G15N06K NFET Vgs(th) Variation with Temperature at 100°C Inquiry

Sunnysky wrote:

We are doing Sync Buck with G15N06K NFETs. (15vout , 3Aout, 24Vin, 150khz) We have "turn-off-fast" diodes in the gates. But what is the Vgs(th) at say 100degC?......our turn off diode may not be able to get under that. Why does the datasheet not tell?
It only tells Vgs(th) at 25degC.

Hello,

For your synchronous buck converter design using G15N06K N-channel FETs, you've brought up an important point regarding the variation of the gate threshold voltage (Vgs(th)) with temperature. It's true that many datasheets provide the Vgs(th) at a standard temperature of 25°C, but may not always include the temperature coefficient for this parameter, which would allow you to calculate it at higher temperatures such as 100°C.

The Vgs(th) can indeed vary with temperature, generally increasing with an increase in temperature. Without specific data from the datasheet, a rule of thumb is that the threshold voltage can change by about -2 to -4 mV/°C for silicon power MOSFETs. However, this is a very rough estimate and it can vary significantly between different devices.

If the datasheet does not provide the temperature coefficient for Vgs(th), you have a few options:

1. Contact the manufacturer to request this information.
2. Perform your own measurements of the Vgs(th) at various temperatures, if you have the necessary equipment.
3. As a conservative design approach, you can assume a worst-case scenario based on typical temperature coefficients for similar devices and ensure that your gate drive voltage will still reliably turn off the FET at the highest expected temperature.

Regarding your turn-off diode, you are correct to be concerned about whether it can pull the gate voltage below the threshold at higher temperatures. You may need to ensure that your drive circuit can handle a lower Vgs to guarantee turn-off under all conditions, or choose a turn-off diode with a lower forward voltage drop to help achieve this.

Could you provide more details about your turn-off diode and the gate drive circuit you are using? This would help in giving you a more specific recommendation. Also, have you observed any issues in practice, or is this a theoretical concern at this stage?