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Boost DC-DC help

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sololf21

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desperate urgent help needed!!!

I need to design a boost DC-DC from scratch but is totally lost by the overwhelming information i can find online... i need help in choosing the correct diode specifications, power mosfet the inductors rating, the filtering capacitors at input and output, the correct sensing resistors to use and the calculation for the PWM frequency which is controlled by the PIC16F876.

My specifications are as below; would appreciate if some guys could direct me to the correct calculations cos I need to finish my project in 3 months time...

Input voltage: 18V
Input current: 1A
Output voltage: 50V
Efficiency: 90-95%

:confused::confused::confused:
 
Sounds like you are designing a switching power supply design with the µP in the feedback loop. That requires a quite complex design. Suggest you Google "digital control loop power supplies" and do some reading to understand how to stabilize the control loop. You might also look up "fuzzy logic power supplies" which is an alternate (and some say easier) way to use a µP to realize a stable switching supply control loop.

The size of the inductor and capacitors are mostly determined by the amount of ripple in the output voltage you can tolerate and the switching frequency you will use.

The transistor and diode choice are determined by the current and voltage of your circuit. To get 90-95% efficiency is not easy. You will need a transistor with a low on-resistance and a diode with a low forward voltage drop.

This is a difficult task if you are starting from scratch and only have three months left. You'll need a good supply of midnight oil. Good luck.
 
Rehaps I should can this resonse as this seems to be a very common suggestion.

Look at the Linear Technology, National and Maxim semiconductor websites. They make ICs purpose built for this kind of thing.

What do you want 50V at 1A for?

Assuming 85% efficiency, it's 3.27A at 18V, are you sure your primary PSU can cope with this?
 
Sounds like you are designing a switching power supply design with the µP in the feedback loop. That requires a quite complex design. Suggest you Google "digital control loop power supplies" and do some reading to understand how to stabilize the control loop. You might also look up "fuzzy logic power supplies" which is an alternate (and some say easier) way to use a µP to realize a stable switching supply control loop.

The size of the inductor and capacitors are mostly determined by the amount of ripple in the output voltage you can tolerate and the switching frequency you will use.

The transistor and diode choice are determined by the current and voltage of your circuit. To get 90-95% efficiency is not easy. You will need a transistor with a low on-resistance and a diode with a low forward voltage drop.

This is a difficult task if you are starting from scratch and only have three months left. You'll need a good supply of midnight oil. Good luck.

hi Carl,

yup, u r correct... I'm using a uP for the design. I'm trying to solve each problem at a time. but for now, i think it's pretty important that i get all the components right cos i am reali puzzled with the different equations i see online calculating the peak current, inductor value etc...:eek:
 
Rehaps I should can this resonse as this seems to be a very common suggestion.

Look at the Linear Technology, National and Maxim semiconductor websites. They make ICs purpose built for this kind of thing.

What do you want 50V at 1A for?

Assuming 85% efficiency, it's 3.27A at 18V, are you sure your primary PSU can cope with this?


hi there...

the input; 18V is from a solar panel... my lecturer mentioned that the output current from the solar panel is 1A. I'm setting the output current at 5A if possible. btw, i'm not supposed to use any dc-dc ICs in the market... :(
 
I'm trying to solve each problem at a time. but for now, i think it's pretty important that i get all the components right cos i am reali puzzled with the different equations i see online calculating the peak current, inductor value etc...:eek:
If you tell us the exact equations you are puzzled about with specific questions about them, we can then perhaps give you some help.
 
hi there...

the input; 18V is from a solar panel... my lecturer mentioned that the output current from the solar panel is 1A. I'm setting the output current at 5A if possible. btw, i'm not supposed to use any dc-dc ICs in the market... :(

Then your lecturer is either pretty stupid or he's making you think (more likely) by setting you an impossible assignment.

Are there any batteries in the system?

Energy out can't exceed the energy in.

You could have a battery on the input or output which chould be charged and provide surges but you can't have 18V 1A in and 50V 1A out continiously.
 
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You could have a battery on the input or output which chould be charged and provide surges but you can't have 18V 1A in and 50V 1A out continiously.

I re-read the OP first post two times and can't seem to locate anywhere it mentions 1A at 50V?
 
I re-read the OP first post two times and can't seem to locate anywhere it mentions 1A at 50V?

hi,

it was my lecturer's instruction to output a higher current, but pardon me for being dumb... but i have a few questions hoping for some enlightment...

1. Is it impossible for me to boost the output current if the input current is 1A? maybe abt 5A?

2. The other requirement is to use a battery at the output to store the energy from the boost-ed supply. but I have not gone into tat yet. <-- any suggestions on the type of battery to use?

