When designing a motor management circuit, it’s important to find out how you can ship the excessive present required to drive the motor. Designers should select whether or not to make use of monolithic built-in circuits (ICs) which have inner energy units, or gate driver ICs and discrete exterior energy MOSFETs.
This text discusses the benefits and drawbacks of every method and gives steering on when to decide on both resolution.
Monolithic drivers
The primary possibility is to make use of a monolithic driver IC to drive a motor. An built-in IC is comprised of 1 silicon die in a bundle; this die integrates logic, assist, and safety circuitry, in addition to the ability units like MOSFETs that drive the present by the motor.
As a result of the MOSFETs in a monolithic resolution are fabricated on the identical die because the management circuitry, these options present the advantage of correct present measurement. Monolithic ICs additionally present sturdy safety options, resembling over-current safety (OCP) and over-temperature safety (OTP), since this circuits might be positioned in shut proximity to the MOSFETs on the silicon.
Built-in drivers are restricted to voltage and present scores which might be appropriate with IC processes, which implies that the very best accessible voltage ranking is between 80 V and 100 V. As well as, these drivers can drive as much as about 15 A.
Monolithic drivers are used virtually completely in high-volume purposes resembling printers, the place provide voltages are usually under 35 V and motor currents are under 5 A.
An instance of an built-in driver is MPQ6541, an automotive-specified, 3-channel energy stage machine. It’s rated at a provide voltage of as much as 45 V and a steady load present of 8 A, or a peak present of 15 A per channel. This motor drive integrates six MOSFETs which have an RDS(ON) of 15 mΩ every. It is available in a TQFN-26, 6 mm x 5 mm flip-chip bundle.
Determine 1 The block diagram highlights the important thing constructing blocks of an built-in driver. Supply: Monolithic Energy Techniques
Determine 1 exhibits the MPQ6541’s block diagram. Observe that it integrates present measurement for every channel. This eliminates the necessity for giant and costly current-sensing resistors or current-sense amplifiers.
Gate drivers
The second possibility makes use of discrete energy MOSFETs—or in some instances, different energy units—to drive the present by the motor, and the MOSFETs are managed by way of a gate driver IC, pre-driver, or a number of gate drivers.
Monolithic options don’t exist for purposes that require excessive voltages exceeding 100 V or very excessive currents. In these instances, a gate driver, plus discrete MOSFETs, are required.
Since a number of units are required on this situation—generally as many as three gate drivers and 6 energy MOSFETs—the answer measurement, which is the PCB space occupied by the motor driver, is way bigger than what’s required by a monolithic driver.
An instance of a extremely built-in gate driver is the MPQ6533, a 3-channel gate driver IC with built-in options like slew price management and inner diagnostic capabilities. This machine is accessible in a 5 mm x 5 mm QFN-32 bundle.
Determine 2 The block diagram highlights the important thing constructing blocks of a gate driver. Supply: Monolithic Energy Techniques
Determine 2 exhibits the MPQ6533’s block diagram. Observe that this resolution requires six energy MOSFETs. Usually, three twin MOSFETs (two MOSFETs packaged collectively into one IC bundle) are used.
Value concerns
Analog and mixed-signal IC processes are way more advanced than devoted discrete MOSFET processes. Since fabricating low RDS(ON) MOSFETs in an IC course of takes a big space of silicon, the price of a tool with the identical RDS(ON) and voltage in a MOSFET course of is often increased than it might be to manufacture an identical machine in a devoted discrete MOSFET course of.
For lower-current and/or lower-voltage motor drivers, the penalty for fabricating the MOSFETs within the IC course of is small. Because the management and safety capabilities take up an enormous a part of the die, the added space for the MOSFETs doesn’t improve the associated fee as a lot as utilizing exterior MOSFETs.
For top-current purposes, nevertheless, the price of the MOSFETs in an IC course of begins to dominate the price of the machine. Despite the fact that there are monolithic motor drivers that may assist a 15-A motor present, they usually are dearer than an implementation utilizing a gate driver plus discrete MOSFETs.
There are instances the place the small measurement of a monolithic half is valued so extremely that it justifies a dearer resolution. For instance, some techniques require an built-in driver inside a motor, the place there may be little house accessible. In these eventualities, an answer utilizing a gate driver plus MOSFETs merely might not match within the constrained house.
To get a tough thought of the relative value of monolithic options vs. a gate driver resolution, we are able to examine the price of a monolithic IC plus a gate driver IC with three twin MOSFETs and three current-sense resistors. Different supporting elements, resembling bypass capacitors, have related costs between each options. Observe that these prices are based mostly on low amount catalog costs; precise quantity manufacturing costs are usually a lot decrease.
Desk 1 A value comparability is proven between a devoted monolithic IC and gate driver IC. Supply: Monolithic Energy Techniques
Desk 1 exhibits the associated fee comparability between a devoted monolithic IC and gate driver IC with discrete MOSFETs.
Answer measurement
Monolithic drivers are virtually all the time smaller than the equal resolution utilizing gate drivers and discrete MOSFETs.
For example, we are able to examine the PCB space occupied by the MPQ6541 to the MPQ6533 with extra energy MOSFETs (Determine 3). Each elements considerably differ in measurement, with the MPQ6541 occupying 130 mm2 and the MPQ6533 occupying 520 mm2, which is 4 instances bigger. Observe that the gate driver resolution proven right here makes use of twin MOSFETs in small packages; in different instances, the MOSFETs might be a lot bigger, which additional will increase the answer’s PCB space.
Determine 3 A measurement comparability is proven between a devoted monolithic IC and gate driver IC. Supply: Monolithic Energy Techniques
Thermal concerns
To successfully dissipate the warmth that’s generated within the energy MOSFETs, the PCB is often used as a heatsink. Bigger packages usually have higher thermal conductivity to the PCB, which implies that greater options are higher from a thermal dissipation standpoint. This may work in favor of options that use gate drivers, for the reason that energy MOSFETs are usually giant. Low RDS(ON) energy MOSFETs are available, so in some instances—particularly with purposes that must function in harsh environments—thermal concerns might preclude using a monolithic driver.
Monolithic drivers are available in smaller packages. To compensate for the upper thermal resistance in these packages, the RDS(ON) for a given present have to be decrease than it might be for a comparable resolution utilizing a discrete MOSFET.
Take into account the MPQ6541 monolithic driver and its smaller measurement. If the PCB is designed accurately, a major present might be pushed by this half. Determine 4 exhibits the MPQ6541’s temperature on a 5 cm x 5 cm, 2-layer PCB, whereas delivering 6 A of present to a 3-phase brushless motor. The case temperature measured was 38°C above the ambient temperature. A 4-layer PCB with inner planes would additional decrease the temperature rise.
Determine 4 The thermal picture of a monolithic driver highlights temperature situations. Supply: Monolithic Energy Techniques
Fastidiously contemplate tradeoffs
Desk 2 summarizes the primary variations between built-in and gate driver options.
Desk 2 An element comparability highlights main variations between a devoted monolithic IC and gate driver IC. Supply: Monolithic Energy Techniques
The selection between monolithic motor drivers and gate drivers with exterior MOSFET options to drive motors is advanced. Tradeoffs between value, resolution measurement, and thermal traits have to be thought-about.
For very small motors, monolithic drivers are one of the best resolution. Likewise, for very high-power motors, an answer utilizing gate drivers and discrete MOSFETs ought to be used. Nevertheless, there’s a giant overlap between each options, so designers ought to contemplate the specs of their software when making their selection.
Pete Millet is employees technical advertising and marketing engineer at Monolithic Energy Techniques (MPS).
Associated Content material
