Near Field Notes

Families10 notes

Power

A FET rings, runs hot or a rail misbehaves.

SwitchingInductors and capacitorsBatteriesSupplies and converters

Find your problem

What is the board doing wrong?

  1. aRings past its ratingGate ringing
  2. bHard switching heats the FETsZero-voltage switching
  3. cSwitching loss limits the frequencyGaN vs silicon
  4. dYou picked a FET by its Rds(on)FET choice
  5. eA trace near the inductor picks up switching noisePower inductors
  6. fA 22 µF MLCC holds far less on its railCapacitors
  7. gA pack reads full but one cell is over 4.20 VBattery balance
  8. hA LiFe battery's gauge jumps from full to nearly emptyLiPo vs LiFePO4
  9. iA supply has to sink current tooPower quadrants
  10. jNo room for three inductorsSIMO converters

Switching

4 notes

How a FET turns on and off, and what each flip costs.

Voltage on the FET
Fig. 4cV0488080 V rating0100200 ns1 A gate drive60 mAFig. 4cV0488080 V rating0100200 ns1 A gate drive60 mAFig. 4cV0488080 V rating0100200 ns1 A gate drive60 mA
  • 1 A
  • 60 mA

Note 4Gate ringing

How hard should you drive a half bridge?

81 Von an 80 V FET at full drive

DRV8323 · TPN19008QM · DRV8302 · moteus-c1

Where it goes
Fig. 10cLost per turn-on, µJ012Hard1.170.800.240.362.6 µJZero-voltage0.430.45 µJFig. 10cLost per turn-on, µJ012Hard1.170.800.240.362.6 µJZero-voltage0.430.45 µJFig. 10cLost per turn-on, µJ012Hard1.170.800.240.362.6 µJZero-voltage0.430.45 µJ
  • Overlap
  • Capacitance
  • Diode recovery
  • Dead time

Note 10Zero-voltage switching

What is zero-voltage switching, and what does it save?

2.6 → 0.45 µJper turn-on of a pair of 80 V FETs at 48 V and 10 A

TPN19008QM

Energy lost per flip
Fig. 11cSiliconDrain charge 1.40Gate 0.321.73 µJGaNDrain charge 0.40Gate 0.010.42 µJ00.511.52 µJFig. 11cSiliconDrain charge 1.40Gate 0.321.73 µJGaNDrain charge 0.40Gate 0.010.42 µJ00.511.52 µJFig. 11cSiliconDrain charge 1.40Gate 0.321.73 µJGaNDrain charge 0.40Gate 0.010.42 µJ00.511.52 µJ
  • Silicon
  • GaN

Note 11GaN vs silicon

Why are USB-C chargers going GaN?

1.73 → 0.42 µJenergy lost per flip, switch and gate drive, in a 65 W charger at 120 V AC

IPD65R190C7 · GS-065-011-1-L

Leg loss against gate current
Fig. 20eLeg loss at 30 kHz, W024681060 mA300 mA1 AGate currentCross at 290 mA9.08 W2.41 W0.77 W1.98 WFig. 20eLeg loss at 30 kHz, W024681060 mA100 mA300 mA1 AGate currentCross at 290 mA9.08 W2.41 W0.77 W1.98 WFig. 20eLeg loss at 30 kHz, W024681060 mA100 mA300 mA1 AGate currentCross at 290 mA9.08 W2.41 W0.77 W1.98 W
  • IRFS7530
  • TPN19008QM

Note 20FET choice

Is the FET with the lowest Rds(on) the best one?

9.08 vs 2.41 W1.2 mΩ vs 15 mΩ FET at 60 mA of gate current

IRFS7530 · TPN19008QM · DRV8323 · moteus-c1

Inductors and capacitors

2 notes

The parts that hold a rail’s energy, and how each kind behaves.

On a trace 1 mm away
Fig. 12i1 Unshielded drum7.5 mV2 Semi-shielded drum3.8 mV3 Shielded drum0.67 mV4 Molded powder0.83 mV5 Multilayer chip0.44 mV02468 mVFig. 12i1 Unshielded drum7.5 mV2 Semi-shielded drum3.8 mV3 Shielded drum0.67 mV4 Molded powder0.83 mV5 Multilayer chip0.44 mV02468 mVFig. 12i1 Unshielded drum7.5 mV2 Semi-shielded drum3.8 mV3 Shielded drum0.67 mV4 Molded powder0.83 mV5 Multilayer chip0.44 mV02468 mV

Note 12Power inductors

Does it matter how a 4.7 µH inductor is built?

