Power owe Electronic ec o c Devices ev ces and d Circuits (EEL-209) By: Prof. Bhim Singh Department of Electrical Engineering, Indian Institute of Technology Delhi, Hauz Khas, New Delhi-10016, India- 110016 email:
[email protected] Ph.:011-2659-1045 1 Gate driver circuits Gate Driver Circuit • Interface between control ( (low p power electronics) ) and ( (high g power) switch. • Functions: – Amplification: amplifies control signal to a level required to drive power switch – Isolation: provides electrical isolation between power switch and logic level • Complexity of driver varies markedly among switches. – MOSFET/IGBT drivers are simple – GTO and BJT drivers are very complicated and expensive. Gate Driver Circuit Control Circuit Driver Circuit Power switch The switching g speeds p and on-state losses of a g given controlled switch depend on how it is controlled, so it is important to design the proper drive circuit Functionality of Gate/Base Drive Circuits Turn T power switch it h from f off-state ff t t to t on-state t t Minimize turn-on time through active region where power dissipation is large Provide adequate drive power to keep power switch in on-state Functionality of Gate/Base Drive Circuits Turn T power switch it h from f on-state t t to t off-state ff t t • Minimize turn-off time through active region where power dissipation is large • Provide bias to insure that power switch remains off Functionality of Gate/Base Drive Circuits • Control power switch to protect it when over voltages or over currents are sensed • Signal g processing p g circuits which g generate the logic g control signals not considered part of the drive circuit. • Drive circuit amplifies control signals to levels required to drive power switch • Drive circuit has significant power capabilities compared to logic level signal processing circuits Drive Circuit Design Considerations Drive circuit topologies • Output signal polarity - unipolar or bipolar • AC or DC coupled • Connected in shunt or series with power switch Drive Circuit Design Considerations Output current magnitude • Large Ion shortens turn-on time but lengthens turn-off delay y time • Large Ioff shortens turn-off time but lengthens turnon delay y time Provisions for power switch protection Over currents cu e s • Ove • Blanking times for bridge circuit drives Drive Circuit Design Considerations Wave shaping to improve switch performance • • • • Controlled diB/dt for BJT turn-off Anti-saturation diodes for BJT drives Speedup capacitors Front-porch/back porch currents Component layout to minimize stray inductance and shielding from switching noise Power MOSFETs (high speed, voltage (high-speed voltage-controlled controlled switches that allow us to operate above the 20kHz audible range) D: Drain D If desired, a series blocking diode can be inserted here to prevent reverse current G: Gate G Switch closes when VGS ≈ 4V, 4V and d opens when h VGS= 0V S S: Source N channel MOSFET equivalent circuit Controlled turn on, controlled turn off (but there is an internal antiparallel diode) 11 We Avoid vo d the t e Linear ea (Lossy) ( ossy) Region, eg o , Using Us g O Only yt the e On and Off States MOSFET “on” D MOSFET “off” D S S when VGS = 12V when VGS = 0V 12 We Want to Switch Quickly to Minimize Switching Losses Turn Off VDS(t) VDS(t) Turn On 0 I( ) I(t) Δtoff ff 0 I( ) I(t) Δton 0 PLOSS(t) 0 Energy lost per turn off 0 PLOSS(t) 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) 13 Consider, for example, the turn off Turn Off VDS(t) V Energy lost per turn off is proportional to V • I • Δtoff , 0 I(t) I 0 PLOSS(t) 0 Δtoff Energy lost per turn off so we want to keep turn off (and turn on) times as small as possible. possible The more often we switch, the more “energy loss areas” we experience per second. Thus, , switching g losses ( (average g W) ) are proportional to switching frequency f, V, I, Δtoff, and Δton. And, of course, there are conduction losses that are proportional to squared I 14 ELECTRICAL ISOLATION FOR DRIVERS Isolation is required to prevent damages on the high power switch to p propagate p g back to low p power electronics. Normally opto-coupler (shown below) or high frequency magnetic materials (as shown in the thyristor case) are used. Many standard driver chips have built-in isolation. For example TLP 250 from Toshiba, HP 3150 from Hewlett-Packard uses optocoupling isolation. ELECTRICAL ISOLATION FOR DRIVERS Power semiconductor devices can be categorized into 3 types based on their control put requirements: equ e e ts: input a) Current Current-driven driven devices – BJTs, BJTs MDs, MDs GTOs b) Voltage-driven devices – MOSFETs, IGBTs, MCTs MCT c) Pulse-driven devices – SCRs, TRIACs CURRENT DRIVEN DEVICES (BJT) Power BJT devices have low current gain due to constructional consideration, leading current