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Filter Calculation for PWM

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aggpankaj2

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Hi,

In the attached figure, you can see that a P-FET (Power MOSFET) is to drive a big solenoid which operates at 8A constant current. A current sensing circuit and one or two filters on the line. one MOSFET driver is driving the P-FET (not shown in figure) at certain frequency (assume 10KHz).

Now to calculate the center frequency of PWM, I need to find out the filter values which will help me keeping my output constant (considering with noise my current to Solenoid can vary in between 7.8 to 8.2A) and determining the PWM frequency.

Can somebody make me understand how we calculate those values?

1.jpg
 

For got to tell that Output current sense is going to an MCU, which further is generating a PWM and driving the MOSFET Driver to Drive the switching element.
 

Solenoids really operate by current, and have large inductance.
You do not need to enforce a fixed output voltage and so you
don't really need another filter between the chopper and the
solenoid.

It seems like you have a ripple current defined (8.2-7.8=0.4A).
Maybe this is more than just Iripple. But you can figure Iripple
from V=LdI/dt, letting V be your max raw supply, L be the
solenoid inductance and dt being roughly 1/2*(1/fSW). The
allowed current ripple sets your minimum desired switching
frequency.

A peak-mode current control will do as well as an average,
in this kind of application. That can be a simple SRFF kicked
by oscillator for "on" and a current limit comparator for "off".

You can achieve some efficiency by setting a peak/hold
bilevel if it is an actuating solenoid; a positioning solenoid
would need closed loop based on some position feedback.
 

dick_freebird said:
Solenoids really operate by current, and have large inductance.
You do not need to enforce a fixed output voltage and so you
don't really need another filter between the chopper and the
solenoid.

It seems like you have a ripple current defined (8.2-7.8=0.4A).
Maybe this is more than just Iripple. But you can figure Iripple
from V=LdI/dt, letting V be your max raw supply, L be the
solenoid inductance and dt being roughly 1/2*(1/fSW). The
allowed current ripple sets your minimum desired switching
frequency.

A peak-mode current control will do as well as an average,
in this kind of application. That can be a simple SRFF kicked
by oscillator for "on" and a current limit comparator for "off".

You can achieve some efficiency by setting a peak/hold
bilevel if it is an actuating solenoid; a positioning solenoid
would need closed loop based on some position feedback.
Click to expand...

I agree with the point that 2nd filter won't be needed.. only one LC filter or CLC will do the purpose.
I dont have the inductance of the solenoid defined, so Iripple I only to consider from myself, assuming some approximate value.. What does this SRFF mean? and What kind of controlling you are suggesting.. i am not getting? plz explain a bit more with diagram if possible.

My idea for this circuit is to create a constant current source using PWM as you know.
 

Pankaj,
I think you need solenoid inductance to defined your PWM frequency, without that, there is no way to define it.
 

hi umesh!! i have defined my pwm frequency.. thats 20K aith +/- 5% I ripple.. but now my problem is that i need to regulate current in solenoid to 8 A, so i need a circui or better call a filter to store current for current regulation.. and i am puzzled about its calculation..

basic idea is when my solenoid is switching so fast with my pwm freq say 20KHz i need a filter (or current storage element say inductor) in between load and my switching element so that my current take a gradual path rather than steep.. so that its almost maintained at 8A.

I am not sure if i making my concern clear or not to you guys.. but i need your help on this.

umesh49 said:
Pankaj,
I think you need solenoid inductance to defined your PWM frequency, without that, there is no way to define it.
Click to expand...
 

First part of your circuit is buck chopper or step down convertor. You need to connect a diode, to allow the flow of current when MOSFET is turned off. For filter design you can refer any textbook, covering buck chopper. One good reference is Buck converter - Wikipedia, the free encyclopedia

Hope this helps.
 
You can try searchnig for some solenoid driver IC. you might not get for 8A current, but you should get some driver IC with external switch to implement the same.
 
thank you guys!! i hope now i can do it.. atripathi is right.. its calculation will be more like a buck regulator.. will let you know the result as soon i try with my calculations.. thank you guys!
 

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