Dear All,
Here is details of design and problem we are facing with LM5007.
We currently using LM5007 Switching IC as Non Isolated Buck Converter with following Specifications.
- Vin : 12 to 75 VDC
- Output Voltage : 12 Volt , 4000mA
The reference design is taken from datasheet of LM5007 .
We currently facing following queries for reference design.
A) With above design we tested it for Vout =12 V,I=250mA for Vin=48 VDC,for this its working fine.
However when we using it for higher current rating i.e 300 mA or up to 400 mA,we facing Heating of IC which is not desire for our applications.
We also conducting EMI/EMC test,for that we added FUSIBLE resistor at input,if we ignore this resistor then EMI/EMC Surge test gets failed.
By keeping resistor we passing Surge test but we getting heating of resistor up to 92°C above 300 mA.So for Surge test we can not ignore this resistor.
B) When we using same design with Vin = 24 VDC for Vout = 12 VDC ,I=250 mA,the source current drawing from input is 120mA.So we getting heating of input resistor.
We also added PTC at Input that also gets open when source current 120 mA.When we removing PTC ,the circuit is working but resistor at input gets Hot.
For this case No heating of LM5007 were observed. We cannot ignore input resistor for EMI/EMC.
Please also suggest whether source current is desire or not for this case.
C) In LM5007 Specification sheet the current rating is given up to 700 mA , we want to use this device up to 500 mA.
Please suggest design changes for same.
The resistor and PTC used is as below
- R=47E /5 W
- PTC 60 Volt ,100mA
I have a attached reference design which we are using currently and reading taken for 48 VDC and 24 VDC.
| Power Supply Specifications : | ||||||||
| Supply Voltage = 48 VDC , +/-20% i.e 38.4 VDC to 57.6 VDC | ||||||||
| Output Voltage = 11.709 VDC at 48 VDC input | ||||||||
| Load Current = 250mA | Programmable resistor values R1 =3K and R2=820E | |||||||
| Readings of LM 5007 for above specifications | ||||||||
| LM5007 | ||||||||
| Source Voltage |
Source Current (mA) |
Pin no.8 (VIN) |
Pin no.7 (VCC) |
Pin no.5 (F/B) |
Pin no.1 (SW) |
Output Voltage After Inductor |
||
| 35 | 122 | 27.973 | 6.981 | 2.5052 | 11.672 | 11.686 | ||
| 38 | 109 | 31.7 | 6.992 | 2.507 | 11.665 | 11.692 | ||
| 39 | 107 | 32.86 | 6.996 | 2.5072 | 11.661 | 11.695 | ||
| 42 | 98 | 36.38 | 7.007 | 2.5086 | 11.652 | 11.701 | ||
| 45 | 91 | 39.66 | 7.017 | 2.5096 | 11.644 | 11.705 | ||
| 48 | 86 | 43 | 7.027 | 2.5105 | 11.637 | 11.709 | ||
| 52 | 79 | 47.41 | 7.039 | 2.5114 | 11.629 | 11.715 | ||
| 55 | 75 | 50.61 | 7.049 | 2.5121 | 11.625 | 11.718 | ||
| 57 | 74 | 52.79 | 7.054 | 2.5123 | 11.623 | 11.719 | ||
| 58 | 73 | 53.74 | 7.057 | 2.5125 | 11.622 | 11.72 | ||
| 60 | 71 | 55.88 | 7.062 | 2.5129 | 11.619 | 11.724 | ||
| For 24 Vdc input by removing 47 ohm and PTC | ||||||||
| 24 | 117 | 23.25 | 6.966 | 2.503 | 11.675 | 11.676 | ||