TECHINCAL FIELD
Embodiments of the present disclosure relate to voltage regulator. More particularly  the embodiments relate to series voltage regulator with built-in short-circuit protection for automotive application.

BACKGROUND OF DISCLOSURE
Presently  the available regulated power supply is one that controls the output voltage or current to a specific value. The regulated power is obtained using a series voltage regulator. The output of the series voltage regulator is held nearly constant despite variations in either load current or voltage supplied by the power supply’s energy source. In automotive engine control units (ECU’s) regulated power supply is designed to provide supply for microcontroller and sensors in the vehicle.

The disadvantage of a series voltage regulator is that the pass transistor is in series with the load. If a short circuit is developed in the load  a large amount of current will flow in the regulator circuit. The pass transistor gets damaged by the large amount of current which is excessive than the required current.

Figure 1 illustrates a short circuit protection with current limiting circuit. The conventional short circuit protection as illustrated is mainly used to limit the current flowing through the series pass transistor Q1  rather than switch it off. The series pass transistor Q1 is connected to a power supply source Vin through a resistor R1. The resistor R1 103 is a source resistance to input circuit of power supply. The circuit has a current sense resistor R3 and current sense transistor Q2. By increasing the value of sense resistor the current may be limited to different values. As the short circuit occurs the voltage across the current sense resistor increases to more than 0.6volts. So  the current sense transistor starts to conduct  thereby the voltage at the base of series pass transistor Q1 decreases. This action causes series pass transistor to conduct less. Hence  preventing any increase in the output current of the power supply. However  with this technique  the pass transistor is exposed to excessive power dissipation at short-circuit thus  limiting allowable current limit. This severely restricts application of this technique in even moderate current applications exceeding few hundred of mA.

Hence  there is a need of a solution to protect the series pass transistor during output short circuit to ground. Further  the solution needs to be simple with minimal usage of hardware.

OBJECTIVES OF THE DISCLOSURE
The objective of the present disclosure is to provide a low cost series voltage regulator with built-in short-circuit protection for automotive application.

Another objective of the present disclosure is to provide a circuit to protect series voltage regulator from short circuiting of load.

Yet another objective of the present disclosure is to provide a method to protect series voltage regulator from short circuiting of load.

SUMMARY
The shortcomings of the prior art are overcome and additional advantages are provided through the provision as claimed in the present disclosure.

Additional features and advantages are realized through the techniques of the present disclosure. Other embodiments and aspects of the disclosure are described in detail herein and are considered a part of the claimed disclosure.

In one embodiment  the disclosure provides a series voltage regulator circuit with short circuit protection. The voltage regulator circuit comprising a transistor connected in series with a resistor to receive an input voltage. The transistor produces a predetermined output voltage on its output terminal based on the input voltage. The output terminal of the transistor is connected to an output load. The resistor is connected to an input terminal of the transistor. Also  the voltage regulator circuit comprises a resistor in series with an opto-coupler transmitter (LED) connected to the transistor output terminal  wherein the opto-coupler transmitter (LED) produces an output illumination based on current flowing through the transistor. Further  voltage regulator circuit comprises a photo transistor in parallel combination with a start-up capacitor (C1) is connected across the input terminal and a third terminal of the transistor. The photo transistor switches ON in responsive to a predetermined magnitude of illumination. The start-up capacitor (C1) provides start-up current to the transistor to get switched ON during start-up of the voltage regulator circuit. Further  a voltage regulator is connected between the third terminal of the transistor and the opto-coupler transmitter to provide regulated output voltage. During the short circuit condition of the output load  the opto-coupler transmitter (LED) turns OFF as the voltage or current required to forward bias is reduces because of short circuit condition and LED provides almost zero illumination output thereby  turning OFF the photo transistor to protect the transistor.

In one embodiment  the disclosure provides a method for short circuit protection of a series voltage regulator. The method comprises generating an output voltage using a series transistor and a voltage regulator. A photo transistor is connected across the transistor for protecting the transistor; the voltage regulator is connected between the third terminal of the transistor and the opto-coupler transmitter. Also  the method comprises detecting a short circuit in output load of the series voltage regulator using generated light output of the opto-coupler transmitter. The opto-coupler transmitter gets switched OFF if the output load is short circuited. Further  the method comprises automatically switching OFF the photo transistor thereby switching OFF transistor for protecting from the detected short circuit.

The foregoing summary is illustrative only and is not intended to be in any way limiting. In addition to the illustrative aspects  embodiments  and features described above  further aspects  embodiments  and features will become apparent by reference to the drawings and the following detailed description.

BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES
The novel features and characteristic of the disclosure are set forth in the appended claims. The disclosure itself  however  as well as a preferred mode of use  further objectives and advantages thereof  will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying figures. One or more embodiments are now described  by way of example only  with reference to the accompanying figures wherein like reference numerals represent like elements and in which:

Figure 1 illustrates a circuit diagram of a conventional voltage regulator with a short circuit protection using current limiting circuit; and

Figure 2 shows a circuit diagram of series voltage regulator with short circuit protection of series transistor  in accordance with one embodiment.

