International Journal on Recent and Innovation Trends in Computing and Communication
Volume: 4 Issue: 3
ISSN: 2321-8169
496 - 501
_______________________________________________________________________________________
Associate Professor, Department of Electronics, S. P. University, V. V. Nagar
Email: swamibhavin@gmail.com
Abstract : In most of the electrical systems mainly the damage is caused by overvoltage and excess current. This paper addresses the methods and issues related with the overvoltage protection for high voltage loads. If the voltage exceeds beyond the withstanding voltage capacity of the device it may damage it. The design and development of primary and secondary protection circuits are described. This includes use of IC
MPS2400 for protecting the loads from damage due to overvoltage. This overvoltage protection controller is used with an external N-channel
MOSFET to isolate the sensitive loads from destructive voltage spikes and surges. The Gate output voltage and Gate sourcing current are measured. Output Turn on response and Output turn off response are characterized. Results are discussed.
Keywords : Overvoltage protection, Crowbar circuit, Electrical systems, Voltage transient, Gate voltage.
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1.
INTRODUCTION
In most of the electrical and electronics systems, failure and damage is due to the overvoltage or excess current or combination of both. High voltage can cause unintended current paths such as forward or reverse breakdown of diodes or oxides, reaching their breakdown voltage within integrated circuits[1]. There is also a direct thermal damage due to the excess current. Thus, it is necessary and of prime importance to protect the device against such adverse conditions.
Protection can be done with either voltage activated circuit elements that open low resistance paths to prevent excess voltage or with the help of current limiting devices or a combination of the two. There are various protection methods which are widely used. Amongst this, the use of primary and secondary protection is very common. In telecommunication industry, it is used for lighting and surge protection. In advanced integrated circuit technology, it is used for ESD protection [2,3].
2.
METHODS OF PROTECTION
(i) Primary Protection :
The primary protection element which is connected closer to the external source of transient is intended to carry the bulk of the current stress. Fig.1 shows the scheme of primary and secondary protection for a data line entering in a building. The primary protection element is located at the building entrance which prevents surges from lighting causing fires or electrocution in the building. This protection element which can carry the large current from a lightning induced surge normally do not have the fast turn on time and low trigger voltage needed to protect the sensitive electronic instrument.
Thus, protection element at an entrance of the building will also not be able to protect circuits from surges generated within the building.
(ii) Secondary Protection :
Near sensitive circuits, protection is provided by secondary protection element. It protects against the let through stress (Transient) from the primary protection and surges generated within the building. It is possible to customize the secondary protection to the special need of the circuit being protecting. Also, as soon as the secondary protection begins to carry current, voltage drop in the resistance between the primary and secondary protection, either parasitic or intentional, can help turn on the primary protection. One can also place a current limiting element between the primary and secondary protection elements to enhance the safety.
3.
CLASSIFICATION OF PROTECTION
DEVICES
Overvoltage protection scheme is classified as
Unidirectional or Bidirectional and Voltage Clamping or
Crowbar circuits. Current limiting devices can be considered as one time use and resettable.
(i) Unidirectional Protection Methods :
For electrical nodes, protection requirements with only positive or only negative voltage differ from nodes whose voltage extends above and below zero volts. This can be addressed using diode based Transient Voltage Suppressors
(TVS) devices. Figure 2 shows unidirectional TVS protection
496
IJRITCC | March 2016, Available @ http://www.ijritcc.org
_______________________________________________________________________________________

International Journal on Recent and Innovation Trends in Computing and Communication
Volume: 4 Issue: 3
ISSN: 2321-8169
496 - 501
_______________________________________________________________________________________ of an input. The current-voltage curve for a unidirectional
TVS device is same as that of a standard diode.
(iii) Clamp & Crowbar Devices :
Protection devices are also classified as clamp and crowbar devices. TVS devices are clamp devices.
When any transient occurs or during any stress event, they clamp the voltage at a defined level. When a trigger voltage is reached, a crowbar device attempts to create a short circuit. It is like putting a metal crowbar across the high voltage to provide a short. This is illustrated in Figure 4.
If such TVS system is inserted into the circuit as shown in
Fig.2, the signal voltage will be undistorted if it remains between 0V and the TVS’s reverse breakdown voltage. The
TVS device protects by reverse bias breakdown for positive stress on the signal line where as such system protects by forward bias operation for negative voltage stresses. The
Unidirectional TVS device can be inserted with the opposite polarity when the signal ranges between 0V and a negative voltage.
(ii) Bidirectional Protection methods :
A bidirectional TVS device can be formed by using two anti-parallel diodes in series as shown in Fig. 3.
When it is used as a protection device, the input voltage can range over positive and negative values.
4.
SELECTION OF PROTECTION DEVICES
The choice of protection element depends on the nature of the circuit being protected and the nature of the external stress. While choosing various protection elements one has to take into account several important technical considerations.
