Latch Up

Have you ever faced a situation in which a component fails immediately after power up? or did you observe your circuit recovering after power cycle? Then you might have observed the latch up condition unknowingly. Latch up occurs when unintentional low impedance path is formed in the digital circuits of a integrated circuit. The integrated circuit is generally built with mosfet structures. So, most of the latch issues are seen in CMOS logic. The parasitic paths formed within the mosfet structure leads to high current and hence damage the mosfet.

To explain it more from the mosfet structure, latch is the generation of a low-impedance path in integrated chip between the power supply and the ground rails due to interaction of parasitic pnp and npn bipolar transistors. One has to remember that the circuit designer using CMOS logic integrated circuit can't avoid latch up as this can be eliminated only when integrated circuit physical design is done.

Have you tried power sequencing in a circuit, or tried power states in the board? During these conditions, we will try to power on/power off various circuits on the board. In that case, when signals are applied at the input of unpowered CMOS circuit, latch up occurs, where the unintentional current may be transferred. Latch up may also occur when the signals applied to a CMOS logic cross the power thresholds. 

Inrush current limit - Part 1

Are you expecting inrush current in your circuit? or have you heard you heard about the surge current? Have you ever dealt with surge current? Are you a beginner and don't know what component/circuit to use? Then after reading this article you must be able to decide after need to be done. Also, if we see our SMPS designs, inrush current limiter is commonly used.

             To start this discussion, let us take a scenario when we power on the circuit. Initially, there will be a surge current that flows. This is because of the capacitor bank that is present in the circuit which draw a significant current instantaneously to charge themselves. Also, we can put it the other way, when the current drawn by the load is greater than steady state current of the circuit, it is called surge current. Surge current may not be significant in some cases. This depends on the underlying circuitry used. If the surge current is of small duration such that the circuitry doesn't get affected by it, then we need not have any protection. Also, the affect of surge current may not be straight forward, it may reduce the life time of a component. The graph here shows the surge current drawn in a circuit over a time.

                 The tradition way of limiting the current is to use a fuse to limit the current. But if the current exceeds the fuse limit, then fuse blows off and only way to recover the circuit functionality is to replace the fuse with a new one. But this is a tedious job. One way to avoid fuse blow is to use some other circuit which handles the inrush current. Thermistor is one such component. we always use resistors in circuits to limit the current, for example, if we take the case of LED, it is always preferable to use a series resistor. But series resistor will not suffice if the inrush current is in amperes range and in fact the resistor may blow away. Also, resistors doesn't have the property to change their resistance unlike thermistor. Thermistors are better suited for these applications.

Thermistors are of type positive temperature co-efficient (PTC) and negative temperature co-efficient (NTC). NTC type are used as inrush current limiters. NTC thermistors are designed to be used in inrush limiter circuits. The principle of NTC is that resistance decreases with increase in temperature. so, initially, thermistor has very high resistance. As the current starts flowing, the resistance decreases as drop/temperature increases. 

     Using thermistor is also a cheaper option rather than having a complex circuitry (Active circuit) to limit the current. NTC is the most commonly used in the industry. 

General specifications to be checked while selecting a NTC/PTC as current limiter:

1. Resistance offered vs temperature 

2. Hold current (current at which thermistor provides low resistance)

3. Trip current (the current at which the thermistor offers high resistance)

4. Recovery time (for thermistor to come back to high resistance after the circuit has switched off)

Advantages of using thermistors:

1. Reliable

2. Stable

3. Lower cost

4. Lesser footprint

5. Easy to include in the circuit and easy to select

6. Longer life time

Some of the disadvantages of using inrush current protection:

1. When thermistor is used, there will be a voltage drop across it.

2. For a small form factor boards, with sizable power consumption (current draw by load), we need to include a bigger size thermistor which may be a limiting factor

3. High recovery time for thermistors.

Important points of thermistor as Inrush current limiter:

1. Resistance drops to milli ohms within in milliseconds

2. The resistance offered by thermistor during idle state will be in hundreds of ohms.

How to select the resistance of inrush current limiter?

Let us assume that we have a circuit with input voltage 12V and load current of 1A. In such cases, the value of thermistor is selected such that it allows min. 1A and cuts off above that current. So, here resistance if V/I = 12 ohms. So, have a thermistor of at least 12 ohms.

Board Fault finder

Are you struggling to debug the board you have in hand? Have you ever thought of an easy option to find a fault in the circuit? Even though understanding the circuit and using the manual procedure to find the fault, there are some automated systems that can do the job for you. If you are planning to The device fault locator can do the following things for you:

1. Multi-channel input for debug

2. Functional test (analog & digital devices)

3. Various logic levels

4. Short test

5. Waveform generation

6. Voltage measurement

7. Current measurement

8. Standard sequence generated to test a circuit

9. EEPROM verification

10. Complete front end GUI interface for diagnostic view

11. Basic multi meter functionality

The board fault locators gives the ability to test the desired devices/ICs for desired functionality. The devices/ICs can be tested in circuit as well as stand alone. Generally, these kind of instruments provide support for common type of digital ics available. the integrated software in the unit is very robust and supports wide range of all these test cases. The board uses a integrated variable power supply to provide all the desired power supply outputs to the board in which fault need to be found out.

