NOR Latch Question

MOSFET KILLER · May 22, 2010

Hello All,
I have been designing a high speed optocoupler circuit for a pair of halfbridge gate drivers. My design uses a NOR latch and the leading edges of the optocoupler waveforms cause it to change states. The NOR gate that turns on first is unpredictable and if the wrong one turns on first before the OFF optocoupler switches it off it would trip my desaturation detection circuit and shut down the whole thing.

Questions:
1. How would I make sure that the right NOR gate changes state first when power is applied?
2. What would happen if both optocouplers were on at the same time?

ericgibbs · May 22, 2010

MOSFET KILLER said:
Hello All,
I have been designing a high speed optocoupler circuit for a pair of halfbridge gate drivers. My design uses a NOR latch and the leading edges of the optocoupler waveforms cause it to change states. The NOR gate that turns on first is unpredictable and if the wrong one turns on first before the OFF optocoupler switches it off it would trip my desaturation detection circuit and shut down the whole thing.

Questions:
1. How would I make sure that the right NOR gate changes state first when power is applied?
2. What would happen if both optocouplers were on at the same time?

hi,
I assume its a 4 gate NOR package.?
Do you have any free NOR gates on the project.?

Hero999 · May 22, 2010

Please post a schematic.

If it's an RS bistable you're talking about an RC circuit can be used to ensure it's kicked in to the desired state when the power is first applied.

MOSFET KILLER · May 22, 2010

Ya, I have two more. Its a 4 gate package.

MOSFET KILLER · May 22, 2010

Here is a schematic of what I was thinking about.

ericgibbs · May 22, 2010

MOSFET KILLER said:
Ya, I have two more. Its a 4 gate package.

hi,
You need to add a power up RESET pulse to set the NOR latch to the desired state.

Look at this option using a couple of diodes, you can use the spare NOR gates for a similar RESET.
Ignore the inverter, thats for test only

ericgibbs · May 22, 2010

hi MOSFET,
This is a spare NOR gate solution.
The 4049 is for SIM only.

MOSFET KILLER · May 22, 2010

How does this look?

Hero999 · May 22, 2010

Good suggestion Eric.

Just one possible improvement: rather than using a separate NOT gate, he could use a spare NOR by tying both inputs together.

MOSFET KILLER · May 22, 2010

I corrected my schematic. The capacitor and resistor were in the wrong places. I rushed it the first time.

MOSFET KILLER · May 22, 2010

What would happen if both inputs were high at the same time?

ericgibbs · May 22, 2010

MOSFET KILLER said:
What would happen if both inputs were high at the same time?

hi,
Its a NOR gate, OUTPUT = 1 when neither A OR B are HIGH [ NOT A OR B]

Code:

B      A      OUT
0        0           1
0        1           0
1        0           0
1        1          0

Should this answer your query.

jakeselectronics · May 23, 2010

To those that are interested.... Jake's Electronics | Theory Box

ericgibbs · May 23, 2010

jakeselectronics said:
To those that are interested.... Jake's Electronics | Theory Box

hi Jake,
Nice techno, but the blank background is a put off, very hard to read thru.

MOSFET KILLER · May 23, 2010

Would this make the flip flop start in the desired state?

ericgibbs · May 23, 2010

MOSFET KILLER said:
Would this make the flip flop start in the desired state?

hi,
What is the desired state at power up.?
If its that the OUTPUT of the top right side of the latch is LOW then it will be.
What are the purpose of the two 10K pull down resistors on the opto collector outputs.????

MOSFET KILLER · May 23, 2010

They are to prevent the inputs from floating when the optocoupler is off.

ericgibbs · May 23, 2010

MOSFET KILLER said:
They are to prevent the inputs from floating when the optocoupler is off.

The inputs will not float, they are tied to +V12, via the 4k7's ???

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