Simple Modular Half-Bridge

Transcription

1 Simple Modular HalfBridge Shane Colton Maachuett Intitute of Technology Rev March, 2009

2 Simple Modular HalfBridge Module Overview V i V i Iolated DCDC Supply: Supplied by 12V from any battery or power upply. Create 12V upply referenced to any MOSFET ource. Create highide drive voltage. Iolate gate drive circuitry uing AC coupling. A K V G GateDrive Optocoupler: Supplied by 12V referenced to MOSFET ource. Input opticallyiolated PWM ignal. Output 2.0A peak gate drive. V G D S MOSFET: Could be virtually any ingle or parallel combination of MOSFETS. Chooe to et power and voltage level of the halfbridge. Mut include appropriate gate reitor.

3 Simple Modular HalfBridge Iolated DCDC Supply: Individual Unit V i V i V i V i 1μF 1μF Texa Intrument DCP Iolated 12V DC/DC Converter The Texa Intrument DCP i an iolated 12V to 12V DCDC converter. It take a 12V input, referenced to a common ground, and output 12V referenced to ome other voltage. It ue AC and magnetic coupling to iolate the output from the input. Thi i ued to upply 12V referenced to a MOSFET ource pin, no matter where that i with repect to the ytem ground. The two 1μF capacitor are ued to mooth the input and output voltage. Thi i the mot expenive ingle component, at approximately $12. However, it help create a very welliolated power ytem.

4 Simple Modular HalfBridge Gate Drive Optocoupler: Individual Unit A K V G A V Avago Technologie HCPL3120 GateDrive Optocoupler G V K V The Avago HCPL3120 i an opticallycoupled MOSFET or IGBT gate driver. The purpoe of optical coupling i to iolate the input ignal from the highpower electronic. When the LED i on, the MOSFET gate i ourced by V. When the LED i off, the gate i unk to V. Important Specification 1 : LED Forward Voltage, V f : 1.5V LED Forward Current, I f : 10mA 1 HCPL3210 Dataheet:

5 Simple Modular HalfBridge Gate Drive Optocoupler: ShootThrough Delay Pair H in R To LowSide MOSFET C To HighSide MOSFET L in The above circuit i a paive way to enure that hootthrough cannot occur in a halfbridge. It introduce a delay between turnoff of one MOSFET and turnon of the other. Note that the optocoupler LED are in revereparallel configuration. The delay hould be ignificantly longer than the turnon/turnoff time of the MOSFET. A few microecond i uually ufficient. The exact delay i omewhat difficult to etimate, but a good approximation can be made uing: t delay R C V V f ig t delay R C 2V V f ig For inglequadrant control. More to come on thi. For twoquadrant control (ynchronou rectification). More to come on thi.

6 Simple Modular HalfBridge Gate Drive Optocoupler: Chooing Paive Component H in R To LowSide MOSFET C To HighSide MOSFET L in R hould be choen like a tandard LED currentlimiting reitor: R V ig I V f f C hould be choen baed on the deired hootthrough delay time: C t R delay V V ig f Singlequadrant control. or C t R delay V 2V ig f Synchronou rectification.

7 Simple Modular HalfBridge Gate Drive Optocoupler: ShootThrough Delay Example H in R To LowSide MOSFET C To HighSide MOSFET L in Condition: Predicted Delay: V ig 3.3V, R 200Ω, C 0.047μF, Synchronou Rectification 8.5μ (uing the equation for the previou lide) Voltage on C Voltage on Gate Turnoff i almot intant. Turnon i delayed by 7.89μ.

8 Simple Modular HalfBridge MOSFET: Flexible Configuration D G S Almot Anything Becaue the floating upply and optocoupled gate driver are very modular, the MOSFET ued can be virtually anything. They can be individual mall MOSFET, many mall MOSFET in parallel, or large MOSFET with huge currenthandling capabilitie. They can be in almot any configuration, including half and fullbridge. The modular gate driver treat high and lowide MOSFET the ame. There are a few general guideline to follow: 1. Ue gate reitor. The optocoupled gate driver can handle a peak output of 12V. So the total effective gate reitance hould be no le than 6Ω. 2. Ue TVS diode from ource to drain to protect MOSFET from inductive pike. 3. For parallel MOSFET, oberve good gate reitor practice to prevent ringing. See next lide. 4. Ue a pulldown reitor on all gate to enure that MOSFET turn off in the event of a gate driver failure. 5. Check the gate capacitance pecification in the MOSFET data heet to determine the approximate turnon and turnoff time, given your choen gate reitance. Set the hootthrough delay at the optocoupler accordingly.

