Buck converter question

[renzen]

Hello all..., how are you?

I have some question

1. Should I use R3? I tested with and with out R3, both are working fine.
2. Is there any difference if I use high frequency transistor or fast switching transistor (Q1)?
3. I want use P-mosfet for reverse protection at the input. What kind P-mosfet should I use? Low Rd? or any kind P-mosfet will do?
4. LM2576 question. Can I stack LM2576 for higher current?(2 LM2576 with 1 inductor... etc).

Thanks in advance

 

[ronsimpson]

Please post any files you have about the MC34063. I can't run your design with out them.
1) I normally have 10 to 22 ohms in that spot but it is not critical.
2) You aren't running fast. I like faster transistors but what you have is good.
3a) You don't need low turn on voltage because you gate is driven by 11 volts.
3b) Try a current rating of 4x your load current. (I don't see a load)
3c) Voltage rating could be 50V or 35V. You will only have 13V on the part.
4) I would not stack LM2576s together. They will need to be synchronized. Because the oscillator does not come out they can not be made to march in step.
I notices the LM2576 comes in 1A or 3A depending on who makes it and the part number.

 

[crutschow]

3. The P-MOSFET for reverse bias protection should have a voltage rating above whatever value of reverse voltage you think could be applied. It's ON resistance, Rd, should be low enough so its voltage drop, when conducting the maximum input current, will generate an acceptably low voltage drop (whose value is determined by the difference between the minimum input voltage and the minimum operating voltage of the circuit).

 

[renzen]

Thank you all for the reply.
Sorry forgot the MC34063 files

More question,
1. I've read somewhere, people said that MC34063 can run up to 150khz. Is it true? In what condition MC34063 can run up to 150khz?
2. What parameter of P-mosfet(M1) should I see beside low Rds for high efficiency? Low Qg, low Ciss and low VGS(th) ( < -3v is better)?

 

[crutschow]

.................................
1. I've read somewhere, people said that MC34063 can run up to 150khz. Is it true? In what condition MC34063 can run up to 150khz?
2. What parameter of P-mosfet(M1) should I see beside low Rds for high efficiency? Low Qg, low Ciss and low VGS(th) ( < -3v is better)?

1. The data sheets shows a limit of 100kHz so I would stay below that frequency for reliable operation.

2. A low Qg and Ciss will help efficiency. Vgs(th) generally has little effect as long as Vgs is sufficient to fully turn on the MOSFET (typical magnitude of 10V for standard devices and 3v-5V for logic-level devices).

 

[renzen]

I've finished my MC34063 buck converter. Thank you all
When I buy MC34063, the seller offer me UC3845. He said UC3845 is much better than MC34063.

Now I have some question about UC3845,

1. The red area. I want to change with lm358 opamp for current sense and I've read lots of article about lm358 opamp for current sense. But I can't make it run in this circuit. Can any one help or give me advise? And which one is better? Using transistor for current sense or op amp?
2. The blue area. I've downloaded Application notes about UC3845 and I can't found about how to calculate the resistor and capacitor for compensation/error amplifier. Does anyone know how to calculate the resistor and capacitor for compensation/error amplifier?
3. The green area. Do I have to use it?
4. Is this circuit will work?

Thank you in advance

 

[dr pepper]

A faster switching transition will give you less dissipation in the switching transistor, but it brings an issue of emi with it.
The lm2576 can be paralled up, however each will need to have current limitong so its max o/p current isnt exceeded.

 

[MrAl]

Hi,

I havent used this chip myself but here are a few tips that might help.

RED:
Transistors are faster than the LM358 so the LM358 might be too slow. If that part does not react fast enough something may blow before it gets to protect anything. How the transistor circuit works might come into question too though, as to the matching of the transistors. You may have to adjust the sense resistor and test over temperature.

GREEN:
You probably need this cap to eliminate the diode recovery spike current effect on the sense voltage.

BLUE:
Try downloading the TI version of the data sheet and see if that helps. There are certain requirements for this chip.

It looks like the circuit could work but you should simulate it first.
Some data sheets seem to indicate that the chip is obsolete, but that may be only for that company.
The chip is probably quoted as being better than the MC chip because for one it goes to a much higher frequency.