Pay Consideration to the Gate-Driver Energy Provide


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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 every kind, sizes, and speeds have been the glamorous topics of common 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 offer particular greenback quantities right here, as it’s so laborious to measure; there are numerous legit methods of trying on the specifics and some ways to interpret that focus. Nonetheless, there’s little doubt that energy units are the place loads of the seen exercise is occurring proper now, pushed by electrical automobiles, inexperienced initiatives, and plenty of different components.

However power-related circuit designs don’t reside by their energy units alone. The gate driver, which is normally a separate gadget for all however the smaller energy units, performs a key position in managing the ability gadget, guaranteeing it meets its efficiency potential and guaranteeing 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 an in depth and appropriate companion of the switching energy gadget is a vital a part of the design course of. The correct selection could make that energy gadget meet goal specs, whereas a suboptimal mismatch can negate a lot of its potential and even harm or destroy it. In consequence, gate drivers have their very own units of utility notes and design pointers; some are device-specific, whereas others are extra common (see References).

Nevertheless, there’s one other, considerably silent companion within the relationship that’s behind the scenes and simply ignored till late within the cycle: the gate driver’s personal energy provide, virtually at all times a DC/DC converter. Whereas the ability gadget has its personal provide rails through the load, the gate driver wants its personal rails that have to be matched to the specifics of the motive force/energy gadget mixture (Determine 1).

Determine 1: On this stylized view of an entire power-system chain, there’s normally a lower-voltage rail for the processor and its help, in addition to rails for the load, whereas the gate driver usually wants its personal provide relying on the motive force and energy gadget pairing.

Why fear concerning the gate driver’s energy supply?

I used to be reminded of this apparent however simply ignored actuality after I got here throughout the applying notice “Gate Drive Utility Notes: IGBT/MOSFET/SiC/GaN gate drive DC-DC converters” from Murata Energy Techniques. This notice appears at a few of the driver issues and the way they mirror again onto the motive 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 motive force and its DC/DC converter must be galvanically remoted from floor together with high-side energy gadget; in some circumstances, each high-and low-side units must be remoted.

However that’s simply the beginning. It additionally seems that the bipolar DC/DC energy converter might not must be symmetrical however will work successfully with the motive force with a detrimental rail of decrease voltage than the optimistic 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 frequent symmetrical bipolar values.

Because the Murata notice factors out, “Whereas 0 V on the gate could be sufficient for a lot of units, a detrimental voltage usually between –5 V and –10 V permits fast switching managed by a gate resistor. Any emitter inductance L between a change and the motive force reference [Point X in Figure 2] causes an opposing gate-emitter voltage when the change 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 only a few millimeters of wired connection.) An acceptable detrimental drive ensures that the gate-emitter off voltage is at all times really zero or much less.”

Determine 2: As a result of unavoidable stray inductance L, a detrimental di/dt produces a detrimental voltage on the emitter, opposing the turn-off voltage at switch-off. (Supply: Murata Energy Techniques)

There are additionally issues on the transient response of the DC/DC converter, because the load introduced by the motive force all of a sudden adjustments because it drives the ability gadget on and off. The Murata notice makes it clear as properly: “IGBTs/MOSFETs shouldn’t be actively pushed by PWM indicators till the drive-circuit voltage rails are at appropriate values. Nevertheless, as gate-drive DC/DCs are powered up or down, a transient situation may exist the place units could possibly be pushed on, even with the PWM sign inactive, resulting in shoot-through and harm. 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 side correctly taken into consideration? The notice factors out some issues when selecting an acceptable 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 end in no fairly good closing design, as there are simply too many conflicting expectations within the face of unavoidable tradeoffs. But regardless of my fear, I see many excellent designs that by some means handle to fulfill, to an appropriate and even required stage, the various electrical, environmental, regulatory, thermal, effectivity, price, manufacturing, and different overlapping calls for positioned on them.

The attributes of the device-driver DC/DC converter are yet one more set of those challenges with many levels of freedom to juggle. It’s unlucky that most of the people doesn’t admire this and certain thinks it’s straightforward. That’s why I discover it particularly worrisome when politicians legislate the form and fee of progress towards some laudable objective. If solely it have been that easy.

What’s been your expertise with publicity to gate drivers and particularly their energy converters? Has the efficiency of the converter even turn out to be an sudden limiting issue or downside even when the motive force and energy gadget have been correctly matched?

References

  1. Texas Devices. “Fundamentals of MOSFET and IGBT Gate Driver Circuits
  2. IR/Infineon. “Gate Drive Traits and Necessities for HEXFET Energy MOSFETs,” Utility Be aware AN-937.
  3. Texas Devices. “Frequent Mode Transient Immunity (CMTI) for UCC2122x Remoted Gate Drivers
  4. Drive Circuits for Energy MOSFETs and IGBTs,” Utility Be aware AN524.

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