There are many main circuit structures for step-down DC/DC. Among them, the BUCK type DC/DC has become one of the preferred conversion circuit topologies due to its simple structure and high conversion efficiency.
DC/DC is generally composed of control chips, inductors, diodes, transistors, and capacitors. The topological diagram of the basic step-down Buck DC/DC circuit is shown in Figure 1, Uin is the input voltage, U. Is the output voltage, Cin is the input capacitor, S is the main power switch tube, VD is the main power diode, and L is the energy storage inductor coil.
The working process of the basic step-down Buck DC/DC circuit is as follows: When the switch S is turned on, the current flows through the load and the inductor L through S and increases linearly, and the electrical energy is stored in the inductor L in the form of magnetic energy, and at the same time, the load is supplied with power , The capacitor Cin, the load L, and S form a loop. At this time, the anode of the diode VD is connected to the negative, and VD is in the cut-off state. When S turns from on to off, the energy stored in the inductor is released and maintained by VD freewheeling Supply power to the load, L, VD and the load form a loop. If the switch tube S is periodically controlled to turn on and off, the energy can be transferred from Uin to U0. The output voltage of the circuit U0=δUin (δ is the switch The duty cycle of the conduction channel of the tube S). In order to achieve the above-mentioned step-down transmission, the open tube S and the diode VD must be turned on and off in turn, with frequent commutation between the two.
In fuel cell electric vehicles (FCV), fuel cells only generate electricity from fuel, but cannot store electricity, so unidirectional DC/DC is used. The power supply used by FCEV has its own characteristics. The fuel cell only provides direct current, and the voltage changes with the output current. It is impossible for the fuel cell to accept the charging current of the external power supply and only flow in one direction. The auxiliary power source (battery and super capacitor) used by FCEV also flows in the form of direct current during charging and discharging, but the current can flow in the reverse direction.
The voltage and current of various power sources on FCEV are unstable due to changes in working conditions. In order to meet the voltage and current requirements of the drive motor and the control of the multi-power supply power system, between the power supply and the drive motor, a computer is used to realize the comprehensive control of the FCEV’s multi-power supply to ensure the normal operation of the FCEV. FCEV fuel cells need to be equipped with unidirectional DC/DC, and batteries and supercapacitors need to be equipped with bidirectional DC/DC.
(1) Full bridge DC/DC for fuel cell
The output voltage of the fuel cell is generally 240~450V, and the output voltage of the fuel cell decreases as the output current of the fuel cell increases. In addition, because the fuel cell cannot be charged, a one-way full bridge DC/DC is configured to convert the fluctuating current of the fuel cell into a stable and controllable direct current. The full-bridge DC/DC input end uses four conduction switch tubes and four rectifier diodes to form a high-power DC converter (IGBT), the middle part is a high-frequency transformer Tr, and the output end uses four rectifier diodes to form a rectifier. The principle of the insulated full-bridge DC/DC converter circuit is shown in Figure 2.
When the switch tube VT1 is turned on first, the switch tube VT4 is turned on after a certain α potential angle is delayed, and VT2 and VT3 are stopped. VT1 and VT4 alternately conduct 180° potential angle. At this time, the voltage U1=Uin. Then the switch tube VT2 is turned on first, and after a certain potential angle is delayed, the switch tube VT3 is turned on, and VT1 and VT4 are turned off, and VT2 and VT3 are turned on at a potential angle of 180° in turn. At this time, the voltage U1=-Uin. When the four switch tubes are controlled to conduct, alternating voltage and current will be generated, and an alternating square wave voltage and current can be obtained at two points A and B.
A capacitor C2 is connected in series with the primary side circuit of the AC square wave voltage to prevent the magnetic eccentricity of the transformer, and then the AC square wave voltage U1 is input to the primary side of the transformer Tr, and the transformer adjusts the output voltage U0 by adjusting the duty cycle to control and maintain The stability of the two side output voltage U0. The rear of the secondary side is connected with a four-tube rectifier, and a DC voltage can be obtained at two points of C and D after rectification. A filter composed of an inductor Lf and a capacitor Cf is added to the C and D circuits to filter out the high frequency components in the DC square wave voltage to obtain a flat DC voltage.
As long as the conduction time is changed, the value of the output voltage U0 can be adjusted, and the intelligently controlled high-power full-bridge DC/DC converter can be selected, which can have good self-protection ability and service life.
