A fully integrated 3 phase IGBT switching assembly with a very low loss DC Link Capacitor -- Ed Sawyer, SBE Inc. Scott Leslie, Powerex Inc.

A fully integrated 3 phase IGBT switching assembly with a very low loss DC Link Capacitor -- Ed Sawyer, SBE Inc. Scott Leslie, Powerex Inc.

Thermal characteristics of the Power Ring shape SBE has conducted extensive modeling and thermocouple testing of its Power Ring Film Capacitor shape Based on those results we can very accurately relate current and Trise of our devices SBE has conducted, and continues to conduct, comparative tests of most of the typical DC Link capacitor solutions in the market vs. the Power Ring solution

Thermal characteristics of the Power Ring shape 5000 second simulation temperature profile indicating predicted hot spot

Thermal characteristics of the Power Ring shape From this analysis, we can intelligently place thermo-couples on Film Capacitor to monitor critical temperature rise locations This presentation utilizes that data to compare traditional Inverter volumetric power density to that which is possible utilizing the Power Ring shape An assumption is made that for any given acceptable Trise implemented the resulting film capacitor reliability is the same, all else being equal

Concept 1 Modules around a ring 3 phase individual switches around a ring

Concept 1 Modules around a ring SBE has experimented with innovative ways to populate IGBT switch modules around a ring of capacitance Goals Most volumetric power density Low inductance (reduce or eliminate snubbers) Symmetric current density in ring for lowest max Trise point

Concept 2 Module inside the ring center A 3 phase IGBT module inside a Power Ring

Concept 2 Module inside the ring center SBE can vary the size of the center hole Capacitor volume is small near the center Modules inside the hole offer challenges and advantages but may be a path to a very power dense solution Powerex and SBE are investigating the feasibility and development costs The opportunity of the result is high

Interconnect detail and concerns/advantages E1C2 C1 Low Inductance Internal Terminal Configuration E2 IGBT FWD Standard IGBT Module Example

Interconnect detail and concerns/advantages C1 E2 FWD E2 C1 IGBT E1C2 Low Profile IGBT Module Example E1C2

Interconnect detail and concerns/advantages The connection opportunity is enticing The connection tabs lend themselves to short connection to both sides Lowest possible length of connection from die to DC Link capacitor The lowest inductance and best current distribution would result from a new module die lay-out

Other low inductance configurations Stacked capacitance and module If the designer becomes less concerned with capacitor radiated heat, tighter configurations are possible Lowest possible length of connection from IGBT switch to DC Link capacitor is the goal Result is increased power density, reduction or elimination of snubbers, and more efficient cooling

Interconnect detail and concerns/advantages Example of DC link capacitor/igbt stacked

How big a capacitor? Capacitor current for a 3-phase inverter is a complex function of: -inverter loading -DC bus impedance -gate drive timing algorithm Tough to calculate a minimum value capacitance! Best way is to back into a reasonable value Assumptions for an example case: 260ADC inverter input for peak load(70 100kw motor) 10v p-p ripple voltage maximum 20khz switching frequency

How big a capacitor? Assume a worst case scenario, a converter where the capacitor has to source all current half the time. Capacitor waveform triangle, 10Vp-p V=It/C. C=I*t/V P=50us [20KHz fs] t=25us C=260*25e-6*/10 = 650uF This sets upper bound on capacitance required. Better estimate for Capacitance? Sure!

How big a capacitor? Common formula for minimum capacitance: From AVX appnotes p58 C=Irms/(2*pi*fsw*Vrms ripple) This makes the tenuous assumption that capacitor ripple voltage is triangular, and approximately the same RMS value as a sine wave.and that ripple current is the same as the DC load current Rms ripple 3.54V Calculated value for C=584uF.

How big a capacitor? Do you really need 584uF to keep ripple less than 10Vp-p? NO! 3phase inverter will never require the capacitor to supply entire load current half the time! Capacitor RMS current will be significantly less than DC bus current. 500uF will be more than sufficient. A lower value should be considered!

Then why use 1000-2600µF? Current ratings of capacitors are the limiting factor in size reduction Typically 5-10µF/Arms for film caps Typically 20-40µF/Arms for electrolytics The equivalent Power Ring shape capacitor 1.2-2µF/Arms Rating depends on thickness of ring and current distribution within ring (bus and/or connection design) For all capacitor types, thermal environment is important

Comparison of Power density vs. traditional layout The 260Arms example DC Link Capacitor A 2600µF Kemet A 2098µF Lexus A 500µF SBE Standard Power Ring A possible 300µF SBE Optimized Design Ring and Powerex IGBT arrangement All have essentially the same Trise from comparative tests (within 3 C) (to be verified on the 300µF example)

Comparison of Power density vs. traditional layout Calculated Power Density of IGBT and DC Link Capacitor examples (no cooling/bus volume) 2600µF Kemet 37W/cc 2098µF Lexus 37W/cc 500µF SBE Standard Power Ring 70W/cc 300µF SBE Optimized Design Ring and Powerex IGBT arrangement 87W/cc An 80-130% increase in power density vs. these traditional designs!

Comparison of Power density vs. traditional layout The 300µF optimized solution is assumed to be in the stacked configuration for this analysis The circle of capacitance around a module example needs further analysis to understand the efficiency of connections and current distribution. 300-500µF final size and resulting power density expected

Conclusions SBE Power Ring shape allows for new design ideas for inverter power density Combined with the Thin Pack IGBT products from Powerex provides for the leading edge in inverter power density with no sacrifice in reliability

Conclusions The challenge for the Powertrain Systems Designer is to decide what to do with this Power Density advantage Reduce the size of inverter for equivalent power Reduce the capacitor cooling requirements Increase the available power Improve the reliability