than would normally be expected for a given load or collector current. The h main i problem bl with i h this hi circuit i i is i the h slow l turn-off ff time. i ELECTRICALLY ISOLATED DRIVE CIRCUITS EXAMPLE : GATE DRIVE FOR THYRISTORS Pulse transformer is used for isolation. isolation R1 is to limit the gate current Normally a pulse with length 10us with amplitude of 50mA is sufficient to turn-on turn on the thyristors. thyristors It is quite common to fire the thyristors with successive pulses to ensure proper turn-on. It is i not t possible ibl to t turn-off t ff a thyristor th i t with ith the th above b circuit EXAMPLE: SIMPLE MOSFET GATE DRIVER Note: MOSFET requires VGS =+15V for turn on and 0V to turn off. LM311 is a simple amp with open collector output Q1. When Wh B1 is i high, hi h Q1 conducts d t . VGS is i pulled ll d to t ground. d MOSFET is off. When B1 is low, Q1 will be off. VGS is pulled to VGG. If VGG is set to +15V, the MOSFET turns on. Gate Driver Design Techniques Reduce IRR ( (diode reverse recovery y current) ) by: y reducing di/dt, which means increasing gate series resistances Reduce VCE,overvoltage by reducing di/dt balancing b l i gate t timing ti i and d voltage lt sharing h i among the series-connected IGBTs In both cases cases, need a better, better independent control of di/dt and dv/dt to optimize the gate driver for speed, minimum losses, , and reliability y Two-Stage Two Stage Gate Driver To reduce IRR and VCE,overvoltage Turn-on: RGon2 << RGon1 Stage Stage-2 2 is off initially Cgate charged through RGon1 (larger) to keep IRR small After diode has recovered, stage-2 turn on (triggered by VREF in comparator) Driver resistance is now RGon1 G 1||RGon2 G 2 (smaller) Two-Stage g Gate Driver Turn-off: RGoff2 << RGoff1 Stage-1 & 2 is on initially for p discharge g of Cgate rapid (RGoff1||RGoff2 smaller) When VCE has risen to DC, link voltage, voltage stage-2 stage 2 turns off Driver resistance is RGoff1, reducing current fall rate After VCE is settled, stage-2 turns on again to ensure small driver impedance and prevent against dv/dt induced turn-on GCT Gate Drive Equipment GCT & Gate Driver Board IGBT Gate Driver Circuit IGET Gate Driver Equipment Injection enhanced insulated gate bipolar transistor Calculated Reliability of Gate Drivers Power Devices Losses Switching Losses Power Devices Switching Losses Power Devices Switching Losses Power Switch Voltage and Current Ratings Power Switch Voltage and Current Ratings MOSFET gate driver From control circuit +VGG R1 Rg Q1 LM311 G + VGS _ + D VDC S _ MOSFET Gate Driver Note: MOSFET requires q VGS =+15V for turn on and 0V to turn off. LM311 is a simple amp with open collector output Q1. When B1 is high, Q1 conducts. VGS is pulled to ground. MOSFET is off. When B1 is low, Q1 will be off. VGS is pulled to VGG. If VGG is set to +15V, the MOSFET turns on. Effectively, the power to turn-on the MOSFET comes form f external l power supply, l VGG Isolation Isolation using Opto coupler Need for Isolation for Gate Driver Circuit Opto-Coupler Opto Coupler Isolated BJT Drives Trans former Coupled BJT Drives Opto-Coupler Opto Coupler Isolated MOSFET Drives Over p protection with Drive Circuit Snubber Circuits and Heat Sinks 45 Need for protection in power semiconductor devices? For reliable operation of a device specified ratings should not be exceeded. exceeded But In practice, device may be subjected to over voltages or over currents. During turn-on, di/dt may be prohibitively large. There may be false triggering by high value of dv/dt. A spurious signal across gate-cathode terminals may lead to unwanted turn-on. The Th device d i must t be b protected t t d against i t all ll such h abnormal b l conditions diti f for satisfactory and reliable operation of SCR circuit and the equipment. The object of this section is to discuss various techniques adopted for the protection of SCRs. Power semiconductor devices are commonly protected against: 1. Over-current; 2. di/dt; 3. Voltage spike or over-voltage; 4. dv/dt ; 5. Gate-under voltage; 6 Over voltage at gate; 6. 7. Excessive temperature rise; 8 Electro-static 8. El t t ti discharge; di h di/dt protection: p If the rate of rise of anode current, i.e. di/dt, is large as compared to the spread velocity of carriers, local hot spots will be formed near the gate connection on account of high current density. This localised heating may destroy the thyristor. Therefore, the rate of rise of f anode d current at the h time i of f turn-on must be b kept k b l below the h specified ifi d limiting value. The value of di/dt can be maintained below acceptable limit by using a small inductor, called di/dt inductor, in series with the anode circuit. Typical di/dt limit values of SCRs are 20-500 A/µ sec. Local spot heating can also be avoided by ensuring that the