Fig.5 shows very elementary circuit indicating use of protection element. The signal line is connected to an input of sensing circuit. The signal line enters the system from an unprotected electrical environment. The signal line can be exposed to a variety of external stresses with various voltage and current levels well beyond those that the input can withstand. The protection element ensures that the voltage on the input remains within safe limits.
Protection is provided by reverse bias breakdown in series with a forward bias diode in both polarities.
497
IJRITCC | March 2016, Available @ http://www.ijritcc.org
_______________________________________________________________________________________

International Journal on Recent and Innovation Trends in Computing and Communication
Volume: 4 Issue: 3
ISSN: 2321-8169
496 - 501
_______________________________________________________________________________________
Figure 6 shows the normal voltage range of the input, as well as the maximum range of voltage above which damage may
The protection element turns on at 5 V, safely above the normal operating voltage and can carry in excess of 25A occur.
The current-voltage curves of two protection elements viz. voltage clamping device and a crowbar device is shown in
Fig.6
For normal operating voltage range, the resistances of both protection elements are high, ensuring integrity of input signal. The voltage clamp protection looks well suited because the voltage never enters the damage region. The crowbar circuit is also capable for providing protection.
How much low resistance is required to prevent the voltage reaching the danger zone depends on how much current the external stress can provide. Test standards specify the voltage and current waveforms that a product needs to survive in a given environment. Fig. 7 illustrates that the system is required to survive a stress with a peak current of 25A. The circuit being protected has an input with an operating voltage of 0V to 3.6V and damage to the input is expected if the voltage exceeds 8V. without the voltage exceeding the unsafe operating voltage.
This type of protection is very powerful tool for protecting against an ESD stress. Most interfaces are more complex than a single line with respect to ground. Differential signals and systems such as the telephone network which use a combination of primary and secondary protection need special considerations. The capacitance of the protection element is often more important than the low voltage resistance of the protection element, especially for high speed circuits.
5.
OVERVOLTAGE PROTECTOR FOR HIGH
VOLTAGE LOADS
Electrical transients in the form of voltage surges have always existed in electrical distribution systems. Many electronic and electrical instruments/devices can be damaged by voltage transients. The difference between them is the amount of energy they can absorb before any damage occurs. Since most of the modern semiconductor devices such as low voltage
MOSFETs and integrated circuits can be damaged by disturbances that exceed only 10 to 12 volts or so, their probability to survive is poor in unprotected environments.
It is observed that in many cases, as semiconductors has evolved, their ruggedness has diminished. The widespread use of MOSFET and GaAs based FET technologies as well as trend to produce smaller and faster devices has led to an increased unsafe protection[4]. High impedance inputs and small junction sizes limit the ability of these devices to absorb energy and to conduct large currents. Therefore, it is required to use safe electronic components with devices specially designed to cope with these hazards[5,6,7].
TPS 2400 is one of the most suited overvoltage protection controller. It is used with an external N-channel MOSFET as shown in Figure 8.
It isolates the sensitive electronics/electrical system from destructive voltage spikes and surges. It is specially designed to prevent large voltage transients associated with automotive environments ( Load dump) from damaging sensitive circuitry. When potentially damaging voltage levels are
498
IJRITCC | March 2016, Available @ http://www.ijritcc.org
_______________________________________________________________________________________

International Journal on Recent and Innovation Trends in Computing and Communication
Volume: 4 Issue: 3
ISSN: 2321-8169
496 - 501
_______________________________________________________________________________________ detected by the overvoltage protection controller, the supply is disconnected from the load before any damage can occur.
TPS 2400 can be used to protect high voltage loads having up to 100V overvoltage protection. It is having under voltage
(3V) shutdown threshold and overvoltage(6.9V) shutdown threshold. It is available in 5 pin SOT-23 package. Its advantages include – fast response time and survival during external overvoltage events.
Figure 9 represents the block diagram showing the application of TPS 2400 to protect the load
The large in-rush current may damage power connectors P1 and J1 and power switch S1. If power supply output resistance and inductance is increased, it helps in reducing the inrush current. But increase in R1 and L1 increases system power dissipation where as increase in inrush current causes decrease in switch and connector reliability by way of allowing contacts to arc when they bounce.
(ii) Turn On voltage slew rate control :
When the overvoltage protector disconnects the load from power supply, the power supply output – voltage spikes as the stored energy in inductor L is released. A zener diode ZD1 or a small capacitor can be used to keep the voltage spike at a safe level.
6.
EXPERIMENTAL, RESULTS & DISCUSSION
The overvoltage protector is used to control the Load inrushcurrent and Turn-on voltage Slew Rate. This is described in detail.
(i) To control Load inrush-current :
Figure 10 shows the representation of an appliance with a plug-in power supply. When power is first applied to the load, the large filter capacitor C1 connected across load acts like a short circuit. This produces immediately an in-rush current that is limited by the power supply output resistance and inductance R1 & L1 respectively. This in-rush current is much higher than the steady-state load current.
