Ultra‑high temperature and high‑pressure (HTHP) oil/gas wells dominate deep and ultra‑deep reservoir exploration. As shallow reserves deplete, drilling extends beyond 6,000 m, often exceeding 8,000 m. Downhole temperatures reach 200 ℃–250 ℃ with formation pressure above 100 MPa and peak values up to 150 MPa. Downhole high‑voltage power supplies serve as core power units for Logging While Drilling (LWD), Measurement While Drilling (MWD), formation testers, production logging tools and intelligent completion systems. They deliver stable high‑voltage bias and drive power for gamma, density, neutron, acoustic and resistivity sensors, signal amplifiers, transmit drivers, servo actuators and data telemetry modules. Converting power from downhole lithium batteries or turbine generators into regulated high‑voltage DC, their miniaturization, structural compatibility, power density, shock resistance and long‑term reliability directly determine logging accuracy, operating duration, drilling efficiency and overall deep reservoir development success. Downhole HTHP conditions impose extreme requirements far beyond conventional logging standards: 1. Permanent stability above 225 ℃ and 150 MPa. Standard semiconductors suffer thermal runaway and exponential leakage at temperatures exceeding 200 ℃; insulation softens, magnetic losses surge and capacitors degrade severely. Ultra‑high pressure causes enclosure deformation, potting compression and mechanical circuit damage. Power units must operate continuously at 225 ℃ (extended to 250 ℃ premium grade) withstanding 150 MPa hydrostatic pressure, output drift below ±1.5 % and irreversible performance degradation prohibited. 2. Cylindrical ultra‑compact integration for drill collar mounting. Internal cavities are narrow, elongated cylindrical bores typically ≤45 mm outer diameter, sometimes below 30 mm for slimhole tools, with axial installation limited to 300 mm. Power supplies must fit within ≤40 mm outer diameter and ≤250 mm length with power density ≥150 W/in³ using fully conformal cylindrical stacking. 3. Severe drilling vibration and shock immunity. Downhole tools endure continuous harsh vibration, torsional excitation and impacts exceeding 1,000 g across 10 Hz–2,000 Hz, risking cracked pins, detached solder, potting delamination and winding failure. Compliance with petrochemical downhole environmental standards requires survival of 1,000 g shock and 20 g random vibration. 4. Low power consumption and high efficiency under extreme heat. Downhole energy from turbines or high‑temperature batteries is limited and degrades rapidly at elevated temperatures. Static power must remain below 10 mW with peak efficiency ≥88 % maintained across 20 %–100 % load to minimize self‑heating and thermal accumulation. 5. Ultra‑low noise and precision output for weak reservoir signals. Logging sensors detect nanovolt to microvolt formation signals, which are easily corrupted by switching noise at high temperature. Output stability better than ±0.8 % and ripple below 0.2 % peak‑to‑peak are mandatory with minimal conducted and radiated EMI at 225 ℃. 6. Maintenance‑free long service life. Single drilling runs last dozens of days with extremely high operational costs; unplanned tripping causes severe financial loss. Required MTBF ≥2×10⁴ hours with design life ≥8 years and autonomous thermal protection plus fault recovery. 7. Ultra‑wide input voltage adaptability. Turbine and battery voltage fluctuates drastically from 5 V to 40 V under varying rotation, discharge depth and thermal stress. Stable startup and full load operation down to 5 V input are essential. This methodology establishes a complete framework covering HTHP optimized topologies, cylindrical 3D integration, full thermal range tuning, pressure sealing, vibration hardening and low‑noise precision output. It standardizes power design for LWD, MWD, formation testing and intelligent completion, supporting domestic breakthroughs in deep reservoir equipment. Addressing extreme heat, tight cylindrical space, intense shock and low‑power reliability, the universal architecture adopts high‑temperature optimized isolated flyback + cylindrical axial/radial 3D stacking + fully digital adaptive thermal control reinforced by monolithic pressure‑resistant potting and full vibration shielding. It overcomes traditional limitations of poor thermal resilience, incompatible planar layout and weak mechanical ruggedness. The high‑temperature flyback topology minimizes component count and size, enabling integration within 40 mm diameter while supporting high isolation and high voltage gain. Silicon carbide active clamping achieves soft switching at 250 ℃, reducing thermal runaway risk and EMI. Eight core design principles are defined: 1. HTHP enhanced topology optimization. Active clamp soft switching reduces switching loss and recycles leakage inductance energy using high‑temperature SiC MOSFETs stable at 250 ℃. Mixed PWM/PFM control ensures stable operation from 5 V to 40 V input across all loads with high‑temperature current sensing eliminating drift and subharmonic oscillation. Symmetrical voltage doubling rectifiers reduce transformer winding complexity and radial size; SiC Schottky diodes minimize reverse leakage; high‑temperature multi‑layer ceramic capacitors eliminate electrolytic/tantalum failure modes above 200 ℃. 