questions about buck DC-DC converter

[arsenal]

hi,

As to buck DC-DC converter from 12v to 3.3v,
1. in synchronous rectification, some use MOSFET with the body diode integrated instead of the external diode, then how is this body diode integrated?
2. The 12V input voltage has to be scaled down to act as the power for the controller, then how to scale it down? by resistors? If so, will it affect the efficiency?
3. As to the saw-tooth pwm, is it generated by some oscillator with an integrator?

thanks
arsenal

 

[nxtech]

If heat dissapation is not a concern then use an LDO. they can be had for very little money.

If you need to use a switch mode supply, a switching buck does use a PWM scheme as part of step down process.

See www.ti.com and search for power supply

 

[edwintsu]

Hi arsenal,

1.Body diode is parasitic in the synchronous Power MOS;

2.Power MOS use 12V power supplies, Others use Low (Typical 5v) power supplies. For some special circuit, you need level shifter from high voltage to low voltage or from low voltage to high voltage. It has littel influence on efficiency;

3.For saw-tooth pwm, u can get it by the discharge and charge capacitor at the same time of getting OSC clock.

Hope it can help you!

B/R
Edwin.

 

[arsenal]

thanks, edwin


as to 2, that means I have to get 5v out of 12v using resistor devider?

best regards,
arsenal

Hi arsenal,

1.Body diode is parasitic in the synchronous Power MOS;

2.Power MOS use 12V power supplies, Others use Low (Typical 5v) power supplies. For some special circuit, you need level shifter from high voltage to low voltage or from low voltage to high voltage. It has littel influence on efficiency;

3.For saw-tooth pwm, u can get it by the discharge and charge capacitor at the same time of getting OSC clock.

Hope it can help you!

B/R
Edwin.

 

[edwintsu]

U need a voltage regulator for internal 5v (Typ. 3.6 or below for better effi.) power suppy.

B/R
Edwin.

 

[max0412]

as to 2, that means I have to get 5v out of 12v using resistor devider?

Use a simple zener shunt.

 

[xcoder]

you can use depletion n-fet to shift the high voltage to low voltage (-vth), connecte the gate to ground, as a nmos ldo without feedback.