Overview
The"freewheelingdiode"weusuallycallgetsitsnamebecauseitplaystheroleoffreewheelinginthecircuit.Generally,fastrecoverydiodesorSchottkydiodesareselectedas"Freewheelingdiode",whichisgenerallyusedinthecircuittoprotectthecomponentsfrombeingbrokendownorburntoutbytheinducedvoltage.Itisconnectedinparalleltothetwoendsofthecomponentthatgeneratestheinducedelectromotiveforceandformsaloopwithittogeneratehighelectromotiveforce.Intheloop,thecontinuouscurrentconsumptionisusedtoprotectthecomponentsinthecircuitfrombeingdamaged.
Function
Freewheelingdiodesareoftenusedtogetherwithenergystorageelementstopreventsuddenchangesinvoltageandcurrentandprovideaccess.Theinductancecanprovidecontinuouscurrenttotheloadthroughit,soastoavoidsuddenchangesintheloadcurrentandsmooththecurrent.Intheswitchingpowersupply,youcanseeafreewheelingcircuitcomposedofdiodesandresistorsconnectedinseries.Thiscircuitisconnectedinparallelwiththeprimarysideofthetransformer.Whentheswitchtubeisturnedoff,thefreewheelingcircuitcanreleasetheenergystoredinthetransformercoiltopreventtheinducedvoltagefrombeingtoohighandbreakingdowntheswitchtube.Generally,youcanchooseafastrecoverydiodeorSchottkydiode,whichisusedtoconsumethereversepotentialgeneratedbythecoilintheformofcurrent.Itcanbeseenthatthe"freewheelingdiode"isnotasubstantialelement,itisjustafunctioninthecircuit.Theresultingroleiscalled"continuousflow".
Workingprinciple
Freewheelingdiodesareconnectedinparallelatbothendsofthecoil.Whenthecoilpassescurrent,itwillgenerateinducedelectromotiveforceatbothendsofthecoil.Whenthecurrentdisappears,itsinducedelectromotiveforcewillproduceareversevoltageonthecomponentsinthecircuit.Whenthereversevoltageishigherthanthereversebreakdownvoltageofthecomponent,itwillcausedamagetothecomponentssuchastriodesandthyristors.Freewheelingdiodesareconnectedinparallelatbothendsofthecoil.Whenthecurrentflowinginthecoildisappears,theinducedelectromotiveforcegeneratedbythecoilisconsumedbytheworkofthecircuitformedbythediodeandthecoil,therebyprotectingthesafetyofothercomponentsinthecircuit.Thefreewheelingdiodeisconnectedinreverseparalleltothetwoendsoftherelayortheinductorinthecircuit.Whentheinductorispoweredoff,theelectromotiveforceatbothendsdoesnotdisappearimmediately.Atthistime,theresidualelectromotiveforceisreleasedthroughadiode.ThediodethatplaysthisroleiscalledthecontinuousFlowdiode.Infact,itisstilladiode,butitplaysafreewheelingrolehere.Forexample,thediodeconnectedinthereversedirectionatbothendsoftherelaycoilorthereverseconnectionatbothendsoftheunidirectionalthyristorisalsothecase.Whyconnectadiodeinreverse?Becausethecoiloftherelayisaverylargeinductance,itcanstoreelectricalenergyintheformofamagneticfield,sowhenitpullsin,itstoresalargeamountofmagneticfield.Whenthetransistorofthecontrolrelaychangesfromontooff,thecoilisde-energized,butthereisamagneticfieldinthecoil.Atthistime,areverseelectromotiveforcewillbegenerated,andthevoltagecanbeashighas1000v,whichcaneasilybreakdownthetriodeorothercircuitcomponents.Thisisbecausetheconnectionofthediodecoincideswiththedirectionofthereverseelectromotiveforce,andthereversevoltageisneutralizedintheformofcurrentthroughthefreewheelingdiode,therebyprotectingothercircuitcomponents.Therefore,itisgenerallyadiodewitharelativelyfastswitchingspeed.Likethethyristorcircuit,becausethethyristorisgenerallyusedasacontactswitch.Ifyouarecontrollingalargeinductiveload,ahigh-voltageback-EMFwillalsobegenerated,andtheprincipleisthesameasthatofarelay.Freewheelingdiodesarealsousedindisplays,generallyusedinthecoilsofdegaussingrelays.