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Shunt current monitor in DIP package

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kathmandu

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[FONT=Verdana, Arial, Tahoma, Calibri, Geneva, sans-serif]Hello,[/FONT]
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[FONT=Verdana, Arial, Tahoma, Calibri, Geneva, sans-serif]I need to design a short-circuit protection using a high-side shunt (200A/60mV). [/FONT]I've checked the TI/Linear/Avago/AD websites but I didn't find any current shunt monitor in DIP(8) package. I do prefer a dedicated high-side monitor but any low-side one could be adapted.


[FONT=Verdana, Arial, Tahoma, Calibri, Geneva, sans-serif]Does anyone have any evidence of such a IC? Thanks in advance fo any clue. [/FONT]
 

DIP8 Package cannot resist to 200A current density.
**broken link removed**

Allegro Microsystems manufactures some current sensors, look at this, it may work for you.
 

I had no luck in finding one in a DIP either.
DIP packages are generally no longer made.
I hate to hear that.. ;) They want to bury the DIY or something??

I've seen some soic-to-dip8 adapters (pcb) but still I can't solder those 0.9mm step terminals. Avago has a isolation amplifier in DIP8 but the specs don't look good enough.

What about some good instrumentation amplifiers in DIP packages? I need rail to rail inputs though..
 

I hate to hear that.. ;) They want to bury the DIY or something??
Not deliberately.
But it's a matter of demand and volume, and hardly anyone uses DIP's anymore except the DIYers. :-(

Here's the LTspice simulation of a discrete version of the high side current monitor IC's which may work for you.
You just need to find a rail-rail op amp in a DIP that will meet your voltage requirements (what are they?).

Capture.PNG
 
Last edited:
A dedicated IC would have been better, because of the built-in calibration. But looks like your solution is the only choice. Btw, it's a 48V battery bank (64V maximum).
 

A dedicated IC would have been better, because of the built-in calibration. But looks like your solution is the only choice. Btw, it's a 48V battery bank (64V maximum).
Okay.
That means an added zener is needed to keep the opamp supply voltage within its limits, as shown below:
The LT1638 is a rail-rail opamp with a 44V rating and is available in PDIP.

Capture.PNG
 
@crutschow:

Thank you very much for your simulation. As I need a very quick response (short-circuit protection), I'm a little bit concerned about the LT1638 slew rate (0.4V/us). Does it have any effect on the above circuit topology? Also, does the Mosfet affect the output linearity?

Wouldn't be better to put a comparator on the high-side and only translate its output on the low-side (when a short-circuit condition occurs), to drive a realy and to shutdown the power Mosfets (H-bridge) drivers?
 

Thank you very much for your simulation. As I need a very quick response (short-circuit protection), I'm a little bit concerned about the LT1638 slew rate (0.4V/us). Does it have any effect on the above circuit topology? Also, does the Mosfet affect the output linearity?

Wouldn't be better to put a comparator on the high-side and only translate its output on the low-side (when a short-circuit condition occurs), to drive a realy and to shutdown the power Mosfets (H-bridge) drivers?
The slew rate won't significantly affect the response time since the opamp output voltage change is only what is required to control the MOSFET (a fraction of a volt).

The MOSFET is part of the feedback loop so has no effect on linearity.

Yes, for fastest response to a sudden overload you could use a rail-rail comparator.
You may have a problem finding one in a DIP.

But if you are concerned about speed you certainly don't want to use a (mechanical) relay to shut down the MOSFETs.
The bridge MOSFETS will be long gone before the relay even starts to operate. 8-O
 

Yes, for fastest response to a sudden overload you could use a rail-rail comparator.
Actually, I was thinking of using the opamp as a fixed gain & low offset/drift amplifier (60mV -> 3V) followed by a comparator. The output of the comparator would be translated on the low-side using a BJT or something.

But if you are concerned about speed you certainly don't want to use a (mechanical) relay to shut down the MOSFETs.
The bridge MOSFETS will be long gone before the relay even starts to operate.
The realy is used to disconnect the main 230V loads (most probably, the cause of the short-circuit). But, like I've just said, I'm using the comparator output signal to shutdown the H-bridge MOSFETs drivers (this is where I need a very fast response).
 

Actually, I was thinking of using the opamp as a fixed gain & low offset/drift amplifier (60mV -> 3V) followed by a comparator. The output of the comparator would be translated on the low-side using a BJT or something. .............
Okay.
Here's the response to a step change in current.
It takes about 5µs to respond which should be fast enough to protect the bridge.

Note I had to add resistor R3 to suppress oscillations I found when looking at the step response.

Capture.PNG
 
Thanks again for the simulation.

What do you think about this design?

shunt-mon.png

Could it be faster?

I hope I could get a higher slew rate comparator and a high frequency BJT with very low propagation delays.
 

........What do you think about this design?

View attachment 140463

Could it be faster?

I hope I could get a higher slew rate comparator and a high frequency BJT with very low propagation delays.
Looks okay.

How fast do you think you need?
 
I want to stay bellow 10us (from the short-circuit moment to the drivers shutdown command).

It may look too conservative but the threshold (200A) is quite high for my inverter ratings hence there has to be an abnormal condition for such a step change to occur.

With your proposed circuit, I could get 5us propagation delay at the low-side resistor terminals?
 

.............
With your proposed circuit, I could get 5us propagation delay at the low-side resistor terminals?
Yes, that's what the simulation shows.

Here is a simulation of the circuit with an added ground side comparator.
For a step current rise from 190A to 210A, the comparator responds in <2.5µs with the comparator set for 3V (200A).

Capture.PNG

What is the nominal running current?
 
The rated current is around 100A (though the average load current is 10-20A). I could further set the short-circuit threshold to 150A or so (I'll try to simultaneously start some heavy loads to check for the transient currents).

I just want to be sure that from the moment I've got 45mV (150A load current, by example) across the shunt resistor, I could send a shutdown command to MOSFET drivers in less than 10us.

Unfortunately, I can't run LTSpice on my PC box (I'm on linux and I have a problem with wine installation, too) so I have to ask you once again to run a simulation of a load current step change of 100-200A, while the comparator threshold is set to 2.25V (150A).

Thank you very much for your time. Btw, I have already ordered few LT1638 and some low-power P/N-channel MOSFETs. I hope to be able to make some test on Tuesday.. ;)
 

Here's the simulation with the threshold set at 2.25V (150A) and a current step of 100A to 200A.
The comparator response time is still <2.5µs.

Capture.PNG
 
That's perfect! I've already start designing the pcb. ;)

Thank you very much for your kind support. I'll post few scope screens when it's finished.
 

That's perfect! I've already start designing the pcb. ;)

........................... I'll post few scope screens when it's finished.
Make sure you add some decoupling capacitors to the power, such as 0.1µF ceramics between the opamp and comparator plus power pins to ground (don't add one to the minus opamp pin or you will zap the op amp).

Yes, I'd be interesting in hearing how it turns out.
 
I have some bad news. The supplier has just notified me that the DIP8 package (LT1638) is out of stock. I've searched a lot but I didn't find any other similar circuit (rail-to-rail input) in DIP package.

I'm afraid I'll have to design some sort of "over the rail" power supply to accomodate a regular (not rail-to-rail) opamp. Maybe a bare implementation of a voltage doubler could do the job.

shunt-over-rail-supply.png

Any other ideas??
 

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