Fairchild AN-7502 Step-Voltage Gate Drive, Application Note, Experimental Verification, Vgsth/Vg

Application Note 7502

100
	75
DSS
V	50
RATES
		RFM15N15
		IT = 1mA
%		VG = 10 VOLTS
	25	RL = VDSS/ID(RMS)
	0
		20IT/IG	40IT/IG	60IT/IG	80IT/IG

TIME - microseconds

FIGURE 9. NORMALIZED RFM15N15 SWITCHING WAVE- FORMS FOR CANSTANT GATE-CURRENT DRIVE.

Step-Voltage Gate Drive

The majority of power MOSFET applications employ a step gate-voltage input with a finite source resistance RO. Often RO for turn-on is not the same as RO for turn-off. How can switching times for these situations be estimated using the switching characterization curves just described? The analy-

sis for resistive step voltage inputs, which is complex because the gate current is no longer constrained to be con- stant, but is a function of device gate-voltage response, is covered in Appendix A. (A second, shorter appendix, B, has been added to illustrate the estimation of RO for some practi- cal gate drive circuits.) Table 1 summarizes the common switching equations, and indicates the appropriate 1G to be used in each state for relating step voltage drives to the char- acterization curves.

Experimental Verification

Since the switching equations for step currents and voltages differ only by gate-current magnitudes for the same device type, one would expect a plot of switching time versus 1/RO to be of the same form as those obtained for a step current drive. This is exactly the case, as Figure 10 is merely a vari- ation of Figure 8. Using the relationships of Table 1, the observed differences between Figures 7 and 9 can be pin- pointed. The two sets of experimental curves confirm that, on the basis of the short-circuit drive current VG/RO equal-

ling the constant IG, tD(on), tR, tD(off), and tF will all be longer, as predicted by the ratios of the gate drive currents of

Table 1. Notice also that tR, tF switching symmetry is dis- rupted by the use of a step voltage with source resistance RO. For states 2 and 6 the time ratio is:

TABLE 1. COMMON SWITCHING EQUATIONS

	CONSTANT CURRENT			STATE 1: MOS OFF, JFET OFF	CONSTANT VOLTAGE
	t =	CISS VGS(TH)			[1]
					t = RO CISS In
T		IG				[1 - VGS(TH)/VG]
U	IG = IT			STATE 2: ACTIVE, ACTIVE	IG = (VG - VGS(TH))/RO
R
				[VDD - VDK] [CGS + Cx (1 + gM/gMJ)]
N		t =
O				gMRLIG
N
	IG = IT			STATE 3: ACTIVE, SATURATED	IG = (VG - VG(SAT))/RO
		t =		(VDK - VD(SAT))CX
				IG
	IG = IT			STATE 4: SATURATED, SATURATED	IG = -VG/RO
	t =	(CGS + CX)(VG - VG(SAT))			t = RO(CGS + CX) In (VG/VG(SAT))
T		IG
U	IG = IT			STATE 5: ACTIVE, SATURATED	IG = (VG - VG(SAT))/RO
R
N			t =	(VDK - VD(SAT))CX
O
				IG
F
F	IG = IT			STATE 6: ACTIVE, ACTIVE	IG = (VG - VG(SAT))/RO
			t =	[VDD - VDK] [CGS + CX (1 + gM/gMJ)]
				gMRLIG

©2002 Fairchild Semiconductor Corporation Application Note 7502 Rev. A1