Battery powered portable devices are more and more important in today's life. This trend also depends on the development of high-energy storage technologies, such as Li ion batteries and supercapacitors. These energy storage devices are connected to renewable energy systems (solar and wind energy) to collect and store energy and provide it to users stably. Some of these applications require fast charging or discharging.

Here we will introduce a bi-directional DC-DC converter, which allows the current generator to charge and discharge simultaneously. Bidirectional controller can provide excellent performance and convenience for vehicle dual battery system. Moreover, using the same circuit module in step-down and step-up mode can greatly reduce the complexity and size of the system, and even achieve 97% energy efficiency, and can control the two-way transmission of large current.

Electrical principle

Figure 1 shows a simple but fully functional electrical diagram with a symmetrical configuration that allows the user to choose four different modes of operation. It consists of four single phase quadrants of cascaded buck boost converter, including four switches, one inductor and two capacitors. Depending on the function of the electronic switch, the circuit can reduce or increase the input voltage. The switch module is composed of silicon carbide (SIC) MOSFET uf3c065080t3s, which can be replaced by other modules.

Figure 1: wiring diagram of bidirectional buck boost converter

Four operation modes

The user can simply configure four MOSFETs to determine the operation mode of the circuit, including the following four types:

The battery is located at the a end, the load is located at the B end, and the step-down is from a to B;

The battery is located at the a end, the load is located at the B end, and the step-up voltage is from a to B;

The battery is located at the B end, the load is located at the a end, and the step-down is from B to a;

The battery is located at the B end, the load is located at the a end, and the boost voltage is from B to a;

In this circuit, SiC MOSFET can operate in three different ways

On, positive voltage to ground;

Turn off, voltage is 0;

Pulsing, with square wave and 50% PWM; The frequency should be selected according to the specific operating conditions.

According to these standards, the function of SiC MOSFET follows the table shown in Figure 2

Figure 2: operation mode and function of four SiC MOSFETs

Mode 1: Buck a-b

Select mode one, the circuit as a buck, that is, the output voltage is lower than the input voltage converter. This circuit is also called "step-down", its voltage generator should be connected to the a side, and the load should be connected to the B side. The load efficiency depends on the MOSFET module 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.

Figure 3 shows the input and output voltages in Buck A-B mode; The input voltage is 12V and the output voltage is about 9V, so the circuit can be used as a buck. The switching frequency is 10kHz, the output load is 22ohm, and the power consumption is about 4W.

Figure 3: input and output voltages in Buck A-B mode

Mode 2: boost a-b

Mode 2 provides boost operation, that is, as a converter whose output voltage is higher than the input voltage. This circuit is also called "step-up". The voltage generator should be connected to the a side and the load should be connected to the B side. The load efficiency depends on the MOSFET module 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 10kHz square wave frequency.

Figure 4 shows the input and output voltage in boost A-B mode. The input voltage is 12V and the output voltage is about 35V, so the circuit can be used as a booster. The switching frequency is 10kHz, the output load is 22ohm, and the power consumption is about 55W.

Figure 4: input and output voltages in boost A-B mode

Mode 3: depressurization B-A

In mode 3, the circuit also works as a buck, that is, a converter whose output voltage is lower than the input voltage. The voltage generator is connected to the B side and the load is connected to the a side. The load efficiency depends on the MOSFET module. 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: switching at 100 kHz square wave frequency;

SW4: turn off, that is, turn off the switch.

Figure 5 shows the input and output voltages in Buck B-A mode. The input voltage is 24 V and the output voltage is about 6.6 V, so the circuit can be used as a buck. The switching frequency is 100kHz and the output load is 10ohm.

Figure 5: input and output voltages in Buck B-A mode

Mode 4: boost B-A

Select mode 4, the circuit operates as a booster, that is, the converter whose output voltage is higher than the input voltage. This kind of circuit is also called "step-up", its voltage generator should be connected to the B side, and the load should be connected to the a side. The load efficiency depends on the MOSFET module used. The specific configuration is as follows:

SW1: turn off, that is, turn off the switch;

SW2: switching at 100 kHz square wave frequency;

SW3: turn on, that is, turn off the switch (grid level power supply);

SW4: turn off, that is, turn off the switch.

The input and output voltages in boost B-A mode are shown. The input voltage is 18V and the output voltage is about 22V, so the circuit can be used as a booster. The switching frequency is 100 kHz, the output load is 22 ohm, and the power consumption is about 22W.

epilogue

The efficiency of the circuit depends on many factors. The first is the MOSFET on resistance RDS (on), which determines whether the current is easy to pass through (as shown in Figure 7). In addition, the circuit with four power switches needs careful safety inspection; If SW1 and SW2 (or SW3 and SW4) are on at the same time, it may cause a short circuit and damage the components.