“With the widespread use of high-efficiency energy accumulators such as batteries and supercapacitors, better current control is a trend. Introduced today is a bidirectional DC-DC converter whose bidirectionality allows the current generator to have both charging and discharging capabilities. Bi-directional controllers can provide outstanding performance, convenience, and longevity for automotive dual-battery systems. Moreover, using the same circuit block in buck and boost mode greatly reduces the complexity and size of the system, and even achieves up to 97% energy efficiency and can control the maximum current delivered in both directions.
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With the widespread use of high-efficiency energy accumulators such as batteries and supercapacitors, better current control is a trend. Introduced today is a bidirectional DC-DC converter whose bidirectionality allows the current generator to have both charging and discharging capabilities. Bi-directional controllers can provide outstanding performance, convenience, and longevity for automotive dual-battery systems. Moreover, using the same circuit block in buck and boost mode greatly reduces the complexity and size of the system, and even achieves up to 97% energy efficiency and can control the maximum current delivered in both directions.
Electrical Principles
Figure 1 shows a simple but fully functional electrical diagram with a symmetrical configuration that allows the user to select four different operating modes. It consists of four single-phase quadrants of cascaded buck-boost converters, including four switches, one Inductor, and two capacitors. Depending on the function of different Electronic switches, the circuit can step down or step up the input voltage. The switching element is composed of silicon carbide MOSFET RSM065030W, of course, other devices can also be used instead.
Figure 1: Bidirectional Buck-Boost Converter Wiring Diagram
Four working modes
Users can simply configure four MOSFETs to determine the working mode of the circuit, including the following four:
・ The battery is at the “A” end, the load is at the “B” end, and the voltage is stepped down from “A” to “B”;
・The battery is at the “A” end, the load is at the “B” end, and the voltage is boosted from “A” to “B”;
・ The battery is at the “B” end, the load is at the “A” end, and the voltage is stepped down from “B” to “A”;
・The battery is at the “B” end, the load is at the “A” end, and the voltage is boosted from “B” to “A”;
In this circuit, the silicon carbide MOSFET can work in three different ways:
・Conducting, positive voltage to ground;
・Turn off, the voltage is 0;
・ Pulsing, with square wave and 50% PWM. Its frequency should be selected according to the specific working conditions.
Based on these criteria, the functionality of the SiC MOSFET follows the table shown in Figure 2.
Mode 1: Buck (Buck) AB
Selecting mode one, the circuit will operate as a step-down, a converter with an output voltage lower than the input voltage. This circuit is also called “step-down”. Its voltage generator needs to be connected to the A side, and the load is connected to the B side. Load efficiency depends on the MOSFET device used. The specific configuration is as follows:
・ SW1: switch at 10 kHz square wave frequency;
・ SW2: Turn off, that is, turn off the switch;
・ SW3: Turn off, that is, turn off the switch;
・ SW4: Turn off, that is, turn off the switch.
The graph in Figure 3 shows the input and output voltages in Buck AB mode. Its input voltage is 12 V and the output voltage is about 9 V, so the circuit can be used as a step-down. The switching frequency is selected as 10 kHz, the output load is 22 Ohm, and the power consumption is about 4W.
Figure 3: Input and output voltages in Buck AB mode
Mode 2: Boost AB
Mode two provides boost operation, that is, as a converter with an output voltage higher than the input voltage. This circuit is also called “step-up”. The voltage generator needs to be connected on the A side and the load on the B side. Load efficiency depends on the MOSFET device used. The specific configuration is as follows:
・ SW1: turn on, that is, turn off the switch (gate power supply);
・ SW2: Turn off, that is, turn off the switch;
・ SW3: Turn off, that is, turn off the switch;
・ SW4: Switch at a 10 kHz square wave frequency.
The graph in Figure 4 shows the input and output voltages in Boost AB mode. Its input voltage is 12 V and the output voltage is about 35V, so the circuit can be used as a booster. The switching frequency is selected as 10 kHz, the output load is 22 Ohm, and the power consumption is about 55W.
Figure 4: Input and output voltages in Boost AB mode
Mode 3: Buck BA
In mode three, the circuit also works as a buck, a converter whose output voltage is lower than the input voltage. Its voltage generator needs to be connected to the B side, and the load is connected to the A side. Load efficiency depends on the MOSFET device used. The specific configuration is as follows:
・ SW1: Turn off, that is, turn off the switch;
・ SW2: Turn off, that is, turn off the switch;
・ SW3: switch at 100 kHz square wave frequency;
・ SW4: Turn off, that is, turn off the switch.
The graph in Figure 5 shows the input and output voltages in Buck BA mode. Its input voltage is 24 V and the output voltage is about 6.6V, so the circuit can be used as a step-down. The switching frequency is chosen to be 100 kHz and the output load is 10 Ohm.
Figure 5: Input and output voltages in Buck BA mode
Mode 4: Boost BA
In mode four, the circuit works as a booster, that is, a converter whose output voltage is higher than the input voltage. This circuit is also called “step-up”. Its voltage generator needs to be connected to the B side, and the load is connected to the A side. Load efficiency depends on the MOSFET device used. The specific configuration is as follows:
・ SW1: Turn off, that is, turn off the switch;
・ SW2: switch at 100 kHz square wave frequency;
・ SW3: turn on, that is, turn off the switch (gate power supply);
・ SW4: Turn off, that is, turn off the switch.
The graphs in Figure 6 show the input and output voltages in Boost BA mode. Its input voltage is 18V and the output voltage is about 22V, so the circuit can be used as a booster. The switching frequency is selected as 100 kHz, the output load is 22 Ohm, and the power consumption is about 22W.
Figure 6: Input and output voltages in Boost BA mode
in conclusion
The efficiency of a circuit depends on many factors, starting with the on-resistance Rds(on) of the MOSFET used, which determines how easily current can flow (see Figure 7). Also, this circuit with four power switches requires serious safety checks. If SW1 and SW2 (or SW3 and SW4) are on at the same time, a short circuit may result, which could damage the device.
Figure 7: Pulse voltage and current curves on the inductor in Boost AB mode
Risen-Silicon Carbide MOSFET Selection
・ Benchmarking Cree, Rohm, ST;
・ Has successfully mass-produced 650V, 1200V, 1700V;
・It is fabricated by channel self-alignment process, and the device has excellent consistency;
・Competitive Ronsp, compared with the 1st generation product, the Ronsp is reduced by 20%, which is also competitive with the 2nd generation product;
・ Product specification: 650V-1700V 30mΩ-1Ω;
・ Application areas: solar inverter, high voltage DC/DC converter, UPS, new energy vehicle charging pile.
Risen semiconductor
REASUNOS is a national high-tech enterprise dedicated to the R&D, sales, technical support and service of power semiconductor devices. The members of the R&D team are mainly from the top technical elites and well-known universities in the industry. Existing product lines include power management ICs, silicon-based power devices, silicon-based ESD protection devices, and silicon-carbide-based power devices (SiC diodes and SiC MOSs). After years of technology accumulation and market development, Risen Semiconductor has become a long-term partner in the global switching power supply, green lighting, motor drive, digital home appliances, security engineering, photovoltaic inverter, 5G base station power supply, new energy vehicle charging pile and other industries. .
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