[Introduction]The current sales trend in the global automobile market is still that traditional fuel vehicles account for about 80%, and the remaining 20% are occupied by new energy vehicles including various hybrid vehicles and electric vehicles. One of the most important market segments.
In fact, the mild-hybrid platform is already a concrete alternative to traditional vehicle architecture, as the mild-hybrid system addresses the need for a larger power reserve while reducing the overall cost of the powertrain. The mild hybrid system has the following advantages:
● Integrated drive motor, faster acceleration after stopping;
● Help the start-stop system to improve fuel efficiency;
● Turbocharging system reduces exhaust emissions;
● Safer than high-voltage bus solutions in the event of maintenance and failures
Among the STPOWER STripFET 80V-100V power transistors, the F7 series has obtained the AEC-Q101 vehicle-qualified product certification, and the new vehicle-qualified products have entered the prototype development stage. STripFET power transistors have excellent switching performance and energy efficiency, are robust enough to meet all automotive requirements, and are the right choice to address high frequency radiated immunity issues in 48V-12V DC-DC power converters.
DC-DC Conversion in Mild Hybrid Architectures
In mild-hybrid vehicles, the DC-DC converter transfers some of the energy stored in the 48V lithium battery to the 12V lead-acid battery, keeping the 12V lead-acid battery charged while powering low-power loads and infotainment systems. The converter also supports current flow in both directions, and in some cases, a 12V battery can charge a 48V lithium battery to drive the car to the nearest garage if it breaks down.
Common technical specifications of this converter are as follows:
The output power in buck mode is 2kW to 3.3kW, and the output power in boost mode is up to 1.5kW;
● The output current is about 250A;
● 12V – 14V output voltage;
● Input voltage 24V to 56V;
● Energy efficiency is higher than 93%.
Figure 1 shows the schematic of a multiphase DC-DC buck converter.
Figure 1: Schematic of a multiphase DC-DC buck converter
DC-DC converter A power Module, the components include:
● MOSFET half bridge (HB) and gate driver with an internal comparator for current detection;
● 48V lithium battery high-voltage safety switch, in the event of a fault, protects the electric drive system and disconnects the system from the lithium battery; the switch is usually composed of several 80V MOSFETs in parallel. Static drain-source on-resistance (RDS(on)) and switch tube with high current handling capability;
● The low-voltage safety switch, used to disconnect the system from the 12V battery, is composed of several parallel branches, including two on-resistance (RDS(on)) very low 40V MOSFET switch;
● The controller is responsible for the synchronization, activation and regulation of the phases according to the load level, and shuts down the system when the phase transition is dangerous;
● Provides more protection against single-phase overcurrent events and output overvoltage due to battery disconnection.
In this topology, the high-side (HS) MOSFET is optimized to improve the switching performance of the converter and reduce noise radiation, thereby improving the converter’s energy efficiency at light loads, while the low-side (LS) MOSFET It has also been optimized to minimize conduction losses, thereby increasing the converter’s energy efficiency at high loads.
Therefore, the main characteristics of the upper and lower side MOSFETs in the 48V-12V DC-DC converter can be summarized as follows:
● Drain-source breakdown voltage (BVDSS) between 80V and 100V;
● Gate threshold voltage (VGS(th)) is the standard voltage;
● High-side MOSFET static drain-source on-resistance (RDS(on)) is lower than 7.0mΩ, and the lower arm is lower than 3.5mΩ;
● The total gate charge of the high-side MOSFET (QG)very low;
● The reverse recovery charge of the low-side MOSFET (Qrr) is lower;
● STMicroelectronics’ PowerFLAT 5×6 package is used for paralleling multiple MOSFET switches, while the H2PAK package (2 leads or 6 leads) is used for a single switch.
The switching loss (PSW) of the high-side MOSFET is calculated using the following equation (Equation 1):
VINis the input voltage of the DC-DC converter;
IOUT is the load current;
fSWis the switching frequency of the converter;
QG,SW = Qgd + Qgsis the amount of charge required to turn on the MOSFET (is the gate-drain charge Qgd and gate-source charge Qgs Sum);
IGATE is the gate current of the MOSFET.
The conduction loss of the low-side MOSFET (PCOND) is calculated with the following formula (Equation 2):
RDSon[T] is the on-resistance of the MOSFET at operating temperature T;
ID is the MOSFET drain current;
D is the duty cycle of the converter.
The best compromise between minimizing switching losses and optimizing conduction losses is to choose a low on-resistance (R) for the low-side MOSFET.DS(on)) switch, the high-side MOSFET selects low gate leakage charge (Qgd) switch tube, this trade-off consideration has a great impact on improving system energy efficiency and suppressing electromagnetic interference (EMI) noise radiation.
