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Registered Member #1334
Joined: Tue Feb 19 2008, 04:37PM
Location: Nr. London, UK
Posts: 615
...when there are so many chips capable of driving FETs/IGBTs in half-bridge configurations without using a GDT? It seems that standard motor or inverter driver chips from a number of manufacturers are pretty much ideal for hitting big drivers, with the added bonus of coming with a raft of protective circuitry...
So why does everyone use GDTs? Yes, they are simple and pretty foolproof, but they are big and carry their own baggage...
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
The largest shortcomings of the high-side gate drive chips are as follows:
1) propagation delays are often longer than transformer coupled methods and often un-even (shorter rise time, longer fall time type thing).
2) they dont tolerate poor layout. Negative spiking of the high side driver source connection will destroy the isolation barrier internal to the chip. I found many half-bridge drivers failed this way when designing motor controllers... it would surely be worse for tesla coils.
3) they typically only work up to 600V
4) they require isolated power supplies, which often have hidden problems like extra parasitic capacitance. Boot strapping has its own limitations at start up, or when the drive sits idle.
5) often cost more money than GDTs.
6) difficult to get negative Vge bias that is integral to transformer drive.
I honestly cant think of why you would not want to use a GDT for tesla coil work where the drive waveform is 50% duty cycle and no additional deadtime. After having explored many other gate drive options (also part of my job as an R&D engineer designing inverters) i find GDTs to be simple, robust and very effective. Of course that can all go wrong with poor design, but on the other hand, its much easier to screw up other types of high-side gate driving.
Registered Member #1334
Joined: Tue Feb 19 2008, 04:37PM
Location: Nr. London, UK
Posts: 615
Thanks for the comments - I was interested as no-one seemed to be doing anything else, and being curmudgeonly & contrary, I wondered why...
Being specific, I was looking at the ST L6384E, and AN2738 from them.
If you think its a no-go, I'll stick with the GDT approach, but they sure looked interesting. The app note is a good read anyway about general practice/design theory.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
I "sploded" many of the IC gate drivers until I found the failure mode - the floating supply's ground spikes often many volts under the -V buss, overvolting the chip/gate or making it to act funny in other ways (like both outputs go HIGH at the same time). They work fine for applications where there's not much inductive current, but for TCs, they are mostly unusable.
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Why use GDTs....
