Limitation of the charging current of the capacitor of the mains rectifier of the SMPS. Scheme, description

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Limiting the charging current of the capacitor of the mains rectifier SMPS

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Encyclopedia of radio electronics and electrical engineering / Chargers, batteries, galvanic cells

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One of the important problems in network switching power supplies is the limitation of the charging current of a large-capacity smoothing capacitor installed at the output of the mains rectifier. Its maximum value, determined by the resistance of the charging circuit, is fixed for each specific device, but in all cases it is very significant, which can lead not only to blown fuses, but also to failure of input circuit elements. The author of the article offers a simple way to solve this problem.

A lot of works are devoted to the solution of the problem of limiting the starting current, in which devices of the so-called "soft" switching are described [1 - 3]. One of the widely used methods is the use of a charging circuit with a non-linear characteristic. Typically, the capacitor is charged through a current-limiting resistor to the operating voltage, and then this resistor is closed with an electronic key. The simplest is such a device when using a trinistor [4].

The figure shows a typical diagram of the input node of a switching power supply. The purpose of elements that are not directly related to the proposed device (input filter, mains rectifier) is not described in the article, since this part is performed in a standard way [5].

Limiting the charging current of the capacitor of the mains rectifier SMPS

The smoothing capacitor C7 is charged from the mains rectifier VD1 through the current-limiting resistor R2, in parallel with which the trinistor VS1 is connected. The resistor must meet two requirements: firstly, its resistance must be sufficient so that the current through the fuse during charging does not lead to its blowout, and secondly, the power dissipation of the resistor must be such that it does not fail before fully charged capacitor C7.

The first condition is satisfied by a resistor with a resistance of 150 ohms. The maximum charging current in this case is approximately equal to 2 A. It has been experimentally established that two resistors with a resistance of 300 ohms and a power of 2 W each, connected in parallel, meet the second requirement.

The capacitance of the capacitor C7 660 μF is selected from the condition that the amplitude of the ripple of the rectified voltage at a maximum load power of 200 W should not exceed 10 V. The values of the elements C6 and R3 are calculated as follows. Capacitor C7 will be charged almost completely through resistor R2 (95% of the maximum voltage) during the time t=3R2 C7=3 150 660 10-6 -0,3 s. At this point, the trinistor VS1 should open.

The trinistor will turn on when the voltage at its control electrode reaches 1 V, which means that the capacitor C6 must be charged to this value in 0,3 s. Strictly speaking, the voltage on the capacitor grows non-linearly, but since the value of 1 V is about 0,3% of the maximum possible (about 310 V), this initial section can be considered almost linear, so the capacitance of the capacitor C6 is calculated using a simple formula: C \uXNUMXd Q /U, where Q=l t is the charge of the capacitor; I - charging current.

Let's determine the charging current. It should be slightly larger than the current of the control electrode, at which the trinistor VS1 turns on. We choose the trinistor KU202R1, similar to the well-known KU202N, but with a lower turn-on current. This parameter in a batch of 20 trinistors was in the range from 1,5 to 11 mA, and for the vast majority of its value did not exceed 5 mA. For further experiments, a device with a turn-on current of 3 mA was chosen. We select the resistance of the resistor R3 equal to 45 kOhm. Then the charging current of the capacitor C6 is 310 V / 45 kOhm = 6,9 mA, which is 2,3 times greater than the turn-on current of the trinistor.

We calculate the capacitance of the capacitor C6: C \u6,9d 10 3-0,3 1 / 2000-1000 μF. The power supply uses a smaller 10 microfarad capacitor for a voltage of 0,15 V. Its charging time has been halved, to about 7 s. I had to reduce the time constant of the charging circuit of the capacitor C2 - the resistance of the resistor R65 was reduced to 310 ohms. In this case, the maximum charging current at the moment of switching on is 65 V / 4,8 Ohm = 0,15 A, but after a time of 0,2 s, the current will decrease to approximately XNUMX A.

It is known that the fuse has a significant inertia and can pass short pulses, much higher than its rated current, without damage. In our case, the average value for a time of 0,15 s is 2,2 A and the fuse transfers it "painlessly". Two resistors with a resistance of 130 ohms and a power of 2 W each, connected in parallel, also cope with such a load. During the charging of the capacitor C6 to a voltage of 1 V (0,15 s), the capacitor C7 will be charged by 97% of the maximum.

Thus, all conditions for safe operation are met. Long-term operation of the switching power supply has shown the high reliability of the operation of the described node. It should be noted that a smooth voltage increase on the smoothing capacitor C0,15 for 7 s favorably affects the operation of both the voltage converter and the load.

Resistor R1 is used to quickly discharge the capacitor C6 when the power supply is disconnected from the mains. Without it, this capacitor would be discharged much longer. If in this case you quickly turn on the power supply after turning it off, then the trinistor VS1 may still be open and the fuse will burn out.

Resistor R3 consists of three, connected in series, with a resistance of 15 kOhm and a power of 1 W each. They dissipate about 2 watts of power. Resistor R2 - two MLT-2 connected in parallel with a resistance of 130 ohms, and capacitor C7 - two, with a capacity of 330 microfarads for a nominal voltage of 350 V, connected in parallel. Switch SA1 - toggle switch T2 or push-button switch PkN41-1. The latter is preferable because it allows you to disconnect both conductors from the network. The KU202R1 trinistor is equipped with an aluminum heat sink 15x15x1 mm in size.

Literature

Sources of secondary power supply. Reference manual. - M.: Radio and communication, 1983.
. Eranosyan S. A. Network power supplies with high-frequency converters. - L .: Energoatomizdat, 1991.
3. Frolov A. Limiting the charging current of the capacitor in the mains rectifier. - Radio, 2001, No. 12, p. 38, 39, 42.
4. Mkrtchyan Zh. A. Power supply of electronic computers. - M.: Energy, 1980.
5. Integrated circuits of foreign household video equipment. Reference manual. - S.-Pb: Lan Victoria, 1996.

Author: M.Dorofeev, Moscow

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