High Current Power Supply Circuit Diagram 28V10A

Introduction

The demand for high current power supply circuits has significantly increased in various industries and applications. To meet this requirement, a circuit utilizing the LM723 type adjustable voltage regulator IC, LM317L, 5 x 2N3055 transistors, and a 250V MOV (Metal Oxide Varistor) has been developed. This circuit diagram provides a reliable and efficient solution for delivering a stable 28V power supply with high current capabilities. In this article, we will discuss the individual components used in this circuit and their functions, highlighting their key features and benefits.

Many modern high power transistors used in RF power amplifiers require 28V d.c. collector supplies, rather than the traditional 12-V supply. By going to 28 V (or even 50 V), designers significantly reduce the current required for an amplifier in the 100-W or higher output class. The power supply shown in the following circuit diagrams, is conservatively rated for 28 V at 10 A (enough for a 150-W output amplifier)-continuous duty! It is designed with simplicity.

High Current Power Supply Circuit Diagrams

This project can be built using a few basic components. The circuit diagram of this project is shown below.

**Fig(a)** Unregulated Power Supply for 28V-10A Circuit Diagram

**Fig(b)** 28V High Current Power Supply Circuit Diagram

**Fig(c)** Crowbar for Protection of Supply Circuit Diagram

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Components List of High Current Power Supply Circuit

Following is the list of all components used in this project:

C1 ………… Electrolytic capacitor, 22,000 μF, 50 V
C2, C3 ……. AC rated bypass capacitors
C4 …………. Electrolytic capacitor, 100 μF, 50 V
IC1 ………… Three-terminal adjustable voltage regulator, 100 mA (LM-317L or equivalent)
IC2 ………… 723-type adjustable voltage regulator IC, 14-pin DIP package
Neon …….. Neon indicator lamp, 230 V a.c.
T1 – /T5 ….. NPN power transistor, 2N3055 or equivalent
R2 – R5 …… Power resistor 0.1 2, 5 W (or greater), 5%
R7 ………… Power resistor, 0.067 2, 10 W (or greater), made from three 0.2-2, 5-W resistors in Parallel
Rectifier .. Bridge rectifier, 50 PIV, 25 A
Transformer.. Power transformer, primary 230 -V a.c.; secondary, 32 V, 15 A
ZD1 ………. 250-V MOV (Metal Oxide Varistor)

Circuit Detail of High Current Power Supply Circuit

The schematic diagram shown in Fig(a) is the section which draws its power from mains supply, steps down the voltage, rectifies it and filters it to provide an unregulated d.c. supply to the regulator circuit.

The secondary of the transformer is rated for 32 V at 15 A, continuous duty. The primary is bypassed by two 0.01 uF capacitors and protected from line transient by Z1, which is an MOV (Metal Oxide Varistor).
A 25-A bridge rectifier module is used to rectify the output of the transformer secondary. It requires a heat sink in this application. Filter capacitor C1 is a computer grade 32000 μF electrolytic. Bleeder resistor R1 is included for safety because of the high value of C1; bleeder current is about 12 mA.

There is a trade off between the transformer secondary voltage and the filter capacitor value. To maintain regulation, the minimum supply voltage to the regulator circuitry must remain above approximately 31 V. Ripple voltage must be taken into account. If the voltage on the bus drops below 31 V in ripple valleys, regulation may be lost.

The regulator circuitry shown in Fig(b) takes the advantage of commonly available parts. The heart of the circuit is IC2, a 723 voltage regulator IC. The resistors R8, R9, and R10 are chosen to allow the output voltage to be adjusted between 20 V and 30 V. The 723 has a maximum input voltage of 40 V, which is slightly lower than the filtered bus voltage. IC1 is an adjustable 3-terminal regulator that provides approximately 35 V to power IC2. IC2 drives the base of T1, which then drives the series pass transistors T2 to T5 in parallel. This configuration allows for the use of commonly available components. Although it may seem excessive, the use of four transistors is necessary due to the need to dissipate approximately 120 W of heat generated by the high input voltage. If a transformer with a significantly different secondary potential is used, consideration must be given to how the generated heat will be dissipated.

Resistor R9 is used to adjust the output voltage of the power supply. It is internally mounted and is a “set and forget” control since the supply is primarily designed for 28-V applications. A 25-turn potentiometer allows for precise adjustment of the voltage. If desired, R9 can be mounted on the panel for easy variation of the output voltage.

