Pre-Charge Apparatus and Control Method

An apparatus includes a power converter configured to provide a constant current charge to a capacitor coupled to a high voltage bus through a mechanical contact, an isolation interface configured to receive a pre-charge signal on a primary side of the isolation interface, and convert the pre-charge signal into a bias voltage signal and a control command signal on a secondary side of the isolation interface, and based on the bias voltage signal and the control command signal, a constant current control unit configured to generate a gate drive signal for the power converter.

PRIORITY CLAIM

This application claims priority to Chinese Patent Application No. 202210225212.4, filed on Mar. 9, 2022, which is hereby incorporated by reference in its entirety.

TECHNICAL FIELD

The present invention relates to a pre-charge apparatus and control method, and, in particular embodiments, to a pre-charge apparatus for charging an energy storage capacitor in an electric vehicle.

BACKGROUND

As technologies evolve, electric vehicles have been widely adopted. A typical electric vehicle may include a power source (e.g., a rechargeable battery pack) and an inverter (e.g., a three-phase motor inverter). The rechargeable battery pack is configured to establish a high voltage bus for driving an electric machine through the inverter. An energy storage capacitor or a plurality of energy storage capacitors is employed to establish an input voltage bus for the inverter. The energy storage capacitor is coupled to the high voltage bus through a mechanical contact (e.g., a power relay).

During a shutdown mode of the electric vehicle, the rechargeable battery pack is isolated from the energy storage capacitor through opening the mechanical contract. Due to various safety requirements, the energy storage capacitor must be discharged within a specific time once the rechargeable battery pack is disconnected from the energy storage capacitor. When the electric vehicle is turned on, the energy storage capacitor has to be charged up to a voltage level approximately equal to the voltage on the high voltage bus before the power relay can be closed, otherwise a huge surge current can occur. The huge surge current may damage the power relay.

To reduce or eliminate the huge surge current, a pre-charge circuit including a current limit resistor and a low power pre-charge relay may be used to charge up the energy storage capacitor to a voltage level approximately equal to the voltage on the high voltage bus prior to closing the power relay.

In operation, the current limit resistor may cause reliability issues and unnecessary power losses. In addition, the current limit resistor is of a bulky size for handling the energy losses dissipated in the resistor. If the relay opens during the pre-charging time for any reasons, an arc could occur and damage the contacts of the relay. It would be desirable to have a simple apparatus to pre-charge the energy storage capacitor to reduce power consumption and improve reliability. The present disclosure addresses this need.

SUMMARY

These and other problems are generally solved or circumvented, and technical advantages are generally achieved, by preferred embodiments of the present disclosure which provide a pre-charge apparatus for charging an energy storage capacitor in an electric vehicle.

In accordance with an embodiment, an apparatus comprises a power converter configured to provide a constant current charge to a capacitor coupled to a high voltage bus through a mechanical contact, an isolation interface configured to receive a pre-charge signal on a primary side of the isolation interface, and convert the pre-charge signal into a bias voltage signal and a control command signal on a secondary side of the isolation interface, and based on the bias voltage signal and the control command signal, a constant current control unit configured to generate a gate drive signal for the power converter.

In accordance with another embodiment, a method comprises prior to closing a power relay between a high voltage bus and a capacitor, transferring a pre-charge control signal from a primary side of an isolation interface to a secondary side of the isolation interface, obtaining a bias voltage signal and a control command signal based on the pre-charge signal received at the secondary side of the isolation interface, based on the bias voltage signal and the control command signal, generating a gate drive signal to control a power conversion apparatus so as to provide a constant current charge to the capacitor, and closing the power relay after a voltage across the capacitor exceeds a predetermined voltage level.

