# Different load ratios for two boost converters design help

#### zazas321

##### New Member
Hello. I would like to ask for some guidance and help regarding the excersise that I am doing.

The excersise requires to design two boost converters for a single load. One boost converter is used for solar energy and the second one for the wind energy. The requirement is that the load should receive 40% of the power from the wind and 60% from the solar. Assume that both solar energy and wind energy are the same and are available 100% of the time.

If I assume that the voltage of the both sources are 10V, I am trying to do calculations for the Inductor value but I need to calculate the ripple current first. For the calculations see picture: I was able to get two equations for ripple current ( one when the switch is clsoed and the second one when the switch is open)

My first question is whether these 2 equations are equal?? Can I use them both to derive the equation for the Inductance ?

Secondly, I need to know the duty cycle meaning that I also need to know the output voltage. How can I determine the Output voltage if it is not specified anywhere. If I assume that the Voltage output required is 100V, would that mean that Solar boost output would be 60V (because 60%) and Wind boost output would be 40V?

#### MichaelaJoy

##### Active Member
Hello. I would like to ask for some guidance and help regarding the excersise that I am doing.

The excersise requires to design two boost converters for a single load. One boost converter is used for solar energy and the second one for the wind energy. The requirement is that the load should receive 40% of the power from the wind and 60% from the solar. Assume that both solar energy and wind energy are the same and are available 100% of the time.
Are both converters producing the same polarity? The top one is positive.

I would think that the bottom one should be positive as well.

#### zazas321

##### New Member
Oh it could be my mistake from the drawing but I assume yes

#### ronsimpson

##### Well-Known Member
I don't have enought information but......
Assume a discontinuous boost PWM. (inductor current starts at 0, ramps up, ramps back down to 0, then there is a dead time)
I am going to assume that both switches open/close together. (same duty cycle)
Input current is a function of input voltage, inductance, and time. (time=time because the two switches are working together)
You want a 60/40 share of the current.
current = input voltage x time /inductance
if (time=time and if voltage=voltage) then you are left with current=1/inductance.
If one inductor has 40uH its current will ramp up faster than the inductor of 60uH.
Thus your current will have a 60:40 ratio when the inductors have a 40:60 ratio.
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If the input voltages are different:
Input1 voltage/inductor1 = current1, input2 voltage/inductor2 = current2 , current1:current 2=60:40
example if InputVoltage1 = 60 volts and InputVoltage2=40 volts and the inductors are both 100uH then the currents will be 60:40.
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I think continuous mode boost PWM will not work right like this because the discharge time will not be the same. Hard to explain.

#### zazas321

##### New Member
I think we have to assume that both sources produce same ammount of current and voltage we just have to change their duty cycle to achieve the required load
I do not require to build it, i just need to show a concept therefore any assumptions are allowed to make it less complicated

#### zazas321

##### New Member
Oh okay i have misread of what you just said. So how would i calculate the values of the inductance required? I assume that I need to know the ripple current before I start choosing inductor. In some research file I have read that the most popular ripple current is 30% max output current.

#### zazas321

##### New Member
I don't have enought information but......
Assume a discontinuous boost PWM. (inductor current starts at 0, ramps up, ramps back down to 0, then there is a dead time)
I am going to assume that both switches open/close together. (same duty cycle)
Input current is a function of input voltage, inductance, and time. (time=time because the two switches are working together)
You want a 60/40 share of the current.
current = input voltage x time /inductance
if (time=time and if voltage=voltage) then you are left with current=1/inductance.
If one inductor has 40uH its current will ramp up faster than the inductor of 60uH.
Thus your current will have a 60:40 ratio when the inductors have a 40:60 ratio.
------------
If the input voltages are different:
Input1 voltage/inductor1 = current1, input2 voltage/inductor2 = current2 , current1:current 2=60:40
example if InputVoltage1 = 60 volts and InputVoltage2=40 volts and the inductors are both 100uH then the currents will be 60:40.
------------
I think continuous mode boost PWM will not work right like this because the discharge time will not be the same. Hard to explain.
Hey. I have been thinking and I cant get my head arround how does changing inductor value will change the output current .

If I understand correctly changing the value of inductor would affect how big or small is the output ripple but no effect on the rms output current

#### ronsimpson

##### Well-Known Member
If I understand correctly changing the value of inductor would affect how big or small is the output ripple but no effect on the rms output current
Yes.
If you double the inductance the peak to peak current is 1/2. The average current will not change. Current change in a inductor is related to voltage * time / inductance. The slope (delta) of current is VT/inductance.

#### zazas321

##### New Member
So how does changing the inductor change the load ratio then? How does having 40mh and 60mh current inductors divide the currents to 60/40 if the rms will be the same for both of them? Isnt the rms curent that matters to the load?

#### ronsimpson

##### Well-Known Member
There are two different types of boost power supplies. This is very important.
Continuous:
Switch current, Diode current, and switch voltage. Discontinuous:
Switch current starts at zero and ramps up. (slope is set by input voltage/inductance)
Diode current starts high and ramps down to zero.
Voltage has three states.
There are three times. Input power, output power, dead time.
Dead time = no current. Continuous:
The red circuit has 80uH so the current ramps up slow. The black circuit has 40uH so the current ramps up faster.
Assuming that the two batteries are of the same voltage. The input current is set by the inductor. Both switches have the same duty cycle.
The two batteries are your two sources of power. (solar & wind)
The two outputs are connected together to charge up one battery.

#### zazas321

##### New Member
These are great drawings thanks! So looking at your drawings, it seems to me that the rms voltage of both(red and black) is clearly different. The red one will have much lower RMS current and less contribution towards the load. Am I right?

I am still a little bit confused how does changing the ripple change the boost converter output current. I understand that

I believe I am confusing the continous and discountinous mode. In continous mode, the changing the ripple current does not change the rms current value because it is compensated by the negative side of the ripple.

But in discontinous mode, twice larger ripple would mean twice larger rms current isnt it?  If a boost converter is working in a discontinous mode, I can apply the triangle area formula or integrate it to calculate the average current. It is clear that the average current is twice bigger in red case. But continous mode is not the same. Am I right?

#### ronsimpson

##### Well-Known Member
Right.

Another option is to measure the current and adjust the duty cycle to get the right current.

#### zazas321

##### New Member
I assume that changing the duty cycle will increase/decrease the output current by the same amount as a output voltage? Because Power should be equal so reducing voltage would meam increase in current

#### ronsimpson

##### Well-Known Member
The requirement is that the load should receive 40% of the power from the wind and 60% from the solar
Assume that both solar energy and wind energy are the same
Because Power should be equal so reducing voltage would meam increase in current
The only requirement is to keep a 60:40 ratio of power.
VinSolar=VinWind.
If the current is a 60:40 ratio then the power will also be a 60:40 ratio.
I do not see a requirement to keep the power constant.

#### zazas321

##### New Member
I was talking about individual power delicered by each supply 