0.1uF DF value bad analysis
Time:
2022-07-19 14:10
Source:
Author:
0.1μF/275V D2 High DF Cause Analysis
Capacitor is one of the most used components in electronic equipment, and DF value is one of the four parameters to measure capacitor performance (capacity, DF, insulation, withstand voltage), which directly leads to capacitor Fever during use. When the heat generated inside the capacitor is greater than the heat radiated to the outside, the temperature inside the capacitor is getting higher and higher, and finally the thermal breakdown of the capacitor is caused. Therefore, the increase in the internal temperature of the capacitor in actual use is caused by the loss of the capacitor itself.
Metallized film capacitors are wound with metallized film, sprayed with metal on the end face and welded with leads, so in the production process, the DF value will become a key factor in quality control. The loss of the metallized film capacitor is mainly composed of the loss of the dielectric part and the metal part (plate, plate and gold spray layer, gold spray layer and lead, lead).
1. Loss of the dielectric part
Because this product is produced by polypropylene film, the dielectric loss is very small, so the high loss is mainly caused by the metal part.
2. The loss of the metal part The loss of the metal part of the
metallized film capacitor is composed of the lead loss (CP line) of the capacitor, the contact loss between the CP line and the gold spray layer, the contact loss between the gold spray layer and the end face of the element and the plate loss. . It can be expressed by the following formula:
tgδ=tgδlead+tgδ1+tgδ2+tgδpole In the
formula: tgδlead
: lead loss
tgδ1 : contact loss between CP line and gold spray layer
tgδ2 : contact loss between gold spray layer and element end face
tgδ pole: Plate loss
2.1 Lead loss
tgδ = wC*r = 2πfc*4/π*ρι/d2*kd
In the formula:
ρ: resistivity/Ω.cm; d: diameter/cm; ι: length/cm; k: constant copper k=0.106 related to wire material The company uses tinned copper-clad steel wire, so the above formula is simplified as :
tgδ = 0.85*ρι/d2*f*C
, it can be seen that the loss is proportional to the length and inversely proportional to the diameter. When used at a higher frequency, the lead loss is a non-negligible part.
2.2 Plate loss
The schematic diagram of the metallized film capacitor plate is as follows:
The reason for the increase of the r pole is that the R□ of the metallized film increases during the processing process, and the other reason is that the square resistance increases due to the oxidation of the metal layer. Therefore, the tension should be uniform during the winding process of the element, and the element should not leave any gaps. , to prevent oxidation of the metal layer.
a) The contact loss between the CP wire and the gold spray layer and the contact loss between the gold spray layer and the end face of the element
In the case of good welding, the contact loss tgδ1 between the CP wire and the gold spray layer should be 0. The gold spray layer and the end face of the element Due to the existence of a certain contact resistance, the quality of the gold spray layer is very important. The main reason for the increase of the capacitor DF is also determined by these two contact points.
Output 2.1KV Energy Storage 2.5KV Pulse Test
1 Test Condition
Temperature: 25℃ Use Instrument: Pulse Tester Output Voltage: 2100Vdc
Humidity: 75% Valid Date: 2008/10/29 Energy Storage Voltage: 2500Vdc (
Test Sample) 0.1UF
/275V D2
Numbering |
Before the test |
After the test |
change value |
anatomy of prime |
determination |
||||
Capacitance |
DF |
Capacitance |
DF |
Capacitance change rate % |
DF value change |
OK& OF |
|||
1 |
100.460 |
1 |
97.553 |
3 |
-2.89% |
2 |
normal |
OK |
|
2 |
100.870 |
1 |
100.950 |
1 |
0.08% |
0 |
normal |
OK |
|
4 |
96.301 |
1 |
94.078 |
25 |
-2.31% |
twenty four |
End face depression |
of |
|
5 |
98.286 |
1 |
98.825 |
4 |
0.55% |
3 |
normal |
OK |
|
6 |
97.608 |
2 |
98.267 |
8 |
0.68% |
6 |
normal |
OK |
|
7 |
99.332 |
1 |
99.142 |
19 |
-0.19% |
19 |
Welding pressure is too high |
of |
|
8 |
97.202 |
1 |
97.193 |
2 |
-0.01% |
1 |
normal |
OK |
|
9 |
98.886 |
1 |
99.318 |
2 |
0.44% |
1 |
normal |
OK |
|
10 |
98.174 |
1 |
98.735 |
2 |
0.57% |
1 |
normal |
OK |
The power loss of the capacitor p=V2* W*C*tgδ It can be seen that the increase of tgδ under the condition of constant voltage and power will directly lead to the increase of power loss of the capacitor. From the above analysis, it can be seen that improvement measures should be taken in the following aspects.
1. The end face of the element is flat, not recessed & core-pulling.
2. The gold spraying operation is strictly based on the SOP operation, and its parameters cannot be changed at will.
3. The excitation adjustment between the welding heads is appropriate, and the welding pressure is moderate.
4. Welding current should not be too large, set according to SOP. If the instantaneous current is too large, the end face of the element near the lead will shrink, causing the contact resistance to increase. The use of short-circuit charging and discharging high frequency (10KHz) can eliminate early failure products.
Speaker: Zheng Shikang
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