Eric Persson, Infineon Technologies
With industrial, office and home automation all expanding, motors and their drives are increasingly used for a wide range of functions from robotic arm control to the humble domestic clothes washer. Motors have of course been used for over a century, but today they must be ‘smart’ to enable more flexibility in motion control, for better functionality and energy savings. However, there are constraints – industrial products have to be small, light and energy efficient, while commercial products also have to be very low cost.
Enhanced control is achieved with electronic motor drives or ‘voltage source inverters’ which typically generate three-phase AC at variable frequency and amplitude to control motor speed, torque and direction. Switched-mode techniques are used in the drive, typically operating around 16 kHz, with pulse width modulation to achieve output control. Devices such as silicon MOSFETs have for decades been able to switch an order of magnitude higher, but in motor drives, the advantages of high frequency – smaller magnetics – is not so apparent; the ‘magnetic’ element is the motor itself which is sized independently for the application. Keeping frequency low is therefore sensible for minimum switching losses. The fast voltage transitions seen with high frequency devices such as MOSFETs actually present their own problems; high ‘dV/dt’ causes motor winding insulation stress with voltage overshoot or ringing, risking breakdown and ‘partial discharge’ degradation. Additionally, EMI increases, requiring additional filters, and common mode EMI currents can find their way through the bearings of the motors to ground, producing mechanical wear in the form of ‘fluting’ in the bearing races.
Integrated power modules
With the seeming lack of advantage of high-frequency switching, IGBTs are still commonly used but in a bid to improve efficiency, Si MOSFETs are also popular with measures to reduce the switching edge rates by slowing gate drives, introducing snubbers and incorporating external series and common mode filters in the three-phase drive output, all impacting efficiency to an extent. At low and medium powers, the MOSFETs can yield lower conduction losses than IGBTs and certainly better switching losses. Both device types are offered in ‘Intelligent Power Modules’ or IPMs which integrate the typically six switches required, along with gate drives and protection features.
The wide bandgap switch approach using GaN
IPMs with Si MOSFETs are efficient, but there is inexorable pressure to improve further; not only does better efficiency save energy and money, but it can lead to smaller, lighter products with less expensive heatsinking. If an improvement can remove the need for an unreliable fan or allow a smaller drive to be placed alongside the motor in a robot arm instead of in a remote cabinet, the benefits are clear. More efficient devices are now available in the form of gallium nitride (GaN) high electronic mobility transistors(HEMTs). This wide bandgap technology, available from Infineon, exhibits lower conduction losses than otherwise-comparable silicon MOSFETs and can be the key to unlocking the benefits outlined, outweighing their premium in unit cost by far.
Controlling dV/dt is key
GaN switches, such as CoolGaN, are phenomenally fast, capable of edge rates measured in hundreds of kV/µs. This is a great attribute in miniature AC-DC and DC-DC converters operating at 1 MHz and higher, but in motor drives at low frequency, there is little benefit over the already minimal switching loss in the older technologies, and the EMI, ringing, breakdown and bearing wear problems are exacerbated. Edge rates therefore have to be controlled to more benign levels. If the aim is to maintain high efficiency, lossy snubbers and external filters are not preferred, so slowing the gate drive signal to the device can be considered. The typical method is to add series gate resistance which forms an R-C filter with the gate capacitance, slowing switching, with two resistors often used with diode steering to independently control positive and negative transitions. The method is common with Si-MOSFETs but with GaN there is a problem – the gate capacitance varies over a three-decade range under different operating conditions, whereas with MOSFETs, the range is much smaller. This means that the introduced delay is highly variable, producing a corresponding variation in dV/dt. For reliable motor operation the edge rate should be no more than about 5 kV/µs (5 V/ns) so if the resistors are set for this as a worst case, under other conditions, the edges will be much slower, risking significant loss in efficiency (Figure 1).
The variation in total gate capacitance CRSS is mainly due to the ‘Miller’ effect where gate-drain capacitance CGD is effectively amplified by the action of the drain transitioning from high to low voltage and back, with the variable device output capacitance COSS and input capacitance CISS also playing a part.
An effective solution to optimize edge rates is to sample the drain voltage through a capacitor which generates a current proportional to dV/dt. This can then be fed back to the gate drive circuit to control gate charge and discharge currents, for constant edge-rate under all conditions. Implementation is problematic though, with the addition of a high-voltage capacitor which is not easily incorporated into IPMs as a discrete component. Cost is also increased along with the complexity of additional wire bond attachments to the controllers in an IPM. It has also been observed that parasitic inductance in the capacitor connection can lead to sustained oscillation and device failure.
The capacitor could be fabricated as part of the GaN die and connection brought out through wire bonds, but Infineon engineers realized that simply including a very small capacitor from drain to gate in the GaN die had a significant ‘linearizing’ effect on overall capacitance. The value chosen is tiny, of the order of 1.2 pF, resulting in about double the existing total gate charge figure. Drive losses increase, but are anyway negligible at around 50 µW switching at 16 kHz. The effect is shown in Figure 2 where edge rates are accurately limited to around 5 V/ns, with dV/dt naturally falling to lower values at lighter loads.
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Figure 2: Edge rates are limited under all conditions with GaN linearizing capacitor included
The technique developed has allowed Infineon to design IPMs with significantly better efficiency than Si-MOSFET types in the same application, while controlling edge rates to acceptable values. A comparison between technologies in Table 1 shows near halving of losses for the same temperature rise in a motor drive application, leveraging the better on-resistance of the GaN device in the IPM. The result could be the difference between operation with and without a heatsink or ability to drive larger motors with the same overall drive size. In both cases costs are saved.
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Table 1: IPM capability compared across MOSFET and GaN switch technologies
GaN die in IPMs for motor drive applications with integrated linearizing capacitors show dramatic reduction in power loss over silicon technologies, without the high edge-rate problems that could impact reliability and EMI compliance. Parts from Infineon now have a proven pedigree and have been shown to be robust against stress and short circuits often present in motor drive applications.
GaN switches are no longer exotic curiosities - their value is already seen in DC output power converters and now they are shown to potentially reduce system costs in motor drives, outweighing the small premium for the switch technology.