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<h2 class="center" id="title">HIGH-SIDE MOSFET SWITCHING</h2>
<h5 class="center">22 JUNE 2025</h5>
<br>
<div class="twocol justify"><p>Needed low-power switching for the <a href="../fpm-door-lock-lp/">fingerprint door
lock</a>. Servo and FPM draw high quiescent current–had to
cut power electronically during sleep. MOSFETs can do this.</p>
<p>Schematics belong to <a href="https://electronics.stackexchange.com/users/292884/simon-fitch" class="external" target="_blank" rel="noopener noreferrer">Simon Fitch</a>.</p>
<h2 id="problem-with-simple-low-side-switching">Problem with simple low-side switching</h2>
<p>Typical approach: GPIO → gate of N-channel MOSFET on low side, pull-down
resistor between gate and drain. Works if MCU and load don’t share common
ground. Doesn’t work when they do (like controlling a component powered by the
same MCU).</p>
<p>Issue: source potential = gate potential - threshold voltage. Example: 3.3V
gate - 1.5V threshold → 1.8V at load–not nearly enough for a servo. Raising
the gate potential above source is not always practical. Solution: high-side
switch.</p>
<h2 id="p-channel-high-side-switch">P-channel high-side switch</h2>
<p><img src="p_high_side.png" alt="P-channel high-side switching circuit" /></p>
<p>M1 is P-channel (high-side), M2 is N-channel (level converter). MCU output low
→ M2 off → R1 pulls M1 gate to +6V → servo off. MCU output high → M2 conducts →
M1 gate drops to 0V → servo on.</p>
<p>Note: IRF9540 in the schematic doesn’t work. V<sub>GS</sub> (-10V) for
RDS<sub>on</sub> too much for 3.3V ATmega328P to drive. NDP6020P is the only
suitable through-hole MOSFET I could find.</p>
<h2 id="n-channel-high-side-switch">N-channel high-side switch</h2>
<p><img src="n_high_side.png" alt="N-channel high-side switching circuit" /></p>
<p>Less common but works if you have voltage high enough to drive the gate. Both
M1 and M2 are N-channel. MCU low → M2 off → M1 gate rises above threshold →
servo on. MCU high → M2 on → M1 gate drops → servo off. R2 prevents
high-impedance power-up from switching servo on.</p>
<p>M2 needed in both topologies for level conversion (0V ↔ +6V or +9V). Carries
<1mA. Gate-source threshold must be lower than MCU supply. Common choices:
2N7000, 2N7002, BSS138.</p>
<p>Note: D1 flyback diodes protect MOSFETs from voltage spikes caused by inductive
loads (servos, relays).</p>
<h2 id="a-bjt-alternative">A BJT alternative</h2>
<p><img src="bjt.png" alt="BJT architecture" /></p>
<p>Simpler, cheaper, more available. Q2 conducts when MCU outputs high. Q2
amplifies Q1’s base current. Unlike MOSFETs (voltage-driven), BJTs are
current-driven. R3 and R4 must be calculated for desired base currents. <a href="https://teachmetomake.wordpress.com/how-to-use-a-transistor-as-a-switch/" class="external" target="_blank" rel="noopener noreferrer">Guide on BJT
switches</a>.</p>
<h2 id="which-topology">Which topology?</h2>
<p>MOSFETs preferred in professional work—more efficient when on. Harder to drive
at 3.3V due to V<sub>GS</sub> requirements for full saturation (low
R<sub>DS(on)</sub>).</p>
<p>N-channel: Lower on-resistance, cheaper, more efficient than P-channel. Harder
to drive high-side (gate must be above source—requires extra circuitry like
MOSFET drivers).</p>
<p>Used P-channel high-side for the door lock redesign. Simpler to drive from 3.3V
MCU, no driver needed.</p>
<h2 id="further-reading">Further reading</h2>
<ul>
<li><a href="https://www.embeddedrelated.com/showarticle/98.php" class="external" target="_blank" rel="noopener noreferrer">Different MOSFET
topologies</a></li>
<li><a href="https://www.embeddedrelated.com/showarticle/809.php" class="external" target="_blank" rel="noopener noreferrer">How to read
MOSFET datasheets</a></li>
<li><a href="https://teachmetomake.wordpress.com/how-to-use-a-transistor-as-a-switch/" class="external" target="_blank" rel="noopener noreferrer">How to use a
transistor as a switch</a></li>
<li><a href="https://forum.digikey.com/t/guide-to-selecting-and-controlling-a-mosfet-for-3-3-vdc-logic-applications/42606" class="external" target="_blank" rel="noopener noreferrer">Guide to
selecting and controlling a MOSFET for 3.3 VDC logic applications</a></li>
<li><a href="https://forum.digikey.com/t/driving-a-large-relay-from-a-3-3-vdc-microcontroller-using-an-npn-darlington-transistor/41751" class="external" target="_blank" rel="noopener noreferrer">Driving a large
relay from a 3.3 VDC microcontroller using an NPN Darlington transistor</a></li>
</ul>
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<p class="post-author right">by W. D. Sadeep Madurange</p>
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