Introduction
Electrical automobiles (EVs) usually function a big DC hyperlink capacitor (CDC LINK) to attenuate voltage ripple on the enter of the traction inverter. When powering up an EV, the aim of precharging is to soundly cost up CDC LINK earlier than working the car. Charging CDC LINK as much as the battery stack voltage (VBATT) prevents arcing on the contactor terminals, which might result in catastrophic failures over time.
The standard precharge technique includes implementing an influence resistor in sequence with the CDC LINK to create a resistor-capacitor (RC) community. Nonetheless, as the full CDC LINK capacitance and VBATT improve, the required energy dissipation grows exponentially. On this article, we’ll current an easy method to designing an environment friendly, lively pre-charge circuit utilizing a spreadsheet calculator.
Understanding lively precharge
Whereas passive precharge employs an influence resistor to create an RC circuit that expenses the capacitor asymptotically, lively precharge can make use of a switching converter with a buck topology that makes use of hysteretic inductor present management to ship a continuing cost present to the capacitor (Determine 1).
Determine 1 The lively precharge circuit the place a buck converter makes use of a hysteretic inductor present management to ship a continuing cost present to the capacitor to allow the linear cost of the capacitor voltage (VCAP) as much as the identical voltage potential because the battery (VBATT). Supply: Texas Devices
This fixed present permits linear charging of the capacitor voltage (VCAP) as much as the identical voltage potential as that of the battery. Determine 2 and Equation 1 characterize this linear habits.
Determine 2 Lively precharge linear habits utilizing a buck topology with hysteretic inductor present management. Supply: Texas Devices
Step one is to find out the required cost present (ICHARGE). ICHARGE is the quotient of the full DC hyperlink cost (QDC LINK) and the required precharge time (tCHARGE) proven in Equation 2.
QDC LINK is the product of CDC LINK and VBATT, as proven in Equation 3.
Calculator overview
This lively hysteretic buck circuit has a floating floor potential driving on the change node, so powering the management system requires an remoted bias provide. The calculator instrument will be certain that the facility consumption of this management circuitry stays inside the sourcing functionality of the remoted bias provide, or else the voltage will collapse.
The Excessive-Voltage Strong-State Relay Lively Precharge Reference Design from Texas Devices (TI) introduces an lively resolution that enhances vitality switch effectivity and reduces sensible cost time. TI’s TPSI3052-Q1 is a totally built-in remoted bias provide used within the lively precharge reference design, which might supply and provide as much as 83 mW of energy to the remoted secondary. Gate drive present, gadget quiescent currents, and resistor dividers are the first contributors to energy consumption. Equation 4 characterizes the gate drive energy (PGATE DRIVE) because the product of the gate drive present (IGATE DRIVE) and gate drive voltage (VS GATE DRIVER) which is 15 V, within the case of the reference design.
Equation 5 characterizes gate drive present because the product of the metal-oxide semiconductor field-effect transistor (MOSFET) whole gate cost (QG) and switching frequency (FSW).
Equation 6 expresses how FSW varies in response to VCAP all through the charging interval, creating the upside-down parabola within the FSW versus VCAP curve in Determine 3. As proven within the Determine, the gate drive present peaks on the most switching frequency (FSW_MAX), which happens when VCAP reaches half of VBATT. Equation 7 expresses the connection between FSW_MAX, VBATT, inductance (L) and peak-to-peak inductor present (dI):
Determine 3 Calculator curve displaying FSW versus VCAP and FSW LIMIT. Supply: Texas Devices
Utilizing the calculator instrument
The calculator prompts you to enter varied design parameters. The yellow cells are the required inputs whereas grey cells signify optionally available inputs. The default values within the grey cells mirror the parameters of the reference design. A person can change the grey cell values as wanted. The white cells present the calculated values as outputs. A purple triangle within the upper-right nook of a cell signifies an error; customers will be capable to see a pop-up textual content on methods to repair them. The target is to realize a profitable configuration with no purple cells. This may be an iterative course of the place customers can hover their mouse over every of the unit cells to learn explanatory data.
Precharge system necessities
The primary part of the calculator, proven in Determine 4, computes the required cost present
(ICHARGE REQUIRED) primarily based on the VBATT, tCHARGE, and CDC LINK system parameters.