3. I'm using equations from Boost Switching Converter Design Equations to obtain the inductance and the Irms, Inductor value and the capacitor values...

i input my requirements for Vf for diode to be 0.95V and transistor voltage drop be 2.6V (using power mos BUZ11) and got the following results. the Inductor Current Ripple and Output Voltage Ripple were set to 5%.

Volts In 18 V
Volts Out 50 V
Load Current 1 A
Freq. 300 KHz
Vripple 2.5 V
Duty Cycle 68.148914167528 %
Ipp Inductor 2 A
Ipk Inductor 2 A
Irms 0.577 A
L 17.49 uH
C 0.908 uF

4. Btw, Are there any parameters I need to calculate? before i get my components?

5. Attached is a reference circuit i've found, prob using it to design for my requirements. I'm not reali sure abt the following
- the current amp at the input, wat is it used for?
- is the voltage divider at the output for error feedback?

Thanks a million...
 

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I re-read the OP first post two times and can't seem to locate anywhere it mentions 1A at 50V?
Correct. Only the output voltage was mentioned, not the current. Isn't it interesting how misinformation propagates from one posting to the next.
 
Well, I will assume the OP understands conservation of energy so that he knows he has implicity specified an outptu current of 320-340mA (depending on the efficiency range he has given).

But if that is the case, then his second post does not make sense:

hi there...

the input; 18V is from a solar panel... my lecturer mentioned that the output current from the solar panel is 1A. I'm setting the output current at 5A if possible. btw, i'm not supposed to use any dc-dc ICs in the market... :(

Well, I will assume the OP understands conservation of energy so that he knows he has implicity specified an output current (from the converter, not the solar panel) of 320-340mA (depending on the efficiency range he has given).

hi there...

the input; 18V is from a solar panel... my lecturer mentioned that the output current from the solar panel is 1A. I'm setting the output current at 5A if possible. btw, i'm not supposed to use any dc-dc ICs in the market... :(
You can't set the output current at 5A. It is whatever you are trying to pull from the solar panel and whatever the solar panel can provide. You could rate your components for an output current of 5A however to make sure things don't burn out. If you have to build this from scratch you are in for a hard time...but it means you have to look at the basic schematic of a boost converter:
**broken link removed**

And figure out what real components will do the trick. Hopefully your instructor does not want it to be regulated and yuo can assume a 18V input with 50V output (even the input will fluctuate in real life). Then you can just figure out a constant square wave duty cycle from the equations to drive the transistor with so for ~18V input you will get ~50V output. If the input changes, so will the output but that's why it's called unregulated.
 
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Well, I will assume the OP understands conservation of energy so that he knows he has implicity specified an outptu current of 320-340mA (depending on the efficiency range he has given).

A LM2577 from National Semiconductor might do it for you. THere are also equivelant parts from Texas Instruments and variations of the part from both manufacturers.

i must use discrete components, i prob wont run into problems if i am able to use ICs in replacement :)
 
i must use discrete components, i prob wont run into problems if i am able to use ICs in replacement :)
Does the output voltage need to regulated?
 
erm Carl, wat does 'output voltage need to be regulated mean"?
It means, does the output voltage need to be stable with a variation in input voltage or output load?
 
It means, does the output voltage need to be stable with a variation in input voltage or output load?

hi Carl,

the input comes from a solar panel, hence i guess there might be variations at the input. I would say the output load is a battery which is capabale of retaining the boosted power.

btw, i've posted my equations in an earlier post.... do help take a look...
 
it was my lecturer's instruction to output a higher current, but pardon me for being dumb... but i have a few questions hoping for some enlightment...

1. Is it impossible for me to boost the output current if the input current is 1A? maybe abt 5A?

This is getting serious. I have the feeling that you have misunderstood what your lecturer had said. You definitely need to clarify the output current value with him/her. Higher current is only possible with lower output voltage.

Maybe your aim is to: "Design a converter that can output 50V, max 5A from a battery that uses 18V 1A solar panel as its charging source."
 
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5. Attached is a reference circuit i've found, prob using it to design for my requirements. I'm not reali sure abt the following
- the current amp at the input, wat is it used for?
- is the voltage divider at the output for error feedback
Question 1 - Page 2 of the reference discusses obtaining the maximum power from the solar panel by optimizing its I-V operating point. The current and voltage feedback from the panel output to the PIC allows the PIC to adjust the PWM duty-cycle to achieve this maximum power point. This point varies with the intensity of sunlight on the panel. (I don't think your project necessarily requires that but it is a desirable way to get power from a panel).

Question 2 - If you notice, the output voltage divider goes to a box labeled "Overvoltage Detector". It detects when the battery voltage indicates a fully charged battery and reduces the converter output to avoid overcharging.
 
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