Can I use 6A Inductor for LC low pass filter for my 350W Inverter?

Can I use 6A Inductor for LC low pass filter for my 350W Inverter ?

Question: Can I use 6A Inductor for LC low pass filter for my 350W Inverter ?

I am designing a 350W Inverter using 2x PIC18F46K22.

12V 50Ah battery is used and so charging current for battery is 5A.

I want to know two things.

My calculations are like this.

I want to be able to use a 350 W max load and so 20% of 350W is 350 * 0.2 = 70.

350 + 70 = 420W

I am designing Inverter for 420W (because there will be power losses even with good low Rds(on) Mosfets).

As max load is 350W

Irms = 350W/220V = 1.59A

Ipk = 1.59 * sqrt(2) = 2.25A

So, can I use a 6A pk current inductor for my inverter's output ? (LC low pass filter).

Filter calculation is like this.

My SPWM frequency is 16 KHz.

I chose fc = approx 2500 Hz.

I choose L = 0.68 mH 6A and C = 5.6 uF 560V.

fc = 1 / ( 2 * pi * sqrt( L * C))

fc = 1 / (2 * 3.14159265 * sqrt (0.68 * 5.6 * n))

fc = 1 / (3.977 * 10^(-4))

fc = 2579 Hz

It is a pure sinewave inverter.

So, finally my questions again are

1. Can I use 6A Inductor for my purpose ?

2. Will I get clean sinewave output after filter ?

3. If max load current is say 3A then how to calculate DC-DC converter's switching transformer's primary winding current ?

Re: Can I use 6A Inductor for LC low pass filter for my 350W Inverter ?

The output capacitor causes a reactive current of about 25 % of the rated current, probably unwantedly high.

That's the price for choosing a relative low inductor value and respectively large capacitor. The cutoff frequency seems fine. Something in a 5 to 20 mH range seems more appropriate for a 350 W converter.

1. Can I use 6A Inductor for my purpose ?

Click to expand...

Basically yes. Do you consider a specific type?

2. Will I get clean sinewave output after filter ?

Click to expand...

Some residual PWM ripple of a few volts according to filter cut-off frequency and respective 16 kHz attenuation.

3. If max load current is say 3A then how to calculate DC-DC converter's switching transformer's primary winding current ?

Click to expand...

You didn't tell a circuit or converter topology.

Re: Can I use 6A Inductor for LC low pass filter for my 350W Inverter ?

The output capacitor causes a reactive current of about 25 % of the rated current, probably unwantedly high.

That's the price for choosing a relative low inductor value and respectively large capacitor. The cutoff frequency seems fine. Something in a 5 to 20 mH range seems more appropriate for a 350 W converter.


Basically yes. Do you consider a specific type?

Click to expand...

https://www.coilcraft.com/ga3199.cfm

Inductor looks like this.

https://www.3dcontentcentral.com/download-model.aspx?catalogid=4610&id=404920

Some residual PWM ripple of a few volts according to filter cut-off frequency and respective 16 kHz attenuation.

Click to expand...

Ok. I will try to get 20 mH 6A Inductor.

You didn't tell a circuit or converter topology.

Click to expand...

I will be using a 2 switch forward converter with Half-Bridge signals from the 1st PIC18F46K22. The 1st PIC18F46K22 will generate 2x Half-Bridge signals. One for 325V DC to 14.6V SMPS battery charger and another for 12V (battery V) to 325V VBUS voltage for H-Bridge (Full-Bridge Inverter).

There will be no active/passive power factor correction.

Half-Bridge PWM frequencies will be 50 KHz.

Re: Can I use 6A Inductor for LC low pass filter for my 350W Inverter ?

O.K., I see that sine inverter and DC-DC converter are separate circuits. Insufficient information about the DC-DC to discuss transformer details.

Ok. I will try to get 20 mH 6A Inductor.

Click to expand...

You probably won't find a suitable off-the-shelf inductor. Compromises might be necessary if you don't want to make it yourself or order it custom made. I would select a rated inductor current nearer to actual converter output, consider that inductor size is proportional to I²L in the first order.

Re: Can I use 6A Inductor for LC low pass filter for my 350W Inverter ?

FvM said:

O.K., I see that sine inverter and DC-DC converter are separate circuits. Insufficient information about the DC-DC to discuss transformer details.

You probably won't find a suitable off-the-shelf inductor. Compromises might be necessary if you don't want to make it yourself or order it custom made. I would select a rated inductor current nearer to actual converter output, consider that inductor size is proportional to I²L in the first order.

Click to expand...

Ok. I will get the inductor manufactured for 20 mH 6A Toroidal.

I have a question. I have seen 20 mH 6A Common mode chokes. Can I use half of the Choke as Inductor for my requirement ?

That is can I leave one side winding unconnected and use another side winding of this choke for my filter ?

