应用指导 | CGAN003: GaN switching behavior analysis

 

 

众所周知，GaN 功率器件具有传统功率器件无可比拟的性能优势，如大幅提升的开关速度和显著降低的开关损耗，从而提供更优的整体效率，这使得GaN 器件在高频、高效率的应用中展现出明显优势。我们的应用指导将帮助客户更好得使用云镓的 GaN 器件，更大限度去挖掘云镓的 GaN 器件的能力。本次发布云镓应用手册第三弹: GaN switching behavior analysis。此外，客户可以通过云镓半导体官方网站或者微信公众号了解到更多云镓 GaN 产品资讯和相关技术文档。

 

 

 

Part 1 : Overview of DPT

 

 

01

Why do we need DPT to measure GaN?

Whydo we need double pulse tester to measure GaN FETs?

Extracting switching parameters (ton, toff, tr, tf)

Extractingswitching loss (Eon, Eoff)

Emulating system behavior, e.g. turn-off drain spike, switching time and etc.

Evaluating dynamic performance, e.g. dynamic Ron, D-HTOL, switching SOA, and etc.

Noneed of high-power equipment, low-cost scheme

02

An example of DPT -650V/30mΩ CloudSemi GaN

 

 

 

Part 2 : Detailed analysis 

 

 

01

1st pulse turn-on: zero-current switching
 

 

Stageof start-up,zero-current turn-on (capacitive-load switching)：

• Vds of rectifier GaN: Vbus→ 0; Vds of synchronous GaN: 0 → Vbus
• The setup can be treated as a capacitive-load switching:
• Oscilloscope-captured Ids comes from charging Coss of Syn GaN: Ids = C(Vbus-Vds)*dv/dt
• Discharging Coss of Rec GaN cannot be directly captured by oscilloscope: ICoss= Coss*dv/dt
• Current through Rec GaN channel: Ich, rec = Ids + Coss*dv/dt

02

HSW turn-on transient: Vgoff to Vth

 

t0~t1,gate drive charges Ciss of Rec GaN:

Vgs of Rec GaN rises to Vth. No current flows through Rec GaN.

Load current flows through inductor and Syn GaN.

Until Vgs of Rec GaN reach to its Vth, load current starts to commutate and flow through it.

 

03

HSW turn-on transient: current commutation

 

 

t1~t2,current commutation:

As Vgs of Rec GaN reaches Vth,current starts to flow through Rec GaN. Load current gradually commutates from Syn GaN to Rec GaN, as Vgs increases;

During current commutation, the gate driver charges Ciss until Rec GaN has currentcapability of IL.[Ids|(Vgs=Vgp) = IL]

When Vgs = Vgp (Miller plateau, = Vth + IL/gm),all the inductive-load current flows through the Rec GaN.

In this duration, Rec GaN Ids overlaps with Vds, and I-Voverlap loss dominates and can be calculated by integrating I&V.

04

HSW turn-on transient: voltage commutation

 

t2~t3,voltage commutation:

As Vgs of Rec GaN reaches Vgp, allthe load current flows through Rec GaN;

Then, RecGaN Coss starts to discharge from Vbus to Von & SynGaN Coss starts to charge to Vbus

RecGaN Coss discharging cannot be captured,but SynGaN Coss charging can be recognized from Ids waveform

During this duration, the loss on Rec GaN consists of 2main parts:

I-V overlap loss

Cap loss from Syn/Rec GaN: Vbus*Qoss

 

GaN features much lower Qoss and switching loss Psw comparing to Si SJ

 

05

HSW turn-on transient: gate over-drive

 

 

t3~t4,gate over-drive to make GaN fully turn-on:

Gatedriver continue to charge Ciss from Vgp to Vdrive (e.g. 6V), making GaN FET fullyturn-on

Von= IL* Rdson (Vgs = Vdrive). Therecommended gate drive voltage for CloudSemi E-mode GaN is ~6.0V.

Aftert4, inductive load will continue to excite, following dIL = Vbus/L * dt

Conductionloss on Rec GaN FET dominates in this duration

Comparing to Si SJ, GaN FET features much lower gate charge QG

 

 

06

Turn-on period: excitation
 

 

t4~t5,inductive load excitation:

dIL = Vbus/L *dt, Ids = IL

Ciss stops charging

Currentflowing through GaN channel gradually increases

RecGaN FET works in linear region.

Conductionloss on Rec GaN FET dominates in this duration

 

07

HSW turn-off transient: VGS → Vgp
 

 

t5~t6, Ciss starts discharging:

Rec GaN Ciss starts to discharge, until GaN enters saturation mode (Vgs = Vgp, Idsat = IL).

Here,Vgp =Vth + IL/gm

RecGaN works from linear region to saturation region

Conductionloss on Rec GaN FET dominates in this duration

 

08

HSW turn-off transient: voltage commutation

 

t6~t7,voltage commutation (Rec GaN Vds rises to Vbus):

Loadcurrent charges the switching node (charging Coss ofRec GaN, discharging Coss of Syn GaN)

Parameterof Qoss is crucial, and determinescharging/discharging speed (i.e. switching speed)

Powerloss on Rec GaN mainly comes from I-V overlap loss

 

09

HSW turn-off transient: current commutation

 

t7~t8,current commutation:

Load current gradually commutates from Rec GaN to Syn GaN, as Vgs decreases;

Duringcurrent commutation, the gate driver discharges Ciss until Rec GaN has been turned off(Vgs = Vth)

WhenRec GaN Vgs = Vth (i.e. after t8), all the inductive-load current flows through theSyn GaN.

Powerloss mainly comes from I-V overlap loss

 

 

Part 3 : Summary 

 

 

01

Summary

 

更多关于 GaN 器件在系统中的损耗，我们将以 200V→400V 同步 boost 和 48V→12V 同步 buck 来进行实例说明，敬请期待。