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Registered Member #42796
Joined: Mon Jan 13 2014, 06:34PM
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Posts: 195
from datasheet the gate capacitance is quite high adding a bit of complexity to the gate driving circuits the Switching Characteristics are not that bad so i think those IGBT's could work up to 100-150kHz the recovery speed of the reverse diode is ok for the above frequencies
anyway i'm fairly new to this so i might be wrong...
Registered Member #3704
Joined: Sun Feb 20 2011, 01:13PM
Location: Vermont, U.S.A.
Posts: 92
dexter wrote ...
from datasheet the gate capacitance is quite high adding a bit of complexity to the gate driving circuits the Switching Characteristics are not that bad so i think those IGBT's could work up to 100-150kHz
Hi dexter,
You actually just strengthened my point, as my TC has a resonant frequency of around 240kHz. I'm thinking I posted the specs earlier in the thread, in case you'd like to see them. I appreciate your feedback!
By the way guys, I constructed a very simple quick-replace system for the TO-247 IGBTs in case one blows. I'm using some MSTB connectors that have the socket on the top and the adjustment screws on the side. The bottom is supposed to be plugged into a header, but since I don't have one I decided to use some 1.5mm diameter tubing in place of the header "prongs". I was then able to plug standard headers into the tubing, which allows the whole thing to be mounted to the perfboard.
This is not ideal, as the connectors are only rated for 10A and the headers are fairly wimpy. I'm thinking that it should be ok though, considering the headers are connected using the tubing, and the current will be pulsed. Here are a few pics of the connectors and the setup:
I'm thinking I'll probably shorten the tubing, as it really doesn't need to be that long.
Regards, Matt
EDIT: I have also had a growing concern regarding whether my IGBTs will be able to handle the current. In case you don't feel like reading back, I found a good price on a bunch of HGTG20N60A4Ds. The datasheet says they should be good for 600V, 280A pulsed. That alone concerns me, as I am not sure how to calculate the primary current that they will be seeing--I think one of you mentioned that the calculation for that will be quite complicated. If someone could help me out with that, I would appreciate it. Secondly, I am not sure what the maximum allowable duty cycle will be, or how to calculate it. I think to calculate the maximum allowable duty cycle will require the current calculations, so without knowing the current there's not much more I can do.
Registered Member #3704
Joined: Sun Feb 20 2011, 01:13PM
Location: Vermont, U.S.A.
Posts: 92
dexter wrote ...
so you are now making a DRSSTC....
quick-replace system for TO-247 IGBTs is not something new but i will try to reduce any extra joints as possible, use some of these:
but i doubt this could handle the currents in a DRSSTC setup
as for the peak current people have built DRSSTC with just 100A tank current
I used the connectors shown because of how the IGBTs will be mounted, both to the heat sink and to the board. Once again I will be shortening the tubing, so it will really just be the connector and the header. It's completely experimental, and I do have a heavy-duty terminal block if I need to use that instead.
Sure people have built 100A tank circuits, but my point is that mine is already designed--I have all the components chosen already and need to calculate the expected current. At resonance the reactance of the tank circuit is equal to zero, if I am not mistaken, so I would have to find the voltage that the primary rings up to and divide it by the total resistance of the tank circuit, am I correct? And yes, I decided quite a while ago that I was building a DRSSTC
By the way folks, here's a simulation video of my Arduino-based interrupter. It's still very much a work in progress, but at least it is functional:
The interrupter has four modes:
Mode 0: Output off Mode 1: Manual -- Arduino switches power to a 555 timer and a 393 comparator to allow independent frequency and duty cycle control Mode 2: PC MIDI input -- Arduino accepts MIDI signal over USB and converts it to a square wave. Will probably switch to a MEGA so that I can include a channel select with readout, or even better an LCD. Mode 3: External MIDI input -- Not yet functional, but this mode will accept a MIDI signal directly from a musical instrument.
Sorry for the delays in the video, it's caused by Proteus loading my CPU. I also apologize for the crowded screen--Most of the windows do not have the option to adjust their size.
