lead acid battery charger - low voltage transformer problems

[CrabMan]

low voltage transformer problems

Hi all,

I have a problem with my lead acid battery charger that i am working on, I have a Transformer which is converting 220VAC to 30VAC and then there is a bridge rectifier with a capacitor filter, the problem is that at low current the circuit works fine but when i try to get the current above 5Amperes the circuit responds in a weird way, the current flows for around 300msec and then it drops to 0amperes for around 500msec and then starts to flow again, I think the transformer is getting saturated, am i right? and if I am is there a way to solve this problem?

P.S i am using mosfet and driving its gate at a frequency of 30Khz with PWM i have tried and changed the frequency of gate drive from 30Hz to 70KHz but it dosnt works.

Regards,
CrabMan

 

[madhub63]

transformer pwm output

Can u give us the circuit.
Whether u r using linear regulator if so where this mosfet comes from.

 

[CrabMan]

battery charger transformer problems

This is the circuit diagram, i am not using any regulator it is an unregulated supply... and the mosfet is 2SK1522

 

[pauloynski]

whathappens when a transformer saturates

Where are you taking the power supply for the PWM circuit from? What happens to the voltage on the 10000uF cap when the current is very high? How is connected the battery? What do you intend the circuit you´ve posted to do? I think there is a little chance you are having saturation problems.

 

[CrabMan]

Re: Transformer Problem

Well the power supply for the pwm driving circuit is from another source but I have made the grounds common, secondly the battery is connected in place of 0.1ohm resistor, i just showed it to identify load, and the purpose of this circuit is to charge a lead acid battery,
The capacitor voltage has small ripples i.e within 5% of supply voltage. when the current is flowing and when it is not flowing the voltage across capacitor is smooth dc.

 

[pauloynski]

Re: Transformer Problem

With a large current (say 15A average) your circuit will show ripples of about 10V. Please make sure all four rectifier diodes are connected. If not you can have transformer saturation. I suppose your PWM circuit frequency is very high compared to the mains frequency. If not it can imbalance the transformer´s current and go into core saturation. The value of R1 (0.1Ω) will produce very high current peaks on the circuit and does not simulate a real battery since it is connected to ground (you need to simulate the battery´s voltage too). Give a try using a higher value (say 3Ω) to keep things under control and see how the time constant of 0.5s you´ve mentioned is affected. May be your battery simulation is the real problem.

 

[CrabMan]

Re: Transformer Problem

Thanks for a detailed reply, I am not simulating this circuit (i am doing the experiment in lab), and I havnt connected the 0.1Ohm resistance physically, It was just to show the maximum current limit, anyhow, physically I have connected the battery inplace of 0.1ohms and secondly I noted another thing today i.e. during the dead time(the time during which no current flows) I don't see any PWM output from the gate drive circuit while the battery is connected and furthermore when I removed the battery and connected a 0.4 ohm resistor in its place the current control was perfect and I went upto around 40Amperes(and yes there were some ripples across capacitor which i rectified by connecting a 40,000uF capacitor bank) and there were no signs of core saturation and pwm output came out fine all the time, however when i replaced the load resistor with a battery the dead zone came back during which 0 current flows.....

So clearly its not core saturation, then what is it.

 

[pauloynski]

Re: Transformer Problem

Could you upload your PWM circuit?

Added after 53 minutes:

If there is a current limiter inside it may be causing the 0.5s delay you´ve mentioned.

 

[Alliegro]

Re: Transformer Problem

The battery is a non linear load. Imagine a capacitor connected instead of a battery. This would alter the overall mosfet capacitance (in short).The best option would be to use the mosfet like a linear regulator, the negative of the battery will be at ground potential. The only draw back is an isolated drive for the gate.

 

[wiserguy]

Re: Transformer Problem

It sounds like the circuit thinks you could be overcharging the battery by a small amount hence the PWM circuit momentarily ceases output pulses - this can be made worse when the Battery measurement is done at the output of the circuit rather than at the battery.

If the PWM is over compensated or if the feedback around the error amplifier are wrong it could cause similar issues.

I think it would be helpful if you post the overall circuit with the complete PWM drive circuit details for better help. The fact the circuit seems to work with a resistive load is encouraging.