================================================================================ SCHEMATIC: Linear Regulated Power Supply — 13.8V DC TM-PWR-001 Rev A 5A (QRP bench), 10A (station), 20A (high-power HF) Output noise: < 1 mV RMS | Ripple: < 5 mV p-p | Regulation: ±0.5% ================================================================================ LINEAR PSU THEORY ────────────────── A linear supply dissipates excess voltage across a series pass element (transistor). For 13.8V output from a 16–18V rectified supply, the pass element drops 2–4V. Power dissipated in pass element: P_pass = (V_in - V_out) × I_out At 10A output, V_in=17V: P_pass = (17 - 13.8) × 10 = 32W This is the fundamental efficiency limit: η = V_out / V_in = 13.8 / 17 = 81% (theoretical max) Practical: 70–78% (transformer copper losses, rectifier drops, etc.) Despite lower efficiency, linear supplies offer: - Near-zero switching noise (no PWM harmonics) - Output noise < 100 µV RMS (vs. SMPS 1–5 mV RMS with filtering) - No conducted or radiated EMI - Superior for sensitive HF receiver operation ================================================================================ STAGE 1: TRANSFORMER AND RECTIFIER ================================================================================ TRANSFORMER SELECTION: Required secondary voltage: V_sec(rms) ≥ V_out + V_drop_rectifier + V_drop_regulator + V_ripple V_sec(rms) ≥ 13.8 + 1.4 (bridge) + 3.0 (regulator headroom) + 2.0 (ripple) V_sec(rms) ≥ 20.2V → use 18V secondary (18V × √2 = 25.5V peak; ample headroom) Or: 22V secondary is common for 13.8V/20A supplies. Rectified DC (no load, full-wave bridge): V_dc = V_sec × √2 - 2 × V_f = 18 × 1.414 - 1.4 = 24.1V Under full load with filter capacitor: V_dc(load) ≈ V_dc - ΔV_ripple where ΔV_ripple = I / (2f × C) VA rating of transformer: VA = V_out × I_out / η × 1.8 (1.8 factor for capacitor input filter) 5A supply: VA = 13.8 × 5 / 0.75 × 1.8 = 165VA → use 200VA transformer 10A supply: VA = 13.8 × 10 / 0.75 × 1.8 = 331VA → use 350VA transformer 20A supply: VA = 13.8 × 20 / 0.75 × 1.8 = 662VA → use 750VA transformer BRIDGE RECTIFIER SELECTION: Peak inverse voltage (PIV): 2 × V_sec × √2 = 2 × 22 × 1.414 = 62V → use 100V rated Peak current: I_peak ≈ 3 × I_dc (capacitor charging pulse) Average current: I_dc = output current 5A: BR610 (6A, 1000V) or GBPC1510 (15A, 1000V) — 10A rated for margin 10A: KBPC2510 (25A, 1000V) — bridge module on heatsink 20A: KBPC5010 (50A, 1000V) — large bridge on dedicated heatsink FILTER CAPACITOR: C = I_out / (2 × f_line × ΔV_ripple) Target ΔV_ripple = 2V (will be further filtered by regulator) f_line = 60 Hz (USA), 50 Hz (EU) 5A, 60Hz: C = 5 / (120 × 2) = 20,833 µF → use 22,000 µF / 35V 10A, 60Hz: C = 10 / (120 × 2) = 41,667 µF → use 47,000 µF / 35V (2× 22,000µF in parallel) 20A, 60Hz: C = 20 / (120 × 2) = 83,333 µF → use 3× 33,000µF / 35V or 4× 22,000µF CAPACITOR VOLTAGE RATING: V_peak = V_sec × √2 = 22 × 1.414 = 31.1V Use capacitors rated ≥ 35V; 50V preferred for reliability margin. Electrolytic capacitors: general purpose OK; 105°C rated for long life. RECTIFIER BLOCK SCHEMATIC: AC IN (L) ──────┬──────────────────────────┐ │ │ T1 18–22V SEC │ BRIDGE D1–D4 │ ├──[D1]+──[D3]──┬──── (+) DC OUT ──┬──── to regulator │ │ │ └──[D2]+──[D4]──┘ [C1] 22000–100000µF │ / 35–50V AC IN (N) ──────────────────────────────────────────┴──── GND [F1] 5A fuse on AC primary side [F2] 5A fast-blow fuse on DC output side (in addition to regulator protection) [MOV1] 275V MOV across AC input (surge protection) [CX1] 0.1µF X2 film cap across AC input (EMI) [CY1,CY2] 2.2nF Y2 caps (L-to-chassis, N-to-chassis) — common mode EMI ================================================================================ STAGE 2: VOLTAGE REGULATOR — 5A VERSION (LM338) ================================================================================ LM338 ADJUSTABLE REGULATOR (5A TO-3 package): V_out = 1.25V × (1 + R2/R1) + I_adj × R2 I_adj ≈ 50µA (typical); negligible for R2 < 10kΩ For V_out = 13.8V: R2/R1 = (13.8/1.25) - 1 = 10.04 Using R1 = 240Ω: R2 = 10.04 × 240 = 2,410Ω → use 2.4kΩ + 100Ω trim pot Fine-adjust with 100Ω pot: range covers 13.5–14.2V LM338 5A REGULATOR SCHEMATIC: V_unregulated (+) ──[1µH inductor]──────────────── IN (pin 3) │ ADJ (pin 1) ──┤ │ LM338 [R1] │ 240Ω OUT (pin 2) ────── V_out (13.8V) │ │ [R2] [C_out] 2.4kΩ+pot 1000µF/25V │ + 0.1µF GND ─────────────────── GND PROTECTION COMPONENTS: [D_prot]: 1N4002 from OUT to IN (protects against input short with charged output cap) [D_adj]: 1N4002 from ADJ to OUT (protects against output capacitor discharge through R2) [C_adj]: 10µF across R2 (improves ripple rejection; adds 20 dB to PSRR) [C_in]: 0.1µF ceramic on IN pin (oscillation prevention) HEATSINK CALCULATION: P_max = (V_in_max - V_out_min) × I_max With V_in = 24V, V_out = 13.8V, I = 5A: P = (24 - 13.8) × 5 = 51W LM338 θ_jc = 1°C/W; for Tj_max = 150°C, Ta = 40°C ambient: R_heatsink = (150 - 40 - 51×1) / 51 = 1.16°C/W Use 0.8°C/W heatsink (Hammond HS151) with thermal grease. ================================================================================ STAGE 2 (ALT): REGULATOR — 10A AND 20A (LM723 + PASS TRANSISTORS) ================================================================================ For 10A and 20A, the LM338 alone is inadequate. Use LM723 as control/error amplifier with external Darlington pass transistors. LM723 + TIP35C/2N3055 DARLINGTON PASS STAGE: V_in(unregulated) ─────────────────────── COLLECTOR (Q1) │ LM723 V+ (pin 12) ─── V_in │ TIP35C (10A NPN TO-218) LM723 Vref (pin 6) ──[R_ref 5.1kΩ]── +Vref │ or 2N3055 (15A NPN TO-3) LM723 (+) (pin 5) ──[R_top]──┬── ADJ trim BASE LM723 (−) (pin 4) ──────[R1]─┤ │ Q1 │ SETPOINT DIVIDER │ [R2] LM723 output ── R_drive (100Ω) ─── BASE │ GND EMITTER ──── V_out For 10A DARLINGTON: Q1: TIP35C (25A, 100V, NPN TO-218); drive from LM723 output (150mA max drive) Or: 2× 2N3055 in parallel (two TO-3 NPN, 15A each) For 20A DARLINGTON: PRE-DRIVER: LM723 output → Q_pre (BD139, 1.5A) → BASE of Q_main Q_main: 3× 2N3055 in parallel with 0.1Ω emitter resistors for current sharing EMITTER SHARING RESISTORS (critical for parallel transistors): Each parallel transistor must have equal emitter resistance to force current sharing. If Q1 has higher hFE, it takes more current → thermal runaway in parallel NPN. Emitter resistors: 0.1Ω to 0.22Ω per transistor, 3W–10W wirewound. Equalizes current: ΔI = ΔV_be / R_emitter ≈ 60mV / 0.1Ω = 600mA max imbalance. 10A REGULATOR BLOCK DIAGRAM: V_unrg ──────────────────────────── TIP35C Collector │ LM723──[100Ω]── TIP35C Base ──── TIP35C Emitter ─── 0.1Ω ─── V_out 13.8V │ V_out ──[R_sense 0.1Ω]── LM723 CL+ (pin 2) [C_out] V_out ──[R_CL 1kΩ]──── LM723 CL− (pin 3) 1000µF (current limit comparator: if V_sense > V_CL, reduce output) │ GND CURRENT LIMITING (LM723 BUILT-IN): V_sense = I_out × R_sense Current limit trips at V_sense ≈ 0.65V (internal V_be) R_sense = 0.65 / I_limit 5A limit: R_sense = 0.65/5 = 0.130Ω → use 0.1Ω + 30Ω trim 10A limit: R_sense = 0.65/10 = 0.065Ω → use 0.05Ω wirewound 20A limit: R_sense = 0.65/20 = 0.0325Ω → use 0.033Ω (parallel 0.1Ω resistors) FOLDBACK CURRENT LIMITING: Standard current limit: when I > I_limit, V_out stays, I stays at limit. Foldback: as overload voltage drops, I_limit ALSO reduces. Prevents transistor from being destroyed in dead-short condition. FOLDBACK CIRCUIT (added to LM723 CL pins): V_out ──[R_fb1: 1kΩ]──┬── LM723 CL− (pin 3) │ [R_fb2: 2.7kΩ] │ GND At full output voltage: V_CL− = V_out × R_fb2/(R_fb1+R_fb2) = high; limit = high As V_out sags (overload): V_CL− drops → limit set point drops → foldback Recovery: must remove overload; V_out recovers automatically. ================================================================================ OUTPUT NOISE FILTER ================================================================================ A well-designed linear PSU already has < 5 mV p-p output ripple from the regulator's PSRR (~74 dB for LM723, ~65 dB for LM338 at 120 Hz). For receiver-quiet operation (< 1 mV RMS goal): OUTPUT FILTER (Π-TYPE): V_reg_out ──[L1: 10µH, 20A ferrite]──┬──── V_filtered_out │ [C3: 1000µF, 25V] [C4: 100µF, 25V] (before L1) │ │ [C5: 0.1µF ceramic] GND │ GND [L_CM1: common-mode choke, 10A, 1mH] — in series with both V_out and GND Suppresses common-mode RF currents that ride on supply rails. Additional: [C6: 0.01µF] from V_out to chassis, [C7: 0.01µF] from GND to chassis (differential-to-common-mode decoupling; reduces RF on supply leads) MEASURED PERFORMANCE TARGETS: Input ripple (unregulated): 2–4V p-p Output ripple (regulated): < 5 mV p-p (at 13.8V/10A) Output noise (wideband, 10 Hz–100 kHz): < 500 µV RMS Output noise (with output filter): < 100 µV RMS Line regulation: < ±0.1% for ±10% line change Load regulation: < ±0.5% from no-load to full load ================================================================================ PARTS LIST — LINEAR PSU ================================================================================ 5A VERSION: Item | Qty | Description | Source --------|-----|--------------------------------------|------------------- T1 | 1 | 200VA 22V CT or 18V secondary | Antek, Hammond Mfg BR1 | 1 | KBPC1510 (15A 1000V bridge) | Mouser C1 | 1 | 22,000µF 50V electrolytic | Mouser C2 | 2 | 0.1µF 50V ceramic (bypass) | Mouser IC1 | 1 | LM338T (TO-220, 5A adj regulator) | Mouser Q1 | — | (not needed at 5A) | — R1 | 1 | 240Ω 1% metal film | Mouser R2 | 1 | 2.2kΩ 1% + 200Ω trim pot | Mouser D1,D2 | 2 | 1N4002 protection diodes | Mouser C_adj | 1 | 10µF 25V electrolytic | Mouser L1 | 1 | 10µH 5A ferrite core choke | Mouser C_out | 1 | 1000µF 25V + 0.1µF ceramic | Mouser HS1 | 1 | 0.8°C/W heatsink for LM338 | Mouser MOV1 | 1 | 275V MOV (AC line) | Mouser F1 | 1 | 5A slow-blow (primary) | Mouser F2 | 1 | 5A fast-blow (DC output) | Mouser 10A VERSION (additions/substitutions): IC1 | 1 | LM723 (DIP-14 voltage regulator) | Mouser Q1 | 1 | TIP35C (25A NPN TO-218) | Mouser R_sense | 1 | 0.05Ω 5W wirewound | Mouser HS1 | 1 | 0.5°C/W heatsink for Q1 | Mouser C1 | 1 | 47,000µF 50V (2× 22,000µF parallel) | Mouser 20A VERSION (additions/substitutions): Q1–Q3 | 3 | 2N3055 (15A NPN TO-3) | Mouser Q_pre | 1 | BD139 (1.5A NPN pre-driver) | Mouser R_eq | 3 | 0.1Ω 5W emitter sharing resistors | Mouser T1 | 1 | 750VA 22V transformer | Antek AS-7T22 C1 | — | 4× 22,000µF 50V in parallel | Mouser HS1 | 1 | 0.25°C/W chassis heatsink (bolted) | Custom/Hammond ================================================================================