Hybrid electric vehicles (HEVs), Plug-In HEV (PHEVs) and full battery electric vehicles (BEVs) are increasing their market share of the vehicles sold each year. As those years move on so do their warranties. That means the independent repair technician must be able to tackle diagnostics on these complicated vehicles. This article will help increase your knowledge on high voltage PE (Power Electronics) modules. What exactly are the “power electronics” on a HEV/BEV? Conventional automotive electronic modules monitor information (inputs) and control conventional voltage/low current outputs. PEs are used to control high voltages (HV) and high currents used in HEVs, PHEVs and BEVs. The AC inverter and DC-DC converter will be the two PE modules we’ll cover (Figure 1). We’ll discuss what these modules do, what makes them tick and how to diagnose them when they fail.
PE 101
Very briefly (and basically), AC inverters change DC to AC (and vice versa) while DC-DC converters drop high voltage DC down (a.k.a. “buck”) to the approximate 13-15 volts we’re used to seeing outputted from a belt driven alternator on a conventional vehicle (Figure 2). Inside the inverter are six Insulated Bipolar Transistors (IGBTs) (Figure 3) long with digital control electronics, diodes, resistors and capacitors. The inverter allows high voltage (and high amperage) DC power from the HV battery pack to basically switch on and off very rapidly (while reversing polarity) in order to create the simulated three-phase AC sinewave of electric power that Motor Generators (MGs) need to operate.
Within the inverter there may be a step DC converter called a “boost converter” to assist with performance (Figure 4). As with a conventional alternator, the AC produced while the MGs are in generator mode (engine idling/regenerative braking) is rectified by the same inverter (with some help from diodes and other components) back to DC to charge the HV battery pack.
OK, enough theory! Let’s get into some diagnostics for power electronics problems.
Cranks but won’t start – HEV style
On most HEVs and all PHEVs the inverter’s first job is to create AC power to run the MG that starts up the I.C.E. whenever it needs to run. The root causes for a HEV’s Internal Combustion Engine (I.C.E) with a crank/won’t start condition are the same as with a conventional (gas only) vehicle regardless of how complex the vehicle is. With a conventional vehicle the customer might report that their engine will turn over but won’t come to life. If you’re a service advisor, you type “cranks/won’t start” into the R.O. and away we go with testing the usual spark/fuel/air components necessary for an engine to run. Not so easy with an HEV utilizing both an I.C.E. and one or more high voltage AC three-phase electric MGs. On all but the mild HEVs (GM BAS, for example) there is no conventional 12-volt starter to crank over the I.C.E. A high voltage MG performs the cranking function of a conventional gear reduction starter. For the MG to get power to start the I.C.E. (or move the vehicle in EV mode), the HV battery pack supplies HV DC power to the HEV’s inverter. The inverter changes HV DC power into HV three-phase AC power. The I.C.E. cranks over, but with some major differences compared to a conventional gas only powertrain. One major difference is the sounds you DON’T hear when a full HEV’s MG turns over the I.C.E. You simply don’t hear a starter solenoid or the gear mesh of a starter drive/Bendix into the flywheel. Nor is there a very noticeable rhythmic sound of the I.C.E. turning over at approximately 150 RPMs. Considering we’re past the 20th anniversary of HEVs in this country, today’s techs should know the familiar sound of an HEV’s I.C.E. cranking/starting. Just in case the sound is not firmly in your mind, compare to something you may recall from some years back. Think back to the hand push start of a manual trans equipped vehicle when the battery was a little low. As soon as you were rolling you released the clutch (while in gear) and the engine came to life almost instantly. If you recall that experience, you know the sound of a HEV’s MG starting an I.C.E. An HEV’s I.C.E. is cranked at much higher RPMs. A HEV will attempt to crank its I.C.E. whenever the accelerator pedal is pushed/held to the floor. I’ve disconnected the coils or disabled fuel on numerous HEVs only to have experienced techs swear the sound they were hearing was an idling I.C.E. Try that with a PHEV (Chevy Volt, Chrysler Pacific, etc.) and you’re liable to have nothing happen unless the HV battery pack is discharged enough for a demand call on the I.C.E. to run.
Methods to determine if a HEV is being unsuccessfully cranked by a MG at near idle speeds or successfully running
1. Will I.C.E. rev up (at least a little) on accelerator pedal W.O.T. applies?
1a. Yes? I.C.E. is running
1b. No? I.C.E. is probably NOT running – its being cranked by an MG
2. Use diagnostic equipment to determine if I.C.E. is running
2a. Use scan data / search for I.C.E. fail to start DTCs
2b. Use an infrared pyrometer / thermal imager on the exhaust to determine if combustion is being detected.
PE (Power Electronics) diagnostic bottom line
If you have an HEV/PHEV verified “I.C.E. cranks but won’t start” complaint in your bay – it will NOT be due to a fault in the power electronics (inverter or DC-DC converter) on the vehicle. To begin with, a problem with the DC-DC converter will likely result in a discharged 12-volt battery. With an insufficient 12-volt power source, the various modules including the one that controls the HV contactors (relays) in the HV battery pack won’t have power to operate. That stops everything! The fact that the inverter changed HV DC into HV three-phase AC for use by a MG to crank the I.C.E. means the inverter is functioning – at least at this point.
