このリファレンス・デザインには、高電圧バスからシャーシ・グランドへの絶縁抵抗を監視する機能があります。結合デバイスと部品の高電圧からシャーシ・グランドへの絶縁強度を監視することは、HEV や EV において必須の機能です。これは、バッテリ管理システム、トラクション・インバータ、DC/DC コンバータ、オンボード充電器、その他のサブシステムが高電圧 (60V を超える) で動作するためです。
TIDA-01513 | デザイン・フォルダ |
AMC1301-Q1 | プロダクト・フォルダ |
TPSI2140-Q1 | プロダクト・フォルダ |
OPA2348-Q1 | プロダクト・フォルダ |
OPA320-Q1 | プロダクト・フォルダ |
SN6501-Q1 | プロダクト・フォルダ |
TL4050B25-Q1 | プロダクト・フォルダ |
TPS763-Q1 | プロダクト・フォルダ |
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In response to the latest changes in global environmental conditions and to reduce greenhouse gases, there is a need to have hybrid or electric traction units, which have very low or zero emissions. In a hybrid electric vehicle (HEV) or electric vehicle (EV), high-voltage batteries are used as storage elements to power the wheels. High-voltage batteries for automotive systems are defined as those with ≥ 60 V. Onboard chargers or external DC converters are used to source the power. Meanwhile, high-voltage batteries are used to store that energy. DC/DC converters and motor control inverters are used to power the wheels and other subsystems such as heating, ventilating, and air conditioning (HVAC). All these subsystems are working on high voltage.
High-voltage components in HEV or EV systems are typically isolated from the chassis for functional and occupant safety reasons. The level of isolation in systems completely depends on the application, subsystem location within the vehicle, and the effective peak operating voltage. In general, HEVs and EVs use functional, basic, and reinforced isolation-based devices (see Figure 1-2). Functional isolation is used for protecting ground loops and the operation of those devices. Basic isolation is a single level of isolation which provides basic protection against electric shock. Reinforced isolation is a double level of isolation which provides higher protection against electric shock. Automotive power-train system developers should select basic or reinforced isolated components based on the voltage of the battery and peak voltages of the onboard charger and inverter.
Isolation is a critical parameter for the safety of HEV and EV systems. Due to many factors such as improper motor winding, deteriorating wiring harnesses, general aging, and power dissipation, the operating temperature and peak electrical stress on semiconductors may lead to degradation or loss of isolation in these HEV and EV systems. Any single point for failure of isolation loss does not have much impact on the operation of the system, but it does become a potential life risk when operators make contact with this high-voltage operating environment. Vehicle manufacturers need to have a mechanism to detect every single point failure of isolation in a complete system and have a necessary preventive action in place. Measure isolation resistance and insulation leakage currents to check the safety of occupants in the HEV or EV system. As per FMVSS 305 specification, at least 500 Ω/V of isolation resistance must be maintained from high-voltage systems to chassis ground. Depending on the leakage current measured, HEV/EV system error-handling functions may be designed to take appropriate actions.. Functions or systems will be built to disconnect high-voltage relays and discharge the DC-link capacitors.
Checking the leakage or low ohmic resistance paths from high-voltage nets to the low-voltage chassis ground is important. The necessary isolation resistance is calculated based on battery voltage, creating a isolation breakage path and monitoring the deflections as explained in this design guide. Based on the vehicle architecture, the number of sampling points for isolation leakage measurements varies.
