Design of Boost Converter

[PhysicsTest]

Homework Statement

Designing a Boost Converter

Relevant Equations

V = L di/dt

I want to do hands on designing a buck converter mainly understanding the selection of component values Inductor, Capacitor, Resistor, Mosfet, performing the calculations of Inductor current, Output voltages and Load currents. My first question is, i want to consider the below configuration of 10V, 4.8A -> 24V, 2A. Is the selection Ok? or i am failing in the first step itself, there is no particular reason for considering the above specifications, or please suggest better specifications which would help. Thank you in advance.

Brief circuit of Boost Converter

 

[berkeman]

I want to do hands on designing a buck converter

You mean boost converter there, right? That's what you've shown and you mention boosting the input voltage to make the higher output voltage.

 

[Baluncore]

My first question is, i want to consider the below configuration of 10V, 4.8A -> 24V, 2A. Is the selection Ok?

Your specs assume 100% efficiency, which will not be possible. The best you can do will be about 95%.

 

[PhysicsTest]

You mean boost converter there, right? That's what you've shown and you mention boosting the input voltage to make the higher output voltage.

Sorry yes it is Boost converter can I go with above specifications assuming 95% efficiency.

 

[berkeman]

What are your L and C values and what is the switching frequency? What duty cycle would an ideal boost converter need to run at for your input-->output voltages? How does the lower real efficiency affect the duty cycle?

 

[Baluncore]

First, ignore the input current.
Design for a specified output current.
You know the input and output voltages.
Write equations that can be solved for D, the duty cycle.
Only then will the values of C and L be determined.

 

[PhysicsTest]

I am really new to hardware design so please excuse me for any mistakes or i am going in an entirely different direction. I have attempted this, i have no idea if it is correct
R = V/I = 24/2 = 12 Ohm
$$ V_{out} * I_{out} = 0.95 *V_{in}*I_{l} - eq 1 $$

When switch closed let the ON time be T_on
$$I_l = \frac{V_{in} * T_{on}}{L} - eq3$$ substitute in eq-1
$$\frac{V_{out}*I_{out}}{R} = \frac{0.95 *V_{in}^2 * T_{on}}{L} eq-4$$
$$V_{out} = V_c$$
Using the capacitor equation
$$i = C*\frac{dv}{dt} eq- 5$$ For linear ramp
$$V_c = \frac{I_{out}*T_{off}}{C} - eq6 $$
$$\frac{I_{out}^2 *T_{off}}{C} = \frac{0.95 *V_{in}^2 * T_{on}}{L} $$
$$V_{in} = 10 V; I_{out} = 2 A; $$
$$\frac{T_{on}}{T_{off}} = \frac{4*L}{C*100*0.95} $$
$$1 + \frac{T_{off}}{T_{on}} = 1 + \frac{23.5*C}{L}$$
$$\frac{1}{D} = 1 + \frac{23.5C}{L} $$
In case it is wrong please guide me with the rules / laws i have to apply to derive the values.

 

[DaveE]

When switch closed let the ON time be T_on
Il=Vin∗TonL−eq3

This is the change in the inductor current; how much it increases.

Read up a bit about continuous vs. discontinuous conduction mode. There's a ton of information on the web. Plus everyone here will assume your switching frequency is fast compared to the circuit dynamics, which I assume you will too. The inductor value is usually chosen initially by how much current ripple you want.

https://en.wikipedia.org/wiki/Boost_converter

https://www.ti.com/lit/an/slva372d/slva372d.pdf?ts=1769844593676

Sorry, I don't have time to go through the rest of your equations now, but you'll find the equivalent in the links I gave.

 

[Baluncore]

You can know the duty cycle, but you cannot know the switch on/off times, until after you select the MOSFET switch, and design the gate drive.

The thing that decides the frequency of operation, is the speed of the MOSFET switch, and that depends on the gate charge, miller capacitance, and your gate drive circuit.

The energy dissipation of one switch cycle, will tell you how many times you can switch per second, and not exceed the package dissipation. Then you can specify output voltage ripple, which will size the reservoir capacitor, and then the inductor.

 

[DaveE]

The thing that decides the frequency of operation, is the speed of the MOSFET switch, and that depends on the gate charge, miller capacitance, and your gate drive circuit.

There are many other factors involved in selecting the switching frequency. I guess this would be a good description of the maximum speed. However, much also depends on the commentating diode (or Mosfet for synchronous rectification). At turn on the FET must support the current ramp up to the load current plus the diode recovery time at full output voltage. You'll want a very stiff (high current) gate drive to keep the Miller effect from also adding extra time to this turn on stress.

Honestly my advice would be to skip the switching frequency optimization for now and just use 100KHZ. If your inductor is too big, use 200KHz. You're still learning the basic stuff, you can optimize this on your next attempts. Faster than this will just make things harder.

Also 95% efficiency is definitely doable with optimization, but plan on 90% at first, that's probably what you'll get.

If you're actually going to build this, PCB layout, snubbers, and bypass caps, are super important. You can read about that or ask us.