In accordance with yet another embodiment, a system comprises a capacitor configured to be coupled to an input of an inverter, a mechanical contact coupled between a first terminal of the capacitor and a high voltage bus, and a constant current pre-charge module having a first terminal coupled to the high voltage bus, a second terminal coupled to a common node of the capacitor and the mechanical contact, and a third terminal coupled to a second terminal of the capacitor, wherein the constant current pre-charge module is configured to provide a constant current charge to the capacitor.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The present disclosure will be described with respect to preferred embodiments in a specific context, namely a pre-charge apparatus for charging an energy storage capacitor in an electric vehicle. The disclosure may also be applied, however, to a variety of power conversion systems. Hereinafter, various embodiments will be explained in detail with reference to the accompanying drawings.

FIG.1illustrates a block diagram of a capacitor pre-charge system in accordance with various embodiments of the present disclosure. The capacitor pre-charge system comprises a capacitor C1, a mechanical contact101and a constant current pre-charge module102. The capacitor C1 is configured to be coupled to an input of an inverter (not shown). The capacitor C1 functions as an energy storage capacitor of the inverter. In some embodiments, the inverter a three-phase motor inverter configured to drive a motor in an electric vehicle. The mechanical contact101is coupled between a first terminal of the capacitor C1 and a high voltage bus (HV_BUS). The mechanical contact101may be implemented as a power relay. In some embodiments, the high voltage bus is coupled to a rechargeable battery pack of the electric vehicle. As shown inFIG.1, a second terminal of the capacitor C1 is connected to ground. Throughout the description, this ground is alternatively referred to as a high voltage ground (HV_GND) as shown inFIG.1.

The constant current pre-charge module102has a first terminal coupled to the high voltage bus, a second terminal coupled to a common node of the capacitor C1 and the mechanical contact101, and a third terminal coupled to a second terminal of the capacitor C1. In some embodiments, the constant current pre-charge module102comprises a power converter, an isolation interface and a constant current control unit. The power converter is configured to provide a constant current charge to the capacitor C1. The detailed schematic diagram of the power converter will be described below with respect toFIG.2. The isolation interface is configured to receive a pre-charge signal on a primary side of the isolation interface, and convert the pre-charge signal into a bias voltage signal and a control command signal on a secondary side of the isolation interface. The detailed schematic diagram of the isolation interface will be described below with respect toFIG.3. The constant current control unit is configured to generate a gate drive signal for the power converter based on the bias voltage signal, the control command signal and a current sense signal indicative of a current flowing through the power converter. The detailed schematic diagram of the constant current pre-charge module will be described below with respect toFIG.4.

In operation, during a shutdown mode of the electric vehicle, the rechargeable battery pack is isolated from the energy storage capacitor C1 through opening the mechanical contact101. Due to safety requirements, the energy storage capacitor C1 is discharged within a specific time once the rechargeable battery pack is disconnected from the energy storage capacitor C1. When the electric vehicle is turned on, the constant current pre-charge module102is configured to charge the energy storage capacitor C1 up to a voltage level approximately equal to the voltage on the high voltage bus before the mechanical contact101is closed. During the process of charging C1, the constant current pre-charge module102is configured to provide a constant current charge to the capacitor C1.

One advantageous feature of having the capacitor pre-charge system shown inFIG.1is that the constant current pre-charge module102helps to improve reliability and reduce the physical size of the capacitor pre-charge circuit. In particular, a switching circuit is employed to generate a controllable current to charge the capacitor C 1. The switching circuit is efficient, therefore reducing the physical size of the capacitor pre-charge circuit. Furthermore, in comparison with the conventional solutions, a semiconductor switch is used to replace the mechanical relay widely used in the conventional solutions. The replacement helps to solve the arcing issues caused by the mechanical relay. As a result, the reliability of the pre-charge circuit is improved. Furthermore, a peak current mode control scheme is employed to control the power converter. The peak current mode control scheme can respond any short circuit cycle by cycle and prevent the circuit from being damaged.

FIG.2illustrates a diagram of the constant current pre-charge module shown inFIG.1in accordance with various embodiments of the present disclosure. The constant current pre-charge module102comprises a power converter, a constant current control unit202and an isolation interface213.

