Just another mosfet AC switch thread

Discussion in 'General Electronics Chat' started by abuster, May 25, 2011.

  1. abuster

    abuster Thread Starter New Member

    Joined:
    May 24, 2011
    5
    0
    Hi there!
    I've used days searching and reading forums for finding a good solution to switch AC with mosfets in totem, source-to-source. TRIAC is not an option, as I want to switch of in other times than phase-over(PWM, reverse phase dimmer, etc). SSR are overkill, and to expensive.

    Like many others, I stranded upon the problem with the floating source, and found solutions with a photovoltaic optocoupler. But, the photovoltaic optocouplers available from my supplier are pretty expensive.

    So, I made a circuit with LTspice, using a rectifier and a optocoupler with transistor output. That way, I can feed the mosfet gate with a positive/negative pulse.

    The cheap optocouplers usally have about 10+us on/off time, so that will be a limitation with high frequency switching(for example 100khz PWM).

    This seems to work, but I wondered if any of you had real life experince with this type of setup?

    The application will be fed with 12VAC. The load will be max 50-100W resistive. The models used are random, and I will adjust that to my application(Rds on, max current, max gate-source/drain-source voltage, type of optocoupler, etc).

    [​IMG]
     
    #1
  2. abuster

    abuster Thread Starter New Member

    Joined:
    May 24, 2011
    5
    0
    And, the controlling pulse will be 5V from a micro controller. :)
     
    #2
  3. JDT

    JDT Well-Known Member

    Joined:
    Feb 12, 2009
    658
    83
    The simple way to do this is by connecting two N-channel mosfets in series as shown.

    This works because fets are actually bi-directional. A positive voltage on the gate (with respect to the source) switches the channel to a low resistance regardless of the direction of current in the channel. So if you choose a fet with a sufficiently low on resistance, the body diode on the reversed fet will not conduct. This can make a very low voltage drop AC switch.

    I have used this circuit to switch 24VAC 250W halogen lamps with high efficiency.

    Be careful though: Make sure that the voltage on the gate is either zero or sufficient to switch full-on. Even during power up. Or your fet will be quickly destroyed. I know from experience!

    Another complication: Your drive circuit will probably have to be isolated from your micro-controller. As the sources are probably at some unknown AC voltage. A small floating power supply or DC-DC converter might have to be used. Unless your complete micro-controller circuit is attached to the sources.
     

    Attached Files:

    #3
  4. abuster

    abuster Thread Starter New Member

    Joined:
    May 24, 2011
    5
    0
    Thanks for sharing your experience.

    As you say, the complication lays in ground level, and floating mosfet source.

    The way I planned to do it, was to set one of the AC phases as common ground(the circuit will be totally isolated from touching), and have +/- DC floating from that level. That way, the sources at the mosfets will float to a minus voltage when the body diode of the upper mosfet is forwarded. As you state, this will potentially burn the mosfets, if the gate is not at a lower voltage.

    A problem with this, might be that gate-source voltage gets to high. When not conduction, we have potentially -17V(minus body-diode drop) amplitude plus the +/-17 voltage.

    Do you have any schematics how you made it work with isolated DC on the micro-controller?
     
    #4
  5. abuster

    abuster Thread Starter New Member

    Joined:
    May 24, 2011
    5
    0
    Actually, as my simulation shows, the sources are pulled against ground level when load is above the mosfet totem and gate rising from -16V. Maybe the optocouplers rise-time is enough to get a smooth transistion, and not blow the mosfets because of the +-20V gate-source voltage limitation.
     

    Attached Files:

    #5
  6. JDT

    JDT Well-Known Member

    Joined:
    Feb 12, 2009
    658
    83
    There are many ways of doing this but consider that

    • The MOSFET must always be fully OFF or fully ON
    • The gate has a high capacitance, so to switch the transistor fast the driver must be able to sink and source high currents.
    • There needs to be an under-voltage lockout so that the transistor can never be partially on when the supply voltage is low.
    I suggest using a mosfet driver IC. It's all on the chip. An example driver circuit is attached. I have used an opto-coupler and a small DC-DC converter. Gate current is only needed when switching so only a small supply current is needed. It is possible to get the supply current from the AC supply itself using diodes and resistors.
     

    Attached Files:

    #6
    cmartinez likes this.
  7. abuster

    abuster Thread Starter New Member

    Joined:
    May 24, 2011
    5
    0
    Thanks. One step further to prototype:D
     
    #7
  8. domingojordan

    domingojordan New Member

    Joined:
    Aug 11, 2014
    17
    0
    JD, thanks for this useful information. I am not a technical person. I am going to abuse by asking to give me more details in the way to connect two N channel mosfets to switch AC current. In the diagram that you posted where is the input AC and how can I get the driving voltage and current for the mosfet gate. I am sorry about my ignorance but I can not figure out how is that done when it alternates. Thanks for the help.
     
    #8
  9. Papabravo

    Papabravo AAC Fanatic!

    Joined:
    Feb 24, 2006
    7,794
    951
    The two MOSFETS are placed in series on the hot lead. DO NOT under any circumstances connect the MOSFETS across the hot and neutral, in parallel with the load. This is similar to the mistake of trying to measure current with an ammeter by placing the leads across a resistor.

    The driving voltage for the gates comes from the DC-DC converter that produces a +12V output to the MC34151 gate-driver(pin 6). The input to the DC-DC converter was not specified.
     
    #9
  10. crutschow

    crutschow AAC Fanatic!

    Joined:
    Mar 14, 2008
    8,630
    1,730
    Below is a simulation of an SSR circuit using a 4N25 opto isolator and two back-to-back N-MOSFETs to control the load. The switching looks good at a 25kHz control frequency and 60Hz AC power frequency. The input is isolated from the output (Rsim is just to avoid a floating node error in the simulation). Power for the MOSFET drive circuits is generated using a half-wave rectifier from the 12Vac supply.

    You may have to add a driver if you want to drive the opto from a micro.

    SSR.gif
     

    Attached Files:

    Last edited: Nov 16, 2014
    #10
  11. cmartinez

    cmartinez AAC Fanatic!

    Joined:
    Jan 17, 2007
    1,911
    614
    Excellent circuit crutshow! could you please share the LTspice file for that circuit?
     
    #11
  12. crutschow

    crutschow AAC Fanatic!

    Joined:
    Mar 14, 2008
    8,630
    1,730
    I added the LTspice .asc file to post #10.
     
    #12
Loading...