Vendors that provide these devices are:

1. GSAS Micro systems

Product name: BoardMaster 8000 Plus

2. ABI electronics

SYSTEM 8 Board Fault Locator (BFL)

Thermal Imagers

Are you working on a board with temperature (components) crossing dangerous limits or reaching the thresholds? Then how do you track them? what you do to have your board safe? The one solution is to use thermal Imager. Thermal Imagers are used in the industry to mainly detect the temperature variations. Thermal Imager can be said Heat sensor. The principle of thermal imager depends on the fact that every object emits infra red radiation and the extent of emission depends on temperature. More the temperature more is the radiation. We can say that infrared radiation is the signature of temperature. Thermal Imager has the capability to capture the limited variations in temperature. Once the thermal imager collects the data it represents the data in a visual form which indicates the intensity of the temperature. Thermal Imagers add color variations based on the temperature. As commonly used, red indicates the high temperature area.

               

The basic ingredients of thermal Imager are:

• Special Optical lens
• Infrared detector (array)
• signal processing unit
• Display

The key specifications that define the thermal Imager are:

• Infrared Resolution
• Thermal Sensitivity
• Ability to create real image that is sensed
• Temperature measurement range
• Range of measurement
• Frame rate
• Detector type (Cooled/uncooled)
• Storage capability
• Battery operating time

The various vendors that provide Thermal Imagers are:

• Keysight technologies
• Fluke
• Testo
• FLIR

General facts:

• Thermal Imagers are accurate
• Generally, on board we have number of temperature sensors but they give a approximate of the value and may not always be accurate. Thermal Imagers can access any component with accurate results.
• Thermal Imagers can sense higher temperatures of  up to 1500+ degrees
• Thermal Imagers can't measure lower temperatures below 20 deg
• Greater the resolution of the sensor more the accuracy of the measurement.
• Thermal Imagers are fast and reliable
• Thermal profile is created by the signal processing unit o the thermal Imager
• If there are multiple devices in order, the thermal Imager can only catch the image of first device
• Thermal Imagers are also used for security purposes during night to catch any intruders
• Thermal Imagers are available as hand held devices
• Thermal Imagers can be used safely as they are used hand-held at a distance from the board

Battery Charging Basics - 1

There are several batteries available in the market. The classification is mainly on the battery chemistry. Several types available are Lead-Acid, Li-ion, Ni-Cd, Ni-Mh. Of these Li-ion is mostly used in the mobiles phones widely available in the market today. Some of the batteries we use in real world are rechargeable and some are non-rechargeable. There are several options available to charge these batteries as well. The below block diagram shows the generic pattern for charging the batteries:

What are the main differences between various battery types when it comes to charging:

1. Ni-Cd, NiMH are difficult to charge than Li-ion batteries. Charging in Nickel based batteries is based on the current flow through the battery. The voltage variation must only be
2. All the battery types can undergo fast as well as slow charging. Prefer not to use fast charging on NiCd, NiMH cells.
3. NiCd, NiMH must be charged from a constant current source.
4. End of charge detection happens in NiMH, NiCd as the voltage gets reduced with overcharging.
5. The batteries have a in-built thermistor to check the heat produced within the battery. this is an indicator of increase of battery temperature.
6. All the battery chemistry can be trickle charged. Trickle charging is the charging that must be applied when the battery is fully discharged.
7. Ni-Cd are not preferred in solar applications as they need constant current charging.
8. Ni-Cd batteries can charge to full capacity and beyond with less reduction in battery capacity.
9. Ni-Cd battery absorbs the heat while charging unlike other battery chemistry.
10. Li-ion batteries charging work on the constant voltage with variable current. The current variation is based on the level of battery charging.

Electromagnetic Compatibility - 1

EMI/EMC is a very big subject in electronics. It itself has a prominent place and without EMI/EMC precautions your product may not get sold in the market. In this modern world there are radiators, absorbs everywhere. To get our product withstand such an environment, your product has to comply to standards of EMI/EMC.

Most of us basically, work on Low voltage digital Electronics products. There are several standards for each of these products and one of the primary regulation is the FCC Part 15. In FCC part 15, there are several classifications of the products like A,B,C, D,E,F among which Category B corresponds to the unintended radiation. FCC Part 15 basically mentions the allowable limits of Conducted Emission and Radiated emission.

Conducted Emission -> 150 KHz to 30 MHz

Radiated emission -> 30 MHz to 40 GHz

There are test procedures defined for each and every certification and product has to undergo these certifications before being said as compliant.

Website formally Launched

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