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Old 01-12-2011, 09:36 PM   #3 (permalink)
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Gate driver isolation.

Gate driver isolation.

The Cougar design uses a DC-DC converter and optoisolators to provide
voltage isolation between the logic section (run from the +12V car body power) and traction power control (run at traction battery voltage).

These are some of the most expensive parts on the logic board. The converter provides lots of power and excellent regulation, but a MOSFET gate driver only needs modest power and is tolerant of the supply being a
volt or two off.

There are easier, better, and sometimes less expensive ways to provide the voltage isolation.

In some cases it is possible to use an ADUM5230 or ADuM6132 chip. These are "magic" gate drivers that use an exotic internal
chip structure to both provide an isolated power supply and isolated
gate drive signal. They are usually not considered for power devices
because their gate drive is weak. They peak at 0.1 amp and 0.2 amp
respectively, while Cougar uses a MIC4451 that can put out 12A
pulse. But you can use them as only the power supply converter and
signal isolation, stacking a powerful gate driver (e.g a MIC4422
non-inverting driver) for a fast gate drive pulse.

There are drawbacks to this approach. The power converter in the 6132
only puts out about 22mA of current at 15V. This is (barely) enough for
driving the Cougar MOSFET array at 8KHz, and may not be enough for
more/bigger/worse MOSFETs or higher traction voltage. It's very likely
not enough for an IGBT.

The other drawback is that these chips are power pigs. They suck down
over 200mA from the regulated 5V supply when doing nothing, and use even more when the gate driver
draws current. The big converter on the Cougar starts at only 5mA, and
draws it from the unregulated +12V supply.

And finally, these are Wide-SOIC parts. They are big enough to easily
solder, but you'll need a protoboard with surface mount pads.

An intermediate approach is using a DC-DC converter that operates from
the +5V regulated supply. Often these are smaller and cheaper because they don't need to handle a wide input voltage range.

VBSD1-S5-S15-SIP 5V->15V 1W, 1KV isolation Digikey 102-1362-ND $3.96

Small converters tend to require more design thought. For instance,
this part requires a minimum 7mA current draw. That's close to the basic power draw of some isolators, or easily done with
a "power on" LED. But you have to do those calculations and make a note that the specific part can't be upgraded to a lower power version, or that you can't skip
installing the LED on the board.

Even keeping a separate converter, you can still simplify and improve the gate
driver circuit. The Si8233 and Si8235 chips provide both voltage isolation
and two reasonable (4 amp) gate drivers for about the same price as the
TC4451. Even one of the gate drivers is enough for the Cougar design
(ten IRFP4668 MOSFETs) if you drop the gate resistor values, but you may need to use both drivers (and do some design calculations) to use an IGBT.

If you do decide to use an optoisolator be certain that your drive current is high enough. A rule of thumb is double the minimum current that you calculate from "typical performance" graphs or experimentation. The parts have significant batch to batch variation, and decrease in performance as they age.

And finally, if you need precise timing or a balanced drive (e.g. for synchronous rectification), strongly consider solutions beside optoisolators. Their turn-on and turn-off delays and slopes are asymmetric, and they change with temperature.

Last edited by DJBecker; 01-21-2011 at 02:53 PM..
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