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It’s considerably just like the fairy-tale tales of Cinderella or the Ugly Duckling: For a few years, processors of all kinds, sizes, and speeds have been the glamorous topics of normal media consideration in addition to main R&D investments. On the identical time, energy units — primarily silicon-based MOSFETs in addition to IGBTs — have been apparently underappreciated and languished within the background as a part of the supposedly boring energy area of interest.
Issues have actually modified in the previous couple of years, as energy units, particularly these based mostly on gallium nitride and silicon carbide course of applied sciences, are getting loads of consideration and funding. It’s not sensible to supply particular greenback quantities right here, as it’s so laborious to measure; there are lots of official methods of wanting on the specifics and some ways to interpret that focus. Nonetheless, there’s little question that energy units are the place loads of the seen exercise is going on proper now, pushed by electrical automobiles, inexperienced initiatives, and lots of different elements.
However power-related circuit designs don’t reside by their energy units alone. The gate driver, which is often a separate machine for all however the smaller energy units, performs a key function in managing the facility machine, making certain it meets its efficiency potential and making certain that working boundary associated to voltage swing, present, turn-on/-off slew charges, parasitic inductance and capacitance, and extra are revered and adopted.
In reality, making the gate driver to be a detailed and appropriate associate of the switching energy machine is a vital a part of the design course of. The precise alternative could make that energy machine meet goal specs, whereas a suboptimal mismatch can negate a lot of its potential and even injury or destroy it. Consequently, gate drivers have their very own units of utility notes and design pointers; some are device-specific, whereas others are extra normal (see References).
Nevertheless, there’s one other, considerably silent associate within the relationship that’s behind the scenes and simply ignored till late within the cycle: the gate driver’s personal energy provide, virtually all the time a DC/DC converter. Whereas the facility machine has its personal provide rails by way of the load, the gate driver wants its personal rails that should be matched to the specifics of the driving force/energy machine mixture (Determine 1).
Why fear in regards to the gate driver’s energy supply?
I used to be reminded of this apparent however simply ignored actuality once I got here throughout the applying word “Gate Drive Software Notes: IGBT/MOSFET/SiC/GaN gate drive DC-DC converters” from Murata Energy Programs. This word seems to be at a few of the driver concerns and the way they mirror again onto the driving force’s DC/DC converter performing as its energy provide.
The primary level is that in lots of circuit topologies, such because the broadly used bridge association, the driving force and its DC/DC converter have to be galvanically remoted from floor together with high-side energy machine; in some instances, each high-and low-side units have to be remoted.
However that’s simply the beginning. It additionally seems that the bipolar DC/DC energy converter could not have to be symmetrical however will work successfully with the driving force with a damaging rail of decrease voltage than the constructive one. Needing solely a decreased negative-output voltage offers extra flexibility in designing or specifying the power-handling capabilities of the DC/DC converter. For instance, among the many Murata DC/DC pairing are 20-V/–5-V and 18-V/–2.5-V items, along with the extra widespread symmetrical bipolar values.
Because the Murata word factors out, “Whereas 0 V on the gate could be satisfactory for a lot of units, a damaging voltage usually between –5 V and –10 V allows fast switching managed by a gate resistor. Any emitter inductance L between a swap and the driving force reference [Point X in Figure 2] causes an opposing gate-emitter voltage when the swap is popping off. Whereas the inductance could also be small, simply 5 nH would produce 5V at a di/dt of 1,000 A/μs, which isn’t uncommon (5 nH is just some millimeters of wired connection.) An applicable damaging drive ensures that the gate-emitter off voltage is all the time really zero or much less.”
There are additionally concerns on the transient response of the DC/DC converter, because the load introduced by the driving force out of the blue adjustments because it drives the facility machine on and off. The Murata word makes it clear as properly: “IGBTs/MOSFETs shouldn’t be actively pushed by PWM alerts till the drive-circuit voltage rails are at right values. Nevertheless, as gate-drive DC/DCs are powered up or down, a transient situation would possibly exist the place units might be pushed on, even with the PWM sign inactive, resulting in shoot-through and injury. The DC/DC outputs ought to due to this fact be well-behaved on power-up and -down with monotonic rise and fall.” Sure, we knew that, however was that facet correctly taken under consideration? The word factors out some considerations when selecting an appropriate DC/DC converter for the gate driver.
There are occasions when I’ve a nagging fear that the multidimensional calls for positioned on engineers will lead to no moderately good ultimate design, as there are simply too many conflicting expectations within the face of unavoidable tradeoffs. But regardless of my fear, I see many superb designs that one way or the other handle to fulfill, to a suitable and even required degree, the various electrical, environmental, regulatory, thermal, effectivity, value, manufacturing, and different overlapping calls for positioned on them.
The attributes of the device-driver DC/DC converter are one more set of those challenges with many levels of freedom to juggle. It’s unlucky that most of the people doesn’t recognize this and certain thinks it’s straightforward. That’s why I discover it particularly worrisome when politicians legislate the form and charge of progress towards some laudable aim. If solely it have been that straightforward.
What’s been your expertise with publicity to gate drivers and particularly their energy converters? Has the efficiency of the converter even turn into an sudden limiting issue or downside even when the driving force and energy machine have been correctly matched?
References
- Texas Devices. “Fundamentals of MOSFET and IGBT Gate Driver Circuits”
- IR/Infineon. “Gate Drive Traits and Necessities for HEXFET Energy MOSFETs,” Software Notice AN-937.
- Texas Devices. “Widespread Mode Transient Immunity (CMTI) for UCC2122x Remoted Gate Drivers”
- “Drive Circuits for Energy MOSFETs and IGBTs,” Software Notice AN524.
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