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Help understanding and deciding on DRSSTC

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Ken M.

Sat Apr 19 2008, 04:33AM

Registered Member #618 Joined: Sat Mar 31 2007, 04:15AM
Location: Us-Great Lakes
Posts: 628

OK well I orderded all the parts I could not sample, And so before I go into prototype testing I'd just like to see if any of you guys can spot something I might have missed.

1208579577 618 FT43271 Drsstc Driver W Ocd

Tonic

Sat Apr 19 2008, 08:36AM

Registered Member #528 Joined: Fri Feb 16 2007, 10:32PM
Location: Warsaw, Poland
Posts: 166

What 1N5819 Schottkies paralled with gate resistors do and how it works?

The capacitors at LM78XX's outputs are quite big, I think few hundreds of microfarads are enough. Somewhere I've readed it's a good idea to put diode across input and output to return energy to rail instead through voltage regulators, preventing from damage, but this practice doesn't appear to be a common thing there.

Another question, decoupling capacitors have to be placed across voltage rails, or elements inputs, or everywhere?

c4r0

Sat Apr 19 2008, 09:11AM

Registered Member #151 Joined: Sun Feb 12 2006, 02:53PM
Location: Poland
Posts: 153

Tonic wrote ...

What 1N5819 Schottkies paralled with gate resistors do and how it works?

I'm not sure but i think that it allows IGBTs to turn off faster than they turns on. Maybe it gives some dead time?

HV Enthusiast

Sat Apr 19 2008, 04:31PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

The 1N5819 across the gate resistor is simply to allow fast turn-off. Turn-on is through the resistor, turn-off through the diode.

Tonic

Sat Apr 19 2008, 10:30PM

Registered Member #528 Joined: Fri Feb 16 2007, 10:32PM
Location: Warsaw, Poland
Posts: 166

I'm not sure if I understand how it does work. Is it faster for gate charge to discharge through diode instead of resistor (because resistor has coil, it has inductance which at high frequencies gives much higher reactance than diode)?

If yes, would I have right saying that's useful with large charges (especially in IGBT modules)? I mainly see this diode in DRSSTCs with powerful IGBTs, not in SSTCs, hence this question.

And would you mind answering question regarding decoupling capacitors?

Sorry to hijack topic, Ken, but I guess it might be useful for you, too :)

Ken M.

Sun Apr 20 2008, 12:43AM

Registered Member #618 Joined: Sat Mar 31 2007, 04:15AM
Location: Us-Great Lakes
Posts: 628

An IGBT is similar to a MOSFET both have gates that are insulated from the rest of the unit, when the Gate is made to turn the unit on It charges like a capacitor, but when the IGBT or Fet turns off THe Gate discharges like a cap, If you just have the Gate Resistor then the discharge takes longer, but With the Diodes in the position they are in It allows for faster Discharging of the gate or Allows the FET\IGBT to turn off faster.

Am I right about that? (Just trying to help Tonic and myself understand it)

ALso So far this Is What I've got setup.

1208651896 618 FT43271 Drsstc Driver Topbottom

1208651896 618 FT43271 Drsstc Driver Bottom

1208651896 618 FT43271 Drsstc Driver Top

1208651896 618 FT43271 Drsstc Power Topbottom

1208651896 618 FT43271 Drsstc Power Bottom

Once Again if you notice something that could or needs to be changed Please let me know.

Off Topic Real quick Anyone know how to get Picture links like these to be placed Side by side, so I can make this post be shorter while still keeping everything on it?

Tonic

Sun Apr 20 2008, 06:13AM

Registered Member #528 Joined: Fri Feb 16 2007, 10:32PM
Location: Warsaw, Poland
Posts: 166

Remove br (in square brackets) attributes.

Ken M.

Sun Apr 20 2008, 02:05PM

Registered Member #618 Joined: Sat Mar 31 2007, 04:15AM
Location: Us-Great Lakes
Posts: 628

Thanks Tonic!

Josh Johnson

Sun Apr 20 2008, 03:47PM

Registered Member #793 Joined: Sun May 20 2007, 06:50PM
Location: Minneapolis, MN
Posts: 35

Ken, Tonic,

Generally speaking MOSFETs turn on about as fast as they turn off and the gate resistor is there to limit large transient spikes. IGBTs though tend to have have a much longer turn off already, so a gate resistor is not needed to limit spikes and the resistor would just make the turn off even longer; so it's bypassed with a diode.

Ken,
What are C19 and C20 in your power section layout (the IGBT PCB). I didn't see them on the schematic.

Josh

Ken M.

Sun Apr 20 2008, 04:27PM

Registered Member #618 Joined: Sat Mar 31 2007, 04:15AM
Location: Us-Great Lakes
Posts: 628

Crap..... Sorry for the double post I needed to repost the schemes, I thought I had screens of the newer ones with revised part #'s, at any rate THESE are the corrected Revised schematics.

C19\20 are were previously C17's, also J4 (HV IN) is so I could place pads for the power to enter the board.

1208708793 618 FT43271 Drsstc Driver W Ocd

Could I trouble a Mod to delete that previous post. (My apologies for double posting!)

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