Author Topic: UD2.7 dual Purpose Use  (Read 1754 times)

Offline FonziDaytona

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Re: UD2.7 dual Purpose Use
« Reply #20 on: September 26, 2022, 07:55:31 PM »
I wonder if I can disable UVLO, at startup, and then re-enable? Or, use a switch instead of a jumper on the Interruptor output to toggle?

-Matt

Offline davekni

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Re: UD2.7 dual Purpose Use
« Reply #21 on: September 27, 2022, 03:12:49 AM »
Quote
I wonder if I can disable UVLO, at startup, and then re-enable?
Might help, but still leaves power-up state of OCD FF indeterminate.  Random chance FF will power up enabled.

Quote
Or, use a switch instead of a jumper on the Interruptor output to toggle?
Yes, that is what I was trying to suggest.  A normally-closed push-button switch would make starting easiest.  Just a push of the button rather than toggling a switch off and then back on.
David Knierim

Offline FonziDaytona

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Re: UD2.7 dual Purpose Use
« Reply #22 on: October 12, 2022, 05:11:46 PM »
Okay, so I’ve been running this for a little now and it seems to be doing the job, but I’ve found that my IGBT gate drive resistors (5.1ohm .5watt) are getting extremely hot, and I don’t understand why.

My math:

Cm300dy-12nf has 1200nC per the data sheet. Current needed should be:
1200nC*35khz = 42mA, right? I^2R heating should only be 9mW? I don’t see why my .5w resistors are getting so hot…

-Matt
« Last Edit: October 12, 2022, 05:15:33 PM by FonziDaytona »

Offline davekni

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Re: UD2.7 dual Purpose Use
« Reply #23 on: October 13, 2022, 05:07:57 AM »
Quote
Okay, so I’ve been running this for a little now and it seems to be doing the job, but I’ve found that my IGBT gate drive resistors (5.1ohm .5watt) are getting extremely hot, and I don’t understand why.

My math:

Cm300dy-12nf has 1200nC per the data sheet. Current needed should be:
1200nC*35khz = 42mA, right? I^2R heating should only be 9mW? I don’t see why my .5w resistors are getting so hot…
First, induction heating etc. applications typically run much higher duty cycles than DRSSTC.  That is why DRSSTC's have less issue with gate resistor heating.

Analog simulation is one way to see details of gate resistor power dissipation.  For simple approximations, two changes from your math:

Gate charge of 1200nC is for 0-15Vge swing.  If UD2.7 is being fed 24V, then Vge is swing -24V to +24V (48Vpp rather than 15Vpp).  Gate charge will be roughly proportional to Vpp, so 1200nC * 48V / 15V =  3840nC.  3840nC * 35kHz = 134mA.  This presumes gate resistor is paralleled by a diode, so conducting current only during Vge rising edges.

134mA is average current.  RMS current is much higher.  Presume that UD2.7 output resistance and GDT wire resistance are negligible compared to gate resistor.  Presume no internal brick gate resistors.  (I don't see any listed in specification for Cm300dy-12nf.)  Also presume instant UD2.7 output switching.  With those assumptions, average voltage across gate resistor during current flow is 24V (48V down to 0V during each Vge rising edge).  So resistor power is 134mA * 24V = 3.2W (at 100% duty cycle).

The above assumptions end up being worst-case.  Some of that 3.2W per gate will be dissipated within UD2.7 output FETs.  (A higher percentage of the additional 3.2W/gate power from falling edges is dissipated within UD2.7 output FETs.)

I've concluded that bricks are overkill for most high-duty-cycle applications, since they require high gate drive power.  TO247 IGBTs are a better fit for such uses, or small bricks.  0-15Vge drive also works better for such, presuming layout is clean enough to make negative Vge unnecessary.  (And presuming no internal gate resistors, which then require negative Vge to turn off sufficiently fast.)

Good luck with modifications to fix gate resistor power issues!
David Knierim

Offline FonziDaytona

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Re: UD2.7 dual Purpose Use
« Reply #24 on: October 13, 2022, 04:47:02 PM »
Hi David,

I knew I was missing a few things :-)

That would explain the heating, then.

I ordered some 3W 5ohm resistors…I should be able to run at a little lower frequency (30khz) and be within their ratings. I may also incorporate my Interruptor to drop the on time down a bit. I had originally planned to do this for helping with current limit if my PSU winds up exceeding expectations.

Thanks!
Matt

High Voltage Forum

Re: UD2.7 dual Purpose Use
« Reply #24 on: October 13, 2022, 04:47:02 PM »

 


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