I'm and ME so I'm a little lost here. I needed an H-Bridge to control a
small motor (about 10A) so I sat down and got aquatinted with Google. I
read and read and then read some more then I finally designed an
H-Bridge. I used all NPN transistors for the corners and it works fine
from what I can see; however, most H's I read about used a combination
of PNP and NPN. Just wondering why this mixing of xsters is used
instead of simply using one variant or the other.
They explain what is required if you use all of the N type mosfets in
your design, i.e. there is a problem to overcome with the high side gate
drive.
_______________________________________
Roy
Tauranga
New Zealand
_______________________________________
I used all NPN transistors for the corners and it works fine
from what I can see; however, most H's I read about used a combination
of PNP and NPN. Just wondering why this mixing of xsters is used
instead of simply using one variant or the other.
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I'm sorry, I can't find the article you're referring to. The site you
linked seemed (I didn't go through each and every link but I did go
through quite a few of them and didn't see anything related to my
question on the site map) more like a long description of 4QD
controllers rather than a technical description of H-bridges.
On Mar 12, 2005, at 7:44 PM, Hopkins wrote:
> Have you read the explanations at http://www.4qd.co.uk/
>
> They explain what is required if you use all of the N type mosfets in
> your design, i.e. there is a problem to overcome with the high side
> gate
> drive.
>
> _______________________________________
> Roy
> Tauranga
> New Zealand
> _______________________________________
>
> I used all NPN transistors for the corners and it works fine
> from what I can see; however, most H's I read about used a combination
> of PNP and NPN. Just wondering why this mixing of xsters is used
> instead of simply using one variant or the other.
>
>
> --
> No virus found in this outgoing message.
> Checked by AVG Anti-Virus.
> Version: 7.0.308 / Virus Database: 266.7.2 - Release Date: 11/03/2005
>
>
> I'm and ME so I'm a little lost here. I needed an H-Bridge to control a
> small motor (about 10A) so I sat down and got aquatinted with Google. I
> read and read and then read some more then I finally designed an
> H-Bridge. I used all NPN transistors for the corners and it works fine
> from what I can see; however, most H's I read about used a combination
> of PNP and NPN. Just wondering why this mixing of xsters is used
> instead of simply using one variant or the other.
I won't even attempt to explain all of the details of why a
matched pair of PNP and NPN is better, but the short answer is
that using a matched pair of NPN (on the low side) and PNP
(on the high side) allows all 4 transistors to turn fully on.
And I'm sure that someone will respond with the correct
circuit theory.
Forgive my ignorance, but I'm still not seeing a problem which needs to
be overcome. I read that page already and what I saw was their
description of why the LM3524 was a poor choice. The other issue they
addressed was the need to have the gate voltage a little greater than
the source. How is that a "problem"? Also, that doesn't explain why NPN
or PNP transistor (my original question was about transistors not FETS)
are used together instead of simply using all NPN's.
Thanks for the reply.
David
On Mar 12, 2005, at 10:41 PM, Hopkins wrote:
> Ok - check this out http://www.4qdtec.com/pwm-01.html#soph
>
>
> _______________________________________
> Roy
> Tauranga
> New Zealand
> _______________________________________
>
> {Original Message removed}
It is not a problem regarding the gate voltage as long as you are aware
of the need to have the gate voltage higher than the source.
_______________________________________
Roy
Tauranga
New Zealand
_______________________________________
The other issue they
addressed was the need to have the gate voltage a little greater than
the source. How is that a "problem"? Also, that doesn't explain why NPN
or PNP transistor (my original question was about transistors not FETS)
are used together instead of simply using all NPN's.
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> Forgive my ignorance, but I'm still not seeing a problem which needs to
> be overcome. I read that page already and what I saw was their
> description of why the LM3524 was a poor choice. The other issue they
> addressed was the need to have the gate voltage a little greater than
> the source. How is that a "problem"? Also, that doesn't explain why NPN
> or PNP transistor (my original question was about transistors not FETS)
> are used together instead of simply using all NPN's.
Hi David,
There's nothing wrong with using NPN transistors, even on the high
side. However, you will end up using some PNP transistors somewhere
in your circuit, or else do some heating with low-value resistors and
suffer from not being able to turn the final stage fully on(or maybe
multiple power supplies or bootstrap capacitors?). Also, you will
likely do some quasi-darlington thing with a PNP driving an NPN, so
you are really creating a PNP anyway, unless you really want to argue
the finer points.
> I'm and ME so I'm a little lost here. I needed an H-Bridge to
> control a small motor (about 10A) so I sat down and got aquatinted
> with Google. I read and read and then read some more then I finally
> designed an H-Bridge. I used all NPN transistors for the corners and
> it works fine from what I can see; however, most H's I read about
> used a combination of PNP and NPN. Just wondering why this mixing of
> xsters is used instead of simply using one variant or the other.
Summary: Using PNP high side drivers allows a simpler circuit and no
extra high side driver power supply.
Or: It's just that it's easier to use PNPs for the high side
drivers. It can be done with all NPNs but there are additional design
requirements to meet.
