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[DPRG] Cheap high power H-bridge

Subject: [DPRG] Cheap high power H-bridge
From: Chuck McManis cmcmanis at mcmanis.com
Date: Tue Nov 27 17:30:23 CST 2001

At 02:27 PM 11/27/01, David Peterson wrote:
>Good to see your controller Chuck. Interesting choice of FET the IRF1010E.
>The IRF1405 speced for the OSMC does have a few different charecteristics,
>looks pretty decent. Rds-on for the 1010 is 12 mohms, where the 1405 is 5.3
>mohms. Vds for the 1010 is 60v, for the 1405 it's 55v. Input capacitance is
>3210 pF for the 1010, a bit better than the 5480 pF for the 1405, perhaps
>improving PWM capabilities of an h-bridge. But because of that first Rds-on
>value, the OSMC should carry more current with the same cooling as your
>h-bridge. How are you driving your FETs on your board? Any luck with the
>Black Ice water cooler yet?

The choice of the 1010 was driven primarily by gate capacitance and the 
desire to minimize switching losses which implies a decent gate drive 
circuit. I've managed to get near optimal switch times (enough that I'm 
probably going to add a couple more ranks of FETs). The importance of this 
is that while PWM'ing I'd like to spend < 1% of my time in the "transition 
region." This become especially true in higher current situations, as the 
during the brief transition to "on" if you sit at an Rds of 100 mOhms too 
long you have to dissipate waaay to much power. By the same token, running 
the FETs in the 'hundreds' of Hz has implications in matching power 
delivery to the expected LR circuit that the motor represents.

With 8 ranks (double the current number), the 1010's achieve a respectable 
1.5 mOhms. At 200Amps dissipation is about 75 watts (given the raising of 
Rds at temperature). They also need only dissipate about 7 or 8 watts per 
package. That means one can simply air cool the resulting load and get 
reasonable performance.

The real winner is the IR1404 of course, but its only rated to 40 volts, is 
a pig to drive, but only 3 mOhms makes it tasty none-the-less. That's what 
I'm using in the replacement for the 'low end' bridge.

The "design envelope" is, as usual, a combination of factors.
         1) Fast switch times to minimize losses and excess heat
            generation.
         2) Multiple packages to distribute the heat
            load in the total overall system.
         3) Gate drive requirements (to achieve #1)
         4) Total system cost/complexity for marketability
            and reliability.

The Black Ice radiator rocks, although for battle toughness it looks like 
I'm going to end up designing it out of the system. For other applications 
it adds 50% "headroom" to the design though. A 48V 300A system can be used 
on a passenger vehicle (one per traction motor, two or four motors).

--Chuck


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