The power converter comprises a switch Q1, a sense resistor Rs and a diode D1 connected in series between the high voltage bus HV_BUS and the high voltage ground HV_GND. The power converter further comprises an inductor L1 connected between a common node of the sense resistor Rs and the diode D1, and a common node of the capacitor C1 and the mechanical contact101. The common node of Rs and D1 is a low voltage ground (GND) as shown inFIG.2.

In some embodiments, the power converter is a buck converter. The power converter is configured to provide a constant current charge to the capacitor C1 coupled to the high voltage bus HV_BUS through the mechanical contact101.

In accordance with an embodiment, the switch Q1 ofFIG.2may be a metal oxide semiconductor field-effect transistor (MOSFET) device. Alternatively, the switch Q1 may be implemented as any suitable semiconductor devices such as bipolar junction transistor (BJT) devices, super junction transistor (SJT) devices, insulated gate bipolar transistor (IGBT) devices, gallium nitride (GaN) based power devices and/or the like.

It should be noted whileFIG.2shows the switch Q1 is implemented as a single n-type transistor, a person skilled in the art would recognize there may be many variations, modifications and alternatives. For example, depending on different applications and design needs, the switch Q1 may be implemented as a p-type transistor. Furthermore, Q1 may be implemented as a plurality of switches connected in parallel. Moreover, a capacitor may be connected in parallel with one switch to achieve zero voltage switching (ZVS)/zero current switching (ZCS).

It should further be noted whileFIG.2shows D1 is employed as a freewheeling device, this is merely an example. A person skilled in the art would recognize there may be many variations, modifications and alternatives. For example, D1 may be replaced by a MOSFET device to further improve the efficiency of the power converter.

The isolation interface213comprises a primary winding P1, a secondary winding S1, a primary side circuit212and a secondary side circuit214. The secondary winding S1 is magnetically coupled to the primary winding P1. The secondary winding S 1 and the primary winding P1 form a transformer for transferring a pre-charge signal from the primary side to the secondary side. In addition, the transformer provides isolation between the primary side and the secondary side. The isolation provided by the transformer helps to connecting circuits with grounds (e.g., HV_GND and GND shown inFIG.2) at different potentials.

As shown inFIG.2, the primary winding P1 is connected to the primary side circuit212. In some embodiments, the primary side circuit212is implemented as a high frequency oscillator. The secondary winding S1 is coupled to a secondary side circuit214. In some embodiments, the secondary side circuit214comprises a first rectifier and a second rectifier.

In operation, a pre-charge signal is fed into the primary side of the isolation interface213. The pre-charge signal is modulated by the primary side circuit212. The modulated pre-charge signal is transferred to the secondary side through the primary winding P1 and the secondary winding S1. The first rectifier converts the received pre-charge signal into a bias voltage signal VCC. The second rectifier converts the received pre-charge signal into a control command signal CMD. As shown inFIG.2, the bias voltage signal VCC and the control command signal CMD are fed into the constant current control unit202.

As shown inFIG.2, the constant current control unit202is configured to receive a current sense signal CS across the sense resistor Rs. The current sense signal CS is proportional to the current flowing through the inductor L1. Based on the bias voltage signal VCC, the control command signal CMD and the current sense signal CS, the constant current control unit202is configured to generate a gate drive signal Vg for the power converter.

FIG.3illustrates an implementation of the isolation interface shown inFIG.2in accordance with various embodiments of the present disclosure. The isolation interface213comprises a primary winding P1, a secondary winding S1, a primary side circuit and a secondary side circuit. The primary side circuit is implemented as a high frequency oscillator302. The high frequency oscillator302is configured to receive a pre-charge signal. In some embodiments, the secondary side circuit comprises a first rectifier and a second rectifier.

