Gate resistor value tradeoff / Tesla Coils / Forums

Forums

4hv.org :: Forums :: Tesla Coils

« Previous topic | Next topic »

Gate resistor value tradeoff

1 2 next

Move Thread

LAN_403

chris_inkubate

Mon Sept 15 2014, 06:03PM

Registered Member #46007 Joined: Wed Apr 30 2014, 08:02PM
Location: Walsall, UK
Posts: 36

Hi all,
I have been experimenting with gate driving a Littelfuse MG06400D-BN1MM half bridge brick using UD2.5 and don't fully understand the trade offs associated with the gate resistor value. This brick already has an integrated 2.5R gate resistor so I'm not sure if this is enough or if to add more. From tests adding an additional 4R7 caused the rise time of the gate waveform to be around 1us while testing at around 60KHz. Reducing this value to 2R near enough halved this to 500ns. I suspect that 1us is a bit slow but am also aware that dead time is created by this and is also desirable to avoid shoot through.

Can anyone clarify this for me please so I can determine and optimum value?

Many Thanks

IGBT:
Gate charge: 1.8uC
Integrated gate resistor: 2.5R

loneoceans

Mon Sept 15 2014, 06:13PM

Registered Member #4098 Joined: Fri Sept 16 2011, 09:26PM
Location:
Posts: 236

I did a write-up on how to choose a good gate resistor value. Too small and you will be at risk of shoot-through. Too large and the gate doesn't turn on fast enough. The transistions can be seen by scoping the gate and collector and adjusting them for best performance. See here (somewhere in the middle with scope shots):

chris_inkubate

Mon Sept 15 2014, 07:18PM

Registered Member #46007 Joined: Wed Apr 30 2014, 08:02PM
Location: Walsall, UK
Posts: 36

Thanks for that link to your site - very useful information. Its made some sense to me so I can work towards perfecting the value when I also adjust my phase lead.

I know this may be a silly question but I assume from what I read that I scope the outputs of my full bridge where one connection goes to my primary and the other my tank cap? I've yet to test as my current scope probe isn't rated for this although having a USB scope and laptop should allow measurement without using two probes (I think!). I must admit its a scary prospect connecting my scope and laptop direct to my bridge!

Chris

loneoceans

Mon Sept 15 2014, 09:15PM

Registered Member #4098 Joined: Fri Sept 16 2011, 09:26PM
Location:
Posts: 236

You will have to be careful when scoping your bridge to avoid blowing things up. The best and safest way to do this is to isolate your entire bridge completely. I usually use a 60V isolated power supply for low power bridge testing, and then move on to an isolation transformer which then goes to my variac for higher power. Use one scope at the GATE of one IGBT and another at the Collector of the same igbt (for say the bottom IGBT so the ground goes to the emitter and the negative rail). This allows measurement of gate voltage and inverter output. For primary current, just measure the voltage across a burden resistor across a current transformer. You can make your own transformer just like making FB or OCD CT.

chris_inkubate

Mon Sept 15 2014, 10:10PM

Registered Member #46007 Joined: Wed Apr 30 2014, 08:02PM
Location: Walsall, UK
Posts: 36

Cheers for the info on testing. I'll add some waveform pix when I've done to show how I get on. I have a 48V SMPSU which I can use so I'll rig that up and check that its isolated first.

I made a little current transformer for testing that works well for current monitoring so I'll be able to finally measure and design this DRSSTC better then my previous attempt as I really don't want to blow a brick!

Dr. Dark Current

Mon Sept 15 2014, 10:33PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Use the smallest resistor you can while the gate waveform still looks clean without too much ringing and overshoot. If the output resistance of your gate drivers is high enough, you can even get away with no resistor.

Steve Conner

Tue Sept 16 2014, 07:40AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

I believe too small a gate resistor can cause reliability issues due to excessive gate voltage overshoot and collector voltage spikes caused by turning the IGBT off faster than it was designed to go.

If you study the datasheet of an IGBT brick, you'll probably see a particular value of gate resistance that was used when measuring most of the parameters and charts. I take this as the recommended value.

To introduce deadtime, a larger gate resistor is commonly used along with a diode that shorts it out in the reverse direction, so the device turns off faster than it turns on. This is not ideal because the large gate resistor slows the turn-on rise time as well as the desired effect of lengthening the delay period. But it's simple and seems to work fine for most DRSSTCs.

Solid-state tesla coiling is all about connecting expensive things to dangerous things.

Dr. Dark Current

Tue Sept 16 2014, 03:48PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Steve, I'm aware of that, even though the "too fast gate drive can damage a transistor" belief is somewhat unconfirmed.
With the gate charges of usual bricks and the way most people make GDTs, the "smallest resistor value while the gate waveform still looks clean without too much ringing and overshoot" will most often be larger than a value which could cause problems by too fast switching. I was just trying to be helpful without too much information

chris_inkubate

Tue Sept 16 2014, 05:32PM

Registered Member #46007 Joined: Wed Apr 30 2014, 08:02PM
Location: Walsall, UK
Posts: 36

Thanks for the additional information from you both and it is always interesting to know a little more stuff as these things sometimes make sense when a dodgy waveform appears on the scope!

The pointer to the test Rg was handy as the Td on/off etc were with a Rg o 3R which added to the internal Rgint of 2.5R gives 5.5R which is in the ball park of what many people seem to be using on IGBT gates. So I'll start with that and see what happens.

Chris

chris_inkubate

Sat Sept 20 2014, 09:17PM

Registered Member #46007 Joined: Wed Apr 30 2014, 08:02PM
Location: Walsall, UK
Posts: 36

I've setup my bridge with 2R gate resistors which added to the internal ones gives 4.5R. I put a resistive load across my bridge output and run it up on 20VDC just to check the basic output of the bridge with a scope. I notice there are some spikes on the rising edges from time to time - would this be a symptom of too small a gate resistance? I just want to get this as good as possible before putting the tank circuit in the mix!

Many Thanks

Bridge output (above)
Here is my gate waveform too (below)

1 2 next

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob