Discussions#

Q1: Trace width#

PCB Trace Inductance and Width: How Wide is Too Wide?

  • Goal: minimize PCB trace inductance per unit length while keeping impedance constant

  • IPC 2142 formulas are only highly accurate within a particular impedance range

  • Waddell’s implementation produces less than 0.7% error which is much better than IPC-2141

\[\begin{align*} L &= \dfrac{Z_0(\frac{w}{h}, \frac{t}{h})}{c} \sqrt{\epsilon_{eff}(\frac{w}{h}, \frac{t}{h})} \\[1em] C &= \dfrac{L}{Z_0(\frac{w}{h}, \frac{t}{h})^2} \\[1em] Z_0 &= \text{Waddell’s equations for microstrip impedance} \end{align*}\]
\[\begin{align*} \text{Minimize }L \bigg(\dfrac{w}{h}, \dfrac{t}{h} \bigg) &= \dfrac{Z_0 (\frac{w}{h}, \frac{t}{h})}{c} \sqrt{\epsilon_{eff}(\dfrac{w}{h}, \dfrac{t}{h})} \\[1em] \text{s.t. } & 0 < \dfrac{w}{h} < a \\[1em] & Z_0 = 50 \Omega \\[1em] & 0 < \dfrac{t}{h} < b \end{align*}\]
../../_images/trace-geometry.png

Fig. 7 Trace Geometry#

Q2: Filling unused area in signal planes with copper pours#

On Shaky Ground—the Arguments Against Copper Pours

  • Ground pours resemble patch antennas and can emit noise

  • When the copper is thick, desoldering and service operations are more difficult

Q3: Current sensor location#

Current Sensing: Where to Place the Sense Resistor

  • Placing sense resistor after inductor provides best signal to noise ratios. Ultimately, resistor should be replaced with hall sensor to improve dissipated power if it’s within budget ($).

../../_images/Rsense.jpg

Fig. 8 Sense Resistor in Switch Mode Power Supply#

Q4: Gate driver protection#

Gate Resistor for Power Devices
Peak Current of Isolated Gate Drivers

../../_images/TypicalApplication.png

Fig. 9 Typical Application#

Design

Component

Gate Driver

STDRIVEG600

MOSFET

TPH8R903NL

Given gate driver specs, find in series resistance where \(R_G = R_g + r_g\).
\(R_g\) is the additional resistance added.
\(r_g\) is the MOSFET contact resistance.

\(V_{BO}\)

Source

Sink

\(R_{\text{DS_on}}\)

\(R_{\text{DS_off}}\)

6.0V

1.3A

2.4A

2.0\(\Omega\)

1.2\(\Omega\)

15V

5.5A

6.0A

1.25\(\Omega\)

0.9\(\Omega\)

High Side Calculations

\[\begin{align*} I_{\text{charging_peak}} &= \dfrac{V_{BO}}{R_{\text{DS_on}} + R_{\text{G_on}}} \\[0.5em] 5.5A &= \dfrac{20}{1.25 + R_{\text{G_on}}} \\[0.5em] R_{\text{G_on}} &= 2.39 \Omega \\[0.5em] I_{\text{discharging_peak}} &= \dfrac{V_{BO}}{R_{\text{DS_off}} + R_{\text{G_off}}}\\[0.5em] 6.0A &= \dfrac{20}{0.9 + R_{\text{G_off}}} \\[0.5em] R_{\text{G_off}} &= 2.43 \Omega \\[0.5em] \end{align*}\]

Low Side Calculations

\[\begin{align*} I_{\text{charging_peak}} &= \dfrac{PVCC}{R_{\text{DS_on}} + R_{\text{G_on}}} \\[0.5em] 1.3A &= \dfrac{5}{2.0 + R_{\text{G_on}}} \\[0.5em] R_{\text{G_on}} &= 1.85 \Omega \\[0.5em] I_{\text{discharging_peak}} &= \dfrac{PVCC}{R_{\text{DS_off}} + R_{\text{G_off}}}\\[0.5em] 2.4A &= \dfrac{5}{1.2 + R_{\text{G_off}}} \\[0.5em] R_{\text{G_off}} &= 0.89 \Omega \\[0.5em] \end{align*}\]

Resistor Size

Additionally, power dissipated through the resistor is

(1)#\[\begin{equation} P = C V^2 f \end{equation}\]

for selecting components.