In operation, both the power signal (e.g., VCC) and the pre-charge control signal (e.g., CMD) are extracted from single pre-charge control input. In particular, the high frequency oscillator302is configured to generate a high frequency oscillation voltage and applies it to the isolation transformer formed by P1 and S1. The first rectifier and the second rectifier form a signal receiver at the secondary side of the transformer. The signal receiver extracts both the power signal (VCC) and the pre-charge control signal (CMD). The power signal (VCC) is used to power the downstream constant current control unit202, and the pre-charge control signal (CMD) is to turn on or off the constant current control unit202. In other words, CMD is used to enable and disable the constant current control unit202. Throughout the description, the power signal VCC may be alternatively referred to as a bias voltage signal. The pre-charge control signal CMD may be alternatively referred to as a control command signal.

As shown inFIG.3, the first rectifier comprises a first diode D1 and a second diode D2 connected in series between a first signal bus and a ground bus. The common node of the first diode D1 and the second diode D2 is coupled to a first terminal of the secondary winding S1. The first signal bus is configured to generate the bias voltage signal VCC.

As shown inFIG.3, the second rectifier comprises a third diode D3 and a fourth diode D4 connected in series between a second signal bus and the ground bus. The common node of the third diode D3 and the fourth diode D4 is coupled to the second terminal of the secondary winding S1. The second signal bus is configured to generate the control command signal CMD.

As shown inFIG.3, a first capacitor C1 and a first resistor R1 are connected in parallel between the first signal bus and the ground bus. A second capacitor C2 and a second resistor R2 are connected in parallel between the second signal bus and the ground bus.

FIG.3further illustrates the pre-charge signal applied to the high frequency oscillator302is a pulse. The pre-charge signal is a combination of an energy signal and a control signal. After this pre-charge signal is transferred from the primary side to the secondary side, the first rectifier and the second rectifier convert the pre-charge signal into the bias voltage signal VCC and the control command signal CMD, respectively. As shown inFIG.3, both bias voltage signal VCC and the control command signal CMD are in phase with the pre-charge signal.

It should be noted that the isolation interface shown inFIG.3is merely an exemplary structure and is not meant to limit the current embodiments. Other suitable isolation structures may alternatively be used. For example, VCC may be transferred from the primary side to the secondary side through a signal transformer. CMD may be transferred from the primary side to the secondary side through an opto-coupler.

FIG.4illustrates a block diagram of the constant current control unit shown inFIG.3in accordance with various embodiments of the present disclosure. The constant current control unit202comprises a first narrow pulse generator404, an inverter402, a comparator412, a first AND gate408, a second AND gate410, a constant off time generator416, a second narrow pulse generator414, a flip-flop406and a buffer418.

As shown inFIG.4, the first narrow pulse generator404is configured to receive the control command signal CMD through the inverter402, and generate a first narrow pulse. The comparator412is configured to receive a current sense signal CS indicative of a current flowing through the power converter (the current flowing through the inductor L1), compare the current sense signal with a predetermined current reference Iref, and generate a comparison result.

The first AND gate408has a first input configured to receive the control command signal CMD through the inverter402, and a second input configured to receive the comparison result generated by the comparator412. The second AND gate410has a first input configured to receive the comparison result generated by the comparator412, and a second input configured to receive the first narrow pulse generated by the first narrow pulse generator404.

The constant off time generator416is configured to receive an output signal of the second AND gate410, and generate a constant off time for the switch (e.g., Q1 shown inFIG.2) of the power converter. The second narrow pulse generator414is configured to receive the constant off time generated by the constant off time generator416, and generate a second narrow pulse.

The flip-flop406has a set input configured to receive the second narrow pulse generated by the second narrow pulse generator414, and a reset input configured to receive an output signal of the first AND gate408. The flip-flop406is configured to generate the gate drive signal of Q1. As shown inFIG.4, the buffer418is configured to receive the output signal of the flip-flop406and feed the gate drive signal into the switch of the power converter.

FIG.5illustrates various waveforms associated with the capacitor pre-charge system shown inFIG.1in accordance with various embodiments of the present disclosure. The horizontal axis ofFIG.5represents intervals of time. There may be four vertical axes. The first vertical axis Y1 represents the control command signal CMD. The second vertical axis Y2 represents the current flowing through the inductor L1 of the power converter. The third vertical axis Y3 represents the gate drive signal Vg applied to the gate of Q1. The fourth vertical axis Y4 represents the voltage Vc across the capacitor C1.