**broken link removed**

Regarding DC-DC converter transformer details, here it is.

Secondary current max will be 3A. I want to know what will be the switching transformer's primary winding current.

For DC-DC Converter

Push-Pull Topology (2 switch forward converter)

Npri = (Vin(nom) * 10^8) / (4 * f * Bmax * Ac)

Core chosen is ETD39 Ferrite.

Power source is 12V 50Ah battery.

Output DC will be 325V after rectification with ultrafast recovery diodes (bridge) and LC filter.

Switching frequency is 50 KHz

Lowest possible battery voltage is 10.5V.

Highest possible battery voltage is 14.6V

Vin(nom) = 12V

f = 50 KHz

Bmax (ETD39) = 1500 G (allowed range 1300 G - 2000 G)

Ac - Effective cross sectional area in cm^2 = 1.25 cm^2

Npri = (12 * 10^8) / (4 * 50K * 1.5K * 1.25)

Npri = 3.2

Npri(rounded) = 3 turns

At minimum input voltage Vin = Vin(min) duty cycle will be max (98%)

(not 100% because FET Drivers can't switch at 100% PWM duty)

98% of 10.5V = 10.29V

Voltage ratio (secondary : primary) = 330V : 10.29V = 32.1

Nsec = N * Npri = 32.1 * 3 = 96.3 (rounded to 96)

Primary with a center tap (3 + 3 turns)

Secondary 96 turns

-------------------------------------------------------------------------------

I have another question related to SMPS battery charger. I will ask it in my next post after I get answers for DC-DC converter transformer's primary current rating.

Re: Can I use 6A Inductor for LC low pass filter for my 350W Inverter ?

Hi,

Can I use half of the Choke as Inductor for my requirement ?

Click to expand...

No, it is not designed to store much energy. It will saturate.

Klaus

Re: Can I use 6A Inductor for LC low pass filter for my 350W Inverter ?

Does the standard transformer equation holds true for switching transformer calculation also ?

Vs/Vp = Ns/Np = Ip/Is

Re: Can I use 6A Inductor for LC low pass filter for my 350W Inverter ?

Hi,

Yes. In push pull mode.

But in flyback mode the transformer is used as an energy storage device.
In one time the primary side is conne ted to the power supply .... and current is only at the primary side,
Next time the primary side is switched OFF, no current, but the energy is pushed to the secondary side --> secondary side current.

Klaus

Re: Can I use 6A Inductor for LC low pass filter for my 350W Inverter ?

I am using push-pull mode for DC-DC converter and SMPS Battery charger.

I have another question.

Np/Ns = Vp/Vs = Is/Ip

So, I know Np and Ns

Ns = 96 turns

Np = 3 + 3 turns.

So, for Np should take value as 3 or 6 for doing the calculation ? because only half of the primary will be conducting at a time in push-pull mode. So, I have to consider Np as 3 in my case ?

Re: Can I use 6A Inductor for LC low pass filter for my 350W Inverter ?

Hi,

The relationship between voltage and winding count is true always.

Sum (current x winding_count) of each side should be equal. In push pull mode.

Klaus

Re: Can I use 6A Inductor for LC low pass filter for my 350W Inverter ?

My previous question was not that. If I know Np then I can calculate Ip using standard transformer equation. My previous question was should I take Np as 3 or 6 because only half of the winding will be conducting at a time in push-pull mode.

So, I have to take Np = 3 in my case according to my transformer calculations ?

If yes, then

Np/Ns = Is/Ip

3/96 = 3A/Ip

Is this true ?

Ip = 96A ?

So, I have to use min 12V 120Ah battery ?

Mine is 12V 50Ah battery.

Re: Can I use 6A Inductor for LC low pass filter for my 350W Inverter ?

Hi,

Ip = 96A ?
So, I have to use min 12V 120Ah battery ?

Click to expand...

Don´t mix units. A is not Ah.

If you want to travel from one town to the other, maybe 100km apart.
You don´t need a vehicle that runs at least 100km/h, a bicycle is enough.

Klaus

Re: Can I use 6A Inductor for LC low pass filter for my 350W Inverter ?

But I want 2 hours backup time.

Re: Can I use 6A Inductor for LC low pass filter for my 350W Inverter ?

baileychic said:

My previous question was not that. If I know Np then I can calculate Ip using standard transformer equation. My previous question was should I take Np as 3 or 6 because only half of the winding will be conducting at a time in push-pull mode.

So, I have to take Np = 3 in my case according to my transformer calculations ?

If yes, then

Np/Ns = Is/Ip

3/96 = 3A/Ip

Is this true ?

Ip = 96A ?

So, I have to use min 12V 120Ah battery ?

Mine is 12V 50Ah battery.

Click to expand...

You have to consider 3 as your primary turns