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Should I use an optocoupler here? (Read 327 times)
sunrise7747
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Should I use an optocoupler here?
Apr 18th, 2018, 1:56am
 
HI everyone,

I have finally succeded in making a decent DC to DC converter. A this point, I am only testing how fast I can charge a cap to 300V. To discharge it, I am using a SCR. This got me thinking....I may need more protection.
 
In my current, I have 2 connections from my pic that controls the charging pulse and the discharge of the secondary.
 
1. The transistor that controls the primary switching is a logic level mosfet. I used this so I could easlily switch my 12v rail with no extra parts. I have a winding ratio of 1:10, so in event of current production from the secondary to the primary, the mosfet will only see 30V and it is rated to 100V.
 
-In this case, is an optocoupler needed?
-should I place a zenner from the gate to the drain?
 
2. The other transistor is the SCR that discharges the 300v cap on the secondary side. So far, I have been just using a resistor to a 5v signal to open it and discharge. I need to control this with a pic pin.
 
-Should I use an optocoupler here?
 
I may have some questions about optocouplers themselves though i have checked out the opto basics and learned how Opto works (maybe you haven't checked out something like this, so you could pick up here: http://www.apogeeweb.net/article/69.html), I know opto are often used for physical separation of the control signals of the power circuit and are always welcome. I have some here, but online circuits look a little odd to me.

I bought some high speed optocouplers. They are the 6n137 models. They are 2 channel also, so that will be perfect. I am using the NTE2987, as mentioned earlier .However, the other transistor is the 2n6504 SCR...and I am getting quit irritated with this. First of all, it looks like I am going to need a SCR dedicated optocoupler. Second, I am disappointed with it getting stuck in the on state because of my highly inductive load. And third...Im not using AC current, so I dont think the SCR is necessary. I am discharging 300V and 480uF, which comes to around 20J of energy. Can you also recommend a non-scr transistor to discharge a high voltage? I have some IRF730 units here. They are rated at 400V...but the single pulse avalanche rating says 300mJ...so I dont know if this will blow her.

Thanks a lot in advance!
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