Designing Safe and Fast DC Charging Stations with OptocouplersDecember 17, 2018 by Hong Lei Chen
Rapid growth of electric vehicle (EV) fleet drives strong demand for charging infrastructure to extend EVs’ travel range. DC fast charging stations can
Rapid growth of electric vehicle (EV) fleet drives strong demand for charging infrastructure to extend EVs’ travel range. DC fast charging stations can shorten the charging time from hours to minutes. In designing DC fast charging stations, one of the key aspects is electrical safety, which can be addressed by using optocouplers.
The grid transmits power in AC form, and energy stored in the onboard battery is in DC, therefore a charger is required to do the conversion job. Depending on whether the charger is installed inside of the vehicle or not, chargers can be categorized into the onboard charger (OBC) and off-board charging station. An OBC accepts AC power source from the mains supply and converts to DC to charge the battery, which is slow due to the limited power rating of the charger. DC charging is often used in off-board charging stations. It supplies regulated DC power directly to the batteries inside the vehicle. As the DC charging equipment is installed at fixed locations with a little constraint of size, its power rating can be as high as several hundreds of kilowatt. DC fast charging method shortens charging time from hours to minutes , . Figure 1 illustrates the AC and DC charging methods. DC fast charging is a key instrument in the successful roll-out of electric vehicles to reduce or eliminate range anxiety.
Designing a Charging Station with Safety Isolation
A DC fast charging station typically includes functional blocks such as AC to DC rectifier, power factor correction (PFC) stage, DC to DC conversion to regulate the voltage level suitable to charge the battery in the vehicle. Energy delivery and charger-vehicle communication are done through the charger coupler interface. Figure 2 shows a simplified block diagram of a DC charging station design. In this diagram, there is a safety isolation barrier designed in the functional blocks. This is important to ensure the design safety comply with regulatory standards.
Using Optocouplers in a PFC Stage
A power factor correction (PFC) stage is to transform the input current close to a sinusoidal waveform that is in phase with the grid voltage. This is to reduce the harmonics injected into the power grid and improve the power factor in order to comply with various standards. The PFC stage also generates a regulated DC output voltage to supply the downstream DC-DC converter. Figure 3 shows an example of interleaved PFC stage.
In this stage, MCU (micro-controller unit) alters PWM (pulse-width modulation) signals to switch the power MOSFETs or IGBTs on and off and the duration of each status according to the control algorithm. Gate drivers are used to amplifying the PWM signals with larger voltage and current magnitude in order to drive the power switching devices at the desired frequency.
Figure 4 shows an example gate drive circuit. In this circuit, the ACPLW349 features 2.5 Ampere output current, rail-to-rail output voltage range, 55 ns very short propagation delay time. Packaged in an SSO-6 small surface-mount device, this part has isolation voltage rating of 5000 Vrms for 1 minute per UL1577 standard, and 1140 Vpeak per IEC/EN/DIN EN 60747-5-5 standard. These standard approvals ensure the safety of the controller and the user side.
In the PFC stage, various voltage and current signals are required to implement the control algorithm. These include the rectified input voltage, the current of each of the interleaved phases, the total current and the DC bus capacitor voltage.
A typical method of measuring high voltage is to use a resistive potential divider to step down the voltage to a suitable level for a linear sensing chip to measure and send to the MCU. A current sensing circuit often employs a precision shunt resistor to convert the current to a small voltage, which is sent to the MCU via some signal conditioning devices. To transmit the signals accurately from high voltage areas such as the PFC and DC-DC converter stages to the low voltage MCU side, isolation amplifiers such as the ACPL-C87X series and ACPL-C79X series are handy to carry out the voltage and current sensing functions , .
Using the ACPL-C87X isolated voltage sensor is straightforward as shown in Figure 5. Given that the ACPL-C87X’s nominal input voltage for VIN is 2 V, choose resistor R1 according to R1 = (VL1-VIN)/ VIN×R2. The down-scaled input voltage is filtered by the anti-aliasing filter formed by R2 and C1 and then sensed by the ACPL-C87X. The isolated differential output voltage (VOUT+-VOUT-) is converted to a single-ended signal VOUT via a post-amplifier U2. VOUT is linearly proportional to the line voltage on the high voltage side and can be safely connected to the system microcontroller. With the ACPL-C87X typical gain of 1, the overall transfer function is simply VOUT = VL1/ (R1⁄R2+1) .
