CONSIDERATIONS DRIVING POWER MOSFETS HIGH-CURRENT, AN-22 SWITCH MODE R

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CONSIDERATIONS DRIVING POWER MOSFETS HIGH-CURRENT, AN-22 SWITCH MODE R

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CONSIDERATIONS DRIVING POWER MOSFETS HIGH-CURRENT, SWITCH MODE REGULATORS
CONSIDERATIONS DRIVING POWER MOSFETS HIGH-CURRENT, AN-22 SWITCH MODE REGULATORS
DRIVING MOSFET
on-resistance high current carrying capability power MOSFETs make them preferred switching devices SMPS power supply design. However, designing with these devices straightforward with their bipolar counterparts. Unlike bipolar transistors, power MOSFETs have considerable gate capacitance that must charged beyond threshold voltage (VGS-TH) achieve turn-on. gate driver must provide high enough output current charge equivalent gate capacitance (CEI) within time required system design.
MUCH GATE CURRENT?
most common error calculating gate current confusing MOSFET input capacitance (CISS) applying equation. C(dv/dt)
Equation
VGS-TH
(Gate Charge) [nC]
Figure Gate Charge Characteristic
equation form: (CEI)(VGS)
Equation
calculate required peak gate current. actually much higher, must derived from MOSFET manufacturer's gate charge (QG) specifications. total gate charge (QG) that must dispensed into total gate capacitance MOSFET achieve turnon given
Equation Equation
QG/t(transition)
Equation 4.quation
where: total gate charge gate-to-source charge gate-to-drain charge (Miller) "overdrive charge" after charging Miller capacitance. curve Figure typical those supplied MOSFET manufacturers. Notice that, order achieve strong turn-on, well above that required charge (and well above VTH) required. effective gate capacitance determined dividing given into corresponding gate charge. required drive current (for transition within specified time) determined dividing gate charge desired transition time.
where: total gate charge, defined above equivalent gate capacitance gate-to-source voltage gate current required turn MOSFET time period t(transition) t(transition) desired transition time example: GIVEN:
N-Channel MOSFET 25nsec. Peak gate drive current.
FIND:
From MOSFET manufacturer's specifications, 50nC 10V. Using Equation QG/t(transition) 10-9/25 10-9 2.0A
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CONSIDERATIONS DRIVING POWER MOSFETS HIGH-CURRENT, SWITCH MODE REGULATORS AN-22
Table guideline matching various TelCom MOSFET drivers Industry-standard HEXFETs. Table Selecting MOSFET Drivers Rise Time Rated Load (nsec)
Device
TC1426 TC1427 TC1428 TC4426 TC4427 TC4428 TC4423 TC4424 TC4425 TC4420 TC4429 TC4421 TC4422 TC4469 TC4468 TC4467
Drive Current (Peak)
1.2A 1.2A 1.2A 1.5A 1.5A 1.5A 3.0A 3.0A 3.0A 6.0A 6.0A 9.0A 9.0A 1.2A 1.2A 1.2A
Output Number Rated Type Load Inverting Non-Invert. (pF)
Dual Single Dual Single Dual Single Single Single Single Dual Single Dual Single Dual Single Non-invert Inverting 1000 1000 1000 1000 1000 1000 2200 2200 2200 4700 4700 10,000 10,000 1000 1000 1000
Fall Time Rated Load (nsec)
Rising Edge Prop. Delay (nsec)
Falling Edge Prop. Delay (nsec)
Input Protected Latchto Below Proof Rail
Single Quad Quad Quad NAND
Table MOSFET Size Suggested Drive Family MOSFET Size
Parallel Modules
Size (mm)
0.89 1.09 1.75 2.41 3.40 2.21 4.44 2.79 7.04 4.32 6.45 6.45 Various
Total MOSFET (pF)
1500 3000 6000 12,000 15,000 16,000 48,000
Suggested Driver Family 12V)
TC1426/4426/4469 TC1426/4426/4469 TC1426/4426/4469 TC1426/4426 TC4423 TC4423 TC4429/4420 TC4429/4420 TC4421/4422
Faster Rise/Fall Times
TC4423 TC4423 TC4429 TC4429 TC4421/4422 TC4421/4422
DEDICATED MOSFET DRIVERS?
Traditional SMPS controllers have on-board drivers suitable some applications. Typically, these drivers have peak output currents less, limiting their scope applications. addition, heat generated these drivers causes on-chip reference voltage change. need "smarter" power supplies forcing SMPS controllers grow sophistication. Many newer
AN22-1 3/17/97
SMPS controllers fabricated smaller geometry CMOS process technologies, precluding high voltage (i.e. voltages greater than 12V). such cases, external MOSFET driver also acts level shifter, translating TTL-compatible levels MOSFET drive voltages. device like TC4427A example, furnishes rail-to-rail output voltage swing (from maximum 18V) from input swing 0.8V 2.4V.
CONSIDERATIONS DRIVING POWER MOSFETS HIGH-CURRENT, SWITCH MODE REGULATORS AN-22
