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Power Budget — Ozone Generator Control Board
Status
Updated from the user's clarified input/load requirement on 2026-05-16.
The board is intended to operate from either:
Table
| VIN mode | Approx. input/load current | Approx. input/load power |
|---|---|---|
| 12 V DC | 3.5 A | 42 W |
| 24 V DC | 1.8 A | 43.2 W |
The J1 input and the VIN-powered field devices on J2 and J3 are intended to be matched to the selected input voltage. This means the switched loads are not fixed-voltage loads; they must be the 12 V version when VIN is 12 V and the 24 V version when VIN is 24 V.
Key sizing conclusion
The 12 V / 3.5 A case is the worst-case current case for connectors, fuse, D2, MOSFETs, VIN copper, and switched-load copper.
Using the 1.5x margin rule for protection and connector sizing:
I_DESIGN_CONTINUOUS_MARGIN = 3.5 A × 1.5 = 5.25 A
Therefore, the input path and any load path that may carry the full load current should be rated for at least 5.25 A continuous-equivalent margin, plus any startup/inrush or inductive transient margin.
Power tree
Table
| Stage | Source | Destination | Key parts | Notes |
|---|---|---|---|---|
| Raw input | J1 | F1 | J1, F1 | J1:P1 is raw 12 V or 24 V DC input; J1:P2 is GND |
| Protected input | F1/D2 | VIN / Net 3 | F1, D2, D1, C1/C2/C3/C4 | VIN feeds load connectors and buck regulator |
| Buck rail | VIN / Net 3 | Approx. 5 V / Net 42 | U2 TPS54531DDA, L1, D3, C5, feedback network | R2/R3 imply about 5 V if U2 reference is 0.8 V |
| Logic rail | Approx. 5 V / Net 42 | 3.3 V / Net 47 | U3 AMS1117-3.3, C13/C14/C18/C22/C23/C24 | Powers MCU, sensors, instrumentation amp, headers |
| Analog rail | 3.3 V / Net 47 | AVCC / Net 17 | L2, C15 | Filtered supply for MCU analog domain |
Known onboard loads on 3.3 V rail
Table
| Load | Qty | Rail | Current basis | Budget status |
|---|---|---|---|---|
| U1 ATMEGA328PB-AU | 1 | 3.3 V | MCU active current depends on clock, voltage, firmware, peripherals | Small compared with VIN load; still verify from firmware mode |
| U5/U6 2SMPP-03 pressure sensors | 2 | 3.3 V | Sensor ICC not extracted in this pass | Small compared with VIN load; verify from datasheet |
| U7 INA2331AIPWT | 1 | 3.3 V | Amplifier quiescent current not extracted in this pass | Small compared with VIN load; verify from datasheet |
| LEDs | 3+ | GPIO/3.3 V | Resistor-limited; approximate per LED depends on Vf and duty cycle | Small compared with VIN load |
| J6 external 3.3 V pin | 1 | 3.3 V | External load unknown | Must still be specified if J6 powers external hardware |
| J7 programming header VCC | 1 | 3.3 V | Normally reference/programming only | Usually low, but external programmer behavior should be confirmed |
High-current VIN loads
Table
| Connector | Positive side | Switched side | Switch | Protection | Current requirement |
|---|---|---|---|---|---|
| J2 | VIN / Net 3 | Net 29 | Q1 AO3400A through U4 gate driver | D5, D4 TVS, snubber-like network | Field device matched to 12 V or 24 V input; design switched path for up to 3.5 A continuous and 5.25 A margin unless actual branch current is lower |
| J3 | VIN / Net 3 | Net 14 | Q2 AO3400A from MCU PB0 | D6 flyback/clamp | Field device matched to 12 V or 24 V input; design switched path for up to 3.5 A continuous and 5.25 A margin unless actual branch current is lower |
| J4 | VIN / Net 3 | Net 34 | Q3 AO3400A from MCU PD7 | No dedicated clamp found in returned data | Current and load identity still not confirmed; add clamp if inductive |
Input path current sizing
The input connector, fuse F1, D2, VIN copper, and bulk capacitance must be sized for the complete input envelope:
I_INPUT_TOTAL_12V = 3.5 A continuous nominal
I_INPUT_TOTAL_24V = 1.8 A continuous nominal
I_INPUT_DESIGN_MARGIN = 5.25 A minimum continuous-equivalent target
This sizing target must be applied to:
- J1 input connector contacts and mating harness.
- F1 fuse/PPTC or replacement fuse.
- D2 input diode, if retained.
- Net 1 raw input copper.
- Net 2 fuse-to-diode copper.
- Net 3 protected VIN copper.
