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Guarding electronics against electrical overstress

Posted: 22 Nov 2013     Print Version  Bookmark and Share

Keywords:Electrical overstress  EOS  electrostatic discharge  power supply  electromagnetic interference 

Another disadvantage of this circuit is that there is tremendous increase in temperature on the order of 90°C. So, we cannot keep other devices near this circuit, therefore increasing the effective board area used by this circuit. There is a similar device called a Polyzen device that has both a resettable fuse and Zener diode on a single component, thus occupying limited space on the board. However, we will still have a series voltage drop due to this device.

The two main parameters of a resettable fuse are the 'Ihold' current and the series resistor. The 'Ihold' current is the current up to which the fuse will not break. This current value must be equal to or slightly more than the systems maximum current consumption. To keep the series voltage drop across the fuse minimum, a fuse having the least series resistance has to be chosen.

A Zener diode should be chosen such that the breakdown voltage of the Zener diode is equal to or slightly more than the working voltage of the system being protected. However, the break down voltage should not exceed the maximum voltage limit of the system being protected.

Option #4: MOSFET-based protection circuit. As discussed the primary disadvantage of the above three protection circuits are the voltage drop cased by the protection components and thermal dissipation. A MOSFET-based circuit help eliminate these voltage and thermal losses while providing effective protection. Due to negligible voltage drop, this protection can even protect analogue pins used to sample/monitor an external voltage.

An ideal over-voltage protection circuit needs to satisfy two criteria. The first is to prevent over or reverse voltage to be applied to the device pins. The second is to not to intrude on the normal function of the circuit (i.e to avoid any series voltage drop).

Consider an IC that can be operated from 1.71V to 5.5V. Such devices typically get damaged if a voltage above 6V is applied. Thus, the protection circuit must pass all the working voltages (1.71V to 5.5V) without any voltage drop. The protection circuit must also cut-off the supply to the microcontroller when the applied voltage is greater than 5.5V.

The main disadvantage of the circuit is that there are six components in the circuit and this circuit does not protect form over current (short circuit) EOS scenarios.

The functional block diagram of the protection circuit is shown in figure 5. The first block is just an enhancement mode P-MOS which does not allow the reverse polarity input to pass to the next voltage check (cheque for banks) block. The voltage check (cheque for banks) block consists of a Zener Diode and a PMOS to control the switch.

Figure 5: Functional block diagram of the circuit.

The protection circuit consists of two P-channel MOSFETs (figure 6) on the power line allowing the power/current to flow from input to output depending on the voltages applied to the line. The protection circuit will protect from a maximum over-voltage or reverse-voltage of 12V (max of Q1VDG). The cut-off voltage on the 5-V line is 5.7 V and on the 3.3-V line is 3.6 V. This means, if you apply more than this voltage level, the P-MOS Q5 will turn off, thus protecting the device. The current consumption of these protection circuits is less than 6 mA.

Figure 6: Schematics of the 5V and 3.3V protection circuits.


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