9 Simple Modular HalfBridge MOSFET: One Example D International Rectifier IRFB3207 MOSFET (x4) TVS Diode 60V, 5kW D G 20Ω 20Ω 20Ω 20Ω S G 5Ω 10kΩ S IRFB3207: Thee are eriou workhore MOSFET. Even though they are in a mall TO220 package, each one can handle 75A at 48V if properly heatinked. Gate Reitor: Four 20Ω reitor in parallel and one 5Ω reitor in erie with all. Thi give a 10Ω effective gate reitance a een by the optocoupled gate driver, but a larger gate reitance a een from one MOSFET to the ret. Thi help prevent ringing iue in parallel MOSFET. TVS Diode: Thi protect againt large inductive pike, aborbing energy when the voltage acro it i too high. It may or may not be neceary, depending on the application.

10 Simple Modular HalfBridge MOSFET: Computing the TurnOn/TurnOff Time The witching time of the MOSFET module can be found with the gate capacitance pecification of the MOSFET. For example, the gate capacitance of a ingle IRFB3207 MOSFET i 7.6nF 1. The capacitance of parallel MOSFET i ummed. For a quick etimate of the witching time, the RC time contant of the total gate drive circuit can be calculated. A full witching period might take four time contant: 10Ω 12V (4)(7.6nF) 30.4nF 4 4R C (4)(10)(30.4nF) 1. 2 w g g Thi ha a few practical implication. For one, the hootthrough delay hould be ignificantly longer than one witching time period. In thi cae, a few microecond i enough. Alo, the time pent witching hould be everal order of magnitude le than the time pent fullyon or fullyoff, to enure high efficiency. In many cae, PWM frequencie up to 10100kHz may till atify thi condition. 1 IRFB3207 Dataheet:

11 Simple Modular HalfBridge MOSFET: Heat Diipation The MOSFET will create heat in three way: conductive, witching, and diode diipation: Conductive: When the MOSFET i on, it behave like a very mall reitor. The onreitance i pecified in the MOSFET dataheet. Switching: Energy i lot a the MOSFET croe through a partiallyon tate. Thi diipation i proportional to witching frequency. Diode: In inglequadrant control, one MOSFET module i ued a a diode. Diode diipation i uually greater than conductive diipation becaue of the relatively high voltage drop, which i why ynchronou rectification i beneficial. Diode diipation i eay to etimate and make a good wortcae cenario in many cae. Example: 4 x IRFB3207 in 300A ( 4 x TO220 package limit). Diode drop i 1.3V 1. Total thermal reitance juntion to greaed heat ink i 0.95ºC/W 1. T P di IV ( 300A)(1.3V ) 390W 1 Pdi Rth (390W )(0.95º C / W ) 93º C (Shared by four MOSFETS) Thi i amazingly till within operating temperature a long a the heat ink remain near ambient. 1 IRFB3207 Dataheet:

12 Simple Modular HalfBridge Putting It All Together: Completely Iolated HalfBridge H in L in (gate drive upply ground), (main power upply ground), and logic ground for optocoupler input are all iolated! You can do whatever you want with them. If everything i upplied by one battery, all ground will be the ame. You can tie ground together with mall reitor and ue lot of extra capacitance to protect the logic from noie on the power ground. Or, you can upply the logic and/or gate driver with a eparate battery for complete iolation.

13 Simple Modular HalfBridge Putting It All Together: Input Capacitance H in L in The input capacitance hold the voltage acro the halfbridge table againt inductive voltage pike from the battery cable. It exact value depend on many factor, but a very conervative wortcae etimate can be made by conindering the voltage ripple if the battery cable inductance wa very large: I 1 Keep ΔV reaonable baed on working voltage. max V Keep battery cable hort and together. C Thi capacitor may need a precharge circuit. f PWM

14 Simple Modular HalfBridge Putting It All Together: Driving the HalfBridge H in L in Hin HIGH LOW HIGH LOW Lin LOW LOW HIGH HIGH X X Note On Coat Coat Brake Drive H in with PWM while L in i low for inglequadrant control. The lowide MOSFET will act a a flyback diode. Drive H in with PWM and L in with complementary PWM for twoquadrant control / ynchronou rectification / regenerative braking.

15 Simple Modular HalfBridge Putting It All Together: Phyical Layout The iolated DCDC upplie and optocoupler can be eaily built onto a breadboard, protoboard, or printed circuit board. One example of a printed circuit board layout for four et of thee module i hown below. Thi board could handle two halfbridge, or one fullbridge. (The PCB manufacturing file are alo available.)

16 Simple Modular HalfBridge Putting It All Together: Phyical Layout Phyical contruction of the MOSFET module i very important to achieving the maximum performance and reliability, epecially if your application puhe the limit of the MOSFET ued. Good heat inking i critical! You can ue mooth, facedoff aluminum bar or commerciallyavailable heat ink. Ue thermal pate on the drain tab and fan to cool the heat ink. One quick and eay way to achieve good thermal and electrical conductivity i to ue the heat ink to carry current a well. Bra crew through the drain tab can give a very low reitance electrical connection. Thi alo minimize oldering and make replacing individual MOSFET very imple. The following how a highcurrent halfbridge made with minimal oldering: