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  #1  
Old 02-09-2008, 06:59 PM
Marie Marie is offline
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Default Optocoupler and gate drive

Hi,

I've built an optocoupler circuit to isolate the control circuit from the power circuit.

1) The optocoupler circuit is built using 6N137.

2) The power circuit consists of a MOSFET driving a resistive load. The gate drive I used is IR2111.

The optocoupler circuit works well on its own. And the gate drive circuit also works well stand alone, i.e., i use the signal generator as input. However, when I connect the optocoupler output to the gate drive input, I am unable to switch on the MOSFET.

The input whether from the signal generator or optocoupler are the same and the I've checked; the ICs are ok.

I've attached a schematic of the circuit i use. I'm wondering whether it could be due to the bootstrap diode, IN4001 has a high trr. However, the circuit work on its own with this diode!!

I've also posted the datasheet for 6N137 and IR2111 (sorry for the size).

I am currently testing the circuit on breadboard.

Did I miss something?

Thanks
Attached Files
File Type: doc opto+gate.doc (33.0 KB, 694 views)
File Type: pdf 6n137vo2.pdf (176.6 KB, 497 views)
File Type: pdf IR2111.pdf (250.2 KB, 265 views)
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  #2  
Old 02-09-2008, 07:16 PM
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Ron H Ron H is offline
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Quote:
Originally Posted by Marie View Post
Hi,

I've built an optocoupler circuit to isolate the control circuit from the power circuit.

1) The optocoupler circuit is built using 6N137.

2) The power circuit consists of a MOSFET driving a resistive load. The gate drive I used is IR2111.

The optocoupler circuit works well on its own. And the gate drive circuit also works well stand alone, i.e., i use the signal generator as input. However, when I connect the optocoupler output to the gate drive input, I am unable to switch on the MOSFET.

The input whether from the signal generator or optocoupler are the same and the I've checked; the ICs are ok.

I've attached a schematic of the circuit i use. I'm wondering whether it could be due to the bootstrap diode, IN4001 has a high trr. However, the circuit work on its own with this diode!!

I've also posted the datasheet for 6N137 and IR2111 (sorry for the size).

I am currently testing the circuit on breadboard.

Did I miss something?

Thanks
The first thing I see is that the maximum output voltage (and Vcc) of the 6N137 is 7 volts, and you are applying 15V through 1k. This may not harm the 6N137, but it's output may be be breaking down, or have an internal diode to Vcc (more likely). This will not allow the output to go high enough to switch your 2111. It looks to me like you need a transistor or some other level shifter between your 6N137 and your 2111.
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  #3  
Old 02-09-2008, 07:45 PM
Audioguru Audioguru is offline
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Hi Marie,
The max allowed supply voltage for the load resistor for the 6N137 is 7V. You have 15V.
You don't show a current limiting resistor for the input of the 6N137 and don't show its current.
You have a slow 60Hz rectifier diode instead of a high speed diode for the charge-pump in the IRF2111.
You don't show the operating frequency of the input pulses.

The max allowed output voltage of the 6n137 is too low for the minimum input voltage of the IRF2111.
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  #4  
Old 02-10-2008, 03:28 AM
Marie Marie is offline
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Hi,

Sorry, I forgot to mention certain things:

1) the switching frequency is 5KHz.

2) there is a current limiting resistor at the 6N137 input which is 330ohm and the current is about 10mA

3) i'm applying the 15V at the output because the optocoupler tends to float such that i'am unable to get any signal without the pull up resistor.

4) i tried to use a transistor at the output of 6N137 but it does not work without the pull resistor and i still fail to get any result at the IR2111 output.

5) I combined the pull resistor and the transistor circuit, but then again no response from the gate drive.

6) I've checked the voltage for the input for the gate drive; it is 15V.

Any idea of what might have gone wrong?

Thanks
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  #5  
Old 02-10-2008, 04:30 AM
nanovate nanovate is offline
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Quote:
3) i'm applying the 15V at the output because the optocoupler tends to float such that i'am unable to get any signal without the pull up resistor.
The max you should apply is 7V. If you insist on usinf 15V then try a 4N25 instead of the 6N137.

Quote:
4) i tried to use a transistor at the output of 6N137 but it does not work without the pull resistor and i still fail to get any result at the IR2111 output.
5) I combined the pull resistor and the transistor circuit, but then again no response from the gate drive.
Please post a schematic of what you did here. This probably should have worked unless you damaged your 6N137 by applying 15V.

Last edited by nanovate; 02-10-2008 at 04:35 AM. Reason: added more info
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  #6  
Old 02-10-2008, 04:48 AM
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SgtWookie SgtWookie is offline
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1) Disconnect and remove the pull-up resistor from pin 6 from 15V
2) Connect a 220 Ohm resistor from pin 6 of the 6N137 to 5V. This will limit the current through the 6N137's phototransistor to 23mA, less than 1/2 it's max rating. It will also prevent exceeding the specified max limit of 7v on the output phototransistor.
3) Get an NPN transistor, such as a 2n2222 or 2n3904. Connect it's emitter to ground, base to pin 6 of the 6N137 using a 1K resistor, and collector to 15V using a 1K resistor. The output of this will be taken from the collector, and will be inverted from the input of the 6N137.
4) Get a 4093 CMOS Schmitt-input quad NAND gate, and use 15V for it's Vdd, and ground for Vss. Connect the collector output of the NPN transistor to both inputs of one of the NAND gates using a 10K resistor. Connect the output of that NAND gate to the IN (pin 2) of the IR2111.

The NPN transistor is used to translate levels between 5V and 15V, but it inverts the signal in the process. The 4093 is used as an inverter to get back the positive logic, and to "square up" the output of the NPN transistor. The 4093 has Schmitt inputs, so it will tolerate slow-rising waveforms without heating or falsely triggering. The IR2111 also has Schmitt input, so the 4093 may be superfluous - but unless you wish to invert your logic prior to the optoisolator, it'll need to be done in hardware before the IR2111. You could simply use an inverter gate, or make an inverter using a PNP transistor and a resistor or two.
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Last edited by SgtWookie; 02-10-2008 at 05:11 AM.
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  #7  
Old 02-10-2008, 09:52 AM
Marie Marie is offline
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Hi,

well the circuit is quite similar to that proposed by SgtWookie except for the fact that i connected the output of isolator direct to 15V through a resistor of 1kohm before applying it to the transistor.

The npn transistor I use is Q2N222. I've attached a schematic of the system i tested and the one proposed by SgtWookie. I'll test the circuit tomorrow hoping that the mistake was to use 15V.

By the way, I think it is not a problem if the output is inverter as 6N137 is enabled by a low input.

Thanks a lot.
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File Type: doc opto+gate drive revised.doc (47.0 KB, 339 views)
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  #8  
Old 02-10-2008, 05:48 PM
nanovate nanovate is offline
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If the output of the 6N137 doesn't drop low enough then increase the 220 Ohm resistor to 470R. Sometimes the CTR is not high enough to allow the 6N137 output phototransistor to sink 20mA.
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  #9  
Old 02-11-2008, 12:41 PM
Marie Marie is offline
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Hi,

Thanks a lot for all ur advices. I managed to get output at the optocoupler. However, I still fail to turn on the MOSFET, probably there is something wrong with my gate drive circuit. I just hope the IC is not damaged.

Again, Thanks a lot.
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  #10  
Old 02-11-2008, 04:17 PM
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Hi Marie,
1) When you apply a pulse to the input logic level to the 6N137, does the pin 2 (IN) input to the IR2111 change from 0v to 15v and back to 0v?

2) What voltage are you measuring on IR2111 pin 8?

3) Try increasing the voltage across the IRF740 and 1k resistor to around 20 to 25v. Does the voltage on IR2111 pin 8 increase accordingly?
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