Isolated ground planes on a MCU controlled high voltage IGBT

[stube40]

I am putting together a PCB that has a CPU to control the on-off state of a high power IGBT. There are two power supplies associated with this:

1) A 24 DC lab PSU that is regulated on the PCB to both 5V (CPU) and 15V (TC427 MOSFET driver)

2) A meaty 150V / 40A DC PSU that flows through the IGBT and into a massive inductor

The trouble is, I'm nervous about connecting the grounds of both PSUs together due to my suspicion that in certain conditions in my application I'll get large negative currents on the ground plane and/or other nasty stuff that the CPU and related electronics will hate.

However, if I dont I have a concern regarding getting the correct 15V gate voltage to turn the IGBT on and also another circuit where I use a 2-resistor voltage divider to downscale the 150V to a meagre 5V so that it can be fed into one of the CPU's ADC inputs to measure the incoming voltage.

Can anyone suggest a way forward?

 

[FvM]

Re: Isolated ground planes on a MCU controlled high voltage

Funny question. You decided to use a direct connected gate driver for your design, so you obviously have to connect the ground nets.

40A isn't actually a high current in power electronics. It's also good, that you drive only one IGBT, so the "natural" common ground
point is near the IGBT E terminal. You should be able to get a suitable accurate DC measurent using this ground reference as well.
For higher accuracy requirements, use a differential amplifier.

 

[stube40]

Re: Isolated ground planes on a MCU controlled high voltage

What if I used an opto-coupler instead?? The switching frequency is only around 10Hz, but reaction time has to be within 1ms - I think a good opto-coupler could achieve this.

If I used an opto-coupler, the only remaining problem is what to do with the ground for the resistor-divider voltage measurement feed that allows the CPU to measure the amplitude of the high-power voltage source. Maybe using the CPU's differential ADC inputs?

 

[IanP]

Re: Isolated ground planes on a MCU controlled high voltage

If I used an opto-coupler, the only remaining problem is what to do with the ground for the resistor-divider voltage measurement feed that allows the CPU to measure the amplitude of the high-power voltage source. Maybe using the CPU's differential ADC inputs?

Use linear optocoupler ..
Here are examples:
**broken link removed**
**broken link removed**

Rgds,
IanP

 

[stube40]

Re: Isolated ground planes on a MCU controlled high voltage

Great, I never knew of linear optocouplers! You learn something new everyday....

Having decided to go down the opto-isolator route, I'm struggling with the logistics of powering the IGBT-side of the opto-isolator.

To elaborate, there are actually 4x IGBTs and drivers in an H-bridge formation. They are routing the 150V / 40A power source through are large superconducting coil. We have found from previous experiments that all sorts of strange things happen when the coil is being charged and switched including zero voltage and negative voltages. Hence, this cannot be the source of the 15V for the IGBT side of the opto-coupler and the TC247 driver. Yet, to turn the IGBT on I need Vge to be 15V, but I'm worried about connecting the ground of the 150V PSU to the 15V PSU.

Does this make any sense at all, or should I upload a diagram?

 

[IanP]

Re: Isolated ground planes on a MCU controlled high voltage

To provide power to the isolated side you need a dc/dc converter ..
There are hundreds of them available on the market and they come in several voltage and wattage ratings ..
For example, the NMA1215 takes 11-13V (as input) and produces +/-15V (as output) ..

If you have, look in the RS Components book, page 240-250, or visit their website ..

Rgds,
IanP

 

[stube40]

Re: Isolated ground planes on a MCU controlled high voltage

To provide power to the isolated side you need a dc/dc converter ..
There are hundreds of them available on the market and they come in several voltage and wattage ratings ..
For example, the NMA1215 takes 11-13V (as input) and produces +/-15V (as output) ..

If you have, look in the RS Components book, page 240-250, or visit their website ..

Rgds,
IanP

Thanks Ian,

I looked at the datasheet for the NMA1215. It sounds good in that I could give it input power from the CPU side of things and still maintain isolation.

Any ideas how to make this work with the IGBT emitter, which is effectively the GND of the 150V / 40A PSU that I'm scared of? In order to get my Vge to be the required 15V, will I need to connect the minus rail of the DC-DC output to the GND of the 150V / 40A PSU? And if so, might I still be in some dangerous territory if the high-power PSU has zero or negative voltages invoked upon it due to the mention weirdness during charging/switching of the superconducting coil?

Jeez, this is my first power electronics project - I am normally a digital electronics engineer. It sure is a baptism of fire!

 

[IanP]

Re: Isolated ground planes on a MCU controlled high voltage

RBGTs have diodes to protect them against EMF kick-backs, in addition you can add a snubber circuit .. see:
https://www.fairchildsemi.com/an/AN/AN-9020.pdf

As far as the isolated 15V is concerned, you may have to add a 16V Zenner and/or Transient Voltage Suppressor ..
https://www.microsemi.com/micnotes/125.pdf

will I need to connect the minus rail of the DC-DC output to the GND of the 150V / 40A PSU

Without connecting grounds there is no way you can provide any reference, but with good surge protection that shouldn’t cause problems ..

