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diff --git a/_site/log/mosfet-switches/index.html b/_site/log/mosfet-switches/index.html index 3b8f83b..97bba07 100644 --- a/_site/log/mosfet-switches/index.html +++ b/_site/log/mosfet-switches/index.html @@ -2,12 +2,12 @@ <html> <head> <meta charset="utf-8"> - <title>On the use of MOSFETs as electronic switches</title> + <title>High-side MOSFET switching</title> <head> <meta charset="utf-8"> <meta name="viewport" content="width=device-width, initial-scale=1"> - <title>On the use of MOSFETs as electronic switches</title> + <title>High-side MOSFET switching</title> <link rel="stylesheet" href="/assets/css/main.css"> <link rel="stylesheet" href="/assets/css/skeleton.css"> </head> @@ -41,104 +41,76 @@ <main> <div class="container"> <div class="container-2"> - <h2 class="center" id="title">ON THE USE OF MOSFETS AS ELECTRONIC SWITCHES</h2> + <h2 class="center" id="title">HIGH-SIDE MOSFET SWITCHING</h2> <h6 class="center">22 JUNE 2025</h5> <br> - <div class="twocol justify"><p>Recently, I needed a low-power circuit for one of my battery-operated projects. -Much of the system’s power savings depended on its ability to electronically -switch off components, such as servos, that draw high levels of quiescent -currents. My search for a solution led me to MOSFETs, transistors capable of -controlling circuits operating at voltages far above their own.</p> - -<h2 id="acknowledgments">Acknowledgments</h2> - -<p>This article is a summary of what I learnt about using MOSFETs as switches. -I’m not an electronics engineer, and this is not an authoritative guide. The -circuits in this post must be considered within the narrow context in which -I’ve used them. All credits for the 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="preamble">Preamble</h2> - -<p>For a typical MOSFET-based switch, we can connect a GPIO pin of a -microcontroller to the gate of a logic-level N-channel MOSFET placed on the low -side of the load and tie the gate and the drain pins of the MOSFET with a -pull-down resistor. This would work as long as the power supplies of the -microcontroller and the load don’t share a common ground. Things become more -complicated when they do (e.g., controlling power to a component driven by the -same microcontroller).</p> - -<p>In that scenario, the source potential visible to the load is the difference -between the gate and the threshold potentials of the MOSFET. For example, when -the gate and the threshold potentials are 3.3 V and 1.5 V, the potential the -load sees is 1.8 V. So, to use a low-side N-channel MOSFET, we need the gate -potential to be higher than the source potential, which may not always be -practical. The alternative would be a hide-side switch.</p> + <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> -<h2 id="p-channel-high-side-switch">P-channel high-side switch</h2> +<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>The following schematic shows how a high-side P-channel MOSFET (M1) could -switch power to a 6 V servo driven by a 3.3 V 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>When the microcontroller outputs low, the M2 N-channel MOSFET stops conducting. -The R1 resistor pulls the gate of the M1 P-channel MOSFET up to +6 V, switching -the servo off. When the microcontroller outputs high on the GPIO pin, M2’s -source-drain connection starts conducting, causing M1’s gate potential to drop -to 0 V, which switches on power to the servo.</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> -<h2 id="n-channel-high-side-switch">N-channel high-side switch</h2> +<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> -<p>The P-channel high-side switch would be the typical architecture for our use -case. However, if we have access to a potential high enough to safely raise the -gate potential above the threshold such that their difference outputs the source -potential required to drive the load, we can switch on the high side using an -N-channel MOSFET:</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>In the schematic, both M1 and M2 are N-channel MOSFETs. When the -microcontroller output is low, M2 stops conducting. This causes the M1’s gate -potential to rise above the threshold, turning the servo on. Conversely, a high -output on the GPIO line switches M2 on, which lowers M1’s gate potential. This -switches the servo off. The R2 pull-up resistor prevents the high impedance of -the output pins at power-up from switching the servo on.</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>Both topologies require M2 to act as a level converter between circuits -containing the microcontroller and the servo, converting between 0 V and +6 V -or +9 V. M2 is a low-power signal converter carrying less than a milliamp of -current. The gate-source threshold voltage of M2 must be lower than the MCU’s -supply voltage. 2N7000, 2N7002, and BSS138 are popular choices for M2.</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>The D1 flyback diodes used in the two topologies safeguard the MOSFET from -voltage spikes caused by inductive loads such as servos.</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>A Bipolar Junction Transistor (BJT) is a simpler, cheaper, and more widely -available type of transistor that can be used as a switch.</p> - <p><img src="bjt.png" alt="BJT architecture" /></p> -<p>In the schematic, when the MCU outputs high, Q2 starts conducting. Q2 amplifies -Q1’s base current. Unlike MOSFETs, which are voltage-driven, BJTs are driven by -base current. Resistors R3 and R4 must be chosen carefully to output the -desired base currents. <a href="https://teachmetomake.wordpress.com/how-to-use-a-transistor-as-a-switch/" class="external" target="_blank" rel="noopener noreferrer">“How to choose a -transistor as a switch”</a> is an excellent guide on using BJTs as electronic -switches.</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> -<h2 id="which-topology-to-choose">Which topology to choose?</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>The professional community appears to prefer MOSFETs over BJTs. MOSFETs are -more efficient when the switch is on. However, they are more challenging to -drive, especially with a 3.3 V MCU, due to the V<sub>GS</sub> potentials -required to achieve specified R<sub>DS(on)</sub> values (i.e., to turn them on -fully).</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>N-channel MOSFETs have lower on-resistance values, making them more efficient -than P-channel ones. They are also cheaper. However, they are harder to drive -on the high side as their gate potential must be higher than the source -potential. This often requires extra circuitry such as 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> @@ -147,11 +119,11 @@ potential. This often requires extra circuitry such as MOSFET drivers.</p> 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 src="https://teachmetomake.wordpress.com/how-to-use-a-transistor-as-a-switch/" class="external" target="_blank" rel="noopener noreferrer">How to use a + <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 src="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 + <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 src="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 + <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> </div> |