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Three terminal voltage regulator integrated diffuser flow analysis method

Author: released in: 2015/3/13 12:41:05 hits:

Three terminal voltage regulator integrated diffuser flow analysis method

In electronic circuits, DC regulated power supply is an essential part of them. Linear regulated power supply because of its good stable performance, small output ripple voltage, simple circuit, low cost and other advantages of universal application. Voltage stabilizing part linear regulated power supply in most realized by three terminal integrated voltage regulator. Convert the DC output voltage stability by DC voltage will not stable.

Three terminal integrated voltage regulator can generally be divided into fixed output and adjustable output two types. Three terminal integrated voltage regulator chip only leads to the three terminals are respectively connected with the input, output, end and the common end (or adjust the end), basically no external components, and the internal limiting protection, overheat protection and overvoltage protection circuit, the use is very safe, convenient.

The maximum output current three terminal integrated voltage regulator, related to product model, specific 0.1A, 0.3A, 0.5A, 1.0A, 1.5A, 3A, 5A and 10A etc. several. In actual use, if the maximum output current and voltage stabilizing circuit required more than three terminal integrated voltage regulator, needs to design the current amplification circuit to expand output current in the original basis. In this paper, based on three terminal adjustable voltage regulator of LM317K expansion flow as an example to illustrate the method of three terminal voltage regulator integrated diffuser flow, and through analyzing the specific circuit.

1 the working principle of LM317K

LM317K is a three terminal adjustable voltage regulator, the output voltage range, the maximum output current of 1.5A. The typical application circuit shown in figure 1.

Typical application circuit of Figure 1 LM317K

In Figure 1, and as the sampling resistor. The calculation formula for the output voltage

(1)

Type (1), [1]. So, when =0V,; it was, at the time.

There are two kinds of methods: one is the value, minimum load current at LM317K, the selection of 6.8K adjustable resistor; the other one is taken, the optional 3.4K, adjustable resistance. We can get the two regulated output value adjustment at the time of. And is the role of the self-excited oscillation elimination of input and output of the possible, is the role of ripple filter at both ends, and as a protection diode, prevent the input end or the output end when a short circuit fault occurs on the LM317K internal discharge and damage the chip [2].

If use need to the maximum output current of 3A voltage regulator circuit composed of LM317K, several methods can be realized by following the presentation of the expansion flow.

Methods 2 expansion flow

At present, the method of three terminal voltage regulator integrated expansion flow are the main transistor expansion flow, resistance and three parallel expansion expansion flow.

The 2.1 transistor current amplification

This method is mainly realized through the voltage stabilizing circuit is connected to large power transistor.

(1) the NPN transistor

This is a frequently used method, between the stabilizing circuit input end and the output end of the access of a NPN type power transistor, as shown in figure 2.

Figure 2 external NPN transistor expand output current circuit

High current load are required to provide a high power transistor T, and the base of the transistor is composed of three terminal integrated voltage regulator output to drive.

Fig. 2 shows the regulator circuit output voltage, after the expansion flow after the value has changed, the output voltage is inherent, three terminal voltage regulator integrated value, is an external T emitter junction voltage value, so using this method the regulator output voltage value after the expansion flow reduces the output voltage value than the original natural. In order to overcome the expansion flow after the output voltage value decrease malpractice, can do to improve the circuit of Figure 2, as shown in figure 3.

Figure 3 Figure 2 improved circuit

Figure 3 access a diode D, is used to compensate the transistor emitter junction voltage, output voltage of the circuit is basically equal to the output voltage of the three terminal integrated voltage regulator. As long as the appropriate choice of diode model, and through regulating resistance

Current, resistance to change through the diode can be obtained, at this time, can be seen from Figure 3

At the same time, access diode D also compensates for the effect of temperature on the transistor, the output voltage is stable [3].

This method of direct access to the NPN type power transistor current amplification is common, but due to the internal circuit of LM317K limit, LM317K does not directly with a NPN type power transistor.

(2) the PNP transistor

LM317K is suitable for PNP type high power transistor, circuit for PNP type high power transistor current amplification are shown in Figure 4, the input base PNP type high power transistor changed access three terminal integrated voltage regulator, therefore no longer affect the output voltage of the three terminal voltage regulator integrated inherent value, i.e..

Figure 4 external PNP type power transistor extended output current circuit

Diagram of resistance of R 4 is the bleeder resistor three terminal integrated voltage regulator, the role is to base make static current three terminal integrated voltage regulator is no longer flowing through the external expansion flow transistor T. Because all the bias current and adjust the end of the current three terminal integrated voltage regulator flows from the input, the base if all through T, and after T amplification, then the output terminal of the three terminal integrated voltage regulator will be connected with the dummy load to discharge these to the real load useless current. The principle of choosing R is:

(2)

Among them: for the quiescent current three terminal integrated voltage regulator, which is about 10mA, the general set the minimum output current design is about tens of Ma, so R chooses the value about a few ohms can be [4]. Here you can choose R for 3.3.

The output current of the expanded. When the only work when, LM317K;, LM317K and external PNP type power transistor working together.

(3) the NPN+PNP transistor

If you must use the NPN type transistor to expand the LM317K output current situation encountered in use, can increase through a PNP type tube matched with the drive to achieve the expansion flow requirements, the circuit shown in figure 5.




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