conduction spreads to the whole area as rapidly as possible. This can be achieved by applying pp y g a g gate current nearer to ( (but never g greater than) ) the maximum specified gate current. dv/dt protection: p If the rate of rise of forward voltage dVa/dt is high, the charging current i will be more. This charging current plays the role of gate current and turns on the SCR even when gate signal is zero. Such phenomena of turning-on a thyristor, called ll d dv/dt d /d turn-on must be b avoided id d as it i leads l d to false f l operation i of f the h thyristor circuit. For controllable operation of the thyristor, thyristor the rate of rise of forward anode to cathode voltage dVa/dt must be kept below the specified rated limit. Typical values of dv/dt are 20 – 500 V/µsec. False turn-on of a thyristor by large dv/dt can be prevented by using a snubber circuit in parallel with the device Overview of Snubber Circuits for HardS it h d Converters Switched C t Function to Protect the Semiconductor devices by y • Limiting device voltages during turn-off transients • Limiting device currents during turn- on transients • Limiting the rate-of-rise (di/dt) of currents through the semiconductor i d device d i at device d i turn-on • Limiting the rate-of-rise (dv/dt) of voltages across the semiconductor device at device turn-off or during forward blocking voltages • Shaping the switching trajectory of the device as it turns on/off 50 Hard switching •Hard switching refers to the stressful switching behavior of the power electronic devices. devices •During the turn-on and turn-off time, the power device has to withstand high voltage and current simultaneously, resulting in high switching g losses and stresses. Dissipative passive snubbers are usually added to the power circuits so that the dv/dt and di/dt of the power devices could be reduced, So the switching loss and stress are di diverted d to the h passive i snubber bb circuits. 51 Classification of snubbers There are mainly 2 types of classification of snubbers 1 Active snubbers 1. 2. Passive snubbers • Active snubbers can importantly reduce switching losses, these include transistors and other active switches that often imply extra circuitry. Disadvantages of active snubbers • But they need extra circuitry to control the active switch making more complex circuits which are not appropriate to all applications and the increase in complexity of controlling these elements. P i snubbers Passive bb • Passive snubbers are relatively simple to design and they can reduce switching losses. Passive snubbers are limited to resistors, capacitors, inductors and diodes. 52 Classification of passive snubbers Passive snubbers are classified into two types 1. 2 2. Dissipative passive snubbers N di i i passive Non-dissipative i snubbers bb If the energy stored in the snubber is dissipated in a resistor the snubber is classified as dissipative. dissipative If the energy is moved back to the input or to the output the snubber is classified as non-dissipative. Another type of classification is 1. 2. Polarized Non polarized Snubber can be classified as polarized or non-polarized depending on the direction of the energy energ moves, mo es in or out o t the snubber. sn bber 53 Classification of non dissipated snubber circuits 54 Types of dissipative Snubber Circuits 1. Unpolarized series R-C snubbers • Used to protect diodes and thyristor 2. Polarized R-C snubbers • Used as turn-off snubbers to shape the turn-on switching trajectory of controlled switches. • Used as over voltage snu1bbers to clamp voltages applied to controlled switches to safe values. • Limit dv/dt during device turn-off 3 Polarized L-R 3. L R snubbers • Used as turn-on snubbers to shape the turn-off switching trajectory of controlled switches. • Limit di/dt during device turn-on 55 Unpolarized snubber Forward Polarized snubber Turn-on Turn on and Turn Turn-off off snubber for Thyristor Reverse Polarized snubber Turn-off Turn off snubber for transistor Turn-on Turn on snubber for transistor Over voltage snubber for transistor Turn-off Turn off snubber for MOSFET Snubber for GTO References 1. N. Mohan, T. M. Undeland and W. P. Robbins, “Power Electronics, , Converter, , Application pp and Design”, g , Second Edition, John Willey & Sons, 1995, New York, ISBN 997151-177-0. M. H. Rashid, “Power Electronics, circuits, Devices and Applications”, Second Edition, Prentice-Hall, 1995, India, ISBN 81-203-0869-7. B. W. Williams, “Power Electronics: Devices, Drivers and Applications”,Wiley,1987.NewYork,ISBN: li i il k 0470206969. W. C. Lander, "Power Electronics", 3rd Edition, McGrawHill, 1993, New York, ISBN: 0077077148S. HOW TO SELECT A HEAT SINK. Seri Lee, Director. Advanced Thermal Engineering. Aavid Thermal Technologies, Inc. Laconia, New Hampshire. 65 2. 3. 4. 5. Thank you 66