Technical Article

Extending Unified AC-Input Light Industrial Application Output Power Via Low RDS(ON) SiC MOSFETs

May 30, 2023 by Simon Kim

For light industrial applications, a unified platform for single and 3-phase AC input can be designed using a 1200 V SiC MOSFET and a diode in the power factor correction stage to maintain a constant DC link voltage of 540 V.

Commercial fridge applications of 5.25 kW can be realized using a 30 mΩ SiC MOSFET in the power factor correction (PFC) stage. Using a low RDS(ON) 14 mΩ, 7 mΩ, or two 30 mΩ SiC MOSFETs in parallel can mitigate the thermal bottleneck at PFC and extend the output power to 7 kW for commercial air conditioner applications. Paralleling SiC MOSFETs gives better thermal performance by effectively reducing the case-to-heat sink thermal resistance by half.

Commercial refrigerators and air conditioners (CAC) are two major types of light industrial applications, with maximum power levels of 5.25 kW and 7 kW, respectively, with single-phase AC input. The power electronics circuit of these applications is composed of a single-phase input rectifier, a PFC boost circuit, and a 3-phase inverter circuit (often with forced air cooling) [1][2]. The requirement for different topology and voltage classes in the input stage and inverter stage can be substantially simplified by a unified platform that uses 1200 V class power devices for the rectifier, inverter, and PFC stages. An example of such a unified platform with a power integrated module (PIM), a SIC MOSFET, and a diode is shown in Figure 1.

 

Figure 1. Concept of the unified platform with a 1200 V PIM IGBT module, a SiC MOSFET, and a diode. Image used courtesy of Bodo’s Power Systems [PDF]

 

Resolving Power Factor Correction Thermal Bottlenecks

SiC MOSFETs and diodes are essential for the PFC stage due to their low losses and high switching frequencies. In this platform, a 6-channel gate driver IC, 6ED2230ST, and an IGBT module, FP25R12W1T7, have been used in the inverter stage. When a 230 Vac, single-phase AC is used, the DC link voltage is boosted to 540 V by the PFC stage. On the other hand, when a 380 Vac, 3-phase AC is used, the PFC circuit is bypassed by the diode or relay, but the DC link voltage can still reach 540 V. As shown in previous works, for a 5.25 kW commercial fridge inverter application a 30 mΩ SiC MOSFET in the PFC stage will help meet all thermal requirements [1]. When the same design is used for 7 kW CACs, the junction temperature, TVJ, of all power devices remains within the thermal limit, except for the PFC SiC MOSFET, and this becomes a thermal bottleneck [3]. This article discusses how using a SiC MOSFET with lower RDS(ON) to reduce power loss can extend the system power level from 5.25 kW to 7 kW for both commercial refrigerators and air conditioner applications.

To resolve the thermal bottleneck created at the PFC stage of a 7 kW CAC while using a 30 mΩ SiC MOSFET (IMW120R030M1H), a SiC MOSFET with an RDS(ON) of 14 mΩ and a junction-to-case thermal resistance, RthJC, of 0.25 K/W (IMZA120R014M1H) was investigated; assuming an RthCH of 1 K/W [4], and a junction-to-heat sink thermal resistance RthJH of 1.25 K/W. A demo board was used to measure the loss at operating conditions to get a more accurate simulation model than the one prepared using datasheet conditions. Power loss simulation using PLECS showed a total loss of 41.8 W with 29.4 W of switching loss and 12.4 W of conduction loss, which was significantly lower than that with IMW120R030M1H. Mersen R-TOOLS thermal simulation [5] gave the heat sink temperature, TH, and the junction temperature, TVJ, of the PFC SiC MOSFET as 82.1°C and 134.4°C, respectively, as listed in Table 1. The thermal design requirement for 7 kW systems was thus met.

 

Table 1. Device temperature for 7 kW output power when using a 14 mΩ SiC MOSFET, IMZA120R014M1H, with VDC = 540 V
Item

PFC

SiC M

PFC

SiC D

Rec.

Inv.

IGBT

Inv.

FWD

Ploss [W] 41.8 21.3 18.1 16.4 3
TH[°C] 82.1 79.1 95.8 95.8 95.8
RthJH [K/W] 1.25 1.7 1.54 1.55 2.04
TVJ[°C] 134.4 115.3 123.7 121.2 101.9

 

A SiC MOSFET, IMZA120R007M1H, with an even lower RDS(ON) of 7 mΩ and an RthJC of 0.15 K/W was also evaluated for the 7 kW CAC, assuming an RthCH of 1 K/W and RthJH of 1.15 K/W. The power loss by simulation of this SiC MOSFET was 41.7 W, with an estimated TVJ of 130.1°C, as listed in Table 2. The thermal performance improved only marginally because the reduction in conduction loss was offset by an increase in the switching loss. RthJH, however, also reduced only marginally.

