BI-DIRECTIONAL dc-dc CONVERTER FOR HYBRID VEHICLES O. García, L.A. Flores1, J.A. Oliver, J.A Cobos, J. de la Peña2 Universidad Politécnica de Madrid División de Ingeniería Electrónica José Gutiérrez Abascal, 2 28006 Madrid SPAIN
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1
Instituto Tecnológico de Aguascalientes División de Estudios de Postgrado Adolfo López Mateos 1801 OTE 20256 Aguascalientes MEXICO
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Hybrid vehicles need a high voltage DC bus to supply power to the motor. Architectures of these vehicles usually include a dc-dc bi-directional converter between this voltage bus and the conventional battery. When the DC bus is held by a big capacitor, the selection and design of the aforementioned converter has an additional difficulty since the converter has to work with an output voltage ranging from 0 to 420V in steady-state conditions.
A
A bi-directional dc-dc converter for a hybrid vehicle is proposed in this paper. It can be used in the case that a big capacitor holds the voltage in the high side. The application forces three different operation modes being the converter able to operate in those conditions. Experimental results of a 1500W prototype are included in the paper.
B
L
Figure 2.- Two topological solutions for bi-directional DC-DC converters depending on the position of the inductor
INTRODUCTION
In some cases, the high voltage bus is hold by a fuelcell or a high voltage battery. However, in our case, this voltage bus is maintained by a bank of capacitors. This results in added difficulty because they can be fully discharged, forcing to look for a topology able to operate with wide voltage range.
Hybrid vehicles have several advantages over conventional cars and there are some models available in the market. From the point of view of the Power Electronics field, in the power chain there are two circuits that have to be developed (see figure 1). The inverter to drive the motor and the dc-dc converter placed between the battery and the high voltage bus. This dc-dc converter should be bi-directional since the energy can flow from the battery to the DC link or in the opposite direction.
MOTOR
HIGH VOLTAGE BUS
BI-DIRECTIONAL DC-DC CONVERTER
Therefore, there are several operation modes as shown in figure 3. In down mode the energy flows from the high voltage capacitor to the battery (the motor is a generator) being the battery voltage between 10 and 16V; in up mode, energy is taken from the battery and capacitors at the DC-link have to be charged. Moreover, the converter can be forced (by the central controller) to operate with any voltage between 0 and 420V in a stable way. Thus, in up mode, two zones can be distinguished: buck (start-up and under-voltage mode) and boost (normal operation). The border between them is fixed by the battery voltage referred to the other side of the transformer.
LOW VOLTAGE BUS
C BATTERY ISOLATION
Figure 1.- Simplified block diagram of a hybrid vehicle showing the position of the bi-directional converter
Several solutions for this converter can be found in the state of the art [1-6]. They can be classified in two types as shown in figure 2: (A) circuits where the inductor is placed close to one of the inputs and (B) circuits where the inductor is between active switches and close to the power transformer. In type (A) the converter is voltage-fed or current-fed depending on which side is the input [1-4], being advisable to place
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the inductor in the high current side (close to the battery); in type (B) the converter is ‘more symmetrical’ but more reactive energy is required [56]. The main advantage of this last group is that the leakage inductance of the power transformer (very large in this application due to the isolation requirements of the transformer) is integrated in the power stage.
ABSTRACT
INVERTER
2
Topologies of the state of the art that have an inductor in the high current side are not valid to be used here since they work like a boost converter and, therefore, they can not operate in steady-state conditions with an output voltage smaller than the input voltage.
1881
In this paper a new solution is proposed to solve this problem. It is able to work with a bank of capacitors holding any voltage smaller or higher than the battery voltage referred to secondary and, therefore, operate in the three regions shown in figure 3. This is the main difference of this idea compared with others already proposed in the state of the art.
In both modes, during normal operation, the full-bridge works with 50% duty cycle and the converter acts as a buck in down mode or boost converter in up mode. In the case of start-up and under-voltage in the storage capacitor, the converter can operate by varying the duty cycle of the full-bridge and without switching the buck converter; as a result the equivalent circuit is a conventional full-bridge. These three operation modes are explained below.
