Measurement of high power with Microcontroller

Thread Starter

Victor07

Joined Apr 16, 2019
21
The circuits below show how I wanted to measure ADC with my c2000 launchpad but got blown up and affected my pc also. The reason was that I wanted to isolate to provide one on one mapping so that will can get an accurate measurement of my voltage and current value, the first isolation I used for pc817 did not give me the actual value of the ADC I wanted to measure for feedback purpose.

To cut the story short I need an isolation method for my project, any suggestion please I need a different method so I can select the place of application.

Thank you for stopping by.
 

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Papabravo

Joined Feb 24, 2006
20,133
The circuits below show how I wanted to measure ADC with my c2000 launchpad but got blown up and affected my pc also. The reason was that I wanted to isolate to provide one on one mapping so that will can get an accurate measurement of my voltage and current value, the first isolation I used for pc817 did not give me the actual value of the ADC I wanted to measure for feedback purpose.

To cut the story short I need an isolation method for my project, any suggestion please I need a different method so I can select the place of application.

Thank you for stopping by.
I need some additional explanations because I cannot quite follow your post.
The circuits below in your post do not show me anything I can use because they are too small. You say you want to measure ADC, which makes no sense. I don't know what a c2000 launchpad is and I'm sorry it got blown up and damaged your PC.

I'm going to take guess here and ask if you want to measure the voltage and the current of the AC power line with an A/D converter.

Then you say: "I wanted to isolate to provide one on one mapping". Then you said that a PC817: "did not give me the actual value of the ADC I wanted..."

It is neither advisable, nor safe to connect ANY circuitry directly to the power line. I hope this lesson has been impressed on you, and you will not foolishly attempt this in the future.

Let us agree that for now you will refrain from trying to measure the power line directly. If you agree to that we can talk about how to proceed with alternative methods.
 

Thread Starter

Victor07

Joined Apr 16, 2019
21
Thank you for the response.

Let me see if I can explain it better, I am trying to measure the output voltage of a 400VDC dc to dc converter, so I used a voltage divider to reduce the voltage to about 2.8VDC, then I fed the ground and the 2.8VDC to the microcontroller pin to measure the voltage without isolation.

The other circuit is the sine factor to measure a dc voltage but this is about 250VDC so I used a resistor divider or voltage divider to reduce it to 3VDC, so I fed it to the microcontroller for measurement without isolation, so spoilt a lot of things, I don't know if its the ground issue or no isolation that caused the problem, but the voltage was reduced to 3VDC why can't I feed it to the microcontroller directly for measurement.

If isolation is necessary what are the different methods of isolation of high voltage from low voltage before I can measure the voltage, in such a way that the input and the output of the isolator are the same for accurate measurements?
 

Irving

Joined Jan 30, 2016
3,427
Can you post better schematics that can actually be read.

Measuring HV DC shouldn't prove difficult and a voltage divider is normally sufficient. However, the fact you blew things up suggests your DC output wasn't floating with respect to your expected 'common ground'. Howeveryour first diagram says 'AC' not 'DC' and you appear to be using a differential amp approach, which can work if done correctly but I suspect from the little I can see isn't quite right.

I don't understand what the 'sine factor' of a DC voltage is- is this a typo?
 
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Thread Starter

Victor07

Joined Apr 16, 2019
21
The diagram is so big but I just decided to cut the part I need help which is to measure the voltage using the microcontroller.

The sine factor is to measure the voltage before the rectification, which I used half bridge rectification.

The point is that what are the different ways that I can isolate the high voltage side from the low voltage side before feeding it to the microcontroller?
For the 400Vdc that I spoke about, I have not measured it, I am just skeptical if it going to work as aspected without causing any damage.

The diagram given shows voltage at a different level.

I want to know the issue with the grounding of the circuit,
 

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Reloadron

Joined Jan 15, 2015
7,192

Irving

Joined Jan 30, 2016
3,427
In isolation your circuit works as expected as shown in the simulation.

1661187390074.png


The concern I have with this circuit is where the common ground - your dotted red line - is going elsewhere. In any external equipment, eg 'scope, PC, etc, there is a good chance that their ground, case, etc. is connected back to the protective earth (PE).

In most installations PE is bonded to Neutral at the generator or more commonly at the entry to the building. But you have now also connected it to the 'centre tap' of L & N.... This ought to trip any RCD in circuit but older installations might not have them.. Adding that to the simulation I(PE) shows the current that flows through the ground connection to the external circuits...

1661187520873.png

You caan only use this approach if the 'ground' is purely local. It would be fine if your MCU is standalone, or the connection to your PC was opto-isolated.

Where is this 400v DC buss going?
 
