# Urgent HELP- Designing a buck converter!

#### diya khan

Joined Nov 29, 2010
18
Hey,

im required to design a buck converter that will step down the voltage to supply a piece of equipment.I also require to show the results of simulation of my circuit.

My circuit should meet the following specifications:

Vin=24V
Vout=18V
Max ripple compared to DC input=10%
Switching frequency must lie between 50kHz to 200kHz
Assume ideal devices have been used!

ok..so to start of with, I no the circuit im required to design (not sure about the which switching device to choose yet).i have decided to use Pspice Student Version to simulate my circuit.
I have calculated:

ΔI=0.1*3.6=0.36A
D=Vout/Vin=0.75

Now for continuous conduction mode:

we know that L=$$\frac{(Vin-Vout)*(D/Fs)}{Ir}$$ whr Ir=ΔI

Now the issue is that i really cannot figure out how to choose Fs!!Once i choose Fs correctly, i can find the value for my filter inductor.
its mentioned in the ques that i can choose between 50kHz to 200kHz, but is there any appropriate logical way to selecting the Switching frequency?
I would also like to know whether its better to use a p/n type MOSFET or IGBT for my circuit and why!

thx

#### crutschow

Joined Mar 14, 2008
33,348
A higher switching frequency allows the use of smaller inductors, but the switching losses so it's a tradeoff between those two parameters.

IGBTs have a relatively slow switching speed so they are normally only suitable for lower frequency applications. MOSFETs are much faster so are typically used in switching power supplies, since they would have lower switching losses.

#### diya khan

Joined Nov 29, 2010
18
A higher switching frequency allows the use of smaller inductors, but the switching losses so it's a tradeoff between those two parameters.

IGBTs have a relatively slow switching speed so they are normally only suitable for lower frequency applications. MOSFETs are much faster so are typically used in switching power supplies, since they would have lower switching losses.
ok. but could you give me a range, like as you said, MOSFETs are much faster which means that they have high switching frequency.So is 50kHz high enough??
and what difference does it make whether i use a p-type or an n-type MOSFET?

#### bountyhunter

Joined Sep 7, 2009
2,512
Now the issue is that i really cannot figure out how to choose Fs!!Once i choose Fs correctly, i can find the value for my filter inductor.
its mentioned in the ques that i can choose between 50kHz to 200kHz, but is there any appropriate logical way to selecting the Switching frequency?
YES, but it requires understanding gained by actually building buck converters, not simulating them. The capacitors have losses and internal resonances, the inductor core has losses that increase with frequency, the FET and diode both have switching losses. You basically push the switching frequency up as far as you can before the switching losses get too high (and you define that, but probably less than 10% of load watt power). Understand, there are ways to reduce losses by "upgrading" to better caps and inductors but they cost more so it's all a matter of engineering judgement. No free lunch.

In the real world, pushing a 3A buck converter above 100 kHz usually starts getting too pricey for what consumers want to pay for things, but it's possible to do. I would stay between 50 kHz and 100 kHz and live with a slightly larger inductor.

#### bountyhunter

Joined Sep 7, 2009
2,512
and what difference does it make whether i use a p-type or an n-type MOSFET?
Power P-FETs are usually more expensive for a given voltage/On resistance value because of larger die size.

#### SgtWookie

Joined Jul 17, 2007
22,230
Along with P-ch being more expensive, if all other parameters are roughly equal (Rdson, Vdss, logic/standard, Id rating) the gate charge of a P-ch will be roughly 2.5 times as much as that for an N-ch MOSFET. Also, the selection of P-ch is far more limited than for N-ch.

#### bountyhunter

Joined Sep 7, 2009
2,512
The bigger die size of the P-FET means a larger gate capacitance which the switching driver circuit has to force to charge as fast as possible, so P-FETs take "stronger" drivers that can deliver higher peak currents to maintain the same fast switching speed.

However, the P-FET buck circuit is simpler than the N because the N-FET buck requires some kind of way to generate a higher driver voltage to pull the N-FETs gate up to to turn it on fully. There are always trade offs. Most of the time they go with the N-FET to save money and use a cheap charge pump to boost up to get a gate driver rail.