Future Electronics : Power MOSFETs and IGBTs: Not So Simple After All

FutureLogo31.jpg

By: David DeLeonardo, Analog Specialist AE, Future Electronics

Power MOSFETs and IGBTs are actually fairly complex devices with a number of parameters that must be well understood to properly apply them. In this article, we will review the important operational parameters and key application issues for each device. We will then compare and contrast the performance and applicability of MOSFETs vs. IGBTs and discuss when to use one vs. the other.

A Simplified MOSFET Model
At the simplest level, a MOSFET can be thought of as a voltage dependent resistor. Referring to Figure 1 below, the voltage between the gate and the source determines the resistance between the drain and the source. However, we must consider a number of other elements in order to arrive at a useful conceptual model of any MOSFET component.

pg20_Future_1_figure1

Figure 1: Simplified MOSFET model

pg20_Future_2_figure2

Figure 2: MOSFET schematic symbols

In Figure 1, the resistance (Rds) is a function of the gate – source voltage. However, internal parasitic capacitance between the terminals, represented by Cgs & Cdg, must also be considered. There is also an “intrinsic” body diode that is effectively in parallel with the drain-source channel. This diode has generally very poor performance characteristics in that it is relatively slow to turn off and has a high forward voltage, so for high performance applications, a separate “fast recovery” diode must be added in parallel with the FET.

A Simplified IGBT Model
Again, at the simplest level, an IGBT can be thought of as a MOSFET driving a PNP Bipolar transistor. Here, the MOSFET gate acts to pull current from the base of the PNP transistor thereby turning it ON. Thus, the IGBT is something of a voltage dependent current source. As in the case of the MOSFET model above, the MOSFET inside the IGBT has similar input capacitances that affect performance.

pg20_Future_3_figure3

Figure 3: Simplified IGBT model

pg20_Future_4_figure4

Figure 4: Simplified IGBT model


In Figure 3, just as in the case of the MOSFET model above, there are two parasitic capacitances that substantially affect IGBT performance. Here, they are Ccg and Cge, respectively. Now, unlike the MOSFET where the conductive path can be modeled by a resistor, here the conductive path is a PNP structure that transitions from a high impedance in the OFF state to a saturation voltage Vce(sat) in the ON state.


Critical MOSFET and IGBT Performance Parameters

While good MOSFET and IGBT datasheets can have well over 25 electrical parameters, here we will limit our discussion to the most critical parameters that must be considered for nearly every application. Where possible, we will compare and contrast the MOSFET and IGBT parameters “in tandem”.

1. MOSFET Vds & IGBT Vce(max): For the MOSFET, the Vds is the maximum voltage between the drain (+) and the source (-) that the device is rated to withstand in the OFF state. Vce(max) is the analogous spec for the IGBT. There is some temperature dependency here, so care must be taken to account for the thermal effects of transients. Units are in volts measured across the drain source or collector emitter terminals.

2. MOSFET RDS(ON) & IGBT Vce(sat): For the MOSFET, RDS(ON) is the Drain-to-Source ON State resistance. Units are in ohms measured across the drain – source terminals. This parameter has a POSITIVE temperature coefficient which allows MOSFETs to load share very easily when connected in parallel. For the IGBT, Vce(sat) is the saturation voltage when the device is in the ON state. Units are in volts measured across the collector emitter terminals. Vce(sat) has a NEGATIVE temperature coefficient which means it is hard to get IGBTs to load share in parallel configurations. This also makes them subject to thermal runway. However, Vce(sat) does NOT increase substantially for increasing current, which is one of the IGBTs main performance advantages over MOSFETS/IGBTs.

3. Qg in both MOSFETs & IGBTs: This is a measure of the total charge needed to raise the gate to its full rated Vgs or “ON” voltage. Units are in nCoulobs. In the past, values were only given for Cgs/Cge and Cdg/Ccg which, along with Vth (threshold voltage) and the MOSFET drain/IGBT collector voltage, collectively determine Qg. These two internal capacitances are very influential in determining the dynamic behavior of the MOSFET/IGBT. For example, Cdg/Ccg, also known as the Miller Capacitance, provides “negative feedback” to the Gate, especially during fast switching events. Thus, when trying to turn the FET/IGBT ON by raising the gate voltage, the voltage at the drain/collector will fall very quickly. This, in turn, will pull charge away from the Vgs/Vge via the Cds/Ccg and thus tend to turn the device OFF again. Care must be taken in the drive circuit to see that there is sufficiently low impedance present to hold the gate ON against this effect. Further, Cgs/Cge has the effect of slowing down changes in Vgs and thus changes in the channel resistance. This, in turn, slows down the switching of the MOSFET/IGBT and increases switching losses.

pg20_Future_5_figure5

Figure 5: MOSFET/IGBT driver circuit

A MOSFET/IGBT drive circuit is often “asymmetric” in that it presents very different impedances in the ON mode vs. the OFF mode. That is, when PWM is high, current is driven through R1 to turn on the MOSFET/IGBT. To turn it OFF, PWM goes low and current is pulled through R2. R1 is usually substantially greater than R2 since the ON time is often “throttled back” to control EMI while there is a need to hold the gate low against the Miller Capacitance effect.

