Polarity or sense of MOSFET driver ICs?

[j_writer]

I haven't seen this discussed before - do you need to worry about the sense of a MOSFET driver output? If your control signal disappears, shouldn't the FET default to the off condition? Notice in the figure that the FET turns on if the LED turns off. Are driver ICs available in both senses, inverting and non-inverting? Thanks.

 

[FvM]

Obviously, the "disappearing control signal" has a special meaning with an optical isolated driver. In this case, you can expect a
failsafe behaviour, as implemented with FOD3150 and - as far as I'm aware of - with all similar types from othe vendors. In so
far, the signal polarity will be always "normally off".

In case of galvanically coupled drivers, that are available with different signal polarities, you may want to specify, what a
"disappearing control signal" means. It's not obvious, and you usually set the default input level by pull-up or pull-down
resistors. If the control signal source has a dedicated default level, e.g. the reset state pin level of a microprocessor, it should be
considered when designing your logic. Failsafe behaviour should and can be implemented in any design.

 

[dick_freebird]

Most of the MOSFET drivers I've seen, and the couple I've
been tasked to design, have complementary inputs (AND-NOT
logic) so as to be general-purpose (pick your polarity and
strap the other).

 

[j_writer]

Most of the MOSFET drivers I've seen, and the couple I've
been tasked to design, have complementary inputs

Good to hear this. Any part numbers for opto isolated devices? I have not seen any so far.

In this case, you can expect a failsafe behaviour, as implemented with FOD3150 and - as far as I'm aware of - with all similar types from other vendors.

I'm sorry, I really don't understand what you are talking about. This is a P-channel device.

 

[FvM]

This is a P-channel device.

Thank's for the clarification, I missed this point respectively, I didn't expect it. Unfortunately, a FOD3150 does not provide fail
safe behaviour for P-channel switches, and I fear, you won't find any other optical isolated gate driver that does. Simply because
P-channel FETs are very unusual in power stages, the product won't be demanded at all. The same problem applies to the undervoltage
lockout feature of usual gate drivers, that's always designed for N-FET switch polarity.

A possible solution for an isolated P-channel driver would be a TC426/427, which has no undervoltage lockout, and a separate logic level opto coupler.

 

[j_writer]

The attached circuit may work. There is just sufficient margin between the two devices for the logic levels to work properly. It is unclear what will happen when the two IC's power up simultaneously, but the circuit should stabilize with the MOSFET off.

If anybody has other part suggestions, please holler out!

 

[FvM]

It is unclear what will happen when the two IC's power up simultaneously.

Sharing the same supply, both IC's will always "power up" simulataneously. FOD3150 behaviour is clear from the datasheet,
but TC42x behaviour below 4.5 V supply is unspecified. Depending on the MOSFET threshold voltage, you may have difficulties to
guarantee safe behaviour. A gate-source load resistor may help, but I'm not sure about. Generally, it should be preferred to power
the driver supply before the power bus.