Five free practice questions on power electronics and charging: content area D of the ASE L3 Light Duty Hybrid/Electric Vehicle Specialist test, 13 of its 45 scored questions (28.9%). They are in the ASE format: four options, one best answer. Answer each one before opening the explanation. No account, no email.
01The questions
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When an electrified vehicle is accelerating on electric power, what does its DC/AC inverter do?
- Switches high-voltage DC into three-phase AC for the MG
- Steps high-voltage DC down to charge the low-voltage battery
- Converts AC from the EVSE into DC for the HV battery
- Closes the main contactors to connect the HV battery
Show the answer
A. Correct. The motor/generators are three-phase AC machines, so the inverter's power switches turn the battery's DC on and off in sequence to build three-phase AC, setting the MG's speed and torque by the frequency and current they produce. That is why an inverter fault can stop electric propulsion even with a fully charged HV battery.
Why not the others
B. That is the DC/DC converter's job: it replaces the alternator by stepping high voltage down to keep the low-voltage battery charged. The inverter feeds the MGs, not the low-voltage system.
C. Turning EVSE alternating current into DC for the pack is the on-board charger's job on a plug-in vehicle, and no EVSE is connected while the vehicle accelerates. While driving, the inverter turns battery DC into three-phase AC for the MG.
D. Contactors are relays in the HV battery's circuit that the battery control module closes when the HV system is enabled. The inverter receives the voltage they connect and converts it to three-phase AC for the MG.
ID L3-D-001 · Task L3-D-8 (ASE task list)
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An inverter DTC is stored, and the manufacturer's DTC chart points toward the inverter. Before the inverter is replaced, a technician should do each of these EXCEPT:
- record the DTCs and freeze frame data.
- check for a bulletin or software update.
- replace it on a repeat of the same DTC.
- inspect the inverter's connectors for damage.
Show the answer
C. Correct. A DTC that returns after clearing proves only that the same test fails again; a damaged connector or a fault that a software update corrects sets the same DTC and returns just as reliably. Replacing an expensive assembly on that basis skips the checks that find those causes.
Why not the others
A. Recording DTCs and freeze frame data before anything is cleared keeps the conditions under which the fault set, which guide the diagnosis and the road test that confirms the repair. It belongs before replacement.
B. Manufacturers release software updates and bulletins for known inverter DTCs, and an update can be the whole repair. Checking first is proper; replacing the inverter just because the DTC returns is the exception.
D. A loose, damaged or corroded inverter connector can set the same DTC as an internal fault, so the connectors are inspected before the assembly is condemned. The exception is replacing the inverter just because the DTC returns.
ID L3-D-010 · Task L3-D-4 (ASE task list)
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A hybrid is parked in READY TO DRIVE with its engine stopped by auto stop. Technician A says the high-voltage system is switched off whenever the engine stops like this. Technician B says that selecting OFF is by itself enough to disable the high-voltage system before the inverter is opened. Who is right?
- A only
- B only
- Both A and B
- Neither A nor B
Show the answer
D. Correct. Technician A is wrong: at an auto stop the vehicle is still in READY TO DRIVE, so the contactors stay closed, the MG can still move the vehicle and the engine can restart on its own. Technician B is wrong: OFF does not guard against a welded contactor or someone restarting the vehicle, so the manufacturer's full disable procedure, isolating the battery and verifying zero volts, is still followed.
Why not the others
A. Technician A is wrong because an auto stop only shuts the engine off; READY TO DRIVE keeps the high-voltage system active, and the engine can restart without warning. Technician B is also wrong, since OFF alone is not a disable procedure.
B. Technician B is wrong: OFF is only the first step, and the manufacturer's disable procedure still isolates the battery and verifies zero volts at the inverter. Technician A is wrong too, because READY TO DRIVE keeps high voltage active during an auto stop.
C. Neither technician is right. Technician A mistakes a silent engine for a shut-down system, but READY TO DRIVE keeps high voltage live; Technician B skips the battery isolation and voltage check that OFF cannot replace.
ID L3-D-015 · Task L3-D-6 (ASE task list)
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In an electrified vehicle's inverter, what job do the IGBTs do?
