Diode

Diodes are commonly used to control current direction, but the model of "constant 0.7 V drop in forward bias and zero current in reverse bias" only solves some rough calculations. Determining current for an LED, handling stored energy when a coil turns off, and limiting voltage during a pulse for a TVS all rely on different parameters. This article connects device types, curves, and selection criteria based on actual tasks.

Reading Parameters from the V-I Curve

Anode A and Cathode K define the common forward reference: V_D = V_A−V_K, with positive current flowing from A to K. The cathode is identified by a bar in the circuit symbol; color bands on the device body, package markings, and internal multi-pin connections must be confirmed via the datasheet.

Standard PN diodes are based on the PN junction, while Schottky diodes are based on the metal-semiconductor barrier. Reverse leakage, breakdown, and capacitive currents may all exist, so "current can only flow from A to K" is not a complete physical definition.

Three Common Models Solve Different Problems

The ideal Shockley equation I = I_S[exp(V_D/(ηV_T))−1] describes the relationship under specific mechanisms and conditions. I_S is not a parameter that uniformly falls between nA and μA for all devices, nor does η remain 1 or 2 across all current ranges. The actual forward operating point is determined jointly by the device curve and the external circuit; current does not start flowing only after the voltage exceeds a fixed V_F.

For example, if a power supply V_S connects to a diode via a resistor R, the resistor provides I = (V_S−V_D)/R. The intersection of this load line with the diode curve is the operating point. The V_F in tables usually also specifies I_F and temperature; comparing typical values of two devices at different test currents may lead to incorrect conclusions about losses.

Current and Voltage Ratings Cannot Simply Be Multiplied by a Fixed Margin

Forward average loss is more generally the average of v_D(t)i_D(t). It is only approximately V_F·I_avg when current and voltage drop do not change significantly; reverse and dynamic losses must also be included. The final junction temperature depends on the actual thermal path; the package name itself does not equal heat dissipation capability.

Who determines the forward voltage?

Blue belongs to the device model and gold to the external circuit. The dot is their numerically solved operating point. Reducing supply reduces current smoothly, without a sudden turn-on switch. Differential resistance is the inverse local slope, not VD/I.

$$I_S(e^{V_D/V_T}-1)=\frac{V_S-V_D}{R}$$

Type Differences: Structure Determines Trade-offs

Low-voltage silicon Schottky diodes are often used to reduce forward loss at a certain operating point, but leakage and temperature characteristics may become limiting factors. SiC Schottky diodes already have products like 1200 V, so one cannot treat a breakdown voltage below 200 V as a structural law for Schottky diodes. Infineon: Example of 1200 V SiC Schottky Product

Majority carrier devices usually do not have minority carrier storage recovery like standard PN junctions, but they still have capacitive charge that needs to be charged and discharged during commutation, forming transient currents and energy. Infineon: Capacitive Charge in Schottky Turn-off

LED Current-Limiting Example: Checking with Worst-Case Conditions

An LED should be driven by an appropriate current-limiting mechanism, which can be a resistor, linear constant current, or switching constant current circuit; not all applications require a series resistor. For simple indicator lights, the resistor model is intuitive and sufficiently practical.

Assume the power supply is 5 V ±5%, the selected LED has a V_F of 1.8–2.2 V within the target range, the required current does not exceed 10 mA, and the resistor tolerance is ±1%. These are hypothetical design inputs and should be replaced with actual LED and power supply specifications.

Maximum current occurs at the highest power supply (5.25 V), lowest V_F (1.8 V), and lowest resistance. Thus, R_actual,min ≥ (5.25−1.8)/0.01 = 345 Ω, and the nominal value should be at least 345/0.99≈348.5 Ω. If a 360 Ω ±1% resistor is selected, the maximum current is approximately 9.68 mA.

Minimum current uses the lowest power supply (4.75 V), highest V_F (2.2 V), and highest resistance (363.6 Ω), resulting in approximately 7.01 mA. The worst-case power dissipation of the resistor is approximately (5.25−1.8)²/356.4≈33.4 mW; select the power rating considering temperature and device conditions. This yields a current range, but does not guarantee identical brightness across different LED units; brightness is also affected by luminous efficacy, binning, and temperature.

Rectification: Utilizing Both Half-Cycles Does Not Equal Double Efficiency

Half-wave rectification allows only one half-cycle to power the load; bridge full-wave rectification maintains current in the same direction through the load during both half-cycles, passing through two diodes each time. For an ideal sine wave, purely resistive load, and no filter capacitor, the full-wave output average is 2V_peak/π, and the half-wave is V_peak/π.

The common 40.5% and 81.1% figures come from this specific model's "output DC component power / total load power": the half-wave voltage RMS is V_peak/2, and the full-wave is V_peak/√2, hence the ratios are 4/π² and 8/π² respectively. These are not the actual energy efficiency of any half-wave and full-bridge power supplies, nor are they universal diode loss percentages.

