Page 41 - PEN eBook October 2025

DESIGN DESIGN

considerations: The luminous intensity of LEDs will degrade over
R , R 2 1 MΩ time. According to service life predictions by major
1
▶ ADC input voltage consideration: The voltage optocoupler suppliers, after 1,000 hours of operation,
across R must always remain below the R , R 4 6 MΩ the threshold current will increase by 10%; after
3
5
maximum input voltage of the ADC, ideally close 11 years, the threshold current may rise by a factor
to its full-scale range to ensure high output R 5 12 kΩ of over 5. If the design strictly follows the threshold
accuracy of the ADC. Therefore, current specified in the datasheet, insufficient optical
R ÷ (R + R ) × V BUS should be near the ADC’s Table 2: Recommended resistor values for 800-V systems energy received by the photodiode will lead to
4
5
5
maximum input voltage, determining the ratio bit-error failures. The only solution is to increase the
between R and R . Relay selection input current to compensate for LED aging. However,
4
5
When the insulation resistance degrades below this introduces another challenge: Higher current
▶ ADC internal impedance consideration: Because 500 × V BUS , the system must detect this condition and means elevated temperatures, which degrade both
R serves as the ADC sampling input, its trigger an alarm. Taking a system with the LED’s electro-optical conversion efficiency and
5
resistance should be significantly lower than the V BUS = 800 V as an example, the insulation resistance the photodiode’s optical-to-electrical conversion
internal impedance of the ADC must be greater than 400 kΩ per industry standards. If performance. Additionally, due to thermal stress and
Figure 1: Recommended insulation monitoring circuit (e.g., R < R ADC ÷ 10) to ensure negligible effects the leakage current through K and K is less than 1 μA, aging, PhotoMOS devices are typically limited to a
2
1
5
from the ADC’s internal impedance. their equivalent impedance exceeds 800 MΩ, far higher maximum operating temperature of 85°C. These factors
When K is open and K is open, the voltage across R than the insulation resistances R and R , introducing pose significant reliability challenges for optocoupler
2
n
1
5
p
is V , then V = V × (R + R ) ÷ R , V = V BUS − V , and ▶ Impact on insulation resistance: During switching a measurement error of less than 0.05%. If the leakage products.
p0
5
n0
5
4
0
0
n0
Equation (1) is derived: of K and K , R and R are respectively connected current increases to 10 μA, the equivalent impedance
1
1
2
2
in parallel with R and R . If R and R are too drops to 400 MΩ, causing a measurement error of Replacing PhotoMOS relays (similar to optocouplers)
2
1
n
p
small, the measured insulation resistance values approximately 0.5%. with SSRs that adopt capacitive or magnetic isolation
(R // R and R // R ) may drop significantly. mechanism can significantly improve system reliability.
1
2
n
p
Therefore, higher leakage currents introduce larger The NSI7258 from Novosense is a high-reliability
▶ Detection accuracy consideration: As can be measurement errors. Using Novosense SSR products isolation device that meets the electromagnetic
When K is closed and K is open, the voltage across concluded from Equation (4), when R // R with small leakage current (<1 μA) to enable turn-on interference (EMI) CISPR25 Class 5 standard.
2
3
1
p
R is V , then V = V × (R + R ) ÷ R , V = V BUS − V , and degrades below the critical threshold and turn-off functions of K and K will help enhance Combining robust technological innovation with
1
1
4
1
n1
5
p1
5
n1
5
2
Equation (2) is derived: 500 × V BUS , the MCU should detect this measurement accuracy and avoid false alarms. automotive quality management systems, the NSI7258
degradation and trigger an alarm if offers a high-performance, high-reliability solution for
× R < 500 × V BUS . To compensate for Impact of relay reliability on system safety insulation monitoring systems.
2
ADC measurement accuracy errors, R should Traditional mechanical relays suffer from limited
2
be relatively smaller to allow sufficient voltage switching cycles. Once the maximum number of
differences between V and V . switching cycles is reached, the insulation monitoring
n2
p2
From Equations (1) and (2), we derive Equation (3): function fails, posing significant personal safety risks.
▶ Settle time consideration: In real-world As a result, mechanical relays have largely been made
applications, some customers connect a obsolete in this application.
Y-capacitor between chassis ground and battery
ground (parallel to R ), introducing RC charge/ Currently, traditional PhotoMOS relays are widely
n
When K is open and K is closed, the voltage across discharge behaviors. Smaller R and R reduce the used. In a PhotoMOS relay, when current flows through
2
2
1
1
R is V , then V = V × (R + R ) ÷ R and V = V BUS − V , settle time. However, excessively small values can the LED on the control side, the photodiode receives
n2
2
4
n2
p2
2
5
5
5
and Equation (4) is derived: cause too small equivalent parallel impedances the emitted light and generates current via the
−R // R // R and R // (R + R ) // R , potentially photoelectric effect, thereby controlling the conduction
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p
2
n
1
4
5
compromising personnel safety. of the high-side MOSFET.
Based on the analysis above, Tables 1 and 2 show the
From Equations (1) and (4), we derive Equation (5): recommendations for insulation monitoring circuits.
R , R 2 500 kΩ
1
Equations (3) and (5) are the results for insulation
resistance calculation. R , R 3 MΩ
3 4
COMPONENT SELECTION R 12 kΩ
In a typical circuit design, it is common to set 5
R = R < R = R , with the following design Table 1: Recommended resistor values for 400-V systems Figure 2: PhotoMOS operating principle Figure 3: NSI7258 EMI test results
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3
1
2
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