Electronics > Beginners
Losing Dead time after trifilar. Good on driver side, no DT on other side....
MagicSmoker:
--- Quote from: Andy-In_over_my_head on February 19, 2019, 10:15:00 am ---...
When I scope the 2 outputs of the Fet drivers (ground clips to driver ground), I have a dead time that adjusts perfectly and I'm stoked.
When I probe the other side sides of the trifilar, I have no dead time between the 2 outputs and then I'm not so stoked.
The outputs are beautiful mirrors of each other.... with no dead time. When I adjust the dead time Pot, their duty cycle just varies...
Would there be a reason I would lose my dead time through the transformer?
...
--- End quote ---
This is quite common with gate driver transformers and is the result of the leakage inductance; it generally does not cause any problems in the actual circuit, but if you want to make sure do these 2 tests:
1. Look at the waveform across both secondaries with just a shunt 50 - 100 Ohm resistor present across each winding (no MOSFETs or other components).
2. Look at the current waveform going to the MOSFET drain(s) to see if there are short spikes at every transition from one leg to the other (ie - during when there should be deadtime).
The first test puts a real load on the transformer which (should) totally swamp the energy stored in the magnetizing and leakage inductances; you will almost certain want some shunt resistance across each secondary of the GDT in the actual circuit for the same reason.
The second test shows cross-conduction current. Note that some cross conduction current is okay - perhaps even desirable, as it suppresses ringing between the MOSFET Coss and main transformer leakage inductance - but once it exceeds about 1% of the full load current you can start having problems.
EDIT: grammar
Andy-In_over_my_head:
--- Quote from: xavier60 on February 19, 2019, 10:15:01 pm ---Both outputs from the TL494 needed to be buffered with the same phase. Like both non-inverting.
--- End quote ---
They were both Non-inverted. I don't see how they could not be with pull up resistors on the 2 TL494 outputs. (I know I didn't show that, but I didn't think it was relevant.)
Andy-In_over_my_head:
--- Quote from: xavier60 on February 20, 2019, 07:42:32 am ---BTW,
Early TL494's had a problem that would cause the steering flip-flop to toggle multiple times if too much noise became present on pin 3.
This can be bad news for MOSFETs and IGBTs.
Other ICs at the time had double pulse suppression to prevent this problem.
The TI data PDF says "Internal Circuitry Prohibits Double Pulse at Either Output", Yet I can't see anything relevant in the block diagram.
Ihttp://www.ti.com/lit/gpn/tl494
--- End quote ---
That's funny you say this. I've bought the TI brand, Sony brand and a Cheap Aliexpress brand to see if the different brands and manufacture times made a noticeable difference. Just haven't done testing yet.
(I have a sign in my lab that says "It's almost done though")
There's a lot of half finished projects during the learning curve. I just set things aside until I've learned enough to revisit the issues.
Andy-In_over_my_head:
--- Quote from: MagicSmoker on February 20, 2019, 12:06:28 pm ---This is quite common with gate driver transformers and is the result of the leakage inductance; it generally does not cause any problems in the actual circuit, but if you want to make sure do these 2 tests:
1. Look at the waveform across both secondaries with just a shunt 50 - 100 Ohm resistor present across each winding (no MOSFETs or other components).
2. Look at the current waveform going to the MOSFET drain(s) to see if there are short spikes at every transition from one leg to the other (ie - during when there should be deadtime).
--- End quote ---
Mucho Mahalo.
I'll definitely give these a go.
I did check for spikes where deadtime would normally be. I was hoping for just that.
It just looks like it's filtering the deadtime completely though. I did have spikes on the rise and fall of the square waves but they were minimal.
I'll post some clippings from my scope of the responses of these tests.
Aloha.
Andy-In_over_my_head:
--- Quote from: xavier60 on February 20, 2019, 07:29:50 pm ---The diagram shows one TC4429 inverting buffer and one TC4420 non-inverting buffer being used. So they are both really TC4420?
--- End quote ---
Oh... Negative. You are correct on the drivers. I assumed you meant the outputs of the 494.
I think the term "buffers" threw me off.
So I was under the impression the inverting/non-inverting pair was to create a push/pull on the transformer.
I would be stoked if replacing the 4429 with another 4420 corrected this.
I have a lot of "buffers" I've purchased for experimenting with. This one seemed fool proof since it only had an input and output while others have enable input pins as well.
I will swap the inverting one out for a non inverting and see what happens.
Mahalo for your time. It's greatly appreciated.
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