Added on November 04, 2016

Introducing the ISL78226, a 12V to 48V 6-phase bidirectional synchronous PWM controller for the automotive industry.

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12V/48V 6-Phase Bidirectional Synchronous PWM Controller

I would like to introduce the ISL78226, a 12V to 48V 6-phase bidirectional synchronous PWM controller.

Introducing the ISL78226 6-Phase Bidirectional DC/DC Controller

The ISL78226 is the industry’s first 6-phase bidirectional PWM controller. It is designed to perform power conversion between 48V and 12V power nets in vehicles with a 48V mild hybrid powertrain. It can buck from the 48V net down to 12V net or boost from 12V net up to 48V net. It simplifies the bidirectional DC/DC converter system design. No longer do designers have to put together two unidirectional systems or try to implement a complex digital solution using a DSP. A single ISL78226 IC can control up to six phases and manage power transfer between the two nets of as high as 3.75kW. And, even higher power DC/DC converters can be made by paralleling two to four ISL78226 ICs.

ISL78226’s current balancing architecture with automatic phase adding and dropping allows it to achieve very high efficiency across a very wide load range. It dynamically adjusts the average current in each phase and the active number of phases to optimize the system under varying load demands. And, it includes a digital PMBus interface for advanced diagnostic and control functionality.

Greener Automotive 48V Power Trends

The ISL78226 was developed to capitalize on the introduction of 48V mild hybrid systems. By 2020, most European OEM hybrids will be mild hybrids and have 48V systems in order to comply with Europe’s tightening regulations on CO2 emissions. The 48V hybrid powertrain increases the available electric power in a light vehicle from around 3kW to over 10kW. With this added power, automakers can convert more of the mechanical systems to electric, reducing cost and weight. They are also able to downsize the combustion engine and can avoid negatively impacting the vehicle’s drivability, or acceleration, by adding an electric supercharger. Regenerative braking is also vastly improved with the 48V system. All of this taken together results in improvements in CO2 emissions and fuel economy of between 10% and 20%.

48V Mild Hybrids will have 12V and 48V Board Nets

This is a sketch of a vehicle with a 48V mild hybrid powertrain. It looks very similar to a standard non-hybrid powertrain; however, a few changes have been made. First, a 48V battery has been added. Next, the 12V starter and alternator have been replaced by a 48V starter-generator. Finally, a DC/DC converter is added that bridges the 48V and 12V buses. This allows the power generated by the 48V starter-generator to be used to charge the 12V battery and operate all of the unchanged 12V electronics. Once these basic elements are in place, each automaker can then customize the system by adding new 48V components like a 48V supercharger, or converting some of the legacy 12V systems to 48V, like the various pumps and motors.

The ISL78226 forms the heart of the DC/DC converter.

ISL78226 Key Functional Elements

The bidirectional DC/DC converter used in mild hybrids is typically required to support power transfer between the 12V and 48V power nets of up to roughly three and a half kilowatts. The ISL78226 was designed specifically for this application.

The ISL78226 is a 6-phase controller that uses peak current mode to regulate the output voltage. It also limits average total output current. This is important when a battery is connected to the output and charging must occur at a constant current. It also protects the sourcing power bus from being overloaded, protecting the vehicle power system. Since the controller uses peak current mode, it will naturally balance the peak current across all active phases. However, ISL78226 also balances the average current in each phase, allowing it to compensate for inductor mismatches. This is important for equalizing the thermal stress across the entire system. The outputs from the controller are tri-level or three-state signals, designed to pair with Intersil’s tri-level input half-bridge drivers.

An external half-bridge driver is required for each phase. Some controllers integrate the FET gate drivers, but this forces either long runs and high parasitic inductance on the FET driver outputs or necessitates that the controller be placed very close to the FETs. The ISL78226 specifically avoided integrating the FET drivers so that the controller could be placed away from the FETs and the associated noisy switching nodes without negatively impacting the FET gate drive loops. This system partition allows for optimal EMI, efficiency and regulation accuracy.

The ISL78226 also includes an optional dual output flyback controller. The flyback is used to generate a 12V supply for the half-bridge drivers and a 6V supply that the ISL78226 can use, once it becomes available. At startup, ISL78226 draws power from its VIN pin, presumed to be supplied from either the 12V or 48V bus. If the flyback is enabled, the ISL78226 will switchover to its 6V output in order to limit internal power dissipation.

The ISL78226 also comes with a PMBus interface. Through PMBus, the local microcontroller will be able to configure theISL78226 for boost or buck, change the operating modes, and can even disable any of the phases if a major failure has been detected. Via PMBus, the ISL78226 will also communicate detailed status that the local controller can use to ensure all systems are operating as intended. This diagnostic capability allows faults to be detected and identified, supporting ISO26262 requirements for functional safety.

The ISL78226 also includes a 200mA linear regulator that can be used to provide power to the local microcontroller and for other functions, as required by the system.

As with all of Intersils automotive ICs, ISL78226 is AEC-Q100 qualified. It is rated to Grade 1 for operating temperatures from -40 to 125 degrees Celsius. And, it comes in a 64-pin TQFP package.

Bidirectional PWM Controller Overview and Key Features

Let’s look in a little more detail at the capabilities of the bidirectional PWM controller.

First, it supports both buck and boost modes, using peak current mode control. It offers independent loop and slope compensation for both buck and boost modes. It uses a single pinto select buck or boost mode and offers a PWM-enable pin that can disable the PWM outputs, effectively putting the controller into an idle state – neither bucking nor boosting, with all internal circuits still active.

A single IC supports up to six phases. Up to four ISL78226 ICs can be used in parallel to support up to 12 phases. The internal PLL is limited to 30-degree phases shifts, so while 4 ICsshould be expected to control 24 phases, the controller is limited to operate with a maximum of 12 phases. A single IC can also be configured to operate with fewer phases and each phase is also individually disabled in the case of faults. This capability allows the controller to offer a limp-home mode where the entire system can be operated at reduced capacity, even in the face of a major failure on one or more of the phases.

As mentioned in the prior slide, ISL78226 will balance average current in each phase. It will do this for all six phases within the IC. If multiple ICs are used in parallel, it will also balance current across the ICs.

In addition to the voltage regulating loop, ISL78226 also includes a dedicated average current control loop. This loop limits the sum of the currents in all phases to a user defined maximum. The maximum current is set via a pin but can be adjusted digitally using the PMBus interface.

ISL78226 offers several selectable modes that can be used to improve light load efficiency. First is diode emulation, which eliminates negative current at light load. Second is phase dropping, which allows the controller to reduce the active number of phases switching when the load decreases, and increase the number of active phases when the load increases. Alternatively, the controller can be setup for forced-PWM mode operation to reduce ripple and control switching noise, even at light load.

There is a global fault response, either hiccup mode or latch-off, that can be selected by a pin. Through the PMBus interface, the user can select the fault response of each fault.

The controller is designed to startup smoothly with pre-biased outputs and also offers programmable driver bootstrap cap refresh pulses. This ensures the driver is always able to turn on the HS FET whenever the controller commands.

The Track pin is provided to adjust the regulation voltage on the fly. This pin accepts either an analog reference signal or a digital, pulse-width-modulated (PWM) clock.

The switching frequency is adjustable and externally sychronizable from 40kHz to 750kHz.

The controller includes an internal 1.6V bandgap reference that is offered with 1.5% accuracy over the entire temperature range.

Average current balancing provides 5% accuracy across all phases within an IC and 10% across multiple ICs.

The ISL78226 senses current in the inductor on the BAT12 power net. The current sense inputs can tolerate load-dump up to 45V. The common-mode range is limited on the low end, so there is an under voltage lockout (UVLO) on the BAT12 power bus that requires a pre-bias on the buck output or the controller will not start. This means that in the lab, customers should make sure they have their 12V battery connected for buck testing. Inductor DCR sensing is not supported.

ISL78226 reports the sum of the currents in the six phases on the IMON pin. This is an analog output that provides 1.5% typical accuracy.

The ISL78226 offers very low shutdown current when fully disabled.

Finally, extensive fault diagnostics and protections are available. These include cycle-by-cycle current limiting and protection, average current limiting and protection, over and undervoltage for both power nets, under voltage lockout and over voltage protection for the supply to the controller, over temperature protection, open/short detection, negative current limiting and protection, clock open/short detection, and power good signal. For over and under voltage, ISL78226 offers both a detection threshold, where a warning flag is asserted, and a protection threshold, where the fault response is triggered. This is designed to allow the system controller to take corrective system-level actions if either the input or output voltage begins to move away from the expected operating level.

In the previous slide, we looked at some of the key features of the ISL78226 PWM controller. Now, let’s examine the benefits of those features.

Independent buck and boost loop and slope compensation simplifies the task of compensating both directions.

Having both a direction control pin for selection of buck and boost as well as a PWM enable pin allows the controller to offer an idle state that maintains the controller in a ready state. When changing directions, some systems may need to allow other reconfigurations to take place after switching has stopped in one direction and before it resumes in the other.

The dedicated phase-to-phase average current balancing works to equalize stress on the power stages. With each stage delivering 30A to 60A of average current, small mismatches can lead to large temperature variations and uneven stresses that can lead to reduced operating life and reliability concerns.

The optional phase dropping and diode emulation modes allow the user to do everything possible to improve light load efficiency or to optimize for reduced EMI and ripple.

The boot refresh pulses are designed to keep the bootstrap capacitor of the driver charged at all times. With out a boot refresh function, the bootstrap capacitor would become discharged during periods of inactivity, such as during phase dropping. If the cap were allowed to become discharged, the driver will be unable to respond immediately in the case that phase is commanded to become active again. This could occur if the load immediately steps up and phases that were previously dropped need to be added back.

