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[FAQ] TIDM-THREEPHASEMETER-F6779: Current front-end diode clamps? Why 2 diodes in series?

Daniel Tomlinson
Prodigy 30 points
Part Number: TIDM-THREEPHASEMETER-F6779
Other Parts Discussed in Thread: MSP430F6779

With Reference to the reference design: 

SLAA577G–January 2013–Revised August 2015
Implementation of a Three-Phase Electronic Watt-Hour
Meter Using the MSP430F677x(A)

On page 7, the analogue current front-end schematic diagram is shown (see image posed below). The SD24 ADC input has a differential input swing of +-930mV and an abs max withstand voltage of (AVss-1) for the lower limit and AVcc for the upper limit. In this reference design AVcc is 3.3V. 

My Questions:

1. Diodes D44 to D51 seem to be a clamping circuit to clamp the voltage across the burden resistor (R104) to the two supply rails AGND and AVcc. Is this assumption correct?

2. If question 1 is true, why then are there two diodes placed in series? The diodes used in the design are PMLL4148 which has a maximum forward voltage of 1V. By placing two diodes in series, the voltage drop towards a supply rail is 2V. The circuit will therefore clip at +5.3V or -2V, which is way beyond the withstand limit of the SD24 input specification. Why are two diodes used in series and not just one?

3. My observation is that diodes D44 to D51 together with the TVS diode (SMAJ5.0CA) provides ESD protection (high dI/dt or dV/dt pulses) and NOT overload protection? Is this assumption correct?

4. The TVS diode (SMAJ5.0CA) clamps at 5V. It is assumed that the TVS and D44 to D51 should complement each other to provide ESD protection AND am I correct to say that the protection will only be effective if  the L2 and L2 ferrite beads (omitted in the reference design) are populated? 

 

  • 0
    TI__Mastermind 49692 points

    Hello,

    Daniel Tomlinson said:
    1. Diodes D44 to D51 seem to be a clamping circuit to clamp the voltage across the burden resistor (R104) to the two supply rails AGND and AVcc. Is this assumption correct?

    Yes, that's correct.

    Daniel Tomlinson said:
    2. If question 1 is true, why then are there two diodes placed in series? The diodes used in the design are PMLL4148 which has a maximum forward voltage of 1V. By placing two diodes in series, the voltage drop towards a supply rail is 2V. The circuit will therefore clip at +5.3V or -2V, which is way beyond the withstand limit of the SD24 input specification. Why are two diodes used in series and not just one?

    Assuming that your max negative input reaches -930mV when your RMS current through the CT is 100A, a single diode with a max forward voltage of 1V would most likely start conducting under normal operating conditions, which is not ideal. This would explain why two diodes were used in series to increase the limit to 2V. Thankfully, the MSP430F6779 has ESD diodes that offer a small amount of protection to low, secondary over-voltage conditions.

    Daniel Tomlinson said:
    3. My observation is that diodes D44 to D51 together with the TVS diode (SMAJ5.0CA) provides ESD protection (high dI/dt or dV/dt pulses) and NOT overload protection? Is this assumption correct?

    I would agree with your assumption. If overload protection is a concern, I'd recommend using ferrite beads for R/L1 and R/L2 rather than resistors. Series resistors can offer low levels of ESD protection when combined with the clamps. The diodes at the I/O pins divert the ESD current to the primary clamp (between VCC and VSS), and only the residual stress appears at the device I/O pins. The decoupling capacitor at the supply pins helps limit the initial fast transients of the ESD pulse.

    Daniel Tomlinson said:
    4. The TVS diode (SMAJ5.0CA) clamps at 5V. It is assumed that the TVS and D44 to D51 should complement each other to provide ESD protection AND am I correct to say that the protection will only be effective if  the L2 and L2 ferrite beads (omitted in the reference design) are populated? 

    See my comments above. For more information about ESD protection, take a look at the following resources.

    Hope this helps!

    Regards,

    James

    MSP Customer Applications

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