107 lines
4.8 KiB
Markdown
107 lines
4.8 KiB
Markdown
# Ignition Output Module
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## Purpose
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This module drives the existing dual-ended, two-pin dumb ignition coil. The
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coil primary is battery-fed and switched on its low side by an STMicroelectronics
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VBG08H-E smart ignition IGBT (`VBG08HTR-E` for tape-and-reel assembly).
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The VBG08H-E integrates the ignition IGBT, high-voltage clamp, coil-current
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limit, thermal protection, soft shutdown, and a coil-current threshold flag.
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Its 8 A maximum operative coil-current rating is compatible with V1 only after
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the final coil's primary characteristics and required dwell current are
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confirmed.
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## Power path
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- `HVC` connects to one coil-primary terminal; the other terminal connects to
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`VBAT_PROT`.
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- `VS` is supplied from `VBAT_PROT`. Place a 100 nF ceramic and 10 µF local
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capacitor at `VS`, following the VBG08H-E reference circuit.
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- `VBAT_PROT` protection must maintain the VBG08H-E within its 5.4-28 V
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operating supply range during normal operation and expected local transients.
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- Do not place a conventional flyback diode across the coil primary. The
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VBG08H-E's internal high-voltage clamp provides the rapid primary-current
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collapse required to generate a spark.
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## Command and inhibit path
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`INP` is active high: a rising edge begins coil dwell and a falling edge
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causes the normal spark event.
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The VBG08H-E input-high requirement is 3 V. A 5 V buffer is used rather than
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direct STM32 drive to preserve high-level margin across MCU output tolerance,
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temperature, and ignition noise.
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`EN` is active low:
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```text
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SAFE_IGNITION_INHIBIT = NOT(ENGINE_PERMIT)
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```
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When `ENGINE_PERMIT` is removed by the two-channel deadman interlock or the
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MCU, `SAFE_IGNITION_INHIBIT` becomes high. If a dwell is active, this invokes
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the VBG08H-E soft shutdown so coil current is removed without intentionally
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generating a spark. The 1 kOhm resistors on `INP` and `EN` follow the
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manufacturer's reference circuit and limit transient current into the device's
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control pins.
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The command traces shall be short, have a continuous quiet reference, and be
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routed away from the `HVC` switching node and coil connector. The VBG08H-E
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already includes input filtering; no external RC filter shall be added until
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its timing effect has been analysed.
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Fit a low-capacitance Zener clamp directly at each `INP` and `EN` power-IC pin,
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on the VBG side of its 1 kOhm series resistor. Each clamp returns to the VBG
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local Kelvin-ground reference at the `IGN_PGND` joining point, not to remote
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`DGND`. The clamp voltage shall tolerate the valid 5 V logic high while
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limiting positive transients before the VBG08H-E input-overvoltage threshold.
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On `EN`, the protection network must preserve the fail-safe high/inhibit state
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and must not create a path that can pull `EN` low during a fault.
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## Current-flag diagnostic
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`C.F.` is an open-drain output. It is low while coil current is below its
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internal threshold and releases when the current reaches the nominal 6.5 A
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threshold. The threshold is not externally programmable.
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The pull-up voltage must not exceed 5.5 V; `+3V3_MAIN` is suitable. The signal
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is a diagnostic confirmation of primary-current buildup and is not part of the
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hardware ignition shutoff path. Firmware shall use it to detect and report an
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under-current/late-current condition while retaining a separately defined
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maximum dwell time.
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## Ground and layout
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Define `IGN_PGND` as the ignition module's high-current return domain.
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- `PGND1` and `PGND2` must use equal-length, equal-width, and similarly
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shaped copper paths to the `IGN_PGND` joining point. The VBG08H-E uses these
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paths in its internal current-sensing scheme; imbalance shifts its current
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limit and current-flag threshold.
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- The VBG08H-E `GND` pin connects locally to that same joining point through a
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short, separate Kelvin/reference trace. It does not carry coil current.
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- Do not separately connect the VBG08H-E `GND` pin to the remote `DGND` plane.
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`IGN_PGND` and `DGND` meet only at the deliberate power-entry ground star,
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preventing parallel ground-return paths.
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- Keep the coil-current loop compact and isolate its routing from MCU, ADC,
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trigger-input, and communication traces.
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## Remaining validation before schematic freeze
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1. Identify the coil and measure/obtain its primary resistance, inductance,
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operating current, and permitted dwell characteristics.
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2. Confirm that worst-case stored energy and current remain within the
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VBG08H-E repetitive operating limits at the 16.8 V battery maximum.
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3. Verify thermal performance with the selected PCB copper area and enclosure.
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4. Validate spark noise, `C.F.` behavior, input margins, and the effective
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engine-permit shutdown behavior on hardware.
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