Boost Converter

New User ^_^ · Nov 19, 2004

Hi All,

First of all, I'd like to thank to those who answered my question
before regarding boost converter IC. Actually, I'm not sure the
difference between boost converter and boost switching regulator.

For example, I want to boost the input voltage by 10V, i.e. Vout = Vin
+ 10V. Shall I use boost converter or boost switching regulator??

What does it mean by the following terms?
Fixed 3.3-V High-Efficiency Boost Converter
Adjustable 3.3-V High-Efficiency Boost Converter
Fixed output voltage?? Adjustable output voltage??

For my case, shall I use UC2577-ADJ or LM3478 if I just want to boost
Vin by 10V for 3V<=Vin<=10 ??

Thanks for your help in advance.

Regards,
Will

Anders F · Nov 19, 2004

New User ^_^ said:
Hi All,

First of all, I'd like to thank to those who answered my question
before regarding boost converter IC. Actually, I'm not sure the
difference between boost converter and boost switching regulator.

Same! The first is just short for the latter...

What does it mean by the following terms?
Fixed 3.3-V High-Efficiency Boost Converter
Adjustable 3.3-V High-Efficiency Boost Converter
Fixed output voltage??

Set by the device (controller)

Adjustable output voltage??

Set by you (usually by a resistor divider)

For my case, shall I use UC2577-ADJ or LM3478 if I just want to boost
Vin by 10V for 3V<=Vin<=10 ??

Most designs boost to a stable voltage. You might do some design to add the
input voltage to the feedback. Bust most probably you will be satisfied with
ie. 20V out from 3-10V input?

/A

Fred Bloggs · Nov 20, 2004

New said:
For my case, shall I use UC2577-ADJ or LM3478 if I just want to boost
Vin by 10V for 3V<=Vin<=10 ??

In order to obtain Vout= Vin + 10V which is equivaleNt to
(Vout-Vin)=10V=~8*Vref of the UC2577, you will have to do something like
this:

View in a fixed-width font such as Courier.

..
..
..
.. L1
.. Vin >---+---UUUUU-----+
.. | ===== |
.. | |
.. +------------+ |
.. | Vin | | D1
.. | sw +----+-----|>|--+---+---------+------> Vin + 10
.. | | | | |
.. | | === | /
.. | | | | 20K OA=LT1013
.. | UC2577 | GND | /
.. | | | \ or other
.. | | OA |/| |
.. | | 6.0K /+|-------+ single-
.. | (2)+----+---/\/\--+---< | |
.. | FB | | | \-|--+ / supply
.. | | / | |\| | 10K
.. | | 2,0K | | | / 1MHz GBW
.. | | / | gnd | \
.. | | \ | | |
.. | GND | | | | gnd
.. +-----+------+ gnd | 10K |
.. | +---/\/\--+
.. +-----gnd |
.. | 20K |
.. | Vin >---/\/\--+
.. |
.. -------+--------------------------------------------> GND
..
..

Mac · Nov 20, 2004

Hi All,

First of all, I'd like to thank to those who answered my question
before regarding boost converter IC. Actually, I'm not sure the
difference between boost converter and boost switching regulator.

For example, I want to boost the input voltage by 10V, i.e. Vout = Vin
+ 10V. Shall I use boost converter or boost switching regulator??

It is VERY unclear what you are trying to do.

I'm going to ask some questions, and I suggest you take the time to answer
them. If you don't know the answers to some of them, then you can just
say that you don't know. If you don't know the answers to ANY of them,
then you are not ready to select an IC.

1) What range might Vin be?

2) How much current do you need to supply from Vout?

3) How does Vin vary with time? Is it a non-rechargeable battery which
drains slowly over time? Is it a rechargeable battery which slowly
drains but might experience a rapid rise when the charger comes on-line?
Is it a music signal? Voice? What?

4) What if, instead of producing Vout = Vin + 10, we could just produce
some nice fixed voltage such as 20 volts or 48 volts or something? Would
that meet your needs? It might be easier to do this than to create Vin +
10 volts.

5) What, if any stable voltage supplies do you have available to you?

It is important that you answer these questions, because a variety of
people are probably going to give up trying to help you if you don't start
telling us a little bit more about your application.

[snip]

Thanks for your help in advance.

You can best show your appreciation by supplying answers to the questions
that I and others have asked you. ;-)

Regards,
Will

--Mac

kell · Nov 20, 2004

I am wondering about your post. I can't think why you would
want your otuput voltage to wander up and down.
Because your input voltage will probably vary with load.
Do you really want a converter with an output that is not regulated?

