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AN-35 参数 Datasheet PDF下载

AN-35图片预览
型号: AN-35
PDF下载: 下载PDF文件 查看货源
内容描述: LinkSwitch的设计指南\n [LinkSwitch Design Guide ]
分类和应用:
文件页数/大小: 16 页 / 504 K
品牌: ETC [ ETC ]
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AN-35  
cross sectional area Ae (cm2), primary inductance LP (µH), core  
effective path length Le (cm) and relative permeability µr:  
The secondary diode peak voltage was measured as 0.7 V, the  
secondarywindingresistanceas0.15andthecableresistance  
as 0.23 . Therefore VSEC is defined as:  
2
0.4 ×π × NP × Ae Le  
(18)  
Lg =  
×10  
VSEC  
=
=
VO + VRCABLE + VDOUT + VRSEC  
VO + (IO × RCABLE ) + VDOUT  
+ (ISEC(PEAK) × RSEC )  
LP ×100  
µr  
The gapped effective inductance ALG (nH/t2), required by the  
transformer manufacturer, is calculated from the primary  
inductance LP (µH) and the number of primary turns NP:  
(21)  
= 5.5 V + (0.5 A × 0.23 ) + 0.7 V  
+ (1.96 A × 0.15 )  
= 6.61 V  
LP  
NP2  
(19)  
ALG = 1000 ×  
Voltage VSEC allows the exact VOR to be calculated:  
Clamp, Bias, Bypass and Feedback  
NP  
An RCD clamp, formed by RFB, CCLAMP, and DCLAMP  
(Figure 1), safely limits transformer primary voltage, due to  
transformerleakageinductance,tobelowtheLinkSwitchinternal  
MOSFET breakdown voltage BVDSS each time LinkSwitch  
turns off. Leading-edge voltage spikes (caused by transformer  
leakage inductance) are filtered by RLF and CCLAMP, such that  
CCLAMP effectively charges to the transformer reflected voltage.  
VOR =  
× VSEC  
NS  
116  
15  
(22)  
=
× 6.61 V  
= 51.1 V  
Resistor RFB, a 1%, 0.25 W resistor, converts clamp voltage to  
Feedbackisderivedfromthereflectedvoltage,thatapproximates  
closely the transformer secondary winding output voltage  
(VSEC inFigure1)multipliedbythetransformerturnsratio. Due  
to effects of leakage inductance (causing peak charging),  
calculated VOR may be slightly different from actual voltage  
measured across CCLAMP. Since LinkSwitch is in the upper rail,  
reflected voltage information is now relative to the LinkSwitch  
SOURCE pin and independent of the input voltage.  
LinkSwitch bias and control current.  
Feedback voltage VFB is calculated from VOR and the error due  
to leakage inductance, VLEAK  
.
The value for VLEAK varies depending on the value of leakage  
inductance,thesizeoftheclampcapacitorandthetypeofclamp  
diode selected. For a leakage inductance of 50 µH, a value of  
5 V is a good initial estimate.  
Reflected voltage is directly converted by RFB to LinkSwitch  
CONTROL pin current for duty cycle control and bias. The  
CONTROL pin capacitor CCP provides bypass filtering, control  
loop compensation, and the energy storage required during  
start-up and auto-restart.  
VFB = VOR + VLEAK  
(23)  
Once a prototype has been constructed, the value of VFB can be  
found directly, by measuring the voltage across CCLAMP at the  
power supply peak output power point, using a battery powered  
digitalvoltmeter. Thesehavesufficientcommonmoderejection  
to be unaffected by the switching waveform and provide  
accurate results. The voltage measured is VFB. By subtracting  
VOR thevalueforVLEAK canbedetermined, usefulasanestimate  
in future designs. For the design in Figure 3, VFB was measured  
as 56.7 V, giving VLEAK as 5.6 V.  
Feedback Resistor (RFB)  
Clamp and feedback circuit design begins by first considering  
reflected voltage. Using the schematic in Figure 3 as an  
example. With primary turns NP = 116 and secondary turns  
NS = 15 the peak secondary current can be calculated from  
Equation 20, where IPRI(PEAK) is equal to the LinkSwitch  
typical current limit ILIM(TYP)  
.
An initial value for RFB is calculated from the feedback voltage  
VFB, the CONTROL pin voltage VC(IDCT) and current IDCT at the  
CC/CV transition point, specified in the LinkSwitch  
data sheet.  
NP  
NS  
116  
ISEC(PEAK)  
=
=
× IPRI(PEAK)  
× 0.254  
(20)  
15  
= 1.96 A  
B
4/03  
6
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