Aint this considered a automotive modification which is strictly prohibited on this forum ?

 

Aint this considered a automotive modification which is strictly prohibited on this forum ?

I consider it an electrical project that I am getting excellent help on. I understand that you may get lots of bonehead threads with topics like, "how can I boost voltage to my car to make it go faster?", and so I can see the need to restrict "automotive modification" posts, but look at the content here. I am trying to develop a circuit that charges a capacitor to a certain voltage in a certain amount of time. It could just as easily be for a rail gun.... wait, bad example, it could just as easily be for an arc lamp starter or a welder. And anyway, I am not modifying anything. I am developing a new product.

 

Below is your circuit driven by an NPN totem-pole driver which seems to work. Noticed that I reversed the switch polarity to allow for the signal inversion from the driver. You might try that driver in your breadboard.

I have tried to hold off posting here until I had a little more to report, but since I may get banned, here is what I have found so far:

I really didn't want to accept that the 5V control signal on the gate side of Q2 could be limiting the max voltage that the circuit can produce. I figured that even if it took a long time for the control circuit to turn on the transistor, I could still just leave it on longer to build up the current. after lots of testing and beating my head against the wall, I think I finally understand the point of the totem pole driver for the switching transistor (Q2). And let me know if I am wrong here:

It is not so important how long it takes to turn the transistor on, because indeed it can be left on as long as it takes to get up to the necessary primary side current. What matters is how fast the transistor can be turned off. If the 25mA@5V control signal takes 2uS to slew the switching transistor (Q2) from on to off, then the primary side inductor has all that time to drain energy back through its self rather than push it to the coupled inductor on the secondary side. I am working with small inductances and low resistance, so the time constant of my switching circuit has to be super low.

I ran crutschow's version of my circuit with the totem pole driver. The current into the gate of the switching transistor spikes to ~450mA during transition from on to off. Since 450 is greater than 25, I must conclude that my control circuit is woefully inadequate.

Thanks for all the help so far. I will rebuild, retest, and post the results.

 

OK, so I reworked the circuit and added a driver for Q2. I used 1/2 of a high/low side driver chip I had available (see attached circuit). The extra gate drive definitely helped. Before, the circuit would only charge to 120V and now it goes to 180V. Unfortunately, it still levels off, albeit at this higher voltage. Could it be that Q2 still can't switch fast enough?

And here is something I do not understand: The LTSpice simulation shows that the pulsed voltage on the primary side is always a scaled version of the secondary side, I mean that they increase together (see X6.png). My circuit behaves exactly like the simulation right up to 190V, then the secondary side levels off while the primary side continues to increase in voltage (see X7.png). Aren't the two supposed to pretty much follow each other?

 

What component number is D1?

 

D1 is a Vishay BYG20J.

 

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And here is something I do not understand: The LTSpice simulation shows that the pulsed voltage on the primary side is always a scaled version of the secondary side, I mean that they increase together (see X6.png). My circuit behaves exactly like the simulation right up to 190V, then the secondary side levels off while the primary side continues to increase in voltage (see X7.png). Aren't the two supposed to pretty much follow each other?

Yes, the two voltages should track. Since they don't I'm not sure where the problem lies. Perhaps something is breaking down in the secondary. What is the voltage rating of Q1?

 

Q1 is an IGBT, Fairchild ISL9V5036S3. I have run the circuit without the, ummm, welding coil, attached and it does behave the same way. My next thought is to connect a sense resistor inline with the primary so I can scope the primary current directly. Any ideas for other tests I can run?

 

Please disregard the scope trace I posted last. It turns out that my desktop scope does not like voltage input signals higher than 20V. It seems as if the DAC saturates at this point. So the reason the primary/secondary signals did not match was all scope related.

With the driver installed, I scoped the output with a better scope and shazam! there is my 300V! A little more tuning and I will be at my goal of 340V in 1mS.