The 723 IC features current foldback in case the load draws excessive current. This is set by resistor R7 to be approximately 14 A. If a problem occurs, fuse F2 should blow. However, the output terminals can be shorted indefinitely without causing damage to any of the components. In the event of a failure in the regulator circuitry or a shorted pass transistor, the unregulated supply voltage will appear at the output terminals. To protect against this, the optional over-voltage protection circuit shown in Fig(c) can be added. This circuit connects across the output terminals and can be easily added or removed without affecting the rest of the supply. If the crowbar circuit is not used, fuse F2 should be connected between points A and B of Fig(b).

The Crowbar Circuit:

The crowbar circuit operates by blocking the positive regulated voltage from reaching the base of T6 until its avalanche voltage is exceeded. This voltage level, set at 33 V in this circuit, allows for small overshoots that may occur when the output load is suddenly removed. If the output voltage exceeds 33 V, diode D3 conducts and biases on transistor T6, which then turns on and shorts the raw DC source by turning on SCR T7. This action blows fuse F3. Resistor R24 is used to shunt any internal leakage current in the SCR.

Construction Hints:

In terms of construction, it is recommended to mount the transistors to the heat sink using insulating washers and thermal heat-sink compound to aid in heat transfer. The surface under the transistors on the heat sink must be smooth. All holes should be deburred and the edges lightly sanded with fine emery cloth. Resistor R7 can be made using two 0.1 ohm, 5-W resistors in parallel.

Air Cooling:

For cooling, a five-inch fan is helpful in circulating air inside the cabinet. Forced air cooling is necessary when the heat sinks are mounted inside the cabinet. A single point ground is recommended to avoid ground loop problems, and the negative lead of the supply should be connected directly to the minus terminal of C1. High-current runs, including the wiring between various components, should use #14 or #12 wire. Similar wire should also be used between the output terminals and the load.

Testing:

To test the power supply, first connect the transformer, bridge rectifier, and C1 and verify that the no-load voltage is around 44 V DC. Then, connect the unregulated voltage to the regulator circuitry and leave the base lead of T6 disconnected to disable the crowbar circuit. The output voltage should be adjustable between approximately 20 V and 30 V, and it should be set to 28 V. Finally, short the output terminals to verify that the current foldback is working. When the shorting wire is disconnected, the voltage should return to 28 V. Testing is complete after these steps.

Major Components of High Current Power Supply

723-type Adjustable Voltage Regulator IC:

The 723-type adjustable voltage regulator IC is a versatile and widely used integrated circuit that offers precise voltage regulation and adjustable output. It can provide both positive and negative voltage outputs, making it suitable for a wide range of applications. The IC has built-in overcurrent and thermal protection, ensuring safe and reliable operation. Its adjustable voltage range and high accuracy make it an ideal choice for high current power supply designs.

LM-317L Adjustable Voltage Regulator 100 mA:

The LM-317L is a low dropout voltage regulator capable of providing a regulated output voltage with a maximum current rating of 100 mA. It is designed to operate with input voltages greater than its regulated output voltage, making it efficient for applications where a high input-to-output voltage differential is required. The LM-317L is easy to use and offers excellent line and load regulation, making it suitable for use in the 28V high current power supply circuit.

2N3055 Transistor:

The 2N3055 transistor is a high-power NPN bipolar junction transistor commonly used in power amplifier circuits. In the 28V high current power supply circuit, five 2N3055 transistors are utilized in parallel to handle the high current requirements. This configuration allows for increased current capability and improved heat dissipation. The 2N3055 transistor’s robust construction and ability to handle high currents make it an ideal choice for high power applications.

250V MOV (Metal Oxide Varistor):

The 250V Metal Oxide Varistor (MOV) is a voltage-dependent resistor that acts as a transient voltage suppressor. It provides protection against voltage spikes and transients that could damage electronic components in the circuit. MOVs have a high surge current capability, fast response time, and low clamping voltage characteristics, making them suitable for safeguarding the 28V high current power supply circuit from voltage surges and spikes.

Conclusion of High Current Power Supply

The 28V high current power supply circuit utilizing the LM723 adjustable voltage regulator IC and LM317L, offers a reliable and efficient solution for applications requiring a stable and high current power source. The 723-type adjustable voltage regulator IC provides precise voltage regulation, while the LM-317L offers low dropout voltage and excellent line and load regulation. The 2N3055 transistors handle the high current requirements, ensuring reliable power delivery. Finally, the 250-V MOV protects the circuit from voltage spikes and transients. This circuit diagram is a robust and efficient solution for various high current applications.

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