Fig 11 is an improvement of Fig 10 which limits the inrush current within a safe limit. Here, IC U1, charges the transistor
Q1 gate capacitance C
(G)
with a 5 µA connected as a source follower so that load voltage slew rate and gate voltage slew rate are same and equal to ∂ V
L
/∂ t = 5 µA /C
G
-----(1)
The corresponding inrush current is,
I
INRUSH
C
L
X ∂ V
L
/∂ t = ( C
L
/C
G
) X 5 µA ----(2)
If we select gate capacitance as 2 nF, as per eq.1 we get 2500
V/S. According from eq.2 the inrush current can be
499
IJRITCC | March 2016, Available @ http://www.ijritcc.org
_______________________________________________________________________________________

International Journal on Recent and Innovation Trends in Computing and Communication
Volume: 4 Issue: 3
ISSN: 2321-8169
496 - 501
_______________________________________________________________________________________ approximated as 250mA.Figure 12 shows voltage startup curve with input voltage of 3V.
(i)Undervoltage lockout threshold :
An external capacitor and a series 1KΩ resistor can be connected to the gate of Q1 and ground to further reduce inrush current. For this gate capacitance (CG) is considered as sum of the internal and external MOSFET gate capacitance.
The external resistor (1KΩ) decouples the external gate capacitor. So the TPS2400 device can rapidly turn off transistor Q1 in response to an overvoltage condition[8].
(iii) Driving a High voltage load :
Figure 13 is used to drive a high voltage load. This circuit provides overvoltage protection to a load with an operating voltage of up to 100V.
V’UV = (1+2.49KΩ/5.11KΩ)X(3V+0.5 V)
Therefore,V’UV = 5.21 V
(ii)Undervoltage hysteresis :
V’hyst(UV)=(1+2.49KΩ/5.11KΩ)X100mv
Therefore, V’hyst =144mv
(iii)Overvoltage protection threshold:
V’UV+(1+2.49KΩ/5.11KΩ)X(6.9v+0.5v) = 11V
(iv)Overvoltage hysteresis:
V’hyst(OV) = (1+2.49KΩ/5.11KΩ )X150mv=223mv
Capacitors C1 and Crss(i.e gate to drain capacitance) of transistor Q1 set the turn on voltage slew rate that is given by
∂ VL/∂ t = 5 µA /C1║ Crss
The value of R3 is typically in the range of 1KΩ –
10KΩ.This resistor decouples capacitor C1 from the circuit so that the TPS2400 can rapidly turn off transistorQ1 in response to an overvoltage transient. Zener diode D1 is required only if transistor Q1 has a gate to source voltage rating less than 20V.
If 100V transients are present, transistors Q1 and Q2 are rated at 100V or higher for V
DSS
and V
CEO
.
Various parametes of overvoltage protection controller are measured.
Figure 14 shows graph of gate voltage versus gate sourcing current.
The threshold and hysteresis levels for this figure are defined by,
V t
(threshold) = ( 1 + R1/R2) X ( V t (UV or OV)
+ 0.5 )
V hyst
= ( 1 + R1/R2) X V hyst
The V t
represents the device undervoltage lockout or overvoltage protection threshold and the parameter V hyst
is the corresponding hysteresis.
If we consider R1 = 2.49 KΩ and R2 = 5.11 KΩ , then the
As shown in Figure14, Gate current is constant up to certain
Gate Voltage and then it decreases. In case of higher junction temperature the Gate current is more compared to low junction temperature[9].
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IJRITCC | March 2016, Available @ http://www.ijritcc.org
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International Journal on Recent and Innovation Trends in Computing and Communication
Volume: 4 Issue: 3
ISSN: 2321-8169
496 - 501
_______________________________________________________________________________________
Fig 15 shows characteristic of input supply voltage versus gate output voltage.
As shown in Figure 15, for a 3V input supply voltage, Gate output is approximately 11V. It increases sharply up to 18 V between input voltage of 3 to 5V. When V
IN
is between 5 to
7V, it is almost constant. When V
IN
is above 7V Gate voltage is reduced to zero[10].
Fig 16 shows output Turn On response when a load resistance R
LOAD
of 100Ω is connected and a step input of 5v is applied then output starts increasing after 600µs and becomes constant after saturation.
Fig.17 shows output turnoff response for a load resistance of
100Ω. When the gate is triggered, output is high. As soon as gate voltage starts decreasing output reduces very fast. The output turnoff response is about 40ns.
CONCLUSION
The primary and secondary overvoltage protection methods are discussed. The I-V characteristics of unidirectional and bidirectional protection methods using crowbar circuits are addressed. The overvoltage protector for high voltage loads represents the limits of threshold voltage and hysteresis voltage. The application of TPS2400 to protect high voltage loads is in case of overvoltage is designed and developed. The use of TPS2400 to control the load inrush current and turnon voltage slew rate are represented. Results shows that device turnoff response is very fast. Thus, protecting the loads safely in case of overvoltage.
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