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Forum: Analog Circuits Comparator with adjustable hysteresis


Author: Harald (Guest)
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Hello,

I have to built a current source that controls the current in a coil. I 
want to built a ripple regulator that measures the current by a 
hall-sensor and switches the half bridge high side on if the current is 
too low and switches the half bridge low side on if the current is too 
high. Dead time to prevent cross-conduction is in the driver.

I need a high switching frequency for small currents (100kHz @ 20A) and 
a low switching frequency for high currents (20kHz @ 200A). I have to 
use a current control to have a permanent over current protection.

I want to realize this by adjusting the hysteresis of the comparator.
(see http://www.ti.com/lit/an/snva170b/snva170b.pdf,  Page 2,  Figure 3)

The only schematic I found was this:
https://www.google.com/patents/US5166550

PLEASE has someone a schematic of a comparator with adjustable 
hysteresis for me. Best would be a ground referenced voltage input for 
the hysteresis adjustment.

THANKS for your help!!!!

Best Regards
Harald

Author: catal (Guest)
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The easiest way to accomplish independent setpoints is by using two 
comparators and combine their outputs with a SR latch (similar to a 
window comparator circuit, SR replaces the AND).

For the example attached, I assumed a sensitivity of 20mV/A and a ripple 
of 10%. To fit 20kHz@200A the current rise and fall times are set to 
0.8A/µs.
Unfortunately, using the same settings, the frequency at 20A is 200kHz. 
But I'm not shure, if it's your intention to allow a ripple of 20% to 
keep the freqeuncy as low as 100kHz. Anyhow, if it's necessary to set 
the ripple otherwise than by percentage of the maximum value (or to a 
fixed value), I would use a microcontroller (D/A-Converter, PWM).

Author: Harald (Guest)
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Thanks for your help !!!!

I did it with 2 comparators and a edge triggered RS-FF made of 2 74hc74.

I made the hysteresis adjustable (current mirrors) and proportional to 
the current value.
I also added a frequency/voltage converter and increase the hysteresis 
if the switching frequency is to high.

THANKS for your help!!!!

Best Regards
Harald

Author: MaWin (Guest)
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Harald wrote:
> PLEASE has someone a schematic of a comparator with adjustable
> hysteresis for me

It's quite easy: Use an NE555, it has a turn-on point of 2/3 VCC and a 
turn off point of 1/3 VCC, and amplify your current signal measured by 
the hall sensor so that it will reach the switching points at the 
appropriate measured current.

You know hot to build an op-amp amplifier with adjustabnle gain and 
offset ?

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