SNVAAB6 Application note

SNVAAB6 March 2026 BQ76907-Q1 , LM5190-Q1 , TPS55287-Q1

3.2 Door Unlock Motor Parameters and Power Consumption Calculation

A typical door lock system includes at least four door locks, and depending on the vehicle type, it may also feature four door-handle locks and two safety locks. The motors used are brushed unlock motors operating in a single direction, and a half-bridge topology can be employed for the motor drive.

The maximum current of lock motors varies from 7A to 20A. The figure below depicts a typical profile of door unlock motor current over time, which is divided into four stages from startup to shutdown: an inrush current of 7A lasting approximately 3ms; normal motor operation at 5A lasting 60ms; a heavy-load current of 7.5A lasting 40ms; and a locked-rotor current of 8A lasting 150ms.

Figure 3-1 Diagram of Unlock Motor Current over Time

Taking the 5s supercapacitor as an example, the energy consumption and voltage variation during motor operation are calculated for a setup including four door locks and two safety locks, with an initial capacitance of 5F and a voltage of 12.5V. Since the motors are started sequentially and the voltage continuously drops during the unlocking process of each motor, for simplicity of calculation, the voltage is approximated as constant during a single motor's unlocking sequence (i.e., it does not fluctuate with actions like startup, rotation, or locked-rotor conditions). Additionally, the duty cycle of the motor's PWM control is approximated at 50%. The calculation process for one round of discharge is as follows:

$∆ W = U I \times t$	W	$U = \sqrt[]{2 W / C}$
	$390.63 J$	12.50V
$12.50 V \times 50 % \times (7 A \times 3 m s + 5 A \times 60 m s + 7.5 A \times 40 m s + 8 A \times 150 m s) = 1 1.38$	$37 9.25 J$	12.32V
$12.32 V \times 50 % \times (7 A \times 3 m s + 5 A \times 60 m s + 7.5 A \times 40 m s + 8 A \times 150 m s) = 1 1.21 J$	368.04J	12.13V
$12.13 V \times 50 % \times (7 A \times 3 m s + 5 A \times 60 m s + 7.5 A \times 40 m s + 8 A \times 150 m s) = 1 1.05 J$	356.99J	11.95V
$11.95 V \times 50 % \times (7 A \times 3 m s + 5 A \times 60 m s + 7.5 A \times 40 m s + 8 A \times 150 m s) = 1 0.88 J$	346.11J	11.77V
$11.95 V \times 50 % \times (7 A \times 3 m s + 5 A \times 60 m s + 7.5 A \times 40 m s + 8 A \times 150 m s) = 1 0.71 J$	335.40J	11.58V
$11.58 V \times 50 % \times (7 A \times 3 m s + 5 A \times 60 m s + 7.5 A \times 40 m s + 8 A \times 150 m s) = 1 0.54 J$	324.85J	11.40V

A single round of discharge for the six locks consumes a total of 65.78J, leaving a residual voltage of 11.40V after discharge. To satisfy redundant safety requirements, the system is generally required to support at least two rounds of discharge. Since the energy storage of the 3s and 5s topologies is 196.79J and 188.12J respectively, both can support at least two rounds of discharge.