Hertz

Those MOSFETs are perfect.

Now the only problem is delivering 20V to the gates on top of the 325V.

I would use pulse transformers to do this as they can do this and isolate the high voltage side from the low voltage side.

Is there a way of getting 240vac constant output because the pump stops if the voltage drops below 230vac

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short of boosting the DC-link.. no
either use a transformer on the input to raise the link a bit, or add a boost stage on the DC-link.

@ that FET... 400V blocking should be ok UNLESS you have a really poor layout and lots-o-stray inductance creaps into your power layout, then that 80V headroom is going to vanish!
There is already going to be a voltage overshoot at turn-off (even with the optimum layout you could do... cause it won't be the ideal) so just be careful. I would be tempted to go for 500V FET's but that is just me


depending on yr switching frequency a pulse-transformer (with simple rectifier & cap) will be good, otherwise a bit more in-depth drive will be needed (ie a DCDC with opt-driver)

Hero999 said:

Now the only problem is delivering 20V to the gates on top of the 325V.

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I thought it was +/- 20v? or have i read the data sheet wrong

Hero999 said:

I would use pulse transformers to do this as they can do this and isolate the high voltage side from the low voltage side.

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How would i do that?

Styx said:

I would be tempted to go for 500V FET's but that is just me

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They do a 600v verious aswell suppose this would be safer

yer take the 600V one (as long as the current is still OK).
NOTE FET's have a nasty (ie to calculate with) SOA, so do your homework before you commit. But that is just me I have managed to kill a 1200V diode (in a basic chopper configuration) from a 350V link because I was driving the IGBT too hard (trying to turn it on in 500ns with 600A flowing in the main coil... really should of been happy with 900ns) and there were a bit to much stray inductance around

at the post above
Yes a FET is driven with +/-20V at its gate... W.R.T its source. Have a look at the two FET's which are on the positive rail (ie their DRAIN's attached to the 325V bus). Assuming that the FET is on, the source and drain can be assumed to be shorted (yes in practice Ron and current means this isn't the case... but for illistration...). So if we now see that the source potential is the same as the drain potential (which is 325V), if we want to keep this FET conducting we need the Gate potential to be 20V above the source potential (ie 345V) IF we were to reference w.r.t. the low-side of the DC-link.

That is what isolated gate-drive cct are needed. Sure measure the gate potential w.r.t. to the low-side of the dc-link and you will see it at 345V. But using a transformer to provide the power (and the signal in some cases) means that you develope the 20V across that trasformer BUT it can "float" with the FET as it switches, everybody wins

Basically get a small pulse transformer which has enough blocking potential to cope with yr link (and then some...) and on the "floating" side rig up a small recitifer using Shokky diodes and a small cap (plus a gate terminating resistor + series gate resistor). On the primary side of said transformer some FET's that you then pass a square-wave two when you want to fire the FET

I have a cct somewhere, will find. Personally I prefer putting down a small 1W Newport DCDC with a FET opto-drive, alot simpler

Now im really confused (A diagram would help )

What are Shokky diodes? I cant find an referrence to them?

*schottky*

From what ive read on pulse transformers they amplify the wave input, If the wave input is 5v from the oscillator how does it produce the 345v that is needed to trigger the FET? Is it the winding configuration or does it need to be conected to the 325v source?

no and yes...
Well yes a transformer does "transform" the voltage/current to a higher/lower level, but that isn't what said transformer is used for in this case. It is needed to provide a floating/independent reference point (as well as isolation). A simple 1:4 pulse transformer is all that is needed (even a 1:2 depending on FET and XFMR drive ).

Yes the GATE of the upper FET's will be at a potential of 345V w.r.t the negative side of the link (when they are ON), but that doesn't mean you are going to be supplying 345V to said gate.


So take figure1. This is a crude setup of what you want. The FET drives are almost what you want (don't go exact, there are bit missing... personally I prefer DCDC+Opto over XFMR but thats just me)

Anyway the potential of the Gate of FET driven by G1 will be at 345V w.r.t. to 0V1, however... with respect to its source it will only be 20V.
That is what the XFMR in the gate-drive is doing, it is allowing a local supply (between gates & sources) to float to whatever potential it has to BUT still providing a potential across the secondary of the XFMR (and thus the FET's gate&source of 20V (if designed such).

Try not to think of voltage w.r.t. to a common voltage, think of them w.r.t. whatever at the time of query, then you can see THAT as long as a FET's gate potential (w.r.t to its source) is ... +20V then the FET will turn on. The source may be at a completly different potential w.r.t. another voltage reference point, but w.r.t. the gate is all the FET cares about.


Also I have put in an isolated gate-drive for the lower two FET's as well. Some may argue it is a waste since 0V1 and 0V2 could be the same potential so why bother with the isolation and just simplify the design ... HOWEVER I will say this now that 0V1 is gonna be noisy and you really don't want that noisy polluting yr control rails, so don't pollute it

I understand now

Thanks for all your help, Ill do a schematic and post it up

tha attached PDF is a schematic of what i have so far Please tell me if there is anything wrong

Styx;

The oscilator in your diagram, is the 0 and I0 0v and +5v respectivly or -5v and +5v also what frequency does this need to run at?

Does G1 need to be a + or - trigger?

I'm sorry, I know this isn't helpful but it just looks worng, unfortutnately I don't have time at the moment to tell you what's wrong with it at the moment as I need to go now.

1) As stated the "gate-drive" I sketch won't really work - it was ment as a concept (it should have a couple of IC's really), ie how to get a floating voltage to teh gate (as it stands it suffers from slow turn-off. As I said I don't really like transformer-based gate-drives (hence why no real detail in that quick sketch).
I much prefer just dropping down say a: HCPL-3140-000E (opto gate-drive) with a IE0515D (5V in, 15V out isolated DCC converter). Add a couple of cap's across the output of the DCC (6uF and a 100nF), add a 200R gate resistor with gate terminating resistor and you have a very capable gate-drive

2) Again with the gate-drive, you have a local-cap of 100uF, it definitly won't work with that! - if you were going for the XFMR isolated gate-drive, said cap needs to be small to enable fast switching times (few nF). A fully-ON FET won't draw hardly any current through its gate and that cap is more just to smooth out what the rectifier pushes out

3) yr main AC rectifier really should have a transformer on it to provide isolate for safety

4) there is no DC-link capacitor for this converter, as it stands yr DC-link will follow a fully-recitifed profile. IE humps. Put a big capacitor down (NOTE ripple-current rating depending on how much current you will be sinking into load)

5) the FET's need reverse diodes otherwise the whole thing will blow up with an inductive load

Thanks for the diagnosis heres a new schematic

johnl69 said:

Thanks for the diagnosis heres a new schematic

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So will this one work or have i missed something?

That still looks wrong to me.

What's the purpose of the potential dividers and capacitors on the MOSFETs' gates?

What's T2? Is it an isolation transformer? If so good idea, well done!

There's no filter capacitor on the 240V rectifier.

Hero999 said:

That still looks wrong to me.

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Its lacking some finite details but concept-wise it is virtually there

Hero999 said:

What's the purpose of the potential dividers and capacitors on the MOSFETs' gates?

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At a guess it would be a series gate-resistor to control the charging of the FET and a terminating resistor to pull the gate down to its source
a 1k & 1k values however are wrong.
A 200R for the series resistor and maybe a 10k from gate-source maybe

Hero999 said:

What's T2? Is it an isolation transformer? If so good idea, well done!

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Yes, but he is gonna have to check the VA rating and all that jaz\

Hero999 said:

There's no filter capacitor on the 240V rectifier.

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THAT is definitly needed, no questions asked at all

The opto-coupler gate drives and DC-DC converters are incorrectly connected to the MOSFETs.

It looks like he's connected the output from the DC-DC converter to the gates via potential dividers with capacitors from the gates to sources.

He wants to connec Vcc and Vee on the gate drive opto-isolator IC to the + and 0V of the DC-DC converters, the gate drive outputs to the gates (a 10hm: resistor or ferrite bead might be required here) and the LEDs to the PIC's outputs.

Ive got to admit that most of this is confusing me I have done a bit of electronic before but usually same simple timer and switching circuits and nothing with mains.

The values on the schematics are just the standard one that are in the programming im not sure what values they should be.

these are the dc-dc converters that i was going to use **broken link removed** I took the pin configurations from them.

Hero999 said:

the gate drive outputs to the gates (a 10hm: resistor or ferrite bead might be required here) and the LEDs to the PIC's outputs.

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This bit i dont understand