SMPS Battery Charger using PIC18F26K22 - Transformer current calculation

[baileychic]

SMPS Battery Charger using PIC18F26K22 - Transformer current calculation

I am converting 250V AC max to DC and so, max DC to SMPS transformer is 250 * 1.4142V = 354V DC

My SMPS transformer secondary output voltage is 16V and I will get 16 - 1.4 (diode drop) = 14.6V for boost charging of the battery.

I have calculated the transformer specifications.

No, my question is, is the below calculation correct for SMPS transformer primaty current rating ?

I need 10A charging current for battery so that with same SMPS battery charger I can charge 10Ah to 100Ah batteries.

Vp/Vs = Is/Ip

354V/16 = 10/Ip = 22.125

Ip = 10 / 22.125 = 0.452A

What SWG wires can I use for 10A and 0.452A transformer winding currents ?

 

[KlausST]

Hi,

The transformer is a HF one (not a mains frequency one)?
What SMPS topology?

Klaus

- - - Updated - - -

Added:

Vp/Vs = Is/Ip

This is true for the average current.

But for winding loss calculation you need to calculate with RMS current.
RMS current = average_current / sqrt (duty_cycle). For square wave currents.

In detail it depends on topology.

Klaus

 

[baileychic]

One switch Forward Converter.

Transformer output power max = P = VI = 16V * 10A = 160W and as power is less than 300W I am using one switch forward converter topology.

Please show calculation.

 

[baileychic]

I also need to calculate output LC filter for the SMPS Battery charger. So, can I use

fc = 1/2Pisqrt(LC) equation and set cut of frequency to say 212 Hz (switching frequency is 50 KHz).

So,
fc = 1/2*pi*sqrt(560uH*1000uF) = 212.6 Hz

Can I use this filter for my 12V and 24V dual 15A outputs ?

This is my circuit. How can I improve it ? What value capacitor I have to use in series with primary of transformer ? How to calculate its value ?

What other components I have to add to get clean signals on the scope ?

 

[KlausST]

Hi,

Filter: it is not critical for a battery charger.
As with all LC filters I recommend to use an RC damping circuit to avoid resonance problems.

212Hz is not very likely to interfere with your 50kHz switching frequency...but I assume there is some kind of current regulation loop...

Klaus

 

[baileychic]

How to calculate value of RC damping circuit and where should I connect it ? Across output inductor ? (you mean a snubber across output inductor ?)

This is my simulation file. I am not getting 28.3V and 14.6V exact charging voltages in simulation.

 

[KlausST]

Hi,

low loss if the RC is across the output capacitor.

I´d start with 100R, 10uF. Not calculated.

Klaus

 

[FvM]

The secondary rectifier circuit is just incredible. A shallow comparison with text book circuit should reveal that it misses free wheeling diodes.

I am not getting 28.3V and 14.6V exact charging voltages in simulation.

Do you expect to achieve a calculated output voltage without feedback loop. That won't work. Rectifier operation mode is varying between continuous and discontinuous depending on the load current, output voltage is varying between average and peek of the output pulse respectively.

 

[baileychic]

@Klaus

I will try that damper.

@FvM

I will put a PIC18F26K22 into the circuit to adjust PWM duty based on feedback.