The external characteristics of DC/DC are shown in Figure 3, and the control block diagram of a single DC/DC is shown in Figure 4. According to FCEV’s power performance design requirements, determine the given value of DC/DC output voltage. When the fuel cell current gradually increases, the voltage remains basically stable. Through the closed-loop control of the output voltage, a constant voltage output of DC/DC is realized (see section AB in Figure 3). When the fuel cell current continues to increase and the voltage drops rapidly, the DC/DC constant power output is realized by controlling the output power (see section BC in Figure 3). Since the voltage of the fuel cell is affected by the reaction temperature, pressure and environment when it reaches the lower limit, the power of the BC section in Figure 3 cannot be given in advance, but the output voltage and current through the fuel cell at this time Measure and adjust the output power of DC/DC in real time, which is a key measure to ensure that the fuel cell does not over-discharge. When the DC/DC reaches the maximum output current, the voltage drops rapidly (see the CD section in Figure 3) as a constant current section, and its current value determines the maximum output current of the DC/DC.
The control chip controls the turn-on and turn-off of the power semiconductor. There are two modulation methods: PFM (Pulse Frequency Modulation) and PWM (Pulse Width Modulation). During PFM modulation, the switch pulse width is constant, and the output voltage is stabilized by changing the pulse output time. The frequency of the switching pulse is constant during PWM modulation, and the output voltage is stabilized by changing the pulse output width. Normally, the performance of DC/DC using PFM and PWM are different.
(2) Two-way DC/DC
In a hybrid power supply composed of a battery and a super capacitor, the battery generally works in the form of steady-state charging/discharging; while the super capacitor can work in the form of large current discharge when the electric car needs to be started. The fed-back electric energy can work in the form of high-current charging. The current of the battery and the super capacitor flows in two directions. Therefore, a buck-boost two-way DC/DC is installed between the battery and the super capacitor and the power bus to control and adjust the input and output current in both directions. The buck-boost bidirectional DC/DC circuit is shown in Figure 5.
The input end of the buck-boost bidirectional DC/DC uses two conduction switch tubes and two rectifier diodes to form two high-power DC converters (IGBT) respectively. An inductor L2 and a capacitor C are installed at the input end. When the two-way DC/DC with inductor L1 at the output is in charging mode, the on-switch tube VT1 is cut off, and the on-switch tube VT2 is turned on, and the current fed back by the charger or brake is charged to the battery or super capacitor via the power bus. When passing through the inductor L1, part of the current is temporarily stored in the inductor L1. After the on-switch tube VT2 is turned off, the current stored in the inductor L1 is transferred to the capacitor C through the rectifier diode VD2. The bidirectional DC/DC is in a Buck state when charging the super capacitor. Installing the inductor L1 on the super capacitor circuit can also reduce the current pulses entering the super capacitor circuit.
When the bidirectional DC/DC is in the discharging condition, the conduction switch VT1 is turned on, and the conduction switch VT2 is cut off. When the battery or super capacitor is discharged, the charge stored in the capacitor C is also discharged at the same time. The current direction is from the super capacitor to the direction of the power bus, and the DC/DC external discharge is in a boost state. Installing inductor L2 on the bus circuit can reduce the current pulse entering the bus.
The TDC-320-12IG 1000W car -mounted DC converter shown in Figure 6 can be installed on electric cars. The high-voltage side line is thin and the low-voltage side line is thicker, providing 14V low-voltage DC power supply for the car. The output terminal can be connected to a 12V auxiliary battery, and the DC/DC automatically manages the charging of the auxiliary battery. The shell is a fully sealed waterproof and dustproof structure, high temperature resistance and vibration resistance.
The connection relationship between DC/DC and power battery, 12V auxiliary battery, and low-voltage equipment is shown in Figure 7. DC/DC high-voltage DC input is directly connected to the positive and negative electrodes of the power battery, and the DC output is connected to the 12V auxiliary battery, and then output Go to a 12V electrical appliance.
The auxiliary battery is generally a traditional lead-acid battery. The role of the auxiliary battery is. When the battery is completely discharged or the DC/DC fails, an auxiliary battery provides approximately 700W of power to ensure the safety and reliability of the car. The capacity of the auxiliary battery is usually determined as the energy storage capacity for 1H of continuous work under emergency load. Another advantage of using an auxiliary battery is that it can prevent the voltage fluctuation of the auxiliary subsystem of the electric car and the electromagnetic interference pollution caused by the electric drive.
Figure 8 shows the Mercedes-Benz 400 two-way DC voltage rectifier, which supports 12V battery charging (high-voltage battery→auxiliary battery), supports high-voltage battery (12V battery→high-voltage battery) that realizes the assist effect, and is bridged by 12V charger or other cars Start (12V battery-high voltage battery), self-discharge through capacitor.