7.5 → 0.83 mVon a trace 1 mm from a 4.7 µH inductor, unshielded drum to molded powder

CD43NP-4R7MC · SRN4018BTA-4R7M · SRR4018-4R7Y · XAL4030-472 · LQM2HPN4R7MG0L

The drop on a 1 A spike
Fig. 13jDrop in the part’s voltage, V00.250.50.751100 ns10 µs1 ms1 s12345Drops least12450.31 µs12 µs0.29 s1 MLCC2 Polymer tantalum3 Aluminum electrolytic4 Supercap5 Lithium-ion capacitorFig. 13jDrop in the part’s voltage, V00.250.50.751100 ns1 µs10 µs100 µs1 ms10 ms100 ms1 s1 MLCC2 Tantalum3 Electrolytic4 Supercap5 Li-ion capacitorDrops least12450.31 µs12 µs0.29 sFig. 13jDrop in the part’s voltage, V00.250.50.751100 ns1 µs10 µs100 µs1 ms10 ms100 ms1 s1 MLCC2 Tantalum3 Electrolytic4 Supercap5 Li-ion capacitorDrops least12450.31 µs12 µs0.29 s

Note 13Capacitors

Which kind of capacitor should feed a load?

1.1 s → 40 hsix parts that store charge, each running the same 1 mW load

GRM21BR61E226ME44L · T520B107M006ATE040 · EEE-FK1C221P · SCCR12B105SRB · HS1016-3R8306-R · BCT1812M101AG

Batteries

2 notes

How lithium cells charge, age and stay balanced in a pack.

A plain charge
Fig. 15dThe pack, V11.512.012.5Each cell, V3.84.00102030 min4.2Charger’s limit, 12.60 VFull, 4.20 V4.25 V4.17 VFig. 15dThe pack, V11.512.012.5Each cell, V3.84.00102030 min4.2Charger’s limit, 12.60 VFull, 4.20 V4.25 V4.17 VWeak cellFig. 15dThe pack, V11.512.012.5Each cell, V3.84.00102030 min4.2Charger’s limit, 12.60 VFull, 4.20 V4.25 V4.17 VWeak cell

Note 15Battery balance

Why does a LiPo pack need balancing?

12.60 V, one cell at 4.25 Va 3S pack charged without its balance lead, one cell faded to 90%

BQ76920 · LTC3300-1

Each cell at rest
Fig. 18cCell voltage at rest, V2.02.53.03.54.00255075100%Charge, of a full cell299 mV141 mVLiPoLiFeFig. 18cCell voltage at rest, V2.02.53.03.54.00255075100%Charge, of a full cell299 mV141 mVLiPoLiFeFig. 18cCell voltage at rest, V2.02.53.03.54.00255075100%Charge, of a full cell299 mV141 mVLiPoLiFe

Note 18LiPo vs LiFePO4

How does a LiFePO4 cell differ from a LiPo?

42–71% vs 47–53%a half-full LiFe and LiPo cell read with the same 10 mV meter error

ANR26650M1 · ANR26650M1-B · LF280K

Supplies and converters

2 notes

What makes a rail: a bench supply that can take power back, or one inductor shared by three rails.

One solar cell, three supplies
Fig. 21hCurrent out, mABench supply4% of the curve−1001002-quadrant supply43% of the curve−100100SMU100% of the curve−100100−11Voltage across the output, VFig. 21hCurrent out, mABench supply4% of the curve−1001002-quadrant supply43% of the curve−100100SMU100% of the curve−100100−11Voltage across the output, VFig. 21hCurrent out, mABench supply4% of the curve−1001002-quadrant supply43% of the curve−100100SMU100% of the curve−100100−11Voltage across the output, V
  • The cell
  • What each supply read

Note 21Power quadrants

What are 1-, 2- and 4-quadrant power supplies?

4 / 43 / 100%of a solar cell’s curve traced by a 1-, 2- and 4-quadrant supply

2200-20-5 · 2281S-20-6 · 2460 · KXOB25-14X1F · moteus-c1

Battery life and board area
Fig. 22cBattery life, h0510 hThree LDOs4.3 hThree bucks8.8 hOne SIMO8.6 hBoard area, mm²02040 mm²Three LDOs10.8Three bucks38.7One SIMO23.5Room, 33 mm²Fig. 22cBattery life, h0510 hThree LDOs4.3 hThree bucks8.8 hOne SIMO8.6 hBoard area, mm²02040 mm²Three LDOs10.8Three bucks38.7One SIMO23.5Room, 33 mm²Fig. 22cBattery life, h0510 hThree LDOs4.3 hThree bucks8.8 hOne SIMO8.6 hBoard area, mm²02040 mm²Three LDOs10.8Three bucks38.7One SIMO23.5Room, 33 mm²

Note 22SIMO converters

Can one inductor feed three rails?

8.6 h in 61% of the areaa watch's three rails from one 300 mAh cell: one SIMO against three bucks at 8.8 h and three LDOs at 4.3 h

MAX77654 · TPS62840 · TPS7A02