Jaguars vs Victors

[techhelpbb]

Originally Posted by Matt Krass

I'm curious, what effect are you expecting to see during the off cycle part of things that would be a problem?

Synchronous rectification on the black Jaguars.
http://www.ti.com/lit/an/spma033a/spma033a.pdf

I like this description, so with respect to ST Microelectronics I quote it here from this datasheet http://www.datasheetcatalog.org/data...onics/7048.pdf page 8.
(This is a courtesy to the audience that may not understand what this is. I'm quoting this because I like the description in this datasheet. The datasheet does not pertain to any part in our systems.)

A motor is an inductive load. When driven in PWM mode, motor current is switched on and off at the
25kHz frequency. When the MOS is switched off, current can not instantaneously drop to zero, a so-called
”free-wheel” current arises in the same direction than the power current. A path for this current must be
provided, otherwise high voltage could arise and destroy the component. The classical way to handle this
situation is to connect a diode in an anti-parallel configuration regarding to the MOS, so that current can
continue to flow through this diode, and finally vanishes by the means of ohmic dissipation, mainly in the
diode due to its 0.8V direct voltage. For high currents, dissipation can be an important issue (eg: 10A x
0.8V makes 8 W!). Furthermore, high speed diodes have to be used, and are expensive.

A more efficient way to handle this problem is to use the high side MOS as a synchronous rectifier. In this
mode, the upper MOS is switched ON when the lower one is switched OFF, and carries the free-wheel
current with much lower ohmic dissipation. Advantages are : one expensive component less (the fast
power diode), and more reliability due to the lower dissipation level.

However, we have to take care not to drive the two MOS simultaneously. To avoid transient problems
when the MOSare switched, a deadtime is inserted between the opening of one MOS, and the closing of
the other one. In the TD340 device, the deadtime is fixed to about 2.5 microseconds. This value is the time
between the commands of the gate drivers, not the deadtime between the actual MOS states because of
the rising and falling times of the gate voltages (due to capacitance), and the MOS characteristics. The
actual value of the deadtime for a typical configuration is about 1.5 microseconds.

Please keep in mind that people often use the body diodes of these MOSFETs as the anti-parallel high speed Schottky. So these diodes get hot and that heats up the MOSFET transistor as a whole. That's the advantage of this feature in the black Jaguars as apposed to the previous generation.

Course it might be trouble if other things are injecting power in the bridge. Usually motors can't return more power than you put into them unless something turns them faster than the speed control (it may be more voltage than you put into them with less current, but in the sense of power it must be less than you put in unless something adds mechanical energy and makes the motor into a generator). I may be misunderstanding but from your original proposal it would seem that a black Jaguar might dissipate the energy in the load from the Victor....that would be quite a bit more energy than I think would be normal even for a motor that had extra mechanical energy being added to it's rotation (at least in the scope of what we can build with these parts).