[Introduction]Over the years, Power Integrations (PI) has continuously brought many products for consumer electronics AC-DC to the market, with a considerable number and market share. Now, with its long-term commitment to high-efficiency power applications, PI has extended its AC-DC to automotive applications, especially light-duty electric vehicle high-voltage power architectures, which is a new application area for PI.
Mr. Jinguang Yan, Senior Technical Training Manager at PI, said: “We hope to take advantage of high volume delivery capabilities and high quality, low failure rate (PPM) products to achieve the safety and very high reliability requirements of automotive applications. Consumer electronics AC- DC’s application experience is applied to new fields, allowing automotive customers to use PI’s high-quality products with confidence.”
Several firsts for new products
Two automotive-grade high-voltage switching ICs with 1700V silicon carbide (SiC) MOSFETs integrated inside are part of the InnoSwitch™ 3-AQ (AQ stands for automotive-qualified product) product lineup, and are the industry’s first automotive-grade switching power supply ICs using SiC primary switching MOSFETs , not only can significantly reduce the number of components, but also greatly improve the efficiency of electric vehicles and industrial applications.
Two newly released devices (INN3947CQ and INN3949CQ) add AEC-Q100 qualified products to the InnoSwitch™ 3-AQ family, delivering up to 70W of output power, primarily for 600V and 800V pure battery and fuel cell passenger vehicles , can also be used in electric buses, electric trucks and various industrial power applications.
Not only is the new device the industry’s only AEC-Q100 qualified and highly integrated 1700V switch IC, but there is currently no comparable SSR (secondary feedback) solution on the market. PI is the only company that can not only do high voltage input and high bus voltage applications, but also realize the precise control scheme of secondary voltage.
Another first is that in all the previous series of PI products, the three letters of SiC have not been seen at all. Now the new product is PI’s first integrated silicon carbide device.
Why launch high voltage products?
Yan Jinguang said that the main purpose of the new products is the emergency power supply of electric vehicles, and the integration of 1700V silicon carbide MOSFET is mainly to comply with the current trend of higher and higher bus voltage of automobiles. These products are suitable for 600V to 1200V bus voltage automotive applications. The trend dictates.
He said that PI had previously launched the 900V withstand voltage InnoSwitch3-AQ, and its main application was electric passenger cars with a 400V bus voltage. As high-end luxury cars, such as Porsche, Audi, and Lucid Air use 900V bus voltage, the requirements for automotive-grade switching power supply ICs continue to increase. Since power is equal to voltage × current, under the same power, increasing the bus voltage can reduce the current, and reducing the current can reduce circuit losses. No matter how the internal current flows, it must be connected through conductors and wires, and the loss on the resistance is equal to I2×resistance value, so if the busbar voltage and current are increased under the same power, the current-related wire loss will be reduced. Thinner wires will reduce the weight and volume of the vehicle, which is very helpful for cruising range. The comparison shows that compared with 400V, the heat loss generated by using 900V bus voltage is 1/5 of the original, which greatly improves the efficiency of electric vehicles.
In addition, increasing the bus voltage can significantly shorten the battery charging time. The forecast a year and a half ago shows that about 20% of the cars will be changed to 900V bus voltage, and now about 50% or more of the new models are using 900V bus voltage. The future trend will even have electric vehicles with 1000V to 1200V bus voltage on the market.
How to meet the special requirements of emergency power supply?
Yan Jinguang pointed out that the emergency power supply has higher requirements on the withstand voltage of the switch tube, which is also the reason for the release of a 1700V silicon carbide MOSFET. In the emergency power supply, it is not only necessary to adapt to the higher input bus voltage, but also to ensure that it can work normally at 30VDC, which is the index requirement of the functional safety of the automotive emergency power supply. That is to say, under normal circumstances, the 800V bus powers the traction inverter, and the 12V low-voltage battery powers the control circuit of the traction inverter. Once the 12V battery fails, the emergency power supply will start to work. The purpose is to convert the 800V bus voltage. down to low voltage to power the traction inverter drive and other control circuits.
For the sake of safety, as a mobile vehicle, the car cannot cause safety problems due to the breakdown of the 12V battery in the event of a failure or a crash, so the car has very high requirements for safety and reliability. The input voltage of the emergency power supply has a wide adaptability range, and it is required to be able to provide the power required by the control circuit when the busbar voltage is lower than 60V.
InnoSwitch3-AQ IC adopts InSOP-24D package and common AC-DC flyback power supply topology. The voltage at both ends is connected to the battery bus, and its output is the traction inverter drive and other control circuits. In the traditional solution, for such a high input voltage (1200V bus voltage), the withstand voltage of the MOS tube should be increased accordingly. Therefore, for high bus voltage applications, StackFET is used, and another MOS transistor is superimposed on the low withstand voltage InnoSwitch3-AQ, in order to increase the withstand voltage through series connection. But to turn on the tubes above, some extra circuits are added. Now with the 1700V InnoSwitch3-AQ, there is no need to add these additional MOS tubes and corresponding drive circuits.
