Author Topic: DC Bus Voltage sensing cirquit  (Read 555 times)

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Offline eliafavero

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DC Bus Voltage sensing cirquit
« on: April 08, 2019, 11:52:40 pm »
Hi All

----disclamer----
first time posting on the blog and i am not sure this is the right section for this topic. Feel free to point me to the more appropriate section or to a post that already discusses the topic.

General / open question (Below more context is given):
Can anyone point me to documentation or suggest me some DC voltage sensing cirquit? I am working on a project where i need to measure a voltage of roughly 270V.
Design restriction:
   - No microcontroller, all analog
   - Low budget
   - Accuracy of ~1% with the voltage between 270 and 320V
   - Low power consumption 

context:   
I need to design a voltage sensing cirquit for a DC bus of 270V. Multiple AC motors and the respective driver are connected to the DC bus. The circuit needs to "regulate"  the dc voltage by powering a "braking resistor" (Pnominal = 500W) in case that the dc voltage raises above 300V. The option that i found to be the best at the moment is the "typical application Circuit" proposed on the datasheet of the ACPL-C87 sensor with some sort of schmit trigger or comparator stage at the end to drive the Mosfet of the braking resistor (see the attachements).

My concern with this design is precision. To measure the voltage i first need  a voltage divider which is already pretty inaccurate. The ACPL-C87 accepts an imput voltage of 0-2V. The variation of the DC voltage is about 10% (((300-270)/270)*100) and since Vin is proportional i have roughly 200mV to work with.
Now,to me, it sounds a bit unrealistic to have a reliable measurement in an environment with high AC Noise.
Should i not be worried? Is there a valid alternative? 

if something is unclear(maybe i should delete the "if") just ask, i'll be glad to give further explanation.
Thanks! :)

« Last Edit: April 09, 2019, 12:05:35 am by eliafavero »
 

Offline schmitt trigger

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Re: DC Bus Voltage sensing cirquit
« Reply #1 on: April 09, 2019, 12:01:57 am »
A rssistor divider is as accurate as the tolerance of its individual resistors.

The 1% tolerance resistors are cheap as dirt nowadays, and you can also get 0.1% easily (at least in major countries).

Using exclusively 0.1% resistors, and by selecting the proper values, the resulting divided down DC sample will retain that accuracy level.
 
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Online Zero999

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Re: DC Bus Voltage sensing cirquit
« Reply #2 on: April 09, 2019, 12:09:09 am »
There's no need for the ACPL-C87, as the circuit being switched, isn't isolated for the voltage being measured.

I would go for the TL431 controlling some power transistors, with most of the power dissipated by a resistor.
 
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Offline Marco

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Re: DC Bus Voltage sensing cirquit
« Reply #3 on: April 09, 2019, 02:14:20 am »
I'd call it a bleeding resistor since the bus voltage is already providing the breaking.

You can use a TL431, but it will need a few auxiliary components. How about something like this? Switching won't be very fast.
 
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Offline eliafavero

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Re: DC Bus Voltage sensing cirquit
« Reply #4 on: April 09, 2019, 03:44:07 pm »
There's no need for the ACPL-C87, as the circuit being switched, isn't isolated for the voltage being measured.

I would go for the TL431 controlling some power transistors, with most of the power dissipated by a resistor.
you are definitly right! Don't know why I came up with the isolated OP amp.:palm:

I am not very familiar with the TL431, is the circuit proposed by Marco the same you had in mind? or would you propose something different? i'd really appreciate some schematics :)

I'd call it a bleeding resistor since the bus voltage is already providing the breaking.

You can use a TL431, but it will need a few auxiliary components. How about something like this? Switching won't be very fast.

I think i haven't done a good job at explaining the situation.
the AC/DC converter actually generates a DC Voltage of 540V which is then divided in two with capacitors in series. this gives me virtually 2 dc lines: 270-->"0" and "0"--> (-270). I then have several motor units on the "High"dc bus and the same number on the "low" dc bus. in normal operation every motor has the same mechanical, and therefore electrical, load. this keeps the bus and the "0" stable. so far so good.
it can occour that one or several motors need to stop for a while. This causes an unbalanced load and the vitrual "0" drifts. which leads the "high" dc bus to go up to 350V and the "low" one to go as low as 190V. Is there some sort of circuit used for this applications? does anyone have an idea on how to adress the problem or which approach should i take?
« Last Edit: April 09, 2019, 04:09:14 pm by eliafavero »
 

Offline jbb

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Re: DC Bus Voltage sensing cirquit
« Reply #5 on: April 09, 2019, 07:00:32 pm »
Ah, that’s interesting. So you’re more interested in balancing than braking?

