ECE1750, Spring 2018 Week 5 MOSFET Gate Drivers 1
Power MOSFETs (a high-speed, voltage-controlled switch) D: Drain D If desired, a series blocking diode can be inserted here to prevent reverse current G: Gate G S: Source Switch closes when V GS 4Vdc S N channel MOSFET equivalent circuit Controllability? - Controlled turn on, controlled turn off (but there is an internal antiparallel diode). Thanks to the diode it can conduct in both directions but it cannot block D-S voltages in which VD<VS. Controlled through the gate by voltage. If VGS>VGS,th it conducts. Otherwise, it does not conduct (in the forward direction). 2
We Avoid the Linear (Lossy) Region, Using Only the On and Off States Ideal MOSFET on D Ideal MOSFET off D S when V GS = 20 V S when V GS = 0 V 3
Main characteristics Static Characteristic Power MOSFETS Ohmic Region ON State Active Region i id id ON State Higher VGS Lower Cutoff (OFF State) VGS<VGS,th v vds VGS,th VDS vgs 4
Power MOSFETS Main characteristics Behavioral models Static model (sw) sw = ON when VGS>VGS,th OFF when VGS<VGS,th Dynamic Model G Cgd Cgs id Capacitances (particularly D Cgs) ) need to be charged or rd(t) discharged to turn the MOSFET ON or OFF, respectively Cds RDS,ON S 5
We Want to Switch Quickly to Minimize Switching Losses Turn Off Turn On V DS (t) V DS (t) 0 0 I(t) t off I(t) t on 0 0 p LOSS (t) p LOSS (t) 0 Energy lost per turn off 0 Energy lost per turn on Turn off and turn on times limit the frequency of operation because their sum must be considerably less than period T (i.e., 1/f) 6
Consider, for example, the turn off V DS (t) 0 I(t) Turn Off V Energy lost per turn off is proportional to V I t off, so we want to keep turn off (and turn on) times as small as possible. 0 I The more often we switch, the more energy loss areas we experience per second. p LOSS (t) t off 0 Energy lost per turn off Thus, switching losses (average W) are proportional to switching frequency f, V, I, t off, and t on. And, of course, there are conduction losses that are equal to the product of RDSon and I squared 7
Switching losses With a resistive load, voltage and current waveforms are approximately like (linear commutation) turn-on losses: V off turn-off losses: I on Voff I pon () t tvoff t on poff () t tion t ton ton t off t off Total switching losses: 1 I V I V P p t dt f p t p t dt f t t f t T 6 6 T T on off on off sw () ( on() off ()) ( on off ) sw 0 0
Switching losses With in inductive load and an ideal switch, voltage and current waveforms are approximately like More realistic behavior Approximate behavior turn-on losses: pon () t 0 Total switching losses: turn-off losses: V off I on poff () t Ion t Voff t ( tloff, /2) ( tloff, /2) 1 I V I V P p t dt f t f t T 2 2 T on off on off sw () ( L, off ) sw 0
Advantages of Operating Above 20kHz Yes, switching losses in power electronic switches do increase with operating frequency, but going beyond 20kHz has important advantages. Among these are Humans cannot hear the circuits For the same desired smoothing effect, L s and C s can be smaller because, as frequency increases and period T decreases, L s and C s charge and discharge less energy per cycle of operation. Smaller L s and C s permit smaller, lighter circuits. Correspondingly, L and C rms ripple currents decrease, so current ratings can be lower. Thus, smaller, lighter circuits. AC transformers are smaller because, for a given voltage rating, the peak flux density in the core is reduced (which means transformer cores can have smaller cross sectional areas A). d db d B sin( ) max t v( t) N NA NA NAB max cos( t ) dt dt dt Thus, smaller, lighter circuits. N = number of turns, ϕ = magnetic flux, B = magnetic field, A = x-sectional area 10
Fast Switching Frequencies Previous slide was just to illustrate how, with increased switching frequency, one can reduce the size of AC transformer cores needed: d db d B sin( ) ( ) max t v t N NA NA NABmax dt dt dt Thus, smaller, lighter circuits. cos( t) N = number of turns, ϕ = magnetic flux, B = magnetic field, A = x-sectional area The drawback, of course, to high frequency switching is increased power loss, since: P Total (loss) = P switching loss x number of switching events (or, the switching frequency) This is the downside of high-frequency switching. Thus, one must work to ensure overall losses are reduced by working to reduce the individual switching transition time. 11
MOSFETS are usually controlled with a pulse-width modulation (PWM) strategy. In essence, the PWM process involves comparing a sawtooth (or any other periodic function with linear transitions) with a voltage level. If the voltage level is constant, it will tend to produce a constant (dc) output. Sawtooth or triangle adjustable analog input (duty cycle control) Linear portions of sawtooth allows to directly translate voltage levels into time intervals (on-times) 12V Gate Driver (e.g. TC1427) Comparator (e.g., LM393) 12
PWM D = 8/12 = 075 0.75 Gate Driver (e.g. TC1427) Comparator (e.g., LM393) 13
PWM D = 6/12 = 05 0. Gate Driver (e.g. TC1427) Comparator (e.g., LM393) 14
PWM D = 4/12 = 1/3 Gate Driver (e.g. TC1427) Comparator (e.g., LM393) 15
MOSFETS are Very Sensitive to Static Electricity Touching the gate lead before the MOSFET is properly mounted will likely ruin the MOSFET But it may not fail right away. Instead, the failure may be gradual. Your circuit will work, but not correctly. Performance gradually deteriorates. When that happens, you can spend unnecessary hours debugging Key indicators of a failed MOSFET are Failed or burning hot driver chip Burning hot gate driver resistor (discolored, or bubbled up) Board scorches or melts underneath the driver chip or gate driver resistor Avoid these problems by mounting the MOSFET last, by using an antistatic wristband, and by not touching the gate lead 16
MOSFET Switch Turn-Off Overshoot.. Simple snubber circuit 200kHz, 0.01µF snubber VDS VGS 200kHz, no snubber VDS 100kHz, 0.01µF 01µF snubber VDS VGS VGS 200kHz, 0.0022µF snubber VDS 50kHz, 0.01µF snubber VDS VGS VGS 17
MOSFET Safe Operating Area (SOA) Pulsed drain current must never be exceed Maximum continuous drain current can be exceeded but only for a brief time Operation naturally limited by R DS,on (Ohm s law) Thermal limit (power limit) can be exceeded but only for a brief time Breakdown voltage must never be exceeded 18
MOSFET Datasheet 19
Small 10Ω, 100W Resistor. 22Vdc. Vpeak = 240V. MOSFET opens V DS ON V GS OFF DT Note: Ringing is caused by the interaction of parasitic capacitors and inductors. 20
Small 10Ω, 100W Resistor. 22Vdc. Vpeak = 40V. 0.022µF Snubber Cap MOSFET opens 21
Small 10Ω, 100W Resistor. 22Vdc. Vpeak = 60V 0.0068µF Snubber Cap MOSFET opens 22
Gate Drivers 23
Integrated PWM f 1.2 R T C T MC34060A, Fixed Frequency, PWM, Voltage Mode Single Ended Controller f 1 C R T T 24