If the answer to that one is yes, one way is to use a
converter with an isolated output, like
a push-pull converter. That way you can just take the output
and stack it on top of your input power source.

Push-pull converters differ from the boost
converter topology in that they use a transformer instead
of a single inductor. Push-pulls will put out more power.

What is the application?

New User ^_^ · Nov 21, 2004

Hi Mac,

Sorry for making trouble. I should have made it clear about my
question.

Actually, I'd like to step up Vin by 10V to Vout which is the supply
voltage of a AND logic gate. This AND gate drives the gate(Vg) of a
N-channel power MOSFET. But at the same time, the value of Vs is more
or less the same as Vin. That's why I want to step up Vin by 10V in
order to turn on the MOSFET completely.

Hahaha, one thing I should have verified is that Vin is FIXED. The Vin
given before is just a range of possible voltages.

What I really want to know is: For Vin=3V, I make a boost converter
(boost converter IC + different values of components, e.g. L, R, C )
to step up Vout to 13V. Up til now, I think everything should be
correct. But, then, I change Vin=10V. MY QUESTION is: Is it possible
to step up Vout to 20V without changing the values of the components??

1) What range might Vin be?

Vin could be 3V-10V or higher

2) How much current do you need to supply from Vout?

About 100mA

3) How does Vin vary with time? Is it a non-rechargeable battery which
drains slowly over time? Is it a rechargeable battery which slowly
drains but might experience a rapid rise when the charger comes on-line?
Is it a music signal? Voice? What?

Vin is FIXED. The range is mentioned above.

4) What if, instead of producing Vout = Vin + 10, we could just produce
some nice fixed voltage such as 20 volts or 48 volts or something? Would
that meet your needs? It might be easier to do this than to create Vin +
10 volts.

It would be nice if I could make Vout = Vin + 10V as I explained
above.

5) What, if any stable voltage supplies do you have available to you?

I have stable voltage supplies, 3V or whatever stable voltage source.

Thanks for your help in advance.

Best regards,
Will ^_^

Fred Bartoli · Nov 21, 2004

Fred Bloggs said:
In order to obtain Vout= Vin + 10V which is equivaleNt to
(Vout-Vin)=10V=~8*Vref of the UC2577, you will have to do something like
this:

<Snipped diagram>

Or if the OP doesn't mind a -2mV/K temperature dependency, here's the
simpler :

.------------------------------.
| |
. | L1 |
. Vin >---o---UUUUU-----+ |
. | ===== | |
. | | |
. +------------+ | |
. | Vin | | D1 |
. | sw +----+-----|>|--+-----)-------+------> Vin + 10
. | | | | |
. | | === +---. |
. | | | | | .-.
. | UC2577 | GND | | | |20K
. | | | V | |
. | | | - '-'
. | | --- | |
. | (2)+-----------+--------/ ^--+---'
. | FB | |
. | | .-. Small sig PNP
. | | | |
. | | | |2K7
. | | '-'
. | GND | |
. +-----+------+ |
. | ===
. +-----gnd GND
. |
. |
. |
. -------+--------------------------------------------> GND
(created by AACircuit v1.28 beta 10/06/04 www.tech-chat.de)

Fred Bloggs · Nov 21, 2004

Fred said:
<Snipped diagram>

Or if the OP doesn't mind a -2mV/K temperature dependency, here's the
simpler :

.------------------------------.
| |
. | L1 |
. Vin >---o---UUUUU-----+ |
. | ===== | |
. | | |
. +------------+ | |
. | Vin | | D1 |
. | sw +----+-----|>|--+-----)-------+------> Vin + 10
. | | | | |
. | | === +---. |
. | | | | | .-.
. | UC2577 | GND | | | |20K
. | | | V | |
. | | | - '-'
. | | --- | |
. | (2)+-----------+--------/ ^--+---'
. | FB | |
. | | .-. Small sig PNP
. | | | |
. | | | |2K7
. | | '-'
. | GND | |
. +-----+------+ |
. | ===
. +-----gnd GND
. |
. |
. |
. -------+--------------------------------------------> GND
(created by AACircuit v1.28 beta 10/06/04 www.tech-chat.de)

Very nice!- and good enough for high side MOSFET(s) gate drive bias.

