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  #1  
Old 05-25-2011, 10:42 AM
abuster abuster is offline
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Default Just another mosfet AC switch thread

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).

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Old 05-25-2011, 10:44 AM
abuster abuster is offline
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And, the controlling pulse will be 5V from a micro controller.
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Old 05-25-2011, 12:00 PM
JDT JDT is offline
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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.
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Old 05-25-2011, 12:22 PM
abuster abuster is offline
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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?
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Old 05-25-2011, 12:52 PM
abuster abuster is offline
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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.
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  #6  
Old 05-25-2011, 03:22 PM
JDT JDT is offline
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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.
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File Type: png AC-MOSFET-DRIVER.png (138.2 KB, 372 views)
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  #7  
Old 05-26-2011, 08:05 AM
abuster abuster is offline
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Thanks. One step further to prototype
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