The figures depict embodiments of the disclosure for purposes of illustration only. One skilled in the art will readily recognize from the following description that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles of the disclosure described herein.

DETAILED DESCRIPTION
The foregoing has broadly outlined the features and technical advantages of the present disclosure in order that the detailed description of the disclosure that follows may be better understood. Additional features and advantages of the disclosure will be described hereinafter which form the subject of the claims of the disclosure. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present disclosure. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the disclosure as set forth in the appended claims. The novel features which are believed to be characteristic of the disclosure  both as to its organization and method of operation  together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood  however  that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present disclosure.

Referring now to the drawings wherein the drawings are for the purpose of illustrating an exemplary embodiment of the disclosure only  and not for the purpose of limiting the same.

To overcome the drawbacks mentioned in the background  the disclosure provides short circuit protection circuit for a series voltage regulator  which is also a cost effective solution.

Exemplary embodiments of the present disclosure provide a voltage regulator circuit 100 with short circuit protection. The voltage regulator circuit 100 comprising a transistor 107 connected in series with a resistor R1 103 to receive an input voltage from a power supply source 101. The resistor R1 103 is a source resistance to input circuit of power supply. The transistor 107 produces a predetermined output voltage 102 on its output terminal based on the input voltage 101. The output terminal of the transistor 107 is connected to an output load. The resistor 103 is connected to an input terminal of the transistor. Also  the voltage regulator circuit 100 comprises a resistor 111 in series with an opto-coupler transmitter (LED) 112 connected to the transistor output terminal  wherein the opto-coupler transmitter 112 produces an output illumination based on current flowing through the transistor 107 or output voltage 102. Further  voltage regulator circuit 100 comprises a photo transistor or opto-coupler receiver 105 in parallel combination with a start-up capacitor (C1) 106 are connected across the input terminal and a third terminal of the transistor 107. The photo transistor 105 switches ON in responsive to a predetermined magnitude of illumination produces by the opto-coupler transmitter 112. The start-up capacitor (C1) 106 provides start-up current to the transistor to get switched ON during start-up of the voltage regulator circuit 100. Further  a voltage regulator 108 is connected between the third terminal of the transistor and the opto-coupler transmitter 112 to provide regulated output voltage. During the short circuit condition of the output load  the opto-coupler transmitter 112 turns OFF as the voltage or current required to forward bias is reduces because of short circuit condition and opto-coupler transmitter 112 provides zero output or zero illumination thereby  turning OFF the photo transistor 105 to protect the transistor 107.

Figure 2 illustrates a circuit diagram 100 of series voltage regulator with short circuit protection of series transistor. The circuit comprises an opto-coupler to protect the series transistor in case of short circuit of the output pin of the voltage regulator circuit 100 of the load connected to the output. Under normal operating condition  an opto-coupler or phototransistor 105 turns ON when the opto- coupler transmitter (LED) 112 gets forward biases to provide required illumination. The phototransistor 105 upon turning ON supplies current to base of the series pass transistor 107. Thereafter  the series transistor 107 becomes continuously ON because of the base current and provides a regulated output.

In an embodiment  the opto-coupler transmitter 112 is connected at the output via a resistor R3 111. When the output pin of the voltage regulator circuit 100 is short circuited to ground or GND 110 then the opto-coupler transmitter 112 of the opto-coupler switches OFF the phototransistor 105  and hence the series voltage regulator gets switched OFF. An opto-coupler is used to isolate the series pass transistor during short circuit  so as to avoid damage of series pass transistor.

As shown in Figure 2  an opto-coupler (U1) is divided into two blocks i.e. opto-coupler transmitter (LED) 112 and optical receiver or photo-transistor 105. A simple series voltage regulator comprises an NPN transistor (Q1) 107 also known as series transistor and a Zener diode (Z1) 108 in one embodiment. The transistor 107 used may be PNP transistor. This circuit is called a series regulator because collector and emitter terminals of the transistor are in series with the load. The unregulated dc supply is fed to the input terminal (Vin) 101 and regulated output voltage (Vout) 102 is obtained.

Initially  when voltage at Vin 101 is provided capacitor C1 106 acts as short circuit and zener diode 108 establishes the value of the base voltage  thereby current for the series transistor 107. The series transistor switches it ON based on the current supplied through the capacitor 106. The opto-coupler transmitter 112 at the regulator output gets forward biased because of the current supplied through the series transistor 107. The current is passed through the opto-coupler transmitter 112 thereby illuminates or which produces light as output. The produced light output triggers the phototransistor 105 or opto-coupler receiver  causing the phototransistor 105 to produce output current continuously  thereby supplying base current to the series transistor 107.

