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LM317 and L200 Power Supplies with Soft Start

2014-11-01 04:38  
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Figure 1 

The two power supply circuits are presented to illustrate the soft start procedure using LM317 and L200 integrated circuits where each reaches the desired output of 15 V gradually for a period of 5 second.

Soft Start – an electronic feature that controls the increase of output voltage by reducing the excess of current flow during initial setupLM317 – a 3-terminal positive and linear voltage regulator with adjustable outputL200 – a monolithic integrated circuit built for the purpose of regulating programmable current and voltage2N2907 – a small general-purposeBJTtransistor used for switching and linear amplification applications

The LM317 voltage regulator is capable of supplying more than 1.5 A over a range of voltage from 1.25 V to 37 V. Some of its features include the automatic reduction of its output current for the duration of under load when an overheat takes place. It also has an internal short circuit current limiter to prevent over current rating, floating operation for high voltage applications, eliminates stocking many fixed voltages, safe operating area protection and internal thermal overload protection. These features makes LM317 virtually blow-out proof. The soft start operation of LM317 is made possible by connecting to a universalPNPtransistor, which is 2N2907 in this circuit. It is designed to operate at higher speeds with low power, medium voltage and low to medium current.

Once the circuit has been switched ON, the electrolytic capacitor gradually charges on the initial conduction and the increasing voltage on the positive side causes the transistor to turnOFF. This type of capacitor is significant in low frequency and high current electrical circuit. An electrolytic capacitor contains filters that allow very low cutoff frequencies due to its very high capacitance. With reference to the ground, the event turningOFFthe transistor raises the voltage on the adjustment pin of the LM317 voltage regulator. The LM317 is better than any 3-pin regulator because it only requires less current via the adjustment pin. However, it can also obtain large amount of current through the ground pin if the value is near the dropout voltage. To maximize power handling capability and life span, the use of heatsink is recommended during dissipation of large amount of heat at medium to high current loads.

For the L200 circuit, in order to perform the soft start function, it utilizes its Pin 2 as its internal comparator. Just like LM317, it has internal protection to lessen the possibility of damage to the device which consists of power limiting, thermal shutdown, current limiting and input over-voltage protection. It also has low bias on regulation pin, low standby current drain, and adjustable output current and output voltage. An internal safe operating area protection circuitry of the L200 controls the power dissipation. Nevertheless, it would be compulsory to use an appropriate heatsink at higher power levels to prevent reaching excessive temperatures. When high voltage precision is necessary, the L299 can be used as a replacement with other fixed voltage regulators to eliminate the need to stock a range of fixed voltage regulators. The electrolytic capacitor in the L200 circuit reduces the current-regulation loop within the L200 as the capacitor voltage increases. Since Pin 2 acts as a comparator, it switches to whichever value is higher between the comparison of two voltages or two currents.

The versatility of LM317 makes it possible to be suited in many applications like in local on-card regulation and programmable output regulation. It is sometimes preferred for high precision fixed voltage applications due to being adjustable. Different package forms for different applications are available for LM317 which includes surface mount and heatsink mounting applications. Packages for high current applications with common form factors include TO-220 and TO-3. The L200 may be utilized in DC-DC converters, battery chargers and power supplies. Soft start function may not only be used in electronics but also in conveyors, pumps, compressors, mixers, shredders, saws, and injection molding machines.


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