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Diode on PWM controller gate drive pin

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Jun 22, 2008
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I am writing with regard to the MOSFET gate drive pin on the LT1241 PWM controller.

Is a diode needed to be connected to it.?

In an offline SMPS , it is said to be necessary to put a Schottky diode at the gate drive pin of any PWM controller…… seen here by D5……….

Schottky (here D5) at gate drive output pin .


To collaborate

On page 306 of Raymond Mack’s book “Demystifying switching power supplies” (2005) an MBR170 schottky diode (here D1) is clearly seen at the gate drive output pin of the LT1241 PWM controller.

(The topology is a one-transistor-forward-converter).

The explanation for this diode is given on page 309, at the top of page, it says …..

“This diode prevents transient voltages coupled by the FET internal capacitances from taking the output pin more than a Schottky diode drop below ground. Large negative transients can cause instabilities in the IC (LT1241 PWM controller) because the parasitic diode in the output transistor and substrate will be turned on.”

So Howcome not all SMPS's have this diode there?

As a first point, there's a clear statement in the datasheet, which should be the basic source of information before any third party literature.
In some applications the parasitic LC of the external MOSFET gate can ring and pull the OUTPUT pin below ground. If the OUTPUT pin is pulled negative by more than a diode drop the parasitic diode formed by the collector of the output NPN and the substrate will turn on. This can cause erratic operation of the device. In these cases a Schottky clamp diode is recommended from the output to ground.
The diode is also present in a LT1241 data sheet's application circuit.

I have this additional comments: Susceptibility of an IC pin to latch-up and similar effects is basically a matter of non-optimal IC design (and available technlogy in part). Thus it's primarly a LT1241 issue, athough it may be present with other chips, too. Furthermore, as said in the data sheet, ringing can occur in some applications. It mainly depends on PCB layout and lead inductances of the used MOSFET package. So if you are designing a switcher from the scratch without previous prototyping, you may want to include the diode as a precaution, but you can possibly omit it after a first measurement.

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