NeoECU-Hardware/Architecture/IO_MODULES/IGNITION.md

Ignition Output Module

Purpose

This module drives the existing dual-ended, two-pin dumb ignition coil. The coil primary is battery-fed and switched on its low side by an STMicroelectronics VBG08H-E smart ignition IGBT (VBG08HTR-E for tape-and-reel assembly).

The VBG08H-E integrates the ignition IGBT, high-voltage clamp, coil-current limit, thermal protection, soft shutdown, and a coil-current threshold flag. Its 8 A maximum operative coil-current rating is compatible with V1 only after the final coil's primary characteristics and required dwell current are confirmed.

Power path

VBAT_PROT --> coil primary --> VBG08H HVC
                                 |
                              internal IGBT
                                 |
                         PGND1 + PGND2 --> IGN_PGND --> power-entry ground

• HVC connects to one coil-primary terminal; the other terminal connects to VBAT_PROT.
• VS is supplied from VBAT_PROT. Place a 100 nF ceramic and 10 µF local capacitor at VS, following the VBG08H-E reference circuit.
• VBAT_PROT protection must maintain the VBG08H-E within its 5.4-28 V operating supply range during normal operation and expected local transients.
• Do not place a conventional flyback diode across the coil primary. The VBG08H-E's internal high-voltage clamp provides the rapid primary-current collapse required to generate a spark.

Command and inhibit path

INP is active high: a rising edge begins coil dwell and a falling edge causes the normal spark event.

STM32 ignition timing output
  --> 3.3 V-to-5 V non-inverting buffer --> 1 kOhm --> INP
                                                     |
                                              low-capacitance
                                                Zener clamp
                                                     |
                                      VBG local Kelvin-ground reference

SAFE_IGNITION_INHIBIT
  --> 1 kOhm --> EN
                  |
           low-capacitance
             Zener clamp
                  |
   VBG local Kelvin-ground reference

The VBG08H-E input-high requirement is 3 V. A 5 V buffer is used rather than direct STM32 drive to preserve high-level margin across MCU output tolerance, temperature, and ignition noise.

EN is active low:

SAFE_IGNITION_INHIBIT = NOT(ENGINE_PERMIT)

When ENGINE_PERMIT is removed, SAFE_IGNITION_INHIBIT becomes high. If a dwell is active, this invokes the VBG08H-E soft shutdown so coil current is removed without intentionally generating a spark. The 1 kOhm resistors on INP and EN follow the manufacturer's reference circuit and limit transient current into the device's control pins.

The command traces shall be short, have a continuous quiet reference, and be routed away from the HVC switching node and coil connector. The VBG08H-E already includes input filtering; no external RC filter shall be added until its timing effect has been analysed.

Fit a low-capacitance Zener clamp directly at each INP and EN power-IC pin, on the VBG side of its 1 kOhm series resistor. Each clamp returns to the VBG local Kelvin-ground reference at the IGN_PGND joining point, not to remote DGND. The clamp voltage shall tolerate the valid 5 V logic high while limiting positive transients before the VBG08H-E input-overvoltage threshold. On EN, the protection network must preserve the fail-safe high/inhibit state and must not create a path that can pull EN low during a fault.

Current-flag diagnostic

C.F. is an open-drain output. It is low while coil current is below its internal threshold and releases when the current reaches the nominal 6.5 A threshold. The threshold is not externally programmable.

VBG08H C.F. --> reference-style R/C filter and 3.3 V pull-up --> STM32 GPIO

The pull-up voltage must not exceed 5.5 V; +3V3_MAIN is suitable. The signal is a diagnostic confirmation of primary-current buildup and is not part of the hardware ignition shutoff path. Firmware shall use it to detect and report an under-current/late-current condition while retaining a separately defined maximum dwell time.

Ground and layout

Define IGN_PGND as the ignition module's high-current return domain.

PGND1 -- matched heavy copper --+
                                +--> IGN_PGND local joining point --> power-entry ground
PGND2 -- matched heavy copper --+
                               |
VBG08H GND -- short, separate Kelvin reference trace --+

• PGND1 and PGND2 must use equal-length, equal-width, and similarly shaped copper paths to the IGN_PGND joining point. The VBG08H-E uses these paths in its internal current-sensing scheme; imbalance shifts its current limit and current-flag threshold.
• The VBG08H-E GND pin connects locally to that same joining point through a short, separate Kelvin/reference trace. It does not carry coil current.
• Do not separately connect the VBG08H-E GND pin to the remote DGND plane. IGN_PGND and DGND meet only at the deliberate power-entry ground star, preventing parallel ground-return paths.
• Keep the coil-current loop compact and isolate its routing from MCU, ADC, trigger-input, and communication traces.

Remaining validation before schematic freeze

1. Identify the coil and measure/obtain its primary resistance, inductance, operating current, and permitted dwell characteristics.
2. Confirm that worst-case stored energy and current remain within the VBG08H-E repetitive operating limits at the 16.8 V battery maximum.
3. Verify thermal performance with the selected PCB copper area and enclosure.
4. Validate spark noise, C.F. behavior, input margins, and the effective engine-permit shutdown behavior on hardware.