Compared with the previous StackFET architecture, the biggest difference is that the peripheral components are further reduced on the basis of the original simpler circuit.
What should be considered in AC-DC application design?
Returning to AC-DC applications, there are currently two bus voltages on the automotive market: 400V and 800V. 400V low-end vehicles are used more, it is the nominal voltage, and the lowest voltage may be 320V. Now, all electric vehicle manufacturers are trying to increase the voltage as much as possible, the purpose is to reduce the current and reduce the loss, the 400V range is between 380V and 450V, and the 800V maximum voltage will reach 950V.
How is this calculated? Yan Jinguang explained that taking the voltage of 800V as an example, when the electric vehicle brakes, the reverse electromotive force will appear at both ends of the bus; when the electric vehicle is charging, the charging voltage must be higher than the voltage at both ends of the battery, and generally a 15% margin should be left. , so 800V bus voltage × 15% equals 920V. When designing a flyback power supply, the influence of output reflected voltage and leakage inductance should also be considered, because in addition to the input voltage, the MOS tube must also withstand the reflected voltage on the transformer winding. The reflected voltage is generally designed according to the normal turns ratio, which may be around 120V. Therefore, when the MOS tube is turned off, the maximum value of 920V is added to 120V, and the voltage at both ends can reach 1040V.
A certain margin should be left in the design, such as 80% derating, taking the derating into account, the withstand voltage of 1040V should reach 1300V. Corresponding to the nominal 900V bus voltage, the actual voltage at both ends of the MOS tube will reach 1300V. Therefore, the previous solutions on the market often use automotive certified MOS tubes with a withstand voltage of 1500V for high input bus voltage applications.
PI now uses 1700V silicon carbide MOS tube, and there is no problem with the margin. Even if the bus voltage is increased to 1200V and 1500V in the future, the current 1700V silicon carbide MOS tube can be used. In the InnoSwitch3-AQ, there are two controllers on the primary side and two on the secondary side. The controller uses the secondary side controller as the main controller, that is, the turn-on of the primary MOS transistor comes from the instruction of the secondary side controller. After the primary side controller receives the secondary side control command, the MOS transistor is turned on. The turn-off after turn-on is controlled by the primary side. When the switch tube reaches a certain peak current, the primary-side MOSFET turns off.
To improve efficiency, there is a synchronous rectifier drive on the secondary side, which comes from the secondary side controller. If done with discrete components, there is often another independent synchronous rectifier controller on the secondary side. This kind of independent controller may have undesired driving pulses in some fault conditions, which may cause the switch to be turned on by mistake. PI uses the same controller to control the primary side power tube and the secondary side synchronous rectifier tube, so that these two tubes can be either one or the other, and there will be no common conduction phenomenon. This is an advantage over discrete components, and co-conduction is unacceptable in flyback power supplies.
FluxLink™ is almost standard
Synchronous rectification can improve efficiency, and the output is very accurate due to secondary-side voltage sensing. Of course, there is no need for an optocoupler between the primary and the secondary, but the FluxLink™ magnetic inductive coupling, which is available in many of PI’s previous products, is used for communication.
Yan Jinguang explained that in automotive applications, many car manufacturers do not like to use optocouplers, because optocouplers will change their transmission characteristics with the extension of temperature and service life. Its safety and reliability are difficult to meet applications such as automobiles with high safety requirements. Therefore, many schemes are done with PSR, that is, constant voltage on the primary side. In fact, the output voltage regulation accuracy of PSR is not so high, because it is detected by the bias winding on the primary side. When using SSR, the output voltage and current are directly detected on the secondary side. Compared with the working mode of the primary voltage regulator circuit, it can achieve higher high precision. The increase in accuracy provides significant cost savings for the overall system.
In the traditional way, if the voltage regulation accuracy is not high, a second-stage voltage regulator circuit will be added, which will not only increase the number of components, but also affect the overall power efficiency. In the application of increasing the busbar voltage, the use of FluxLink™ can fully guarantee the dielectric strength, meet the AEC-Q100 automotive grade certification, and integrate 1700V silicon carbide MOS transistors on the primary side.
In the discrete component scheme, some start-up resistors are sometimes added to control the start-up. Generally, the emergency power supply of the car is required to work below 60V, and the performance advantage of the new device is to support 30V start-up. Thanks to the InnoSwitch3 drain self-powered start-up, the full-load output power can be guaranteed at 30W to 70W.
