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300V IGBTs Displace MOSFETs in Power Supplies Jonathan Dodge, P.E. Senior Applications Engineer Jeff Morrison Senior Characterization Engineer Advanced Power Technology 405 S.W. Columbia Street Bend, OR 97702 USA Abstract With recent reductions in both conduction and switching losses, PT IGBTs now have superior performance over MOSFETs rated from 200 to 300 Volts. This paper compares the overall performance of these 300 Volt IGBTs with 300 Volt power MOSFETs. On-state voltage is greatly reduced due to conductivity modulation in the IGBT, yet total switching losses are almost identical to that of MOSFETs. Due to active minority carrier lifetime control, these PT IGBTs are suitable for high frequency power supply applications. Tests in a hard-switched circuit show that the 300 Volt IGBTs provide a lower cost, higher performance alternative to power MOSFETs. lowest on resistance has the lowest conduction loss. For power MOSFETs with blocking voltage above about 200V, the majority of on resistance is in the drift region rather than the channel. IGBTs address the on resistance issue at its core by significantly reducing the drift region resistance. This is done by injecting minority carriers into the drift region (conductivity modulation and bipolar current flow) [1]. Introduction IGBTs are replacing MOSFETs in many high frequency SMPS designs [1]. The use of 300V IGBTs in power supplies is a relatively new development. Recent advancements in PT IGBT technology such as Power MOS 7 IGBTs make it possible for 300V IGBTs to replace power MOSFETs, even with the lower RDS(on) at 200V to 300V ratings. The main advantage of these new 300V IGBTs is lower cost while maintaining performance similar to power MOSFETs. Alternatively, efficiency and power density can be significantly improved at the same cost. 300V PT IGBT Technology IGBTs can be thought of as having on resistance similar to MOSFETs. The relationship at any given voltage and current is simply Ohm’s law: on-state voltage equals current times on resistance. IGBT on resistance is it on-state voltage (VCE(on)) divided by collector current IC, just like MOSFET RDS(on) is drainsource on-state voltage divided by drain current ID. Care must be taken to use the voltage corresponding to the operating temperature. Conduction loss is the product of on resistance squared and current, so the device with the Figure 1 On-State Voltage and Resistance Comparison: 300V Power MOSFET vs. 300V PT IGBT @ 125 C, 15V Gate Bias Figure 1 shows the on-state voltage versus current of APT30M75BLL, a 44A, 300V power MOSFET and APT83GU30B, a 300V, 83A PT IGBT, each at 125 C. On resistance at 22A is shown for each. At current above about 7 Amps, the 300V IGBT has significantly lower on resistance than the 300V MOSFET. This dramatic improvement in on resistance enables: smaller IGBT chip area for the same power converted as the MOSFET – lower cost higher efficiency for similar chip area as the MOSFET higher power density As with higher voltage IGBTs, the tradeoff of lower on resistance by conductivity modulation is a tail current at turn-off due to the minority carriers. This tail current increases the turn-off switching loss Eoff. Conductivity modulation does not negatively impact IGBT turn-on energy Eon but rather reduces it. Total switching losses remain about the same as a 200V to 300V power MOSFET of similar current rating. Note that power MOSFET current ratings are made with a case temperature of 25 C. It is the IGBT IC2 rating, with the case at elevated temperature, which should be compared with the MOSFET current rating. The difference in case temperatures in the current ratings is accounted for by the greatly reduced conduction loss of the IGBT. Minority carrier lifetime control significantly reduces the tail current and the consequent Eoff penalty. Electron irradiation is generally used in PT IGBTs as the method of controlling minority carrier lifetime. Lifetime control has three tradeoffs: 1. Eoff versus VCE(on), or on resistance 2. Wider distribution VCE(on) 3. Increased leakage current at high temperature, which is the reason PT IGBTs are not rated above 150 C. More aggressive lifetime control decreases Eoff but increases VCE(on) at a given current. For any given IGBT design, the Eoff versus VCE(on) operating point can be targeted along a curve of Eoff versus VCE(on). This curve is defined by the technology used in the design of the IGBT. An improvement in technology results in lower Eoff and VCE(on) both. It is a recent improvement in technology that enables 300V Power MOS 7 IGBTs to replace 200 to 300V MOSFETs in power supplies. Previously IGBTs mostly replaced MOSFETs with a blocking voltage of 400V and higher because of the lower drift region resistance of lower voltage MOSFETs combined with the MOSFET’s unrivalled switching speed. When comparing power supply IGBTs with MOSFETs, it may seem that the IGBT on-state voltage is high as stated in a datasheet because it directly correlates to conduction loss. However, if RDS(on) of a similarly rated MOSFET is used to calculate on-state voltage, it becomes clear that the conduction loss of the IGBT is in fact significantly lower. This