Battery chargger for an inverter Current calculations ?

[thannara123]

Hello experts ,
I want to build a battery charger in an inverter using an Hbridge .
How can i vary the current for the battery as constant current charging ,Topping carging ,and the floating charging .
I know that diffrent current can produce with varing PWM.
But how the calculation for the boost converter ,may i get any refferance ?
the charger circuit look like

 

[Easy peasy]

what you have drawn is an active bridge rectifier - there is no way to control the current flow due to internal diodes

 

[thannara123]

typical inverter do these technique . the current flow through each closed path can be controll by PWM-ing the low side MOSFET

--- Updated Jul 23, 2023 ---

This is the circuit for the half wave .The Push button want to change as MOSFET ,how the working

 

[KlausST]

Hi,

As long as the transformer peak voltage (bridge side) is lower than the battery voltage you can control it with the PWM.

For some smooth control you need series inductance. Either designed into the transformer, or an external one.

Then you may use the inductor formula: I = V x t / L

Klaus

 

[FvM]

You can control charging current as long as battery voltage is above AC peak voltage. There must a series inductor for controlled boost operation.

 

[thannara123]

Without serial inductor most of the inverter gets boosted voltage to charge the battery ?
the secondary of the transformer act as an inductor ?
is there any way to get the calculation ?

 

[BradtheRad]

When you abruptly shut off current in a transformer, high voltage spikes are liable to occur.
Perhaps internal arcing. Perhaps welding of copper wires.

It may be more practical to use a voltage doubler built from capacitors. Maximum output is more predictable and more easily managed.

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

Crude method to determine output voltage in a boost converter:

1) Inductor at shut-off instantly generates voltage in order to maintain Ampere flow.

2) The generated voltage is calculated as I * R (where R is resistance through entire current loop).

3) Generated voltage starts to drop immediately. (Notice if load is absent it causes high voltage spike, or maybe a spark.)

4) Pulse duration is governed by inductive time constant L/R

5) Power is greatest as pulse begins. Watts delivered depends on stored Webers which is based on inductor Henry value. W = I * I * R

6) Boost converter has power supply in series with inductor. Therefore add supply voltage.

7) Often a reservoir capacitor is in the output stage. It offers little resistance. Therefore appropriate math is necessary to calculate voltage boost on capacitor.

8) To charge a battery does not absolutely require an output capacitor.

 

[thannara123]

In the first post i redrawed the circuit as follows ,(the values not for its just for drwing ) .
By using PWM how to charge the battery may i get a desgin ideas ,selection of PWM frequency ,current ,duty cycle calculation etc

 

[BradtheRad]

100 times per second a strong half-sine waveform could go through a mosfet after it's turned on. It's a good strategy to wait until a zero crossing to shut off current. Spikes are likely if you shut off current while the transformer is conducting.

Rechargeable batteries often are labelled with a recommended charge rate.
To monitor your charge current you should install a ammeter inline with the battery. It should average pulses to create a steady reading.
A reasonable frequency is 1kHz. You can vary duty cycle with a potentiometer.

Duty cycle can be adjusted so you observe the recommenced reading on the ammeter.

Example, AAA batteries (NICAD & NIMH) had advice to charge at 70mA for 10 hours.

Bias each mosfet so it passes the correct polarity available to it in each half-cycle.

To be on the safe side every current burst should be brief enough so that it doesn't exceed (say) 3x the recommended charge rate of the battery.

D cells sometimes contain a AA cell mounted inside a container which is the size of a D cell. These are comparatively lightweight, whereas a true D cell is heavier.

 

[KlausST]

Hi,



For the math/physics Brad already gave good hints.
You did not reply on his post.

So it´s unclear what you really want from us: A complete design? Doing all the math for you? hardware, software...


**

I start my design by considering the specifications. In your case: battery chemistry, capacity, charging current range, voltage range and all the other values that you need for charging this battery.
Then you need to consider the input conditions: Voltage, AC & frequency, DC ...
And what your charging goal is: CCCV? Fast, slow, long battery life, highest stored energy, charging while load is connected?
How to control it? Using dedicated ICs, using your own controller and your own software? Pot, Display, LEDs?

*******

Klaus

 

[FvM]

I don't understand the purpose of post #8 schematic. Original problem is an inverter with H-bridge that shall be optionally operated as battery charger. In #8, battery voltage is reversed and unreasonably low. Better refer to the original H-bridge if the post #1 problem setting is still valid.

