TOP257YN powersupply design

Pls. post your schematic, PCB layout, and transformer documentation.


You see 25.2V at what load? If this is at light load, measure the voltage on the bias supply at this condition. It should be > 6+ Volts. If not, it may be that the coupling between the bias winding and the secondary is poor. Another possibility is that there are too few turns on the bias winding. And check the waveform on the winding to see if the phasing is correct.

Warm greets....
Ur absolutely right. the problem solved when i make some modifications in the feed back loop... thanks for the supports. i have still some doubt. this circuit is planned to use charge SLA batteries (12V/80AH). Is there any modification needs in this circuit?????? i would like to use a charger controller chip (BQ24450) along with a battery monitor ic, is there any problem working with these components???

looking 4 ur reply...

Thanks& Regds,

Sreejith

The BQ24450 is designed to be used with a series pass transistor.
On of our app notes shows the implementation of the CV/CC characteristic that lead acid batteries require using discrete circuitry.

You may be able to use the BQ24450 with a TOPSwitch by connecting the right side of the sense resistor figure 3 of their datasheet, to the battery + terminal. Then you will need to somehow connect pins 15 and 16 to drive the TOPSwitch main feedback optocoupler LED. You will need to invert its gain. The LED of the TOPSwitch circuit needs more current to *reduce* battery current, whereas the BQ24450 is designed to increase its pin 15/16 drive output current to *increase* battery current. One possibility is to connect a resistor from the battery + to the opto LED anode, and another resistor from the opto LED cathode to ground, then connect pin 16 to the anode, and pin 15 to the cathode. There is a potential problem in that when the battery voltage is very low, there may not be enough drive voltage available for the LED.

If you decide to go this route, watch the compensation, (pin 14), it may need to be a series RC and not just a C.

Good day sir,

Thanks for your response, but if we use the BQ24450, the current to the load(Battery) was controlled by the pass element(Transistor) so is there any need to feedback the output to control the TopSwitch??. i am not sure about that but i tat that the TopSwitch will always provide the stable output current by using its own feedback?????

And can u please suggest a good pass element(Transistor) that can withstand up to 15A????

You are better off designing the SMPS to do the CV/CC characteristic. The total system will be lower cost, simpler, higher efficiency, and cooler.

If you use an SMPS with the BQ22450, the SMPS has to be designed for a CV output with a higher voltage than the required battery voltage. And when the battery is charging at high current, the series pass transistor will dissipate a lot of power and run hot.

Here is an example of a CV/CC TOPSwitch circuit. This one has built-in PFC if you need it (this one is 75W):
http://www.powerint.com/sites/default/files/PDFFiles/der136.pdf

You will probably need more CV accuracy than what the zener in the schematic is capable of, so either use a 1-2% zener, or change the CV portion to TL431 feedback, configured to pull down on the base of Q4 through a resistor.

If you don't need PFC, the circuit will simplify considerably. These circuit sections and parts will be eliminated: D12, R19 and associated parts, C10, Q1, D8, and associated parts, D6, D5, C4, L3, L4, C2. C3 will become a normal sized bulk capacitor, and the power capability of the TOPSwitch will increase back to "normal".

Good day sir,

As you said, Is there any problem if we avoid the CC characteristic??? is it impotent in a battery charger?? When i test the circuit with a half drained battery, the out put current attain about 6.5A(4A Designed) and the voltage dropped by 6.35V from 13.5V. I am doing such type of circuit for d first time.. thats y i have many doubts

pls clarify

Thanks & Regards,

Sreejith PS