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SE555: Same value charging capacitors, different packaging, different results!! Why?

MShakibM
Prodigy 10 points
Part Number: SE555

Tool/software:

Hello Everyone,

I'm trying to make a PWM signal for my MOSFET that is driving a heater.

I'm using the  "Pulse Width Modulation Control" topology from electronics tutorial. 

I designed the PCB and assembled all the components. For my application I wanted a 100mHz signal and based on my potentiometer size, I calculated and simulated (in LTSpice) that the discharging capacitor should be 22u. 

Now if I use a 2220 packaging capacitor of 22u 50V 20% and X7R dielec, I'll get the same results as the one I anticipated and simulated. But when I'm using a 0603 packaging capacitor of 22u 25V 20% and X5R dielec, my frequency gets to 300mHz.
I triple checked my circuit and it's components, and all the values I soldered are 100% correct (I desoldered, measured the vale and then  resoldered again) but I don't know the reason for the increase in frequency. Only the capacitor is changing and everything else is the unchanged.

Can it be because of ESL? but my frequency is not high though, so it shouldn't matter!

  • 0
    TI__Intellectual 2690 points

    Hi MShakibM,

    Have you measured the actual values of the two timing capacitors? The timing capacitor plays a significant role in the timing characteristics of the output, and since both capacitors have 20% tolerances, it could be the real values of the capacitors are the reason for the changes in output frequency.

    Best Regards,

    Alex Curtis

  • 0 in reply to Alex Curtis
    Prodigy 10 points

    Hi Alex, 

    Thank you for your reply. Yes! the differences are not alot. One is 21uF and the other is 20uF.

  • 0 in reply to MShakibM
    TI__Intellectual 2690 points

    Hi MShakibM,

    Have you tried swapping the X5R capacitor for a different unit? Given that the output timing matches what you expected with the X7R capacitor, it sounds like the X5R capacitor is the issue.

    Best Regards,

    Alex Curtis

  • +1 in reply to Alex Curtis
    Prodigy 10 points

    Hi Alex,

    Yes, I changed the 22u 25V 20% and X5R with 2.2u 50V 10% and X5R (the value and the max voltage stress changed). I anticipated with the new cap to see a 1s period or a 1Hz frequency plus minus the effect of the tolerance of the cap. To my surprise it completely worked. I saw 1.2Hz in the scope.

    I searched a little and found that capacitors have a voltage coefficient called VCC and although most manufacturers do not mention it in their datasheet it exists. The voltage supply connected to my 555timer is 12V which is almost half of the rated voltage of the  22u 25V 20% and X5R Cap. But when I changed it to any capacitor with the rated voltage of more than 50V, the system started working correctly.

    VCC or Voltage Coefficient of Capacitance is a decrease in capacitance under applied DC voltages that happens in in ClassII and Class III MLCCs. VCC is often noticed while operating close to the rated voltage of the MLCC or where high capacitance is needed in the circuit. VCC is a result of the contruction of the MLCC. Class 1 multilayer ceramic chip capacitors (MLCCs) are characterized by their small capacitances, low dependencies, and small tolerances. They are made from titanium oxide or calcium zirconate, a dielectric material that is stable across temperatures. Class 1 MLCCs are also known as NP0 or C0G. All the X family (X7, X6 and so on) are either class 2 or 3.

    Please let me know if this makes sense. This is the only justification I can have for what I'm observing.
  • +1 in reply to MShakibM
    TI__Intellectual 2690 points

    Hi MShakibM,

    Thanks for your post! Apologies for the delay in getting back to you.

    I think what you've stated makes sense regarding different grades of capacitors impacting the performance of the device. As you've mentioned, it's best practice to stay well below the rated voltage of a capacitor. We usually recommend using C0G/NP0 ceramic timing capacitors as they tend to have the best temperature coefficients, voltage modulation, and aging characteristics, resulting in more precise timing. 

    If you'd like to learn more, we have some great FAQ posts that have a lot of useful information on designing 555 timer circuits. Specifically, I'd recommend reading the accuracy section in the astable FAQ. 

    Best Regards,

    Alex Curtis