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AMC1200: Have trouble with two reference designs

Damith Pavithra
Expert 1125 points
Part Number: AMC1200
Other Parts Discussed in Thread: AMC1100

I have trouble with following two circuit diagrams. they are from two reference designs for voltage measurement using amc1200. Two circuit diagrams are same except at the points which i circled. In one design, diode and one input terminal is connected to the ground. it uses 0-5V power supply to power the HV side . In the other design, diode is connected to the negative 5v and one input terminal is connected to the ground. It uses -5V-0- +5V supply. My question is, what is the difference between this two. Clearly one circuit design is wrong because all other factors are same except those points.

I obtained these circuits from tidua58  and  tiduby7a.

Thank you
 .   

  • 0
    Guru 140200 points
    Hi Damith,

    yes, you are right, the protection diodes must shunt overvoltages to the supply rails. If the input side of AMC1100 or AMC1200 is powered by a +5V singly supply, then the protection diodes must shunt overvoltages to +5V and GND. And not to -5V!

    Kai
  • 0 in reply to kai klaas69
    Expert 1125 points

    Thank you

    that means 2nd designis the right one ,ok?
    But for the first one, They also built a dual power supply . Are you extremely sure 2nd one is the right one. Because both designs are TI approved designs.
    I have another question. Is it necessary to connect one HV input terminal to the power supply ground terminal?.
    What is the point of that connection. ?

  • 0 in reply to Damith Pavithra
    Guru 140200 points
    Hi Damith,

    if the protection diodes are used to protect the AMC1100 / AMC1200 and if these chips are powered by +5V and GND, then the input voltage must be clamped to the supply rails, which is +5V and GND then. If it's clamped to -5V, then the AMC1100 / AMC1200 is in danger to be destroyed. I think the -5V is just a typo in the appnote.

    So, the second scheme should be the correct one. But what makes me wonder is, why the shunting of overvoltages to the supply rails is done by using two diodes in series. This looks weird and makes no sense to me. There should only be one diode from the input and the according rail, like shown in the first scheme.

    The input side of AMC1100 / AMC1200 is fully isolated from the output side. To avoid that the barrier isolation is damaged by too high differential voltages, the input ground and output ground must be connected to well know potentials. Otherwise static charge could develop across the isolation barrier and finally exceed the maximum isolation voltage and by this destroy the chip.

    Kai
  • 0 in reply to kai klaas69
    Expert 1125 points

    Thank you. Can you explain the static charge developing part. I didnt get it. How static charges develop?.  

    No it cant be a typo because they even built a dual supply just  for it. Two diodes are there because they used a diode package which have two diodes in it.

    .I want to know,

    Is it ok to use a general purpose diode like 1N4007 for this. ?

    Or is it ok to use no diodes because there is two zener diodes already in the second circuit.?

    I plan to power the HV side using a small smps-a mobile phone charger. So if one HV terminal is connected to negative terminal (ground) of the charger , will there be any complications. ?

    I have this doubt because i cant understand why it is necessary to connect one hv terminal to the negative terminal.

  • 0 in reply to Damith Pavithra
    Guru 140200 points
    Hi Damith,

    the AMC1100 and AMC1200 already contain internal ESD protection diodes from the inputs to the rails (+5V and GND). See figure 31 of datasheet of AMC1100. Clamping the inputs by the help of an external diode to -5V would not make any sense, because of the internal ESD protection diode which already clamps to 0V. The internal protection diode would be extremely stressed, long before the external protection diode opens to -5V. So, to me, clamping to -5V makes no sense at all.

    It's a good question why external protection diodes were added at all, although the AMC1100 and AMC1200 already contain a protection scheme. I think, it's done to strenghten the internal protection scheme against ESD, surge and burst. For this, the external protection diodes must be able to act fast. Have a look at the datasheet of DESD1P0RFW-7 which is used in the circuit:

    www.diodes.com/.../DESD1P0RFW.pdf

    You can see that it's a very fast acting diode with ESD specifications. So, the extremely slow acting dino diode 1N4007 should not be used in the circuit of appnote TIDUA58!

    Kai
  • 0 in reply to kai klaas69
    Expert 1125 points

    Ok now it is clear that -5V is not correct.

    However they made following dual power supply to power the high side. The design file is Tidua58.

    Thank you.
    What about my last question which is about using 5V mobile phone charger to power the high side.

  • 0 in reply to Damith Pavithra
    Guru 140200 points
    Hi Damith,

    as safety issues arise from this question, I would need to see the whole circuit to give a proper answer.

    Kai
  • 0 in reply to kai klaas69
    Expert 1125 points

    It is in the 19th page of this pdf under the title "2.2.3 Isolated Voltage Measurement Using AMC1200"

    I plan to use the charger for the High side power.

    tiduby7a.pdf

  • 0 in reply to Damith Pavithra
    Guru 140200 points
    Hi Damith,

    I really need to see the WHOLE circuit!

    Not knowing what you want to connect to the input side of AMC1200, which you intend to power by the help of your mobile phone charger, I would say, that it's not allowed to use the mobile phone charger for doing anything different than to charge the mobile phone. The mobile charger is not expected to have the output side being connected to a dangerous HV voltage!!

    Kai
  • 0 in reply to kai klaas69
    Expert 1125 points

    I plan to use measure 230V AC from this circuit. This is the whole circuit.
    If mobile charger is not suitable, Then what would be. 
    Thank you.

  • 0 in reply to Damith Pavithra
    Guru 140200 points
    Hi Damith,

    the safest way to power the HV side of AMC1200 is to use a tiny safety isolating transformer, like this one, e.g.:

    uk.rs-online.com/.../

    (The shown transformer is only an example! Maybe another power rating and another output voltage is better suited for your application.)

    Put a bridge rectifier and big storage cap to the output of this transformer and regulate the supply voltage of AMC1200 with a zener diode as shown in figure 36 of datasheet of AMC1200.

    Kai
  • 0 in reply to kai klaas69
    Expert 1125 points

    Thank you very much for the support. I built the circuit and it works.