In principle you could use a synchronous buck converter to balance the upper and lower halves, but it’s a lot of work. It’s not very useful for braking the whole system because you can’t dump bulk energy.

Using two simple voltage-triggered dump resistors has the advantage of simplicity but wastes power whenever load imbalance occurs. If that’s infrequent, no problem. If it happens often and for a long time, you’ll be making lots of wast heat.

Dump resistors are also good for braking mechanical loads which might be helpful for your process control.
 
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Offline eliafavero

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Re: DC Bus Voltage sensing cirquit
« Reply #6 on: April 09, 2019, 09:12:17 pm »
Ah, that’s interesting. So you’re more interested in balancing than braking?

In principle you could use a synchronous buck converter to balance the upper and lower halves, but it’s a lot of work. It’s not very useful for braking the whole system because you can’t dump bulk energy.

Using two simple voltage-triggered dump resistors has the advantage of simplicity but wastes power whenever load imbalance occurs. If that’s infrequent, no problem. If it happens often and for a long time, you’ll be making lots of wast heat.

Dump resistors are also good for braking mechanical loads which might be helpful for your process control.

As you said, i'd exclude the buck converter since it seems a bit overkill.
the dump resistor seems an appropriate choice to me since the inbalance will only rarely occour.

Do you have any suggestions for the voltage sensing circuit?
 

Offline schmitt trigger

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Re: DC Bus Voltage sensing cirquit
« Reply #7 on: April 10, 2019, 01:00:55 am »
I would say, utilize a variation of Marco's circuit, but with hysteresis added.

Otherwise, the power Mosfet *could* be biased in its linear region, and would dissipate excessive amounts of power.

When I say *could*, is a circuit operational mode that requires to be verified experimentally.
 
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Offline Marco

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Re: DC Bus Voltage sensing cirquit
« Reply #8 on: April 10, 2019, 01:07:32 am »
the AC/DC converter actually generates a DC Voltage of 540V which is then divided in two with capacitors in series.

Do the motors have grounded metal casings you don't want to have at high voltage when they are driven reverse or something? If you drive them with a full bridge you could just have a single DC rail.

Quote
does anyone have an idea on how to adress the problem or which approach should i take?

Here's a rethought version of the shunt reference based circuit. You should probably use some micropower shunt reference, not a standard TL431 ... for the cost of the circuit it doesn't matter, that will be dominated by the P-MOSFET, a IXTA10P50P is 4 Euro for a single.

PS. it already has hysteresis, that's what R5 is for. R5 would be relatively large relative to the resistor divider, so when the P-MOSFET starts conducting it pulls up voltage on the divider a tiny bit, creating hysteresis.

PPS. re-rethought. This circuit now puts more power on resistors rather than Q2 and the shunt regulator, this allows you to use higher current through the shunt regulator and a smaller gate resistor, speeding up switching.
« Last Edit: April 10, 2019, 02:11:09 am by Marco »
 
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Offline schmitt trigger

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Re: DC Bus Voltage sensing cirquit
« Reply #9 on: April 10, 2019, 01:12:59 am »
You are correct about R5. Thanks for pointing it out.
 

Offline jbb

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Re: DC Bus Voltage sensing cirquit
« Reply #10 on: April 10, 2019, 08:54:15 am »
That circuit looks like a good start. I would add an LED which lights when the dump is on, a flyback protection diode across the dump resistor (it’s likely to be inductive) and a bit of noise filter capacitor (maybe 1uF film across the rails?).

The trip point of the clamp should be set with care. You should do some calculations to see how high the DC link might get if your incoming AC supply is 10% high. You don’t want the dump load trying to pull the AC line voltage down, it’s wasteful.

Hysteresis is good, and can be quite large. The power MOSFET should switch at quite low frequencies (100Hz or slower is my guess). Indeed, seeing the LED go blink blink would be fine. You should also look at how much margin you get between clamp voltage and over voltage for the drives.

Finally, I recommend an earthed metal box for this one in case something goes bang.
 
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Offline eliafavero

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Re: DC Bus Voltage sensing cirquit
« Reply #11 on: April 10, 2019, 03:44:17 pm »
Many thanks to all of you!
i'll run some simulations, prototyping and then i'll come back and post what configuration worked for me.
Very much appreciate all your contributions!
 


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