Question

A DC-DC converter is needed to supply a 5V load from a battery input. For flexibility, it is desired to accommodate many different battery types, so the possible input voltages range from 3.0V to 8.0V. The output ripple is to be less than 1% peak to peak. The load can have a power level between 10W and 50W.

a) Propose a conversion circuit to address these requirements. What switching frequency will be reasonable?

b) Select capacitor and inductor values for this circuit so that it will meet the requirements.

c) What voltage and current ratings will be needed for the switching devices and for the inductor?

Answer 1

Answer:

Consider the output voltage the DC - DC converts as 5V.

Assume the input voltage and output power of the converter from the given level is 4 V and 20 W respectively.

Consider the power remains same as 20 W in both sides. Then, the input and output current 5 A and 4 A respectively.

a)

Propose a conversion circuit to address these requirements. What switching frequency will be reasonable.

For this conversation, the required converter circuit is Boost converter.

The circuit diagram for the boost converter is shown below:

The Voltage gain for the circuit is,

Substitute 5 V for V0 and 4 V for Vin .

D=0.2

Where, D is duty ratio and this can be varied from 0 to 1.

The reasonable swithching frequency of the switch is taken as 20kHz.

​​​​​​

The switching time is,

And

The on -time of the switch is,

The off-time of the switch is,

Thus, the proposed converter is boost converter and the switching frequency is 20kHz.

b)

Select capacitor and inductor values for this circuit so that it will meet the requirements.

Write the expression to find the value of inductor.

Here,

The current ripple, is taken as 0.05A from t 1% of input current (5 A).

substitute 4 V for Vin ,1*10-5s for Ton and 0.05 A for .

Write the expression to find the value of capacitor.

The voltage ripple is taken as 1% of output voltage of 5 V is 0.05 V.

Find .

Substitute 0.05 V for and for   in equation(1).

Therefore, the inductor and capacitor values are 0.8mH and respectively.

c)

What voltage and current ratings will be needed for the switching devices and for the inductor.

From the circuit of boost converter, the inductor is placed in input side. then, the supply voltage and supply current are the voltage and current of an inductor. Therefore, the voltage and current rating needed are 10% more than these input values.

% of 4V and

% of 5A

The same maximum possible voltage and current are input values. Therefore, the same 4.4 V and 5.5 ratings are needed for either MOSFET or IGBT and diode switches.