- They store energy to smooth the DC input
- They switch large currents on and off
- They measure each phase's current
- They step high voltage down to 12 volts
Show the answer
B. Correct. An IGBT (insulated gate bipolar transistor) is a power switch that a small gate signal turns on and off thousands of times a second; the inverter's control board fires them in sequence to build three-phase AC. Each phase has a high-side switch to the DC positive and a low-side switch to the DC negative.
Why not the others
A. Smoothing the DC input is the job of the inverter's input capacitor, which holds charge across the DC terminals. The IGBTs are the switches that turn that DC into AC for the MG.
C. Phase current is measured by current sensors on the phase conductors, which report to the control board. The IGBTs do the switching that the control board directs.
D. Stepping high voltage down for the low-voltage system is the DC/DC converter's job. The inverter's IGBTs switch high voltage to drive the MG.
ID L3-D-028 · Task L3-D-8 (ASE task list)
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A plug-in hybrid sets a "DC/DC Converter Input Voltage Low" DTC, and its low-voltage battery runs down while it is driven. In READY TO DRIVE, scan data shows 0 volts at the converter's HV input while the inverter's HV input reads 348 volts. Which of these could be the cause?
- A low-voltage battery with a shorted cell
- A main positive contactor that will not close
- An open in the converter's 12-volt output cable
- An open in the converter's HV input circuit
Show the answer
D. Correct. The inverter's 348 volts shows the contactors are closed and the HV bus is live, yet the converter sees nothing at its input. The break is in the converter's own HV feed, such as its HV fuse or cable connection, as the wiring diagram shows it; with no input it cannot charge the low-voltage battery.
Why not the others
A. A shorted cell would load the converter's output and drag system voltage down, but it would not make the converter's HV input read zero while the bus holds 348 volts.
B. If the main positive contactor stayed open, the inverter's input would read zero too. It reads 348 volts, so the bus is live and the fault is in the converter's own HV feed.
C. An open output cable would stop the battery charging, but it cannot take away the converter's HV input voltage. The 0-volt input with 348 volts on the bus points to the converter's HV feed.
ID L3-D-038 · Task L3-D-13 (ASE task list)
02Answer key
| Question | Answer |
|---|---|
| Q1 | A |
| Q2 | C |
| Q3 | D |
| Q4 | B |
| Q5 | D |
03What area D covers
This content area covers the inverter, the DC/DC converter and the parts around them: the power modes OFF, ACCESSORY, POWER ON and READY TO DRIVE, high-voltage warning lights and driveability complaints, inverter and converter faults and replacement, power-electronics cooling, capacitor voltage, interlock circuits, HV cables, the low-voltage battery, contactors and precharge, AC and DC charging faults, and data network failures.
The ASE test specification calls this content area “Power Electronics”. It carries 13 of the 45 scored questions, 28.9% of the test.
- Choose the right power mode for service work
- Measure capacitor voltage with a rated meter
- Diagnose inverter faults, cooling and replacement
- Test DC/DC converter output, fuses and grounding
- Trace interlock loop and HV cable isolation faults
- Test the low-voltage battery by its type
- Diagnose contactor, precharge and charging faults
- Diagnose data bus and communication failures
04What the questions turn on
Power modes do not disable HV
At an auto stop the vehicle is still in READY TO DRIVE, so the contactors stay closed and the engine can restart on its own. Selecting OFF alone does not guard against a welded contactor, so the manufacturer's full disable procedure is still followed.
Read the capacitor across the bus
The inverter's input capacitor shows its charge only between the HV positive and negative inputs. OL on a low range means more voltage than the range can show, and a minus sign means the leads are reversed. Neither is a safe reading.
Start with the low-voltage supply
When inverter, coolant pump and module DTCs all set in the same second with low 12-volt system voltage in freeze frame, the low-voltage supply DTC is diagnosed first. The control boards and the electric pump all run on that system.
Interlock and isolation are separate
The interlock is a low-voltage loop through pins in the HV connectors and covers, and an open loop makes the module open the contactors. An HV cable chafed to the body sets a loss-of-isolation DTC instead, which an insulation test to the shield confirms.
05The other content areas
The L3 test has five content areas. Each has its own page of free questions:
The ASE L3 Light Duty Hybrid/Electric Vehicle Specialist practice test page describes the full question bank. How these questions are written and checked: how our questions are made. Found a mistake? Tell us and we will fix it.
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