Driving a DRSSTC without the need for a GDT
nzoomed, Thu Dec 15 2016, 09:27PMIve been speaking to a friend who is an electrical engineer who has been studying the schematic for the UD and is unsure what the purpose of the GDT is for.
I told him that its more than likely a means of isolation if an IGBT failed and shorted to the gates to protect the UD board from being fried from 600+V.
He tells me there are actually special MOSFETS for this exact purpose that can take the high voltages involved and is looking at doing a redesign of the board with a few improvements without the need for a GDT.
I expect that if you could drive your coil without a GDT, there would be less losses and a better signal would be sent to the driver?
Re: Driving a DRSSTC without the need for a GDT
Alex Yuan, Thu Dec 15 2016, 10:32PM
The IGBT gate need to be >~7V above the emitter to turn on (we usually drive it to 24V to be able to push more current), but the emitter is not at the same potential as ground. The UD drives it +-24V with respect to ground so the GDT is usually just a 1-1 isolation transformer. Instead of a GDT, you'd have to use a floating gate driver, which gets complicated.
Alex Yuan, Thu Dec 15 2016, 10:32PM
The IGBT gate need to be >~7V above the emitter to turn on (we usually drive it to 24V to be able to push more current), but the emitter is not at the same potential as ground. The UD drives it +-24V with respect to ground so the GDT is usually just a 1-1 isolation transformer. Instead of a GDT, you'd have to use a floating gate driver, which gets complicated.
Re: Driving a DRSSTC without the need for a GDT
johnf, Fri Dec 16 2016, 09:20AM
Electrical engineer is not the same as an Electronics engineer.
Any Electronics engineer would know exactly what the GDT was in the circuit for.
The mid point of the half or full bridge is constantly being driven to near each rail so the GDT follows this because of the isolation allowing the upper devices in the bridge to be driven correctly
12 to 230 volt inverters, variable speed motor drives all tend to use GDT's at the cheaper end of the market. Multiple isolated supplies to supply bootstrap voltage etc to run the upper half of a half or full bridge output stage increase cost but give a little more control.
QED
johnf, Fri Dec 16 2016, 09:20AM
Electrical engineer is not the same as an Electronics engineer.
Any Electronics engineer would know exactly what the GDT was in the circuit for.
The mid point of the half or full bridge is constantly being driven to near each rail so the GDT follows this because of the isolation allowing the upper devices in the bridge to be driven correctly
12 to 230 volt inverters, variable speed motor drives all tend to use GDT's at the cheaper end of the market. Multiple isolated supplies to supply bootstrap voltage etc to run the upper half of a half or full bridge output stage increase cost but give a little more control.
QED
Re: Driving a DRSSTC without the need for a GDT
ajacks504, Sat Dec 17 2016, 05:40AM
Its so each transistor sees a Vgs that is reversed and only referenced to the individual IGBT/FET's Gate/Source (Vgs).
There are chips that can do this for you, "isolated half bridge drivers."
One of the good thing about GDTs is that they can have very little parasitic inductance and still (if core sized right), deliver high current drive pulses, useful for filling up large gate capacitance of sizable semiconductors.
ajacks504, Sat Dec 17 2016, 05:40AM
Its so each transistor sees a Vgs that is reversed and only referenced to the individual IGBT/FET's Gate/Source (Vgs).
There are chips that can do this for you, "isolated half bridge drivers."
One of the good thing about GDTs is that they can have very little parasitic inductance and still (if core sized right), deliver high current drive pulses, useful for filling up large gate capacitance of sizable semiconductors.
Re: Driving a DRSSTC without the need for a GDT
Mads Barnkob, Sat Dec 17 2016, 08:03AM
This have been discussed lately on this forum, the only advantages of high side drivers are less space taken up. Even IC manufacturers admit that a "pro" of choosing ICs is because they do not sell transformers.
Mads Barnkob, Sat Dec 17 2016, 08:03AM
This have been discussed lately on this forum, the only advantages of high side drivers are less space taken up. Even IC manufacturers admit that a "pro" of choosing ICs is because they do not sell transformers.
Re: Driving a DRSSTC without the need for a GDT
nzoomed, Sat Dec 17 2016, 10:59PM
OK, well worth reading.
That explain alot. It did cross my mind if that was a reason for isolation.
nzoomed, Sat Dec 17 2016, 10:59PM
OK, well worth reading.
That explain alot. It did cross my mind if that was a reason for isolation.
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