- Any switched-load path that may carry full branch current.
Critical component concern: F1 appears undersized
The current schematic uses F1 = SMD1812B075TF/33. The part number indicates a 0.75 A-class resettable fuse, which is not consistent with the clarified 3.5 A at 12 V / 1.8 A at 24 V input envelope.
Action required before manufacturing:
- Replace F1 with a fuse/protection device whose hold current supports the real continuous current and ambient derating.
- Target at least 5.25 A continuous-equivalent margin for the input path unless a different protection philosophy is intentionally chosen.
- Verify trip current, series resistance, voltage drop, inrush behavior, and thermal derating.
- Update the BOM and layout footprint if the replacement package changes.
D2 input diode check
D2 is SS54-HF, a 5 A Schottky diode family part. Even if average current rating is nominally near the design target, it still requires thermal verification:
P_D2 ≈ I_INPUT × V_F
At 3.5 A, even a 0.4 V forward drop dissipates about 1.4 W. This is a major thermal item. Consider a lower-loss ideal-diode/P-channel MOSFET reverse-protection stage if thermal rise is excessive.
MOSFET switch checks
For each switched load:
P_Q = I_LOAD^2 × RDS_ON
At 3.5 A, MOSFET dissipation rises quickly. The AO3400A devices must be checked at the actual gate voltage:
- Q1 is driven through U4 and may have stronger gate drive depending on U4 VDD.
- Q2 and Q3 are MCU-driven and likely see only 3.3 V gate drive.
Action required:
- Confirm AO3400A RDS(on) at the actual VGS used in this design.
- Confirm each branch current. If any branch can carry 3.5 A continuously, SOT-23 thermal margin may be tight.
- Consider a lower-RDS(on) MOSFET in a larger thermal package for sustained high-current outputs.
Buck regulator U2 input range
U2 TPS54531DDA is on the protected VIN rail and must tolerate both 12 V and 24 V operation plus transients. Verify:
- VIN maximum rating with expected supply transients.
- EN pin rating and UVLO behavior.
- Input capacitor voltage rating: use margin above 24 V, not just nominal 24 V.
- TVS clamp behavior relative to U2 maximum input voltage.
3.3 V LDO U3 thermal
U3 AMS1117-3.3 dissipates:
P_U3 = (V_5V - 3.3 V) × I_3V3_TOTAL
If Net 42 is about 5 V, each 100 mA on the 3.3 V rail produces about 0.17 W in U3. This is separate from the 42 W / 43.2 W VIN load envelope and should still be checked if J6 powers external 3.3 V hardware.
Remaining missing inputs
Table
| Missing item | Why it matters |
|---|---|
| Actual current split between J2, J3, and J4 | Determines MOSFET, clamp, connector, and switched-net trace sizing per branch |
| Whether J2/J3/J4 loads are inductive | Determines need for flyback/clamp/TVS/snubber |
| Expected 3.3 V external current from J6 | Determines U3 thermal margin |
| Ambient temperature and enclosure airflow | Determines regulator, diode, MOSFET, fuse thermal limits |
| Copper weight | Determines allowable current for high-current traces |
| Input supply transient profile | Determines TVS, capacitor voltage ratings, and U2 VIN margin |
Updated recommendations
- Replace or re-rate F1; the current 0.75 A-class PPTC is not suitable for a 3.5 A input envelope.
- Treat 12 V / 3.5 A as the current-sizing case and 24 V as the voltage-stress case.
- Rate J1, F1, D2, Net 1, Net 2, Net 3, and any full-current switched branch for at least 5.25 A continuous-equivalent margin.
- Verify D2 thermal dissipation or replace with lower-loss reverse-protection.
- Verify Q1/Q2/Q3 thermal dissipation at actual branch currents and gate drive voltage.
- Confirm J2 and J3 field devices are always voltage-matched to VIN before firmware enables them.
- If J4 is also a VIN-powered load, clarify whether it follows the same 12 V/24 V matching rule.
- Rename key power nets from auto-numbered names to explicit names: VIN_PROTECTED, 5V_BUCK, 3V3, AVCC, LOAD1_SW, LOAD2_SW, LOAD3_SW.
- Add a power-source annotation or regulator-output modeling fix for Net 47 to clear the U1:VCC missing-power ERC warning.
- Re-run DRC/ERC after routing and after power-net annotation.
Status
Key sizing conclusion
Power tree
Known onboard loads on 3.3 V rail
High-current VIN loads
Input path current sizing
Critical component concern: F1 appears undersized
D2 input diode check
MOSFET switch checks
Buck regulator U2 input range
3.3 V LDO U3 thermal
Remaining missing inputs
Updated recommendations
