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Old 12th September 2014, 10:11 AM   #1
sekhar is offline sekhar  India
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Default Phase shift in gate drive transformer

Hi,
I am working with a half bridge smps where i have used a gate drive transformer(GDT) to drive the high side mosfet . Although the smps works fine no issues regarding working . I realized that that the GDT is introducing a phase shift of 120ns in the gate drive signal .
There are specs of the GDT
Core - N87
Type - toroid
Al - 2900
Ae - 0.20cm2
Primary turns - 13
Primary inductance - 880uh
Secondary turns - 26
Secondary inductance - 4.3mh
Leakage inductance - <2uh
Inter winding capacitance - 20pf
Operating frequency - 37khz

the GDt is driven by a totem pole with a coupling cap of 1uf/50V non polar
Why is the GDT introducing a phase shift ?? Can any one tell me why
I have tried different configuration increasing the primary turn in GDT reducing the coupling capacitor ....the phase shift only reduces by 30ns
Why doesnt the input signal follow the out signal ???

Regards
Sekhar
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Old 13th September 2014, 03:03 AM   #2
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Location: seattle, wa
i couldn't complain about this.

2 uH resonates with 20 pf at 25 Mhz which is a period of 40 nanoseconds.

i suspect you have a lot more than 20 pf, and a lot more than 2 uH leakage inductance.
secondary inductance is 4.3mH so .999 coupling is 4 uH.
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Old 13th September 2014, 03:09 AM   #3
sekhar is offline sekhar  India
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@johansen .......... yup ur are right i have leage of may be 4uh or a bit more have to test again .... is there any way of reducing this phase shift without changing GTD as this is the best coupling i can get out of teflon wires


Regards
Sekhar
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Old 13th September 2014, 03:32 AM   #4
Art M is offline Art M  United States
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Why are you specifying a Phase shift (Deg) as a delay (nSec). Very confusing not knowing for sure at what Frequency ?
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Old 13th September 2014, 04:37 AM   #5
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are you trying to build a 4 MHZ power supply with 1 Mhz of bandwidth?
it shouldn't matter...
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Old 13th September 2014, 05:07 AM   #6
sekhar is offline sekhar  India
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@johansen no i am just trying to make a simple half bridge ..... I just wanted to know if this can be avoided ...

my bad it should be delay of 120ns ...

Regards
Sekhar
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Old 13th September 2014, 05:20 AM   #7
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Quote:
Originally Posted by johansen View Post
i couldn't complain about this.

2 uH resonates with 20 pf at 25 Mhz which is a period of 40 nanoseconds.

i suspect you have a lot more than 20 pf, and a lot more than 2 uH leakage inductance.
secondary inductance is 4.3mH so .999 coupling is 4 uH.

Could you please explain more about how you worked out 4uH leakage inductance.
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Old 13th September 2014, 05:39 AM   #8
sekhar is offline sekhar  India
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i measured it with an LCR at the operating frequency of 37khz with shorting the secondary and measuring the inductance across the primary .

Regards
sekhar
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Old 13th September 2014, 07:25 AM   #9
Cresnet is offline Cresnet  Germany
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Join Date: Nov 2012
I am not sure if I understand you, you said

" i have used a gate drive transformer(GDT) to drive the high side mosfet "

one GDT to drive only one mosfet? a schematic for us would be better to advice you.

what about the low side Mosfet? how you drive it?
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Old 13th September 2014, 09:12 AM   #10
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Using only a GDT for the upper leg introduces a turn on/off difference which you must compensate for in the dead time of your converter.
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