low pass lc filter design

[--BawA--]

Can we use electolytic capacitor to design a low pass lc filter for filtering spwm from h bridge ?

 

[chuckey]

The load is balanced about earth and you need a DC polarising voltage on the caps. So if this is what you really want to do, split the filter into two PI sections, wiring each inductor in series with the load. Connect your two capacitors per section, one to the +ve line, the other to the -ve line. So at initial switch on the charging currents, charge each capacitor directly from the PSU lines and not through the H bridge.
Frank

 

[--BawA--]

I hve used electolytic capacitor for filtration after h bridge nd my mosfets of h bridge blown out. Without using that filter, circuit was running smoothly .
Should I use film capacitor?

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I hve used electolytic capacitor for filtration after h bridge nd my mosfets of h bridge blown out. Without using that filter, circuit was running smoothly .
Should I use film capacitor?

 

[chuckey]

if the filter was not split into two halves, but just wired in series with the load then it is impossible to get any DC bias on the caps. One way is to use two caps of twice the capacitance in series (- to +)with a diode across each one so they are only exposed to the correct polarity voltage. Are you sure that you have got a good filter?, Should be inductive input to the H bridge, or the high frequency components of the PWM will cause the transistors to draw excessive current, because the first cap is a short circuit to the high frequency components. if you make provision for the bias, electrolytic capacitors are cheaper then film.
Frank

 

[--BawA--]

i have only used one capacitor of 47uf 350volts electrolytic type and an inductor 30uh ,,, i havn't connected any load across capacitor ,,,as i turned on my circuit within 3 second my mosfets blown ,,, , i thought this was due to electrolytic capacitor ,, please suggest me a way to overcome this problem?

 

[--BawA--]

also tell me the required calculations for designing a low pass lc filter?

 

[FvM]

A low pass filter capacitor will be loaded continuously with the full inverter output voltage. An electrolytic capacitor must be rated for the respective peak-peak voltage, about 350V for 120V AC or 700V for 230V.

That's mostly a theoretical consideration because the AC current specifications of most electrolytic capacitors won't allow this 50/60 Hz AC voltage level.

In other words, in most cases you have to refer to film capacitors.

 

[--BawA--]

dear fvM
i want to design a low pass filter , how to proceed ,i mean how to take care of impedence?
my inverter rating is 220v ,350 watts,

 

[FvM]

As a rule of thumb, the filter inductor should be selected with a fundamental wave reactance of 1 .. 4 % of the characteristic inverter impedance Vrated/Irated. 1% corresponds to about 4.5 mH. If a second order filter is intended, you can add a parallel capacitor to the load. For a cut-off frequency of 2 kHz you get e.g. 1.6 µF.

 

[--BawA--]

thanx fvm
where is problem in my calculation
inverter rating 220v 350 watt
using p=iv
i get i=350/220 i=1.59ampere
inverter charecterstic impedence will be 220/1.59 =138.36 ohm
now 1% of 138.36=1.383ohm
X=2*p*pi*f*l
1.383=2*3.14*16000*L
L=0.00001376 henry or 1.3uh
and u hve calculated 4.5mh how?

 

[FvM]

The essential point is "fundamental wave", which means 50 Hz. In other words, I'm choosing the maximum inductance value that keeps the 50 Hz voltage drop acceptable.

 

[--BawA--]

Thanx fvm for ur reply
Can u plz tell me any book or any link , where I can read all about filters in inverter for spwm.?
I want to learn more in this field.

 

[--BawA--]

Hiii
I have successfully made my h-bridge stage of my pure sine wave inverter.when I connect the 311vdc bus to h bridge , it works fine I.e no heating or explosion. But as I connect a lc filter, my mosfets blown out;
I hve used 5mh inductor
0.22uf capacitor
My inverter rating is 220v 350watts.
Please reply anyone??

 

[FvM]

I don't believe that it's primarly a problem of the filter. Have you been able to operate the inverter with a resistive load without a filter?

 

[--BawA--]

yes fvm , i have operated a 15watt bulb ,without filter ,, and it was working fine .but when i connect the filter ,my mosfet blown out, also a high current flows through my dc bus,
what was the problem in the circuit ,, i was using a simple lc filter ,,should i use pi or T filter ??

 

[FvM]

15 W load isn't much for a 350 W inverter, the reactive current drawn by the LC filter is already higher.

So as the most simple explanation, I assume the inverter has design faults and can't work with a certain amount of load current.

More specifically, there might be a problem with reactive output current. A MOSFET H-bridge with suitable drive waveforms won't have such problems, however.

Finally, I should ask about the inductor design. There's a certain possibility that it's already saturated at 220V/16 kHz and draws excessive current.

 

[--BawA--]

hi Fvm,
i am giving you complete detail,

i have used irf740 mosfets for h bridge, and ir2110 mosfet driver ic,

for bootstraping, i have used 22uf capacitor,

My hbridge is driven by pic16f877 mcu , and i have simulated the code , and the mcu was generating the SPWM signal fine,

for inductor design i have used a ferrite toroidal core of OD 35.5mm
ID 23 mm
HT 12. mm approx
my DC genarating stage i.e 12vdc to 311vdc also blown out due to excessive current,, and initially i have seperately tested my Dc dc stage with a 200Watt bulb , and it was working fine, today i decided to rectify the 220volts that is coming from wall outlet using in4007 and 47uf,450v capacitor ,and connect the rectified output dc to hbridge, after connection was complete ,i first power up the Hbridge circuit and mcu,, after that i switched on my dc voltage ,,, in output ,i connected a 15watt bulb ,and that was working fine ,,,after thet i removed the bulb and connect the LC filter ,,now as i switched on my dc voltage ,,my hbridge mosfet blown and also mcb of my room tripps off due to excessive current ? and i have made my circuit 6 times again and again , but i get only failure all the times, and m really confused what to do now? where is the problem in my design?

 

[--BawA--]

please reply,,

 

[FvM]

for inductor design i have used a ferrite toroidal core of OD 35.5mm
ID 23 mm
HT 12. mm approx

Core material and number of turns would make a specification.

 

[--BawA--]

i have used ft140, i dont know the material no, because here ,sellers want only outer dia, inner dia and ht, they dont know about material no, i have used online calculator , and from that i found to make 5mh i hve to wind 72 turns ,for ft140-43 , but when i checked it on inductance meter , i got it was only 0.7mh,, so i wound the ferrite core with sufficiently high no. of turns,,
i have some more doubts-
,how to calculate reactance current of the lc filter?
how can i know that my inductor has saturated?