4. I_ave and I_Pulse: These are the drain/collector average and pulse currents, respectively, that the power device can sustain in the ON state under a set of defined conditions including gate drive voltage, case temperature and test current duty cycle. Units are in amps. This parameter is limited by junction temperature and, subsequently, the thermal impedance and RDS(ON)/Vce(sat) of the device.

5. Maximum Power: This is a measure of the maximum power the device can dissipate while its case is held to a given temperature. Units are in watts. It is an indication of the thermal impedance of the package and the thermal limitations of the device material. It is measured at a specified set of conditions such as Vgs/Vge= 10V, Ids/Ice= 10A, case = 25°C.

6. Vth for MOSFETs: This is the Vgs at which the channel resistance (Rds) has decreased from fully OFF to within a specified percentage of its fully ON value. Units are in volts measured across the gate – source terminals. For IGBTs, this is the Vge at which the device is fully in saturation such that Vce is at a minimum. Units are in volts measured across the collector emitter terminals. Test conditions such as Ids/Ice, Vds/Vce, Vgs/Vge are given in the data sheet.

Key Application Points
The most common cause of power MOSFET and IGBT device failure is running the die at an excessive temperature. To avoid this, it is important to consider two critical design concepts. These are Safe Operating Area (SOA) and Transient Thermal Response.

Safe Operating Area can be thought of as the range or set of voltage and current values across which a given device under given conditions can be expected to operate without failure. Often, a family of curves will be presented together with a range of load current pulse conditions represented. See Figure 6.

pg21_Future_1_figure6

Figure 6: Safe operating area

In Figure 6, the power device Safe Operating Area for a range of conditions is shown. There are three distinct regions. At the far right, the device is limited by its ability to dissipate the average power + the pulse power. In the middle region, the device is strictly current limited. In the region on the left, the device is not thermally limited, but rather is functionally limited by voltages induced by excessive currents, which in turn, would cause operational problems.

Transient Thermal Response is the effect that transients load currents have on the die temperature in raising it from its “DC” value towards, but hopefully not beyond, it’s safe thermal limiting value. Here, the relevant graph has a family of curves where each curve corresponds to pulse duration. See Figure 7.

pg21_Future_2_figure7

Figure 7: Power device transient thermal response

To use a “Transient Thermal Response” graph, follow these simple steps. First, calculate the peak power through the power device during the transient by multiplying the peak current and voltage values. Then determine the pulse duration and duty cycle. (tp, T & “§”, respectively). Next, locate the tp of your test pulse on the X-Axis and follow a vertical line from that point up to the appropriate “§” curve on the chart. From that intersection point, follow a horizontal line over to the vertical “Y” axis to determine the “Ztr” for the particular device under your given transient conditions. Now, multiply the Peak Power value of the transient that you calculated earlier by the “Ztr” value you just found from the graph. The product of these two terms is the value, in Kelvins or “°C” of the thermal transient induced in the die by the load transient. Finally, to get the resulting Peak Die Temperature, this transient temperature value must be added to the pre-transient “DC” value.

pg21_Future_3_figure8

Figure 8: Present day performance envelope

Recent device design and process improvements have greatly expanded the ”performance envelope” for both MOSFETs and IGBTs. For the most part, IGBTs retain their voltage and current magnitude advantage. Of course application requirements will militate in favor of one over the other.

When to Use Which Device MOSFETs vs. IGBTs

As MOSFET and IGBT devices continue to evolve, their relative advantages with respect to one another will shift over time. However, due to their respective strengths and weaknesses, some generalities are likely to apply for some time to come. In Figure 9, we see that there are four regions where one device or the other is fairly dominant. While the edges of these regions are always somewhat in flux, their broad outlines are not. For the region in the middle that is bounded by the other for regions, specific priorities of the application at hand are what tips selection one way or the other.

pg21_Future_4_figure9

Figure 9: MOSFET vs. IGBT

pg21_Future_5_figure10

Figure 10: MOSFET vs. Voltage

In Figure 10, we compare the voltage vs. current transfer function for MOSFETs & IGBTs. Due to the linear nature of the drain-source conductive channel in the ON state, MOSFETs have a linear transfer function. However, when an IGBT is in the ON state, there is a three layer bipolar (PNP) structure in saturation and thus its curve is non-linear and very steep above a certain Vce(sat) voltage. Therefore, there is a “crossover” point above which higher device currents will result in lower losses in the IGBT than in the MOSFET. The value of this “crossover” point is changing with yearly process and design improvements for each device.

Leave a Reply

Your email address will not be published. Required fields are marked *

Protected by WP Anti Spam