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STMicroelectronics’ New Iso-Buck A6986I Enables Optimized Isolated Power Supply for High Voltage Applications.

Iso-Buck A6986I
  • Target applications include On-Board Chargers (OBC), DC-DC Converters, Boosters, eCompressors, main traction Inverter, auxiliary Power Supply, Battery Management Systems (BMS), Belt-Starter Generators (BSG), eTurboChargers and eBoosters, BLDC motors, Heating applications and supply of any Gate Driver for high voltage MOSFETs.
  • A6986I is a DC-DC synchronous switching step-down (buck) regulator enabling two output topologies simultaneously:
    • Primary output is conventional step-down (buck) with up to 2A output current.
    • Secondary output is isolated with 5W power capability. It enables to safely supply the high-voltage side of the gate driver channel, while the low-voltage control side can be supplied by the primary conventional step-down output.

Electrifying the vehicle powertrain has brought several new challenges in the design. One of them is the proper management of different voltage classes across components in one same system, as this can create leakage currents, degrade the system performance or damage electronics on the low-voltage side. Therefore, there are design considerations that must be taken into account when developing such systems.

On-Board Chargers, eCompressors, Boosters or the main traction Inverter for hybrid or all-electric battery vehicles include control, sensing and communication elements which require a low-voltage supply (5V or 3.3V for instance) but also high power elements for the actuation stage such as the MOSFETs. Such MOSFETs are supplied at high voltage, depending on the architecture can vary from 400V up to 800V and beyond.

While the low-voltage elements are traditionally supplied from the 12V lead-acid battery via a power regulator that is stepping-down the voltage, the new high-power MOSFETs are supplied directly from the high voltage battery pack. The element that interacts with these two voltage classes is the gate driver, as the control stage (input side) receives the data from the low-voltage side -normally the microcontroller-, while the actuation stage (output side) drives the high-power (and high-voltage) MOSFET.

Therefore, the gate driver becomes a critical element to ensure the two voltage classes remain isolated. Gate drivers used in this high voltage applications are galvanically isolated. Galvanic isolation is not the only method to provide isolation, but one of the most efficient for wide potential differences. The low voltage side (or input side) will handle the communication and control from microcontroller side, including the low voltage PWM input as switching signal for the MOSFET. In order to avoid any conduction path, and to be galvanically isolated, the control data is passed to the high-voltage side (or output side) where the gate driver will drive the external high voltage MOSFET, via a radio-frequency signal (but could also be optical for instance, such as an opto-coupler).

Still, both sides of the gate driver need power supply to feed the different blocks. And the output side also needs the right voltage to drive the MOSFETs gate; which will depend on the selected MOSFET technology; usually in the range of 0V to 20V for silicon-based MOSFETs and -5V to 18V for silicon carbide SiC MOSFETs which enable a faster switching. As such blocks on the high-voltage side actually require low voltage to operate, this will be supplied usually from the 12V power architecture. This could open an alternative conduction by-path from the high voltage battery via the MOSFETs and Gate Driver to the 12V architecture. Hence, to fully guarantee the system isolation across the two voltage classes, also the high voltage side of the Gate Driver needs to be supplied with a regulation stage that provides isolation. 

There are several solutions to achieve that, even sometimes the Gate Drivers provide a flyback solution. Now STMicroelectronics is also proposing an additional efficient way to solve this problem; the isobuck A6986I (portmanteau of isolated buck). The isobuck A6986I enables two outputs: a conventional buck (primary output) and an isolated output (secondary output). The buck (primary output) can be used for instance to supply the microcontroller or the low-voltage side of the gate driver. And the isolated output (secondary output) can supply the high-voltage side of the gate driver. Therefore, with one single device all the blocks of the gate driver can be properly supplied.

The primary output of this SMPS (switched-mode power supply) is a conventional buck converter, also referred as step-down regulator.As any switching regulator, an input voltage (usually the vehicle’s 12V lead-acid battery) will be regulated down to a lower voltage to supply a load (e.g. 5V, 3.3V…) by means of switching an integrated P-channel MOSFET, achieving high power efficiency (low-side MOSFET is also integrated as A6986I is a synchronous switching regulator). A constant output current flowing to the load is guaranteed by the inductor in series, as it stores energy when the P-channel MOSFET switch is closed (on-state) that will be released when the switch goes open (off-state). The output capacitor will ensure a constant voltage output.

