# Any reason why a DC/DC buck converter can't have a very wide input range?

M

#### Michael

Jan 1, 1970
0
Hey there - somebody recently told me that it wasn't possible to have
a very wide input range on a DC/DC converter. Is there any reason why
this would be the case? Say I want an input range of 10-75VDC with an
output of 5VDC. Couldn't I just find a FET that could handle the 75V,
find a driver for the fet, and then find a dc/dc switcher without an
integrated switch? Or is there some reason this won't work (like a
minimum operating voltage on some FETs or something like that?)

Just curious.

Thanks!

-Michael

J

#### John Popelish

Jan 1, 1970
0
Michael said:
Hey there - somebody recently told me that it wasn't possible to have
a very wide input range on a DC/DC converter. Is there any reason why
this would be the case? Say I want an input range of 10-75VDC with an
output of 5VDC. Couldn't I just find a FET that could handle the 75V,
find a driver for the fet, and then find a dc/dc switcher without an
integrated switch? Or is there some reason this won't work (like a
minimum operating voltage on some FETs or something like that?)

Everything you say is possible and is sometimes done. But
if you design for a smaller input range, the minimum
efficiency can be made to be higher. A wide input range
requires added complexity (cost) and larger compromises and
losses, at least, at some operating conditions in that range.

Here is an example with a 4:1 input range (18 to 75 VDC):
http://www.artesyn.com/specs/CXA101.pdf

C

#### colin

Jan 1, 1970
0
Michael said:
Hey there - somebody recently told me that it wasn't possible to have
a very wide input range on a DC/DC converter. Is there any reason why
this would be the case? Say I want an input range of 10-75VDC with an
output of 5VDC. Couldn't I just find a FET that could handle the 75V,
find a driver for the fet, and then find a dc/dc switcher without an
integrated switch? Or is there some reason this won't work (like a
minimum operating voltage on some FETs or something like that?)

Just curious.

Thanks!

-Michael

10v-75v ->5v would be doable, but you pay for the wide range,
basically you have higher peak currents becuase it has to be higher at 10v
than 75v
but at 75v you stil have the same current but at a low duty cycle.

this it would be less efficient in terms of size and power.

some controller chips have quite a wide range,
although at 10-75 i dont think there are that many,
i dont know one off hand.

Colin =^.^=

L

#### LVMarc

Jan 1, 1970
0
Michael said:
Hey there - somebody recently told me that it wasn't possible to have
a very wide input range on a DC/DC converter. Is there any reason why
this would be the case? Say I want an input range of 10-75VDC with an
output of 5VDC. Couldn't I just find a FET that could handle the 75V,
find a driver for the fet, and then find a dc/dc switcher without an
integrated switch? Or is there some reason this won't work (like a
minimum operating voltage on some FETs or something like that?)

Just curious.

Thanks!

-Michael

You can.

However, A key component in the converter is the inductor. This is set
by "delta Voltage" time and current. AS you state a >7:1 input voltage
range, all above the desired out voltage range; the optimum inductor
would have difeernet values. That being said, you could make a
compromise. Also some chips use adaptive control loops,so that you may
have adjustable "time" aka frequency and or Pulse width modulation, to
absorb the variations in the input voltage,and then there is a whole
devil is usually in the real -world details.

also as the VDD increases, 10v,to 20 , 40, 80 etc.. a different cmos
process is needed to provide stand off voltage. as an 75 VDC source is
an odd level?

Furthermore, it can be worse! There are time where the input voltage
range, straddles the desired output range. for example say 5 vdc is
output volts.

then you add an input voltage range of 2- 8 .. very different approach

Good luck

Marco

www.fwt.niat.net amazing antennas

J

#### John Devereux

Jan 1, 1970
0
colin said:
10v-75v ->5v would be doable, but you pay for the wide range,
basically you have higher peak currents becuase it has to be higher at 10v
than 75v
but at 75v you stil have the same current but at a low duty cycle.

this it would be less efficient in terms of size and power.

some controller chips have quite a wide range,
although at 10-75 i dont think there are that many,
i dont know one off hand.

week. I suggested the LM5574, up to 75V. Linear do some up to 90V I
think.

W

#### Wimpie

Jan 1, 1970
0
Hey there - somebody recently told me that it wasn't possible to have
a very wide input range on a DC/DC converter. Is there any reason why
this would be the case? Say I want an input range of 10-75VDC with an
output of 5VDC. Couldn't I just find a FET that could handle the 75V,
find a driver for the fet, and then find a dc/dc switcher without an
integrated switch? Or is there some reason this won't work (like a
minimum operating voltage on some FETs or something like that?)

Just curious.

Thanks!

-Michael

Hi Micheal,

It is possible, only the duty cycle for the power switch will be small

You could look at the LM5010 (for example). This is a solution that
has the power switch on-board.

It has a boost capacitor charge pump to drive the internal N-channel
MOSFET. When, after a transient, the FET stays off for a too long
period, the capacitor runs out of charge and the switchter will not
start again until Vout drops very low (to recharge the boost
capacitor).

Other thing with such devices, the datasheet specifies a 30ns fast
recovery diode. If you want to keep you chip alive, use a high voltage

Best regards,

Wim
PA3DJS
www.tetech.nl

P

#### PeteS

Jan 1, 1970
0
Michael said:
Hey there - somebody recently told me that it wasn't possible to have
a very wide input range on a DC/DC converter. Is there any reason why
this would be the case? Say I want an input range of 10-75VDC with an
output of 5VDC. Couldn't I just find a FET that could handle the 75V,
find a driver for the fet, and then find a dc/dc switcher without an
integrated switch? Or is there some reason this won't work (like a
minimum operating voltage on some FETs or something like that?)

Just curious.

Thanks!

-Michael

Apart from what others have said, a knowledge of which topologies is
best in a particular situation helps. A wide Vin/Vout range will make
life rather difficult if you are using a voltage mode converter (which
is particularly sensitive to Vin/Vout issues in the control loop)
although a current mode converter will still have issues when trying to
choose the inductor.
(Voltage mode converters are sensitive to input voltage variations for a
fixed output voltage and current mode converters are sensitive to output
current variations as far as stabilising the loop is concerned).

Although it's do-able, most of the time I would do a double conversion
if I had a known high voltage at the input.

Cheers

PeteS

C

Jan 1, 1970
0

M

#### MooseFET

Jan 1, 1970
0
Hey there - somebody recently told me that it wasn't possible to have
a very wide input range on a DC/DC converter. Is there any reason why
this would be the case? Say I want an input range of 10-75VDC with an
output of 5VDC. Couldn't I just find a FET that could handle the 75V,
find a driver for the fet, and then find a dc/dc switcher without an
integrated switch? Or is there some reason this won't work (like a
minimum operating voltage on some FETs or something like that?)

Just curious.

Thanks!

-Michael

There were lots of other good replys but one issue didn't come up:

Wide range converters have an extra measure of trouble with the
control loop. The changing input voltage changes the loop gain unless
you compensate for it somehow.

Abandoning the idea of holding to a constant frequency is usually
needed to make a wide range one practical. If you have about the
right amount of inductance for operation at the high end, it is way
too much for the same frequency at the low end. A "constant ripple"
design sort of does the change automatically for you.

Oct 21, 2006
1,325
1 power
2 efficiency

P

#### PeteS

Jan 1, 1970
0
ChairmanOfTheBored said:

My clock is set for EDT, which is where the system originated

Cheers

PeteS

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