PARAMETER | SPECIFICATIONS | MAXIMUM (MEASUREMENTS) |
---|---|---|
Voltage measurement accuracy | Measurements are done at room temperature. Deviations observed in measured values compared to calculated values. | 0.624% |
Isolated leakage current accuracy | 0.621% | |
ISO_POS accuracy | 0.48% | |
ISO_Neg accuracy | 0.126% |
The TPSI2140-Q1 is an isolated solid state relay designed for high voltage automotive and industrial applications. The primary side of the device consists of four differential drivers which deliver power and enable logic information to each of the internal MOSFETs on the secondary side. This device uses capacitive isolation technology in combination with its internal back-to-back MOSFETs to form a completely integrated solution requiring no secondary side power supply. When the enable pin is brought HI, the oscillator starts and the drivers send power and a logic HI across the barrier. When the enable pin is brought LO or the VDD falls below the UVLO threshold, the drivers are disabled. The lack of activity communicates a logic LO to the secondary side and the MOSFETs are disabled.
Key features include:
The AMC1301 device is a fully-differential, precision, isolated amplifier. The input stage of the device consists of a fully-differential amplifier that drives a second-order, delta-sigma (ΔΣ) modulator. The modulator uses the internal voltage reference and clock generator to convert the analog input signal to a digital bit stream. The drivers transfer (TX) the output of the modulator across the isolation barrier that separates the high-side and low-side voltage domains. The received bit stream and clock are synchronized and processed by a fourth-order analog filter on the low-side and presented as a differential output of the device.
Figure 2-4 shows the AMC1301-Q1 block diagram.
The SiO2-based, double-capacitive isolation barrier supports a high level of magnetic field immunity, as described in ISO72x Digital Isolator Magnetic-Field Immunity . The digital modulation used in the AMC1301 device and the isolation barrier characteristics result in high reliability and common-mode transient immunity (CMTI).
Key features include:
The SN6501-Q1 is a transformer driver designed for low-cost, small form-factor, isolated DC/DC converters using push-pull topology. The device includes an oscillator that feeds a gate drive circuit. The gate drive, comprising a frequency divider and a break-before-make (BBM) logic, provides two complementary output signals that alternately turn the two output transistors ON and OFF. The output frequency of the oscillator is divided down by an asynchronous divider that provides two complementary output signals with a 50% duty cycle. A subsequent BBM logic inserts a dead time between the high-pulses of the two signals. The resulting output signals present the gate-drive signals for the output transistors. As shown in Figure 2-5, before either one of the gates can assume logic high, there must be a short time period during which both signals are low and both transistors are high impedance. Known as BBM time, this short period is required to avoid shorting out both ends of the primary.
Key features include:
Isolation leakage measurements are typically performed in one of the subsystems in an HEV or EV system. Predicting the isolation breakage location is difficult and not possible to dousing isolated measurement techniques. The most effective way to measure isolated leakage current is by breaking the isolation of a complete system with a known resistance. If there is no current flowing between the switched path, then there is no parallel path which indicates that system is safe without any isolation breakages.
Designers must understand the type of failure and calculate the accurate isolation breakage parameters such as location (voltage) and resistance to classify the severity level. Isolation leakage resistance provides the information about the possible amount of leakage current for the second path, which can potentially electrocute the operator or passenger. To obtain a complete board diagnosis, break the isolation at two locations with known resistance paths.
Figure 2-6 is one of the examples of isolation breakage measurements using this reference design. S1 and S2 are relays used to switch the measurement paths. Rps1 and Rps2 are resistors used in the high-resistance path from the positive line whereas Rns1 and Rns2 are resistors used in the negative line. Rs1 and Rs2 are the series resistors used for isolation current measurements. An inverting op-amp configuration with VREF (bias supply) is used for the measurements.
During normal conditions when S1 is closed, no leakage enters the circuit because there is no closed path. The ideal condition is to set the VREF potential at Rps1 and Rps2 with respect to chassis ground. ISO_POS must still stay at VREF voltage when S1 is closed. In a practical circuit based on the type of op amp, its input differential voltage, and bias currents, variations will occur in ISO_POS voltage measurements. The circuit behavior is similar when only S2 is closed, as shown in Figure 2-7.