Vehicle won’t move/I.C.E. won’t crank – HEV diagnostics
Now consider the conventional gas only powertrain with a “No Crank” symptom. In those scenarios your customer might complain of:
a. No sound when the key is turned / start button is pressed
b. A single click of the starter solenoid
c. Rapid starter solenoid clicks (enough voltage for the pull in winding but not the hold in winding)
d. Slow I.C.E. cranking sound that trails off to a click / multiple clicks
An of the above we’ll associate with a lack of 12-volt supply to the starter motor due to the usual suspects of a discharged 12-volt battery, corroded battery cables or the starter motor itself. On the other hand, the reason for a true HEV / PHEV “I.C.E. won’t crank / vehicle won’t move” can be traced to one (or more) of 4 major categories with many additional subcategories for most models.
1. HV Battery Pack
1a. HV battery pack SOC (State of Charge) / SOH (State of Health)
1b. HV battery pack internal resistance
1c. HV battery pack leakage / short detected
1d. HV battery pack thermal condition
1e. HV battery pack main system relays (contactors) not closing
2. HV power distribution (orange cables)
2a. Open circuits / low voltage
2b. Leakage / shorts to chassis detected
3. Safety; HV interlock circuit voltage high (P0A0D or similar DTC)
3a. Hybrid service plug (not fully engaged)
3b. Orange (HV) cable connection (not fully seated)
3c. Access panel on or near a HV component (not in place)
4. Motor / Generator (MG)
4a. MG Performance (incudes I.C.E. mechanical cranking resistance)
Note; even a “slight” hydrolock condition such as excess oil blowby on top of the pistons from a crankcase overfill
4b. Leakage / Short detected
4c. Resolver / Position sensor
4d. MG Temperature
4. Power Electronic Problems (inverter & DC-DC converter
5a. Inverter performance
5b. DC-DC converter performance (12-volt charging system)
5c. Power electronics temperature
5d. Leakage / Short Detected
General HV leakage DTC tech tip
If a HV leakage DTC sets always document and then try to clear the DTC to see exactly when it reoccurs. Does it occur immediately on power up? Focus on the battery pack, DC-DC converter and orange DC cables. Does it occur only when the I.C.E. tries to start? Think inverter, three-phase cables to MG1 and MG1 itself. Does it occur only when the vehicle is moved? Think inverter, three-phase cables to MG2 and MG2 itself. Does it occur only when the AC compressor is requested? Think compressor/AC compressor cables. Most A/C compressors have their own mini inverter built into them. A few Toyotas had a small inverter built into the main inverter that fed three-phase AC to the compressor.
Other PE-related potential problems — heat
Many inverters and DC-DC converters are under hood mounted and liquid cooled. The same coolant type is usually used in the I.C.E.’s cooling system on HEVs/PHEVs. With a couple of rare exceptions both systems (PE and I.C.E.) use separate cooling systems. The PE cooling system consists of at least one 12-volt electric pump, hoses/lines to the inverter and DC-DC converter, hoses to the MGs and a heat exchanger (Figure 5). You may see a separate heat exchanger (another cooler near the radiator and A/C condenser) or the I.C.E. cooling system’s radiator may have an integrated but separated heat exchanger dedicated to cooling the PE and MG coolant.
Heat is the enemy of any PE module. Always follow OEM instructions (usually a tall/sealed funnel or Airlift tool) when refilling an HEV’s PE cooling system to prevent any air from getting into the system. Hose leaks, seals internal to the PE devices and faulty 12-volt electric coolant pumps are common failure items (Figure 6).
Always use the OEM’s recommended coolant. Adding universal coolant (with tap water instead of deionized water) can lead to failures down the road due to even a tiny internal leak of a conductive coolant. If a PE device is in the rear of the vehicle as on some Hondas and Fords (Figure 7) the same fan motor that blows air across the HV battery pack cools the PE device(s). Make sure that system is fully functional, duct work in place, airflow free of restrictions (some use filters) and the blower’s squirrel cage itself is not caked with dust/pet hair. Most models allow for the use of a scan tool to check the data PIDs for the inverter and DC-DC converter temperatures (Figure 8) as well as bidirectional capability to activate coolant pumps and blower motors.
Future trends in PE – make them smaller!
Until recently PE modules have used Insulated Gate Bipolar Transistors (IGBTs). IGBTs have allowed for the growth in numbers of HEVs, PHEVs and BEVs we’ve seen throughout the last 20 years. In the last couple of years IGBTs are beginning to transition to smaller and more efficient silicon carbide transistors in power electronics products. Foregoing conventional wire bonds (Figure 9) between the power transistors and the circuit boards, these revolutionary new switching devices will bring down the size (and cost) of power electronics. This will allow for more OEMs to electrify their conventional vehicle lines w/o adding as much to the vehicle’s cost compared to current power electronics technology. That means more and more hybrid and electric vehicles will be turning up for service at your repair shop as time marches on!
