Page 39 - PEN eBook October 2025

AEROSPACE DESIGN

Application of

Solid-State Relay

NSI7258 in Insulation

Monitoring for

High-Reliability BMS

By Kaijie Zheng and Ning Zhang, Novosense

The operating voltage of a battery management insulation monitoring circuit should promptly detect
system (BMS) generally exceeds the safe voltage the change and trigger an alarm.
threshold for the human body. A degradation in
insulation performance may lead to increased leakage The recommended insulation monitoring circuit
current, posing a threat to personal safety. The is illustrated in Figure 1. K and K represent the
1
2
insulation monitoring circuit described in this article main application positions of Novosense solid-state
utilizes the NSI7258 from Novosense to calculate and relays (SSRs) in the insulation monitoring circuit.
monitor the insulation resistance between the positive The chip adopts a low-side control approach, where
bus-to-ground and negative bus-to-ground of the the low-side reference ground is the vehicle chassis
battery, thereby achieving high-precision and ground. This circuit performs insulation resistance
high-reliability insulation monitoring. measurements (R , R ) every two to three seconds
n
p
by switching K and K , while K switching controls
1
3
2
PRINCIPLE OF THE SOLUTION whether the insulation monitoring circuit is active.
According to China’s national standard GB 18384, During insulation monitoring, K remains normally
3
“Electric vehicles safety requirements,” the insulation closed.
resistance must be greater than 500 Ω/V at the
maximum operating voltage. Therefore, for a system In this article, the bus voltage is defined as V BUS , the
with a bus voltage of V BUS , the insulation resistance positive bus-to-chassis ground voltage as V , and the
p
must be greater than 500 × V BUS to ensure personnel chassis ground-to-negative bus voltage as V .
n
safety. When the insulation resistance degrades, the
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