The topology is defined by the secondary isolated output, for which the inductor must be replaced by a flyback transformer. A flyback transformer differs from a conventional transformer, as while in the conventional transformer the input current is immediately transferred in the output (hence no energy stored), the flyback transformer does keep the input current stored as energy in the magnetic core and then at a later stage this energy is released and transferred as output current. In any case, both flyback and conventional transformers behave in the same fashion regarding voltage and current ratios.

Depending on the transformer ratio (number of turns in the winding of each side of the transformer’s coils) a new voltage value can be achieved; and this can be both a step-down voltage (lower voltage) or a step-up (higher voltage) than the input. As the energy is transferred from one side to the other side of the transformer via a magnetic flux, the two sides are galvanically isolated, with no current flowing from one side to the other side.

It is even possible to set multiple secondary windings, enabling accordingly several secondary outputs, all of them isolated and of voltage depending on the number of turns of the specific winding in each secondary stage. When designing the transformer ratio to achieve the desired secondary voltage output the forward voltage drop of the secondary rectifier diode must also be considered, as well as the resistance of the winding of each coil. Also another relevant parameter to evaluate is the leakage inductance.

In any case, the secondary output voltage closely tracks the voltage on the primary output. The primary current shape derives from the secondary current. During TOFF the secondary current implies a negative current in the primary winding. The A6986I design has been optimized to handle this negative current.

To better understand how the isobuck works, we can imagine a generic example. A possible circuit could consist of a converter regulation from 12V battery to a primary output of 5V (VOUT_1). Additionally, two isolated outputs must be generated; one stepping up to 10V (VOUT_2) and one stepping down to 3.3V (VOUT_3). This means the flyback transformer will have two secondary windings with ratios of circa 1:2.4 (Np_1:Ns_2) and circa 1:0.9 (Np_1:Ns_3) to provide the described secondary outputs of 10V (VOUT_2) and 3.3V (VOUT_3) respectively.

When the P-channel MOSFET of the converter is closed (ON state), the current will flow in the primary winding (Iprimary_1), while the current on the secondary windings (Isecondary_2 and Isecondary_3) will remain at zero since the rectifier diodes are reverse biased. Once the converter goes to OFF state, the flyback transformer will release the energy, the rectified diodes will be forward biased and current will flow on the secondary windings (Isecondary_2 and Isecondary_3) which will supply current to the loads (IOUT_2 to Load2 and IOUT_3 to Load3) and also charge the capacitors (C2 and C3). When the converter goes to the next ON state (and diodes again to reverse biased and therefore not conducting), the capacitors (C2 and C3) will supply the loads (Load2 and Load3), therefore ensuring a continuous regulated supply.

Accuracy for a stable isolated VOUT over the whole current range on the secondary output can be more precise by using the STMicroelectronics’ TL43x or TL143x voltage references in the post regulation circuitry.  

Finally, commenting that A6986I is also designed in application that do not need isolation, but just multiple outputs. That is by using a multiple winding flyback transformer several outputs can be generated at different voltage levels with just one regulator, this is not only practical and reduces overall footprint, but also an economic solution which also even reduces further the system costs as fewer inductors are required.

In conclusion, the isobuck A6986I brings several advantages on the power management design such as:

  1. Reduce total number of components (BoM), saving cost, footprint and design time.
  2. Enables both isolated step-up (boost) and step-down (buck) regulation.
  3. Multiple outputs possible to supply several loads.
  4. Isolated supply to handle different voltage classes in one same system.

A6986I can deliver up to 2A in the primary output (buck) while the secondary output (isolated) supports 5W power loads. It is designed as primary regulator from 12V battery and able to support cranking as well as load dump condition as the operating voltage range goes from min. 4V up to 38V. The switching frequency is configurable from 250kHz up to 1MHz and operates in forced PWM mode. 

A6986I also offers standard protection such as overvoltage trip and thermal shutdown. Configurable soft-start, Power Good pin or inhibit pin are also available among other features. For further details on the product specification, the A6986I Data Sheet is available on stm.com. Evaluation Boards are also available for customer evaluation.

Author : Llorenc VALLMAJO RIBAS – Senior Product Marketing Manager at STMicroelectronics

Link: https://www.avnet.com/wps/portal/ebv/products/new-products/npi/2021/stmicroelectronics-a6986i/