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X-11181-002 Datasheet(PDF) 13 Page - Linear Technology |
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X-11181-002 Datasheet(HTML) 13 Page - Linear Technology |
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13 / 20 page  LT3798 13 3798f Switch Voltage Clamp Requirement Leakage inductance of an offline transformer is high due to the extra isolation requirement. The leakage inductance energy is not coupled to the secondary but goes into the drain node of the MOSFET. This is problematic since 400V and higher rated MOSFETs cannot always handle this energy by avalanching. Therefore the MOSFET needs protection. A transient voltage suppressor (TVS) and diode are recommended for all offline application and connected, as shown in Figure 3. The TVS device needs a reverse breakdown voltage greater than (VOUT + VF) • NPS where VOUT is the output voltage of the flyback converter, VF is the secondary diode forward voltage, and NPS is the turns ratio. An RCD clamp can be used in place of the TVS clamp. period, as well. Similarly, initial values can be estimated using stated switch capacitance and transformer leakage inductance. Once the value of the drain node capacitance and inductance is known, a series resistor can be added to the snubber capacitance to dissipate power and criti- cally dampen the ringing. The equation for deriving the optimal series resistance using the observed periods (tPERIOD, and tPERIOD(SNUBBED)) and snubber capacitance (CSNUBBER) is below, and the resultant waveforms are shown in Figure 4. OPERATION Figure 3. TVS & RCD Switch Voltage Clamps 3798 F03 GATE VSUPPLY GATE VSUPPLY In addition to clamping the spike, in some designs where short circuit protection is desired, it will be necessary to decrease the amount of ringing by using an RC snubber. Leakage inductance ringing is at its worst during a short circuit condition, and can keep the converter from cycling on and off by peak charging the bias capacitor. On/off cycling is desired to keep power dissipation down in the output diode. Alternatively, a heat sink can be used to manage diode temperature. The recommended approach for designing an RC snubber is to measure the period of the ringing at the MOSFET drain when the MOSFET turns off without the snubber and then add capacitance—starting with something in the range of 100pF—until the period of the ringing is 1.5 to 2 times longer. The change in period will determine the value of the parasitic capacitance, from which the parasitic inductance can be determined from the initial Figure 4. Observed Waveforms at MOSFET Drain when Iteratively Implementing an RC Snubber TIME (μs) 0 0 10 30 40 50 0.20 90 3798 F04 20 0.10 0.05 0.25 0.15 0.30 60 70 80 NO SNUBBER WITH SNUBBER CAPACITOR WITH RESISTOR AND CAPACITOR CPAR = CSNUBBER tPERIOD(SNUBBED) tPERIOD ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ 2 –1 LPAR = tPERIOD2 CPAR •4π2 RSNUBBER = LPAR CPAR Note that energy absorbed by a snubber will be converted to heat and will not be delivered to the load. In high volt- age or high current applications, the snubber may need to be sized for thermal dissipation. To determine the power dissipated in the snubber resistor from capacitive losses, measure the drain voltage immediately before the MOS- FET turns on and use the following equation relating that |
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