Please help for this SMPS circuit

[xyz9915]

Hi! The attached file is a schematic of 50KHz SMPS power supply which uses 325 VDC (after rectification of Mains 240 V). The required current is approx 15 Amperes. Please give comment if any modification in the circuit is required.

 

[mvs sarma]

Hi! The attached file is a schematic of 50KHz SMPS power supply which uses 325 VDC (after rectification of Mains 240 V). The required current is approx 15 Amperes. Please give comment if any modification in the circuit is required.

I fear, the circuit lacks control on output voltage and load current. Please try to check application notes based on MC3525.

 

[crutschow]

Yes, as mvs sarma noted, the circuit has no regulation, either for input voltage or output load changes. If you can't tolerate poor output voltage regulation, then you will need to add a circuit to detect the output voltage and adjust the 3525 duty-cycle in a feedback loop.

If you need output isolation, then the feedback will need to be through a transformer or opto isolator.

 

[xyz9915]

The voltage regulation is not essential, but Please guide how do I add feedback circuit to limit the current.

 

[xyz9915]

The changes/modification proposed by the friends, and thinking that the half bridge topology is better for converting AC mains voltage to low voltage, the modified circuit is attached herewith with feedback control circuitry to keep the design stable. Please point out if any further modification is required.

 

[xyz9915]

The modified schematic is attached in which both AC mains & secondary sided are separated by two different isolation symbols. the question is that the 0.1 ohms/10W resistor are sufficient to drive the transistor ( about 0.7v) or its value may be increased?

 

[mvs sarma]

The modified schematic is attached in which both AC mains & secondary sided are separated by two different isolation symbols. the question is that the 0.1 ohms/10W resistor are sufficient to drive the transistor ( about 0.7v) or its value may be increased?

Still you need to indicate the way how the 12V is derived? This 12V is expected to be derived from rectified 400V DC and how is it derived and ghow clean is it matters.

 

[xyz9915]

The 12V is derived from a separate small isolation transformer? please point out that the 0.1 ohms/10W resistor are sufficient to drive the transistor ( about 0.7v) or its value may be increased?

 

[mvs sarma]

The 12V is derived from a separate small isolation transformer? please point out that the 0.1 ohms/10W resistor are sufficient to drive the transistor ( about 0.7v) or its value may be increased?

you can use 2 nos of 0.1 ohm resistors( to make it 0.5 ohm) and for 15 amps it would develop .75 V
the wattage needed is 15*15*.05=11.25. So using 10 watt 0.1 ohms 2 nos would be safe enough.

Finally whether it would limit at 15 amps or 10 amps , or higher depends on the voltage needed for the transistor (opto isolator) to switch.
Later if needed, you may use a potentiometer of say 100ohms across the current sense and feed only a part of the drop to the transistor and it would help effective control of the current.

How ever i have my doubts on ETD44 core -- whether it could deliver 15*16= 240 watts ( say 300 watts including the system efficiency).

Try out better core with higher capacity.

 

[mvs sarma]

you can use 2 nos of 0.1 ohm resistors( to make it 0.5 ohm) and for 15 amps it would develop .75 V
the wattage needed is 15*15*.05=11.25. So using 10 watt 0.1 ohms 2 nos would be safe enough.

Finally whether it would limit at 15 amps or 10 amps , or higher depends on the voltage needed for the transistor (opto isolator) to switch.
Later if needed, you may use a potentiometer of say 100ohms across the current sense and feed only a part of the drop to the transistor and it would help effective control of the current.

How ever i have my doubts on ETD44 core -- whether it could deliver 15*16= 240 watts ( say 300 watts including the system efficiency).

Try out better core with higher capacity.

I am attaching a modified schematic. perhaps the non -inv input can be held at 2.5V and the inv-input needs a voltage input from Vref which can be switched by the transistor of opto-isolator.
the actual value can be around checked by you.

 

[Ubergeek63]

i am amazed that no one has yet pointed out that you are applying 650V to a 500V FET.

Try

Digi-Key - IRG4BC20WPBF-ND (International Rectifier - IRG4BC20WPBF)

or

Digi-Key - 497-2785-5-ND (STMicroelectronics - STP9NK90Z)

Either one would only require a single switch on each end

 

[Ubergeek63]

The snubber caps are over voltaged as well

 

[Ubergeek63]

in your original circuit I would use synchronous rectification:

**broken link removed**

 

[xyz9915]

After receiving many criticism in the design, I have changed the design by using UC3842 Current-Mode PWM Controller chip. The data sheet is available with various manufacturer, while one page from Semiconductors - Integrated Circuits - Electrical Components is shown in the attachment. The schematic is then modified by me to fulfill my requirements. So, please point out if any further change is required.

Both schematic (Original & modified are in the same attachment)

 

[mvs sarma]

After receiving many criticism in the design, I have changed the design by using UC3842 Current-Mode PWM Controller chip. The data sheet is available with various manufacturer, while one page from Semiconductors - Integrated Circuits - Electrical Components is shown in the attachment. The schematic is then modified by me to fulfill my requirements. So, please point out if any further change is required.

Both schematic (Original & modified are in the same attachment)

It is fine that you want to shift to 3842. don't claim it on the criticism by others
I for one, would feel, that we learn only by refining a design and not ny shifting ti another.
I some criticises this design, would you, i wonder , change over to a third design.
3525 is also a good chip used many places . I saw them in telecom power supplies.

BTW, if you are really interested how a critical review can be to improve a design, you should see one "improve a 0-30V lab psu from 3 amps to 5 amps, of course on another forum

similarly you may see sensitive FM transmitter.

both are from great AudioGURU. i really love the way he treated the topics, methodically and logically.

 

[Ubergeek63]

i do not often have the time to treat things with that detail. I often answer and try to point people to the relevant site, data sheet, or app note.

One thing that surprises me is his asking for help and then being offended when it is given.

 

[Hero999]

I don't think he has a problem whith the constructive critism offered by us. It's just that English probably isn't his first language so his post was worded in such a way that it could be interpreted as complaining.

 

[xyz9915]

To: Hero999! Thanks for explaining the situation. I don't mind with constructive criticism offered by the forum members to bring improvement in the design. Since English isn't my first language that is why the misunderstanding was happened. Any way I excuse if it hurts someone.