 

Table 2. Device temperature for 7 kW output power when using a 7 mΩ SiC MOSFET, IMZA120R007M1H, with VDC = 540 V
Item

PFC

SiC M

PFC

SiC D

Rec.

Inv.

IGBT

Inv.

FWD

Ploss [W] 41.7 21.3 18.1 16.4 3
TH[°C] 82.1 79.1 95.8 95.8 95.8
RthJH [K/W] 1.15 1.7 1.54 1.55 2.04
TVJ[°C] 130.1 115.3 123.7 121.2 101.9

 

Another approach to 7 kW light industrial applications is to use two pieces of the SiC MOSFET, IMW120R030M1H, with an RDS(ON) of 30 mΩ, in parallel. For devices in parallel, the current sharing imbalance should be considered. Assuming a current imbalance of 10 percent, the power handled by one SiC MOSFET in a 7 kW PFC, for example, would be 3.85 kW. To simplify the evaluation, the output power was set to 7.7 kW with equal current sharing in both the SiC MOSFETs. The power loss of one SiC MOSFET in the PLECS simulation was 21.3 W, as shown in Figure 2. Thermal simulation by R-TOOLS gave the TH of the PFC SiC MOSFET and inverter IGBT as 79.9°C and 96.6°C, respectively, as shown in Figure 3. TVJ of all power devices were within their thermal limit, as listed in Table 3. The thermal design requirement for 7 kW systems was thus met.

It is notable that the TVJ of the PFC SiC MOSFET is lower with two pieces of 30 mΩ devices in parallel than a single 14  mΩ device (112.1°C versus 134.4°C) despite the 10 percent margin added to compensate for the current sharing imbalance. This is due to the effective reduction in RthCH by half when two packages are in parallel. RthCH is a dominant portion of the total RthJH, with a nearly 80 percent share in low RDS(ON) devices such as the 14 mΩ IMZA120R014M1H.

 

Figure 2. PFC and rectifier power loss simulation, using PLECS, with two pieces of SiC MOSFET, IMW120R030M1H, in parallel. The SiC MOSFET’s power loss at rated output power of 7.7 kW with 10 percent current imbalance is 21.3 W with 7.9 W conduction loss and 13.4 W switching loss. Image used courtesy of Bodo’s Power Systems [PDF]

 

Figure 3. R-TOOLS thermal simulation with a heat sink of dimensions 140 mm (L) x 120 mm (W) x 60 mm (H) and an airflow velocity of 3.7 m/s with paralleled MOSFETs for 7.7 kW

 

Table 3. Device temperature for 7 kW output power when using two 30 mΩ SiC MOSFETs in parallel with VDC = 540 V
Item

PFC

SiC M

PFC

SiC D

Rec.

Inv.

IGBT

Inv.

FWD

PSloss [W] 21.3 24.6 18.1 16.4 3
TH[°C] 79.9 78.3 95.8 95.8 95.8
RthJH [K/W] 1.51 1.7 1.54 1.55 2.04
TVJ[°C] 112.1 120.1 124.5 122 103.2

 

The power loss and estimated maximum junction temperature of different SiC MOSFETs used in the PFC stage for 7 kW system output power are shown in Figure 4. Power loss and TVJ of a PFC SiC MOSFET can be reduced using a single device with low RDS(ON) or two devices in parallel. Paralleling SiC MOSFETs gives a better thermal performance despite the current sharing imbalance due to the effective reduction in the case-to-heat sink thermal resistance, RthCH, by half.

 

Figure 4. Power loss and estimated junction temperature of different SiC MOSFETs used in the PFC stage for 7 kW output power. Image used courtesy of Bodo’s Power Systems [PDF]

 

References

[1] S. Kim, and K. W. Ma, “Unified platform with single- and 3-phase input for light industrial application using SiC power device,” ICMRA 2021. Zhanjiang, China, DOI 10.1109/ICMRA 53481.2021.9675728

[2] S. Kim, B. S. Swaminathan, K. W. Ma, and D. W. Chung: “Unified design approach for single- and 3-phse input air conditioning system using SiC device”, KIPE Conference, Busan, Korea, 2020, pp. 205–208

[3] S. Kim, and K. W. Ma, “Extending output power of unified AC input light industrial applications by SiC MOSFET”, PCIM Europe 2022, DOI:10.30420/565822182

[4] Infineon Technologies, “AN2015-13 Explanation of discrete IGBT’s datasheet”

[5] Mersen R-TOOLS MAXX: https://www.r-tools.com

 

This article originally appeared in Bodo’s Power Systems [PDF] magazine

Featured image used courtesy of Adobe Stock