The selectable fault response allows the system to be customized to differing OEM requirements. Furthermore, with use of the PMBus, each fault type can have its response programmed. This provides the system designer a great deal of flexibility and helps to ensure that functional safety requirements can be met. For example, an under voltage fault may not be considered dangerous and a hiccup restart can be offered. However, an over voltage fault may have led to damage, necessitating the controller latch-off until further diagnostics can be performed.

The pre-biased startup support is important so that the controller can identify these conditions and avoid negative current during startup. Since large capacity batteries can be present on each power bus, it is important to avoid a situation where the controller allows large and otherwise unlimited currents to flow out of the batteries at startup.

The Track pin allows the output voltage to be adjusted while the controller is in operation. It will accept both an analog or a digital style input, allowing it to be put into an analog feedback loop or connected directly to a GPIO of a microcontroller.

The wide degree of flexibility on the operating frequency allows the user to optimize the size of the solution versus its efficiency.

The accurate current reporting aids the system in monitoring the present load demanded and communicates this back to an ECU. If the load is beginning to approach the maximum limit, the ECU can then potentially disable or reduce the power consumption of various loading systems.

Finally, the extensive protections are necessary to limit the potential for damage should a fault develop in this very high current system.

Supply Switchover, Aux LDO, Flyback, and PMBus Overview

And just a few points on some of the other key functional elements.

As discussed earlier, the ISL78226 includes an optional flyback controller that allows it to create a low-voltage supply for itself to limit internal power dissipation. At startup, the controller is expected to be supplied by one of the two main buses. There is an internal linear regulator that creates a 5V supply to power the rest of the IC. If the flyback is enabled, it will come up and regulate both a 12V and 6V supply. There is an internal linear regulator on the V6 input pin that creates 5.2V. When the 5.2V supply is within regulation tolerance, the IC will automatically switchover to using this for the internal circuitry. The backup LDO that is supplied by the VIN pin remains active and able to provide the IC with power, in the event of a failure of V6, providing redundancy.

The ISL78226 also includes an auxiliary linear regulator, called the MCU LDO in the datasheet. The output is adjustable from 5V to 1.2V and can support up to 200mA of continuous current. If this linear regulator is used, it is highly recommended to also use the flyback and its 6V output as the IC power supply in order to limit internal power dissipation.

Finally, the PMBus provides extensive configuration and telemetry. Through the PMBus, the user can enable or disable phase dropping and diode emulation. The Phase Disable function is also accessible through PMBus. With Phase Disable, any combination of the output phases (up to 5) can be disabled, allowing the converter to operate in a reduced limp-home capacity. The response for each type of fault can be selected through PMBus. Also, the driver boot refresh pulses, both the number of pulses and the duration of each pulse, can be programmed via PMBus. The output current limit can be adjusted on the fly through PMBus. And lastly, many warning and fault thresholds can be reconfigured via PMBus.

In addition to configuration, PMBus provides communication of vital system status. The interface includes a status alert flag that updates the controller when any of the status bits have changed. In the event of a serious fault, it also includes a system failure flag that can alert the controller to take immediate action.

Parallel Operation Details

As noted earlier, the ISL78226 can be configured to operate in parallel, with up to four ICs and 12 phases. This allows a modular system design to be used to customize the DC/DC converter for various power requirements. Each device is individually addressable. Current information is shared across the ICs so that per-phase current can remain in balance. The interleaved devices also communicate phase dropping and fault information so that the system takes coordinated action. A 12-phase schematic is included in the datasheet to illustrate the various connections between the ICs.

ISL78226 Evaluation Boards

Intersil has developed several evaluation boards. Currently, we are offering a 2.5kW, 6-phase board and GUI that will allow our customers to hit the ground running. This board is designed to be very robust with inductors that can handle various fault conditions, like a short-circuited output. Power levels above 2.5kW can be supported, if the user supplies airflow to cool the power stages.

With the GUI, the user can modify any of the control settings of the IC and can monitor fault flags. The software also allows the user to save configuration files for streamlined testing. It also provides a set of tabs that will show the registers in Hex so that firmware can be easily debugged, and corresponding tabs where the Hex values have been translated to functional descriptions of the modes and status to facilitate understanding. The GUI is a power tool for becoming familiar with the ISL78226’s suite of capabilities.

Efficiency Graphs

These efficiency curves were measured using the 6-phase evaluation board and are published in the datasheet. One key requirement placed on the DC/DC converter by all OEMS is for the efficiency to reach 95% for the normal working range of the converter in buck mode. Typically, this is from 20% to 100% of the continuous load rating. Efficiency is largely determined by conduction losses in the inductor and FETs and switching losses at the FET, so most of this is not necessarily a reflection on the “goodness” of our controller. But, it nevertheless should offer the customer a known good starting point and confidence that when they select ISL78226, they will be able to achieve their efficiency goals.

ISL78226 6-Phase Bidirectional DC/DC Controller

To wrap up, The ISL78226 is the industry’s first 6-phase bidirectional controller and has been tailored to meet the demanding challenges placed on the 12V to 48V converter used in mild hybrids. ISL78226 simplifies the design and makes it easy to achieve performance and functional safety goals.

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