Using an isolation amplifier to sense current can be as simple as connecting a shunt resistor to the input and getting the differential output across the isolation barrier, as shown in Figure 6. By choosing an appropriate shunt resistor, a wide range of current, from less than 1A to more than 100A, can be measured. In operation, currents flow through the shunt resistor and the resulting analog voltage drop is sensed by the ACPL-C79X. A differential output voltage is created on the other side of the optical isolation barrier. This differential output voltage is proportional to the current amplitude and can be converted to a single-ended signal using an op-amp such as the post-amplifier shown in Figure 5, or sent to the controller’s analog-to-digital converter (ADC) directly .
Using Optocouplers in a DC/DC Converter
As shown in Figure 7, a DC/DC converter stage follows the PFC stage, providing a stable DC energy to be transferred directly to the battery. Output voltage and current need to be measured and fed back to the MCU for calculation, which will adjust PWM signals. These PWM signals will then control gate drive optocouplers to drive IGBTs or MOSFETs. In this stage, the galvanic isolation barrier shall be observed along the power transformer and the gate drive, voltage and current sense optocouplers. Refer to Figure 4, Figure 5 and Figure 6 for gate drive, voltage sense and current sense circuits.
Using Optocouplers in a Charger-Vehicle Interface
The advanced control scheme is necessary to implement a charging control protocol between the charging station and the EV. The most popular plug standard CHAdeMO (based on EV sales with fast charging type)  chooses CAN (Controller Area Network) for fast charging, recognizing its high communication reliability. The CHAdeMO standard provides a pair of CAN bus lines connecting the charger side and the vehicle side at the coupler interface. The coupler pin 8 and 9 are assigned as CAN-H and CAN-L, [7, p. “Technological details”] respectively, to which a CAN transceiver can be connected. Adding optical isolation between the CAN transceiver and CAN controller significantly improves system safety since optocouplers provide a safety barrier which prevents any damage from cascading to the system MCU. This arrangement also enables more reliable data communication in extremely noisy environments, such as high-voltage battery charging systems. Figure 8 shows how to use optocouplers to implement isolated CAN bus digital communication for fast charging station designs. A similar circuit is applicable to the vehicle side, where automotive-grade parts are required.
In the example circuit shown in Figure 8, a pair of 10 MBd fast optocouplers ACPL-W61L is used for data transmit and receive. This product requires 1.6 mA very low LED current to work and is delivered in an SSO-6 package that is less than half of the size of a traditional DIP-8 package. Although in a small package, the ACPL-W61L can withstand high voltage of 5000 Vrms for 1 minute, per UL1577 rating.
Designed to transmit signal in the presence of strong transient noises, this part guarantees common mode transient immunity of 35 kV/μs . In case of different design needs, other optocouplers can also be used in place of the ACPL-W61L. These include the 5 MBd-rated ACPL-W21L , and the 25 MBb dual-channel bidirectional ACSL7210 .
As one of the safety measures, it is required to include an insulation resistance monitoring function in the EV charging station . One of the possible implementations is shown in Figure 9. In this circuit, isolation amplifier ACPL-C87X measures voltage signal at its input and sends the output to MCU. The ASSR-601J consists of a LED input side and two discrete high voltage MOSFETs at the output side. In application, the two source nodes of the MOSFETs can be used as two contact points of a switch. They can withstand breakdown voltage of above 1500 V when in OFF mode. Both the ACPL-C87X and ASSR-601J use optical coupling technology to provide galvanic isolation while sending a signal across the isolation barrier, which is certified by IEC 60747- 5-5 with a working voltage of 1414Vpk .
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- Broadcom, “ACPL-W61L Ultra Low Power 10 MBd Digital CMOS Optocouplers,” Data Sheet AV02-2150EN, 2012.
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- Broadcom, “ASSR-601J 1500V High Voltage Photo-MOSFET Data Sheet,” 2017.
About the Author
Hong Lei Chen is a Senior Product Manager at Boardcom Limited. He has a Master of Science in Microelectronics.