Questions about switching mode power supplies

[Kon Zaw]

switching mode pw supply

now i am learning about switching mode power supplys. i have learned that switching pw supply can theoritically give regulated 15v dc from 15v dc source by using PWM method.
1. how little can power comsumption even there are transformer and transistors?(it is like a perpetual motion mechine!!)
2. how can out put transformer operate for dc?
3. how can pwm base current in switching transistor help in load regulation?
please give me big picture or details. i am confusing.
thanks

 

[Mansour_M]

Re: switching mode pw supply

Hi Dear Kon Zaw!

I got you an eBook, it may help.

Thanks!

 

[fazan83]

Re: switching mode pw supply

Hi Dear Kon Zaw!

I got you an eBook, it may help.

Thanks!

Mansour,

I need your help on switching power supply. Why my switching power supply always blow the fuse. My switching power supply current rating is 3A and I check it use 8A fuse.

But somehow it always blow the fuse. I am very confuse how to check this. Because if I put a new fuse then I am going to check it it will blow the fuse.
If I use higher rating fuse then there is possibility the high current will damage other component.

Please advise what should I do on this. I am really Beginner in this field. The switching power supply control system is almost the same as you mention in the Ebooks that you upload above.

Thanks.

 

[engineeralee]

Re: switching mode pw supply

Dear friend,
What I have understood, the problem occuring because ur power supply is short. Check your connections and try again. Best of luck

Yours truly,
Muhammad Ali

 

[wisepie]

Re: switching mode pw supply

I think probaly your switching Mosfet is gone.

 

[Kon Zaw]

Re: switching mode pw supply

dear fazan83
i asked my teacher about your pb. does ur fuse blow just switch on or after sometime? if blow immdiately, the pb may be output section short, transformer short or swithcing transistors short. if u get sometimes to test, check secondary ac volt of output transformer. you may be able to isolate wrong section.
please share me if u get good books about sw power supply.
thanks

 

[wisepie]

Re: switching mode pw supply

dear fazan83
i asked my teacher about your pb. does ur fuse blow just switch on or after sometime? if blow immdiately, the pb may be output section short, transformer short or swithcing transistors short. if u get sometimes to test, check secondary ac volt of output transformer. you may be able to isolate wrong section.
please share me if u get good books about sw power supply.
thanks

Dear Kon Zaw,

Nowdays most of the ic controlled SMPS come with OCP and short circuit protection function. If the Output is short ( the load is short), the supply will turns into burst mode. Normally the fuse blown problem is related to primary components.

 

[fguihot]

Re: switching mode pw supply

Hi !!!

PWM is a great scheme of modulation.... But there are drawback...
The main difficulty in pwm is not there operationnal point : it is the startup.

Does your fuse blow in normal operation, or at the startup ???
If you have a 15V main power rail, and a load of 2 Ohms, at the startup, the pwm controller may not limit the in rush current, thus there will be 15V on 2 Ohms, and your fuse will blow... You should limit the inrush current...

What kind of power bridge do you use ??? The fuse could blow because of shoot through in the command of the Power transistor (The High AND the Low transistor could conduct in the same time... Shortcut of power rail to ground...) You should add dead time to the transistor command.

In SMPS, there is always a energy stockage device. it could be Capacitor in CUK, or inductor in Buck/boost/flyback.... If it is an inductor, watch for saturation current at heavy load....

Last, but not least, there could be oscillation between the load and the pwm controlling scheme.... you should monitor the output of the pwm, and try to introduce perturbation ( in the load, the power rail and in the command....)

Good luck !!!