2. Cylindrical 3D axial–radial integration. Functional blocks stack axially on circular high‑temperature PCBs secured by ceramic supports with high‑temperature metal pin interconnections. Components arrange radially symmetrically: power transformers centered, small passives distributed peripherally for balanced thermal distribution and minimized parasitic inductance. Pot cores or toroidal magnetic components optimize vertical installation to reduce outer diameter. Central hollow channels reserve drilling fluid paths and wiring ports without sacrificing power volume. 3. 250 ℃ qualified component grading. All semiconductors adopt SiC wide‑bandgap devices with junction temperature ≥300 ℃. Control ICs, drivers and references are certified for –55 ℃ to +250 ℃ logging environments with low temperature drift ≤15 ppm/℃. Passives exclusively use high‑temp ceramic/mica capacitors and precision high‑temperature resistors; magnetic cores feature Curie temperature ≥350 ℃. PCBs employ polyimide high‑temperature substrates; connectors use gold‑plated beryllium copper; structural insulation adopts PEEK and alumina ceramic. All components undergo 250 ℃ burn‑in, thermal cycling and vibration screening to eliminate early failure. 4. Fully digital extreme‑temperature adaptive control. Dedicated high‑temperature MCU models real‑time thermal drift across 25 ℃–250 ℃, dynamically adjusting frequency, current limits, compensation and dead time to maintain ±0.8 % output accuracy and stable soft switching. Mixed PWM/PFM optimizes efficiency at light loads; sleep mode reduces static power below 10 mW. Full thermal calibration stores temperature compensation curves for drift suppression below ±1.5 %. Isolated CAN/RS485 telemetry enables downhole parameter configuration and fault logging. 5. Three‑stage ultra‑high pressure containment and sealing. Titanium alloy TC4, duplex stainless steel or Monolithic enclosures are forged for 150 MPa resistance with finite element optimized wall thickness and seamless construction. Multi‑layer metal end sealing employs perfluoroelastomer seals rated ≥260 ℃ with dual redundant sealing and leak monitoring. Full vacuum high‑temperature potting with glass transition ≥260 ℃ and compressive strength ≥200 MPa eliminates internal air gaps, ensuring insulation, thermal conduction and structural integrity. Passive pressure compensation balances internal/external hydrostatic load to reduce enclosure stress. High‑temperature ceramic feedthrough terminals provide double rated insulation with reinforced creepage margins. 6. Full shock and vibration hardening. Monolithic potting integrates all PCBs, transformers and components into a single rigid assembly eliminating relative movement. Multi‑point ceramic support columns secure stacked circular PCBs; heavy magnetic devices are fully encapsulated; solder joints reinforced. Modal simulation avoids structural resonance; qualification testing validates survival of 1,000 g impact and 20 g random vibration per downhole logging standards. 7. Extreme thermal management via conduction cooling. Heat dissipates entirely through high‑conductivity potting into the pressure enclosure and drilling fluid without convection reliance. Power components distribute evenly to eliminate hotspots; junction temperatures maintain below 60 % of maximum rating. Optimized soft switching and SiC technology minimize internal heat generation with efficiency ≥88 % to prevent thermal accumulation at ambient 250 ℃. Aggressive thermal derating ensures long‑term stability under maximum downhole temperature. 8. Full‑range dual protection with autonomous recovery. Ultra‑high speed hardware protection (<1 μs response) covers overvoltage, overcurrent, short circuit and overtemperature with non‑bypassable safety priority. Software protection enables flexible threshold configuration and fault recording. Transient faults auto‑recover after clearance; multi‑level watchdog prevents system lockup. Embedded high‑temperature non‑volatile memory stores operational and fault data for post‑run diagnostics with real‑time health telemetry to surface systems. This framework resolves critical downhole limitations regarding extreme thermal stability, cylindrical miniaturization, pressure resistance, shock immunity and low‑noise precision performance. High‑temperature flyback with SiC ensures reliable operation up to 250 ℃; 3D cylindrical stacking achieves ultra‑compact form factor within 40 mm diameter and 250 mm length at ≥150 W/in³; three‑stage pressure containment withstands 150 MPa; full vibration hardening survives 1,000 g impact; adaptive thermal control guarantees low drift and high efficiency. Widely applicable to LWD, MWD, formation testing and intelligent completion systems, it delivers core technical support for domestic HTHP downhole power localization and performance advancement in deep oil/gas exploration.