Figure 2-8 shows that, if both switches are closed, leakage current from a high-voltage battery flows using the chassis ground of the HEV or EV. Select resistors Rps1, Rps2, Rns1, and Rns2 such that they have very low leakage current (< 1 mA) in the chassis ground for the maximum battery voltage.
As per Equation 6, the VREF and resistors are fixed. Measure the ISO_POS with a 16-bit or 12-bit analog-to-digital converter (ADC) for better precision and resolution. Compare the measured value of ISO_POS with the known parameters of HV battery voltage and resistors used in the design. System calculations must have accurate voltage measurements. If the calculated value is beyond the tolerance level of the system, then consider it to be isolation breakage in the system. To minimize the error in the system, it is important to select high-precision metal electrode leadless face (MELF) resistors and op amps with low offset and bias currents. The designer can measure the offset voltage of the opamp and calibrate it in the system. Calibrating the input bias currents of opamps is difficult and they greatly influence measurements of the isolation leakage currents.
As stated in Section 1, there are multiple root causes for the isolation breakage in the system. If the isolation breakage happens at the positive line of a high-voltage system, the circuit behavior is as shown in Figure 2-9. Riso is the isolation resistance from the high-voltage positive line to chassis ground, it can be as low as mΩ to MΩ. To perform the safety analysis, the designer must first identify the resistance of the isolation breakage and location. When only S1 is closed, there is no closed path for the high-voltage battery. Only leakage current flows from the low-voltage system due to the reference bias potential on the high-voltage line. The leakage current is negligible when only S1 is closed because the reference voltage is low (< 5 V) and the resistors are quite high (Rps1 + Rps2 > 500 kΩ).
When only S2 is closed, the high-voltage battery is in series with Rns1, Rns2, Riso, and VREF. A significant leakage current flows from the high-voltage section to the chassis ground based on the battery voltage, Rns1, and Rns2. This is the actual leakage current from high-voltage to chassis ground, which can be measured at ISO_NEG.
Figure 2-10 shows an equivalent circuit for isolation leakage current measurements when both S1 and S2 are closed. Considering that precision components are used in the circuit, input impedance, bias currents, and offset voltages of op amps are neglected.
Leakage currents in Figure 2-10 can be calculated by using the superposition of voltage sources. With the same reference supply and no offset voltage in the op amps, the leakage currents due to the VREF supply is negligible and canceled in the circuit (because of symmetry). HV_BATT is the significant voltage source which contributes to leakage current in the chassis ground. If Riso is too low, then the I_Leak2 current shown in Figure 2-10 will be negligible. The key contributor variable leakage currents are the HV_BATT power source and isolation resistance. The measurements of ISO_N are significant because they can be used to find the leakage currents and isolation resistance when an isolation error occurs at a high-voltage positive terminal.
A similar analysis of the leakage currents is valid with the error at a negative potential. A couple of equations will change, but most of the theory remains the same. Measurements of ISO_POS are significant when an isolation error at the negative terminal of the high-voltage battery exists.
High-voltage measurement is required to calculate the isolation leakage current. In TIDA-01537 , the AMC1301-Q1 device is used to perform these measurements. The AMC1311B has a high input resistance, a 2-V input range, and can also be used for high-voltage measurements.
As Figure 2-11 shows, the Rsh monitoring resistor is placed in series to a high-ohmic potential divider network. Voltage measurements are performed with a floating ground of the AMC1301-Q1. The OPA320-Q1 device is used to amplify the signal range and give a single-ended output to an MCU or logic interface. The AMC1301-Q1 can measure a bidirectional signal of ±250 mV. In HEV or EV motors, battery voltages are only in the positive range, so the usable range of the AMC1301-Q1 is 250 mV. A potential divider network must be chosen in such a way that the voltage drop in shunt resistance must be ≤ 250 mV at the maximum battery voltages.
Use Equation 5 to calculate the high battery voltage.
where
Equation 5 is a simple equation that does not consider the influences of bias currents or offset voltages, which can lead to deviations in measurements.