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Smart way to scale 1600V to 5V with isolation up to 2KV.

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flote21

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Hello guys,

I am working in the design for a signal conditioning circuit for a high voltage application. I need to measure with a ADC the voltage of one IGBT bridge which are in the range of 0V and 1600V. However the maximum input voltage of the ADC is 5V.

Therefore I want to know if there is an IC with an ISOLATION of up to 2kV which is able to scale this such high voltage to the suitable ADC range.
Currently we are using a resistor divider + optocoupler to scale this voltage, but I would like to hear other solutions. Something like a a Voltage transducer like the LEM LV 25-P but much more cheaper. Any idea?

Thanks in advance!
 

Saying you are now transmitting the signal through optocoupler (IL300?) suggests that you are targeting for rather basic performance specifications, because the accuracy of analog isolation techniques is very limited.

Also the dielectric strength won't be sufficient for a lot of power electronics applications, particularly regarding partial discharge.

Classical hall effect voltage transducers are in fact a solution, also non-isolated differential voltage dividers for some applications.

Recent isolated measurement solutions are using isolated ADC or sigma delta modulators and digital isolation. In case of modern "isolation amplifiers" like ISO122 or HCPL-7520, the digital signal is re-converted to analog.
 

hi FVM thanks for the reply!

I dont understand exactly the solution that you propose.

First of all i want to clarify that the signal what i want to measure is a DC voltage level of normally 1600V.It is like monitoring a power supply. So the plan is to use an ADC to measure this this voltage and monitoring. Then, firstly, I need to scale 1600V to 5V and secondly I nedd to isolate it from the rest of my PCB. Therefore I agree that I can use one of the advised parts (ISO122 or HCPL-7520) for isolation. But what can i use for sacaling? Is ther any part with everyhting included (Volt scaling + Isolation)?

Thanks
 

The usual scaling means is a high voltage resistor respectively a voltage divider. It's inbuilt with said LEM LV25, you'll supply it separately with other solutions. If you are targeting for moderate accuracy and drift specification, a HV resistor isn't the problem. Also LV25 isn't much better than 1 percent accuracy.

According to it's rugged and old fashioned design, LV25 is using a rather large primary current of 10 mA, causing respective losses. You'll probably go for 1 mA or lower.
 

One reasonably economical way to do this is to run a voltage to frequency chip that is isolated at 2Kv.

A small dc/dc converter supplies isolated power to the V/F chip, and a high voltage digital opto isolator isolates the output frequency.

The advantage is fairly decent linearity and accuracy with excellent resolution is possible, and there is only one serial output line to isolate.
By its very nature it also averages over the measurement period, so any ripple or noise gets averaged out.

Most micro controllers now have the ability to measure frequency or period, often up to 16 bit resolution.

It may or may not fit in with other requirements you may have, but its something to think about.
 

I believe precision isolation amplifiers have used V/f conversion up to the nineties. Later they move to SDM. Recent SD ADC with single bit output like AMC1304 can be used with a uC frequency counter to achieve at least similar performance as a traditional V/f converter at lower data rates.
 

Interesting option the v\f converter. I will make a research on that to see if it is suitable for my application. Thanks for the replies buddies!
 

Hi,

maybe a PWM is possible also.
One triangle or sawtooth generator and a comparator as a discrete solution.
Or a microcontroller with built in ADC and built in PWM generation.
Should have good linearity.

****

I don´t know about the safety regulations in your country, but take care about them.
If you just need isolation, then a 3mm gap may be enough.
But for safety a creepage distance of 16mm and more may be expected.

Therefore consider some other "optical" solutions.
* IR LED and IR receiver, data transmitted UART style
* IrDA style
* POF, like AVAGO´s HFBR system.

Klaus
 

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