FURTHER EDITS:
Thanks to a sticky Mads just posted, I believe I found the answer to the current problem. The thread I am referring to is this one:
Uspring wrote ...
Basically you'll add the switching voltage to your MMC voltage every time you switch. In detail: With a mains voltage of 120V and a half bridge you switch between -170V and +170V. Initially you start off with 0V. When voltage is first applied, the MMC voltage will rise to 170V after one half cycle, i.e. 3us. Then you switch input to -170V. After 3us MMC voltage will be 170V + 340V. You have added the switching amplitude (340V) to the initial voltage. After another 3us MMC voltage wil be 170V + 2*340V and so on.
The MMC voltage determines the current: I = V / (2* Pi * Lpri * fres) with primary fres. For 21uH and 160kHz, 100A will be reached, when the MMC voltage is 2100V, i.e. after about 6 half cycles or about 18us.
This is very much an upper limit for the current. Losses in the primary tank and even more the loading due to the secondary will limit the current. If you have a scope, I'd monitor it and go from there. I believe these fets can stand much more current for the short burst times you are aiming at.
I will be running from full-wave rectified mains (120VAC), so most of the values used in the example match what I will be using. The only differences are my primary inductance (mine is 27.8uH) and fres (mine is around 217kHz). Therefore my max current should be (roughly):
Note that in that thread with Uspring's reply that you quoted, he made the mistake that a half bridge switches between +170 and -170, it does not. A FULL bridge does. A Half bridge on rectified 120V mains switches +85 and -85 as the capacitive divider creates a +85v to ground mid point that the primary is tied to.
Registered Member #3704
Joined: Sun Feb 20 2011, 01:13PM
Location: Vermont, U.S.A.
Posts: 92
Sigurthr wrote ...
Note that in that thread with Uspring's reply that you quoted, he made the mistake that a half bridge switches between +170 and -170, it does not. A FULL bridge does. A Half bridge on rectified 120V mains switches +85 and -85 as the capacitive divider creates a +85v to ground mid point that the primary is tied to.
Hi Sigurthr,
Yep, I did notice that. And since my circuit uses a full bridge, I used the original formula he provided.
I gotta say, that thread did clear up a bit, so thanks for that
Registered Member #3704
Joined: Sun Feb 20 2011, 01:13PM
Location: Vermont, U.S.A.
Posts: 92
Hi guys,
I read somewhere that one should not run a DRSSTC at a duty cycle higher than 10%, unless specifically designed otherwise. Is this true? 10% does seem a bit low, but it would make sense, considering the amount of current being delivered to the primary.
Registered Member #3704
Joined: Sun Feb 20 2011, 01:13PM
Location: Vermont, U.S.A.
Posts: 92
Mads Barnkob wrote ...
You should not run a DRSSTC above 500uS on-time at most, it is normal to run between 50 to 300uS on-time depending on coil size.
That is far far less than 10% duty cycle.
The 10% figure was an upper limit, so that makes sense. It looks like I will have to reconsider my interrupter, though. I was planning on playing music through the coil, possibly using the tone library for Arduino. Those tones are played at a 50% duty cycle, which means an A4 tone (440Hz) would have an on-time of around 1.14nS. I guess if I plan to use the tone library, it would be best if I avoid playing notes at 1kHz or lower? Either that or find a way to change the duty cycle of the tone. How do other people with singing DRSSTCs generally do it? I'm leaning towards manipulating the duty cycle, probably with external circuitry.
Thanks for the info, it clears up a few questions.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Three words: Die temperature ripple.
Calculate the peak power dissipation in your transistors and then from the transient thermal impedance graph find out the longest ON-time you can run. The ripple should never be larger than 30 °C. When running higher duty cycles, also check the average dissipation.
In other words, you can destroy the bridge running at 1% duty in one case, where it will run happily at 10% in another case. Besides peak current; lower break rates, higher switching frequencies and hard switching all limit the duty cycle you can run.
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First SSTC design - Need some critique