is because the MOSFET RDS(on) increases quickly with higher currents (See Figure 1) and with normal operating temperature (See Figure 2). In a power supply, a lower VCE(on) (longer minority carrier lifetime) would result in unacceptably high switching losses with conduction loss being a tiny portion of the total IGBT losses. The end result with the IGBT is lower conduction loss and total switching losses similar to a MOSFET with larger chip area, as will be shown. The wider distribution in VCE(on) is the main cause of difficulty in paralleling PT IGBTs. The temperature coefficient is actually a secondary consideration for paralleling. Fortunately for 300V IGBTs, relatively little lifetime control is needed because the drift region is less resistive compared to higher voltage PT IGBTs, so fewer minority carriers are injected into the drift region during conduction. The lower level of lifetime control results in a narrower distribution of VCE(on) compared to higher voltage IGBTs, and hence easier paralleling. Punch-through technology is likely to remain the technology of choice for 300V IGBTs because of the difficulty in manufacturing a non punchthrough technology IGBT with wafers thin enough for competitive performance with less than about a 600V rating. A final distinguishing feature of 300V PT IGBTs is the temperature coefficient of on resistance (analogous to the temperature coefficient of VCE(on)). Figure 2 On Resistance vs. Temperature for a 300V Power MOSFET and 300V PT IGBT, Each at 22A, 15V Gate Bias The slopes of the curves in figure 2 indicate the temperature coefficient of on resistance for an APT30M75BLL MOSFET and an APT83GU30B IGBT. The IGBT has a slightly negative temperature coefficient at 22 Amps, and a slightly positive temperature coefficient above about 44 Amps. The flat temperature coefficient is beneficial for current overload capability. The positive temperature coefficient at higher current makes paralleling easier. The 300V power MOSFET temperature coefficient is always positive and very strong as shown in Figure 1. This is a typical power MOSFET characteristic. Performance Comparison Tests done in a hard-switched inductive circuit reveal the distribution of losses as well as total losses in a 300V Power MOS 7 IGBT and MOSFET. Power Lo ss (Watts) 100 80 Psw(off) 60 Psw(on) 40 Pcond Figure 4 Capacitance Comparison: 300V MOSFET vs. Smaller Size 300V PT IGBT 20 0 IGBT MOSFET APT 32GU30B APT 30M75BLL Figure 3 Total Losses for a 300V MOSFET & 300V IGBT @ 200V, 22A, 150kHz, 50% Duty, Hard Switched, RG = 20 and TJ = 125 C Each Figure 3 shows total losses for APT30M75BLL and APT83GU30B operating at 200V, 22A switched current, and 150kHz hard switching. Psw(off) is turn-off switching power loss (Eoff times switching frequency), Psw(on) is turn-on switching power loss, and Pcond is the conduction loss. In spite of the higher turn-off switching loss, the IGBT has lowest total losses because of lower conduction and turn-on switching losses. Total switching losses are almost the same for the IGBT and the MOSFET, so changing the switching frequency would still result in lower total losses in the IGBT. Clearly the IGBT is a suitable replacement for the MOSFET in this case. Conduction loss in each case is a significant portion of total losses, so choosing a larger device or paralleling devices would result in a noticeable reduction in total power loss. In Figure 3, it is important to remember that the chip area of the IGBT is much smaller than that of the MOSFET. So a lower cost 300V IGBT can replace a 200V to 300V MOSFET at reduced cost, higher power density, and possibly higher efficiency. Alternatively, a 300V IGBT of similar chip area as a MOSFET can result in significantly higher efficiency. The gate drive requirements are very similar to a MOSFET, but lower power gate driver can be used because of the smaller chip area and capacitance of the IGBT for the same power level as a MOSFET, as shown in Figure 4. Because of the lower capacitance, a higher gate resistance value may be required to maintain similar di/dt and dv/dt at turn-on compared to a 200V to 300V MOSFET. Negative gate drive is completely optional and not necessary. Although a 10V gate drive supply voltage can be used, 12 to 15 Volts is recommended for these 300V IGBTs. Soft Switching The turn-on of an IGBT is very similar to a power MOSFET, so these 300V IGBTs are certainly suitable for soft turn-on (either zero voltage or zero current). Zero current turn-off also yields good results with both IGBTs and power MOSFETs. IGBTs tend to perform poorly with zero voltage turn-off soft switching because the IGBT relies on collector-emitter voltage to sweep out minority carriers. This effect is compensated for somewhat by active minority carrier lifetime control in the Power MOS 7 IGBT design. Lifetime control results in a shorter tail current, even with no collector-emitter voltage applied. However, the absence of collector-emitter voltage at turn-off deprives the IGBT of sweepout of minority carriers, so the zero-voltage turnoff speed cannot match that of a power MOSFET. [1] J. Dodge, T. Loder; “Latest Technology PT IGBTs vs. Power MOSFETs”, PCIM China 2003, Advanced Power Technology