I am using an isolated DC/DC converter (A1212S). On the secondary side, I generate a small negative voltage using a resistor and a Zener diode between +VO and –VO, so the midpoint becomes a local reference (virtual ground).


What I am not sure about is where this midpoint should be connected.
Should this virtual ground be connected to anything else, or should it remain floating and only be used as the reference for the secondary-side circuit (gate driver and decoupling capacitors)?


At the moment, I simply do not know where this midpoint is supposed to go.

 

I could not find "A1212S-2W". I know it is isolated supply. 12Vin 2 watts out +/-12V out.
Measure from Pin-4 to 6 is 24V. Measure from Pin 5 you get +12V and -12V.
What is the MOSFET or IGBT?

Look in the data sheet for the Gate Driver. Connect the top of your Zener to the Emitter.
Add high frequency caps across the Zener or Source.
It looks like you want to turn off at -3.3V and turn on at 24-3.3=20.7V That is a little high. Probably the max on the gate is 20V. You might consider using a 4.7V diode. Or picking a different supply voltage. Look at the transistor data sheet.

You have a resistor + Zener across 24V. Check the power in the resistor.

 

I sorry It A129

I could not find "A1212S-2W". I know it is isolated supply. 12Vin 2 watts out +/-12V out.
Measure from Pin-4 to 6 is 24V. Measure from Pin 5 you get +12V and -12V.
What is the MOSFET or IGBT?

Look in the data sheet for the Gate Driver. Connect the top of your Zener to the Emitter.
Add high frequency caps across the Zener or Source.
It looks like you want to turn off at -3.3V and turn on at 24-3.3=20.7V That is a little high. Probably the max on the gate is 20V. You might consider using a 4.7V diode. Or picking a different supply voltage. Look at the transistor data sheet.
View attachment 361881
You have a resistor + Zener across 24V. Check the power in the resistor.

Thanks for the explanation. I understand that the DC/DC output is isolated.

My intention is to use an A1209 isolated supply and then generate about +14.7 V and –3.3 V using a Zener diode, so that I have a small negative gate-off voltage. The midpoint of the Zener network becomes a local reference (virtual ground).

What I am still not completely clear about is where this virtual ground should be connected. Should this midpoint only be connected to the gate driver reference (VEE/emitter/source) and the local decoupling capacitors on the isolated side, or should it be tied to some other ground node?

In other words, I understand how to generate the voltages, but I am unsure where the virtual ground is supposed to go in the circuit.

 

The picture is not clear. R1 is across the top 4.7uF.
3.3V Zener is across the bottom 4.7uF.
VEE2 is -3.3V as measured from the Emitter.
VCC2 is +14.7V as measured from the Emitter.

 

View attachment 361883
The picture is not clear. R1 is across the top 4.7uF.
3.3V Zener is across the bottom 4.7uF.
VEE2 is -3.3V as measured from the Emitter.
VCC2 is +14.7V as measured from the Emitter.

So you mean that this 0 V node is the virtual ground of the A1209 isolated supply, and it should be connected directly to the MOSFET source, or equivalently to the midpoint between the two decoupling capacitors, correct

 

View attachment 361883
The picture is not clear. R1 is across the top 4.7uF.
3.3V Zener is across the bottom 4.7uF.
VEE2 is -3.3V as measured from the Emitter.
VCC2 is +14.7V as measured from the Emitter.

It looks like this, right?

 

The only thing I would change: You have two caps in series (pins 4 to 6) on the isolated power supply. You only need one.

?What are you doing for the bottom MOSFET? The same thing?

 

The only thing I would change: You have two caps in series (pins 4 to 6) on the isolated power supply. You only need one.

?What are you doing for the bottom MOSFET? The same thing?

The bottom MOSFET

The only thing I would change: You have two caps in series (pins 4 to 6) on the isolated power supply. You only need one.

?What are you doing for the bottom MOSFET? The same thing?

For the bottom MOSFETs, yes — the structure is essentially the same.
However, I use a separate isolated supply for the three low-side MOSFETs, and three independent isolated supplies for each high-side MOSFET.
This way, each high-side gate driver is fully isolated and referenced to its own source/emitter, while the low-side drivers share one isolated supply referenced to the DC bus negative.