Simple laboratory power supply

Encyclopedia of radio electronics and electrical engineering / Power Supplies

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Once the author of this article needed a sufficiently powerful and reliable power source with a widely adjustable output voltage. Having studied the available literature, he came to the conclusion that the devices proposed for repetition have disadvantages: linear stabilizers have large dimensions (due to the need to use high-capacity oxide capacitors and heat sinks), PWM stabilizers have a rather narrow control range and high-frequency ripples are present in the output voltage , and devices with improved consumer qualities (current limitation, mode indication, switching of transformer windings, etc.) are relatively complex. I had to look for other solutions, and as a result, a power source was developed that was free from these shortcomings.

The proposed laboratory power supply uses a two-stage rectified voltage conversion: PWM conversion to intermediate voltage and subsequent linear stabilization. The main technical characteristics of the device are as follows: output voltage regulation limits - from 1,3 to 30 V, voltage instability coefficient - 0,07% / V, load current instability 0,1%, maximum input (alternating) voltage - 27 V, Conversion efficiency at maximum load current - not less than 70%. It is possible to change the current limiting threshold up to 1,2 A, there is a non-trigger short circuit protection with light indication. The source is characterized by small dimensions, minimal heat losses (with a load current of up to 0,3 A, heat sinks are not required).

The block diagram of the device is shown in fig. one.

The input voltage UBX is transformed by the PWM converter DA1 into an intermediate Unp, which, in turn, is the input for the analog stabilizer DA2. Feedback through the differential amplifier DA3 maintains the voltage drop required for DA2 (for LM317 - 2,5 V), so that heat losses on DA2 are minimal.

The schematic diagram of the power supply is shown in fig. one.

(click to enlarge)

The rectified voltage from the output of the bridge VD1 is smoothed by the capacitor C1 and fed to the input of the PWM converter assembled on the input element DA1, VT2, VD2, L1. The switching circuit DA1 is a typical step-down [1]. The use of the KR1156EU5 microcircuit minimized the number of passive elements, but imposed a restriction on the maximum input voltage, which in such an inclusion should not exceed 40 V. PWM with the help of an L1 storage choke and a VD2 diode forms an intermediate voltage Upr on the capacitor C4.

A linear voltage regulator is assembled on the DA2 microcircuit stabilizer. Regulate it with a variable resistor R12. Diodes VD3 and VD4 protect the microcircuit from reverse currents and negative voltages and are introduced in accordance with the recommendations for its use [2].

Op-amp DA3 and resistors R7-R10 form a differential amplifier that monitors the voltage drop across the stabilizer DA2. The gain factor DA3 is chosen equal to 1,5, which allows you to maintain the set value in the entire range of voltages and currents, including when the output is short-circuited. The trimmer resistor R2 regulates the voltage drop during adjustment.

On the elements VT1, HL1, R1, a signaling device for a short-circuited state of the output is made. In normal mode, the transistor VT1 is open, and the voltage drop across it does not exceed a few tenths of a volt. When the voltage at the source output drops to 0,7 V or less, the transistor VT1 closes and the HL1 LED starts to glow. The on state of the power supply is signaled by the HL2 LED.

The role of the resistor R5 is very interesting. When the voltage on it is more than 120 mV (average value determined empirically), the internal pulse width limiter of the DA1 chip comes into effect, turning it into a current source. This property of KR1156EU5 can be used to limit the maximum load current. So, for example, with a resistance of this resistor equal to 0,1 Ohm, the source is capable of delivering current up to 1,2 A to the load, and with R5 \u1d 120 Ohm - only up to 0,5 mA. By installing a resistor with a resistance of 240 Ohm and thereby limiting the load current to a value of 2 mA, you can refuse the heat sink for the DA2 chip and the external current switch of the PWM converter (by excluding the transistor VT3, resistor R2 and connecting pin 1 DA1 to the connection point of the inductor L2 and the diode VDXNUMX). In this case, the dimensions of the product will be slightly larger than a matchbox.

As a VT2 key, you can use any transistor with a static base current transfer coefficient of more than 30 and an allowable collector current of at least 3 A. The author used KT805AM. It has good frequency properties, so switching losses are low. The field-effect transistor IRF3205 "behaves" very well in this place - it does not need a heat sink at a current of up to 1 A. The inductance of the inductor L1 can be any from 40 to 600 μH, the only requirement is that it must be rated for a current of at least 1,5 A. Resistors - MLT, C1-4 with a permissible deviation of resistance from nominal ± 10%, trimmer resistor R2 - multi-turn wire SP5-2VB or similar, variable R12 - of any type with a resistance of 4,7 ... 6,8 kOhm. Capacitors C1 and C4 are oxide K50-35 with a capacity of 220 ... 470 microfarads with a rated voltage of 63 V, the rest are ceramic (KD2, K10-7, K10-17it. p.).

Establishing a power supply comes down to setting a trimmer resistor R2 voltage of 2,5 V between pins 2 and 3 of DA2 (at 50 percent load).

Literature

1. Biryukov S. Voltage converters on the KR1156EU5 microcircuit. - Radio, 2001, No. 11, p. 38,39.42.
2. Integrated circuits: circuits for linear power supplies and their applications. - M.: Dodeka, 1996.

Author: S. Muralev, Dimitrovgrad, Ulyanovsk Region

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