Latch-up immunity another consideration. Latch-up immunity particularly important that driven MOSFETs typically drive inductive circuits that generate significant "kickback" currents. MOSFET drivers like TC4427 withstand much 0.5A reverse output current without damage upset. Protection against shoot-through current still another consideration, especially higher speed SMPS designs. Shoot through currents usually caused excessively long driver rise, fall propagational delay times; causing both high side side MOSFETs brief instant. Current "shoots through" (hence name) from supply input ground, significantly degrading overall supply efficiency. dedicated MOSFET drivers minimizes this problem ways: MOSFET gate drive rise fall times must symmetrical, short possible. driver like TC4427 specified 25nsec into 1000pF load. higher peak output current driver selected achieve more aggressive rise fall times desired. propagational delay times through driver must short (and matched higher speed designs) ensure symmetrical turn-on turn-off delays both high side side MOSFET. TC4427A example, rising falling edge propagation delay times matched within 2nsec (see Figure These delays track each other with both voltage temperature. TelCom's 2nsec skew among best available (competing devices have skews least times larger; drivers integrated board SMPS controller worse yet). These concerns (and related cost reliability concerns) usually point direction external, dedicated driver, opposed integrated external discrete component driver solution.
Input: 10mA fast CMOS drive into 10pF typical input capacitance 5nsec rise/fall
2nsec (typ.)
2nsec (typ.)
Competitor Driver Output: 470pF load, 15nsec rise/fall (typ.)
22nsec (typ.)
22nsec (typ.)
OVERLAP (assuming 16nsec threshold) (typ.) 9nsec (typ.)
OVERLAP (assuming 16nsec threshold) (typ.) 9nsec (typ.)
TC4426A Output: 470pF load, 15nsec rise/fall (typ.)
OVERLAP (assuming threshold) 2nsec (typ.)
Figure
28nsec (typ.)
28nsec (typ.)
OVERLAP (assuming threshold) 2nsec (typ.)
28nsec (typ.)
28nsec (typ.)
AN22-1 3/17/97
CONSIDERATIONS DRIVING POWER MOSFETS HIGH-CURRENT, SWITCH MODE REGULATORS AN-22
+5V/+3V
30V)
SIGNAL
TC4431 Pch-FET INDUCTOR
CONTROLLER
VOUT (CPU VCC)
SIGNAL
Nch-FET TC1411N SCHOTTKY DIODE
OUTPUT CAPACITANCE
Figure Portable Power Supply
TYPICAL APPLICATIONS Portable Computer Supply
common application that exploits design benefits dedicated MOSFET drivers switching power supply portable systems, such those found notebook computer applications. circuit topology high efficiency, synchronous buck converter shown Figure accepts input voltage range accommodate AC/DC adapters (14V 30V) battery supply (7.2V 10.8V). TC1411N acts side driver, powered from supply minimize turn-off delay gate over-charge. high side driver Figure TC4431, which peak output current 1.5A. TC1411N peak output current capability They drive MOSFETs capable continuous drain current 30nsec.
Desktop Power Supply
Desktop power supplies also benefit from dedicated MOSFET drivers (Figure synchronous step-down converter shown common CPUs requiring greater than current. also accommodates custom voltages accommodated current "silver box" supplies. Efficiency large concern, since this supply line-powered. topology shown simpler than that Figure TC4427A serves high-side/low-side driver powered from same VDD. N-Channel MOSFETs used save cost. TC4427A sufficient output current drive (continuous drain current) MOSFET active 25nsec.
AN22-1 3/17/97
CONSIDERATIONS DRIVING POWER MOSFETS HIGH-CURRENT, SWITCH MODE REGULATORS AN-22
(+12V)
+5V/+3V
SIGNAL
Nch-FET INDUCTOR
CONTROLLER TC4427A SIGNAL Nch-FET SCHOTTKY DIODE
VOUT
OUTPUT CAPACITOR
Figure Desktop Power Supply
SUMMARY
Power MOSFETs desirable switching elements SMPS designs because their on-resistance high current carrying capability. Using dedicated MOSFET drivers results more optimized SMPS design. Drivers integrated on-board SMPS controller advantageous only sophistication, output power designs. External drivers fashioned from discrete active passive components have neither repeatable high performance, cost dedicated monolithic driver circuit. Dedicated drivers like those offered TelCom feature fast rise, fall delay times, available wide variety topologies suit virtually every application.
Sales Offices
TelCom Semiconductor 1300 Terra Bella Avenue P.O. 7267 Mountain View, 94039-7267 TEL: 650-968-9241 FAX: 650-967-1590 E-Mail: liter@c2smtp.telcom-semi.com
AN22-1 3/17/97
TelCom Semiconductor Austin Product Center 9101 Burnet Suite Austin, 78758 TEL: 512-873-7100 FAX: 512-873-8236
TelCom Semiconductor H.K. Ltd. Chuk Street, Ground Floor Kong, Kowloon Hong Kong TEL: 852-2324-0122 FAX: 852-2354-9957
Printed U.S.A.

CONSIDERATIONS DRIVING POWER MOSFETS HIGH-CURRENT, AN-22 SWITCH MODE R

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