Method and switching arrangement for starting a power supply", Matti Havukainen, Nokia Tele- communications Oy, realized by a differential amplifier and MOSFET field effect transistor. The output of the differential amplifier is realized using two transistors in push-pull arrangement, whereby the first transistor"pushes"the output to the upper voltage and the second transistor"pulls"the output to the lower voltage. The

biasing of this type of output, however, requires a large current, which results in a considerable power loss at high supply voltages. Furthermore, other control circuits cannot be connected in parallel with this type of output, because the output is forced alternatively into one of the two states. Possible other control circuits have to be awkwardly connected to the differential amplifier's input. However, the limiting circuit can be realized using high-quality operational amplifiers, whereby the cur- rent consumption is small, but the price of the apparatus becomes high.

A problem with prior-art advantageous protection circuits is their high level of starting current or, alternatively, great power loss. Another problem is the inaccu- racy of starting current regulation. A further problem is the difficulty in connecting possible other control circuits in parallel with the control means of the protection circuit.

An object of the invention is to eliminate the disadvantages mentioned above by providing an inexpensive protection circuit which is accurate and makes possible both a small starting current and a small power loss.

The invention pertains to an inrush current limiting protection circuit comprising a means for current sensing, control means for current switching upon starting the protection circuit, and switching means for current switching. In accordance with the invention, the control means for current switching comprises a class B amplifier and a pull-up resistor.

Advantageous embodiments of the invention are presented in the dependent claims.

The invention has the advantage that the protection circuit can be realized simply and with low costs using a small number of components, and that it has a good effi- ciency. Furthermore, the regulating circuit according to the invention is accurate and meets the requirements of the standard.

The invention is below described in detail with reference to the attached drawing in which Fig. 1 is a circuit diagram of an advantageous inrush current limiting protection circuit according to the prior art, and Fig. 2 is a circuit diagram of an inrush current limiting protection circuit ac- cording to the invention.

Fig. 1 was discussed above in connection with the description of the prior art.

Fig. 2 shows an inrush current limiting protection circuit according to the invention.

The protection circuit comprises a current sensing means 4, control means 5, and power supply switching transistor M2. In accordance with the invention, the protec- fion circuit control means 5 includes at least a class B amplifier A1, but preferably also an RC damping circuit R10, C4 and a pull-up resistor Rll. In addition the Figure shows a conductor connecting the positive voltage input VIN+ and output Vous+, which conductor is advantageously grounded to zero potential.

The current sensing means 4 of the protection circuit comprises a current sensing resistor R5, a bias resistor R6 and differentiating resistor R7 connected in series between the supply voltage input Vcc and the negative input VIN-, a connection from coupling point A between the bias resistor R6 and differentiating resistor R7 to the control means 5, series-connected impedances Z1, Z2 connected in parallel with the current sensing resistor R5, and a connection from coupling point B between impedances Zl, Z2 to the control means 5. Coupling point A between the bias resistor R6 and differentiating resistor R7 in the current sensing means 4 is con- nected to the positive (+) input of amplifier A1 in control means 5, coupling point B between impedances Zl, Z2 in the current sensing means 4 is connected to the negative (-) input of amplifier A1 in the control means 5, and the open collector output of amplifier A1 is connected to an RC filter R10, C4 in parallel with the sig- nal, to a pull-up resistor Rl 1 and to the control input of the switching transistor M2.

The voltage across the current sensing resistor R5, which preferably has a resistance value as low as possible, say 0.1 Q, is used to limit the inrush of current I by utiliz- ing the connection of impedances Z1, Z2 to control the amplifier A1 in such a man- ner that as the voltage of resistor R5 rises, the voltage between the inputs of ampli- fier Al rises and transistor Q3 in amplifier Al starts to conduct, whereby the output of amplifier Al is pulled towards the potential of the negative input VIN-of the pro- tection circuit and the current-regulating MOSFET transistor M2, controlled by said output, limits the current I. The output transistor Q3 of amplifier A1 is of a type that realizes an open collector output. At the final stages of the starting moment the cur- rent I becomes smaller and as the voltage of the current sensing resistor R5 drops, the switching transistor M2 becomes fully conductive so that the upper limit of the current I will not be exceeded. The current I is substantially equally large in the positive and in the negative conductor.

An RC damping circuit R8, C2 connected to the output of amplifier A1 filters the gate control voltage of transistor M2. Impedances Z1, Z2 preferably consist of a parallel connection of resistances R8, R9 and capacitances C2, C3. The impedances both convey the current sensing data and cause delay in the control signal, regulat- ing the opening of the switching transistor M2 at the rising moment of the current I and, thus, the rising speed of the current I at the beginning of the starting moment.

Also other elements controlling the switching transistor may be connected between points C and D, such as e. g. a control element realizing a restart attempt in a short- circuit situation. This control element may be implemented with a comparison cir- cuit including two identical voltage comparators.

As an example, let us consider the efficiency of the circuit with 10W and 100W 48V power supplies with input powers of 15W and 150W, respectively. In the equations below, Unmn is the nominal voltage of the power supply, Ipr represents the current consumption of the prior-art control circuit according to Fig. 1 and INeW rep- resents that of a control circuit according to the invention, and Inmn is the nominal current obtained by dividing the input power by the nominal voltage. Further, the resistance values Rmpr and RDSonpr are the resistances of the current sensing resistor and the switching transistor in the prior-art control circuit, and RmN and RDSonN are the corresponding values in the control circuit according to the invention when the circuit is closed.

PPr = Unmn # IPr + Inmn2 # (RmPr + RDsonPr) <BR> <BR> <BR> <BR> 0.36mA+=48V# 1.7## <BR> <BR> <BR> <BR> <BR> <BR> =17.3+166 =183mW Unmn#INew+Inmn2#(RmN+RDSonN)PNew+ =48V +(0.3125A)2#0.2#1.0mA <BR> <BR> =48.0+19.5<BR> <BR> <BR> <BR> <BR> <BR> <BR> = 67. 5mW where the nominal current Inmn is 0.3125A corresponding to a power of 15W.

PPr = Unmn # IPr + Inmn2 # (RmPr + RDSonPr) 0.36mA=48V# + 0.5## <BR> <BR> =17.3+4883<BR> <BR> <BR> <BR> <BR> <BR> <BR> =4.9W<BR> <BR> <BR> <BR> <BR> <BR> <BR> <BR> <BR> <BR> Unmn#INew+Inmn2#(RmN+RDSonN)PNew= =48V +(3.125A)2#0.1#1mA <BR> <BR> =48+977<BR> <BR> <BR> <BR> <BR> <BR> <BR> <BR> =1. OW where the nominal current Inmn is 3.125A corresponding to a power of 150W.

From these examples one can deduce that because of the smaller resistance of the current sensing resistor Rm according to the invention the power loss in the control circuit drops by about two thirds from the prior-art solution. This means 2 to 4% in the total power loss.

The invention is not limited to the exemplary embodiments described above, but modifications thereof are possible without departing from the scope of the inven- tional idea defined by the claims attached hereto.