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Old 28th October 2006, 10:11 AM   #1
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Lightbulb A new (Stupid?) class d idea

I got this idea last night of using a GDT instead of lets say a IR2010 which would spare the gate driver ICīs if the fets blow.

Now i only have IRFP460 but heres the schematic of my idea:
Suggestions are welcome.

Click the image to open in full size.
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Old 28th October 2006, 10:16 AM   #2
djQUAN is offline djQUAN  Philippines
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but how would you introduce dead time?

and also, remember the switching frequencies are also modulated by low freq audio so the driver transfo might saturate......
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Old 28th October 2006, 10:24 AM   #3
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I have ran this output stage setup with other stuff and havent experienced any problems with cross conduction, but that was a SSTC and induction heater so i dunno how it will behave here.

Only one way to find out.

But i am not sure on which input of the comparator the feedback should go, actually im not really a fan of self oscillating class d circuits as they tent to lock onto the rails when you push them hard.

Another idea i have is no GDT but putting an optocoupler on the highside gate driver. Also that would require a deadtime circuit, but i think i have one of those laying around on the drive, that one with XOR gates and RCD circuits.
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Old 28th October 2006, 10:26 AM   #4
djQUAN is offline djQUAN  Philippines
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I also thought about optocouplers before but it is very hard to find ones that can switch at high enough frequencies.
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Old 28th October 2006, 10:31 AM   #5
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Yah and they are also expensive.
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Old 28th October 2006, 10:41 AM   #6
djQUAN is offline djQUAN  Philippines
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so the best options are mosfet drivers specifically designed for the purpose or the UCD concept but we already know that.
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Old 28th October 2006, 11:20 AM   #7
Pafi is offline Pafi  Hungary
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Tekko!

If you limit output voltage to 40...60% of supply voltage, and if you modify the feedback, then theoretically it can work. Somehow...

"i am not sure on which input of the comparator the feedback should go"

It depends on the polarity of the transformer, but this fedback method is basically wrong. 2 correct way:
- 2nd order LPF in feedback loop with 200-300 kHz cutoff freq, or
- feedback from after LC, and phase compensation in feedback network. Check UcD!
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Old 28th October 2006, 11:29 AM   #8
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I was also thinking about a transformer drive for the power stage. But djQUAN's note about the low frequency modulation was exactly the reason why I skipped this idea. Already from theory the resulting number turns were unpractical.

Please note the transformer has to transfer the low frequency signal of music. Especially bass is causing issues, when driving at strong modulation, means high music signal levels.
Consider up to 95% modulation, close to clipping.
Consider about +/-22V peak output of your drivers... means your transformer has to handle 0.95x22V/1.41=14.8Vrms at may be 20Hz. Unsaint conditions for small signal transformers. This massive low frequency signal is existing in the gate drive signal in the same way as in the half bridge output....
If you ignore this, then at high levels you might run into trouble with poor gate signals and in worst case with killed MosFets. At low levels probably nothing bad would happen.
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Old 28th October 2006, 11:42 AM   #9
Pafi is offline Pafi  Hungary
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Maybe this way, or something like this.

Though I'm not a big friend of gate transformers. I tried everything, but I couldn't reach low enough leakage inductance. I ordered a sample from a factory, but it was much worse then mine.
Attached Images
File Type: gif trafo gate drive.gif (3.9 KB, 807 views)
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Old 28th October 2006, 12:13 PM   #10
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Iīll just sample a few HIP4080īs

There problem solved.
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Old 28th October 2006, 04:21 PM   #11
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Is it possible to use the internal HIP4080 comparator for the PWM generation ?

That way only an external triangular wave generator would be needed, or a feedback cir if self oscillating.
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Old 28th October 2006, 06:40 PM   #12
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Heres an idea:
Click the image to open in full size.

What do you think ?

Edit: Oops those connections to the left should be before the filter, not after.
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Old 28th October 2006, 06:59 PM   #13
Eva is offline Eva  Spain
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Transformers require a waveform with the same volt*seconds product in both directions. Also, leakage inductance prevents a fast enough turn off.
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Old 28th October 2006, 08:11 PM   #14
Pafi is offline Pafi  Hungary
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Quote:
What do you think ?
Sorry, but I think first you should learn about basic operation principles of PWM modulators.
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Old 28th October 2006, 08:47 PM   #15
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So scrap this idea too then.

Problem is i already ordered the gate drivers.
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Old 28th October 2006, 09:22 PM   #16
Pafi is offline Pafi  Hungary
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There is no problem with the gate driver, but you should design a usable feedback.
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Old 28th October 2006, 09:28 PM   #17
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Then trash the feedback, i hate feedback anyway.

Click the image to open in full size.

This is just a slight modification of my previous class d build.
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Old 29th October 2006, 07:31 AM   #18
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I take your silence as this will work perfectly without hitch.

Actually ids a modified version of my previous class d build, but instead of two comparators in the pwm, heres only one.

If there are other ways of doing the HIP4080 input without adding more ICīs, tell me about it.

I am thinking about skipping the external comparator and use the HIP4080īs internal one.
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Old 29th October 2006, 10:56 AM   #19
Pafi is offline Pafi  Hungary
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Quote:
I take your silence as this will work perfectly without hitch.
I didn't find serious error in it, but I didn't check every details. Just try it!
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Old 29th October 2006, 11:04 AM   #20
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Btw checking yer site pafi, 2kW pwm, yer crazy!!!

How will IRFP460 (all i have for the time beeing) stand up with a 2ohm load with lets say 50-60 volts in from the power supply?

This thing is for driving a sub, so....
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Old 29th October 2006, 11:21 AM   #21
fokker is offline fokker  China
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Quote:
Originally posted by Tekko
Btw checking yer site pafi, 2kW pwm, yer crazy!!!

but I don't think Pafi's amp can be loaded with a lower impedance at all. that just isn't simply how physics works.
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Old 29th October 2006, 11:42 AM   #22
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I trust that he knows what hes doin, fokker.

That thing can do 10kW if the fets have low enuf Rds(on)
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Old 29th October 2006, 12:40 PM   #23
Pafi is offline Pafi  Hungary
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Tekko!

It can work with IRFP460 (somehow), but surely this is not the best choice. IRF540N is much cheaper and better (faster, easier to drive, have higher efficiency) if you have only 50-60V supply.

Thanks! You are right: in this amp I used very fast FETs with Rds=0,036 ohm, Vdsmax=150 V, 2 paralelled, in full bridge. Protection takes account chip temperature, so it allows higher current for short time, this results a ~120 A peak current (I measured it with 2 ms impulses on 0,1 ohm load). In real overload-test I attached 7 pcs of 8 ohm speakers parallel (Eminence Kappa 15 Pro), and overdrived by 2-3 dB with bass signal, and protection tripped 3-4 times in 15 minutes. Temperature of heat sink stayed below 50 degrees. The reason why I call it "2kW amp" is at the time of specification I didn't have big enough stabilized power supply, and also I wanted to have significant headroom for music signal.
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Old 29th October 2006, 01:03 PM   #24
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My problem is that i cannot buy any fets (granny says no) so i have to use what i have.
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Old 29th October 2006, 03:34 PM   #25
Pafi is offline Pafi  Hungary
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This is incredible!

Should I send some?
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