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Thread Starter

Victor07

Joined Apr 16, 2019
21
Thank you all for the help so far and special thanks for the simulation.

let me quickly mention that the 400V is going to buck converter.

I thought that the center tap of the L and N is to force the potential to zero. Then for the isolation what are the ways we can
isolate high voltage from low voltage before we feed it to the MCU for accurate measurement.
 

Irving

Joined Jan 30, 2016
3,427
And the output of the buck converter?

I have some ideas regarding isolation, but you need to look at the system as a whole. Do you have a complete schematic or even a block diagram of the whole?
 

Thread Starter

Victor07

Joined Apr 16, 2019
21
That is the circuit I want to measure the voltage and current of the conversation using a microcontroller.

I need to isolate or connect the ground properly to avoid damaging my launchpad.
 

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Irving

Joined Jan 30, 2016
3,427
That is the circuit I want to measure the voltage and current of the conversation using a microcontroller.

I need to isolate or connect the ground properly to avoid damaging my launchpad.
You need to export your schematic as a PDF so it can be zoomed in. What package are you using to draw the schematic?
 

rsjsouza

Joined Apr 21, 2014
374
Before I start, a piece of advice: whenever trying to probe or evaluate something in high voltage/high power, always start low to minimize damage to components and eventually to your own life, and verify all the possible ground connections and galvanic isolation points to prevent from damaging components.

That said, if you are using a LAUNCHXL-F28027, the device can be galvanically isolated from the PC host by removing all the supply and GND jumpers of the board - this way you can guarantee that at least the PC side will not be affected (the ISO devices go easily up to 2.5kV, so no problems there) while you will keep communications intact via UART and JTAG. However, since the PC is not providing power to the F28027 device anymore (as it is fully isolated), you will have to provide power to it using an external power supply.

Once this is done, then I would start following the suggestions of others above with regards to voltage dividers and such.

Good luck in your probing!
 

Irving

Joined Jan 30, 2016
3,427
So I've taken your schematic and reduced it to a block diagram showing each functional block and the external inputs and outputs to each. You will see that all the high voltage stuff before the isolation transformer has an 'HV common' (purple diamond) though I hesitate to call 'ground' as its floating some 200 - 300v relative to the protective earth. Each of the inputs and outputs of each block are coloured with respect to their 'common' return path. The grey blocks are elements not yet (or not fully) specified on your schematic. All the LV side has a common ground (green diamond) which is connected to the PE only at the charger output. All the HV side is usually isolated (in a quality product) inside a screened box which is grounded to the PE. So you have one-way isolation from MCU to the various PWM/control inputs to each block. Generally these could be:

  • traditional optoisolators such as Vishay VO0600T/0630T,
  • isolated gate drivers, such as Silicon Labs SI8261ABC, becoming more common as reduces parts count, increases reliability.

or you could have a control MCU on the HV side then this would be bidirectional interprocessor comms.

Not sure why you need the buck converters - why not drive the inverter from the 400v directly?

The monitoring side can be more tricky because you want analog data to cross the barrier. There are several options here, eg:

  • Put an ADC on the HV side and use an isolated I2C interface (hence bidirectional). There are also ADC with built-in isolation.
  • Use isolating opamps to transfer the analog signal, eg TI's AMC1211/1311/1411 or AMC1351, Analog Devices' ADuM3190/4190, or Broadcom ACPL-C87A/B, -C870
  • Use a linear opto-coupler such as Vishay's IL300 or IXYS' LOC110/111/112 (this is my preferred approach as its very flexible though uses more parts)

Hope this help. Keep safe!

1661620815892.png
 

Thread Starter

Victor07

Joined Apr 16, 2019
21
There is something bordering me, the number of MOSFET selected to operate in a safe operating area for my inverter of 1KW taking IRF840 for example.

Which formula will I use to calculate the number of MOSFETs to operate in a safe area?
 

Irving

Joined Jan 30, 2016
3,427
There is something bordering me, the number of MOSFET selected to operate in a safe operating area for my inverter of 1KW taking IRF840 for example.

Which formula will I use to calculate the number of MOSFETs to operate in a safe area?
Which MOSFETs are you referring to? The ones in the H-Bridge?

Do you have the transformer characteristics or part #? What is your inverter PWM frequency? And how are you driving them?
 

Thread Starter

Victor07

Joined Apr 16, 2019
21
MOSFET is irf840
PWM is 100KHz
phase shift full bridge topology
driving the MOSFET from MCU to Mosfet driver to MOSFET.
for 1KW rating SMPS how many MOSFET (irf840) will be required for the device to operate in a safe area?
 

Irving

Joined Jan 30, 2016
3,427
Its not that simple, need more info

What drivers?
I'm assuming 250v DC rail?
Confirm you're referring to U13 - U16
What transformer and inductor?
 
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