After connecting a large filter capacitor, the capacitor replenishes charge near the input peak, causing the rectifier current to form charging pulses. The average value, RMS, surge, and transformer burden all change. One cannot select the bridge diodes based solely on the load average current, nor directly apply the sine load formulas without a capacitor. Low-voltage rectification must also consider the proportion of the series V_F to the output voltage.

Where the Inductive Load Energy Goes

When a low-side switch drives a coil, a diode is commonly anti-parallel across the coil: cathode to the power side, anode to the switch node. When the switch is on, the diode is reverse-biased; after turning off, the inductor maintains the original current direction, raising the switch node above the power supply, causing the diode to conduct and form a local freewheeling loop.

A standard freewheeling diode limits the reverse voltage across the coil and also causes slower current decay. Ignoring coil resistance and approximating the diode drop at 0.7 V, 0.1 A in 100 mH takes about LI/V = 14.3 ms to decay to zero; if the coil reverse voltage can be controlled to 10 V, the same simplified estimate is about 1 ms. Real relays also involve winding resistance, mechanical release thresholds, and magnetic circuit changes; these two numbers cannot be directly taken as actuation times.

Higher clamping voltages can accelerate current decay but increase switch voltage stress and protection device dissipation. Combinations of diodes and Zeners, TVS, driver-internal clamping, or appropriate energy recovery paths can be used; design should be based on the complete loop, not "burning out without a separate freewheeling diode." Regardless of the form, one must verify where the turn-off energy ½LI² goes, and whether peak current and repetitive thermal loads are tolerable.

Zener and TVS: Look at Operating Points and Pulse Conditions

Parallel Regulation Requires Ensuring Current Range

A simple parallel regulator consists of an input series resistor R, a reverse-biased Zener diode, and a parallel load. When V_Z is approximately fixed, I_Z = (V_in−V_Z)/R−I_load. At minimum input and maximum load, leave enough diode current; at maximum input and minimum load, check the diode and resistor power dissipation.

For example, using a fixed V_Z = 5.1 V teaching model, input 10.8–13.2 V, load 0–10 mA, assuming I_Z≥2 mA is required. First, get R≤475 Ω; take 430 Ω ±5%. At minimum input and highest resistance (451.5 Ω), I_Z≈2.62 mA. At maximum input, no load, and lowest resistance (408.5 Ω), I_Z≈19.83 mA, Zener power dissipation is about 101 mW, and resistor is about 161 mW.

This only verifies the fixed V_Z model; in reality, V_Z tolerance, dynamic resistance, temperature, and operating current curves must also be included. A nominal 5.1 V does not mean it strictly clamps at 5.1 V at any current, nor does a certain voltage grade automatically equal the most stable temperature.

The Three TVS Voltages Are Not Interchangeable

For standard avalanche TVS diodes, V_RWM is the normal operating reverse voltage condition, V_BR is measured at a specified small test current, and V_CL corresponds to a larger pulse current and specified waveform. The downstream circuit does not withstand V_RWM during a surge, but rather dynamic clamping plus actual circuit overshoot. Nexperia: TVS Selection White Paper

Actual checks should simultaneously answer: Is leakage acceptable at the maximum normal voltage? Can the clamping voltage protect the downstream circuit under specified pulses? Are the device energy and repetitive conditions met? Does trace L·di/dt cause additional overshoot? Devices used for high-speed signals also need capacitance and signal integrity checks. Some ESD devices have snapback behaviors; do not use a single avalanche curve for all protection devices.

When clamping the input to the power rail via a diode, injected current may enter that power rail; if the regulator cannot sink current, the rail voltage may actually be raised. Protection design therefore requires current limiting and energy loops, not just "protected if exceeding Vcc+0.3 V."

from math import isclose, pi
rmin, rmax = 360*.99, 360*1.01
imax, imin = (5.25-1.8)/rmin, (4.75-2.2)/rmax
assert imax < .01 and .007 < imin < .0071
power_r = (5.25-1.8)**2/rmin
iz_min = (10.8-5.1)/(430*1.05)-.01
iz_max = (13.2-5.1)/(430*.95)
assert iz_min >= .002
assert isclose(4/pi**2, .4052847345693511)
print(f"LED Current {imin*1000:.3f}–{imax*1000:.3f} mA, Resistor Worst-Case {power_r*1000:.3f} mW")
print(f"Fixed Vz Model Iz: {iz_min*1000:.3f}–{iz_max*1000:.3f} mA")

Finally, place candidate devices back into real waveforms: readings must include current, voltage, temperature, pulse, and thermal conditions; use typical curves and guaranteed values separately. The next article BJT enters controllable current and amplification from two adjacent junctions.