The zener diode 108 provides the reference voltage or regulated voltage and the transistor 107 acts as a variable resistor  whose resistance varies with the operating conditions. The principle of operation is based on the fact that a large proportion of the change in supply (or input) voltage appears across the transistor and  therefore output voltage tends to remain constant. When the pin Vout 102 is short circuited to GND  voltage across opto-coupler transmitter 112 becomes zero and is turned OFF. This results in deficiency of light on the photo transistor and phototransistor or opto-coupler receiver 105 is switched OFF. The base voltage of the series transistor 107 reduces and the transistor shuts OFF immediately till the short circuit exists. Hence the series transistor 107 is protected during short circuit of the output 102 to GND 110.

One embodiment of the present disclosure is a method for short circuit protection of a series voltage regulator. The method comprises generating an output voltage using a series transistor and a voltage regulator. A photo transistor is connected across the transistor for protecting the transistor; the voltage regulator is connected between the third terminal of the transistor and the opto-coupler transmitter. Also  the method comprises detecting a short circuit in output load of the series voltage regulator using generated light output of the opto-coupler transmitter. The opto-coupler transmitter gets switched OFF if the output load is short circuited. Further  the method comprises automatically switching OFF the photo transistor thereby switching OFF transistor for protecting from the detected short circuit.

The present disclosure is not to be limited in terms of the particular embodiments described in this application  which are intended as illustrations of various aspects. Many modifications and variations can be made without departing from its spirit and scope  as will be apparent to those skilled in the art. Functionally equivalent methods and devices within the scope of the disclosure  in addition to those enumerated herein  will be apparent to those skilled in the art from the foregoing descriptions. Such modifications and variations are intended to fall within the scope of the appended claims. The present disclosure is to be limited only by the terms of the appended claims  along with the full scope of equivalents to which such claims are entitled. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only  and is not intended to be limiting.

With respect to the use of substantially any plural and/or singular terms herein  those having skill in the art can translate from the plural to the singular and/or from the singular to the plural as is appropriate to the context and/or application. The various singular/plural permutations may be expressly set forth herein for sake of clarity.

In addition  where features or aspects of the disclosure are described in terms of Markush groups  those skilled in the art will recognize that the disclosure is also thereby described in terms of any individual member or subgroup of members of the Markush group.

While various aspects and embodiments have been disclosed herein  other aspects and embodiments will be apparent to those skilled in the art. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting  with the true scope and spirit being indicated by the following claims.

We claim
1. A series voltage regulator circuit with short circuit protection comprising:
a transistor 107 connected in series with a resistor 103 to receive an input voltage 101,
said transistor 107 produces an output voltage 102 on its output terminal based on the
input voltage 101, said output terminal of the transistor 107 is connected to an output
load;
a photo transistor 105 in parallel combination with a start-up capacitor 106 is
connected across the input terminal and a third terminal of the transistor 107;
a resistor 111 in series with an opto-coupler transmitter 112 connected to the
transistor output terminal, said opto-coupler transmitter 112 produces illumination on the
photo transistor 105 based on the output voltage 102 which turns OFF the photo transistor
105 to protect the transistor 107 when there is a short circuit at the output load; and
a voltage regulator 108 connected between the third terminal of the transistor 107 and
the opto-coupler transmitter 112 to provide regulated output voltage.
2. The circuit as claimed in claim 1, wherein the voltage regulator 108 is a zener diode.
3. The circuit as claimed in claim 1, wherein the transistor 107 is a bi-polar junction
transistor.
4. The circuit as claimed in claim 1, wherein the transistor 107 provides a variable resistance
to current, said resistance varies with operating conditions of the voltage regulator circuit.
5. The circuit as claimed in claim 1, wherein the photo transistor 105 switches ON in
responsive to illumination produced by the opto-coupler transmitter 112.
6. The circuit as claimed in claim 1, wherein the start-up capacitor 106 acts as short circuit
path during the start-up of voltage regulator circuit 100 to bias the voltage regulator 108
thereby producing a predetermined voltage to switch ON the transistor 107.
7. The circuit as claimed in claim 6, wherein the transistor 107 output provides necessary
bias current to the opto-coupler transmitter 112 to generate illumination.
11
8. The circuit as claimed in claim 1, wherein the photo transmitter 112 gets reverse biased to
turn OFF the phototransistor 105 when the output of the voltage regulator circuit 100 is
short circuited to ground 110.
9. A method for short circuit protection of a series voltage regulator comprising:
generating an output voltage using a series transistor and a voltage regulator, wherein
a photo transistor is connected across the transistor for protecting the transistor, said
voltage regulator is connected between the third terminal of the transistor and the optocoupler
transmitter; and
detecting a short circuit in output load of the series voltage regulator when an optocoupler
transmitting light on the photo transistor is switched OFF; and
protecting the transistor from the detected short circuit by automatically switching
OFF the photo transistor due to switch OFF of the opto-coupler.
10. A series voltage regulator circuit with short circuit protection and a method for short
circuit protection of a series voltage regulator are substantially as herein above described
and as illustrated in accompanying drawings.