In response to the leading edge of the control command signal CMD, the gate drive signal Vg is applied to Q1. The current flowing through the inductor L1 increases in a linear manner to a predetermined current limit (peak current limit shown inFIG.5). Once the current flowing through the inductor L1 exceeds the predetermined current limit, Q1 is turned off. The turn-off time of Q1 is controlled by the constant off time generator416. This turn-on and turn-off of Q1 repeat until the voltage across the capacitor C1 reaches a voltage level approximately equal to the voltage on the high voltage bus HV_BUS.

In order to better illustrate the operating principle of the power converter, one switching cycle of the power converter is discussed below. At t1, the gate drive signal Vg changes from a logic low state to a logic high state. In response to this change, Q1 is turned on. The current flowing through the inductor L1 increases in a linear manner from t1 to t2. The current flowing through the inductor L1 charges C1. As a result, the voltage Vc across C1 increases accordingly.

At t2, the current flowing through the inductor L1 reaches the predetermined current limit. Referring back toFIG.4, the output of the comparator412generates a logic high signal to reset the flip-flop406. As a result, the gate drive signal Vg changes from a logic high state to a logic low state. In response to this change, Q1 is turned off. The current flowing through the inductor L1 decrease in a linear manner from t2 to t3. The duration from t2 to t3 is determined by the constant off time generator416. At t3, Q1 is turned on again. This process repeats until the voltage across the capacitor C1 reaches a voltage level approximately equal to the voltage on the high voltage bus HV_BUS.

FIG.6illustrates a flow chart of operating the capacitor pre-charge system shown inFIG.1in accordance with various embodiments of the present disclosure. This flowchart shown inFIG.6is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. For example, various steps illustrated inFIG.6may be added, removed, replaced, rearranged and repeated.

Referring back toFIG.1, a capacitor C1 is configured to be coupled to an input of an inverter (not shown). The capacitor C1 is coupled to a high voltage bus HV_BUS through a mechanical contact (e.g., a power relay). A constant current pre-charge module102has a first terminal coupled to the high voltage bus, a second terminal coupled to a common node of the capacitor and the mechanical contact, and a third terminal coupled to a second terminal of the capacitor. During a shutdown mode (e.g., a shutdown mode of an electric vehicle), the capacitor C1 is disconnected from the high voltage bus through opening the mechanical contact. The capacitor C1 is discharged through a suitable discharge circuit. Prior to leaving the shutdown mode and entering into a normal operating mode, the constant current pre-charge module102is configured to provide a constant current charge to the capacitor C1 until the voltage across the capacitor C1 reaches a level approximately equal to the voltage on the high voltage bus HV_BUS.

At step602, prior to closing a power relay between a high voltage bus and a capacitor, a pre-charge control signal is transferred from a primary side of an isolation interface to a secondary side of the isolation interface.

At step604, based on the pre-charge signal received at the secondary side of the isolation interface, a bias voltage signal and a control command signal are obtained.

At step606, based on the bias voltage signal and a control command signal, a gate drive signal is generated to control a power conversion apparatus so as to provide a constant current charge to the capacitor.

At step608, the power relay is closed after a voltage across the capacitor exceeds a predetermined voltage level.

The method further comprises discharging the capacitor after a power source is disconnected from the capacitor through opening the power relay. The method further comprises obtaining the bias voltage signal through a first rectifier, and obtaining the control command signal through a second rectifier.

The isolation interface comprises a transformer. A primary winding of the transformer is coupled to a high frequency oscillator having an input configured to receive the pre-charge control signal. A first terminal of the secondary winding of the transformer is coupled to an input of the first rectifier. A second terminal of the secondary winding of the transformer is coupled to an input of the second rectifier. The power conversion apparatus is a buck converter.

The method further comprises applying a peak current control scheme to the power conversion apparatus to achieve the constant current charge.