The emergency power output is to supply power for the traction inverter drive and other control circuits behind, including active short-circuit circuits and some active discharge circuits. Because 800V only supplies power to the traction inverter bus (motor), but the driving voltage under the motor, including some voltages of the control circuit in the car, is relatively low, and it needs to be powered by the 12V bus; and when the 12V battery pack is broken, it must be powered by Emergency power supply. After using 1700V withstand voltage silicon carbide, the number of power components can be reduced by 50%, the area of the PCB board can be made small, and the reduction of the circuit space is conducive to the use of a smaller metal casing, reducing the weight of the vehicle and increasing the Endurance.
Unlike consumer electronics, automobiles are closely related to human life, and failure or unresponsiveness can cause life and property hazards. The reliability of automobiles has a great relationship with the number of components. If the number of components is large, the reliability will decrease. Designing an emergency power supply with fewer components can fully improve the safety and reliability of the automobile.
In terms of regulation rate, the output voltage can achieve an accuracy of plus or minus 2%, because the secondary side voltage detection accuracy is higher, which can save some post-stage DC-DC circuits, reduce costs and the number of components. The loop response speed of FluxLink™ is very fast, and the advantage is that a smaller output capacitor can be used at the output. Although the larger the output capacitance, the better the dynamic response, but increasing the capacitance will increase the PCB area and cost. Moreover, more than 90% efficiency can reduce heat generation, and high efficiency can eliminate the need for heat sinks. Adding heat sinks involves fixing and vibration, which will affect reliability, and the volume cannot be made small.
All car manufacturers have minimum efficiency requirements. The car does not always work under the maximum load. When the speed is slower, the load is lighter, so the light load efficiency also affects the endurance of the entire car. Depending on the control method, the new device can maintain constant power efficiency under different load conditions.
In addition, the car will also supply power if the bus is not turned on for a long time. If the power consumption is relatively high, the self-discharge of the battery will be very serious, which will affect the battery life. Therefore, the no-load power consumption of the power supply has an impact on the self-discharge of the battery. The no-load power consumption of the new device can be achieved less than 15mW.
From the system level, the DC-DC voltage regulator circuit in the rear stage is not needed with high voltage regulation accuracy. The three-phase traction inverter has 6 switching tubes, and the DC-DC circuit driven by the lower tube can be omitted. In the flyback power supply, multiple outputs can also be made to meet the different output voltage requirements of each subsystem.
see you late
Yan Jinguang said that during the vehicle design verification, all components connected to the high-voltage bus are required to undergo a long-term safety test, including open circuit and short circuit, because the bus voltage is very high, if there is a problem with the components connected to the bus voltage, the battery will be damaged. Short circuit, that would be disastrous. After the 1700V silicon carbide switch tube is used in the emergency power supply, the number of components in the solution is greatly reduced, and there are not many components connected to the bus voltage, which greatly shortens the verification time during automobile certification, and the protection features are also integrated inside the IC. , very popular with customers.
When customers of some electric vehicle design manufacturers first got the demo board, they didn’t quite believe that there was such a solution; after doing the test, they felt that it was really good, and they all felt like seeing each other too late. Because there are few products on the market that have both high withstand voltage MOS tubes and secondary side SSR voltage regulation adjustment. Now, two new models with rated withstand voltage of 1700V can output 50W and 70W different emergency power. The previous devices were available in 700V, 750V and 900V and can now cover all EV system voltages, be it 400V, 800V or future 1200V, 1500V bus voltages.
In addition to the product models that meet the automotive certification, PI also integrates 1700V silicon carbide MOS transistors in some non-automotive certified products, which are mainly used in industrial applications with high AC input voltage (600Vac to 1000Vac), such as robots, electric welding machines, three Phase meters and industrial motors.
The 1700V InnoSwitch3-AQ has two reference designs, DER-913Q-INN3947CQ and RDK-919Q-INN3949CQ, the former has an output power of 35W and the latter is 60W, using a planar transformer and an ordinary transformer design respectively. The efficiency of both designs is over 90%. The number of components is only 40, which is greatly reduced compared to the design of more than 100 components in the traditional solution.
The most important trend
In the past, when talking about the bus voltage of automobiles, 400V or 800V was often mentioned, which is mainly for common passenger cars. In fact, for high-horsepower buses and trucks, the bus voltage often used in older designs is 600V. These models tend to be used for longer periods of time, carry larger loads and require shorter charging times. For such high bus voltage applications, new devices are available.
The future trend is for passenger cars, buses or trucks to move towards higher bus voltages. Of course, the bus voltage can come from a pure battery or a fuel cell. In the past, high-end passenger cars only used high bus voltage. Now all vehicles will increase the bus voltage. It is the right time for PI to enter this market.
Source: Power System Design,
Author: Liu Hong, Editor-in-Chief of PSDC