Hi,

This seems to be a very useful component for driving the MOSFET when high voltage and full isolation is required.

Does anyone know the source or part number of similar component in U.K.? Would they cost about the same as common optocoupler?

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photov.gif (3.8 KB, 511 views)

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L.Chung

 

Isn't it just a dual opto-coupler?, they are commonly available, as are quad ones.

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Not quite. When the input LED lights, a voltage(5-10V) is produced across the output pins that can be use to drive the MOSFET directly.

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L.Chung

 

Err no. I am with Nigel on this, it is an opto and is used alot, esp in traction, and when the voltage that shall be required exceeds the isolation of opto's, custom XFMR are wound.


How would the 5V be generated? it cant.
For high voltage you cannot use MOSFETS (and when I say voltage I am talking of 1KV and above). For a fully controlled switch you have to use IGBT's or BJT's

These take current to turn on, with te BJT being a current-control device and teh IGBT being a voltage-control device - but it has a gate capacitance that needs to be charge => current (MOSFETS suffer from this as well)

Where current and voltage is concerned, power is always there already (or at least the potential for power to be utilised)

THIS has no means of transmitting power, just signal, ie it is an Opto.
LED on one side and photodiodes on the other.


Standard practice for isolation.

Look at CNY17 for an trasisitor output trasisotr or 6n137 (very good) for a TTL output

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"Photovoltaic" means getting electricity from light, and that's where the 5V comes from.

Don't understand your argument. A photovoltaic source/cell, at 5uA output current, can more than enough to supply the gate charging current of a MOSFET or IGBT.

I am not surprise that you have such a view, in fact I only know of this device today when I surf the net. The manufacturer place a led and a photovoltaic cell together so that "no external voltage source is required" in order to turn on the gate of a MOSFET or IGBT.

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photov1.gif (26.4 KB, 476 views)

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L.Chung

 

I know what a photovoltaic cell is

My point was MOSFET's and IGBT's as far as a driving cct is concerned are a capacitor that needs to be charged, thus current is needed.

I am working with an IGBT at work that has a gate capacitance of 190nF (bench static), it also has a massive millar capacitance.

Basically I have to blap this IGBT with 11A that decays in 700ns

from that it says it can source 5V at 5uA, since the switching speed of a MOSFET adn a IGBT is directly related to the current-source that feeds the gate 5uA isnt that significant


I am not nocking it, deffenently looks interesting and will look into it since opto's have their limitations that I am starting to come up againsts.

As to stockers of this?? dunno, ill contact our local IR rep abt these to get some more info

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A search on FindChips lists nine different suppliers of this line of chips, including a few very large distributors who probably also ship internationally.

 

Hi Styx, sorry for the bad language I used in which I subsequently amended while you are posting your reply.

I understand that your requirement for an IGBT driver is a very tough one but I'm not talking of fast turn On/Off but some other operations where additional power supply is not available to drive the MOSFET/IGBT. It would then be a "handy method" to use where speed is not a problem.

Yes, its very interesting but I don't seems to find any stores in UK selling the part or similar parts. Thus the original post. Please post back if you have any new findings. Thanks

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L.Chung

 

Thanks TekNoir. I now know it is priced at US$6.7 each for qty 1-9 which is quite expensive for hobby use. But in situation where no additional power source is available at the isolated output side, this is a certainly a reasonable price to pay.

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L.Chung

 

AS long as the 5V output is enough to turn the FET on and saturate it?
that is the other concern I have with this, otherwise goodgood

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Once a problem is realised, the rest is just details

 

When the gate is charging up, current reduces and output voltage increase to 9V, as in image below. Good for saturate the MOSFET, if time is not of concern.

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photov2.gif (12.4 KB, 443 views)

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L.Chung

 

true but I looked at the the datasheet and you are looking at 10s to 100s of us to switch, this means a FET might be in its active region for an extended period of time, the could be bad new if you are switching either significant amps or significant volt or worse both

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Nothing is impossible.
Once a problem is realised, the rest is just details

 

Of course we all know that it is "repetitive" slow switching pass the active zone which generates high dissipation and heat that kills the MOSFET or IGBT.

From the data sheet, a MOSFET with 1000pF gate capacitance takes 6mS to turn ON. If that speed kills the MOSFET, one way is to use several drivers in parallel to increase the gate drive current output and shorten the turn ON time.

The device is a not a magic solution for all applications but comes in handy in some special cases.

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L.Chung