Small, remoted energy provides present energy throughout isolation boundaries in functions from electrical automobile traction inverters to manufacturing facility management modules. On this Energy Tip, I’ll look at totally different remoted bias-supply topologies and their electromagnetic interference (EMI) efficiency. As you’ll see, parasitic capacitance throughout the isolation transformer is a serious contributor to common-mode noise propagation.
In traction inverters, gate drivers actuate the high-power switches—often insulated-gate bipolar transistors (IGBTs) or silicon carbide (SiC) MOSFETs—which convert vitality between the high-voltage battery and the electrical machine (see Determine 1). The gate drivers are usually remoted, with one a part of the gate driver IC linked to the low-voltage area (the first facet) and one other half linked to the high-voltage area (the secondary facet). The gate-drive management alerts come from a microcontroller on the first facet and are handed by way of to the secondary facet to show the facility change on and off.
Determine 1 A traction inverter with remoted gate drivers. Supply: Texas Devices
The secondary facet of the remoted gate driver wants an remoted energy provide with a purpose to actuate energy change turn-on and turn-off (see Determine 2).
Determine 2 An remoted bias provide offers energy throughout the isolation barrier for the remoted gate driver. Supply: Texas Devices
The ability scores for remoted bias provides are often pretty low—lower than 10 W. The equation under estimates their energy necessities as:
PDRV = VDRV x Qg x FSW (1)
the place VDRV is the gate-drive voltage, Qg is the change gate cost, and FSW is the switching frequency of the change (not the remoted bias-supply switching frequency). The gate-drive voltage will depend on which change you choose, however usually ranges from +15 V to +25 V on the optimistic rail and –8 V to 0 V on the unfavourable rail.
Frequent topologies for remoted bias provides embrace flyback, push-pull and inductor-inductor-capacitor (LLC). Some totally built-in energy modules (that embrace the transformer within the bundle) use a full-bridge configuration on the first facet. Flyback converters, such because the LM5180-Q1 from Texas Devices (TI), are well-known, present good output voltage regulation, are pretty environment friendly, may be designed with out an optocoupler (utilizing primary-side regulation), and might have a number of remoted outputs. Their drawbacks are that they are usually restricted in frequency vary (<350 kHz) and have a big transformer measurement. Push-pull converters, comparable to TI’s SN6507-Q1 and LLC converters, comparable to TI’s UCC25800-Q1, are easy however don’t have closed-loop suggestions. Because of this, the output voltage regulation suffers, doubtlessly requiring a pre-regulator, a post-regulator, or each. Built-in energy modules (comparable to TI’s UCC14341-Q1) can regulate the output voltage and are easy and small, however their drawbacks are restricted energy output (usually <1.5 W) and decrease effectivity than different choices.
One query you will have is in regards to the EMI efficiency of various topologies: Do sure topologies have roughly influence on electromagnetic compatibility outcomes? To deal with these questions, let’s first look at the isolation transformer. Some parasitic capacitance does exist between the windings of the transformer, which costs or discharges when the traction inverter change node (VSW) switches between HV+ and HV– nodes. A brief pulse of common-mode present costs or discharges the parasitic capacitance in the course of the switching transition. The common-mode present is instantly proportional to the parasitic capacitance and the switch-node slew fee (dv/dt). Giant capacitance or sooner switch-node slew charges—as you would possibly see with huge band-gap semiconductors comparable to gallium nitride (GaN) and SiC—will lead to extra common-mode present. Determine 3 highlights this parasitic capacitance and the common-mode present used to cost and discharge it.
Determine 3 Frequent-mode present costs the transformer parasitic capacitance when the change node (VSW) transitions. Supply: Texas Devices
Converter topology does affect transformer design and the ensuing parasitic capacitance. Flyback converter transformers (or coupled inductors in case you favor that nomenclature) are designed to have robust coupling between the first and secondary sides in an effort to scale back leakage inductance. Leakage inductance causes undesirable voltage spikes and energy loss in snubber circuits. The unlucky influence of designing for low leakage inductance is that the interwinding capacitance usually will increase, and may be 20 pF or extra. Alternatively, it’s potential to design LLC converters to make use of a transformer’s leakage inductance in its resonant tank. Because of this, you wouldn’t must design the transformers to reduce leakage inductance; they will have a parasitic capacitance round 2 pF. As you’ll see, this helps cut back common-mode currents.
Desk 1 reveals some parameters from a examine of 4 remoted bias topologies to experimentally validate the influence of transformer parasitic capacitance on common-mode present. All the converters had been designed for a 15-VIN, 15-VOUT, 1.5-W software. The switching frequency of every topology is predicated on typical values, with the transformers designed accordingly. As you’ll be able to see, the flyback converter transformer has the bottom leakage inductance and the very best parasitic capacitance. The LLC converter transformer has the very best leakage inductance and the bottom parasitic capacitance.
Desk 1 Transformer parameters for 4 instance remoted bias-supply converters. Supply: Texas Devices
Evaluating these remoted bias-supply topologies concerned performing an intensive set of exams: effectivity, load regulation, enter and output ripple, thermals, and carried out and radiated EMI. To concentrate on the common-mode present measured between the remoted grounds within the system, my colleagues linked a wire between the 2 grounds and measured the common-mode present when the high-power switches (on this instance, a GaN half bridge utilizing the LMG3522R030-Q1) turned on and off at 400 V. Determine 4 and Determine 5 present the outcomes for a high-voltage switch-node slew fee of 40 V/ns and 100 V/ns, respectively.
Determine 4 Frequent-mode present comparability at a 40-V/ns slew fee on the change node. Channel 1 is the high-voltage change node (200 V/div) and Channel 2 is the common-mode present (500 mA/div). Supply: Texas Devices
Determine 5 Frequent-mode present comparability at a 100-V/ns slew fee on the change node. Channel 1 is the high-voltage change node (200 V/div) and Channel 2 is the common-mode present (500 mA/div). Supply: Texas Devices
The measured outcomes show that the common-mode present for the flyback transformer was the biggest (935 mA and 1,420 mA for 40-V/ns and 100-V/ns slew charges). That is to be anticipated, because the transformer had the biggest parasitic capacitance. And since the LLC converter had the bottom parasitic capacitance, the common-mode present measured was the smallest (197 mA and 570 mA for 40-V/ns and 100-V/ns slew charges). Giant common-mode present spikes are detrimental, since they will conduct noise from the high-voltage area to the low-voltage area, trigger floor bounce; and doubtlessly result in poor converter operation together with skipped pulses, misplaced regulation, or surprising shutdowns.
Frequent-mode currents may be particularly troublesome to mitigate. Among the best methods to handle common-mode present points is to keep away from producing common-mode present within the first place. Whereas the appliance mentioned right here was a traction inverter in an electrical automobile, the rules apply to functions comparable to grid-connected converters and server energy provides.
Pradeep Shenoy leads the Energy Design Companies workforce at TI, targeted on automotive programs. He beforehand labored in TI’s Kilby R&D Lab and within the Switching Regulators enterprise unit. He has introduced technical and business shows on the Utilized Energy Electronics Convention practically yearly since 2011, and serves as normal chair in 2023. He acquired the Illinois Worldwide Graduate Achievement Award in 2010, the Jack Kilby Award for innovation in 2015, and the Institute for Electrical and Electronics Engineers (IEEE) Richard Bass Award in 2020. He serves actively within the IEEE Energy Electronics Society as treasurer. Shenoy has a Ph.D. from the College of Illinois Urbana-Champaign.
Acknowledgments
Prashant Kumar and Markus Zehendner accomplished the {hardware} design and testing.
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