All About Circuits Forum How can I float the low side gate of a half bridge?
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#1
01-30-2007, 08:12 AM
 arvin New Member Join Date: Jan 2007 Posts: 5
How can I float the low side gate of a half bridge?

I'm trying to have an N- channel mosfet half bridge work between negative and positive voltages (say -50v and +50V).

If I am not wrong I need around -40 V to trigger the low side mosfet ( for
Vgs = 10)
However when I check the IC s the only side mentioned to be floating
is the high side.
I was able to float the low side using a gate transformer but I wonder if there
is a discrete method to do it using IC s ( ex ir2113 etc)

thanks
#2
01-30-2007, 03:54 PM
 beenthere Senior Member Join Date: Apr 2004 Location: Missouri, USA (GMT -6) Posts: 15,815 Blog Entries: 10

Hi,

The accepted standard is to run the lower leg grounded and all the voltage on the upper. Is there any reason you can't reconfigure your source voltage to run between 0 & 100 volts?
#3
01-30-2007, 08:29 PM
 dragan733 Senior Member Join Date: Dec 2004 Location: Macedonia Posts: 154

Quote:
 Originally Posted by arvin I'm trying to have an N- channel mosfet half bridge work between negative and positive voltages (say -50v and +50V). If I am not wrong I need around -40 V to trigger the low side mosfet ( for Vgs = 10) However when I check the IC s the only side mentioned to be floating is the high side. I was able to float the low side using a gate transformer but I wonder if there is a discrete method to do it using IC s ( ex ir2113 etc) thanks
You can drive directly the low side with a zener diode between GS, for example 12V. Anode of the zener to the G. Or send your schematic with purpose to be clear.

Last edited by dragan733; 01-30-2007 at 08:39 PM.
#4
01-31-2007, 11:19 AM
 arvin New Member Join Date: Jan 2007 Posts: 5
I use rectified ac

Quote:
 Originally Posted by beenthere Hi, The accepted standard is to run the lower leg grounded and all the voltage on the upper. Is there any reason you can't reconfigure your source voltage to run between 0 & 100 volts?
First of all thank you for the messages !

I hope I don't have fundamental flaws

I use a mono phase ac supply which gives + and - when rectified.

............----->|------|---------- ..+volts--\
...........|..................= ...........................mosfet
...........|..................=................... ........mosfet
............-----|<------|---------- ..-volts--/

I'd like to use the whole voltage range,how can I drive the lower mosfets ?

thank you

Last edited by arvin; 01-31-2007 at 11:25 AM. Reason: messed up figure
#5
01-31-2007, 01:34 PM
 beenthere Senior Member Join Date: Apr 2004 Location: Missouri, USA (GMT -6) Posts: 15,815 Blog Entries: 10

Hi,

If that's a transformer and bridge rectifier, all you have to do is use the negative output as the ground reference. The transformer isolation makes this possible. In effect, you will have a +100 VDC source. Just leave the transformer's center tap lead disconnected. Then you can hook up your half bridge in the "conventional" fashion.

Kind of makes you wonder why Mr. Edison fought so hard against AC power?
#6
02-01-2007, 03:36 PM
 arvin New Member Join Date: Jan 2007 Posts: 5

Quote:
 Originally Posted by beenthere Hi, If that's a transformer and bridge rectifier, all you have to do is use the negative output as the ground reference. The transformer isolation makes this possible. In effect, you will have a +100 VDC source. Just leave the transformer's center tap lead disconnected. Then you can hook up your half bridge in the "conventional" fashion. Kind of makes you wonder why Mr. Edison fought so hard against AC power?
Hi again,

I am still confused.
If you could please check out the figure 1 on page
http://www.elektrorevue.cz/clanky/2004/0014/

you'll see the positive and negative rails of 270 / -270 volts.
There it says use galvanic isolation and shift the gate drivers ground to
-270v.
I am not so sure how I'm going to apply it to a gate driver ic like ir2113
Vss and com, are they going to have the low rail voltage and Vdd is it going to be the low rail +15 volts ?

I hope the question is clearer

thanks

Last edited by arvin; 02-01-2007 at 03:38 PM. Reason: correction
#7
02-01-2007, 06:25 PM
 beenthere Senior Member Join Date: Apr 2004 Location: Missouri, USA (GMT -6) Posts: 15,815 Blog Entries: 10

Is this a school assignment? If so, I'll move it to the homework help section where it will get better exposure.
#8
02-01-2007, 07:13 PM
 arvin New Member Join Date: Jan 2007 Posts: 5

Quote:
 Originally Posted by beenthere Is this a school assignment? If so, I'll move it to the homework help section where it will get better exposure.
sure ..

thanks
#9
02-02-2007, 05:41 PM
 beenthere Senior Member Join Date: Apr 2004 Location: Missouri, USA (GMT -6) Posts: 15,815 Blog Entries: 10

That is an odd circuit, in that a bridge driver normally changes the path of the current rather than the voltage polarity.

If I had to do this, I would make up two small power supplies to run the driver circuits. The one for the negative leg would use the negative voltage as its ground reference; the other would use convertional ground and run a floating gate driver. I would use optocouplers to pass drive signals to the driver circuits.

I would prefer to use a conventional H bridge design, though. The hardware is readily available, and there is only one voltage to avoid.
#10
02-02-2007, 06:18 PM
 arvin New Member Join Date: Jan 2007 Posts: 5

Quote:
 Originally Posted by beenthere That is an odd circuit, in that a bridge driver normally changes the path of the current rather than the voltage polarity. If I had to do this, I would make up two small power supplies to run the driver circuits. The one for the negative leg would use the negative voltage as its ground reference; the other would use convertional ground and run a floating gate driver. I would use optocouplers to pass drive signals to the driver circuits. I would prefer to use a conventional H bridge design, though. The hardware is readily available, and there is only one voltage to avoid.

Hi,

Please correct me if I'm wrong. I've been surfing on the net to see if there are similar cases (+inverters +igbt +negative +rail) What I've found out is almost all the industrial ac motors rectify three phase voltage and they connect the mid point to the ground. It is kind of resonable otherwise you would only get the half of the rectified voltage, wouldn't you. Yet for all those industrial applications I dont think they have power transformers to shift the voltage. So its strange that there is no mention of driving those low side igbt's which run under the negative voltage. It seems like I'm missing something but what

 Tags bridge, float, gate, half, low, side

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