Choose MOSFET for PWM signal
Modjo , 05-04-2021, 03:36 AM
Hi all, like said in the title i'm looking for a mosfet that capable to swith from a frequency between 200 KHz to 1 MHz. This Mosfet can be used to synchronise a clock for buck. See under the schematic that i used for the moment :
Vin can vary between 9V to 24V.
With a 2N7002 Mosfet, if i use a pwm with a clock > 10 KHz i have this :
Can you help me (how to calculate) to find a mosfet that can be ok with this parameter ?
qdrives , 05-08-2021, 02:02 PM
What is pulling up the drain? I am amazed that it goes up at this rate.
For fast signals you need a push-pull design. Either with a totempole or something like SN74LVC1G17.
robertferanec , 05-10-2021, 02:30 AM
I agree with @qdrives , maybe using a simple single gate chip instead of the transistor could help.
Modjo , 05-10-2021, 03:08 AM
Thanks for your reply.
I'm a rookie on electronic design and i didn't think to use pushPull design for that !
I made some research on the web about push-pull design with mosfet :
For example i found this one :
but this need to use a anti-shoot-through protection.
Ohterwise, if i want to use SN74LVC1G17, i need to use a resistor divider to go under 5V for the gate ?
What do you think about this design ?
robertferanec , 05-10-2021, 05:11 AM
- Check datasheet what is the required HIGH signal level for the sync pin - maybe 3V3 is fine.
- Also, if VIN is +5V, you can use a schmitt gate powered by 5V ...
Modjo , 05-10-2021, 05:58 AM
@robertferanec the enable theresold is 1.2V --> i can use 3.3V for sync.
In this way i can connect-it directly to the gpio of my uC ? What's the advantage to use a circuit like SN74LV ?
qdrives , 05-13-2021, 09:07 AM
The shoot through in the reference is high due to the voltage difference between input (5V), output (15V) and gate threshold voltage.
@Modjo the advantage of the 74LVC1G is the high drive then most microcontrollers can handle and can be used as a level converter (take the 74AHCT1G17 for worse case 3.3V to 5V).
The enable/sync input is a bit different then the others due the AC coupling capability. From the little I read the datasheet I too would say you can connect it directly to the uC. If so, you also do not need the buffer.
robertferanec , 05-15-2021, 03:07 AM
I agree, you may be able to connect it directly to the MCU.
Modjo , 05-18-2021, 01:36 AM
@qdrives and @robertferanec thanks for your help. I will connect the en/sync directly to uC gpio.
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