Boost converter voltage pulled down when connected in series with another boost converter

Thread Starter

zazas321

Joined Nov 29, 2015
432
I have designed 2 boost converter both of them have same duty cycle and same L and C values. Voltage input is 12V and with 50% duty cycle I expect to see 24V output from each of them. If I connect them in series I should get arround 48V total output, but for some reason, the top boost converter voltage is pulled down and not giving 24V. Can someone notice my mistake or give me some tips? thank you

upload_2019-1-16_13-50-32.png
 

crutschow

Joined Mar 14, 2008
23,314
Fsw1 needs to be connected to the source of Q1, not ground.
Connecting it to ground means the Q1's drive voltage decreases as the bottom voltage starts to rise until Q1 stops working.
 

BBee

Joined Dec 6, 2018
35
I would agree with crutschow's answer. I use commercial boost controllers in this way to get higher voltage output without problem.

Tracy
 

Thread Starter

zazas321

Joined Nov 29, 2015
432
Thank you guys! What a silly mistake. Would you be able to help me understand another phenomena? I am simulating 3 boost converters and spliting the voltage 20/30/50% by changing the duty cycle of each.

upload_2019-1-16_17-51-21.png


When Load resistance 1kOhm is used, I get a nice distribution of voltages as expected.


However, when I use 100Ohm resistance the two bottom boost converters are pulled down for some reason. The higher the duty cycle applied the bigger nonsense values I get.
The voltage across each capacitor is charging and discharging in a very unusual manner.
 

Thread Starter

zazas321

Joined Nov 29, 2015
432
The only difference in this circuit is Load resistor changed to 100 instead of 1k

upload_2019-1-16_18-9-25.png
 

crutschow

Joined Mar 14, 2008
23,314
The problem is likely due to a mismatch between the voltage and current for the particular duty-cycles you are using.
For proper operation, the feedback loop should be closed on each regulator so it can automatically adjust the duty-cycle to whatever is needed for the required voltage and current.
That's how a real regulator operates.
 

Thread Starter

zazas321

Joined Nov 29, 2015
432
My bad, ignore the pictures above!



By changing the load resistor I also had to change the capacitor values to maintain the required ripple.
However, changing the capacitor values did not solve the voltage problems

upload_2019-1-16_18-26-40.png

I am measuring voltage output across C5 and C4. I am expecting to see 20V output with 5%ripple at C5 and 30V with 5% ripple at C4. As you can see the voltages are much smaller than expected. Also, I am a little confused why they seem out of phase. The green waveform has 55% duty cycle and the yellow one has 70% duty cycle.
 

Thread Starter

zazas321

Joined Nov 29, 2015
432
The problem is likely due to a mismatch between the voltage and current for the particular duty-cycles you are using.
For proper operation, the feedback loop should be closed on each regulator so it can automatically adjust the duty-cycle to whatever is needed for the required voltage and current.
That's how a real regulator operates.

I have tried increasing/decreasing all duty cycles but I still could not get anywhere close to 100V total output voltage
 

bertus

Joined Apr 5, 2008
20,048
Hello,

You have three different inverters.
The inverter with the highest voltage will have the lowest current. (when the powers of the inverters are the same).
As the inverters are in series, the current can not be higher than the current of this inverter.

Bertus
 

Thread Starter

zazas321

Joined Nov 29, 2015
432
I have different capacitor and inductor values to have same ripple on all of them. As i change duty cycle ripple changes as well so i change C and L values to compensate that
 

Thread Starter

zazas321

Joined Nov 29, 2015
432
Hello,

You have three different inverters.
The inverter with the highest voltage will have the lowest current. (when the powers of the inverters are the same).
As the inverters are in series, the current can not be higher than the current of this inverter.

Bertus

I thought that using these DC batteries i can draw as much current as I need from them. By decreasing the load by 10 times, the output current i expected to see 10 times higher.

Isnt the output current of each converter dependant on the load?
 

Thread Starter

zazas321

Joined Nov 29, 2015
432
How can I calculate the current output of each converter when they are all connected? Assume my boost converter that is boosting the voltage from 10 to 50V. I know that if the voltage increases by 5 times, the current reduces by the same amount. But what about the load resistance? Do I use the same 100Ohms /?
 

Thread Starter

zazas321

Joined Nov 29, 2015
432
Assume that I have A load resistance of 750 ohms. The voltage output I get is 97V and the current is 130mA. So that means that my current is limited by the boost converter with smallest current.
The output voltage of a boost converter with a highest voltage is 48V and assuming the current is 130mA, the resistance it is seeing is R=V/I=369Ohms. Where is that resistance coming from?
 

Thread Starter

zazas321

Joined Nov 29, 2015
432
Hello,

You have three different inverters.
The inverter with the highest voltage will have the lowest current. (when the powers of the inverters are the same).
As the inverters are in series, the current can not be higher than the current of this inverter.

Bertus
Hey. I have been experimenting with what you have said and it makes sense to me now, however, I have simulated my hybrid power system on Multisim and the results are very strange.
upload_2019-1-18_14-26-14.png

I did calculations for the boost converters individually and I have simulated them individually as well.
The boost converter on the top is boosting the voltage to 200V and across 100omh resistor the current is 2A. Knowing that, how can the output current when I have connected them all in series reach 10A? shouldnt it be clamped to a lowest currrent of a boost converter?
 
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