Monday, 5 January 2015

Golden Rules For Electronic Circuit Design

- All components must conform to the specified temperature range. For resistors, de-rating of power rating at peak temperature must be considered. For capacitors de-rating of capacitance, leakage and increase in ESR at peak temperature may be important. For other components such as diodes and transistors, parameters such as recovery times and current gain vary with temperature and must be considered at extreme temperatures. In summary, no design calculation must be left to chance (as is often the case with designs put together in haste), temperature effect calculations are indispensable to success.

- Devices connected to interfaces must be well protected from ESD (Electrostatic Discharge) damage. ESD damage may occur at the time of prototype construction, during manufacturing or during use. Contrary to common misconception ESD at the time of production may not render a device defunct, but may cause progressive deterioration which may result in field failure. Therefore built in ESD protectors are necessary and Mansi Engineering ensures that all its designs incorporate them where necessary. IEC 61000-4-2 is a widely accepted ESD test conformance standard. ESD protectors include MOVs, TVS diodes, ESD Protection Clamping diodes and the newly introduced Polymer based devices. Selection of the right type of protector depends on capacitance that can be tolerated without signal deterioration and other factors such as tolerable leakage current, cost, pin outs and board area required. In addition, ESD protectors must be placed as close as possible to the signal entry location and as far away as possible from the device being protected so that the resulting large trace impedance attenuates the high frequency ESD energy as it travels towards the protected device. Zener or TVS diodes must be placed between power supply lines and ground to absorb ESD energy traveling via other protector diodes towards power supply line, while the device is powered off.

- Terminals connecting to inductive sources must be protected from switching overshoots or undershoots using clamping diodes or other devices. Long traces that carry heavy currents may act as sufficiently large inductors to cause unwanted switching transients and therefore protection is necessary.

- Signal lines must be well protected from radiated and conducted EMI. Ferrite beads and capacitor filters are effective means to block EMI.

- Within limits of budget, every design must incorporate sufficient protections from wrong connections, such as battery polarity reversal and accidental grounding of an output line. Large resistors are recommended to be used in series with microprocessor outputs driving drivers that drive large inductive loads. This ensures that the processor is well protected from switching transients.

- Functioning of the system at power up must be analyzed. For example it is important to deploy pull-up resistors on microprocessor output signals that need to be high on power up. Similarly functioning of relevant parts when powering down deliberately or accidentally, must be analyzed. For example, when processor state saving is involved during power down, a power down detect functionality might prove indispensable.

- For EMC, it is recommended to use inductors (preferably shielded) to slow down sharp current rise and fall times. Sharp current transients cause a sharp varying and traveling electromagnetic field.

- It is important to assign test points to relevant nets. Especially all ground and power nets, bias or threshold points and other serviceable signals must have a test point, which add convenience during testing and troubleshooting. Test points may be free vias with suitable drill size to accept and retain stainless steel needles or header leads.

- Having an on board power on LED indicator has its own importance. It warns the engineer and people around that the circuit is live and prevents accidents."

Article by
Department of ECE
Vignana Bharathi Institute of Technology - VBIT

Vignana Bharathi Institute of Technology

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