Fast switching 300V

AnalogKid said:

To get faster risetime you need to turn on M6 fast, which means driving its gate below its source with something that can more some current. It is not clear how that is being done because the "opto" boxes need more detail. In, Out, and Ref are not normal optocoupler terms, so what are these devices?

ak

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These are the optoisolators I am currently using. http://www.avagotech.com/pages/en/optocouplers_plastic/plastic_integrated_gate_drive_optocoupler/acpl-312u-000e/

Papabravo said:

Normally when you use optos you want to provide one ground on the input side and a separate ground on the output side. In addition there can be a potentially large potential difference between the two grounds.

In any design where you have a single common ground, optos don't make much sense because in that case they provide very little benefit and introduce some substantial problems.

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Ok, all of my common grounds are earth ground right now.

So with Optoisolators, it would help to float both?

What substantial problems could arise? Perhaps this is contributing to some of what I'm seeing/not seeing?

Yes the IR2111 is made specifically for this.

While you can use optos, i do not recommend it.

But there are the optoisolated HCPL3120 gate drivers, i used them in this project prototype:
http://i.imgur.com/4oglTh4.jpg

Schematic:
http://i.imgur.com/2fMUJih.png

Thank you. I will redesign this. Wish I had the insight to find you out on here 3mo ago.

So with the IR2111, what value of gate resistors should I be looking at?

It depends on the gate capcitance of the mosfets, but i'd say around 22 ohms.

I just thought I would give you another alternative for High Speed - Half Bridge Switching.

I use a single MIC4420ZT 5 pin Gate Driver to run a Gate Drive Transformer. For Half Bridge circuits, I wind a 1" Toroid with 7t Primary and two 13T secondaries. Each secondary leads go to a MOSFET Gate and Source. The leads are reversed on one MOSFET to operate the MOSFET's 180 degrees out of phase.

Using a Gate Drive Transformer, electrically isolates the High Voltage from the Low Voltage Driver Circuit.

The reason I have an increased number of turns on the secondary, is because the MOSFET Gate Drivers send out a pulse at around 1/2 rail voltage. If I were using 13.2V for the driver, the Gate Driver IC puts out 6.5V pluses. Most MOSFET's require at least 10 volts to turn of fully, so doubling the secondary increases the Gate Voltage high enough to drive the MOSFET "Fully On".

You can also use a standard 1:1:1 Pulse Transformer as a Gate Drive Transformer. When I use these, I set up a Voltage Doubler on the secondary side with a 1uF 50v Ceramic Cap and a Schottky diode. This is called "DC Restored Gate Drive"

Just an alternative for you to consider. It is just the way I do it.

Papabravo said:

Yes they can. IIRC they can move 300V in 7-10 nanoseconds. The original computers were build with vacuum tubes and they did alright for their time.

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The EC80 is UHF triode which will go about 950mhz .001053u/s

Papabravo said:

Here's the wiki

http://en.wikipedia.org/wiki/Current_source

Also -- 12AT7 twin triodes make dandy 300V switches

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So is the 15V DC supply into my optoisolator not considered a current source?

No. In the configuration you are using with the output transistor of the opto in a CE configuration driving the RC circuit composed of the gate resistor and the gate capacitance it is a voltage source. The characteristic of a voltage source is that it provides a fixed voltage at whatever current the load demands. The characteristic of a current source is that it provides a fixed current at whateve voltage the load demands.

Your power supply is a source of current, but it is not a current source.

Thanks for the help everyone.

I successfully designed a new circuit which will switch my voltage in less than 1μs.

Way to go. We don't often have folks asking for help and actually showing the results in the original thread.

Papabravo said:

Way to go. We don't often have folks asking for help and actually showing the results in the original thread.

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Thanks! Glad I could contribute back.

One more question though... what would be the best way to invert my output signal...so that I can switch from 300V to 0V and then back to 300V.

Right now the positive waveform I put in gives me a pulse out of +300V for 1μs.

What should I be doing differently to always be at 300V and switch down to 0V for 1μs?

Experimentonomen said:

The IR2111 is a readymade mosfet halfbridge driver with the necessary deadtime ciruitru already built in, all you need to do is to supply your PWM signal and you will get a 0-300V output.

Using separate drivers like you are, you will need more circuitry to generate some deadtime to avoid shoot through/cross conduction in your mosfets which will cause EMI to be radiated as well as heating of the mosfets.

Click to expand...

So I figured out how to get the 0 to 300V succesfully, have spent the past few days unsuccesfully trying to get a 300 to 0V pulse out of this circuit... what should I be doing differently than what I did for the 0 to 300 circuit?



Things that have not worked
- My simplest explanation was to invert my input waveform (keep it high and drop low, but this doesn't work)
- switching the hi and lo output to opposing mosfets
- switching the orientation of 300V and ground

Thanks for all those that have helped. I have successfully figured out how to switch from 300V to 0V and back... more in this thread