Ahh, I see says the blind man to the deaf mute. Mucho gracias all.
On Mar 13, 2005, at 1:04 AM, Russell McMahon wrote:

>> 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
>
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