Limpkin's blog - Tag - mosfet<div>An electronics geek blog, dedicated to sharing and open source. Check out my stores: <a href="https://lectronz.com/stores/stephanelec" hreflang="en" title="Lectronz store">EU</a> / <a href="https://www.tindie.com/stores/stephanelec" hreflang="en" title="tindie store">EU & US</a>.</div>2024-03-29T00:41:47+00:00Mathieuurn:md5:51de6a3d917257edeff5a252fe925b02DotclearWhy you should never forget decoupling capacitors...urn:md5:b6a0683dd1fdc10f385878c8ecef03f32013-02-01T16:27:00+00:002023-04-14T13:52:35+01:00limpkinMy Projectsdecouplingmosfettransients<p>A few words to explain how forgetting one decoupling cap could affect the rest of your system...<br /><br />
<img src="https://www.limpkin.fr/public/noise/noise_3.3_transfo.png" alt="noise_3.3_transfo.png" style="display:table; margin:0 auto;" title="noise_3.3_transfo.png, fév. 2013" /></p> <p>Please have a look at these simple schematics:<br /></p>
<p><a href="https://www.limpkin.fr/public/noise/schem.png" title="schem.png"><img src="https://www.limpkin.fr/public/noise/.schem_m.jpg" alt="Simple schematics" style="display:table; margin:0 auto;" title="schem.png, fév. 2013" /></a>
Nothing really complicated here:<br />
- you have your 12V voltage input from J1<br />
- a ferrite L1 is placed to avoid high frequency parasites<br />
- a mosfet P is present to avoid reversed polarity<br />
- a low dropout regulator (LDO) U1 is connected to VINP to generate 3.3V<br />
- a mosfet driver U4 is connected to VINP to drive the mosfet N Q2 to get sharp voltage changes on J8 (for better PWM)<br /></p>
<p>Let's <strong>remove</strong> the 22uF capacitor C12, connect a <strong>2A load</strong> on J8, send a small pulse to the mosfet driver, and measure the voltage on <strong>VINP</strong>:<br /></p>
<p><a href="https://www.limpkin.fr/public/noise/noise_transfo2.png" title="trace with 1V/div & 1ms/DIV, AC coupling"><img src="https://www.limpkin.fr/public/noise/.noise_transfo2_m.jpg" alt="trace with 1V/div & 1ms/DIV, AC coupling" style="display:table; margin:0 auto;" title="noise_transfo2.png, fév. 2013" /></a></p>
<p>In this example, a <strong>12V 2A AC/DC wall mount power supply</strong> is used.<br />
Therefore, you can see that its regulation is far from optimal: you have <strong>huge overshoots</strong> when the load is applied and released.<br />
Remember that a ferrite is just a <strong>lossy inductor</strong>. Therefore I'd have expected L1 to filter at least the positive rebound, but it seems it doesn't (maybe because in my case it is just a 80ohms ferrite).<br />
Please note that I'm pretty sure the <strong>transient is actually bigger than measured</strong> as my oscilloscope is not the best.<br /></p>
<p>So guess the effect this will have on 3.3VA:</p>
<p><a href="https://www.limpkin.fr/public/noise/noise_3.3_transfo.png" title="trace with 1V/div & 1ms/DIV, AC coupling"><img src="https://www.limpkin.fr/public/noise/noise_3.3_transfo.png" alt="trace with 1V/div & 1ms/DIV, AC coupling" style="display:table; margin:0 auto;" title="noise_3.3_transfo.png, fév. 2013" /></a></p>
<p>Yes... this is the perturbation you'd get on 3.3VA, even with <strong>the ferrite L2 and the decoupling capacitor C3</strong>.<br />
I think it is quite useless to tell you that this is <strong>very</strong> annoying to have this kind of noise (0.6V!).<br />
So why do we get this perturbation you ask?<br />
Well, even if you are generating 3.3V from your 12V, your LDO is not <strong>fast enough</strong> to adjust its output when the <strong>transient occurs</strong>.<br />
In a LDO datasheet, the parameter you'd want to look at is the <strong><a href="http://en.wikipedia.org/wiki/Power_supply_rejection_ratio" hreflang="en" title="PSRR">Power Supply Rejection Ratio (PSRR)</a></strong>, which depends on the perturbation frequency.<br />
But even good LDOs won't be able to handle such <strong>fast transients</strong>.<br />
One thing you could do is add another ferrite in series with the LDO, but i'm pretty sure you'd still have some perturbation.<br /></p>
<p>In my case the AC/DC transformer has a <strong>small series resistance</strong>. If I use my lab power supply, the series resistance will be smaller and therefore <strong>the transient will be even bigger</strong>:<br /></p>
<p><a href="https://www.limpkin.fr/public/noise/noise_3.3_alim.png" title="trace with 1V/div & 1ms/DIV, AC coupling"><img src="https://www.limpkin.fr/public/noise/.noise_3.3_alim_m.jpg" alt="trace with 1V/div & 1ms/DIV, AC coupling" style="display:table; margin:0 auto;" title="noise_3.3_alim.png, fév. 2013" /></a></p>
<p>So what's the solution? <strong>Never forget decoupling capacitors</strong>.<br /></p>
<p>If we <strong>put C12 back and place it close to Q2 and J8</strong>, when the transient occurs, <strong>C12 will provide the required high frequency current</strong>.<br />
And <strong>tada!</strong> no more perturbation on 3.3VA ;-) .<br /></p>
<p>Anyway, this example is for big loads, but you can guess that <strong>at smaller scales the effect is still the same</strong>.<br />
Don't forget your IC's are actually made of tiny transistors that usually switch at the same time :-) .</p>