Digital isolation in hybrid and electric vehicles
Isolation requirements in transportation
Hybrid and electric drivetrains in cars, trucks and two-wheelers are introducing new, previously unknown challenges in the transportation industry. The 12V board net is now complemented with a 400V or higher battery and power system, which introduces a completely new set of requirements for the car OEMs and system module providers. An isolation need is present in all function of the HEV/EV like the high voltage battery, the DC/DC converter, the inverter for driving the electric motor, but also for the charger module connected to the 230V / 380V power grid - see figure 1.
Figure 1: Typical system architecture for electrical vehicles. For full resolution, click here.
At the same time, cost pressure in these areas will drive the need for higher system level integration, so a product roadmap with single-chip, isolated functions like CAN transceiver, ADCs or gate drivers is of advantage.
Types of digital isolation for transport applications
In principle, there are four different methods of digital isolation, optical, inductive, capacitive and RF, the first three are described below.
Optical isolation uses light transmitted via a transparent nonconductive isolation barrier. The digital signal is converted from the electrical into the optical domain by driving an LED (Light Emitting Diode). This optical signal is then transmitted via the isolation barrier and converted back into electrical signals with an optical detector element (photo diode, photo transistor).
The major advantage of optical isolation is the immunity of light to electric or magnetic fields and the possibility to transfer static signals. On the flipside, the optical isolators are limited in transmission speeds due to the comparably slow LED characteristic. For use in HEV/EV applications, one other main drawback is certainly the limited lifetime. The LED will become less efficient over time, which demands increasing signal drive currents (typically starting with 10mA) and over time make the isolator non functional.
Inductive isolation uses the variation of a magnetic field between two coils to communicate across the isolation barrier. One advantage of the inductive method is the difference between the common mode and differential transfer meaning a good immunity against noise. The disadvantage of this method is the potential distortion from magnetic fields, which are quite common for example in a motor control environment of an HEV/EV.
Capacitive isolation uses the change of electric fields across an isolation barrier. The benefits of the capacitive method are higher immunity to magnetic fields and long system lifetime. Also the transmission speed is similar to the inductive method.
Drawbacks of the capacitive method are the missing differential signal (meaning signal and noise share the same channel) and, as in the inductive method, no possibility to transmit static signals (have to be encoded with a clock signal first) directly.
Isolation product devices
Texas Instruments is using the capacitive isolation method for its ISOxxxx product families. A simplified block diagram of the ISO72xx system setup is shown in figure 2. The ISOxxxx devices are built in a single package out of two separate dies on a split lead frame with a transmit and a receive chip. The only connection between the two dies is via the bond wires. The actual isolation is on the receive chip using a silicon-dioxide (SiO2 - or simply 'glass') based capacitor with copper and doped silicon substrate electrodes – see figure 3. The use of SiO2 offers advantages for high reliability and long life.
Fig. 2: Block diagram of TI's ISO 72xx series. For full resolution, click here.
The two channels allow both, DC and AC communication, but are also used for failsafe functions.
The primary AC channel will use the input signal and transmit it (after filtering) via a differential pair of isolation capacitors. It is then detected by the Schmitt-Trigger inputs on the receive chip and made available via the output buffer. This allows a very high-speed transmission, low pulse-width-distortion and low propagation delays, but cannot be used for transmitting DC signals.
For this, the DC channel can be used to transmit DC (or very low speed signals) over the isolation barrier. The signal is encoded via the on-chip oscillator into a PWM signal and transmitted over the isolation barrier with differential signals similar to the AC channel. On the receive chip side, the signal is decoded and provided via the output buffer.
But the DC channel is also used for failsafe functionality. If, for example, the supply voltage on the Transmit side is insufficient, the oscillator will stop working meaning no carrier will be detected on the receive side, which will lead to failure indication and an output high. In normal operation (meaning sufficient data transmission density) the output multiplexer will ignore the DC channel, but in case of no transmission on the AC channel for around 4us, the DC channel will get priority. Once the AC signal has a transient, the multiplexer will immediate switch back to this channel.
There is a range of isolation devices available serving different configurations (single to quad). All of them offer 560V continuous insulation (4kV peak transients) at data rates of up to 150Mbps. What's more, these devices are automotive qualified.
Fig. 3:Die photo of TI's ISO72xx series with SiO2 isolation on the right side receiver chip. For full resolution, click here.
Reliability considerations and immunity against external fields
The harsh automotive and transportation environment combined with long lifetimes of the vehicles demands specific device characteristics. The MTTF (Means Time To Failure) is a standard method to determine the reliability semiconductor circuits. In the case of Isolation devices, it applies to both, the integrated circuit and the isolation mechanism. Applying a 90% confidence level and 125ºC ambient temperature, the MTTF for typical capacitive and inductive devices is more than 2,000 years and FITs (failures in 10exp9 hours) are below 60. The MTTF for typical optical devices is just 30 years and FITs are almost 4,000.
On the immunity side against magnetic fields, figure 4 shows a comparison of inductive versus capacitive devices. Both devices, the inductive and the capacitive ISO721, offer a high immunity against magnetic fields by far exceeding the IEC61000 standards. However, the ISO721 offers additional margin, which is especially important in the use of harsh automotive environments.
Fig. 4: Immunity against external magnetic fields.
To meet increasing cost pressures, it is certainly desirable to combine several functions of the system in dedicated semiconductor devices; this is not different in the isolation area. TI offers a range of devices that are all based on the capacitive isolation described previously. For communication purposes, the ISO1050 offers a fully integrated, isolated CAN transceiver. But also for driving the power devices in a system, the upcoming ISO55xx family will offer isolated gate drivers for up to 30V and currents of 2.5A with various functions like soft start and integrated IGBT protection.
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