For a switching power supply (SMPS)engineer, it is common to make a choice before the conditions that in one pair or more pairs. The topic we are discussing today is a pair under the opposing conditions.
A, First segmented MOS tube VD:
Ⅰ, the input DC voltage VDC;
Ⅱ, secondary reflection primary VOR;
Ⅲ, the main pipe VD MOS margin VDS;
Ⅳ, RCD absorption of the effective voltage VRCD1.
B, Primary MOS tube for more than a few parts of the VD calculation:
Ⅰ, the input DC voltage VDC.
When calculating the VDC, the maximum input voltage is the standard. For example, wide voltage should be AC265V, namely DC375V.
VDC = VAC * √ 2
Ⅱ, secondary reflection primary VOR.
VOR is the highest according to the secondary output voltage, maximum pressure drop calculation of rectifier diodes, such as output voltage: 5.0V ± 5% (calculated according to Vo = 5.25V), diode VF is 0.525V .
VOR = (VF Vo) * Np / Ns
Ⅲ, the main pipe VD MOS margin VDS.
VDS is a MOS pipe by the minimum value of 10% VD. If KA05H0165R' VD = 650 should be selected DC65V.
VDC = VD * 10%
Ⅳ, RCD absorption VRCD.
VD MOS tube minus ⅰ, ⅲ maximum of three to the rest of VRCD. The actual VRCD should be selected maximum 90% (mainly taking the dispersion of each component switching power supply, temperature drift and time drift and other factors into account).
VRCD = (VD-VDC-VDS) * 90%
Note: ① VRCD theoretical value is calculated and then adjusted through the experiment, making the actual value is consistent with the theoretical value.
② VRCD must be 1.3 times greater than the VOR. (If less than 1.3 times the value of the main MOS tube choice too low VD)
③ MOS tube should be less than VDC 2 times. (If more than 2 times, then pipe the main MOS value is too big VD)
④ If VRCD measured value is 1.2 times less than the VOR, then the RCD snubber circuit to affect the power efficiency.
⑤ VRCD is formed by the VRCD1 and VOR
Ⅴ, RC time determined by constant τ.
τ is the operating frequency switching power supply (SMPS) according to set generally choose 10 to 20 switching power supply cycle.
C, Test value adjustment VRCD
First, assuming that a RC parameter, R = 100K/RJ15, C = 10nF/1KV. They should obey Re-listed electricity followed after the first low pressure, and then from light load to the overload principles. In the tests should be closely monitoring the voltage on the RC components, be sure to VRCD less than the calculated value. If reached the calculated values, they should immediately power off until the R value decreases, the repeat test. An appropriate RC value should be at the highest input voltage, the most important power load, VRCD test value equal to the theoretical value.
D, worth attention’ test phenomenon
The lower Input supply voltage, higher VRCD. The heavier the load is, the higher VRCD is. Then the minimum input voltage, heavy load VRCD test value if greater than the value and more than theoretical calculations VRCD and (iii) whether contradict the content? That do not conflict at the highest theoretical value is calculated when the input voltage, and now is the low input voltage. Heavy Load is the possible maximum load of switching power supply, which is measured primarily through testing the limits of the power switching power supply.
E, RCD snubber circuit in the power of choice R value
R measured by the power of choice is VRCD the maximum figures. The actual power should be greater twice than of the computing power.
The Editor: If R-value of RCD snubber circuit is too small, it will reduce the switching power supply (SMPS) efficiency. However, if the R value is too large, MOS tube risks breakdown. |