Rgds,
IanP

 

[stube40]

Re: Isolated ground planes on a MCU controlled high voltage

Thanks IanP,

I've attached a diagram that outlines the schematic. I've replaced the TC427 and opto-coupler with a Vishay VO3120 isolated driver. The 15V power supply can be created from the CPU side via a DC-DC converter with galvanic isolation.

The snubber circuit you mention may cause me problems as we're trying to minimise power loses, but I wont rule it out yet.

Certainly the surge protection sounds like the way to go - any recommendations where the diodes / tranzorbs would be best placed?

 

[IanP]

Re: Isolated ground planes on a MCU controlled high voltage

I'd put on HV TVS on the 150V-section and one LV TVS +Zenner on the 15V-section .. see attached picture ..

Rgds,
IanP

 

[stube40]

Re: Isolated ground planes on a MCU controlled high voltage

Thanks alot Ian,

I assume the 150V section and 15V section have the negative rails commoned, or are they still floating in your scheme?

 

[IanP]

Re: Isolated ground planes on a MCU controlled high voltage

.. commoned .. :!:


*

 

[stube40]

Re: Isolated ground planes on a MCU controlled high voltage

Thanks. All questions answered. Customer 100% satisfied.

Where do I send the cheque to??

 

[IanP]

Re: Isolated ground planes on a MCU controlled high voltage

1c to Bank West ..

Take care mate ..

 

[FvM]

Re: Isolated ground planes on a MCU controlled high voltage

Considering your original post, I wonder how a single IGBT with common ground to logic supply turned into a H-bridge?

a CPU to control the on-off state of a high power IGBT

I have seen clearer questions at edaboard...

The shown circuit doesn't work as is, respectively isn't complete. I strongly suggest to draw the gate driver connections
exactly to see what's missing. I understand, that the shown +/-15 V supplies are connected to VO3120 VCC and VEE respectively,
so you get a a bipolar gate drive voltage, which is optimal for IGBT control, altough not absolutely necessary for a small
40A/150 V IGBT in my opinion. You need however additional power supplies for both high side drivers. In total, you have 4 VO3120,
3 positive and 3 negative supplies.

P.S.: Reviewing the circuit I realized, that it utilizes unipolar gate drive and a single 15V supply only. That's O.K. so far, I
was confused by the V+/V- notation. But you need at least three 15V supplies, though.

 

[stube40]

Re: Isolated ground planes on a MCU controlled high voltage

Yes, you got me there It's what we call in the trade, cutting corners!!

Anyway, as you right fully suggested - there will be 4 VO3120s. However, I had planned to use just GND and 15V going into the VO3120s, with 15V GND being commoned with the 150V GND. Do you see it a different/better way?

I can update the diagram in the morning when I'm back at work if it helps.

 

[IanP]

Re: Isolated ground planes on a MCU controlled high voltage

Just to clarify grounds and dc/dc voltages (see ½ H-bridge drawing):
VEELs on both sides are connected to GND ..
V[CCL1] = V[CCL2] on both sides is fed from the 1st 15V dc/dc ..

V[CCH1] fed from the 2nd 15V dc/dc ..
V[EEH1] connected as per drawing ..

V[CCH2] fed from the 3rd 15Vdc/dc ..
V[EEH2] symmetrically to V[EEH1]

Rgds,
IanP

 

[stube40]

Re: Isolated ground planes on a MCU controlled high voltage

Great diagram Ian!!

Just for clarification, the 15V DC/DC are giving out 0V/15V or +/- 15V?

Also, if I understand correctly you are sharing one DC/DC for the low side of the H-bridge, but using individual DC/DCs for the high side, hence 3 in total? If I wanted to be modular and wasnt worried about cost, could I also use 2 on the low side for neatness?

 

[IanP]

Re: Isolated ground planes on a MCU controlled high voltage

Just for clarification, the 15V DC/DC are giving out 0V/15V or +/- 15V?

Some dc/dc produce -15 -- 0 -- +15 but I'd suggest that you go for single output as from the same power-rating you'll have double output current ..

If I wanted to be modular and wasnt worried about cost, could I also use 2 on the low side for neatness?

2 dc/dc converters on the low side will be even better ..

Rgds,
IanP

 

[FvM]

Re: Isolated ground planes on a MCU controlled high voltage

Looks much better now. I also think, that unipolar (+15V) gate drive is O.K. But it depends on the IGBT properties and intended
switching speed as set by Rg. In high voltage applications, you often use bipolar gate drive to prevent shoot-threw under all conditions.