Fairchild AN-7502 manual New Device Characterization, State 2 MOS ActIve, Jfet ActIve

Application Note 7502

State 2: MOS ActIve, JFET ActIve

This state graphically illustrates the dramatic influence that the JFET has on the power MOSFET drain-voltage wave- form. Instead of having to discharge Cx from VDD to ground, the lateral MOSFET need only swing VX to ground, a much smaller voltage thanks to the grounded gate JFET. Since the interaction of RL with the device capacitances has a second- order effect on the drain voltage, the equivalent circuit of Fig- ure 7 predicts a drain voltage change of:

dVG/dt = gMRLlG/[CGS + CX(1 + gM/gMJ)]

In all but the smallest power-MOSFET devices, Cx is several thousand picofarads and gM/gMJ is of the order of 3:1. Power-MOSFET devices exhibit a high dVD/dt switching rate because of the cascode-connected JFET, not because

CRSS (CRSS = CGD) is a small value, as zero-drain-current data sheet capacitance values might lead one to believe. If

CRSS were, in actuality, small, long drain voltage tails would not exist. The tail response is a direct result of JFET satura- tion. In order to delineate the transition from state 2 to state 3, a drain voltage at which the transition occurs must be defined. VDK is the knee voltage at which linear extrapola- tions of drain-voltage slopes intersect. The time duration of state 2 is:

t2(t6) = (VDD - VDK)[CGS + CX(1 + gM/gMJ)]/gMRLIG

State 3: MOS Active, JFET Saturated

When the JFET saturates, the gMJVX current generator becomes a short circuit and the equivalent circuit predicts:

dVD/dt = gMRLIG/[CGS + CX(1 + gMRL)]

This is the Miller effect so often referred to in older texts that describe the behavior of grounded-cathode vacuum-tube amplifier circuits. Allowing for the fact that 1 + gMRL is approximately equal to gMRL and CX(1 + gMRL) is very much larger than CGS, the expression for drain-voltage tail time is:

t3(t5) = (VDK - VD(SAT))Cx/lG

State 4: MOS Saturated, JFET Saturated (Turn-Off)

In this state, in addition to gMJVX being shorted, the gMVG cur- rent generator is shorted, and IG is occupied with charging CX

and CGS, in parallel, from the peak value of VG to VG(SAT). The time required for this is:

t4 = (VG - VG(SAT))(CGS + Cx)/IG

Since a value for CGS may be measured independently of switching time, the method described is the simplest way of determining CX.

On turn-off, the state time equations are equally applicable, but in reverse order (states 5 and 6); see the idealized wave- form of Figure 4.

Experimental Verification

The four switching states just analyzed indicate that for a given device, all four switching state times are inversely pro- portional to the magnitude of the gate drive current. Figure 8 illustrates the switching performance of a typical power MOSFET across three decades of gate drive current and time. In each case the data slope is almost a perfect -1.

A New Device Characterization

Figure 8 could not be a reasonable device data sheet pre- sentation because it does not give the designer any informa- tion on a typical value for CX, nor does it convey how VDK, gM, gM/gMJ, and VG(sat) vary with drain current. What would be of enormous value to the designer is a plot of VD(t), VG(t) for selected values of VDD and ID within device ratings.

A reasonable characterization would be as follows:

1.The x axis would be normalized in terms of gate current drive.

2.The y axis would be normalized in terms of percent maximum rated BVDSS (0 to 100%).

3.RL = BVDSS/ID(max) would define the drain load resistance.

4.Four plots of VD(t), VG(t) at 100%, 75%, 50%, and 25% BVDSS(max) would be shown.

	10
			RFM15N15
			VDD = 75V
			ID	= 7.5A
			RO	= ∞ Ω
			VG	= 10V
(t) - MICROSECONDS	1
	0.1
		DATA THEORY
		tD(OFF)
		tR
		tF
		tD(ON)
	0.01
	1	10	100	1000
		(IG) - MILLIAMPERES

FIGURE 8. CONSTANT GATE CURRENT SWITCHING TIME

Figure 9 is such a plot for the RFM15N15 power MOSFET. With such a plot, a designer can estimate device switching performance under any resistive gate/drain conditions

©2002 Fairchild Semiconductor Corporation Application Note 7502 Rev. A1