---
title: "Power"
url: "https://nearfieldnotes.com/families/power/"
site: "Near Field Notes"
---

# Power

Notes for when a FET rings, runs hot or a rail misbehaves.

## Switching

- [Gate ringing](https://nearfieldnotes.com/notes/gate-ringing/): How hard should you drive a half bridge? As hard as it goes without ringing. At its 1 A power-on setting, the gate driver on moteus-c1 rings its 80 V FET to 81 V at 48 V. 190 mA stays at 48 V, and 30 mA loses 3.3 W at 100 kHz.
- [Zero-voltage switching](https://nearfieldnotes.com/notes/zvs/): What is zero-voltage switching, and what does it save? Turning a switch on only after a current has already pulled the voltage across it to zero. A pair of 80 V FETs switching 10 A at 48 V then loses 0.45 µJ per turn-on, instead of 2.6 µJ.
- [GaN vs silicon](https://nearfieldnotes.com/notes/gan-vs-si/): Why are USB-C chargers going GaN? Because a GaN switch and its driver lose about a quarter as much energy each time it flips. In the same 65 W charger at 120 V AC, that’s 1.73 µJ a flip for silicon and 0.42 µJ for GaN, so GaN can flip four times as often for the same heat. A charger that flips faster needs a smaller transformer.
- [FET choice](https://nearfieldnotes.com/notes/fet-choice/): Is the FET with the lowest Rds(on) the best one? Only with a fast gate driver. In moteus-c1’s 48 V, 10 A half bridge at 30 kHz, the 1.2 mΩ IRFS7530 loses 0.77 W and the 15 mΩ TPN19008QM 1.98 W with 1 A of gate current. At 60 mA they lose 9.08 W and 2.41 W.

## Inductors and capacitors

- [Power inductors](https://nearfieldnotes.com/notes/inductor-types/): Does it matter how a 4.7 µH inductor is built? Yes. Built five common ways, the same 4.7 µH puts anywhere from 0.44 mV to 7.5 mV on a trace 1 mm away, and loses 30% of its inductance at anywhere from 0.18 A to 4.7 A. How it’s built decides where its field goes and how much current it takes.
- [Capacitors](https://nearfieldnotes.com/notes/capacitor-types/): Which kind of capacitor should feed a load? The one whose time scale matches the load’s. On the same 1 A spike, a ceramic MLCC drops least for the first 0.31 µs, a polymer tantalum until 12 µs, a supercap until 0.29 s, and a lithium-ion capacitor after that. On the same 1 mW load, six kinds last from 1.1 s to 40 h.

## Batteries

- [Battery balance](https://nearfieldnotes.com/notes/battery-balance/): Why does a LiPo pack need balancing? Because a plain charger watches only the total. On a 3S pack with one cell faded to 90%, a plain charge stops at 12.60 V with that cell at 4.25 V, past its 4.20 V limit. A balance charge watches every cell and stops each one at 4.20 V.
- [LiPo vs LiFePO4](https://nearfieldnotes.com/notes/lipo-vs-life/): How does a LiFePO4 cell differ from a LiPo? A LiFePO4 cell, or LiFe, holds less energy for its weight, lasts longer and fails more gently, but its flat voltage hides how full it is. Read with the same 10 mV error, a half-full LiPo could be 47 to 53% and a half-full LiFe anywhere from 42 to 71%. So a LiFe needs a gauge that counts charge and a charger set for 3.60 V a cell.

## Supplies and converters

- [Power quadrants](https://nearfieldnotes.com/notes/power-quadrants/): What are 1-, 2- and 4-quadrant power supplies? They differ in which way current can flow through the output, and at which polarity. A 1-quadrant supply only pushes current out at positive voltage, a 2-quadrant one can also take it back, and a 4-quadrant one does both at either polarity. You need more than one quadrant only when your load gives power back or its voltage has to cross zero.
- [SIMO converters](https://nearfieldnotes.com/notes/simo/): Can one inductor feed three rails? Yes, if the rails take turns. A single-inductor multiple-output converter, or SIMO, runs this watch’s three rails for 8.6 h, against 8.8 h from three bucks and 4.3 h from three LDOs. It takes 61% of the bucks’ board area.

---

Source: https://nearfieldnotes.com/families/power/