Some points have been clarified in the discussion. Said battery charger operation is only feasible in boost mode, battery voltage must be larger than AC input peak voltage. Boost operation is achieved by periodically shorting the transformer output. Unfortunately a respective pulsed current is drawn from the grid, requiring a low pass filter to comply with power quality regulations.

 

[thannara123]

Hi,



For the math/physics Brad already gave good hints.
You did not reply on his post.

So it´s unclear what you really want from us: A complete design? Doing all the math for you? hardware, software...


**

I start my design by considering the specifications. In your case: battery chemistry, capacity, charging current range, voltage range and all the other values that you need for charging this battery.
Then you need to consider the input conditions: Voltage, AC & frequency, DC ...
And what your charging goal is: CCCV? Fast, slow, long battery life, highest stored energy, charging while load is connected?
How to control it? Using dedicated ICs, using your own controller and your own software? Pot, Display, LEDs?

*******

Klaus

I will reply to that post , I am now learnig ,am not have enoghu knolwedge in dc t dc converter or Battery charger , i need to learn from basic , for that i am reffering

aim is to make charger with the circuit which have posted in Post#1 .
that circuit is redrwan in post#8 . for better understannding
that circuit not seems like a boost converter ,but will work the boost converter .
I felt that that circuit look like a a bridge rectifire .

I want to make charger with a good fascility available .
The desgin tutorial or guidence required

The primary output will be 5volt to 7.5 . (which is a inverter transformer )
Brad said some point , thankyou sir .

Need how do I the selection of PWM frequency and duration of duty cycle with some mathamtics .
Or i require trail and error meathod to understat the current of the circuit .

Learned a lot of , from this forum thanks to all members

 

[BradtheRad]

My simulation using your schematic.

By clicking this link:

https://tinyurl.com/26k5eppr

1) Opens website falstad.com/circuit.
2) Loads my schematic into the animated interactive simulator.
3) Runs it on your computer.

Notice bias from the transformer automatically turns on the correct Nmos. To make things easy I eliminated the Nmos body diode. I used an artificial switch at 1 kHz delivering PWM.

--- Updated Sep 2, 2023 ---

The capacitor corrects power factor. Though the PF error isn't severe, the reduced current reveals the effect on the primary winding, when you rapidly switch the secondary winding.

 

[thannara123]

That means it produce the wave as bridge rectifire isnt it ?

 

[BradtheRad]

That means it produce the wave as bridge rectifire isnt it ?

Yes, the mosfet gate receives a Sinewave biasing it. So the mosfet partially turns on most of the cycle. Ordinarily it looks like the mosfet body is in resistive mode, so it overheats and wastes power...
But notice the mosfet body also receives a Sinewave going through it. It's hard to be sure whether a lot or little power is wasted.

We could devise a clocking scheme which does two jobs in one, switching a transistor on & off at 1 kHz, and deciding which transistor to turn on & off. The other transistor should not conduct when it's supposed to be off. Such a clocking scheme could be more efficient, but it adds work to the project and the simulation.

 

[FvM]

Mosfet polarity in post #13 is wrong, please compare with original H-bridge and #8.

 

[thannara123]

With a 0.44 ON time , The charging voltage is attainable . Is there any advice for the next step , by measuring the current throguh thev battery we need to adjust the Duty cyle for diffrent type of charrging .

 

[BradtheRad]

To find which arrangement works, I tried all ways to arrange the two diodes, Nmos or Pmos, oriented one way and the reverse, auto-biased via various nodes. Only two methods charged the battery properly.

Since increasing voltage was part of the initial purpose, here's something which does that. The transformer only needs to produce a few volts. Two diodes and 2 capacitors double voltage. Only one transistor is needed.

--- Updated Sep 3, 2023 ---

 

[Easy peasy]

@ thannara123 - what you appear to have drawn is a bridge rectifier - the fets offer no control - as their diodes are fwd biased - it appears ....

look at what happens each half cycle - the pos goes through a diode, then the load, then the fet diode .... (?)

 

[FvM]

@ thannara123 - what you appear to have drawn is a bridge rectifier - the fets offer no control - as their diodes are fwd biased - it appears ....

As previously discussed, that's the typical situation for H-bridge inverter used as charger. It's operating in boost mode, Vbat > Vac,peak. It has however not current control when connected to a deep discharged battery or a short.

The simulation in post #17 is an adequate solution for the question in post #1. Circuit parameters (transformer leakage inductance, voltages, switching frequency) would be interesting, also pwm current sourced to the grid.