An NPN bipolar transistor must have its base driven higher than it's
emitter to turn on. Actual be voltage is approaching 1 volt and the
drive circuit will usually drop some voltage. The NPN collector is
connected to V+ and the emitter to the load high side. It is very
desirable that the transistor be saturated or very nearly saturated so
the CE voltage drop is typically under a volt. As a consequence you
need a high driver supply which is above V+ by several volts or more.
While this is easily enough provided, it adds complexity and cost and
it would be "nice" to not need this extra high side supply. When an N
Channel FET (still a transistor) is used as a high side driver you
typically need a 12v or higher high side supply.
When a PNP bipolar transistor is used as the high side driver the
emitter is connected to V+ and the collector to load high side. The
base must be driven negative relative to the emitter and V+ to turn it
on. This does not require an additional high side power supply as the
whole V+ voltage is available to derive drive from.
For low power circuits you can use NPN high side drivers without a
high side supply above V+. The transistors do not saturate and the
dissipation in the driver is higher than it would otherwise be. This
solution becomes impractical as power levels increase. eg at 10A and
with 2 volts Vceon you get 20 watts dissipation - less with under 100%
duty cycle. This would generally be unacceptable.
>> I'm and ME so I'm a little lost here. I needed an H-Bridge to control
>> a small motor (about 10A) so I sat down and got aquatinted with
>> Google. I read and read and then read some more then I finally
>> designed an H-Bridge. I used all NPN transistors for the corners and
>> it works fine from what I can see; however, most H's I read about
>> used a combination of PNP and NPN. Just wondering why this mixing of
>> xsters is used instead of simply using one variant or the other.
>
> Summary: Using PNP high side drivers allows a simpler circuit and no
> extra high side driver power supply.
>
> Or: It's just that it's easier to use PNPs for the high side
> drivers. It can be done with all NPNs but there are additional design
> requirements to meet.
>
> An NPN bipolar transistor must have its base driven higher than it's
> emitter to turn on. Actual be voltage is approaching 1 volt and the
> drive circuit will usually drop some voltage. The NPN collector is
> connected to V+ and the emitter to the load high side. It is very
> desirable that the transistor be saturated or very nearly saturated so
> the CE voltage drop is typically under a volt. As a consequence you
> need a high driver supply which is above V+ by several volts or more.
> While this is easily enough provided, it adds complexity and cost and
> it would be "nice" to not need this extra high side supply. When an N
> Channel FET (still a transistor) is used as a high side driver you
> typically need a 12v or higher high side supply.
>
> When a PNP bipolar transistor is used as the high side driver the
> emitter is connected to V+ and the collector to load high side. The
> base must be driven negative relative to the emitter and V+ to turn it
> on. This does not require an additional high side power supply as the
> whole V+ voltage is available to derive drive from.
>
> For low power circuits you can use NPN high side drivers without a
> high side supply above V+. The transistors do not saturate and the
> dissipation in the driver is higher than it would otherwise be. This
> solution becomes impractical as power levels increase. eg at 10A and
> with 2 volts Vceon you get 20 watts dissipation - less with under 100%
> duty cycle. This would generally be unacceptable.
>
>
>
> RM
>
D Platt wrote:
> most H's I read about used a combination
> of PNP and NPN. Just wondering why this mixing of xsters is used
> instead of simply using one variant or the other.
PNPs can be more convenient for the high side drive because the base sources
current instead of sinking it. With NPN high side drivers, the base voltage
needs to be a little above the collector, and requires current coming in.
That can be difficult to arrange when the collector is already tied to the
highest available supply voltage.
This is not to say that using NPNs is "wrong" for high side drivers, just
that the extra difficulties need to be taken into consideration, especially
if you want good efficiency.
*****************************************************************
Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com
D Platt wrote:
> The other issue they
> addressed was the need to have the gate voltage a little greater than
> the source. How is that a "problem"?
In a typical H bridge, the high side of the H is tied to the positive
supply. Unless you have another supply at a higher voltage, where is this
gate voltage supposed to come from? There are ways to do this, but it has
to be carefully considered.
> Also, that doesn't explain why NPN
> or PNP transistor (my original question was about transistors not FETS)
> are used together instead of simply using all NPN's.
Pretty much the same reason. To get a fully saturated NPN transistor, it
requires dumping current into the base from a voltage higher than the
collector, which is presumably connected to the + supply when used as H
bridge high side driver.
Another issue with using NPN or N channel high side drivers is that the
base/gate voltage slews together with the output signal. Some of these
edges need to be very fast for good effeciency, and this puts additional
burdens on the driving circuit.
*****************************************************************
Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com
>> Also, that doesn't explain why NPN
>> or PNP transistor (my original question was about transistors not FETS)
>> are used together instead of simply using all NPN's.
>
Besides the driving differences, the main reason that H-bridges use all
NPN's is because PNP's are just not available in as high voltages and
currents as NPN's.
On smaller drives, say < 6A or < 24V, it's not a problem to get
complimentary pairs of Fet's, transistors, and so the high side driver is
easier/cheaper than going the opto route.
Higher current PNP/ Nfets availability is changing somewhat with newer
technology, but if you use all NPN, it also means only really designing one
driver stage, that is used all round. The high side is then simply
optically isolated.
clemson.edu
