Help selecting correct value FET  - Page 1
Forum section: electronics / beginners

author: Veketti
author_rank: Regular Contributor
timestamp: February 06, 2023, 06:15:32 pm
content: Dear All,I'm trying to understand the N-Channel MOSFET parameters so that I could select correct value component. I'd need to switch 5W 24VDC solenoid. I recently ordered IRLML6344TRPBF but now I'm hesitating that is this actually enough. Datasheet say maximum power dissipation Ta@25°C 1.3W so that makes me wonder is this enough for 5W. However the source to drain current is rated at 1.3A so that should be plenty when 5W@24V is 0.208A. Then on the datasheet fig. 8 Maximum safe operating area if I interpret it correctly it handles only Rds on something between 1ms to 100us @0.2A. This confuses me that what values dictate?Then if I go to digikey selector and filter by power dissipation, there are two wattages. Ta and Tc. Should I filter by 5W Ta Or Tc?When selecting the FET, assuming that the solenoid duty cycle might be 100%. Recommendations on some good low Rds on FET's also welcome.Thank you in advance.

author: redkitedesign
author_rank: Regular Contributor
timestamp: February 07, 2023, 06:20:25 am
content: The power specced for any FET is the power lost in the FET.Since the FET you've selected is specced as 30mOhm Rdson, the power loss in the FET will be about 0.2^2*30mOhm = 1.2mW. About 3 orders of magnitude less than allowed.What you do have to take care for are:- Your solenoid is an inductive devices, so it needs a freewheeling diode or the turn-off transient will fry the (any) FET.- The Rdson is valid for a certain gate drive (i thing 4.5V, check the datasheet). You have to ensure you reach that level, or the FET will be half on, with much higher resistance and ditto power loss.The fig 8 is for pulse-based operation. You wont do that with your solenoid, the switching times of an solenoid are practically DC for such an small FET.

author: Veketti
author_rank: Regular Contributor
timestamp: February 07, 2023, 10:10:01 am
content: Thanks, came to around same conclusions when studying all the instructions and documentation on FET's. Some FET datasheets also include line for DC in SOA figure which has the lowest current values. Ti training materialhttps://training.ti.com/understanding-mosfet-datasheets-safe-operating-area-soa,tells that the SOA chart is used when the FET operates in Saturation mode. Some texts tell that the saturation mode is the non conducting mode, so this is confusing. Is there any explanation for dummies that in what way that SOA chart is used?

author: redkitedesign
author_rank: Regular Contributor
timestamp: February 07, 2023, 02:14:22 pm
content: Quote from: Veketti on February 07, 2023, 10:10:01 amParts usually don't die from to much current, but from heat. More specifically, this transistor can handle a certain continuous current (about 5A). That current produces the maximum heat the device can handle.Short pulses produce less heat compared to continues current. Since the device has some heat capacity, a short pulse of 25A will heat the device up less than 5A continuously.Of course, the devil is in the details. There are limits to the peak current (no dirac pulses allowed), and the repetition frequency (duty cycle). That's where the chart comes in.

author: Veketti
author_rank: Regular Contributor
timestamp: February 07, 2023, 03:15:39 pm
content: makes sense that short pulses produce less heat than continuous current. But this chart tells completely opposite. Vds 10V with drain to source current 1A only 1msec. Shouldn't it handle more amps not lower amps like this chart illustrates?

author: shapirus
author_rank: Super Contributor
timestamp: February 07, 2023, 03:49:08 pm
content: consider this:Voltage * Current * Time = energy_per_pulse = constfor example, lower voltage allows for higher current and longer time. shorter time allows for higher voltage and current.in addition, for both pulsed and continuous operation you need to calculate average power dissipation and the respective junction temperature rise above ambient to make sure it stays below the maximum allowed value.