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Electrical Fast Transients

Satbir Sekhon
Intellectual 570 points
Other Parts Discussed in Thread: MSP430F2272

Hello all,

I am posting here to get some help regarding the Electrical Fast Transients(EFT), I am observing on the VCC line of my MSP430. 

I am using the MSP430F2272 along with N channel FET and a gate driver to power a solenoid valve. The current through the valve is being controlled by a closed loop current monitoring system. I have the proper decoupling Caps for the MCU along with Catch diode on the load side. But, I am seeing the transient as shown in the figure below on the 3.3 V regulated VCC line:

The transient rises up to 9.30-9.80 V in about 25ns, this make the MSP430 unreliable at times. 

What are my options?

Thanks in advance. 

  • 0
    Guru 227245 points

    You can:
    use bigger capacitors (well, only partly reliable),
    capacitors with less ESR (ceramic are good, but don't have much capacitance).
    Use a transil (suppressor) diode. E.g. the P6KE type
    Use a Pi-filter in VCC (capacitor/resistor/capacitor combo)
    Use an ESD protection element (basically the same as the transil diode, just smaller and only available for few voltages)

    Since the transients are fast, plain capacitor blocking is not very effective. The transil diode might help (in fact, it even suppresses voltage spikes of longer duration, as it can stand quite some current) bit I guess, teh PI-filter will serve you best. Of course all this depends on your exact application.
    Well, maybe a second catch diode in parallel to the existign one will give good results too. If the diode has to catch a high current, there might still be quite some momentary voltage arise despite of the diode.

  • 0 in reply to Jens-Michael Gross
    Intellectual 570 points

    Thanks for the quick reply Michael.

    I have tried to use a transil diode by ST, but it had no effect on the transient. 

    http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00070945.pdf 

    I believe that specific diode was not rated for the EFT, and is only good for ESD pulses. 

    I am looking for TVS diodes having a clamping voltage around 5  to 6 Volts, that are rated for EFT.

    Do we have any upper limit for the transient spike the MSP430 can handle?

    What i have observed so far is that a transient >9.80V resets my MSP430F2272. Any thing below that appears to be okay!! 

    Thanks Again.

    Satbir

  • 0 in reply to Satbir Sekhon
    Guru 227245 points

    Satbir Sekhon said:
    I have tried to use a transil diode by ST, but it had no effect on the transient. 

    Well, looking into the datasheet, it ahs its values specified for 1MHz and pulse rise times of 10µs, which is way beyond your 25nS pulse. Unfortunately, the P6KE isn't faster.

    The ESD5B5.0ST1G seems to be much faster. (response time <1ns and only 32pF capacitance). It clamps to 5.8V (5V permanent blocking voltage). However, it's almost to small to handle. (I had to solder 4 of them onto a wire-wrapped circuit just a couple od weeks ago)

    Satbir Sekhon said:
    Do we have any upper limit for the transient spike the MSP430 can handle?

    Yes, the MSp breaking point :) Bit it's not specified. The MSP is a tough guy, but can't stand everything.

  • 0 in reply to Jens-Michael Gross
    Intellectual 570 points

    Thanks for the suggestions Michael. 

    I just found out that I was making a wrong diagnostics. I was connecting the Scope's ground to main supply ground, and that is why i was seeing those huge transients on the scope. But when I connected the Scope's ground to my MCU GND plane ( Connected to the main GND at a single point), the huge transients disappeared. I could still see some transients but those were only up to 3.4 - 3.6 V. 

    Well, that at least tells me that my intentions to keep the digital and analog ground separated worked. 

    Thanks again,

    Satbir

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