Driving Power MOSFETs High-Current, Switch Mode Regulators Author

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AN786
Driving Power MOSFETs High-Current, Switch Mode Regulators
Author: Abid Hussain, Microchip Technology, Inc.
VGS, Gate-to-Source Voltage
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.
VGS(TH)
MUCH GATE CURRENT?
most common error calculating gate current confusing MOSFET input capacitance, CISS, applying equation. C(dv/dt)
Total Gate Charge (nC)
FIGURE Gate charge characteristics.
equation form: (CEI)(VGS)
calculate required peak gate current. actually much higher, must derived from MOSFET manufacturer's total gate charge, specifications. total gate charge, that must dispensed into equivalent gate capacitance MOSFET achieve turn-on given where: total gate charge gate-to-source charge gate-to-drain Miller charge "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 VGS(TH)) required. equivalent gate capacitance determined dividing given into corresponding total gate charge. required gate drive current (for transition within specified time) determined dividing total gate charge desired transition time.
QG/t(transition) 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 (transistion) 25nsec Gate drive current,
Find:
From MOSFET manufacturer's specifications, 50nC 10V. Using QG/t(transition): QG/t(transition) 10-9/25 10-9 2.0A
2002 Microchip Technology, Inc.
DS00786A-page
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Table guideline matching various Microchip MOSFET drivers Industry-standard HEXFETs.
Device TC1410 TC1410N TC1411 TC1411N TC1426 TC1427 TC1428 TC4467 TC4468 TC4469 TC4426 TC4426A TC4427 TC4427A TC4428 TC4428A TC1412 TC1412N TC1413 TC1413N TC4423 TC4424 TC4425 TC4420 TC4429 TC4421 TC4422
Drive Current (Peak) 0.5A 0.5A 1.0A 1.0A 1.2A 1.2A 1.2A 1.2A 1.2A 1.2A 1.5A 1.5A 1.5A 1.5A 1.5A 1.5A 2.0A 2.0A 3.0A 3.0A 3.0A 3.0A 3.0A 6.0A 6.0A 9.0A 9.0A
Output Number Type
Inverting Single Single Single Single Dual Dual Single Single Quad NAND Quad Quad with INV- Dual Dual Dual Dual Single Single Single Single Single Single Single Single Dual Dual Single Single Single Single Single Single Non-Invert.
Rated Load (pF) 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1800 1800 1800 1800 1800 2500 2500 10000 10000
Rise Time Rated Load (nsec)
Fall Time Rated Load (nsec)
Rising Edge Prop. Delay (nsec)
Falling Edge Prop. Delay (nsec)
LatchUp Proof
Input Protected Below Rail
Note: Typical values 25°C.
TABLE Selecting MOSFET drivers.
DS00786A-page
2002 Microchip Technology, Inc.
AN786
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 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
TABLE MOSFET size suggested drive family.
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 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-torail output voltage swing (from maximum 18V) from input swing 0.8V 2.4V. 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 approximately 19nsec 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. Microchip'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.
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 "overdrive 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.
2002 Microchip Technology, Inc.
DS00786A-page
AN786
Desktop Power Supply
Desktop power supplies also benefit from dedicated MOSFET drivers (Figure synchronous stepdown 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 TC4428A serves high-side/low-side driver powered from same VDD. N-Channel MOSFETs used save cost. TC4428A sufficient output current drive (continuous drain current) MOSFET active 25nsec.
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 Microchip feature fast rise, fall delay times, available wide variety topologies suit virtually every application.
Input: 10mA fast CMOS drive into10pF typical input capacitance 5nsec rise/fall
2nsec (typ.)
2nsec (typ.)
22nsec (typ.)
Competitor Driver Output: 1000pF load, 25nsec rise/fall (typ.)
22nsec (typ.)
16nsec (typ.)
Overlap (assuming threshold) 9nsec typ.
16nsec (typ.)
Overlap (assuming threshold) 9nsec typ.
30nsec (typ.)
TC4426A Output: 1000pF load, 25nsec rise/fall (typ.)
30nsec (typ.)
30nsec (typ.)
Overlap (assuming threshold) 2nsec typ.
30nsec (typ.)
Overlap (assuming threshold) 2nsec typ.
FIGURE Matched delay times TC4426A reduce overlap times resulting reduced shoot-through currents.
DS00786A-page
2002 Microchip Technology, Inc.
AN786
+5V/+3V 30V)
Signal
TC4431 P-Channel MOSFET Inductor
Controller
Signal TC1411N N-Channel MOSFET Schottky Diode
VOUT (CPU VCC) Output Capacitance
FIGURE Portable power supply.
+5V/+3V
VDD1 (+12V)
(+5V)
Signal
N-Channel MOSFET Inductor
Controller
TC4428A N-Channel MOSFET Schottky Diode
VOUT
Signal
Output Capacitor
FIGURE Desktop power supply.
2002 Microchip Technology, Inc. DS00786A-page
AN786
NOTES:
DS00786A-page
2002 Microchip Technology, Inc.
AN786
Information contained this publication regarding device applications like intended through suggestion only superseded updates. your responsibility ensure that your application meets with your specifications. representation warranty given liability assumed Microchip Technology Incorporated with respect accuracy such information, infringement patents other intellectual property rights arising from such otherwise. Microchip's products critical components life support systems authorized except with express written approval Microchip. licenses conveyed, implicitly otherwise, under intellectual property rights.
Trademarks Microchip name logo, Microchip logo, FilterLab, KEELOQ, microID, MPLAB, PIC, PICmicro, PICMASTER, PICSTART, MATE, SEEVAL Embedded Control Solutions Company registered trademarks Microchip Technology Incorporated U.S.A. other countries. dsPIC, ECONOMONITOR, FanSense, FlexROM, fuzzyLAB, In-Circuit Serial Programming, ICSP, ICEPIC, microPort, Migratable Memory, MPASM, MPLIB, MPLINK, MPSIM, MXDEV, PICC, PICDEM, PICDEM.net, rfPIC, Select Mode Total Endurance trademarks Microchip Technology Incorporated U.S.A. Serialized Quick Turn Programming (SQTP) service mark Microchip Technology Incorporated U.S.A. other trademarks mentioned herein property their respective companies. 2002, Microchip Technology Incorporated, Printed U.S.A., Rights Reserved.
Printed recycled paper.
Microchip received QS-9000 quality system certification worldwide headquarters, design wafer fabrication facilities Chandler Tempe, Arizona July 1999. Company's quality system processes procedures QS-9000 compliant PICmicro® 8-bit MCUs, KEELOQ® code hopping devices, Serial EEPROMs microperipheral products. addition, Microchip's quality system design manufacture development systems 9001 certified.
2002 Microchip Technology, Inc.
DS00786A-page
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*DS00786A*
DS00786A-page
2002 Microchip Technology Inc.

Driving Power MOSFETs High-Current, Switch Mode Regulators Author

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