PROPOSED CONVERTER The proposed converter is shown in figure 4. It is composed of a buck converter integrated with a current-fed full bridge. This full-bridge works with fixed duty cycle equal to 50%, therefore, the converter can be seen as a buck plus a dc-dc transformer. The converter has isolation and it is able to work in both modes: transferring energy from the high voltage storage capacitor to the battery (down mode); and from the battery to the capacitor (up mode). Since the converter is bi-directional, all switches must be implemented with MOSFETs.
BUCK CONVERTER M1 M3 VC
M5
M7
M9 VB
M2
14V
400V M4
M6
nTR:1
M8
M10
CURRENT-FED FULL-BRIDGE CONVERTER
Figure 4.- Proposed bi-directional dc-dc converter
UP MODE
DOWN MODE VB
Since there are switches at both sides of the inductor, it seems that the converter belongs to type B (see figure 2). However, the bridge works at 50% in two out of three regions shown in figure 3 and, therefore, the converter performance is type A. In the last region, the bridge changes its duty cycle but not the buck being again a type A but ‘moving’ the inductor to the other side. One of the advantages of this converter is its versatility that allows topological changes depending on the state of the switches.
VC 420
16
BOOST
BUCK 10
VB·ηTR
BUCK
0
Figure 3.- Operation modes of this converter
BUCK CONVERTER
FULL BRIDGE CONVERTER AT 50%
FULL BRIDGE CONVERTER AT 50%
Figure 5.- Equivalent circuit in down mode
BOOST CONVERTER
Figure 7.- Equivalent circuit in up mode
Figure 8.- Main waveforms in up mode
Figure 6.- Main waveforms in down mode
1882
The full wave rectifier composed by transformer and transistors M7 to M10 can be replaced by a center tapped transformer and only two transistors. Despite a higher voltage drop, the proposed scheme is better to clamp the voltage spike due the leakage inductance.
Transistors M1 and M2 are open and the circuit is just a full-bridge converter being VC controlled with the duty cycle of low voltage side MOSFETs (M7 to M10). Thus, during start-up, the duty cycle increases from 0 to 50% and, at that moment, the converter enters in up mode normal operation as shown before. Since the gate signals are different in these two up modes, there should be some circuitry dedicated to switch between one and the other.
DOWN mode The equivalent circuit and the main waveforms are shown in figures 5 and 6. The buck converter operates at twice the switching frequency of the dc-dc transformer. Since the full-bridge works at 50%, the output current has the ripple of the inductor referred to the low voltage side. Control of output voltage is achieved with duty cycle of transistor M1. The most important decision when designing this converter is the turns-ratio nTR. VBUS is equal to VBATTERY multiplied by nTR. It has a strong impact in the buck converter but also in the switches of the bridge M3 to M6. Thus, power losses are very dependent of this parameter. After a deep analysis and for the specifications shown in the experimental results section, nTR has been fixed equal to 11. As can be derived form figure 5, there should be overlap in the triggering sequence of the transistors M3 to M6 of the bridge. Otherwise, the inductor current does not have any path to circulate.
Figure 10.- Main waveforms in start-up and under-voltage mode
UP mode (normal operation)
Although it was not used in this application, this converter offers other possibilities to control the voltage. In fact, in up mode, if same duty cycle is applied to bridge switches (M7 to M10) and to M2, the equivalent circuit is like a flyback converter, being able to achieve a large voltage range at the output.
In this mode the converter transfers energy from the battery to the high voltage side. The full-bridge is working with 50% fixed duty cycle and therefore, VBUS is equal to VBATTERY multiplied by nTR. The buck converter acts as a boost converter being the MOSFET M2 the switch that controls the output voltage. Equivalent circuit and waveforms of this mode are shown in figures 7 and 8.
CONTROL STAGE Control stage is the most difficult part of this circuit. There are many transistors to switch and depending on the mode, the duty should be applied to M1 (down mode), M2 (up mode normal operation) or M7 to M10 (up mode start-up and under-voltage).
UP mode (start-up and under-voltage mode) When there is no power source in the high voltage side such as a battery or a fuel-cell, the voltage is hold by a capacitor. In such a case, the voltage on it runs from 0 to 400V approx. Therefore, boost type converters are not advisable since they cannot control the current and the voltage when VC
