Slow optocoupler responce

Discussion in 'General Electronics Chat' started by Dritech, May 9, 2016.

  1. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
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    Hi,

    I am using the attached H-bridge circuit to drive a 12V 5Amax motor. I realised that at 1kHz clock I am getting a falling time at the output of the optocoupler of around 50us. The datasheet of the HCPL-0454-000E states that the maximum Propagation Delay Time to Logic Low at Output is 0.5us at 20kHz clock. So why am I getting a 50us Propagation Delay Time to Logic Low at Output?

    Thanks in advance.
     
  2. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    hi dri,
    Considering Q4/5
    When the opto detector transistor is switched On, the charge on the internal Q4/5 Gate capacitances is fast, but as the opto switches Off, the only Gate discharge path is via the 4k7 [R1]

    So on one half cycle you have an 'active' discharge path and the other half is a 'passive' discharge path.

    E
     
  3. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
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    Thanks for the reply. How can I solve this? should I connect a resistor with a value larger than 4-7kohms between the gates and ground?
    BTW I tested the output at collector of the OPTO not at the h-bridge output terminals.
     
  4. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    hi,
    If you add a resistor to 0V from the Gates to Gnd, the Top MOSFET could be always be conducting, depending upon the Vgs and resistor value.
    I would not try to solve the problem in that way.

    You need to add a transistor such that the P MOSFET Gates are 'actively' driven.

    This image, is a LTSpice sim, showing the Vgate voltage for the PMOS

    E
     
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  5. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
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    Is there a simpler way to solve the problem? Unfortunately I only noticed the problem after designing and ordering the PCB :(
     
  6. DickCappels

    Moderator

    Aug 21, 2008
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    The risetime will be proportional to 1/R of the pullup resistor. If you can drive a 1k load that would get your risetime down to about 10 us
     
  7. Bordodynov

    Active Member

    May 20, 2015
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    See Draft.png
     
  8. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    Being new to all this I may have an over simplified take on things. But this is another case of not using the "corect" device for isolation and control of mosfets or H-bridges. Can't understand why people are so set against using half bridge mosfet drivers when they need 'isolation' from logic control to "high voltage" switching of mosfets. Most all of the half bridge drivers give ~600V isolation, and make the high side switching easier. Can some one explain the reason people resist using them? Instead of opto-isolators and extra components.
     
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  9. ronv

    AAC Fanatic!

    Nov 12, 2008
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    I would have expected it to be slow in the other direction.
    But having said that, I think you are way over driving it. Try it with 15 ma.
    @shortbus
    The optos have physical isolation the drivers do not.
     
  10. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    If not physical, it is still electronic isolation. They have been in use without any problems that have been reported that I know of or seen reported. There are probably millions or more of them in use in motor drives, VFD's and other high power high voltage devices.
     
  11. ronv

    AAC Fanatic!

    Nov 12, 2008
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    It's maybe more important where things communicate with the outside world. Like telephone and power lines where spikes can be quite high and a short from line to device can cause damage or is dangerous.
     
  12. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    All well and good, but think the O/P is driving a "12V 5Amax motor" from the first post. And using your parameters, even an opto coupler/isolator wouldn't save anything but the mosfets and motor. All of the logic/uc parts that connect to the outside world would still fry/smoke.

    So in the end your saying that the use of high voltage half bridge drivers is a waste? And they can't, won't live up to the ratings given them by the manufacturers? Don't understand why or how this can be?
     
  13. ronv

    AAC Fanatic!

    Nov 12, 2008
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    No, I didn't say that at all.
    You ask the question (Can some one explain the reason people resist using them? Instead of opto-isolators and extra components.) I just gave you an answer.:D
    I wouldn't use opto in the OPs application, but there is nothing wrong with it either.
    Again:
     
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  14. Sensacell

    Well-Known Member

    Jun 19, 2012
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    Here is my general take on the situation:

    A) People don't understand the high currents required to rapidly charge / discharge MOSFET gate capacitance. The general mythology is "MOSFET gates draw no current"

    B) People are not familiar with the wide range of gate driver chips available.

    C) People are not familiar with how relatively slow most opto isolators are.
    They use opto couplers because they see them used in other bad gate driver designs.

    D) Most MOSFET bridge circuits floating around on the web are poorly designed, leading people to replicate these bad designs.

    Rather than exploring chipmaker websites and data sheets, people are morbidly drawn to wonky diagrams floating around on the web, this misinformation is considered less intimidating I guess?

    Perhaps a tutorial on MOSFET gate diving is in order?
     
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  15. dannyf

    Well-Known Member

    Sep 13, 2015
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    You should really think of the MOSFETs as capacitors. Charging them up and discharging them takes time / current.

    Before you attempt to solve the problem, you should ask yourself if 50us is fast enough for your application.

    If it is not, one simple way is to pair the option with a current amplifier.
     
  16. MrAl

    Well-Known Member

    Jun 17, 2014
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    Hello there,

    Yeah opto isolators can be a pain sometimes. Their output drive capability is limited and the speed sometimes isnt quite what we would expect after reading the data sheet.

    The solution would have been to use an IC chip that is made for driving MOSFETs. That way we would get fast turn on and turn off times and the implementation is very simple.

    A solution that might work here given that the PC board is already made and so this will be basically a hack of the board, is to lower the value of R1 (and similar resistors) like had already been suggested, and also to get more drive current you can "piggy back" two opto couplers one placed right on top of the other and soldered in place. This will provide more drive current. Unfortunately, it would be better if the input LEDs to each opto are driven separately, so if you can solder all but the anode of the second opto then you can supply a separate resistor to drive that second LED. With two optos you get twice the output current and so about twice the speed you would get with just one.

    Alternately you could look around carefully for a replacement opto coupler that has higher drive current. These may or may not be available.

    Another alternative is to build an SMD circuit on a perf board with the same pinout as the original opto coupler IC, then use that as the "opto coupler" even though it would be an opto plus SMD MOSFET driver or transistor set. That would be a separate circuit mounted on a very tiny board that comes out looking like a DIP package, assuming you are not using SMD packages already. SMD parts are very small so you can fit a lot on a tiny board that is the size of a DIP package.
     
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  17. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    This is the reason for my posting to this question. I still don't understand what RonV means about the "outside world" thing. The high voltage gate drivers, at least in my pea sized brain have isolation of a 'sort'. According to how I read their data sheets. Most of them say they provide isolation to ~25V OVER the mosfet drain voltage. And most of them are rated at ~600V. How much more "isolation" is needed? Don't even opto- isolators have a "voltage creep" value in their data sheet?

    I know that overall I'm not as intelligent as most of you on this stuff but since the project I started learning electronics for is ~100VDC, this has been my focus, "high voltage(at least for electronics), high side switching". I don't want to make a dumb or even fatal mistake.
     
  18. MrAl

    Well-Known Member

    Jun 17, 2014
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    Hi there,

    He was probably talking about electrical isolation, not static isolation. Electrical isolation is also known as "galvanic isolation". This basically means that any two parts of the circuit that are galvanically isolated do not have ANY common voltages. This is like a transformer for example where the primary has no common with the secondary unless we connect one lead of the primary to one lead of the secondary or connect something between them. Optos can have galvanic isolation up into the thousands of volts which basically means the input LED is a separate part from the output transistor or logic gate.
    With most circuits that are isolated in this way you cant get a shock from the output like you can with no isolation or just static isolation. Touching the input and output at the same time should not give a shock, although i would not do it anyway :)
     
  19. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    Thank you MrAl.
     
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