Fred Bartoli · Nov 21, 2004

Fred Bloggs said:
... and good enough for high side MOSFET(s) gate drive bias.

Ah, I missed that. Yes indeed.

Ken Smith · Nov 21, 2004

Fred Bartoli said:
Or if the OP doesn't mind a -2mV/K temperature dependency, here's the
simpler :

You can compensate for the -2mV/K like this:
Editted diagram:

.------------------------------.
| |
. | L1 |
. Vin >---o---UUUUU-----+ |
. | ===== | |
. | | |
. +------------+ | |
. | Vin | | D1 |
. | sw +----+-----|>|--+-----)-------+------> Vin + 10
. | | | | |
. | | === +---. |
. | | | | | .-.
. | UC2577 | GND | | | |20K
. | | | V | |
. | | | - '-'
. | | --- | |
. | (2)+-----------+---+----/ ^--+---'
. | FB | | !
. | | .-. ! Small sig PNP
. | | | | /
. | | R1 | | \ R2
. | | '-' /
. | GND | | !
. +-----+------+ | V
. | === ---
. +-----gnd GND !
. | GND
. |
. |
. -------+--------------------------------------------> GND
(created by AACircuit v1.28 beta 10/06/04 www.tech-chat.de)

R1
-------- = Vfb/10V (about)
R1 + R2

Fred Bloggs · Nov 21, 2004

Fred said:
<Snipped diagram>

Or if the OP doesn't mind a -2mV/K temperature dependency, here's the
simpler :

.------------------------------.
| |
. | L1 |
. Vin >---o---UUUUU-----+ |
. | ===== | |
. | | |
. +------------+ | |
. | Vin | | D1 |
. | sw +----+-----|>|--+-----)-------+------> Vin + 10
. | | | | |
. | | === +---. |
. | | | | | .-.
. | UC2577 | GND | | | |20K
. | | | V | |
. | | | - '-'
. | | --- | |
. | (2)+-----------+--------/ ^--+---'
. | FB | |
. | | .-. Small sig PNP
. | | | |
. | | | |2K7
. | | '-'
. | GND | |
. +-----+------+ |
. | ===
. +-----gnd GND
. |
. |
. |
. -------+--------------------------------------------> GND
(created by AACircuit v1.28 beta 10/06/04 www.tech-chat.de)

This is almost precision:

View in a fixed-width font such as Courier.

.------------------------------.
| |
. | L1 |
. Vin >---o---UUUUU-----+ |
. | ===== | |
. | | |
. +------------+ | |
. | Vin | | D1 |
. | sw +----+-----|>|--+-----)-------+------> Vin + 10
. | | | | |
. | | === | V D
. | | | | -
. | UC2577 | GND | |
. | | .-. .-.
. | | | |20K | |20K
. | | | | | |
. | | '-' '-'
. | | | |
. | (2)+------+--------------|-------+
. | FB | | | |
. | | | | |
. | | .-. +---+---+ |
. | GND | | | | | | |
. +-----+------+ | |2K7 | c | c
. | '-' | |/ | |/
. +-----gnd | +-| T1 +-| T2
. | | |\ |\
. | === e e
. | GND | |
. -------+----------------------------+-------+-------> GND

Winfield Hill · Nov 21, 2004

Fred Bloggs wrote...

This is almost precision:

.------------------------------.
. | L1 |
. Vin >---o---UUUUU-----+ |
. | ===== | |
. | | |
. +------------+ | |
. | Vin | | D1 |
. | sw +----+-----|>|--+-----)-------+------> Vin + 10
. | | | | |
. | | === | V D
. | | | | -
. | UC2577 | GND | |
. | | .-. .-.
. | | | |20K | |20K
. | | | | | |
. | | '-' '-'
. | | | |
. | (2)+------+--------------|-------+
. | FB | | | |
. | | .-. +---+---+ |
. | GND | | | | | | |
. +-----+------+ | |2K7 | c | c
. | '-' | |/ | |/
. +-----gnd | +-| T1 +-| T2
. | | |\ |\
. | === e e
. | GND | |
. -------+----------------------------+-------+-------> GND
.
. Vin-Vbe Vout-Vd 2.7*20
. Ic1= ------- = Ic2 then Vref= ------- * 2.7K- Ic2 ------K
. 20K 22.7K 22.7
.
. => Vout = 8.4 * Vref + Ic2*20K + Vd =
. Vout = 8.4* Vref + Vin + (Vd-Vbe) ~ 10.3 + Vin

Try this way, same part count, but simplified concept and math...

