[moved]Should I use an optocoupler here?

[sunrise264]

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: https://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!

 

[FvM]

I don't believe that you want to discharge the capacitor without a series resistor that absorbs the stored energy.

As for IRF730, you'll refer to the pulse energy in SOA rather than avalanche energy, but it isn't larger than 600 mJ either. So discharging the capacitor through IRF730 without load resistor or current control is a reliable way to destroy the transistor.

SCR survival in lack of a current limiting resistor depends on capacitor ESR. It's probably sufficient for an electrolytic capacitor, backside is respective capacitor heating. Film capacitors achieve many kA discharge current, too much for a small 25A SCR.

 

[KlausST]

Hi,

optocoupler...are used for isolation.
--> is you need isolation then choose an optocoupler.

If you need protection then use a protection device.
I never used an optocoupler as a protection device. But I´ve seen circuits where they use optocuplers to compensate for a bad PCB layout.

***
To be honest it´s not clear to me where and why you want to use the optocoupler. A schematic or at least a sketch could help.

Klaus

 

[sunrise264]

Two situations, which I think should be:
The MOSFET should be driven by a driver type TC4420, not forget that the switch is a highly inductive circuit, the MOSFET on the power must also be protected;

In the case of SCR, no less dangerous in this case, I think, here the opto is good use (please correct me if you think i am wrong).
Yet the use of the zener or TVS is always welcome in the logic circuit for noise immunization...

I was testing and not getting an output from the optocoupler. I sort of scoped a signal when I cranked the voltage dividend up. I guess my optocoupler choice has a cut off frequency of 80kHz, and I am at 240kHz! Open for suggestions if you have used an optocoupler you like.

And I have several of these:
http://www.us.liteon.com/downloads/LTV-817-827-847.PDF
It says that the Max forward voltage on the LED side is 50mA and 1.4V, and I will be driving it from a pic pin, which should be regulated to 4.95v or less. Should I place a 100 Ohm resistor between the pic pin and the input?

As far at the transistors being switched, the setup is confusing. Should the optocoupler be placed on the high side, or the low side of the transistor? I am using: http://www.bader-frankfurt.de/loads/nte2987.pdf
Also, my rail is 12v. should I use a resistor between it and the transistor?

 

[FvM]

How 240 kHz come into play? I feel that you should show a schematic of your intended design, otherwise we might talk completely on cross purposes.