AN805 vishay siliconix document number: 70649 january 1997 www.vishay.com faxback 408-970-5600 1 pwm optimized power mosfets for low-voltage dc/dc conversion designers of low-voltage dc-to-dc converters have two main concerns: reducing size and reducing losses. as a way of reducing size, designers are increasing switching frequencies. but the result has been reduced converter efficiency. to minimize losses, mosfet manufacturers have generally focused on lowering on-resistance. but the results have not been optimal for dc-to-dc conversion designs, since gate charge and switching speed issues have been largely ignored. the dominant losses associated with mosfets were once conduction losses, but this is no longer the case. vishay siliconix's new family of pwm optimized mosfets has been designed to give the highest efficiency available for a given on-resistance in switching applications such as dc-to-dc conversion. these new devices provide a very low gate charge per unit of on-resistance, in addition to fast switching times. the result is reduced gate drive and crossover losses, allowing designers of dc-to-dc converters to simultaneously reduce the design footprint and increase efficiency. mosfet losses a simplistic model of power loss in a mosfet used in a dc-to-dc converter (figure 1) can be calculated if we know the rms, the current through the mosfet, the duty cycle, the gate voltage, and the r ds(on) of the mosfet. this model can then be used to compare the efficiency of designs using vishay siliconix's new pwm optimized mosfets versus conventional and low-threshold power mosfets. the equation that defines the losses associated only with on-resistance and the gate drive is: p i 2 rms r ds(on) v gs t j d q g v gs v gs f ( watts ) eq1 [ ] the value of the parameter before the parenthesis is dependent on the parameter within the parenthesis. figure 1. generic mosfet model with body diode omitted. gate r g ciss crss i rms c oss r ds(on) source drain where: i 2 rms the rms current in the mosfet (a) r ds(on) on-resistance of the device for a given drive voltage and junction temperature. v gs the peak driver gate voltage for the mosfet (v) [t j ] junction temperature of the mosfet d duty factor of the mosfet (ratio of on time to off time) q g total gate charge for the mosfet at a given gate voltage (c) f frequency of mosfet switching (hz) using equation 1 we can obtain a plot of power loss (gate loss + r ds(on) loss) as a function of gate voltage at varying switching frequencies (figure 2). [1] power loss (mw) figure 2. power loss for pwm optimized si6801 p-channel mosfet as a function of v gs and switching frequency. 0 1 2 3 4 5 6 7 40 30 20 10 0 si6801 power loss, qg, r ds v gs r gate charge (nc) v gs 0.8 0.6 0.4 0.2 0 ( ds(on) figure 3. gate losses and on-resistance losses for pwm optimized power mosfet (si6801dq) versus conventional (si6542dq) and low-threshold (si6552dq) power mosfets. 1 v gs 50 40 30 20 10 conduction + gate charge loss (mw) 2 3 4 5 6 7 technology comparison: 1 mhz power loss ) 40 30 20 10 0
AN805 vishay siliconix www.vishay.com faxback 408-970-5600 2 document number: 70649 january 1997 figure 2 shows the respective contribution of on-resistance and gate charge to overall losses for the p-channel si6801dq at three different switching frequencies. at low gate-source voltages, the r ds(on) of the mosfet is high and therefore on-resistance losses dominate. at higher gate-source voltages, on-resistance becomes almost a constant and the gate charge losses controlled by q g dominate. gate losses increase with the switching frequency, causing a narrowing in the optimum gate voltage. therefore, the optimum drive voltage will be at a level which is just enough to take the r ds(on) into its constant region, but no further. typically, this drive voltage is between 3 and 5 v, which is what most controller ics provide. figure 3 compares the power losses, at a switching frequency of 1 mhz, of vishay siliconix's pwm optimized si6801dq, a conventional power mosfet (si6542dq), and a low-threshold power mosfet (si6552dq). power losses for the pwm optimized mosfet at gate drives between 2.5 and 5.5 v are significantly lower than both conventional and low-threshold mosfets, making the optimized device the obvious choice for all switching applications. [7.] the pwm optimized mosfet in a real application the pwm optimized power mosfet is best viewed in the context of a real application. in the example used here, the si6801dq is paired with the si9160bq switching regulator ic to create a synchronous boost converter for cellular telephones with the following specifications: input voltage: 2.7 v to 5 v (single-cell lithium ion battery is 2.7 v to 4.2 v) output voltage: 5 v output current: 1 a maximum gate drive voltage: 4.5 v control scheme: constant frequency voltage mode control switching frequency: varied by rc value from 300 khz to 1.8 mhz all results shown are with v in = 3.6 v, v out = 5 v, i out = 600 ma, f = 1 mhz unless otherwise stated. figure 4. si9160 boost converter test circuit used to compare mosfet technologies. ml c4 10 f 13 si9160 14 15 16 2 3 4 1 10 11 12 5 6 7 9 8 c11 36 pf r6 12 k c3 r3 d r v dd v s nc d max /ss d s comp p gnd fb ni v ref uvlo set c osc r osc gnd enable c2 0.1 f c10 0.1 f r5 100 k c5 0.1 f r1 10 k c4 0.1 f c6 22 pf r4 2.2 k c10 0.33 f 5 6 7 8 2 3 4 1 s 2 d 1 d 2 s 1 s 1 s 2 g 1 g 2 c8 5600 pf r9 100 r10 3.6 k r11 1.2 k c9 0.1 f ml c3 10 f ml c1 10 f d1 ls4148 d2 ls4148 ml c2 10 f 4.7 h si6801 0.1 f 2.2 k r2, 270 sync 1-cell liion 7. neither figure 2 nor figure 3 is intended for exacting power loss calculations. these figures should only be used as a comparative measure for various mosfet technologies.
AN805 vishay siliconix document number: 70649 january 1997 www.vishay.com faxback 408-970-5600 3 the following complementary n- and p-channel mosfets, all little foot tssop-8 devices, represent the three technologies under test: pwm optimized mosfet si6801dq conventional mosfet si6542dq . . . . . . . . . . low-threshold mosfet si6552dq . . . . . . . . . . figure 4 shows test circuit used. pwm optimized mosfet performance reducing gate charge is one way in which pwm optimized mosfets cut power losses. in a real application, another component of power loss is crossover losses. these are also minimized by the pwm optimized power mosfet design, and are discussed in detail in appendix a. figure 5 shows oscillograms of the boost converter switching waveform using the three different types of power mosfets. the switching speeds are 4 ns for the si6801dq pwm optimized mosfet and 11 ns for the conventional mosfet. the si6801dq provides a nearly threefold improvement and thus lower losses. in addition to the increase in basic switching speed, notice that the pwm optimized mosfet does not exhibit a large characteristic step in the voltage waveform. this step is due to the feedback capacitance from drain to gate of the mosfet or amillero capacitance (c rss in figure 1) being charged when the drain voltage is lower than the gate voltage during a switching transition from an off state to an on state. effectively the gate voltage is astalledo while the miller capacitance is charged, and this is reflected in the voltage waveform from drain to source. this is obviously an unwanted characteristic and has largely been eliminated with pwm optimized mosfet technology. a final component that affects the switching speed of a mosfet is the effective gate resistance (r g in figure 1). the effective gate resistance defines how fast the mosfet capacitance can be charged. it is therefore one of the dominant factors in determining how fast a mosfet will switch. vishay siliconix's pwm optimized mosfets provide a minimum effective gate resistance.
AN805 vishay siliconix www.vishay.com faxback 408-970-5600 4 document number: 70649 january 1997
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