..
.. Vin >--+-------------+--------------------------,
.. | | | gate-drive supply for
.. +------------+ L1 | high-side FET switches
.. | Vin | | D1 |
.. | sw |----+--|>|--+----------------- | ----+---> Vout
.. | | | Vin D2 | _|_+ Vin + 10
.. | | '-- R2 --+---|<|---+ ---
.. | | | | |
.. | UC2577-ADJ | e\| |/e gnd
.. | etc | |--+--|
.. | | /| | |\
.. | | 1.23V | | | Vo = Vin + Vfb R2/R1
.. | fb |------+--------------' '----+
.. | gnd | | pnp pair | Vo = Vin + 10 for
.. +------------+ R1 bcm857bs 10k R2/R1 = 8.13
.. | | bc857bs |
.. -------+-------------+------------------------+-----> GND

D2 protects the pnp transistor when Vout is more than 5V below Vin.

Ken Smith · Nov 21, 2004

Fred Bartoli said:
Or if the OP doesn't mind a -2mV/K temperature dependency, here's the
simpler :

[.. diagram snipped ...]

Linear makes an "isolated flyback" converter that contains much the same
function as the PNP transistor. If the OP can stand the inaccuracy it
has, it would be worthwhile looking at it.

Unfortunately the Linear web site is still all screwed up so I can't find
it and post the number. Its something like LT1723 or there abouts.

Mac · Nov 22, 2004

Hi Mac,

Sorry for making trouble. I should have made it clear about my
question.

Actually, I'd like to step up Vin by 10V to Vout which is the supply
voltage of a AND logic gate. This AND gate drives the gate(Vg) of a
N-channel power MOSFET. But at the same time, the value of Vs is more
or less the same as Vin. That's why I want to step up Vin by 10V in
order to turn on the MOSFET completely.

Have you considered using a P-Channel MOSFET with its gate pulled up to
VCC (or VDD)? Then it would be easy to turn it on, provided it is a logic
level MOSFET.

Hahaha, one thing I should have verified is that Vin is FIXED. The Vin
given before is just a range of possible voltages.

What I really want to know is: For Vin=3V, I make a boost converter
(boost converter IC + different values of components, e.g. L, R, C )
to step up Vout to 13V. Up til now, I think everything should be
correct. But, then, I change Vin=10V. MY QUESTION is: Is it possible
to step up Vout to 20V without changing the values of the components??

There is probably some way to do this, but it would not be a standard
application of the IC's that I am familiar with. They are usually designed
to regulate to a particular voltage, and are usually NOT designed to be
put in series with their V_IN.

Vin could be 3V-10V or higher

I see that you specified that in the post I replied to, so my apologies.

About 100mA

Why do you need 100mA to turn on a MOSFET? Is it because you want to turn
it on quickly, and switch it at a (relatively) high rate?

Vin is FIXED. The range is mentioned above.

It would be nice if I could make Vout = Vin + 10V as I explained
above.

Now that I know you are trying to turn on a MOSFET I have a better
understanding of your needs.

I have stable voltage supplies, 3V or whatever stable voltage source.

Thanks for your help in advance.

Best regards,
Will ^_^

There is a fairly standard technique for turning on high-side MOSFET's in
switching regulator applications which may be applicable to your situation
if you are switching the MOSFET rapidly. It won't get you to Vin + 10, but
it could get you nearly double Vin, which might be enough for you if you
choose the right MOSFET.

The idea is to use a diode and a capacitor. The diode is connected between
VCC (or Vin) and the power supply for the chip driving the MOSFET gate.
The capacitor is connected between the MOSFET source and the power supply
for the chip driving the MOSFET gate. See the following ASCII art
schematic (use courier or similar font). I hope I've got this right:

VCC
|
+----------------------------+
| |
----- |
\ / Diode |
--|-- |
+---------------+ |
| |\| | D
----- Cap | \ ||--+
----- _______| \_______|| N-channel
| + | / G || MOSFET
| | | / ||--+
| Gate |/| | S
| Driver | |
| | |
| GND |
| |
| |
+----------------------------+
|
-----
| L |
| O |
| A |
| D |
-----
|
GND

The gate driver has to be able to withstand VCC * 2. The MOSFET has to be
of a type that can be turned on by VCC, and there has to be frequent
enough switching to guarantee that the capacitor will always be charged up
enough to drive the gate.

--Mac

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