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Boost converter

jackorocko

Apr 4, 2010
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Is it impossible to think that I could use a 12V 50A max car battery charger transformer and make a 45V boost converter to run some steppers with a current draw of 2A per motor. total of 6A.

I want something simple and I already have this transformer, so even though it is not that efficient of a design. It fits my budget for this redneck project I am brainstorming.

Something like this, at least from that calculator on that page, it seems very plausible, as long as I can get a 555 or similar device to produce the PWM input.
http://www.ladyada.net/library/diyboostcalc.html
 

OLIVE2222

Oct 2, 2011
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Hi jackorocko, for almost the same price the MC34063 can be better at the task.
He offer a regulation and a current limiting. Just need an external FET, as with the 555 anyway.

Olivier
 

jackorocko

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1.5A output is not nearly enough output current. I need at the very min 6A
 

BobK

Jan 5, 2010
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That is why Oliver said you need an external (MOS)FET.

Bob
 

jackorocko

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I think I get it now. You are saying to add in a MOSFET, the gate to the switch collector of the output transistor of the 34063 and then the source to ground and the drain to the inductor where the switch collector was previously attached?

I will whip a schematic up sometime and see if I am on track.
 

jackorocko

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Ok, so I went over the datasheet for the mc34063. They have a schematic in there for an external NPN transistor. I assume I can just use this design and replace it with a MOSFET.

I went through the formula table and figured out all my values. This is what I got, did I manage to do it right? If the below information is correct then that would mean I need a MOSFET that can handle 45V @ 42.25A D-S? I am also thinking I should find one with a low gate capacitance to make sure I don't slow down my switching freq. Any other suggestions?

Ton/Toff = 2.25uS
Ton + Toff = 10uS (max freq)
Toff = 3.07uS
Ton = 6.93uS
Ct = 277pF
Ipk = 42.25A
Rsc = 0.007Ohms
Lmin = 2.2uH
Co = 1600uF
 

BobK

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Actually, I don't think that chip is good for using an external MOSFET. A high power MOSFET needs a push-pull driver so that it can both charge and discharge the gate capacitance quickly. You might want ot look for other boost converter controllers that are designed for an external MOSFET switch.

Bob
 

jackorocko

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Thanks bob, I took a quick look on digikey. I found something that has a gate driver, but the specs look to low for what I need. Maybe I am interpreting it wrong?

I think this one or the max773 will work for me. I am going through the datasheet now. The max773 datasheet has a schematic for a [email protected] output shunt regulated design. I am trying to read through the design procedure to see if it will work. I was confused when it said in the datasheet output power to load 24W(max1771) and 15W (max773), but I guess my load in this case is the MOSFET gate? How does one go about figuring out that?
http://search.digikey.com/us/en/products/MAX1771CPA+/MAX1771CPA+-ND/1130157
This one looks more promising, but it has an open collector transistor as the output. I don't think this is what they consider a gate driver, is it? They show in the schematic a MOSFET attached to the output though.
http://search.digikey.com/us/en/products/TL5001AIP/296-10203-5-ND/380448
 
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jackorocko

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This is what I come up with. But I have some serious doubts to whether this will work.

Vcc = 14 - 16V transformer is 12VAC @ 50A

1. Datasheet says to set Rsense by the graphs, well none of the graphs are even close to the range I will be using. I used the 1R resistor in the schematic on PG 12 FIG 3e.

2. Input and output capacitors. Again I just took a guess at these. No idea how to figure the values out and the datasheet is no help.

3. Diode? According to the datasheet I might be better off to look for a high speed silicon diode instead of a shottky. Any suggestions?

4. MOSFET I chose was, Vds 60V @ 30A No idea, just took another wild guess.
http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00003405.pdf

5. The inductor I picked with a ILim of 30A.

6. How do I know what my output current will be?

We can start with the above, I might have more questions.
 

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OLIVE2222

Oct 2, 2011
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Cheap FET driver are available from microchip
http://search.digikey.com/us/en/products/TC1427CPA/TC1427CPA-ND/267372 to be used in combination with the MC34063.

Here under a practical design,with technical highlights, based on a UC3843, same generation as the MC34063, i.e. not optimized for FET driving.


http://www.labkit.ru/userfiles/file...shm/95-wat_Laptop_PSU_12V_in_-_19V_5A_out.pdf


Many others components (or components combination's) can be used as you already find out. More recent components use higher frequencies (in the Mhz range) allowing less bulky inductors and caps.
Unfortunately they tend to be squeezed in harder to handle SMD packages.

regarding your design the MAX773 is scheduled to be discontinued (not an issue at hobby level however) also the 1 ohms shunt seems quite high they probably need to be in the tenth of mOhms range with 6Amps

Olivier
 

jackorocko

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Here under a practical design,with technical highlights, based on a UC3843, same generation as the MC34063, i.e. not optimized for FET driving.

You trying to tell me that the UC3843 is for FET driving just not optimized driving? The datasheet says

a totem pole output stage designed to source or sink high peak current. The output stage, suitable for driving N-Channel MOSFETs, is low in the off state

sry, just a little confused on what exactly is an optimized driver. I guess that FET driver chip you linked to could be used in between any converter IC and a FET to give me the needed drive current?
 

OLIVE2222

Oct 2, 2011
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You right jackoroko the UC3843 with the totem pole output stage is well adapted to drive MOSFET compared to the MC34063 open collector output stage. An "optimized" driver must be able to fastly drive the gate despite it's represent a capacitive load.

Olivier
 
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