一般在評估SiC diode操作上是否安全,除了透過datasheet內所標示的規格來對照外,通常還會透過Tj的溫度計算來確認。
SiC diode整體的Tj溫度是來自穩態與暫態的加總。
- 以Infineon SiC diode IDH06SG60C為例來計算Tj
- Steady state(穩態)
- Conduction loss
Pd_con = VF x IF x Don = 3.5V x [25.4*(2/pi)] x 0.27 = 15.28W
- Don = 3.6us / 13.3us = 0.27
- 由下列曲線,找出對應穩態的VF
- Switch loss
Pd_sw = Qc x Vo x fsw = 8nC x 400V x (1/13.3us) = 0.24W
Qc可由datasheet得知,Vo為PFC電壓,fsw為週期倒數 - Ptot_steady state = Pd_con + Pd_sw = 15.28 + 0.24 = 15.52W
- Conduction loss
- Abnormal (暫態)
- 以264Vac AC cycle dropout waveform為例
- Conduction loss
Pd_con = VF x IF x ton x n
= 5.9V x [38.8*(2/pi)] x 18us x (4ms/13.3us)
=0.793W - 由下列曲線,找出對應暫態的VF值
-
- Switch loss
- Pd_sw = Qc x Vo x n = 8nC x 400V x (4m/13.3us) = 0.96mW
- Ptot_abnormal = Pd_con + Pd_sw = 0.793W + 0.96mW = 0.8W
- Junction溫度計算
- 穩態Tj溫度
- Pd = (Tj - Tc) / Rthjc
Tj_steady state = Tc + 2.1K/W x 15.52W = 85 + 2.1 x 15.52 = 117.6℃
由datasheet得知熱阻係數Rthjc為2.1K/W,先前穩態Pd為15.52W,假設Tc溫度為85 ℃ - 暫態Tj溫度
Trise _abnormal = Pd_pulse x Zthjc = 0.8W x 0.16K/W = 0.13℃
透過下列曲線求出暫態熱阻,Zthjc(18us) = 0.16K/W - 因此整體的Tj為穩態加上暫態的溫度
- Tj_total = Tj_steady state + Tj_rise_abnormal = 117.6 + 0.13 = 117.73℃
- 對照datasheet的Tj規格,Tj的計算結為117.73℃,在datasheet所標示之規格內,符合使用規格。