开关电源Forward converter动作原理_rev4

Forward Power Converter 動作原理
Reporter：Sean Hsia(夏存孝) Date：2009 / 04 /03
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1

Outlines
• Forward Converter Introduction • Operation of Forward Converter • Dual-Forward Converter Introduction
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Forward Converter Introduction (1)
優點： (1) 銅損(Copper loss)比較小，變壓器溫升可以減少一些，鐵心之大 小在選擇上也可以小一些 (2) 輸出電感器儲存能量提供負載使用，輸出電容器在數值大小可 減少許多，電容對漣波電流額定值之大小要求也會小些 (3) 流經功率開關峰值電流較小 (4) 流經輸出電感器之電流並非脈動電流，在輸出端之漣波電壓之 大小比返馳式小 缺點： (1) 與返馳式轉換器比較起來，成本較高，這是因為增加輸出電感 器與輸出二極體 (2)在輕載情況下，若轉換器進入DCM操作模式，則在其他無回授 之輸出端上，其輸出電壓可能會過大，一般在輸出端加入 Dummy Resistor解決。順向式轉換器一般都是設計工作在CCM 之情況來操作
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Operation of Forward Converter (1)
S
Vds
Ids
Vlm
Ilm
Vlo
Ilo
Idr
t0
t1
t2
t3
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Operation of Forward Converter - CCM (1)
S
Vds
Ids
等效電路
Vlm
Ilm
Vlo
Ns Vin Np
Ilo
Idr
t0
t1
t2
t3
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Operation of Forward Converter - CCM (2)
等效電路
Np Nr Vin
Lo Ilo Co Ro Vo
t0
t1 t2 t3
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Operation of Forward Converter - CCM (3)
等效電路
Lo Ilo Co Ro Vo
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t0
t1
t2
t3
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Function of Forward Converter for CCM
N p Vin Vin Vin D ⋅ ⋅ Δt ≥ ⋅ D ⋅ Tsw = ⋅ N r Lm Lm Lm f sw ⇒ Np D ⋅ Δt ≥ Nr f sw
Δt MAX
1− D = (1 − D ) ⋅ Tsw = f sw
N p 1− D D ⋅ ≥ N r f sw f sw
⇒
Np D 1 ≥ ⇒ D≤ Nr 1− D 1+ Nr N p
Vo Ns = D⋅ Vin Np
直流電壓轉換方程式：
1 轉換器最大責任週期： D ≤ 1 + N N r p
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Min. Output Inductance for CCM
Vo 1 = ⋅ ΔI = ⋅ (1 − D) ⋅ Ts 2 2 Lo D ⋅ N S ⋅ Vin ⋅ (1 − D) ⋅ Ts = 2 ⋅ N P ⋅ Lo
I LoB
• 若要使轉換器操作在CCM模式則
I o > I LoB = I oB N S ⋅ Vin ⋅ Ts = ⋅ D ⋅ (1 − D) 或 2 ⋅ N P ⋅ Lo
Vo ⋅ Ts Lo > LoB = ⋅ (1 − D) 2 ⋅ I LoB
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MOSFET的選擇 • 功率開關 – 耐壓值
Vds , max
– 耐流值
I ds ,max
⎛ Np ⎞ = Vin ⋅ ⎜ ⎜1 + N ⎟ ⎟ r ⎠ ⎝
NS NS D ⋅ Ts ⋅Vin ΔI ΔI = ⋅ ( I o + ) + I Lm = ⋅ (Io + ) + 2 2 NP NP Lm
– 平均電流值
I ds , avg NS = ⋅ D ⋅ I o = D ⋅η ⋅ I o NP
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DIODE的選擇(1) • 二極體D1 – 耐壓值
VD1,max
NS Vo = ⋅ Vin = NP D
– 耐流值
I D1,max
⎛ NS ⎞ ΔI Lo D = Io + = Io + ⋅⎜ ⋅ Vin − Vo ⎟ ⎜ ⎟ 2 2 ⋅ Lo ⋅ f sw ⎝ N P ⎠
– 平均電流值
I D1,avg = D ⋅ I o
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