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Author Topic: Having probles biasing FETs in an inverter circuit.  (Read 1047 times)
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Ciao!

Im trying to build an small powered inverter. Heres a picture. Sorry for the quality i was in a hurry and my computer kept crashing so had to use paint for finish smiley-grin

The problem is i dont get much current for my load. In the picture R1 = R2 = 50k. The DC source is 12 volts.  So vg = 5-6 volts for every powerfet. For somereason i think i still dont get them to saturate or something?
What might be the problem?

Or is there an easier way to build one?





* arduinopict.png (22.67 KB, 739x401 - viewed 36 times.)
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You arnt turning the hi side fets on properly.
You need at least 10 volts between the gate and source to turn the fets on, and that means that if you only have a 12 V supply
you need at least 22 volts to turn on the hi side fets.
Google H bridge driver to find a suitable driver chip for your circuit.

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You arnt turning the hi side fets on properly.
You need at least 10 volts between the gate and source to turn the fets on, and that means that if you only have a 12 V supply
you need at least 22 volts to turn on the hi side fets.
Google H bridge driver to find a suitable driver chip for your circuit.



So what your saying is that i need to apply 22 volts at the gate of the upper mosfets ?
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Yes, the hi side fets dont their sources grounded, so the gate voltage has to be higher than the supply voltage, by whatever is needed to ensure
that the fet saturates properly.
As well, you have to turn all the fets on and off quickly which means that the gate source capacitance has to be charged and discharged quickly
and as well you have to prevent shoot through all of which a h bridge driver does for you.
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Yes, the hi side fets dont their sources grounded, so the gate voltage has to be higher than the supply voltage, by whatever is needed to ensure
that the fet saturates properly.
As well, you have to turn all the fets on and off quickly which means that the gate source capacitance has to be charged and discharged quickly
and as well you have to prevent shoot through all of which a h bridge driver does for you.


Okay thank you, that was really helpful!
Im not sure how im supposed to choose the right driver, tho.
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How about some part numbers for what you are using?
What is the circuit intended to do?
"small powered inverter" doesn't really say.
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How about some part numbers for what you are using?
What is the circuit intended to do?
"small powered inverter" doesn't really say.

Its going to be a low power prototype for a microinverter. I am using a DC supply to produce 12 volts (it is representing photovoltaic cell). For MOSFETS im using IRZL 44's (DATASHEET: http://skory.z-net.hu/alkatresz/irfz44.pdf).

At the moment im controlling the MOSFET's gates with my arduino as seen in the picture of my first post.

I have a SM72295 gate driver at my posession (http://www.ti.com/product/sm72295), but im not really sure how to use it...
[EDIT] BTW does that SM72295 need any programming or is it ready to go... Im new to this so excuse my dumb questions =).

Im really banging my head in the wall here, so i am really appreciating you guys for helping me out  smiley
« Last Edit: July 26, 2012, 01:02:00 am by houpsi » Logged

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So you want to take 12VDC and chop it up using N-Channel MOSFETs to look like 0-12V at some frequency?
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Really need more Information.
What is the Inverter powering.
What is the ultimate output voltage, and power capability.
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So you want to take 12VDC and chop it up using N-Channel MOSFETs to look like 0-12V at some frequency?

For starters yeah. 50Hz.  After i get that one working, im going to introduce a transformer 12/230 VAC.

EDIT: Its going to power somekind of test load, such as UPS or something.
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You need to think this whole project through more thoroughly.
Its important to know what the Inverter is going to power as feeding a square wave into a 50 HZ transformer
will not produce a very friendly waveform.
Typically, small Inverters for converting 12 V to 230 V dont work like what you are designing.
Usually, the 12 V is stepped up to 310 V using a push pull Inverter running at around 25 KHZ using a ferrite core
transformer, and then the output is rectified to produce 310 V DC , which is fed into a high voltage H bridge
designed to produce a MSW (modified square wave) output.
Even then, its important to know what the Inverter is going to power, as some devices dont like square wave power.

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I think what you were after is this as a starting point. Prior comments would still apply.


* H-bridge_boost_inverter.jpg (55.64 KB, 960x720 - viewed 31 times.)
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That original circuit is crazy - 50k resistors driving the gates?  That's going to switch absurdly slowly - most of the power will end up in the FETs rather than the load!

Several things are important with FET bridges - (1) high and low sides of the same arm must never be on at the same time (called shoot-through this is extremely bad(*) unless there is some current limiting involved, in which case its just bad).

(2) The devices must switch fast enough to avoid wasting power.  A good ballpark might be switching time < 0.5% of cycle time.  During switching the devices dissipate upto as much power as the load, this will dominate the power consumption of the MOSFETs unless switching time is much much shorter than the cycle time.  The MOSFET is supposed to be a switch, not a heater!

(*) bad as in exploding MOSFETs and burnt out circuitry.
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That original circuit is crazy - 50k resistors driving the gates?  That's going to switch absurdly slowly - most of the power will end up in the FETs rather than the load!

Several things are important with FET bridges - (1) high and low sides of the same arm must never be on at the same time (called shoot-through this is extremely bad(*) unless there is some current limiting involved, in which case its just bad).

(2) The devices must switch fast enough to avoid wasting power.  A good ballpark might be switching time < 0.5% of cycle time.  During switching the devices dissipate upto as much power as the load, this will dominate the power consumption of the MOSFETs unless switching time is much much shorter than the cycle time.  The MOSFET is supposed to be a switch, not a heater!

(*) bad as in exploding MOSFETs and burnt out circuitry.

Ah, okay.. The slowliness is due to the RC-timeconstant?
The reason those biasing resistors were so big was that i was trying to minimize the power going to the biasing (P = U^2 / R). I totally forgot the gate-source-capacitance and didnt even think about it..
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