Mosfet/Circuit Advise - Page 1
Forum section: electronics / beginners

author: ElectricGuy
author_rank: Regular Contributor
timestamp: April 19, 2015, 11:58:43 pm
content: Hi;I need to Dim a VFD Module hat works @5V 700mA.I pretend to do that with a micro using PWM and a mosfet like the atached circuit.My question is, what type/reference of mosfet should i use? Since the Supply voltage of the VFD is 5V and the output of the micro is 5V too, a think a logic level mosfet its better, am i right?Is there any aproach for this problem?Thank you

author: mathias
author_rank: Regular Contributor
timestamp: April 20, 2015, 05:34:40 am
content: A Logic level fet is probably the way to go, but lose the capacitor at the gate. The "smoothing-action" needs to take place at the load, not at the gate. I presume you want your mosfet only in the on or off state?

author: Simon
author_rank: Unknown
timestamp: April 20, 2015, 05:54:18 am
content: IRL520N, IRL530N, IRL540N

author: PSR B1257
author_rank: Frequent Contributor
timestamp: April 20, 2015, 07:07:32 am
content: QuoteNo, this corrupts up the whole idea of the PWM. During the on-state you are charging the capacitor, during the off-state the capacitor is a voltagesource for the load. The greater the capacity, the greater the average current, regardless of the dutycycle.See attached simulation. Exemplarily with 40% an 10% dutycycle.QuoteWhat's the deal with this crappy MOSFETs all the timeOnly the IRL540 (regarding this selection) is semi-reasonable, the other ones are hopeless for low voltage applications.

author: Simon
author_rank: Unknown
timestamp: April 20, 2015, 07:40:55 am
content: QuoteWhy are they hopeless for low voltage applications ? I thought the only real differences were in their current handling ability and on resistance, they are all logic level. Oh and gate capacitace is lower for the lower rated ones which is a plus for higher speed stuff where you can't drive as hard.

author: PSR B1257
author_rank: Frequent Contributor
timestamp: April 20, 2015, 07:58:43 am
content: QuoteExactly. The IRL530 for instace has a on-resistance of 160mOhm. Imagine a 5V system with only 1A loadcurrent. You are already losing 160mV.QuoteTrue. But only for low current stuff.

author: Simon
author_rank: Unknown
timestamp: April 20, 2015, 08:02:26 am
content: You will find there is always a trade off, it's like the Rdson x Vds x Vgs x Ids x Gate"c" always make a constant, you make one better you loose elsewhere or your price starts to rise through the roof

author: ElectricGuy
author_rank: Regular Contributor
timestamp: April 20, 2015, 12:16:32 pm
content: Thank you all!

author: mathias
author_rank: Regular Contributor
timestamp: April 20, 2015, 01:09:50 pm
content: Quote from: PSR B1257 on April 20, 2015, 07:07:32 amBut your cap is between Vdd and Drain, the topic starter has placed the cap at the gate. I.e. he will smooth out the Vgs, assuming operation in the triode region, the FET can have to dissipate a lot of power (i.e. linear action, not PWM action).

author: PSR B1257
author_rank: Frequent Contributor
timestamp: April 20, 2015, 01:55:19 pm
content: QuoteYes, across the load, as you sayed. Or did I misinterpreted your statementQuote@ElectricGuy:You need an electrolytic capacitor from Vdd to GND, as close to the MOSFET and output terminal as possible.

author: sync
author_rank: Frequent Contributor
timestamp: April 20, 2015, 02:18:27 pm
content: Quote from: ElectricGuy on April 19, 2015, 11:58:43 pmA vacuum fluorescent display module?They can't be dimmed by PWM the power supply. A lot of these modules have dimming modes build in. Look at the command set.

author: jlmoon
author_rank: Unknown
timestamp: April 20, 2015, 04:45:45 pm
content: Quote from: Simon on April 20, 2015, 08:02:26 am"trade offs" sounds familiar such as HP vs. Torque curves.. Can't have both graphing on a constant incline.Sorry, I know that was slightly off topic..

author: rickey1990
author_rank: Regular Contributor
timestamp: April 21, 2015, 09:42:52 pm
content: This may help,The 555 will deliver 200mA to a load but the chip gets extremely hot (12v supply). The answer is to use a buffer transistor.For 200mA, use a BC547 or equivalent.For 500mA use a BC337 or equivalentFor 1A, use a TIP31 or equivalent.For 3A - 5A use a BD679 or equivalent with heatsinkFor 5A to 10A use TIP3055 with heatsink

author: PSR B1257
author_rank: Frequent Contributor
timestamp: April 22, 2015, 06:52:22 am
content: QuotePossibleQuoteMay just work, but not recommended.QuotePower darlington, would work, but very poorly. Besides it is not suitet for 5A. And this sucker has a collector emitter saturation voltage of about 2,5V.QuoteThat would not work since the current gain is way too small. Once again the VCE,satis in the ballpark of 2 to 3V.If one switch currents above 1A (and at low voltages) it is almost the best idea to use ("low voltage") MOSFETs.BJTs (as well as IGBTs) are usefull when it comes to switching high voltages (V>200 or so).