Compared to the previous generation, STPOWER STripFET F7 series automotive 80V – 100V MOSFETs have improved Miller effect sensitivity and capacitance ratio (Crss /Ciss) softness, as shown in Figure 2.
Figure 2. Comparison of Capacitance Ratio Waveform Softness of F7 Series and Previous Generation
During the freewheeling phase, the drain-source voltage of the MOSFET (VDS) is fixed at the forward conduction voltage of the body-drain diode (VDS). During transient shutdown, VDSAs the voltage increases, the capacitance is more critical at this time. Since at almost VDS = 0V, the capacitance ratio declines softly and gently, and the initial value is low, Crss/CissThe ratio is moderately resistant to the Miller effect and also helps minimize subthreshold turn-on of the MOSFET, reducing susceptibility to EMI.
The main contributor to EMI emissions is related to the reverse recovery charge of the body-drain diode and its softness in springback (Figure 3).
Figure 3: Measured waveforms of the body diode of the F7 MOSFET
Experimental data show that the body-drain diode return-to-zero current waveform of the STripFET F7 MOSFET is very soft, producing only a few oscillations, thus limiting high-frequency radiation.
In addition, the prototype 80V – 100V MOSFET manufactured with the new technology has good switching performance, VDSThe voltage spike waveform is smooth and the oscillation time is very short. Figure 4 shows the low-side MOSFET and high-side MOSFET of the DC-DC converter of the mild-hybrid system at fsw = Switching waveform measured at 250kHz.
Figure 4: Switching waveforms of the new low-side MOSFET prototype and high-side MOSFET prototype
The figure above shows that the V in the low-side MOSFETDSThe maximum peak voltage is only 52V (dark blue line), and the measured energy efficiency of the DC-DC converter using the new MOSFET is 94%, as shown below (Figure 5).
Figure 5. Measured energy efficiency of a DC-DC converter using the new MOSFET prototype
The efficiency of the converter is also affected by the reverse recovery charge of the body-drain diode of the low-side MOSFET. The recovery current flowing to the body diode changes even at high speed (di/dt), and the new evolution technology can significantly improve the energy efficiency, as shown by the measured recovery waveform in Figure 6.
Figure 6. Recovery current measurement waveform of the new MOSFET prototype.
The actual current flowing through this diode is 10 times the current shown in Figure 6 because the current transformer is used to capture ISD = 60A current.
This recovery current also largely determines the noise radiation rate of the converter, as it is the main source of high frequency noise due to oscillations as the current returns to zero. A near-field noise radiation measurement test highlights the EMI immunity of the new MOSFET prototype and compares the noise radiation rate with the F7 series devices. When the device is switched at a current of about 15A, the experimental noise radiation spectrum of the low-side MOSFET in the half-bridge topology is shown in Figure 7, and the experimental noise radiation spectrum of the high-side MOSFET is shown in Figure 8.
Figure 7. Comparison of measured noise emission spectrum of low-side F7 and new MOSFET prototype
Figure 8. Comparison of measured noise emission spectrum of high-side F7 series and new MOSFET prototype
The two graphs show that the noise radiation rate values are higher at lower frequencies corresponding to the applied switching conditions, while at higher frequencies above 1MHz, the new product prototype of the high-side is on the same board and operating conditions (switching). frequency, current and bias voltage), the noise radiation rate is slightly lower than that of the same class F7 device.
For mild-hybrid systems, the new product prototype improves switching characteristics, reduces power losses, and ensures higher energy efficiency in DC-DC converters. Additionally, they retain the good performance of body-drain diodes, with reverse recovery charge and softness like STripFET F7 MOSFETs, helping to minimize high-frequency radiation, which is essential in any automotive power conversion and motor control topology. One of the desirable and widely recognized properties.
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 EA Jones, M. de Rooij and S. Biswas, GaN Based DC-DC Converter for 48V Automotive Applications, IEEE Workshop on Wide Bandgap Power Devices and Applications in Asia (WiPDA Asia), Taipei, Taiwan, 23-25 May 2019.
 N. Mohan, TM Undeland, WP Robbins, Power Electronics Converters, Applications and Design, 2nd edition J. Wiley & Sons NY 1995.
 S. Musumeci, A. Tenconi, M. Pastorelli, F. Scrimizzi, G. Longo, C. Mistretta, Trench-Gate MOSFETs in 48V Platform for Mild Hybrid Electric Vehicle Applications, AEIT International Conference of Electrical and Electronic Technologies for Automotive, Nov . 2020.
 E. Armando, S. Musumeci, F. Fusillo, F. Scrimizzi, Low Voltage Trench-Gate MOSFET Power Losses Optimization in Synchronous Buck Converter Applications, 21st European Conference on Power Electronics and Applications (EPE ’19 ECCE Europe), Sept. 2019 .
By Filippo Scrimizzi and Giusy Gambino, STMicroelectronics, Catania, Italy
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