Determine 4 The required cost present (ICHARGE REQUIRED) primarily based on the VBATT, tCHARGE, and CDC LINK system parameters. Supply: Texas Devices
Inductance and cost present programming
The part of the calculator proven in Determine 5 calculates the precise common charging present (ICHARGE) and FSW_MAX. The typical inductor present basically equates to ICHARGE the place ICHARGE have to be equal to or larger than ICHARGE REQUIRED, this was calculated within the earlier part to fulfill the specified tCHARGE.
Be aware of the connection between L, dI, and FSW_MAX as expressed in Equation 7. L and dI are every inversely proportional to FSW, so it is very important choose values that don’t exceed the utmost switching frequency restrict (FSW LIMIT). Your inductor choice ought to accommodate enough root-mean-square present (IRMS > ICHARGE), saturation present (ISAT > IL PEAK), and voltage rankings, with sufficient headroom as a buffer for every.
Determine 5 Inductance and cost present programming parameters. Supply: Texas Devices
Present sensing and comparator setpoints
The part of the calculator proven in Determine 6 calculates the underside resistance (RB), prime resistance (RT), and hysteresis resistance (RH) across the hysteresis circuit wanted to fulfill the height (IL PEAK) and valley (IL VALLEY) inductor present thresholds specified within the earlier part. Enter the present sense resistance (RSENSE) and RB. These are versatile and may be modified as wanted. Be sure that the comparator provide voltage (VS COMPARATOR) is right.
Determine 6 Part that calculates the underside resistance (RB), prime resistance (RT), and hysteresis resistance (RH) across the hysteresis circuit wanted to fulfill the height (IL PEAK) and valley (IL VALLEY) inductor present thresholds. Supply: Texas Devices
Bias provide and switching frequency limitations
The part of the calculator proven in Determine 7 calculates the facility obtainable for switching the MOSFET (PREMAINING FOR FET DRIVE), by first calculating the full energy draw (PTOTAL) related to the hysteresis circuit resistors (PCOMP. RESISTORS), the gate driver built-in circuit (IC) (PGATE DRIVER IC), and the comparator IC (PCOMPARATOR IC), and subtracting it from the utmost obtainable energy of the TPSI3052-Q1 (PMAX_ISOLATED BIAS SUPPLY). Enter the MOSFET whole gate cost (QG TOTAL), gadget quiescent currents (IS GATE DRIVER IC and ISUPPLY COMP IC), and gate driver IC provide voltage (VS GATE DRIVER IC). The instrument makes use of these inputs to calculate FSW LIMIT displayed as a purple line in Determine 3.
Determine 7 Remoted bias provide and switching frequency limitations parameters. Supply: Texas Devices
The calculator instrument makes sure assumptions and don’t account for components resembling comparator delays and energy losses in each the MOSFET and the freewheeling diode. The instrument assumes the usage of rail-to-rail enter and output comparators. Ensure that to pick out a MOSFET with an acceptable voltage score, RDSON, and parasitic capacitance parameters. Guarantee the facility loss in each the MOSFET and freewheeling diode are inside acceptable limits. Lastly, choose a comparator with low offset and low hysteresis voltages with respect to the present sense peak and valley-level voltages. Simulating the circuit with the ultimate calculator values ensures the meant operation.
Achieved the specified cost profile
Adopting an lively hysteretic buck circuit considerably improves effectivity and reduces the dimensions of the charging circuitry in excessive voltage DC-link capacitors present in EVs. This helps probably decrease the dimensions, price, and thermals of a precharge resolution.
This text presents the design course of to calculate the suitable part values that assist obtain the specified cost profile.
By embracing these methods and instruments, engineers can successfully enhance the precharge performance in EVs, resulting in improved energy administration methods to fulfill the growing calls for of the automotive trade.
Tilden Chen works as an Purposes Engineer for the Texas Devices Strong State Relays crew, the place he offers product assist and generates technical collateral to assist win enterprise alternatives. Tilden joined TI in 2021 after graduating from Iowa State College with a B.S. in Electrical Engineering. Exterior of labor, Tilden enjoys taking part in Brazilian jiu-jitsu and shuffle dancing.
Hrag Kasparian, who joined Texas Devices over 10 years in the past, at the moment serves as an influence purposes engineer, designing customized dc-dc switch-mode energy provides. Beforehand, he labored on growth of battery packs, chargers, and electrical car (EV) battery administration methods at a startup firm in Silicon Valley. Hrag graduated from San Jose State College with a bachelor of science in electrical engineering.
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